Energy on this world and elsewhere

Instructor: Gordon D. Cates Office: Physics 106a, Phone: (434) 924-4792

email: cates@virginia.edu

Course web site available at www.phys.virginia.edu, click on classes and find Physics 1110.

or at http://people.virginia.edu/~gdc4k/phys111/fall17/home.html

November 7, 2017Lecture #21

Visiting today: Prof. Paschke

Nuclear Power: isotopes

Isotopes

So when we speak of isotopes, we are generally referring to a specific element.

The number of protons in the nucleus determines the element associated with the atom

Why is it that the number of protons determines the number of electrons?

Because the number of protons also determines the number of electrons, and the number of electrons

determines the atom’s chemical properties.

• 1 proton: hydrogen

• 2 protons: helium

• 3 protons: lithium

• 4 protons: beryllium

• 5 protons: boron

• 6 protons: carbon

Elemental assignment based on chemistry

• Mendeleev (and others) constructed periodic tables of elements well before the “modern atom” was discovered. Mendeleev’s table shown above reproduced from Seaborg’s article.

• Placement in the table corresponded to its chemical properties, and what we now recognize as the atomic number.

From paper by Glenn Seaborg, Chemistry and Engineering News, vol. 57, pg. 46 (1979)

• Both isotopes have two protons, as they must to be the same element.

• One isotope has two neutrons, the other isotope has one neutron.

A single element can have more than one isotope by having differing numbers of

neutrons

Proton

Neutron

Electron

4He 3HeExample: two isotopes of helium

The number of protons in the nucleus determines what element the atom is

This atom has two protonsand two neutrons in its nucleus, making it an

isotope of helium (He-4).

This atom has three protonsand four neutrons in its

nucleus, making it an isotope of of lithium (Li-7)

Each of the elements shown below has multiple isotopes

Isotopes

• If two nuclei have the same number of protons but different numbers of neutrons, we say that they are two isotopes of a particular element.

• A nucleus is always a particular isotope of an element.

• A nucleus is uniquely identified when you identify both the element and the “mass number”, the total number of neutrons plus protons.

• Two nuclei with the same mass number but different numbers of protons are not the same isotope, they are two particular isotopes of two different elements.

Identifying a particular isotope

The “chemical symbol” (here U for uranium) identifying

the element.

23592 U

The “atomic number”, which is equal to the number of protons.

This number is often left out since it is essentially redundant with the chemical symbol

The “mass number”, equal to the total number of protons plus the

total number of neutrons.Sometimes one would

also just refer to this as U-235

Table of NuclidesNeutroncaptureFusion(stellar)AlphaemissionNeutronemissionProtonemissionBetadecay(N↔Z)

neutrons

proton

s

Table of NuclidesNeutroncaptureFusion(stellar)AlphaemissionNeutronemissionProtonemissionBetadecay(N↔Z)

neutrons

proton

s

Beta decay

• The neutron mass is slightly more than the proton mass.

• When a neutron turns into a proton, some energy is released, so the system settles into a lower energy state (as far as nuclear forces are concerned).

• The net result is that the mass number stays the same, but the atomic number (the number of protons) goes up by one.

Example of Beta DecayTotal number of nucleons

or the “mass number”

Number of protons or “atomic number”

This is a beta particle, which is really just an electron.

Thick cardboard or a small amount of metal will stop beta particles

13755 Cs �⇥137m

56 Ba + �� + ⇥

How plutonium is made• U-238 captures a neutron, becoming U-239

• U-239 Beta decays (half-life of 23 minutes) into neptunium-239

• Np-239 beta decays (half-life of 2.4 days) into plutonium-239

23 minutes

2.4 days

The two isotopes of natural uranium

23892 U

23592 U

# neutrons = 238 - 92 = 146

# neutrons = 235 - 92 = 143

Only one isotope of uranium is easily fissionable

23892 U

23592 U

99.27% of all uranium is U-238 When hit by a neutron it will sometimes undergo fission, but most of the time the neutron is just absorbed.

0.63% of all uranium is U-235 When hit by a neutron it will almost always undergo fission.

99.28%

0.72%

So how does fission work?

Fission reactions in uranium

Uranium-235 can undergo “fission”

Here the liquid drop model illustrates how the addition of a

neutron can make a nucleus unstable

• While both isotopes of uranium can in principle undergo fission, only 235U will undergo fission after absorbing a slow neutron.

• Note that after fissioning, the resulting nuclei are in a size range where a lower neutron to proton ratio is favored.

Uranium-235 undergoing a chain reaction

A chain reaction is similar to the phenomena of burning when considered

in the context of chemistry.

Mousetrap chain reaction

Notice that on average, every mousetrap must release more than one ball or the chain reaction will not grow in size.

Mousetrap chain reaction

Notice that on average, every mousetrap must release more than one ball or the chain reaction will not grow in size.

Nuclear reactors• Pellets of uranium oxide are

combined into what are called fuel rods.

• The fuel rods are arranged into a matrix.

• The space between the fuel rods is filled with a “moderator” that slows down the neutrons after they emerge from a fission reaction.

• Control rods, that are very good at absorbing neutrons, are inserted and withdrawn to control the rate at which reactions take place.

• “prompt” criticality is unstable. Make use of delayed neutrons for criticality helps control process.

file:///Users/gordon/Gordon's%20files/Energy/energy_2013/lectures/

Reactor6.webarchive

Nuclear reactors need “moderators”• A 235U nucleus is increasingly likely to absorb a neutron as the neutron’s speed is reduced.

