- keff < 1. If the multiplication factor for a multiplying system is less than 1.0, then the number of neutrons is decreasing in time (with the mean generation time) and the chain reaction will never be self-sustaining. This condition is known as the subcritical state.
But this statement is not completely correct. To clarify this issue, we use a finer classification of subcritical states. The subcritical state can be subdivided into:
- transitional subcritical state. Typical feature of this state is a decrease in neutron flux. The above definition is, in fact, a definition of the transitional subcritical state, in which the reactivity of a reactor is lower than zero (keff < 1; ρ < 0). In this case, the production of all neutrons by fission is insufficient to balance neutron losses and the chain reaction is not self-sustaining.
- steady-state subcritical state – subcritical multiplication. Typical feature of subcritical multiplication is a presence of source neutrons (supplied by external or internal neutron source). The source neutrons balances neutron losses and the neutron flux is constant. If an external neutron source is present, then any transitional subcritical state inevitably pass in subcritical multiplication. If not, the neutron flux will approach zero.
As can be seen, the neutron flux in a subcritical reactor with source neutrons stabilizes itself at a corresponding level, which is determined by source strength, S, and by the multiplication factor, keff. On the other hand, the amount of time it takes to reach the steady-state neutron level is dependent only on keff.