fusion and fission of nuclear 1

8/3/2019 Fusion and Fission of Nuclear 1

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Introduction

Today nuclear power is universally controversial. Many would say that it is also

universally needed.as an alternative or supplement to power generated by fossil fuels. The

combustion of fossil fuels produces carbon dioxide, now notorious for the threat of global

warming. Nuclear power plants produce neither carbon dioxide nor oxides of sulfur and nitrogen,

as does the burning of fossil fuels. Thus nuclear power reduces the global production of carbon

dioxide and other pollutants, and helps to alleviate many of the pervasive problems of fossil fuel

supply.

Petroleum is least available in regions of widest use; natural gas is, for the time being, plentiful

and sought after by all; and widely abundant coal has come to be regarded as the great Satan of

air pollution. Water power is important, but it offers limited possibility for growth. Solar energy,

while promising, is far from being a mainstay of the worlds energy supply. Thus sources other

than fossil fuels and nuclear power offer little hope to become major suppliers during our

lifetimes.

y Worldwide Nuclear Power Reactorsy There are 440 nuclear power reactors in 31 countries.y 30 more are under construction.y They account for 16% of the worlds electricity.y They produce a total of 351 gig watts (billion watts) of electricity.


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Review of Atomic Structure

Atoms are made up of 3 types of particles electrons , protons and neutrons. These

particles have different properties. Electrons are tiny, very light particles that have a negative

electrical charge (-). Protons are much larger and heavier than electrons and have the opposite

charge, protons have a positive charge. Neutrons are large and heavy like protons, however

neutrons have no electrical charge. Each atom is made up of a combination of these particles.

Molecule is an electrically neutral group of at least two atoms held together by covalent

chemical bonds. Molecules are distinguished from ions by their electrical charge. The nucleus is

the very dense region consisting of protons and neutrons at the center of an atom.


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Nuclear Fuel

y Nuclear fuel is any material that can be consumed to derive nuclear energy. The mostcommon type of nuclear fuel is fissile elements that can be made to undergo nuclear

fission chain reactions in a nuclear reactor

y The most common nuclear fuels are 235U and 239Pu. Not all nuclear fuels are used infission chain reactions

Nuclear Fission

y When a neutron strikes an atom of uranium, the uranium splits ingto two lighter atomsand releases heat simultaneously.

y Fission of heavy elements is an exothermic reaction which can release large amounts ofenergy both as electromagnetic radiation and as kinetic energy of the fragments

U235 + n fission + 2 or 3 n + 200 MeV


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y If each neutron releases two more neutrons, then the number of fissions doubles eachgeneration. In that case, in 10 generations there are 1,024 fissions and in 80 generations

about 6 x 10 23 (a mole) fissions.

ADVANTAGES DISADVANTAGES

y Nuclear power generation does emitrelatively low amounts of carbondioxide (CO2). The emissions of

greenhouse gases and therefore thecontribution of nuclear power plants to

global warming is therefore relativelylittle.

y This technology is readily available, itdoes not have to be developed first.

y It is possible to generate a high amountof electrical energy in one single plant

y The problem of radioactive waste isstill an unsolved one.

y High risks: It is technically impossibleto build a plant with 100% security.

y The energy source for nuclear energy isUranium. Uranium is a scarce resource,its supply is estimated to last only for

the next 30 to 60 years depending onthe actual demand.


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Fusion and Fission of Nuclear

Nuclear Fusion

Another form of nuclear energy is called fusion. Fusion means joining smaller nuclei (the plural of nucleus) to make a larger nucleus. The sun uses nuclear fusion of hydrogen

atoms into helium atoms. This gives off heat and light and other radiation.

In the picture to the right, two types of hydrogen atoms, deuterium and tritium, combineto make a helium atom and an extra particle called a neutron.

Scientists have been working on controlling nuclear fusion for a long time, trying tomake a fusion reactor to produce electricity. But they have been having trouble learning

how to control the reaction in a contained space. What's better about nuclear fusion is that it creates less radioactive material than fission,

and its supply of fuel can last longer than the sun.


