Postgraduate Educational Course in Radiation Protection and the Safety

of Radiation Sources

PART I

REVIEW OF FUNDAMENTALS

Module I.4 Source of radiation

Lecture Notes

Natural radiation

Man is exposed to two main sources of radiation ,namely:

1. Natural radiation2. Human made radiation sources and 3. Radiation generators

Session I 401 Terrestrial Radionuclides

Natural sources of terrestrial radiation originate from radiation coming from the earth.Natural radioactive decay chains occurring in the earth and some important, specific radionuclides contribute to human radiation exposure.

Three very important naturally occurring terrestrial radionuclides are U-238, U-235, and Th-232. They are actually the parents of three long radioactive decay chains, all of which end in stable lead nuclei. Some nuclides, like Th-232 have several members in their decay chains. You can roughly follow the chain starting with Th-232

Th-232 --> Ra-228--> Ac-228 -->Th- 228 --> Ra-224 --> Rn-220--> Po-216 --> Pb-212--> Bi-212 --> Po-212 --> Pb-208 (stable)

Table 4:1 gives a summary of the properties of the three primordial terrestrial radionuclides

Table 4:1 Summary of the properties of the three primordial terrestrial radionuclides

Nuclide Half -life Natural Activity

235U 7.04 x 10

8 yr 0.711% of all natural uranium

238U 4.47 x 10

9 yr

99.275% of all natural U; 0.5 to 4.7 ppm total U in common rocks

232Th 1.41 x 10

10 yr 1.6 to 20 ppm in common rocks

226Ra 1,600 yr 16 Bq/kg in limestone and 48 Bq/kg in

igneous rock

222Rn

3.82 days Noble gas; average annual air concentrations in US range from 0.6 to 28

Bq/m3

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40K 1.28 x 10

9 yr 0.037 to 1.1 Bq/g in soil

Additional important primordial radionuclides include Ra-226, Rn-222, and K-40. Ra-226 decays to Rn-222. K-40 is found not only in the soil but in foods such as bananas.Rn-222, which is formed by decay of Ra-226, is much more important from a dose standpoint than Rn-220, which is formed from the Th-232 decay chain. This is because the half-life of Rn-220 is very short (55 s) compared with the 3.8 day half-life of Rn-222.

Background radiation

There are three naturally occurring radioactive decay chains which give rise to natural back ground radiation. There are three decay “chains” that occur in nature are:

1.the uranium series, beginning with 238U2.the thorium series, which originates with 232Th3.the actinium series, which originates with 235U

Once upon a time there was also a neptunium series, which originated with 241Pu, that has a half-life of only 14 years. The only remaining member of this series is 209Bi with a half-life of 2ˣ 1018 years.

238U decay Chain

The U-238 decay chain is one of the most important from a radiation dose level viewpoint. It forms Rn-222 and ends in stable lead. Half-lives of its member radionuclides vary greatly. Figure 4:1 shows only alpha and beta radiation being emitted, gamma rays and X-rays are also emitted from this decay chain

Figure 4:1 . 238U decay Chain

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Figure 4:1 238U decay chainFigure 4:2 shows 235U decay chain

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Figure 4:2 235U decay chain.

Figure 4:3 shows 232Th Decay series

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Figure 4:3 232Th decay series

Natural radioactivity in soil

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Natural radioactivity occurs soils of different composition. Table 4: 2 show some estimated values.

ElementAssumed

activity per gm

Mass of the element

Activity

Uranium 25 Bq/kg 2,200 kg 31 GBq

Thorium 40 Bq/kg 12,000 kg 52 GBq

Potassium-40 400 Bq/kg 2,000 kg 500 GBq

Radium 48 Bq/kg 1.7 g 63 GBq

Radon 10 kBq/m3 11 mg 7.4 GBq

Natural Radioactivity in Building materials

Naturally occurring radioactivity in building materials contributes to external gamma ray exposure. Ra-226 in the building material can be a source of Rn-222 emanating into the inside air. People who inhale the air and are exposed.

A well-known exposure problem historically in the western US has been the incorporation of uranium mill tailings into concrete block. This has caused concern both from an external exposure pathway (gamma exposure) and, more importantly, internal exposure from inhalation of Rn produced from Ra-226 decay.

Table 4:3 gives some typical specific activity ( mBq /mg)in building materials in the US.

MaterialSpecific Activity ( mBq/mg)

Uranium Thorium Potassium

Granite 63 8 1184

Sandstone 6 7 414

Concrete 31 8.5 89

Wallboard 14 12 89

Gypsum 186 66 5.9

Clay Brick 111 44 666

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Natural radioactivity can also be found in food , water and in the air.

Session I 402 Cosmic radiation

Cosmic radiation are high energy radiation from outer space.. Cosmic radiation interacts with our atmosphere to produce cosmogenic radionuclides. It also is responsible for whole body doses. Cosmic radiation is divided into two types, primary and secondary.

Primary cosmic radiation is made up of extremely high energy particles (up to 1018 eV), and are mostly protons or sometimes larger particles. A large percentage of it comes from outside of our solar system and is found throughout space. Some of the primary cosmic radiation is from our sun, produced during solar flares.

Little of the primary cosmic radiation penetrates to the Earth's surface, the vast majority of it interacts with the atmosphere. When it does interact, it produces the secondary cosmic radiation, or what we actually see here on Earth. These reactions produce other lower energy radiations in the form of photons, electrons, neutrons and muons that make it to the surface.Background RadiationMost of the cosmic rays incident on the earth’s atmosphere are believed to be of galactic origin and to have been accelerated to their present energies by interstellar magnetic fields. During solar flares the sun contributes significantly to the low-energy (mostly less than 1 GeV) cosmic-ray flux arriving at the earth.Another important effect of the sun is the decrease in galactic cosmic-ray flux reaching us during periods of intense sun-spot activity; this is most likely a magnetic phenomenon.Cosmic radiation contributes to the background radiation on earth.The earth’s atmosphere provides shielding from most of the cosmic radiation. The annual dose you get from cosmic radiation depends on what altitude you are at. From cosmic radiation, the average person in the U.S. receives a dose of 0.27 mSv per year and this roughly doubles for every 1,829 m increase in elevation..

Cosmogenic Radionuclides

Cosmic radiation permeates all of space, the source being primarily outside our solar system. The radiation is in many forms, from high speed heavy particles to high energy photons and muons. The upper atmosphere interacts with many of the cosmic radiations and produces radioactive nuclides. They can have long half-lives, but the majority have shorter half-lives than the primordial nuclides. Table 4.4 shows some common cosmogenic nuclides.

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Table 4:4 Cosmogenic radionuclides

Nuclide Half-life SourceNatural Activity

14C 5730 yr Cosmic-ray interactions, 14N(n,p)14C

0.22 Bq/g

3H 12.3 yr Cosmic-ray Interactionswith N and O

1.2 x 10-3 Bq/kg

7Be 53.3 days Cosmic-ray Interactions with N and O

0.01 Bq/kg

Flying can add a few extra mSv to your annual dose, depending on how often you fly, how high the plane flies, and how long you are in the air

There is only about a 10% decrease at sea level in cosmic radiation rates when going from pole to the equator, but at 55,000 feet the decrease is 75%. This is on account of the effect of the earth's and the Sun's geomagnetic fields on the primary cosmic radiations.

UNSCEAR Report 2008 give the world's average effective dose due to natural radioactivity UNSCEAR as follow ( Table 4:5):

Table 4:5 Global annual average and ranges of individual dose s due to natural sources of radiation

Natural sources World annual average dose(mSv)

Typical range of individual doses(mSv)

Remarks

External terrestrial 0.48 0.3-1.0 The dose is high in some locations

Ingestion 0.29 0.2-1.0Inhalation (radon gas ) 1.26 0.2 - 10.0 The dose is nuch

higher in some dwellings

Cosmic radiation 0.39 0.3-1.0 The dose increases with altitude

Total natural 2.4 1-13 Sizeable population groups receive in the

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range 10-20 mSv

Human Made radioactive sourses

Session I 403 Radioactive Sources

Radioactive sources are found in reactors and no- reactors sources of production.In reactor applications, there are basically 3 sources of radionuclides. The fuel itself, the radionuclides produced by fission of the uranium and activation products (stable materials made radioactive by bombardment with neutrons from the fission process).

In materials applications, there are many areas where radionuclides are employed. The radionuclides are obtained either from a nuclear reactor or from particle accelerators. Some sources result from Naturally Occurring Radioactive Material sometimes referred to as NORM.

Radiation emitted by the radioactive sources

Some sources emit alpha, beta and gamma radiation. Some of these sources are used for calibration of instruments. Others are selected because they are pure alpha, beta or gamma emitters. A source which emits alpha, beta and gamma can be converted to a virtually pure gamma emitter by the use of shielding..Americium-241is used in smoke detectors while mantles used for kerosene or propane heating units or lamps contain Thorium-232. These devices are readily available and are often used to demonstrate the presence of radioactive material in commercially available items. There are very few pure alpha emitters. Most also emit betas and gamma radiation. Polonium-210 is one of the few which is almost a pure alpha emitter (it actually emits a gamma ray but with a very low yield).Beta sources are probably the most commonly used in commercially-available items. When combined with a phosphor they can emit light which is useful for night illumination. This slide illustrates some compasses, markers, exit signs and airport runway markers all containing beta emitting radioactive material, usually tritium (hydrogen-3). The item in the lower right hand corner of the slide is a beta applicator for medical treatment of abnormalities of the eye. It uses strontium-90. Promethium-147 is also commonly used..Sealed sources are almost always gamma sources since the encapuslating material acts to shield beta and alpha radiation. Most high energy gamma emitters such as cobalt-60 and

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caesium-137 are also beta emitters. Encapsulating the source results in shielding the beta radiation but permitting the gamma radiation to escape.

Neutron SourcesThere are very few radioactive sources that emit neutrons. Neutrons are usually generated by particle accelerators, nuclear reactors or alpha-beryllium sources. The only radioisotope commonly used for neutron calibration and neutron radiography is californium-252.X-ray SourcesX-ray sources are plentiful. Most medical facilities have numerous X-ray machines for diagnosis. Treatment centers usually have linear accelerators which accelerate electrons. The electron beam can be used directly or it can be converted to X-rays by using a target.X-ray units are widely used in dentistry and in veterinary practice. X-ray machines are also used for scanning inanimate objects such as in airport baggage scanners.

Session I 405 Production of IsotopesMost commercially available isotopes used in Medicine and Industry are produced by neutron bombardment in reactors (Byproduct Material) or by charged particle bombardment in accelerators (NARM).Some isotopes are obtained from the decay of other isotopes which were produced by the processes listed below.. An examples of this type of isotope is 99mTc which is obtained from the decay of 99Mo which is itself derived from fission (Byproduct Material).

