The basic classification of states of a reactor may be in some case insufficient and a finer classification is needed. The finer classification takes into account the two groups of neutrons that are produced in fission.
It is known the fission neutrons are of importance in any chain-reacting system. The fission of fissile nuclei produces 2, 3 or more free neutrons. But not all neutrons are released at the same time following fission. Even the nature of creation of these neutrons is different. From this point of view we usually divide the fission neutrons into two following groups:
- Prompt Neutrons. Prompt neutrons are emitted directly from fission and they are emitted within very short time of about 10-14 second.
- Delayed Neutrons. Delayed neutrons are emitted by neutron rich fission fragments that are called the delayed neutron precursors. These precursors usually undergo beta decay but a small fraction of them are excited enough to undergo neutron emission. The fact the neutron is produced via this type of decay and this happens orders of magnitude later compared to the emission of the prompt neutrons, plays an extremely important role in the control of the reactor.
The fact the prompt neutrons are emitted within 10-14 and have significant shorter mean generation time (~ 10-5s) than delayed neutrons (~0.1s) have crucial consequences. The period of ~10-5s is very short and causes very very fast response of the reactor power in case of prompt criticality. The state of reactor, when the chain reaction is self-sustained only by prompt neutrons, is known as the prompt critical state. This state of the reactor is very unstable, because one neutron generation takes only ~10-5s. Therefore nuclear reactors must operate in the prompt subcritical, delayed critical condition. All power reactors are designed to operate in a delayed critical conditions and are provided with safety systems to prevent them from ever achieving prompt criticality.The prompt critical state is defined as:
- keff > 1; ρ ≥ βeff, where the reactivity of a reactor is higher than the effective delayed neutron fraction. In this case, the production of prompt neutrons alone is enough to balance neutron losses and increase the neutron population. The number of neutrons is exponentially increasing in time (as rapidly as the prompt neutron generation lifetime ~10-5s).
The prompt subcritical and delayed supercritical state is defined as:
- keff > 1; 0 < ρ < βeff, where the reactivity of a reactor is higher than zero and lower than the effective delayed neutron fraction. In this case, the production of prompt neutrons alone is insufficient to balance neutron losses and the delayed neutrons are needed in order to sustain the chain reaction. The neutron population increases, but much more slowly (as the mean generation lifetime with delayed neutrons ~0.1 s).
The prompt subcritical and delayed critical state is defined as:
- keff = 1; ρ = 0, where the reactivity of a reactor is equal to zero. In this case, the production of prompt neutrons alone is insufficient to balance neutron losses and the delayed neutrons are needed in order to sustain the chain reaction. There is no change in neutron population in time and the chain reaction will be self-sustaining. This state is the same state as the critical state from basic classification.
The prompt subcritical and delayed subcritical state is defined as:
- keff < 1; ρ < 0, where the reactivity of a reactor is lower than zero. In this case, the production of all neutrons is insufficient to balance neutron losses and the chain reaction is not self-sustaining. If the reactor core contains external or internal neutron sources, the reactor is in the state that is usually referred to as the subcritical multiplication.