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Nuclear Fission

An atom's nucleus can be split apart. When this is done, a tremendous amount of energy is

released. The energy is both heat and light energy. Einstein said that a very small amount of

matter contains a very LARGE amount of energy. This energy, when let out slowly, can be

harnessed to generate electricity. When it is let out all at once, it can make a tremendous

explosion in an atomic bomb.

A nuclear power plant (like Diablo Canyon Nuclear Plant shown below) uses uranium as a

"fuel." Uranium is an element that is dug out of the ground many places around the world. It is

processed into tiny pellets that are loaded into very long rods that are put into the power plant's

reactor.

The word fission means to split apart. Inside the reactor of an atomic power plant, uranium atoms

are split apart in a controlled chain reaction.

In a chain reaction, particles released by the splitting of the atom go off and strike other uranium

atoms splitting those. Those particles given off split still other atoms in a chain reaction. In

nuclear power plants, control rods are used to keep the splitting regulated so it doesn't go too

fast.


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Figure: Diablo Canyon Nuclear Plant

If the reaction is not controlled, you could have an atomic bomb. But in atomic bombs, almost

pure pieces of the element Uranium-235 or Plutonium, of a precise mass and shape, must be

brought together and held together, with great force. These conditions are not present in a

nuclear reactor.

The reaction also creates radioactive material. This material could hurt people if released, so it is

kept in a solid form. The very strong concrete dome in the picture is designed to keep this

material inside if an accident happens.

This chain reaction gives off heat energy. This heat energy is used to boil water in the core of the

reactor. So, instead of burning a fuel, nuclear power plants use the chain reaction of atoms

splitting to change the energy of atoms into heat energy.


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This water from around the nuclear core is sent to another section of the power plant. Here, in the

heat exchanger, it heats another set of pipes filled with water to make steam. The steam in this

second set of pipes turns a turbine to generate electricity. Below is a cross section of the inside of

a typical nuclear power plant.


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Types of nuclear reactors

Pressurized Water Reactor

Figure 1: Schematic: Pressurized Water Reactor (PWR)

The most common type of reactor -- the PWR uses regular old water as a coolant. The primary

cooling water is kept at very high pressure so it does not boil. It goes through a heat exchanger,

transferring heat to a secondary coolant loop, which then spins the turbine. These use oxide fuelpellets stacked in zirconium tubes. They could possibly burn thorium or plutonium fuel as well.

Pros:

y Strong negative void coefficient -- reactor cools down if water starts bubblingy Secondary loop keeps radioactive stuff away from turbines, making maintenance easy.y Very much operating experience has been accumulated and the designs and procedures have

been largely optimized.

Cons:

y Pressurized coolant escapes rapidly if a pipe breaks, necessitating lots of back-up coolingsystems.

y Cant breed new fuel -- susceptible to "uranium shortage"


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Boiling Water Reactor

Figure 2 Schematic: Boiling Water Reactor (BWR)

Second most common, the BWR is similar to the PWR in many ways. However, they only have

one coolant loop. The hot nuclear fuel boils water as it goes out the top of the reactor, where the

steam heads over to the turbine to spin it.

Pros:

y Simpler plumbing reduces costsy Power levels can be increased simply by speeding up the jet pumps, giving less boiled water and

more moderation. Thus, load-following is simple and easy.

y Very much operating experience has been accumulated and the designs and procedures havebeen largely optimized.

Cons:

y With liquid and gaseous water in the system, many weird transients are possible, making safetyanalysis difficult

y Cant breed new fuel -- susceptible to "uranium shortage"y Does not typically perform well in station blackout events, as in Fukushima.


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Sodium Cooled Fast Reactor

Figure 3: Sodium-Cooled Fast Reactor

The first electricity-producing nuclear reactor in the world was SFR (the EBR-1 in Arco, Idaho).

As the name implies, these reactors are cooled by liquid sodium metal. Sodium is heavier than

hydrogen, a fact that leads to the neutrons moving around at higher speeds (hence fast). These

can use metal or oxide fuel, and burn anything you throw at them (thorium, uranium, plutonium,

higher actinides).