Examples: 137Cs 60Co 131I 90Sr 192Ir

FalloutFallout is a term used to describe radioactive material which has become airborne as a result of nuclear weapons testing in the atmosphere or as a result of large scale accidents such as that which occurred at the Chernobyl nuclear reactorThe airborne material is carried into the atmosphere and is deposited (falls out of the sky) on remote locations

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The most common radionuclides observed are 137Cs, 90Sr and 131I, all of which can be deposited on vegetationThe vegetation may be consumed by people (direct exposure) or by animals which are then consumed by people (indirect exposure)Radionuclides also appear in the products derived from animals which are ultimately consumed by people (e.g., milk from cows)Fallout contributes less than 0.3% of annual radiation exposure in the United States.Due to fallout from Nuclear testing people in some areas of the United States received external and internal radiation exposure from short-lived radioactive materials for a few days or weeks following the weapons tests until the radioactive materials had decayed to levels indistinguishable from normal background radiation. One short-lived radioactive material of interest for internal exposure was iodine-131. Although a wide range of radionuclides are produced during a nuclear explosion, many of them decay and only a few enter the food-chain. The most important radionuclides which enter the food-chain are given in the above table.Of these 131I is important only in foods produced and consumed within a short period following the fallout, such as fresh milk and fresh vegetables. Other radionuclides accumulate in foods, depending on the uptake of these nuclides by plants which can occur via absorption through leaves on which the fallout nuclides are deposited or through root uptake from the soil.In the case of iodine-131, due to its short half-life, foliar absorption is the main route of uptake by plants. It enters man mainly through milk resulting from the grazing of cattle on pastures in the fallout area.Evidence to date indicates that milk is by far the most important contributor of the fallout nuclide 131I to the human diet. However, ingestion of 131I through milk is significant only for a short period, immediately following a nuclear explosion or a disaster and their consequent fallout. Available evidence indicates that iodine-131 in milk from cows left on contaminated pasture decreases with a half-life of about five days.Iodine-131 in contaminated milk is present as iodide. Such contaminated milk may contain about 10 kBq/l (~0.3 µCi/l) of iodine-131. Radioiodine is therefore a problem with fresh milk and does not persist in processed milk products such as milk powder, etc.Milk is an important route of entry of 89Sr and 90Sr from fallout to humans. Following atmospheric nuclear-weapon testing, these nuclides in milk reached a level of about 250 Bq/g (~6 pCi/g) Ca in the UK and 400 – 550 Bq/g (~11-15 pCi/g) Ca in the US in the 1950s. Because of their long lives, 90Sr (28 years) and 89Sr (50 days) can persist in milk products for a long time. Table 4:6 shows some important fallout radionuclides

Table 4:6: some important fallout radionuclides

Radionuclide Half-life Critical Food Group

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90Sr

28 years milk, cereals, vegetables

89Sr

51 days milk

137Cs

30 years milk and meat

131I

8 days milk

Session I 406 Fission and Fusion

FissionThe fission reaction in U-235 produces fission products such as Ba, Kr, Sr, Cs, I and Xe with atomic masses distributed around 95 and 135. Examples of typical reaction products are shown below:235U + n → 141Ba + 92Kr + 3n + 170 MeV 235U + n → 94Zr + 139La + 3n + 197 MeV

In these equations, the number of nucleons (protons + neutrons) is conserved, e.g. 235 + 1 = 141 + 92 + 3. A small loss in atomic mass is the source of the energy released. Both the barium and krypton isotopes subsequently decay and form more stable isotopes of neodymium and yttrium, with the emission of several electrons from the nucleus (beta decays). It is the beta decays, with some associated gamma rays, which make the fission products highly radioactive although the radioactivity decreases with time.Fission may take place in any of the heavy nuclei after capture of a neutron. However, low-energy (slow or thermal) neutrons are able to cause fission only in those isotopes of uranium and plutonium whose nuclei contain odd numbers of neutrons (e.g. U-233, U-235, and Pu-239). For nuclei containing an even number of neutrons, fission can only occur if the incident neutrons have energy above about one million electron volts (1 MeV).The probability that fission or any another neutron-induced reaction will occur is described by the cross-section for that reaction. The cross-section may be imagined as an area surrounding the target nucleus and within which the incoming neutron must pass if the reaction is to take place. The fission cross sections increase greatly as the neutron velocity decreases. In nuclei with an

Module I.4 Sources of radiation 13

odd-number of neutrons, such as U-235, the fission cross-section becomes very large at thermal energies.(Figure 4:4)For the purposes of the fission process neutrons are classified into several categories list in slide 7 of the power point presentation.

Figure 4:4 Fission cross sections for fission of Uranium and Plutonium

About 85% of the energy released is initially the kinetic energy of the fission fragments. However, in solid fuel they can only travel a microscopic distance, so their energy becomes converted into heat. The balance of the energy comes from gamma rays emitted during or immediately following the fission process and from the kinetic energy of the prompt neutrons. A small proportion (0.7% for U-235, 0.2% for Pu-239) are delayed, as these are associated with the radioactive decay of certain fission products. The longest delayed neutron group has a half-life of about 56 seconds.

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Figure 4:5 :Fission processFigure 4:6 shows the fission yield as a function of the mass number

Figure 4:6 Fission yield as a function of the mass numberCriticality

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The delayed neutron release is the crucial factor enabling a chain reacting system (or reactor) to be controllable and to be able to be held precisely critical. At criticality the chain reacting system is exactly in balance, such that the number of neutrons produced in fissions remains constant. This number of neutrons may be completely accounted for by the sum of those causing further fissions, those otherwise absorbed, and those leaking out of the system. Under these circumstances the power generated by the system remains constant. To raise or lower the power, the balance must be changed (using the control system) so that the number of neutrons present (and hence the rate of power generation) is either reduced or increased. The control system is used to restore the balance when the desired new power level is attained.The fission chain reaction can be initiated by introducing some neutrons into the fissile material. This can be done by inserting an Am-Be, Pu-Be, Po-Be or Ra-Be source. The Am, Pu, Po or Ra emits alpha particles which release neutrons from the beryllium as it turns to carbon-12. Using U-235 in a thermal reactor as an example, when a neutron is captured, the total energy is distributed amongst the 236 nucleons (protons & neutrons) now present in the compound nucleus. This nucleus is relatively unstable and it is likely to break into two fragments of around half the mass. These fragments are nuclei found near the middle of the Periodic Table and the probabilistic nature of the break-up leads to several hundred possible combinations as seen in Figure 4.5. Creation of the fission fragments is followed almost instantaneously by emission of a number of neutrons (typically 2 or 3, average 2.5), which enable the chain reaction to be sustained.

Criticality has three stages which are: Sub-Critical (keff < 1) – more neutrons lost by escape from system and/or non-fission

absorption by impurities or “poisons” than produced by fission. Critical (keff = 1) – one neutron per fission available to produce another fission Super-Critical (keff > 1) – rate of fission neutron production exceeds rate of loss

For the control of fission one needs to understand the following.Fission of U-235 nuclei typically releases 2 or 3 neutrons with an average of about 2.5. One of these neutrons is needed to sustain the chain reaction at a steady level of controlled criticality; on average, the other 1.5 leak from the core region or are absorbed in non-fission reactions.Neutron-absorbing control rods are used to adjust the power output of a reactor. These typically use boron and/or cadmium (both are strong neutron absorbers) and are inserted among the fuel assemblies. When they are slightly withdrawn from their position at criticality, the number of neutrons available for ongoing fission exceeds unity (i.e. criticality is exceeded) and the power level increases. When the power reaches the desired level, the control rods are returned to the critical position and the power stabilizes.

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The ability to control the chain reaction is entirely due to the presence of the small proportion of delayed neutrons arising from fission. Without these, any change in the critical balance of the chain reaction would lead to a virtually instantaneous and uncontrollable rise or fall in the neutron population. It is also relevant to note that safe design and operation of a reactor sets very strict limits on the extent to which departures from criticality are permitted. These limits are built in to the overall design.Neutrons released in fission are initially fast (velocity about 107 m/sec, or energy above 1 MeV), but fission in U-235 is most readily caused by slow neutrons (velocity about 103 m/sec, or energy about 0.02 eV). A moderator material comprising light atoms thus surrounds the fuel rods in a reactor. Without absorbing too many, it must slow down the neutrons in elastic collisions (compare it with collisions between billiard balls on an atomic scale). In a reactor using natural (unenriched) uranium the only suitable moderators are graphite and heavy water (these have low levels of unwanted neutron absorption). With enriched uranium (i.e. increased concentration of U-235), ordinary (light) water may be used as moderator. Water is also commonly used as a coolant, to remove the heat and generate steam.Commercial power reactors are usually designed to have negative temperature and void coefficients. The significance of this is that if the temperature should rise beyond its normal operating level, or if boiling should occur beyond an acceptable level, the balance of the chain reaction is affected so as to reduce the rate of fission and hence reduce the temperature. One mechanism involved is the Doppler effect, whereby U-238 absorbs more neutrons as the temperature rises, thereby pushing the neutron balance towards subcritical. Another mechanism in light water reactors is that the formation of steam within the water moderator reduces its density and hence its moderating effect, and this again tilts the neutron balance towards subcritical.While fuel is in use in the reactor, it is gradually accumulating fission products and transuranic elements which cause additional neutron absorption. The control system has to be adjusted to compensate for the increased absorption. When the fuel has been in the reactor for three years or so, this build-up in absorption, along with the metallurgical changes induced by the constant bombardment of the fuel materials, dictates that the fuel should be replaced. This effectively limits the burn-up to about half of the fissile material, and the fuel assemblies must then be removed and replaced with fresh fuel. Fusion ProcessNuclear Fusion is the energy-producing process which takes place continuously in the sun and stars. In the core of the sun at temperatures of 10-15 million degrees Celsius, Hydrogen is converted to Helium providing enough energy to sustain life on earth. Researchers stress that nuclear fusion has an almost unlimited potential to supply electricity. The hydrogen isotopes in one gallon of water have the fusion energy equivalent of 300 gallons of gasoline. A nuclear fusion power plant would also have no greenhouse gas emissions, and would

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generate none of the long lived, high level radioactive waste associated with conventional nuclear fission power plants. Despite its theoretical potential, leading experts predict that the world is still at least 50 years and billions of research dollars away from having electricity generated from nuclear fusion. This is largely due to the enormous size and complexity of a reactor that would be capable of sustaining nuclear fusion. Nuclear fusion involves the binding together of hydrogen atoms, creating helium. The total mass of the final products is slightly less, one percent, than the original mass, with the difference being given off as energy. If this energy can be captured, it could be used to generate electricity. Deuterium and tritium can be combine to produce helium and energy.D + T → 4He + EnergyEnergy release per nucleon is larger for A = 2 or 3 (deuterium or tritium) than for A = 235 (uranium) as shown in Figure 4:7 .

Figure 4:7 : Energy per Nucleon (MeV) for Fusion compared to Fission

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Energy per Nucleon (MeV)

-10

-8

-6

-4

-2

0

50 100 150 200 2500Atomic Mass Number

FissionA 56

FusionA 56

Tritium has two extra neutrons and is very rare, because it is naturally radioactive and decays quickly. Tritium can be manufactured by bombarding the naturally occurring element lithium with neutrons from either a fission or fusion reactor. Current thinking is that tritium would be created by having a "blanket" made of lithium surrounding a containment vessel. A reactor such as this, which "breeds" its own fuel, is called a breeder reactor. Fusion can only be accomplished at temperatures typical of the centre of stars, (~100 million degrees Celsius) .At such temperatures, the fusion components exist in the form of a plasma, where atoms are broken down into electrons and nuclei. No known solid material could withstand the temperatures involved in nuclear fusion. Therefore, a powerful confinement system is required to keep the plasma away from the walls of the vessel in which it is contained. Fusion Fuels Deuterium is abundant as it can be extracted from all forms of water. If all the world's electricity were to be provided by fusion power stations, Deuterium supplies would last for millions of years. For example, 10 grams of Deuterium which can be extracted from 500 litres of water and 15g of Tritium produced from 30g of Lithium would produce enough fuel for the lifetime electricity needs of an average person in an industrialised country.

