Lecture 7

Question 1

What is the role of coolants

Answer 1

1. Keep fuel rods cool by absorbing heat from fission reaction
2. Transport this heat from core to turbine- sometimes
3. Minor role: coolants also cool neutrons- slow the neutrons down to thermal energy to increase the likelihood of fission- 235U more likely to absorb a slow-moving neutron than a fast one
4. Coolants are often also moderators

Question 2

Describe energy balance of 235U fission

Answer 2

1. Energy released 202 MeV
2. 170 MeV - Hot fission products
3. 7MeV - gamma radiation
4. 5MeV- as fast neutrons
5. 20 MeV- delayed beta radiation from fission products

Question 3

What happens to energy produced in fission

Answer 3

1. Collision with surroundings converts most energy into heat which is carried off by coolant
2. Thus, main role of the coolant is to take up energy from the fission products (but also from neutrons which are not absorbed)

Question 4

What is role of moderators

Answer 4

1. Neutrons produced by nuclear reactiosn move too fast to cause fission
2. They must first be slowed to be absorbed by the fuel and produce additional nuclear reactions
3. Primary purpose of moderators is to slow initially formed fast neutrons down by causing them to collide
4. Atoms of similar mass to neutrons are required for efficient energy transfer in collisions i.e. light elements required
5. However moderator should not absorb the neutrons
6. Also need to be highly stable nuclei

Question 5

What are examples of good moderators

Answer 5

1. 2H is the best - in D2O but also water itself
2. 12C- (pure graphite, cheaper)
3. 16O- Water, CO2
4. All stable so unlikely to absorb neutrons

Question 6

Which is the best moderator and why

Answer 6

1. 2H as has very low capture and good scattering
2. 1H has too much capture which is wasteful as absorbs neutrons once they slow down - fine when fast neutrons

Question 7

How can D2O be used as a moderator

Answer 7

1. Light and low absorption
2. Can enrich H2O with 2H by electrolysis- cheaper as low natural abundance of 2H - easy as large mass difference change
3. Electrolysis- Produces H2 more than D2 but leaves D2O leftover with low H2 conc
4. If D2O used no need for enrichment of 235U

Question 8

Describe role of control Rods

Answer 8

1. Used to regulate the distribution of power in the reactor while the reactor is operating
2. Most important function is to insert to shutdown or stop the nuclear fission process when required
3. Can all be inserted into assembly in 2 seconds
4. Controls power by absorbing all neutrons - opposite to moderators

Question 9

What are typical materials used for control rods

Answer 9

1. Boron carbide
2. Silver
3. Indium
4. Cadmium
5. Hafnium - naturally comes with zirconium so need to be separated- Zirconium used for fuel rods
6. Not stable isotopes so happy to absorb neutrons

Question 10

Describe neutron balance from 6 fissions

Answer 10

1. 15 Fast then slow neutrons
2. 6 go to chain reaction
3. 5 absorbed by 235U- produces Plutonium
4. 4 are lost by leakage, control rods

Question 11

Describe problem of nuclear waste

Answer 11

1. When most of the fissile material of the nuclear fuel is exhausted, highly radioactive waste of various fission products remain as nuclear waste

Question 12

What are the 3 groups that radioactive waste can be divided into

Answer 12

1. Highly radioactive, short-lived isotopes: this material will decay under controlled conditions (often under several metres of water) within a few years
2. Isotopes of intermediate half-lifes (<100 years)
3. Long-lived isotope (>100 years)

Question 13

What is half-life of trans-uranium products

Answer 13

1. Often have very long half-lives (>10000 years) - need long term storage
2. Needs to be stored long enough until the radioactivity drops to natural levels

Question 14

Describe idea of nuclear reprocessing

Answer 14

1. When fuel is removed from a reactor, it contains U, Pu from breeding, many fission products and traces of other trans-uranium elements e.g. Np, Am, Cm
2. Basic problem is to separate U and Pu from each other- want Pu as useful
3. Also want to remove most highly radioactive fissile elements from bulk of cladding - Zirconium of rods etc.
4. Method depends on relative stabilities of the oxidation states of U, Pu and the fission products
5. Separation of U, Pu and fission products

Question 15

Describe plutonium uranium Redox extraction ( PUREX, THORP) process

Answer 15

1. Fuel rods made of Zirconium and pellets which are now partially Pu, U and other fission products
2. First chop up rods into small bits
3. Dissolve in Conc HNO3- doesn’t dissolve Zirconium so left behind
4. Solution of high oxidation state elements- UO2(NO3)2(H2O)2, Pu(NO3)4, Am(NO3)3 in H2O + Other fission products
5. Add hydrocarbon- kerosene and TBP- strongly polar bond so coordinates: Get separation of 2 phases
6. Actinide +3 and Ln 3+ elements and most of other waste products- aqueous phase
7. U and Pu in kerosene- Separate phases using long columns to remove aqueous phase leaving just U and Pu
8. Then reduction e.g. U4+ or H+ : U4+ is oxidised to U6+ and reduces Pu4+ to Pu3+ which goes to fresh aq phase
9. Left with pure U as nitrate in kerosene on top layer- separate layers
10. Oxidise products- produce UO2 and PuO2- could combine to make corresponding fuel to enriched U.