Pros:

y Can breed its own fuel, effectively eliminating any concerns about uranium shortagesy Can burn its own wastey Metallic fuel and excellent thermal properties of sodium allow for passively safe operation

Cons:

y To fully burn waste, these require reprocessing facilities which can also be used for nuclearproliferation.

y Positive void coefficients are inherent to all fast reactors. This is a safety concern.y Not as much operating experience has been accumulated


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Canada Deuterium-Uranium Reactors (CANDU)

Figure 1.2 Schematic: Pressurized Heavy Water Reactor (CANDU)

CANDUs are a Canadian design found in Canada and around the world. They contain heavy

water, where the Hydrogen in H2O has an extra neutron (making it Deuterium instead of

Hydrogen). Deuterium absorbs many fewer neutrons than Hydrogen, and CANDUs can operate

using only natural uranium instead of enriched.

Pros:

y Require very little uranium enrichment.y Can be refueled while operating, keeping capacity factors.y Are very flexible, and can use any type of fuel.

Cons:

y Some variants have positive coolant temperature coefficients, leading to safety concerns.y Neutron absorption in deuterium leads to tritium production, which is radioactive and often leaks

in small quantities.

y Are particularly good at producing weapons-grade plutonium.


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Liquid Fluoride Thorium Reactor

LFTRs have gotten a lot of attention lately in the media. They are unique so far in that they use

molten fuel. So there's no worry of meltdown because theyre already melted. The folks over

at Energy from thorium are totally stoked about this technology.

Pros:

y Can constantly breed new fuel, eliminating concerns over energy resourcesy Can be maintained online with chemical fission product removal, eliminating the need to shut

down during refueling.

y Liquid fuel also means that structural dose does not limit the life of the fuel, allowing the reactorto extract very much energy out of the loaded fuel.

Cons:

y Radioactive gaseous fission products are not contained in small pins, as they are in typicalreactors. So if there is a containment breach, all the fission gases can release instead of just the

gases from one tiny pin. This necessitates things like triple-redundant containments, etc. and can

be handled, but is certainly a challenge and disadvantage. All liquid fuel reactors have this

problem.

y Very little operating experience.


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High Temperature Gas Cooled Reactor

HTGRs use little pellets of fuel backed into either hexagonal compacts or into larger pebbles (in

the prismatic and pebble-bed designs). Gas such as helium or carbon dioxide is passed through

the reactor rapidly to cool it. Due to their low power density, these reactors are seen as promising

for using nuclear energy outside of electricity: in transportation, in industry, and in residential

regimes. They are not particularly good at just producing electricity.


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Pros:

y Can operate at very high temperatures, leading to great thermal efficiency (near 50%!) and theability to create process heat for things like oil refineries, water desalination plants, hydrogen

fuel cell production, and much more.

y Each little pebble of fuel has its own containment structure, adding yet another barrier betweenradioactive material and the environment.

Cons:

y High temperature has a bad side too. Materials that can stay structurally sound in hightemperatures and with many neutrons flying through them are hard to come by.

y If the gas stops flowing, the reactor heats up very quickly. Backup cooling systems arenecessary.

y Gas is a poor coolant, necessitating large amounts of coolant for relatively small amounts ofpower. Therefore, these reactors must be very large to produce power at the rate of other

reactors.

y Not as much operating experience


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Albert Einstein Formula

Another major form of energy is nuclear energy, the energy that is trapped inside each atom. One

of the laws of the universe is that matter and energy can't be created nor destroyed. But they canbe changed in form.

Matter can be changed into energy. The world's most famous scientist, Albert Einstein, created

the mathematical formula that explains this. It is:

This equation says:

E [energy] equals m [mass] times [c stands for the velocity or the speed of light. means c times c, or the speed of light raised to the second power or c-squared.]

Proposition. (Mass-energy equivalence) If a body at rest emits a total energy of Ewhile remaining at rest, then the mass of that body decreases by .

fusion and fission of nuclear 1

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