Current Fusion ResearchFusion research was big news in 1989 when it was reported that scientists had achieved fusion at room temperatures with simple equipment. Unfortunately, the scientists involved could not prove their claims. Therefore, present day fusion researchers still cannot avoid their greatest barrier, the ultra hot temperatures required for sustaining nuclear fusion. There are currently two methods of confining the hot plasma which are being studied around the world, "magnetic confinement" and "inertial confinement". Most experts feel that magnetic confinement has the greatest potential and most of the recent research on fusion reactors has been based on the "TOKOMAK" system. Tokomak is an acronym for the Russian words "torroidal magnetic chamber". The tokomak system was developed in the former U.S.S.R. and has been studied extensively in a number of countries. Work began in 2007 on the ITER project at the Cadarache site in France. This is an ambitious project aimed at taking the Tokomak system from the level of scientific research to an level closer to a power-producing reactor. The design intent is that 500 MW of power will be produced for every 50 MW of power input. It is a multi-billion euro project funded by the EU, India, Japan, China, Russia, South Korea and the USA. A torroidal magnetic chamber is a doughnut-shaped steel structure in which the fusion plasma is confined by means of powerful coils of super-conducting material which create a strong magnetic field.

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The other method of confining a fusion plasma is inertial confinement, where small amounts of a deuterium-tritium mixture are rapidly heated to extremely high temperatures with a high powered laser beam or a beam of charged particles. Very high power lasers are needed, and work on inertial confinement is not as far advanced as that on magnetic confinement. The largest test facility is the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory. In November 1991, the British-based Tokomak (JET) reported that it had achieved break even conditions. This occurs when the energy given off by the fusion reaction is equal to the energy input required to sustain the reaction. In order for a fusion reaction to generate useful amounts of electricity, the energy given off must be many times greater than that required to sustain the reaction. Even the most optimistic researchers feel that it will be the second half of the century before this stage is reached.

Fusion Power PlantsA full scale fusion reactor capable of generating 1000 MW (1 MW = 1 million watts) of electricity, comparable to conventional nuclear power plants, would be a very large and complex machine. While fission reactors can be made small enough to be used in submarines or satellites, the minimum size and output of a fusion reactor would be similar to that of today's largest nuclear plants. Although a fusion reactor capable of generating electricity has never been built, the difficult part is creating a sustainable fusion reaction. Capturing the energy given off by the reaction in the form of heat and transforming the heat to electricity is very similar to generating electricity from a conventional fission reactor. A 1000 MW fusion generator would have a yearly fuel consumption of only 150 kg of deuterium and 400 kg of lithium.

Advantages of FusionNuclear fusion, if it can be developed, would have several advantages over conventional fossil fuel and nuclear fission power plants. The fuels required for fusion reactors, deuterium and lithium, are so abundant that the potential for fusion is virtually unlimited. Oil and gas fired power plants as well as nuclear plants relying on uranium will eventually run into fuel shortages as these non-renewable resources are consumed. Like conventional nuclear plants, fusion reactors have no emission of carbon dioxide, the major contributor to global warming or sulphur dioxide, the main cause of acid rain. Fossil fuel power plants burning coal, oil and natural gas are large contributors to global warming and acid rain. One of the barriers to the widespread use of conventional nuclear power plants has been public concern over operational safety, and the disposal of radioactive waste. Major accidents, such as Chernobyl, are virtually impossible with a fusion reactor because only a small amount of fuel is in the reactor at any time. It is also so extremely difficult to sustain a fusion reaction, that should anything go wrong, the reaction would invariably stop.

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Long lived highly radioactive wastes are generated by conventional nuclear plants; these must be safely disposed of and represent a hazard to living things for thousands of years. The radioactive wastes generated by a fusion reactor are simply the walls of the containment vessel which have been exposed to neutrons. Although the quantity of radioactive waste produced by a fusion reactor might be slightly greater than that from a conventional nuclear plant, the wastes would have low levels of short lived radiation, decaying almost completely within 100 years. Session I 407 Nuclear ReactorsEnrico Fermi led the team which produced the first sustained controlled nuclear chain reaction.Types of reactorsTypes of Nuclear Reactors include:1.Light Water Reactors (LWR)2.Heavy Water Reactors (HWR)3.Gas-Cooled Reactors4.Fast Neutron Reactors 5.Fast Breeder Reactors Light Water Reactors (LWR)In a LWR reactor the water is boiled by the core, turned to steam and that steam is used to drive the turbines which generate the electricity. The spent steam is cooled back to liquid and recycled through the core.Pressured Water ReactorIn a PWR, water is heated in the core and circulates around a closed system called the primary loop. The pressure in the primary loop is kept high enough to prevent the water from boiling. The heat from the primary loop is transferred to a separate water supply (the secondary loop) causing it to boil and turn to steam. This is done by using “steam generators” which contain many small tubes. The water in the primary loop travels through the tubes giving up heat to the water surrounding the tubes. The steam in this secondary loop is used to run the turbines to generate the electricity. In this way, the contaminated water supply is always maintained inside the containment unless of course the steam generator tubes leak causing cross contamination in the secondary loop. After passing through the turbines, the spent steam in the secondary loop is cooled back to water and run through the steam generators again.

Main Components of Nuclear Power PlantThe main components of a Nuclear Power Plant:1.Control Building - From this location, the operator controls the reactor in the control room.2.Containment Building- This is the location of the core and primary components including the steam generators if it is a PWR.3.Turbine Building - This is where the steam is converted to electricity. In a PWR it is “clean” whereas in a BWR, the steam is contaminated since it is produced from water which has been in contact with the core. Thus the turbine floor in a BWR has elevated radiation levels.

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4.Fuel Building - This is where the spent fuel is stored onsite in a pool.5.Diesel Generator Building - This is the location of the generators which supply emergency power and the other components which support the water/steam system.6.Auxiliary Building- This is the location where auxiliary equipment are installed.Protective barriersThe fuel pellets are protected by the fuel rod which is in turn protected by the reactor vessel which is in turn protected by the reactor containment. This affords 3 levels of “containment” for the radioactive material.

Steam GeneratorFor a PWR, heat from the pressurized water in the primary loop is transferred to the secondary system via a heat exchange system. The primary steam (which may contain radioactive contamination) travels through “U” tubes similar to the ones pictured here (these are actually from a heat exchanger rather than a PWR steam generator). The tubes are immersed in clean water from the secondary loop which is then turned to steam. Since this device generates steam in the secondary loop it’s called a “Steam Generator”.Advanced Reactors Today's nuclear reactor technology is distinctly better than that represented by most of the world's operating plants, and the first advanced reactors are now in service in Japan while others are under construction in various parts of the world. Reactor suppliers in North America, Japan and Europe have nine new nuclear reactor designs either approved or at advanced stages of planning, and others at a research and development stage. These incorporate safety improvements including features which will allow operators more time to remedy safety problems and which will provide greater assurance regarding containment of radioactivity in all circumstances. New plants will also be simpler to operate, inspect, maintain and repair, thus increasing their overall reliability and economy.The new generation reactors: They have a standardized design for each type to expedite licensing, reduce capital cost and reduce construction time,-are simpler and more rugged in design, easier to operate and less vulnerable to operational upsets,-have higher availability and longer operating life,-will be economically competitive in a range of sizes,-further reduce the possibility of core melt accidents,-have higher burn-up to reduce fuel use and the amount of waste.The greatest departure from most designs now operating is that many new generation nuclear plants will have more 'passive' safety features which rely on gravity, natural convection, etc,

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requiring no active controls or operational intervention to avoid accidents in the event of malfunction.The new designs fall into two broad categories: evolutionary and developmental. The evolutionary designs are those which are basically new models of existing, proven designs. The developmental designs depart more significantly from today¹s plants and require more testing and verification before large-scale deployment.

CANDU ReactorsAll CANDU reactors follow the same basic design, although variations can be found in most units. Power output in currently-operating units ranges from 125 MWe up to over 900 MWe, the main determinant being the number of fuel channels in the core. Ontario Power Generation (formerly Ontario Hydro) units tend to share a single containment system in a multi-unit station, while the commercial units sold to other Canadian utilities, as well as abroad, tend to have stand-alone containment like that found in other nuclear plant designs.All of the CANDU units sold abroad, with the exception of the early units sold to India and Pakistan, are of the "CANDU 6" design - with power output in the 700 MWe range. The other design currently marketed by AECL is the “Advanced Candu Reactor (ACR)", either as ACR-700 (700 Mwe) or as ACR-1000 (1080 – 1200 MWe). In addition, Candu Energy is working on the CANDU X or SCWR which is a variant of the ACR but with supercritical light water coolant. The CANDU reactor uses natural uranium fuel and heavy water (D2O) as both moderator and coolant (the moderator and coolant are separate systems). It is refuelled at full-power, a capability provided by the subdivision of the core into hundreds of separate pressure tubes.Each pressure tube holds a single string of natural uranium fuel bundles (each bundle half a meter long and weighing about 20 kg) immersed in heavy-water coolant, and can be thought of as one of many separate "mini-pressure-vessel reactors" - highly subcritical of course. Surrounding each pressure tube a low-pressure, low-temperature moderator, also heavy water, fills the space between neighbouring pressure tubes. The cylindrical tank containing the pressure tubes and moderator, called the "calandria", sits on its side. Thus, the CANDU core is horizontal.

Pebble Bed ReactorThe pebble bed modular reactor (PBMR) is designed to skirt some of the biggest headaches of nuclear power: pausing to refuel (which takes on average, about 40 days), complex piping and possible melt-down of the core. But the design does not address every objection to nuclear, and it raises some problems while solving others.

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Small pebble bed reactors ran in Germany in the 1970s, and China has recently started a demonstration plant. A larger version was being developed by South Africa's state utility but the development has ceased due to lack of funds and customers. The pebble bed design uses advances that appear to produce a small reactor that can be built cheaply and operated safely. Instead of the typical rod-shaped fuel, the fuel is formed into "pebbles" about the size of a tennis ball. Each pebble is made of grains of uranium sheathed in heat-resistant graphite and silicon carbide. The proposed Chinese reactor will use 520,000 enriched fuel pebbles.

Session I 408 Research ReactorsResearch reactors comprise a wide range of civil and commercial nuclear reactors which are generally not used for power generation. The primary purpose of research reactors is to provide a neutron source for research and other purposes. Their output (neutron beams) can have different characteristics depending on use. They are small relative to power reactors whose primary function is to produce heat to make electricity. Their power is designated in megawatts (or kilowatts) thermal (MWth or MWt). Most range up to 100 MW, compared with 3000 MWt (i.e. 1000 MWe) for a typical power reactor. In fact the total power of the world's 238 research reactors is little over 3000 MWe. Research reactors are simpler than power reactors and operate at lower temperatures. They need far less fuel, and far less fission products build up as the fuel is used. On the other hand, their fuel requires more highly enriched uranium, typically up to 20% U-235, although some older ones used 93% U-235. They also have a very high power density in the core, which requires special design features. Like power reactors, the core needs cooling, and usually a moderator is required to slow down the neutrons and enhance fission. As neutron production is their main function, most research reactors also need a reflector to reduce neutron loss from the core.

