Impact of energy collapsing on the effective neutron lifetime

The effective mean prompt neutron generation time (or effective lifetime) is an integral parameter that is introduced in the point kinetic model for nuclear reactor time-dependent analysis. Although such a model requires a strong simplification of the neutron kinetic process, the value of the effective neutron lifetime can give an immediate and useful information on the physical characteristics of a multiplying system and on its time response to perturbations. Furthermore, point kinetics is still used for the simulation of control and transient situations and, especially, in an inverse fashion for the interpretation of neutronic experiments. Based on the standard separation-projection mathematical procedure to derive point kinetics equations, the effective lifetime is defined as the ratio between the total instantaneous importance within the system and the total importance generated by fission per unit time. The evaluation of the effective lifetime can be carried out by both deterministic and stochastic computational tools. Relevant differences can be observed if different physical models are used. In this paper the attention is particularly focused on the energy structure employed. In the first part some analytical analyses for simplified configurations are carried out, in order to gain some physical insight on the effects associated with the detail of the energy group structure on the computed value of the parameter. Then, some more detailed numerical studies allow to investigate more complex configurations. The study includes both critical and subcritical, source-driven systems.