About the Concept of «Prompt Criticality»
The article deals with methodological issues related to the conceptual and terminological apparatus of the dynamics of nuclear reactors. Based on the analysis of the standard point kinetics equations presented in the form of a balance of the processes velocities, it is shown that the concepts of «prompt criticality» or «prompt neutron criticality» (which are associated with the state of a nuclear reactor when the reactivity is equal to the «effective fraction of delayed neutrons», ρ = β) are appropriate to refer only to the state with zero reactivity. This is due to the fact that at ρ = β, the rate of reproduction of the neutron population is equal to the rate of generation of delayed neutrons, and the prompt neutrons «go out» to generate the precursors of delayed neutrons (PDN), since the rate of reproduction of prompt neutrons is equal to the rate of generation of PDNs. It is shown that, contrary to the traditional approach, it is impossible to oppose «prompt criticality» and «delayed criticality», since these states are identical (simultaneous and joint), coinciding with the state of zero reactivity. Similarly, the state of «prompt supercriticality» is identical to «delayed supercriticality», coinciding with the state of exponential increase in the velocities of all processes taken into account in the point reactor kinetics equation. It is found that the equality of reactivity to the effective fraction of delayed neutrons cannot be interpreted as a «safety limit» or «controllability limit», since the unacceptable period of reactor acceleration is achieved at significantly lower reactivity values. The proposed clarifications are appropriate both from the point of view of a more adequate description of the processes in a nuclear reactor, and in relation to the wording of nuclear safety requirements.
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