Research reactors have a wide range of uses, including analysis and testing of materials, and production of radioisotopes. Their capabilities are applied in many fields, within the nuclear industry as well as in fusion research, environmental science, advanced materials development, drug design and nuclear medicine.The IAEA lists several categories of broadly classified research reactors. They include 60 critical assemblies (usually zero power), 23 test reactors, 37 training facilities, two prototypes and even one producing electricity. But most (160) are largely for research, although some may also produce radioisotopes. As expensive scientific facilities, they tend to be multi-purpose, and many have been operating for more than 30 years.Table 4:8 provides a summary of type of reactors

Type Number

Critical assemblies (zero power) 60Module I.4 Sources of radiation

24

Test reactors 23Training facilities 37Prototypes 2Generating electricity 1Research 160

Fuel Fuel assemblies are typically plates or cylinders of uranium-aluminium alloy (U-Al) clad with pure aluminium. They are different from the ceramic UO2 pellets enclosed in Zircaloy cladding used in power reactors. Only a few kilograms of uranium is needed to fuel a research reactor, albeit more highly enriched (compared with perhaps a hundred tonnes in a power reactor). Highly-enriched uranium (HEU - >20% U-235) allowed more compact cores, with high neutron fluxes and also longer times between refuelling. Therefore many reactors up to the 1970s used it.But security concerns grew, especially since many research reactors are located at universities and other civilian locations with much lower security than military weapons establishments where much larger quantities of HEU exist.These programs have led to the development and qualification of new, high density, low enriched uranium (LEU) fuels. The original fuel density was about 1.3-1.7 g/cm3 uranium. Lowering the enrichment meant that the density had to be increased. Initially this was to 2.3-3.2 g/cm3 with existing U-Al fuel types.The RERTR program concentrates on reactors over 1 MW which have significant fuel requirements. About 40 research reactors in USA and abroad either have been or are being converted to low-enriched fuel. US exports of HEU declined from 700 kg/yr in mid 1970s to zero by 1993.The Soviet Union made similar efforts from 1978, and produced fuel of 2.5 g/cm3 with enrichment reduced from 90 to 36%. It largely stopped exports of 90% enriched fuel in the 1980s.The first generation of new LEU fuels used uranium and silicon (U3Si2-Al), at 4.8 g/cm3. There have been successful tests with denser U3Si-Al fuel plates up to 6.1 g/cm3, but US development of these silicide fuels ceased in 1989.With US RERTR funding, Russia is working on uranium-molybdenum fuel with density of 4-5 g/cm3, the Mo stabilising the uranium and assisting mechanical integrity. Since 1996 the USA and France have been developing new generation fuels based on U-Mo alloys with 6 g/cm3

uranium density and these should be approved by mid 2005. A further stage of the U-Mo program is aimed at developing fuels of 8-9 g/cm3 for reactors requiring such densities to convert from HEU.All fuel is aluminum-clad.

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Session I 409a Nuclear Fuel Cycle overview

Overview of the Fuel cycleThe various activities associated with the production of electricity from nuclear reactions are referred to collectively as the nuclear fuel cycle. The nuclear fuel cycle starts with the mining of uranium and ends with the disposal of nuclear waste. With the reprocessing of spent fuel as an option for nuclear fuel, the stages form a true cycle.Uranium is a slightly radioactive metal that occurs throughout the earth's crust. It is about 500 times more abundant than gold and about as common as tin. It is present in most rocks and soils as well as in many rivers and in sea water.There are a number of areas around the world where the concentration of uranium in the ground is sufficiently high that extraction of it for use as nuclear fuel is economically feasible. Table 4:9 shows a typical Uranium concentration in various mediaTable 4:9 : Uranium concentration in various mediaSource ppm (part per million)

High-grade ore (2% U) 20,000

Low-grade ore (0.1% U) 1,000

Fertiliser 400

Coal deposits 100

Granite (60% of earths crust) 4

Sedimentary rock 2

Seawater 0.003

Properties of natural UraniumTable 4:10 shows the properties of natural Uranium,Table 4:10 : Properties of natural Uranium,Isotope % Abundance

Half Life (106 years)

Isotope % Abundance

238U

4,500 99.284

235U

704 0.711

234U

0.245 0.005

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Table 4:11 shows the specific activity of natural Uranium and type of enrichment,Table 4:11 : specific activity of natural Uranium and type of enrichment,Type (enrichment) Specific Activity (Bq/gram)

Depleted (0.2%) 1.5 x 104

Natural (0.71%) 2.6 x 104

Enriched (4%) 8.9 x 104

Enriched (93%) 4.1 x 106

Uranium compounds associated with the fuel cycle are1. UF6 produced at conversion plants2. U3O8 is yellowcake from milling3. UO2 is dominant fuel type (ceramic) used to produce pellets4. UF4 is intermediate form in conversion5. Uranyl nitrate is important in recovery

Figure 4. 8 shows a illustrative example of for a LWR fuel cycle in the USA

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Figure 4. 8 : LWR fuel cycle in the USASession I 409b - Fuel Cycle - MiningMining is the first step in the process of providing the natural uranium feed material for use as a fuel. Both excavation and in situ techniques are used to recover uranium. Excavation may be underground or open pit mining. In general, open pit mining is used where deposits are close to the surface. Open pit mines require large holes on the surface, larger than the size of the ore deposit, since the walls of the pit must be sloped to prevent collapse. As a result, the quantity of material that must be removed in order to access the ore may be large.

Underground mining is used for deep deposits, typically greater than 120 m deep..Underground mines have relatively small surface disturbance and the quantity of material that must be removed to access the ore is considerably less than in the case of an open pit mine. In the case of underground uranium mines, special precautions, consisting primarily of increased ventilation, are required to protect workers against airborne radiation exposure. The appearance of uranium ore differs depending on its origin.

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An increasing proportion of the world’s uranium now comes from in situ leaching (ISL), where oxygenated groundwater is circulated through a very porous ore body to dissolve the uranium and bring it to the surface. ISL may be conducted with slightly acid or with alkaline solutions to keep the uranium in solution. The uranium is then recovered from the solution as in a conventional mill.

Session I 409c - Fuel Cycle - MillingMilling, which is generally carried out close to a uranium mine, extracts the uranium from the ore. At the mill the ore is crushed and ground to a fine slurry. The uranium is extracted from the crushed and ground-up ore by leaching, in which either a strong acid or a strong alkaline solution is used to dissolve the uranium from the waste rock. It is then recovered from solution and precipitated as uranium oxide (U308) concentrate sometimes known as "yellowcake".Yellowcake is really a mixture of uranium oxides that is hardly ever yellow any more. It is shipped from the mills in a granular solid form that varies in color from yellowish to an almost black olive green, depending on the mineral it was found in and the processing (most notably, the calcination temperature).‘Yellowcake' generally contains more than 80% uranium. The original ore may contains as little as 0.1% uranium. After drying and usually heating it is packed in 200-litre drums as a concentrate.About 200 tonnes of U3O8 is required to keep a large (1000 MWe) nuclear power reactor generating electricity for one year. The remainder of the ore, containing most of the radioactivity and nearly all the rock material, becomes tailings, which are emplaced in engineered facilities near the mine (often in a mined-out pit). Tailings contain long-lived radioactive materials in low concentrations and toxic materials such as heavy metals; however, the total quantity of radioactive elements is less than in the original ore, and their collective radioactivity will be much shorter-lived. These materials need to be isolated from the environment. Radon emanation from mill tailings is a major concern.For example, if mill tailings were to be used in the construction industry, elevated levels of radon emission would result in the structures built. A variation of this actually occurred in the US some time ago when residue from the phosphate industry was used to refill the excavation. Homes constructed on top were found to have elevated radon levels.Radiological Hazards of Uranium MillingSources of radiological hazards include:

1. Ore dust and radon emissions from ore crushing, sorting, and storage2. Yellowcake dust from drying and packaging area

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3. Windblown particulates and radon emission from the tailings disposal area

Session I 409d Fuel Cycle - ConversionThe product of a uranium mill is not directly usable as a fuel for a nuclear reactor. Additional processing, generally referred to as enrichment, is required for most kinds of reactors. This process requires uranium to be in gaseous form and the way this is achieved is to convert the U308 into the gas uranium hexafluoride (UF6) at a conversion plant. Uranium hexafluoride is a gas at relatively low temperatures.At a conversion facility, uranium is first refined to uranium dioxide, which can be used as the fuel for those types of reactors that do not require enriched uranium. Key compound in Nuclear Fuel Cycle are: Solid for storage ; Liquid for feeding/withdrawing ; Gas for processingThe Solid for storage is white, dense, crystalline. Reacts with water vapor to produce toxic and corrosive hydrofluoric acid.The main hazard of the conversion stage of the fuel cycle is the possible presence of hydrogen fluoride as a result of the reaction of UF6 with moisture in the air. In the USA in 1986 an accident at a conversion facility resulted in the release of UF 6. A worker standing nearby was killed, not from any radiological hazard but from inhalation of HF produced by the reaction shown by the reaction below.:

UF6 + 2H2O → UO2 F2 + 4HT .

Uranyl Fluoride hydrofluoric Acid Chemical hazardUranium compounds go through several process such as (i) dry and wet conversion(ii) dry fluoride volatility(iii) reduction(iv) hdrofluorination (v) distillation The final UF6 product which 99.99% pure is packaged and shipped in strong metal containers .Other conversion processes include:

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British process - This process basically converts ADU first to UO3, which is then calcined in the presence of hydrogen to form uranium dioxide. French process - The uranium peroxide is formed by precipitation of an aqueous solution of uranyl nitrate at 70 – 80 oF and a pH of 3 – 4 with H2O2.The French process is also different in that the fuel pellets are made from UO3 first, and then converted to UO2 through reduction by heating ammonia to about 750 oC.

Session I 409e Fuel Cycle - EnrichmentNatural uranium consists, primarily, of a mixture of two isotopes (atomic forms) of uranium. Only 0.7% of natural uranium is "fissile", or capable of undergoing fission, the process by which energy is produced in a nuclear reactor. The fissile isotope of uranium is uranium 235 (U-235). The remainder is uranium 238 (U-238). In the most common types of nuclear reactors, a higher than natural concentration of U-235 is required. The enrichment process produces this higher concentration, typically between 3.5% and 5% U-235, by removing over 85% of the U-238. This is done by separating gaseous uranium hexafluoride into two streams, one being enriched to the required level and known as low-enriched uranium. The other stream is depleted in U-235 and is called 'tails' which is mostly U-238. Currently, two main methods implemented commercially1.Gaseous diffusion (GDP)2. Gas centrifuge (GC)There are two enrichment processes in large scale commercial use, each of which uses uranium hexafluoride as feed: gaseous diffusion and gas centrifuge. They both use the physical properties of molecules, specifically the 1% mass difference, to separate the uranium isotopes. The product of this stage of the nuclear fuel cycle is enriched uranium hexafluoride, which is reconverted to produce enriched uranium oxide. A small number of reactors, notably the Canadian CANDU and early British gas-cooled reactors, do not require uranium to be enriched.