• To slow down the fast-moving neutrons emitted during fission, a moderator is used. Basically, some material is introduced between regions containing fuel. When the neutrons collide with nuclei in the moderator, they are slowed down.

The first reactor used very pure graphite as a moderator. In this case,

natural uranium can be used.

Another option is to use “heavy water” (D2H) as a moderator.

In this case the water is isotopically enriched, but again

natural uranium can be used.

If normal water is used as a moderator, the uranium must be isotopically enriched.

Three approaches are most common, and they have implications for whether or not the fuel needs to be isotopically enriched:

23592 U Nearly pure U-235

(isotope enrichment)

Two paths to a bomb

Two paths to a bomb

23993 Pu Pu-239 that is made in a

reactor

Three paths to nuclear weapons

• A graphite based reactor makes it possible to use natural uranium as a fuel. There are many good reasons, unrelated to weapons, for such a reactor. Nevertheless, it can be used to produce plutonium.

• Heavy water is D2O. That is, it is water in which the two hydrogen atoms are the isotope 2H instead of 1H. If a country is accumulating heavy water, they are giving themselves the capability of using natural uranium.

• Finally, a “light water” reactor, that is, a reactor that uses normal water, can only be operated if it uses isotopically enriched uranium. If the country is enriching their own uranium, they have TWO paths open to nuclear weapons. One is simply producing highly enriched 235U for a uranium-based bomb, the other is using the reactor to produce plutonium

• Only if a country uses fuel produced elsewhere, and gives the fuel back after using it, is there reasonable assurance that a country can have reactors without developing weapons capability.

As we will discuss, when 238U in a reactor absorbs a neutron, it is transformed into plutonium. Thus ANY reactor signals at least the

possibility of developing nuclear weapons.

Isotopic enrichment

• Chemical techniques cannot be used because different isotopes have the same chemical properties.

• It is necessary to take advantage of the slightly different mass of the two isotopes.

• Something called gas-diffusion separation, combined with mass spectrometers were used during WWII.

• The more modern technique involves vacuum ultracentrifuges, invented by Jesse Beams right here at UVa.

It is very difficult to separate out different isotopes of a given element.

23892 U 235

92 U

Enriching Uranium

The vacuum ultracentrifuge

23892 U 235

92 U

Above, Jesse Beams receives the Nation Medal of Science from

President Johnson in 1967.Invented by Jesse Beams at UVa, the ultra-centrifuge spins incredibly fast, separating

things according to their weight. For uranium, the compound uranium hexafluoride (UF6) is used.

Breeding plutonium

0.7%

99.3%

U-238 U-235

In a reactor, U-238 is slowly (or not so slowly) converted into plutonium. When done intentionally, the

process is referred to as breeding.

Plutonium

How plutonium is made• U-238 captures a neutron, becoming U-239

• U-239 Beta decays (half-life of 23 minutes) into neptunium-239

• Np-239 beta decays (half-life of 2.4 days) into plutonium-239

23 minutes

2.4 days

Creating Pu-239 from U-235

• Conventional reactors burn the U-235 almost exclusively.

• ALL reactors that burn U-235 make Pu-239 in the process (more on this shortly), one of the isotopes from which you can make weapons.

- That is why the existence of ANY reactor in a country represents a potential hazard of proliferation of nuclear weapons.

• Separating the plutonium, however, from the highly radioactive nuclear fuel, is still very difficult.

What if we used nuclear energy at the rate it was being used in 2005?

The time scales given above assume that nuclear energy is used solely to produce electricity, and furthermore, to produce electricity at the same

rate at which nukes are used to produce electricity today (actually in 2005). It is also interesting to consider what would happen if we used

nukes for ALL of our energy.

How long would the entire world’s uranium last if it were the world’s only source of energy?

• Here we assume the same total conventional resources as on the previous slide, and total global energy consumption of 411 Quads/year.

• If only conventional reactors were used for ALL of our energy needs, total conventional resources would last something like 23 years.

• If fast breeder reactors were used (and around half the the U-238 were converted into plutonium), around 1,400 years. Thorium resources would greatly extend this number.

How an FBR works

• Conventional reactors use water as a moderator. It slows down the neutrons produced during fission reactions.

• Slower neutrons are more likely to cause the fission of U-235. Conventional reactors typically use 3-5% 235U (LEU)

• Fast Breeder reactors typically use liquid metal as a moderator. The neutrons are not slowed down nearly as much and are thus “fast”.

• The fast neutrons are readily captured by U-238, which converts to plutonium.

• To make up for the fast neutrons, the reactors use a richer mixture of fissionable fuel (HEU, maybe 20% 235U). This also makes them less stable.

Example of a “Generation IV reactor” FBR design

France’s Superphoenix - example of breeder reactor

France’s Superphoenix - example of breeder reactor

Russia’s BN-800 Fast Breeder Reactor

Thorium-based breeder reactors

• Turns (stable) thorium-232, which is nearly 100% of natural thorium, into uranium-233

• You need to start with something other than pure thorium to get the cycle going, but eventually, you could just use the uranium-233 that you breed.

• India is building a fast thorium-based breeder right now, and has a thermal thorium-based breeder in the works.

Shippingport Power Station

• World’s first utility-scale nuke in the U.S. used entirely for peace-time purposes, first went critical in 1957.

• Last of three “cores”, designed to breed U-233 from thorium, was operated from 1977-1982 and produced 2.5 billion kilowatt-hours of electricity.

• Analysis after decommissioning showed that there was 1.4% more fissile material in the last core at the end of its operation than was the case when it was installed.