The loss of neutrons through leakage may be reduced by surrounding the core with a suitable material such that some of the neutrons are ‘reflected’ back into the core. The material should have a high scattering cross-section, low absorption cross-section and, for a thermal reactor, contain a low mass number element so that leaking fast neutrons are not only returned into the core but slowed down in the process. A good moderator is therefore a good reflector and in many thermal reactor designs they are the same material.

The effect of the reflector is illustrated in Fig 1.16; an unreflected reactor is referred to as a ‘bare’ reactor.

The advantages of using a reflector are that:

• Because of the better neutron economy, the critical size is reduced and hence also the mass of fuel required.

• The neutron flux, and hence power production, across the core is more uniform leading to a gain in the overall power output.