Critical Size

• We shall see later in the course that all neutrons produced do not get absorbed in the reactor.
• Many neutrons leak out of the reactor.
• To keep the neutron population steady in a reactor we need to produce more neutrons than what is absorbed in fuel.
• As the number of neutrons produced depends on the volume and number of neutrons leaking is proportional to surface area, there exists a minimum size below which the reactor cannot operate (due to excess leakage).
• This is called the critical size.
• Thus, for a reactor to operate we need a critical size.
• This size depends on the concentration of the fissile elements.
• The size increases with the reduction in concentration.
• Thus conceptually, as fuel burns out we need to increase the size, which is impractical.
• To overcome this, the size of the reactor is increased to begin with and to compensate for the excess production, neutron absorbers are used.
• As fuel burns out the absorbers are gradually removed.

Control of Nuclear Reactors

The process of power addition or reduction is done by insertion or removal of control rods. Originally when the fuel is fresh and the reactor is not operating, these rods are fully in. To start the reactor, these are slowly pulled out and the rector begins operation when the rods are out by a small amount. As the fissile elements deplete, to compensate for them these are continuously pulled out. When they get pulled out completely, then reactor is stopped and fresh fuel added.