Decoupling of Neutron and Delayed-Neutron Precursor Evolutions in Source-Driven Subcritical Systems

The behaviour of neutron and delayed neutron precursors in subcritical systems is considered in time-dependent physical models at different levels of detail. Point kinetics and a simple space model in one-group diffusion theory are assumed at first, in order to be able to construct analytical solutions and to gain a full insight into the problem. More realistic calculations are then performed using a numerical multigroup diffusion code in two-dimensional cylindrical geometry. The results presented clearly show the appearance of a decoupling phenomenon between the evolutions of neutrons and delayed neutron precursors, which is enhanced by the subcriticality level of the multiplying system.