Enrichment Levels:1) LEU = Low Enriched Uranium: assay < 10%2) “IEU” = Intermediate Enriched Uranium (10% - 20%)3) HEU = High Enriched Uranium: assay > 20% (usually focus on assay > 90%)

Gaseous diffusion (GDP)Gaseous Diffusion uses molecular diffusion to separate the isotopes of uraniumThree basic requirements are needed :

1) Combine Uranium with Fluorine to form Uranium hexafluoride (UF6)2) Pass UF6 through a porous membrane

Module I.4 Sources of radiation 31

3) Utilize the different molecular velocities of the two isotopes to achieve separation4) Enrichment of 235U through one porous membrane (or barrier) is very minute5) Thousands of passes are required to increase the enrichment of natural uranium

(0.711%) to a usable assay of 4 or 5% for use in reactorsThe process is an eight stage programme

1) UF6 Feed Storage2) Feed Supply Autoclave3) Enrichment Cascade4) Tails Condensation and Withdrawal5) Tails Storage6) Product Condensation and Withdrawal7) Product Storage8) Product Shipping

Gas centrifuge (GC)GC is a uranium enrichment process that uses a large number of rotating cylinders in series and parallel configurations to produce LEU suitable for commercial power reactor use. The approach is a hundred years oldSeveral large facilities overseas successfully and economically supply LEU using GC plants. No operating GC plants currently exist in the U.S., but there have been “many discussions.” The gas centrifuge is essentially a bowl, in which there is a rotor spinning round at a very high speed. The gas (UF6) directed to the centrifuge is forced to spin by the rotor. Due to the centrifugal force the heavier molecules (those which contain 238U) will accumulate near the wall of the bowl, while the lighter molecules containing 235U will stay closer to the center of the centrifuge. Session I 409e Fuel FabricationThe enriched UF6 is converted to uranium dioxide (UO2) powder and pressed into small pellets.Reactor fuel is generally in the form of ceramic pellets. These are formed from pressed uranium oxide which is sintered (baked) at a high temperature (over 1400° C). The pellets are then encased in metal tubes to form fuel rods, which are arranged into a fuel assembly ready for introduction into a reactor. The dimensions of the fuel pellets and other components of the fuel assembly are precisely controlled to ensure consistency in the characteristics of fuel bundles. In a fuel assembly The pellets are inserted into thin tubes, usually of a zirconium alloy (zircalloy) or stainless steel, to form fuel rods. The rods are then sealed and assembled in clusters to form fuel elements or assemblies for use in the core of the nuclear reactor.Upon final acceptance of the fuel assembly, units are packed in shipping containers for transfer to utility power reactor site.

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In a fuel fabrication plant great care is taken with the size and shape of processing vessels to avoid criticality (a limited chain reaction releasing radiation). With low-enriched fuel, criticality is most unlikely but in plants handling special fuels for research reactors this is a vital consideration.

Session I 409g Fuel Cycle – Power GenerationInside a nuclear reactor the nuclei of U-235 atoms split (fission) and, in the process, release energy. The process depends on the presence of a moderator such as water or graphite, and is fully controlled. This energy is used to heat water and turn it into steam. The steam is used to drive a turbine connected to a generator which produces electricity. The fissioning of uranium is used as a source of heat in a nuclear power station in the same way that the burning of coal, gas or oil is used as a source of heat in a fossil fuel power plant. Several hundred fuel assemblies make up the core of a reactor. For a reactor with an output of 1,000 megawatts (MWe), the core would contain about 75 tonnes of low-enriched uranium producing about 7 billion kilowatt hours of electricity in one year. Typically, more than 45 million kilowatt-hours of electricity are produced from one tonne of natural uranium. The production of this amount of electrical power from fossil fuels would require the burning of over 20,000 tonnes of black coal or 30 million cubic metres of gas.

Session 409 h Fuel Cycle – Spent FuelWith time, the concentration of fission fragments and heavy elements formed in a fuel bundle increase to the point where it is no longer practical to continue to use the fuel. So after 12-24 months the 'spent fuel' is removed from the reactor. When removed from a reactor, a fuel bundle is emitting both heat and radiation, principally from the fission fragments. Spent fuel is unloaded into a storage pond immediately adjacent to the reactor to allow the radiation levels to decrease. In the ponds, the water shields the radiation and absorbs the heat. Spent fuel can be stored safely in these ponds for long periods. Depending on policies in particular countries, some spent fuel may be transferred to central storage facilities. Ultimately, spent fuel must either be reprocessed or prepared for permanent disposal. Spent fuel can be stored in (i) wet storage facility and (ii) dry storage facilityHowever, both kinds of storage are intended only as an interim step before the spent fuel is either reprocessed or sent to final disposal. The longer it is stored, the easier it is to handle, due to decay of the radioactivity. There are two alternatives for spent fuel: (a)reprocessing to recover the usable portion of it and (b) long-term storage and final disposal without reprocessing.Session I 409i Fuel Cycle - Reprocessing

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Spent fuel is about 95% U-238 but it also contains about 1% U-235 that has not fissioned, about 1% plutonium and 3% fission products, which are highly radioactive, with other transuranic elements formed in the reactor. In a reprocessing facility the spent fuel is separated into its three components: uranium, plutonium and waste containing fission products. Reprocessing enables recycling of the uranium and plutonium into fresh fuel, and produces a significantly reduced amount of waste (compared with treating all spent fuel as waste). Recovered uranium can be returned to the conversion plant for conversion to uranium hexafluoride and subsequent re-enrichment. The reactor-grade plutonium can be blended with enriched uranium to produce a mixed oxide (MOX) fuel, in a fuel fabrication plant. MOX fuel fabrication occurs at five facilities in Belgium, France, Germany and UK, with two more under construction. There have been 25 years of experience in this, and the first large-scale plant, Melox, commenced operation in France in 1995. Across Europe about 30 reactors are licensed to load 20-50% of their cores with MOX fuel, and Japan plans to have one third of its 53 reactors using MOX by 2010. The remaining 3% of high-level radioactive wastes (some 750 kg per year from a 1000 MWe reactor) can be stored in liquid form and subsequently solidified. Over the last fifty years the principal reason for reprocessing has been to recover unused uranium and plutonium in the spent fuel elements. A secondary reason is to reduce the volume of material to be disposed of as high-level waste. In addition, the level of radioactivity in such 'light' waste after about 100 years falls much more rapidly than in spent fuel itself.For most types of fuel, reprocessing occurs anywhere from 5 to 25 years after reactor discharge. In the last 2 decades interest has grown in separating ('partitioning') individual radionuclides both to reduce long-term radioactivity in residual wastes and to be able to transmute separated long-lived radionuclides into shorter-lived ones.Transmutation of one radionuclide into another is achieved by neutron bombardment in a nuclear reactor or accelerator-driven device. The objective is to change long-lived actinides and fission products into significantly shorter-lived nuclides. The goal is to have wastes which become radiologically innocuous in only a few hundred years.All commercial reprocessing plants use the well-proven hydrometallurgical PUREX process. This involves dissolving the fuel elements in concentrated nitric acid. Chemical separation of uranium and plutonium is then undertaken by solvent extraction steps. (Neptunium can also be recovered if required, and maybe used for producing Pu-238 for thermo-electric generators for spacecraft.) The Pu and U can be returned to the input side of the fuel cycle - the uranium to the conversion plant prior to re-enrichment and the plutonium straight to fuel fabrication . The remaining liquid after Pu and U are removed is high-level waste, containing about 3% of the spent fuel in the form of fission products and minor actinides (Np, Am, Cm). It is highly radioactive and continues to generate a lot of heat. It is conditioned by calcining and

Module I.4 Sources of radiation 34

incorporation of the dry material into borosilicate glass, then stored pending disposal. In principle any compact, stable, insoluble solid is satisfactory for disposal.

Session I409j Fuel Cycle – High Level Waste DisposalAt the present time, there are no disposal facilities (as opposed to storage facilities) in operation in which spent fuel, not destined for reprocessing, and the waste from reprocessing can be placed. Although technical issues related to disposal have been addressed, there is currently no pressing technical need to establish such facilities, as the total volume of such wastes is relatively small. Further, the longer it is stored the easier it is to handle, due to the progressive diminution of radioactivity. There is also a reluctance to dispose of spent fuel because it represents a significant energy resource which could be reprocessed at a later date to allow recycling of the uranium and plutonium. A number of countries are carrying out studies to determine the optimum approach to the disposal of spent fuel and waste from reprocessing. The most commonly favoured method for disposal being contemplated is placement into deep geological repositories. Wastes from the nuclear fuel cycle are categorised as high-, medium- or low-level waste by the amount of radiation that they emit. These wastes come from a number of sources and include: * low-level waste produced at all stages of the fuel cycle * intermediate-level waste produced during reactor operation and by reprocessing * high-level waste, which is waste containing fission products from reprocessing, and in many countries, the spent fuel itself. The 3% of the spent fuel which is separated high-level wastes amounts to 700 kg per year and it needs to be isolated from the environment for a very long time. These liquid wastes are stored in stainless steel tanks inside concrete cells until they are solidified. After reprocessing the liquid high-level waste can be calcined (heated strongly) to produce a dry powder which is incorporated into borosilicate (Pyrex) glass to immobilize the waste. The glass is then poured into stainless steel canisters, each holding 400 kg of glass. A year's waste from a 1000 MWe reactor is contained in 5 tonnes of such glass, or about 12 canisters, 1.3 metres high and 0.4 metres in diameter. These can be readily transported and stored, with appropriate shielding. This is as far as the nuclear fuel cycle goes at present.The most widely accepted plans are for HLW to be buried in stable rock structures deep underground. Many geological formations such as granite, volcanic tuff, salt or shale will be suitable. The first permanent disposal in the US was expected to occur at Yucca Mountain, Nevada but the funding was scaled back in 2009 by the President. New waste disposal policy being developed? Most other countries intend to introduce final disposal sometime after about 2010, when the quantities to be disposed of will be sufficient to make it economically justifiable. In USA high-level civil wastes all remain as spent fuel stored at the reactor sites. Some HLW is also stored at Department of Energy sites. It was planned to encapsulate these fuel assemblies

Module I.4 Sources of radiation 35

and dispose of them in an underground engineered repository, at Yucca Mountain, Nevada. This program was scaled back by the President in 2009 and it now appears highly unlikely that Yucca Mountain will ever become the national repository. In Europe some spent fuel is stored at reactor sites, similarly awaiting disposal. However, much of the European spent fuel is sent for reprocessing at either Sellafield in UK or La Hague in France. The recovered U and Pu is then returned to the owners (the Pu via a MOX fuel fabrication plant) and the separated wastes (about 3% of the spent fuel) are vitrified, sealed into stainless steel canisters, and either stored or returned. Eventually they too will go to geological disposal. Session I 410 Linear Accelerators, Betatrons and CyclotronsCharged particles can be accelerated

1) Linear Accelerators2) Betatrons 3) Cyclotrons

Linear Accelerators

A linear particle accelerator (often shortened to linac) is a type of particle accelerator that greatly increases the kinetic energy of charged subatomic particles or ions by subjecting the charged particles to a series of oscillating electric potentials along a linear beam line; this method of particle acceleration was invented by Leó Szilárd.

An alternative method of accelerating charged particles is by use of a radio frequency linear accelerator. This device uses a smaller but changing electric field over and over again to increase the energy of the particle. The particles pass through tubes called cavities which are alternately charged + and - by the alternating supply.

Initially, each tube is longer than the last so that, as the particle gets faster, it traverses each one in the same time to keep in step with the changing field. In high energy machines (especially in electron accelerators) as particles approaches the speed of light, their speed remains nearly constant so the tubes are evenly spaced.

