Question

Discuss the need for a supercritical core (Keff > 1) at the beginning of cycle (BOC) and why this is not an issue to the 'health and safety of the public

Answer 1

At the beginnig of the cycle, the output power is small and if Keff >1, the number of neutron emitted are more than one for each fission and these neutrons create more fissions increasing the rate of fission reaction exponentially and thus the out put power increases rapidly.

This is reqiured until a level of output power reaches when the rate of reaction should remain constant and then we need to maintain Keff to 1.

As in the beginnig of the cycle, rate of fission is small, there is not so much energy release and hence it is not an issue as the energy release doesn't harm being limited.

However, once a level of power is reached, the Keff must not become greater than one, as the exponential increase of high energy can be catastrophic.