Questions on nuclear reactors and safety

Question 1

State the function of the moderator in a thermal nuclear reactor.

Accepted Answer

Slows the fast neutrons released by fission to thermal speeds (1); thermal neutrons are much more likely to be absorbed by uranium-235 and cause further fission (1).

Question 2

State the function of the control rods in a nuclear reactor.

Accepted Answer

Absorb neutrons (1); they are raised or lowered to adjust the number of neutrons causing fission, controlling the rate of the chain reaction or shutting it down (1).

Question 3

State the function of the coolant in a nuclear reactor.

Accepted Answer

Transfers thermal energy from the core to a heat exchanger, where steam is raised to drive the turbines (1); it also prevents the core from overheating (1).

Question 4

Give one example of a material used as the moderator and one example of a material used in the control rods of a thermal nuclear reactor.

Accepted Answer

Moderator: water or graphite (1). Control rods: boron or cadmium (1).

Question 5

State two properties needed by the coolant of a nuclear reactor.

Accepted Answer

It must transfer thermal energy effectively out of the core (1); it must absorb few neutrons (or: be stable and non-corrosive at the operating temperature) (1).

Question 6

Explain, in terms of collisions, why the moderator in a thermal reactor is made from atoms of low mass.

Accepted Answer

A neutron transfers the largest fraction of its kinetic energy when it collides with a particle of similar mass (1); light nuclei, such as hydrogen in water or carbon in graphite, have masses close to that of a neutron (1); so each collision removes a large fraction of the neutron's energy and few collisions are needed to reach thermal speeds (1).

Question 7

Explain how control rods are used to keep a reactor operating at a steady rate.

Accepted Answer

The rods are adjusted so that, on average, exactly one neutron per fission causes a further fission (1); lowering the rods absorbs more neutrons and reduces the reaction rate (1); raising them absorbs fewer, increasing the rate, so the output is held steady (1).

Question 8

Describe how the moderator, the control rods and the coolant work together in a thermal nuclear reactor.

Accepted Answer

Fission of the fuel releases fast neutrons and energy (1). The moderator slows the neutrons to thermal speeds so they can sustain the chain reaction (1). The control rods absorb surplus neutrons so that on average one neutron per fission causes another, keeping the rate steady (1). The coolant carries the thermal energy from the core to a heat exchanger to raise steam, preventing overheating (1).

Question 9

Explain what is meant by critical mass in the context of a working reactor.

Accepted Answer

The minimum mass of fuel for which the chain reaction is just self-sustaining (1); on average exactly one neutron per fission goes on to cause a further fission (1); below the critical mass too many neutrons escape and the reaction dies out (1).

Question 10

In a reactor running at a steady power, each fission releases on average 2.5 neutrons. Determine the average number of neutrons per fission that must be absorbed by the control rods, or lost in other ways, without causing fission.

Accepted Answer

For steady power, on average exactly one neutron per fission must go on to cause a further fission (1) So 2.5 − 1.0 = 1.5 neutrons per fission must be absorbed or otherwise lost (1)

Question 11

Beryllium-9 can be used as a moderator. In a head-on elastic collision with a stationary nucleus of mass M, a neutron of mass m keeps a fraction ((M − m)/(M + m))² of its kinetic energy. Taking M = 9m for beryllium-9, calculate the fraction of its kinetic energy a neutron keeps in one head-on collision, and state one property, other than low nuclear mass, that a moderator material must have.

Accepted Answer

Fraction = ((9m − m)/(9m + m))² = (8/10)² (1)
Fraction = 0.64, so 36% of the kinetic energy is transferred in each head-on collision (1)
It must not absorb neutrons strongly (or: it must be chemically and thermally stable in the core) (1)

Question 12

After a reactor has been shut down, the fuel rods must be cooled for a long time. Explain why the fuel continues to release thermal energy even though the chain reaction has stopped.

