Multi-Physics Analysis Considering New Aspects and Neutronic Kinetics to Clarify the Criticality Condition of the Past Incident

A criticality incident occurred at the Windscale Works in the United Kingdom in 1970 by the mixing of an aqueous solution and an organic solvent; however, the detailed criticality scenario remains unclear. In this study, a multiphysics analysis was performed to clarify a possible criticality mechanism, focusing on the time variation of reactivity. Computational fluid dynamics (CFD) calculations were carried out considering the effects of a short-duration inrush and the presence of a stable emulsion, which were not taken into account in the previous study. Based on the space- and time-dependent material distributions obtained from the CFD results, Monte Carlo neutron transport calculations were performed to evaluate changes in the effective multiplication factor. The results show that both the inrush and the stable emulsion might have influenced the criticality, leading to conditions in which the effective multiplication factor exceeded 1. Furthermore, neutronic kinetics analyses using simplified wave-like reactivity variations demonstrated that both the total number of fissions and the duration of criticality are consistent with historical records under certain conditions. These findings support the hypothesis that the Windscale Works criticality incident was governed by a wave-like reactivity variation.