Lecture 8

Question 1

What is a possible alternative to U for a nuclear reactor

Answer 1

1. Liquid fluoride thorium reactor (LFTR)
2. Thorium (232Th) is not fissile in itself but can be bred to fissile 233U

Question 2

Show how production of 233U is possible from 232Th

Answer 2

1. 232Th + 1n –> 233Pa –> 233U (+beta)–> spontaneous fission
2. 233Pa t1/2 - 27 days
3. 233U t1/2 = 160000 years - relatively stable but won’t exist in nature as too short a half-life

Question 3

What are advantages of LFTR

Answer 3

1. Th is 100% 232Th, 4x as abundant as U and available in most countries - no need to enrich
2. Almost only intermediate lived fission products are formed (234U)- thus radioactive waste storage of <500y
3. Less nuclear waste- no Pu produced as in 238U
4. Requires neutron flow: no risk of run-away accident

Question 4

Why is LFTR hard to make atomic weapons with

Answer 4

1. 233U is contaminated by highly gamma-active 232U- can’t work with
2. Therefore isolation of 233 for weapons is very difficult/ expensive
3. This reduced risk of proliferation
4. Little use for atomic weapon production- good
5. Fissile properties- 233U is poor compared to other U isotopes

Question 5

Describe process of LFTR

Answer 5

1. Technology exists best using liquid fluoride salts at 800 degrees with one plant in operation in India
2. Pump material in molten salt solution - in centre have U233
3. U233 does fission reaction - produces neutrons
4. Neutrons picked up by Th-232 producing Pa - wait for decomposition (month)- then extract 233U
5. Inject 233U into core
6. withdraw heat from process into turbine
7. Add new 232Th
8. No need to enrich any isotopes- only need 6Kg of Th to produce same energy as 300kg of enriched U

Question 6

Describe idea of nuclear fusion for energy

Answer 6

1. More energy can be gained by fusion of hydrogen
2. But, process can only occur by collision of positively charged nuclei - high energy particles required ( 1 million K)

Question 7

What is nuclear fusion reaction with lowest ignition temperature and how could it be used

Answer 7

1. 3H + 2H –> 4He + 1n + energy
2. The neutron can be used to generate 3H (difficult to obtain) from much more available lithium

Question 8

Show how 3H could be obtained

Answer 8

1. 6Li + 1n –> 4He + 3H
2. (Or 7 Li + 1n –> 4He + 3H + 1n)

Question 9

What would overall reaction be and what is needed

Answer 9

1. 6 Li + 2H –> 2 4He + 22.4MeV

Question 10

What ratio of Li to D is needed

Answer 10

1. 1:1
2. Can be obtained by LiD

Question 11

How does hydrogen bomb work

Answer 11

1. Have 235U and TNT at one end
2. Deuterium and triterium or LiD at other
3. Detonation of TNT causes fission which generates heat to carry out fusion needs ignition at >10 million K to carry out fusion
4. Energy yield is about 100 times greater than in U or Pu bombs

Question 12

Could you have a fusion power station

Answer 12

1. Once fusion has begun, high energy neutrons radiate from the reactive regions of the plasma, crossing magnetic field lines easily due to charge neutrality
2. Since the neutrons receive the majority of the energy, they are the reactor’s primary source of energy output
3. Beyond the inner wall of the containment vessel blanket modules of lithium slow and absorb neutrons in a reliable and efficient manner, breeding tritium for fuel.
4. But not reaching break even point

Question 13

What is ITER project

Answer 13

1. international thermonuclear experimental reactor
2. Based around a hydrogen plasma torus operating at over 100 million degrees and will produce 500 MW of fusion power
3. Scaling up is more energy efficient

Question 14

What is benefit of nuclear fusion

Answer 14

1. Radiotoxicity (inhalation) of waste from fusion is less than fission and similar to that from coal after 100 years so very low
2. Some radioactivity from neutrons which aren’t caught by Li
3. Can have control over radioactivity - choose which materials you use to make reactor- which ones that will absorb neutrons and become radioactive as not all neutrons are absorbed by Li

Question 15

What would be a better fusion fuel

Answer 15

1. Aneutronic fusion- without generating neutrons- no radioactivity so no nuclear waste
2. 3He + 2H –> 4He + 1H + 18.354 MeV
3. But very low abundance pf 3He on earth - a few ppm of He- not part of typical fusion processes
4. He in atm is very light so goes to space
5. 4He comes from alpha decay of natural gas reservoirs- depends on rock formation- but 3He not produced by this
6. Main source of 3He comes from lunar surface
7. Also need very high temperature 100 million K

Question 16

What is another type of aneutronic fusion

Answer 16

1. 1H + 11B –> 8Be* + 4He + 8.6MeV - Protons heated up with cyclotron and hit plasma of 11B with proton beam
2. 8Be* excited state which decays to produce 2 4He
3. He fusion process insum- exothermic
4. Generates 3x 4He - stable = good - no neutrons
5. Requires 10x more energy than D-T fusion - 100 Million K

Question 17

What are two alternatives to reduce ignition temperature

Answer 17

1. Myon catalysts
2. Cold fusion

Question 18

Describe myon catalyst use

Answer 18

1. Reduce fusion temp
2. The high energy particle myon (mu-) can catalyse fusion
3. 207x heavier than electron (basically heavy electron) thus nuclei in DT is closer to hydrogen - closer to fusion
4. Costs 5000MeV to produce in accelerator
5. Lifetime of only 2 microseconds, long enough to fuse 150DT at 1000K- Not energy neutral atm (need 250)

Question 19

What is cold fusion

Answer 19

1. Pons and Fleischman claimed to have been able to achieve cold fusion of deuterium by electrolysis of D2O on palladium electrodes
2. Thought to be experimental error
3. Similar claims of cold fusion using ultrasound could not be confirmed