Ignoring relativity, the energy gained by the particle is equal to NqV where N is the number of cavities, q is the charge on the particle and V is the maximum voltage of the RF supply. For example, an early machine had 30 cavities with a peak voltage supply of 43 kV to accelerate singly-ionised mercury atoms to 1.3 MeV (30 x 0.043 MeV = 1.29 MeV). Figure 4: 10 an and 10b illustrate the principle of the linear accelerator

Module I.4 Sources of radiation 36

Figure 4:10a Linear accelerator cavities

Figure 4:10b The drift tubes which longer as the particle’s velocity increases so that it spends the same amount of time in each tube.

The design of a linac depends on the type of particle that is being accelerated : electrons, protons or ions. Linacs range in size from a cathode ray tube (which is a type of linac) to the 3.2-kilometre-long (2.0 mi) linac at the SLAC National Accelerator Laboratory in Menlo Park, California.

To reach very high energies, a large number of cavities are needed and so the machine becomes very long. The world’s largest such machine is SLAC, the Stanford Linear Collider, a 20 GeV electron accelerator 3.2 km long. The CLIC accelerator planned by CERN could be nearly 40 km long.

Linacs have many applications: they generate X-rays and high energy electrons for medical application purposes in radiation therapy, serve as particle injectors for higher-energy accelerators, and are used directly to achieve the highest kinetic energy for light particles (electrons and positrons) for particle physics.

Module I.4 Sources of radiation 37

CyclotronA cyclotron is a machine used to accelerate charged particles to high energies. The cyclotron is a particle accelerator conceived by Ernest O. Lawrence in 1929, and developed with his colleague Livingston at the University of California in the 1930s.A cyclotron consists of two “D”-shaped cavities sandwiched between two electromagnets.A charged particle source is placed in the center of the cyclotron and the electromagnets are turned on. The magnetic field bends the path of the charged particle so that the charged particle will circle around inside the D-shaped cavities.This doesn't accelerate the particle but merely causes them to travel in a circular path. In order to accelerate the charged particles, the two “D”-shaped cavities have to be connected to a radio wave generator. This generator gives one cavity a positive charge and the other cavity a negative charge. After a moment, the radio wave generator switches the charges on the cavities. The charges keep switching back and forth. It is this switching of charges that accelerates the particle as they travel across the space between the two “D”-shaped cavities. Similar particles (i.e. with equal charges and masses) will all orbit at the same rate, regardless of their speed.Figures 4:11a and 11b illustrates the principles of operation of the cyclotron .

Figure 4:11a Inside of the "D" -shaped cavity

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Figure 11b : Accelerated particles when the two "D' s are connected to a radio wave generator. In particular how alpha particles can be accelerate in a cyclotron is illustrated in Figure 4:12 and explained below.

Figure 4:12 Acceleration of an alpha particle in a cyclotron Let's say that we have an alpha particle inside our cyclotron. Alpha particles are positively charged (+2) so their paths can be bent by magnetic fields. As an alpha particle goes around the cyclotron, it crosses the gap between the two “D”-shaped cavities. If the charge on the cavity in front of the alpha particle is negative and the charge on the cavity behind it is positive, the alpha

Module I.4 Sources of radiation 39

particle is pulled forward (remember that opposite charges attract while like charges repel). This accelerates the alpha particle.Once the particle is back inside the D-shaped cavity, it is "screened" from electric fields by the copper walls of the cavity. The magnetic field is not screened by the (nonmagnetic) copper cavity so the alpha particle moves in the circular path.

The particle travels through the “D” cavity and again comes to the gap. The radio wave generator changes the charge on the cavities reversing the positive and negative charges so the alpha particle once again sees a negative charge in front of it and a positive charge behind it and is again accelerated forward. As long as the charges on the cavities are reversed just as the particle enters the gap, the alpha particle will always see a negative charge in front of it and a positive charge behind it and the particle will be accelerated to a higher velocity.The faster a charged particle moves, the less it is affected by the magnetic field so that it travels in a circle with a larger radius. The combination of higher speed and larger circle means that the particle always takes the same amount of time to reach the gap. Thus a constant frequency oscillating electric field continues to accelerate the charged particles across the gap.The limitation on the energy that can be reached in such a device depends on the size of the magnets that form the “D”-shaped cavities and the strength of their magnetic fields. The cyclotron was one of the earliest types of particle accelerators, and is still used as the first stage of some large multi-stage particle accelerators. The limiting factor is the basic design - all particles must orbit at the same frequency, whatever their speed, in order to take advantage of the oscillating field which accelerates it across the gap. If the particle does not reach the gap at the correct time, it will not be accelerated. As particles approach the speed of light, they behave as if their mass is increasing and they start to lag behind the oscillating electric field. As cyclotrons approached 20 MeV they began to reach their limits and a new design had to be produced.A particle becomes relativistic once its kinetic energy is comparable to its rest energy. Since the rest energy of the electron is only 0.5 MeV, they cannot be accelerated to a useful energy in a cyclotron.One use of a cyclotron is the production of radionuclides for medical procedures such as Positron Emission Tomography (PET). FDG is an abbreviated name for the radiopharmaceutical Fluorodeoxyglucose, a sugar compound that is labeled with radioactive fluoride (18F) which is produced in a cyclotron by proton bombardment of 18O enriched water. Figure 4:13 shows a picture of a cyclotron

Module I.4 Sources of radiation 40

Figure 4:13 Hospital based cyclotron

BetatronProfessor Donald Kerst built the world’s first magnetic induction accelerator (betatron) at the University of Illinois in 1940. The original betatron is now on display at the Smithsonian Institution.A betatron can be considered as a transformer, with a ring of electrons as the secondary coil. The magnetic field that is used to make the electrons move in a circle is also the one used to accelerate them.If the magnetic field increases, there is a changing flux linking the loop of electrons and so an induced electromagnetic field is produced which accelerates the electrons. As the velocity of the electrons increases, they require a larger magnetic field to maintain a constant radius, which is provided by the increasing magnetic field. The effects are proportional, so the magnetic field both accelerates and maintains the constant radius of the electrons within the betatron.The particles have maximum energy when the magnetic field is at its strongest value, at which point the electrons will be relativistic. Figures 4: 13 a and b illustrates how the betatron works.

Module I.4 Sources of radiation 41

Figures 4: 13 a . The acceleration process of the betatron to hit a target for x-ray production

Figures 4: 13 b. Magnet system of the betatron

Module I.4 Sources of radiation 42

An example of this type of machine is the University of Chicago’s 315 MeV betatron, built in 1949 with a magnetic field strength of 0.92 T and an orbit radius of 1.22 m.

Session I 411 X-ray production x-ray are produced in an x-ray tube. in an X-ray tube, a cloud of free electrons is produced by heating a filament with an electric charge. The filament is similar to that which is found in any commercially available incandescent light bulb. The free electrons would hover around the filament were it not for a potential difference which is established with some electrical source such that the portion of the tube where the filament is located is negatively charged and the target is positively charged. The free electrons are thus attracted to the target. They are accelerated across an open space (preferably a vacuum to preclude loss of energy by interaction with air molecules) and then ultimately strike the target. Although the tungsten target appears to us to be piece of metal, from the viewpoint of the electrons, the tungsten target is in reality a collection of tungsten atoms. Each atom, consisting of positively charged protons and negatively charged electrons orbiting around the nucleus, has the capability of attracting or repelling the accelerated electrons so that they lose the energy imparted to them by the potential difference as they travel to hit the target.In reality, an X-ray tube is a very inefficient method of producing X-rays. Most of the energy of the electrons is dissipated as heat during low energy interactions in the target. However, a few percent of the electrons transfer their energy by emitting X-ray photons. The X-rays are, of course, emitted isotropically (in all directions) as noted previously. However, in a typical medical or industrial X-ray unit, it is desirable to focus the X-rays in a specific direction so they can be applied to the part being studied. For this purpose, shielding usually surrounds the target except for a small “window” through which the useful X-rays are permitted to exit the unit.Low energy electrons dissipate most of their energy as heat in the target (less than 10% results in X-rays). The opposite is the case for high energy electrons. Most of their energy is converted to X-rays.X-rays produced from high energy electrons impinging on a target tend to be scattered in the forward directionX-rays produced by lower energy electrons tend to be scattered at right angles to the direction of the electron beamIndustrial Radiography X-ray units are typically constant potential units with beryllium windows to permit X-rays of all energies to emerge (even very low energy X-rays). After all, we are not concerned about the “skin” dose to the object.Industrial radiography units typically operate between 10 and 300 kVp and 0-10 mA with virtually unlimited exposure times. The tubes are usually water-cooled to dissipate the heat produced by the electrons striking the target.

Module I.4 Sources of radiation 43

For Diagnostic Radiology on people, approximately 2.5 mm of aluminum filtration is used to eliminate the lowest energy X-rays which would deliver a skin dose to the patient but would provide no useful information on the film behind the patient (because of their low penetrating power). Medical megavoltage x-rays produced in linear accelerators are used for radiation therapy.

Session I 412 Filtration and beam qualityThe spectrum of X-rays produced by depends upon the maximum accelerating voltage target material and beam filtration . For a typical tungsten anode X-ray unit with inherent filtration and possibly some added aluminum operating at 100 kVp (kilovolts peak), we expect to see a range of X-ray energies from near zero to a maximum of 100 keV. The lower energy X-rays are attenuated by the inherent and added filtration. Some characteristic X-rays of tungsten are seen superimposed on the spectrum.The spectrum of mammography unit with a molybdenum anode, molybdenum filter X-ray look different . Such a unit is typically operated at a lower potential (25-40 kVp). In the figure shown here, it appears to be operating at 28 kVp. The very dramatic drop in the spectrum at about 20 keV is due to the k-edge of molybdenum and the two prominent peaks are the characteristic X-rays produced by the molybdenum anode-filter combination. This figure also demonstrates that with judicious use of filters, it is possible to “trim” a bremsstrahlung spectrum to more closely approximate a monoenergtic X-ray beam which is very desirable in diagnostic studies. Normally the addition of a filter reduces the lower energy X-rays and leaves the higher energy ones virtually unaffected. However, because of the k-edge effect, the X-rays above 20 keV are dramatically reduced. However, one should not forget that introduction of a filter affects all the X-ray energies to some degree so that although increasing the amount of aluminum filtration in the tungsten tube will continuously reduce the number of lower energy X-rays making the beam more monoenergetic, the filter also reduces (to a lesser degree) all of the other X-rays so that given enough filtration you can end up with only X-rays in a small band near 100 keV (virtually monoenergetic), however, there will be very few such X-rays, not enough to be diagnostically useful.

AttenuationAttenuation is the process whereby the intensity of a photon beam passing a specifies material is reduced by the (i) absorption and (ii) scattering processes. The attenuation coefficient for each material depends on the energy of the photons. The higher the energy, the more likely they are to pass through. The probability of the photoelectric effect which involves total absorption of the initial photon decreases as the energy of the photon increases.The amount of shielding (thickness) depends on (i)the energy of the radiation(ii) the shielding material (its density )

Module I.4 Sources of radiation 44

(iii) the distance from the source

The irradiation geometry is "good" if no scattered radiation reaches the person irradiated. The attenuation process flows an exponential law.