Accepted Answer

The fuel contains fission fragments, which are neutron-rich and radioactive (1); radioactive decay is spontaneous, so it continues at a rate set by the fragments' half-lives and cannot be stopped by the control rods (1); the radiation emitted is absorbed in the fuel and its surroundings, so decay energy continues to heat the rods long after shutdown (1).

Question 13

Use the elastic collision model to explain why water or graphite, rather than a material with heavy nuclei, is chosen as a moderator.

Accepted Answer

In an elastic collision a neutron transfers the greatest share of its kinetic energy to a target of equal mass (1); it transfers very little to a much heavier nucleus, which recoils slowly (1). Hydrogen (in water) and carbon (in graphite) nuclei are comparable in mass to a neutron, so they slow neutrons in few collisions (1); with a heavy moderator, far more collisions would be needed to thermalise the neutrons (1).

Question 14

Describe the main safety features of a nuclear reactor and how high-level radioactive waste is handled.

Accepted Answer

The core is surrounded by thick steel and concrete shielding to absorb neutrons and gamma radiation (1); a sealed containment building encloses the reactor, and in an emergency all control rods drop in to shut the reaction down (1). High-level (spent fuel) waste is intensely radioactive and hot, so it is stored under water for years to cool and to provide shielding (1); it may then be encased in glass (vitrified) and stored securely in a deep, geologically stable repository (1).

Question 15

Explain why spent nuclear fuel must be handled remotely and stored for a long time.

Accepted Answer

Spent fuel contains highly radioactive fission products emitting intense, harmful radiation and generating heat (1); it is handled remotely behind shielding to protect workers from the radiation (1); many of the isotopes have long half-lives, so the activity takes many years to fall to a safe level (1).

Question 16

In a head-on elastic collision with a stationary deuterium nucleus, of mass 2m where m is the neutron mass, a neutron keeps a fraction ((M − m)/(M + m))² of its kinetic energy. Show that this fraction is 1/9, and go on to determine the number of successive head-on collisions needed to reduce a neutron's kinetic energy from 1.0 MeV to below 1.0 eV.

Accepted Answer

Fraction = ((2m − m)/(2m + m))² = (m/3m)² = 1/9 (1)
After n collisions E = 1.0 MeV × (1/9)ⁿ, and 1.0 eV/1.0 MeV = 10⁻⁶, so (1/9)ⁿ < 10⁻⁶ is required (1)
n > log(10⁶)/log 9 = 6.3 (1)
So 7 collisions are needed (1)

Question 17

A campaign leaflet claims that a nuclear power station could explode like a nuclear bomb. Deduce whether the claim is justified.

Accepted Answer

Reactor fuel contains only a few per cent uranium-235, far too dilute to support the runaway growth in fission rate that a weapon requires (1); the control rods and reactor design hold the chain reaction at one neutron per fission, and in an emergency the rods drop fully in and stop it (1); a mismanaged reactor can overheat, which is a serious accident, but a nuclear explosion is not possible, so the claim is unjustified (1).

Question 18

A country with ageing coal-fired power stations is deciding whether to replace them with nuclear power stations. Discuss the benefits and the risks that should be weighed in this decision, giving physical reasons, and state a conclusion.

Accepted Answer

Benefits: fission releases about a million times more energy per kilogram than burning coal, so a nuclear station needs very little fuel and delivers large amounts of reliable power (1); a reactor emits no carbon dioxide in operation, so unlike coal it does not add to the greenhouse effect while running (1); in normal operation the shielding and containment keep the dose to the public very low (1). Risks: accidents are rare, but a loss of cooling can release radioactive material with severe and long-lasting consequences (1); spent fuel contains long-lived isotopes and must be shielded, cooled and then stored securely for thousands of years (1). Conclusion: a reasoned judgement weighing a named benefit against a named risk, for example that the reliable low-carbon supply justifies building the stations provided the waste storage and safety systems are properly funded (1).