In a thermal nuclear power reactor, a moderator is used to sustain and regulate the fission rate safely. Which statement correctly describes the physical principles governing the choice and operation of a moderator?
The moderator must be made of materials with a high neutron capture cross-section, such as boron or cadmium, to actively absorb excess neutrons and keep the reactor subcritical.
The moderator nuclei must have a high nucleon number (AAA) to maximize the momentum transferred back to the neutron, bringing fast neutrons to a complete stop.
The moderator nuclei must have a mass close to that of a neutron to maximize kinetic energy transfer per collision, and a low neutron capture cross-section to avoid absorbing the neutrons.
The moderator's primary safety function is to absorb high-energy gamma photons produced in fission, converting them into kinetic energy of thermal neutrons.
94 exam-style questions on Edexcel GCSE Physics 6.5 Uses and dangers of radioactivity. Each one has a worked solution and a mark scheme showing where the marks go.