I x=I 0 e−μx=I 0 e

− μρ ( ρx)

where:Ix = photon intensity after traversing x cm of some materialIo = initial or incident photon intensityx = thickness of material (cm)m = linear attenuation coefficient (cm-1)r = density (g/cm3)m/r = mass attenuation coefficient (cm2/g)

Two special thicknesses to be noted are (i) half value layer (HVL) and (ii) tenth value layer

The amount of shielding thickness required to reduce the incident radiation levels to ½ is called the “half-value layer” or HVL The amount of shielding thickness required to reduce the incident radiation levels to 1/10 is called the “tenth-value layer” or TVL.

HVL= ln2μ

TVL= ln 10μ

TVL = 3.32 HVL

Session I 4013 Neutron ProductionNeutrons are produced in great numbers by the fission process in reactors.The theoretical basis for fission is the massive energy release which occurs when a heavy nucleus divides into two smaller ones. Only a few very heavy nuclei undergo fission spontaneously, while others can be encouraged to undergo fission by the addition of energy when a neutron is absorbed. Such fissile materials include 235U and 239Pu. During the fission process, a number of neutrons are released, and if these go on to induce new fission events, a chain reaction results. The use of a controlled chain reaction is the basis for all research reactors and nuclear power stations.

Module I.4 Sources of radiation 45

The process of nuclear fission was discovered in 1938 by Otto Hahn and Fritz Strassmann and was explained in early 1939 by Lise Meitner and Otto Frisch. The fissionable isotope of uranium, U-235, can be split by bombarding it with a slow, or thermal, neutron. (Slow neutrons are called “thermal” because their average kinetic energies are about the same as those of the molecules of air at ordinary temperatures.) The atomic numbers of the nuclei resulting from the fission add up to 92, which is the atomic number of uranium. A number of pairs of product nuclei are possible, with the most frequently produced fragments being krypton and barium.Since this reaction also releases an average of 2.5 neutrons, a chain reaction is possible, provided at least one neutron per fission is captured by another nucleus and causes a second fission. In an atomic bomb, the number is greater than 1 and the reaction increases rapidly to an explosion. In a nuclear reactor, where the chain reaction is controlled, the number of neutrons producing additional fission must be exactly 1.0 in order to maintain a steady flow of energy.. The process describe above is illustrated in Figure 4:14 .

Figure 4:14 . Fission process

3He and Neutron Production from FusionNeutrons can be produced by fusion of two deuterons (deuterium ions).( Figure 4:15)For light nuclei, which are of interest in controlled nuclear fusion research Ro may be taken to be approximately equal to a nuclear diameter (5 x 10-13 cm), and since 'e' is 4.8 x 10-10 esu

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(statcoulomb) it follows from the equation that the energy (E)required to surmount the Coulomb barrier is: E = 4.6 x 10-7 Zl Z2 erg = .28 Zl Z2 MeV (million electron volts) Where 1 MeV = 1.6 x 10-6 erg.

Figure 4:15 Fusion of two deuterons to produce neutrons

Neutrons can be produced also by fusion of a deuteron (deuterium ion) and a triton (tritium ion).The first generation of fusion power plants will use the D-T fusion reaction, shown schematically in the above animation. Nuclei of two isotopes of hydrogen, deuterium (D) and tritium (T) react to produce a helium (He) nucleus and a neutron (n).In each reaction, 17.6 MeV of energy (2.8 pJ) is liberated: D + T → 4He (3.5 MeV) + n (14.1 MeV)The first generation fusion reactors will use deuterium and tritium for fuel because they will fuse at lower temperature. Deuterium can be easily extracted from seawater, where 1 in 6500 hydrogen atoms is deuterium. Tritium can be bred from lithium, which is abundant in the earth's crust. In the fusion reaction a deuterium and tritium atom combine together, or fuse, to form an atom of helium and an energetic neutron. This figure shows a deuterium ion (deuteron) combining with a tritium ion (triton) to form an unstable compound nucleus which relaxes into a helium ion and an energetic neutron. The "D-T" reaction has the highest reaction rate at the plasma temperatures which are currently achievable; it also has a very high energy release. These properties make it the easiest reaction to use in a man-made fusion reactor. As the figure shows, the products of this reaction include an alpha particle (Helium-4 nucleus) with 3.5 MeV energy, and a neutron with 14.1 MeV energy. The neutron escapes from the plasma (it has no charge and is not confined) and can be trapped in

Module I.4 Sources of radiation 47

a surrounding "blanket" structure, where the n + Li-6 => He-4 + T reaction can be used to "convert" the neutrons back into tritium fuel.Neutrons can also be produced by a process called “spallation.”A more recent development is the accelerator-based pulsed source which produces neutrons in a totally different manner. In this spallation process, neutrons are released by bombarding a heavy metal target with high energy protons from a powerful accelerator. Spallation is a more efficient way of extracting neutrons than fission, and with recent progress in accelerator technology and computing power it is now clear that the next generation of neutron sources will be based on this concept. The world’s premier spallation neutron source at present is the ISIS facility in the UK. ISIS is the world's most powerful pulsed spallation neutron source. The facility provides beams of neutrons and muons that enable scientists to probe the structure and dynamics of matter in areas encompassing Physics, Chemistry, Earth Science, Materials Science, Engineering and Biology.ISIS is the major facility at the Central Laboratory of the Research Councils’ (CLRCs’) Rutherford Appleton Laboratory site in Great Britain.The source was approved in 1977 and the first neutrons were produced in late 1984.It is well known that tungsten is a better target material than tantalum in two main respects. First, it produces more neutrons per proton and second, the heat produced from the induced activity in tungsten is about one third of that in tantalum. However, tungsten is a more difficult metal to machine and is subject to corrosion by water. A development programme was put in place to establish fabrication methods to construct a target made from tungsten plates clad in tantalum. The cooling system design was optimized to minimize the number of cooling channels in the target and simplify the complex cooling manifold design required for uranium targets and used on the tantalum ones.Other reactions which produce neutrons include capture of protons, deuterons, and alpha particles in various nuclei, with emission of neutrons of various energies.The possible interactions include:

(p,n) reactions (d,n) reactions (α,n) reactions

Many radionuclides can be produced through charged particle (protons, deuterons, alpha particles, 3He+2 ) bombardment of nuclei of stable atoms. Two such nuclear reactions are shown below :p + 68Zn → 67Ga + 2nα + 16O → 18F + p + n.

Module I.4 Sources of radiation 48

The charged particles must have enough kinetic energy to overcome the repulsive effects of a positively charged nucleus (1-100 MeV per nucleon). Such energies are produced by accelerating particles using a linear accelerator or cyclotron.The desired isotope almost always has a different atomic number (Z) with respect to the target material. Charged particle reactions yield radionuclides that are predominantly neutron deficient and therefore decay via positron emission or EC (electron capture). Neutron TherapyNeutrons are high linear-energy-transfer (high LET) radiation used to treat cancer which is more efficient than the Conventional radiation therapy which includes photon (X-ray) and electron radiation, which is available at many clinics and hospitals.Because the biological effectiveness of neutrons is so high, the required tumor dose is about one-third the dose required with photons, electrons or protons.Boron Neutron Capture Therapy (BNCT) is a radiotherapy modality for treating cancerous tumours (e.g. glioma – a cancer of the brain)It utilizes the reaction between the 10B nucleus and slow (thermal) neutrons: 10B + n → 7Li(0.84 MeV) + 4He(1.47 MeV) + γ(0.48 MeV) The released 7Li atom and alpha-particle (4He) have a total energy of 2.31 MeV which is deposited within the range of approximately 5-9 mm (i.e. at a distance corresponding to about one cell diameter).In principle, one boron-neutron interaction liberates enough energy to kill the cell in which the interaction takes place.The short distance deposition of the released energy spares the surrounding boron-free tissue of the radiation damageThe effect of BNCT is dependent on two preconditions:(i) the selective accumulation of boron atoms in the target (tumour) rather than in the healthy cells and ;(ii) the tumour site is reached by a sufficient number of neutrons.In BNCT, cancer cells are not directly destroyed by irradiated neutrons but indirectly by particles generated by nuclear reaction between borons and neutrons. BNCT stands as an effective remedy for malignant brain tumors and melanoma. Boron does not easily enter healthy brain cells owing to the blood-brain barrier function that prevents the invasion of toxic substances into the brain, but it easily enters cancer cells, enabling their selective destruction by irradiation. In this method, boron compounds are injected into the patient's body in advance. After the boron compound has entered the brain tumor cells sufficiently, the affected part is irradiated by neutrons flux generated by a reactor. Boron absorbs neutrons and induces a nuclear reaction, and generates alpha and lithium particles which selectively destroy cancer cells containing it.

Neutron therapy may be directed at the following tumor sites:HEAD AND NECK - Salivary glands, tongue, pharynx, oral cavity, nasopharynx, brain tumors

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CHEST - Localized tumors of lung, mediastinum, pleura, pericardiumABDOMEN - Pancreas, colon, bile duct, gallbladder, ampula of vater, peritoneumPELVIS - Prostate, bladder, uterus, rectosigmoid EXTREMITIES AND TRUNK - Soft tissue, bone, cartilagePALLIATIVE - Large tumors and metastasis from neutron-sensitive tumors Neutron survey are performed by using a proportional counter with ‘Bonner spheres’ to moderate the neutron to thermal energies to be able to detect them.Effective shielding materials contain hydrogen or boron (with high cross sections for thermal neutrons).Features of Neutron Shielding design of facilities include

Long maze design to accommodate many scattering interaction to reduce beam intensity

Provision of an Neutron door - typically filled with borated paraffin wax providing appropriate materials for shielding against generated gamma radiation.

For protection against activation of materials in the Facility it is good practice to let activation products decay prior to entering the room (>10min)Staff must be monitored to detect the level of radiation generated using common personal dosimeters, such as film or TLD badges.If the facility has an accelerator with an energy, E >15 MeV, then radiation surveys should include a neutron survey, especially near the entrance to the maze with a Gas filled (BF3) proportional counter at the centre of a polyethylene block.

Session I 4014 Applications of Ionizing RadiationRadiation Sources find their useful applications in:

Medicine industry and Agriculture consumer products

Medical ApplicationsMedical use of radiation is typically divided into three areas.

1. Diagnostic radiology2. Nuclear medicine3. Radiation therapy

Diagnostic RadiologyMedical x-ray are used to image and diagnose diseases associated with the anatomy of the human body .

Module I.4 Sources of radiation 50

Medical imaging can involve a range of specialties depending on the part of the body to be examined and the pathology under investigation (e.g. cardiologists, vascular surgeons, gynaecologists, urologists, etc):

• Thoracic imaging.• Breast imaging.• Musculo-skeletal radiology.• Gastrointestinal radiology. • Genitourinary radiology.• Head and neck radiology.• Neuroradiology.• Cardiovascular and endovascular radiology and therapy.• Pediatric radiology and.• Dental radiology.

X-ray equipment may be designed and intended for a single purpose or for special purposes for example for:.Dental radiography.General purpose radiography.General purpose fluoroscopy.Mammography .Dedicated fluoroscopy equipment for interventional radiology procedures.Computed tomography (CT).Pediatrics.

