Energy models for the evaluation of the effective neutron lifetime

The point kinetic model is a useful tool in both direct and inverse applications in reactor physics. Among the integral parameters introduced in the model, the effective mean prompt neutron generation time, in short effective neutron lifetime A, determines the time scale of the evolution of the neutron population. The estimation of this parameter is usually carried out as the ratio of the total neutron importance within the system and the total importance generated by fission per unit time, as consistent with the standard derivation of the point kinetic equations. The values is thus dependent on the energy model used for the evaluation of both neutron flux and importance. In this work, the effect of the energy structure on A is investigated, showing that relevant effects, especially for the estimation of the fundamental time constant, are observed when dealing with few-group problems. The analysis is carried out analytically for simple configurations and using numerical codes for more realistic systems.