The chain reaction inside a nuclear reactor is what sustains “combustion" of the fuel which in turn depends on ample supply of thermal energy neutrons within the core. A reflector material is used to ensure that neutrons do not simply fly off the reactor leaving little room for the chain reaction to continue
The principle of reflection is fairly simple and we come across it in our everyday lives. It is the same principle of reflection which lets you see how you look in a mirror by reflecting the light waves. In fact the term reflection refers to any wave or particle being thrown back after hitting a reflecting surface. This principle is extremely useful in the reactor core and helps to maintain an ample amount of thermal energy neutrons, the lack of which could simply extinguish the fission process, rendering the device useless for producing power. Hence a reflector holds an important position amongst the components of nuclear power plant.
The Reflection Process
As we know the reactor consists of the fission process which occurs when a thermal energy neutron is absorbed by the target nucleus leading to its division into two nuclei and emission of 2 or 3 neutrons apart from the heat energy. These neutrons fly randomly in all directions and are usually in the region of fast moving energy neutrons. The moderator is used to control the speed of these neutrons so that they act usefully in creating more fission, but many of these neutrons may simply get lost by flying off the reactor core and thus serving no useful purpose. This might hinder the progression of a chain reaction which is very necessary for the nuclear reactor.
In order to reduce this process of neutron loss the inner surface of the reactor core is surrounded by a material which helps to reflect these escaping neutrons back towards the core of the reactor and these materials are known as reflecting materials.
Materials used as Reflectors
There are a variety of materials which are used as a reflecting medium for neutrons and whatever material is used for the process, it must possess these properties.
Low absorption - this is necessary since if the reflecting material itself starts to absorb the very neutrons it is supposed to reflect back, then the purpose of installing the reflector material would itself be defeated and it would be better not to install any reflector at all.
High reflection - this is an obvious property and does not need any explanation for that is the very purpose for which the reflector exists in the core
Radiation stability - since the reflector material will be exposed to high levels of radiation, it is but natural to assume that it should have a high stability towards radiation
Resistance to Oxidation - the material should not get oxidized otherwise it will fail to serve the requisite purpose
In actual practice there may not be a different material for moderator and reflector for the simple reason that most of the moderators also possess the above mentioned properties of a good reflector as well. Hence they serve the dual purpose of a reflector and a moderator as well. There light water, heavy water and carbon are mostly used as reflectors since they possess the above mentioned characteristics.
The use of a proper reflector helps to reduce the size of the reactor core for a given power output since the number of neutrons leaking are lesser and help to propagate the fission process instead. It also reduces the consumption of the fissile material.