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Nuclear reactors and safety questions
A reactor is a chain reaction held at exactly one neutron per fission. A moderator slows the neutrons so they are absorbed efficiently, control rods absorb the surplus, and coolant carries the heat to the turbines. Around the core sits the safety case in four parts: shut down, cool, shield and contain.
18 original questions · 55 marks · the nuclear reactors and safety notes · Nuclear physics
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State the function of the moderator in a thermal nuclear reactor.
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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).State the function of the control rods in a nuclear reactor.
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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).State the function of the coolant in a nuclear reactor.
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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).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.
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Moderator: water or graphite (1). Control rods: boron or cadmium (1).State two properties needed by the coolant of a nuclear reactor.
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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).Explain, in terms of collisions, why the moderator in a thermal reactor is made from atoms of low mass.
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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).Explain how control rods are used to keep a reactor operating at a steady rate.
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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).Describe how the moderator, the control rods and the coolant work together in a thermal nuclear reactor.
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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).Explain what is meant by critical mass in the context of a working reactor.
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.
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.
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.
Use the elastic collision model to explain why water or graphite, rather than a material with heavy nuclei, is chosen as a moderator.
Describe the main safety features of a nuclear reactor and how high-level radioactive waste is handled.
Explain why spent nuclear fuel must be handled remotely and stored for a long time.
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.
A campaign leaflet claims that a nuclear power station could explode like a nuclear bomb. Deduce whether the claim is justified.
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.
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