Nuclear MedicineThis is a branch of medicine that uses radiation to provide information about the functioning of a person's specific organs or to treat disease. In most cases, the information is used by physicians to make a quick, accurate diagnosis of the patient's illness. In developed countries (26% of world population) the frequency of diagnostic nuclear medicine is 1.9% per year, and the frequency of therapy with radioisotopes is about one tenth of this. Diagnostic techniques in nuclear medicine use radioactive tracers which emit gamma rays while within the body. These tracers are generally short-lived isotopes linked to chemical compounds which permit specific physiological processes to be scrutinized. They can be given by injection, inhalation or orally.Radiopharmaceuticals can be used for diagnosis or treatment, For some medical conditions, it is useful to destroy or weaken malfunctioning cells using radiation. The radioisotope that generates the radiation can be localized in the required organ in the same way it is used for diagnosis - through a radioactive element following its usual biological path, or through the element being attached to a suitable biological compound. In most

Module I.4 Sources of radiation 51

cases, it is beta radiation which causes the destruction of the damaged cells. This is radiotherapy. Short-range radiotherapy is known as brachytherapy. Although radiotherapy is less common than the diagnostic use of radioactive materials in medicine, it has nevertheless become widespread and important. Gamma camera is the first type equipment where single photons are detected by a gamma camera which can view organs from many different angles. The camera builds up an image from the points from which radiation is emitted; this image is enhanced by a computer and viewed by a physician on a monitor for indications of abnormal conditions.SPECT (single photon emission computed tomography) is a nuclear medicine imaging techniques using a gamma camera to acquire multiple 2-D images (also called projections), from multiple angles. A computer is then used to apply a tomographic reconstruction algorithm to the multiple projections, yielding a 3-D data set. This data set may then be manipulated to show thin slices along any chosen axis of the body, similar to those obtained from other tomographic techniques, such as magnetic resonance imaging (MRI), X-ray computed tomography (X-ray CT).To acquire SPECT images, the gamma camera is rotated around the patient. Projections are acquired at defined points during the rotation, typically every 3–6 degrees. In most cases, a full 360-degree rotation is used to obtain an optimal reconstruction. The time taken to obtain each projection is also variable, but 15–20 seconds is typical. This gives a total scan time of 15–20 minutes. A typical SPECT machine is shown in figure 4:17

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Figure 4: 17 SPECT machine

Positron Emission Tomography (PET)A more recent development is Positron Emission Tomography (PET) which is a more precise and sophisticated technique using isotopes produced in a cyclotron. A positron-emitting radionuclide is introduced, usually by injection, and accumulates in the target tissue. As it decays it emits a positron, which promptly combines with a nearby electron resulting in the simultaneous emission of two identifiable gamma rays in opposite directions. These are detected by a PET camera and give very precise indication of their origin. PET's most important clinical role is in oncology, with fluorine-18 as the tracer, since it has proven to be the most accurate non-invasive method of detecting and evaluating most cancers. It is also well used in cardiac and brain imaging.Medical radionuclides used in Nuclear Medicine come from two sources:

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1. Byproduct Materials (Reactor Radioisotopes ) : Typical examples are;Molybdenum-99: Used as the 'parent' in a generator to produce technetium-99m, the most widely used isotope in nuclear medicine. Technetium-99m: Used in to image the skeleton and heart muscle in particular, but also for brain, thyroid, lungs (perfusion and ventilation), liver, spleen, kidney (structure and filtration rate), gall bladder, bone marrow, salivary and lacrimal glands, heart blood pool, infection and numerous specialized medical studies. Chromium-51: Used to label red blood cells and quantify gastro-intestinal protein loss. 2. Accelerator Produced Material : Typical examples are;67Ga ;201Tl ; 123I ; 13N ; 15O ; and 18FEvery organ in our bodies acts differently from a chemical point of view. Doctors and chemists have identified a number of chemicals which are absorbed by specific organs. The thyroid, for example, takes up iodine, the brain consumes quantities of glucose, and so on. With this knowledge, radiopharmacists are able to attach various radioisotopes to biologically active substances. Once a radioactive form of one of these substances enters the body, it is incorporated into the normal biological processes and excreted in the usual ways. A radioisotope used for diagnosis must emit gamma rays of sufficient energy to escape from the body and its half-life must be short enough for it to decay away soon after the imaging is completed. The radioisotope most widely used in medicine is technetium-99m employed in some 80% of all nuclear medicine procedures. It has almost ideal characteristics for a nuclear medicine scan (low energy photon, no beta, short half life) It has a half-life of six hours which is long enough to examine metabolic processes yet short enough to minimize the radiation dose to the patient. Technetium-99m decays by a process called "isomeric“ transition which emits gamma rays and low energy electrons. Since there is no high energy beta emission the radiation dose to the patient is low. The low energy gamma rays it emits easily escape the human body and are accurately detected by a gamma camera. Once again the radiation dose to the patient is minimized. The chemistry of technetium is so versatile it can form tracers by being incorporated into a range of biologically-active substances to ensure that it concentrates in the tissue or organ of interest. Table 4:17 give examples of some common Radiopharmaceuticals Radiopharmaceuticals Primary use

57Co/58Co (cyanocobalamin Schilling test

51Cr (sodium chromate) RBCs

18F (FDG) positron emission tomography (PET) imaging

67Ga soft tissue tumor & inflammatory process imaging

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111In (chloride) labeling monoclonal antibodies

123I (sodium iodide) thyroid imaging and uptake

125I (human serum albumin) plasma volume determinations

131I (sodium iodide) thyroid uptake, imaging and therapy

131I (iodohippurate) renal imaging and function studies

81mKr (gas) pulmonary ventilation imaging

89Sr palliative treatment of bone pain ofskeletal metastases

99mTc (pertechnetate) imaging of thyroid, salivary glands, ectopic gastric mucosa, parathyroid glands, dacryocystography, cystography

RADIOTHERAPY Rapidly dividing cells are particularly sensitive to damage by radiation. For this reason, some cancerous growths can be controlled or eliminated by irradiating the area containing the growth. Radiotherapy uses these sources of ionizing radiation to treat cancer of different types.X-rays machines and radioactive sources are used summarized below:

Linear Accelerators (X-ray or electrons) Teletherapy (External Sealed Source)

- Traditional- Stereotactic Radiosurgery

Brachytherapy (Internal Sealed Source)- Low Dose Rate (LDR)- High Dose Rate (HDR)- Intravascular (IVB)

External irradiation can be carried out using a gamma beam from a radioactive cobalt-60 source, though in developed countries the much more versatile linear accelerators are now widely used as high-energy X-ray sources.

Module I.4 Sources of radiation 55

With any therapeutic procedure the aim is to confine the radiation to well-defined target volumes of the patient. The doses per therapeutic procedure are typically 20-60 Gy. Brachytherapy treatment at short distance is done by administering or planting a small radiation source, usually a gamma or beta emitter, in the target area.Iridium-192 implants are used especially in the head and breast. They are produced in wire form and are introduced through a catheter to the target area. After administering the correct dose, the implant wire is removed to shielded storage. This procedure gives less overall radiation to the body, is more localized to the target tumour and is cost effective.90Sr sources with the appropriate applicators are used for treatment of eye cancers.

Industrial ApplicationsIndustrial radiography is used for Non Destructive Inspection of :

Welds Joints in pipe work (pipe crawler equipment) & storage tanks Castings (valves, engine components) Tyre structure Screening of baggage, parcels and food products

X-Ray Equipment is used a radiation source consisting of : Conventional x-ray equipment (operated at 150 kV to 400 kV) Accelerators

high energy (up to 5 MeV) static, mobile, portable

real-time x-radiographyIndustrial radiography can be performed at a fixed facility ( enclosure radiography) or on site ( Site radiography) .Gamma radiography involves using radioisotopes of Activities of between 100 GBq to 1 TBq listed below:Radionuclide Energy (Mev)

Gamma Factor (Γ γ) ( Sv.m2/GBq.h )

Cobalt-60 1.33 , 1.17 O.351Iridium -192 0.2-1.4 0.130Caesium -137 0.662 0.081Thulium-170 0.08 0.034

Module I.4 Sources of radiation 56

IrradiatorsIrradiators are used for :

Sterilization of medical supplies Preservation of food to improve their shelf life Radiation Effects in biological research Chemical synthesis in chemical research Types of irradiators include : gamma irradiator facilities which use Cobalt -60 and Caesium-137 sources electron beam facilities (< 10 MeV)

Types of Gamma Irradiators Category I

self contained, dry storage irradiator Category II

panoramic, dry storage irradiatorCategory III

self contained, wet storage irradiatorCategory IV

panoramic, wet storage irradiator

Electron Beam IrradiatorsBeam energy up to 10 MeV

no induced radioactivityCategory I

integrally shielded unit with interlocksCategory II

unit housed in shielded room maintained inaccessible during irradiation

Well Logging Welling logging is used for:

Oil Exploration Field Studies

TYPES of Well logging devices: Gamma Logging Gamma-Gamma Logging Neutron Logging

Module I.4 Sources of radiation 57

Tracer Studies Logging While Drilling

GaugesAre used for:

Process Control (fixed) - thickness gauging; level gauging and density gauging Road Paving (portable) - moisture and density gauging

Food and AgricultureA billion people go to bed hungry every night and tens of thousands die daily from hunger and hunger related causes. Radioisotopes and radiation used in food and agriculture are helping to reduce these tragic figures.The beneficial applications include:

Effective use of Fertilizers Fertilizers are expensive and if not properly used can damage the environment. Efficient use of fertilizers is therefore of concern to both developing and developed countries. It is important that as much of the fertilizer as possible finds its way into plants and that the minimum is lost to the environment. Fertilizers 'labeled' with a radioactive isotope, such as nitrogen-15 and phosphorus-32 provide a means of finding out how much is taken up by the plant and how much is lost.

Insect ControlCrop losses caused by insects may amount to more than 10% of the total harvest worldwide, in some developing countries the figure can be as high as 30%. Stock losses due to tsetse in Africa and screwworm in Mexico have also been sizeable. Chemical insecticides have for many years been our main weapon in trying to reduce these losses, but they have not always been effective. Some insects have become resistant to the chemicals used and some insecticides leave poisonous residues on the crops. One solution has been the use of sterile insects. The Sterile Insect Technique (SIT) consists of irradiating laboratory-reared male insects before hatching, to sterilize them. The sterilized males are then released in large numbers in the infested areas. When they mate with females, no offspring are produced. With repeated releases of sterilized males, the population of the insect pest in a given area is drastically reduced.

Food PreservationSome 25-30% of the food harvested is lost as a result of spoilage by microbes and pests. The reduction of spoilage due to infestation and contamination is of the utmost importance. This is especially so in countries which have hot and humid climates and

Module I.4 Sources of radiation 58

where an extension of the shelf life of certain foods, even by a few days, is often enough to save them from spoiling before they can be consumed. In all parts of the world there is growing use of irradiation technology to preserve food. In almost 40 countries health and safety authorities have approved irradiation of many kinds of food, ranging from spices, grains and grain products to fruit, vegetables and meat. Genetic VariabilityIonizing radiation in plant breeding has been used for several decades to produce new genetic lines of sorghum, garlic, wheat, bananas, beans, avocado and peppers, all of which are more resistant to pests and more adaptable to harsh climatic conditions than the original genetic lines.

Water Resources management An adequate water supply is essential for life. Yet in many parts of the world water has

always been scarce and in others it is becoming scarcer.Isotopic techniques are often of great help to trace and measure the extent of underground water resources. Such techniques provide important analytical tools in the

management and husbanding of existing supplies of water and in the identification of new, renewable sources of water. They provide answers to questions about origin, age and distribution, the interconnections between ground and surface water and renewal systems. The results permit informed recommendations for the planning and management of the sustainable use of these water resources.

Module I.4 Sources of radiation 59