Two-region diffusion model for the improved analysis of ADS experiments

A fast lead core and an external neutron source create rapidly varying transients in which the spatial and time effects are important, and, in combination with a thick lead reflector, impose a constraint on an application of the traditional point reactor kinetics approximation. Thereby, for the foreseen GUINEVERE experiments a two-region space and time dependent diffusion approximation was chosen to be solved and analysed. It is necessary to represent the solution for two-region core owing to a presence of an external neutron source, sub-criticality of the system, and an increasing impact of the reflector in a small experimental reactor, as well as due to the inaccuracy of the diffusion approximation around the core reflector, core blanket interfaces, and throughout fast reactor blankets. This two-region solution without separation of space and time gives a significantly improved methodology for the analysis of the future experiments like GUINEVERE. The efficiency of the derived solution over the accurate numerical solutions (like Monte Carlo calculation, for example) lies in a comparatively short calculation time, which is of major importance for the on-line monitoring the reactivity of a subcritical reactor system.