Impact of the Multigroup Velocity Approximation in the Evaluation of the Effective Neutron Lifetime

Reactor kinetics is among the most computationally intense calculations. One possible way to overcome this is issue is to use the point kinetics approximation. Point kinetics allows for the modeling of a transient by using just a few scalar parameters. Among these, the effective neutron lifetime provides an indication of the typical timescale on which prompt neutrons evolve. To compute this parameter, the neutron velocity has to be used. In the multigroup approximation, the rigorous definition of the group velocity would imply a weighted average of the neutron velocity with the energy spectrum. Since the real energy spectrum is unknown at the moment of the production of the multigroup velocities, some approximations are needed to input them into the calculation.

In this paper, we investigate alternative weighting strategies of the velocity and their impact on the effective neutron lifetime, providing considerations and insights about further improvements. The results show that a simple preprocessing strategy of the velocities, similar to the one used with nuclear cross sections, can improve the evaluation of the mean neutron generation lifetime when compared to previous studies that focused on simpler alternatives.