Moderators and Control Rods

If nuclear fission processes are used to generate electricity in a nuclear power station, it is absolutely vital to manage the neutrons that are released during nuclear fission so that their number remains constant and energy is released at a constant rate. This management is achieved using control rods and a moderator, which serve two completely separate purposes.

Control rods absorb excess neutrons. So if a fission event produces three free neutrons and we want to have just one free neutron that goes on to cause further fissions then it is clear that we need to remove two-thirds of all the neutrons that are released. Control rods perform this function.

Control rods can be moved into and out of the reactor core. When they are pushed in they absorb a high proportion of neutrons and when pulled out they absorb none. Control rods are like a “volume control” knob for neutron numbers: they can turn the number up or turn the number down just by being pulled out or pushed into the reactor core respectively. In theory, a nuclear reactor could be shut down by pushing the control rods all the way into the core so that there are no free neutrons left to cause any new fission events.

The moderator slows down neutrons that are not absorbed by the control rods. This is important because neutrons that are released during nuclear fission have very high energy and are moving too fast to be captured by another nucleus. The moderator turns these high energy neutrons into “thermal” neutrons, which can be captured by a nucleus to cause a new fission event.

Without a moderator, the nuclear reactor would not work. Water-cooled reactors that also use water as the moderator are sometimes described as “fail-safe” because if the water were to leak out, or boil away, the rate of nuclear fission events would quickly drop and the reactor would shut itself down. Graphite (a form of carbon) was used as a moderator in some older reactor designs but it had the twin disadvantages of needing a separate coolant and acting as a fuel for combustion if it became too hot in the presence of air.

Things are actually more complicated than this but the exam board only requires you to have a basic overall understanding of the roles of control rods and the moderator, as summarised above.