Nuclear Power

Question 1

Describe the reactor being built at Hinkley Point C.

Answer 1

• Advanced Pressurised Water Reactor
• Uses enriched U235 or Pu (up to 5%)
• Fission in the core generates heat that is removed by the pressurised water (coolant and moderator)
• High pressure hot water (155bar and 300C) goes through a steam generator which transfers heat to another low pressure water loop
• This water steams and turns steam turbines that spin alternators to generate electricity
• Two reactors, each 1.6GWe. Equates to 7% of the UK’s generating capacity and enough for 6 million homes

Question 2

Why does it usually take longer to build a nuclear station than a gas fired one?

Answer 2

• Inherently more difficult
• Safety challenges
• Legislation and approval is more difficult to obtain
• Politics
• Social preferences
• Nuclear proliferation

This all requires extensive consultations and careful consideration

Question 3

Why are nuclear stations considered base load stations, while gas fired ones are load followers?

Answer 3

• Construction of nuclear stations is very expensive
• Nuclear fuel is cheap compared to gas
• Nuclear stations need to be run at high capacity constantly in order to bring thee cost of electricity down
• Gas stations are then switched on and off to match demand

Question 4

What is a moderator? Give some examples of moderators.

Answer 4

A material with the purpose of slowing down fast energetic neutron to thermal speeds.

Carbon, Water, Heavy Water

Question 5

State the essential characteristics of a moderator.

Answer 5

• Able to slow (must have atoms of low atomic mass)

- Will not absorb neutrons (very low cross sectional area of absorption)

Question 6

What is an absorber? Give an element that is a good absorber.

Answer 6

A material which absorbs neutrons easily and readily at a large spectrum of neutron energy levels.

Boron

Question 7

What are delayed neutrons?

Answer 7

Neutrons that are released some time after a neutron has been absorbed into a fissile material. They still contribute to the four factor formula, but in a relaxed manner, making themselves apparent some finite time after the relevant absorption event.

This means a reactor with a positive reactivity sees its neutron population slowly increase. The speed of increase is sufficiently slow to enable the reactivity to be controlled safely by the relaxed movement of absorption material in and out of the core.

Question 8

What is the difference between fission and fusion?

Answer 8

Fission is the break up of large (unstable) nucleus into two or more smaller nuclei with the release of energy.

Fusion is the joining of two light nuclei into one heavier nucleus with the release of energy.

Question 9

State the four factor formula and name each of the factors.

Answer 9

k = ENPF (for infinite size)

N = fast neutron yield
E = fast fission factor
P = resonance escape probability
F = thermal utilisation factor

kn = ENPFn(1-Lf)(1-Ls)

where Lf and Ls are the proportion of fast and slow neutrons lost

Question 10

What is breeding?

Answer 10

Breeding is where a non-fissile material absorbs a neutron to become a fissile material. Th232 becomes U233. This increases the utilisation factor up to 50%. Thorium is also 3x more abundant than Uranium on Earth.