Development and application of a simplified neutron-kinetics model for severe accident recriticality assessment

A simplified neutron-kinetics model for recriticality calculations has been developed and implemented within the severe accident code MELCOR. The model partitions the core into active and inactive regions and determines neutron flux using factorization with adiabatic and prompt-jump approximations. Homogenization of pre-calculated core cell multiplication factors, together with Doppler and xenon reactivity defects calculated by perturbation theory, avoids explicit treatment of cross-sections. This approach allows recriticality analysis to be performed seamlessly within MELCOR at standard time-steps and computational cost. Comparisons with SARA project simulations show reasonable agreement. Integrated with MELCOR, the model extends its capabilities to predict reactivity, recriticality timing, fission power, fuel temperatures, and xenon transients. The model was applied to station blackout scenarios in a Nordic BWR with varying low-pressure safety injection timing and two decay-heat curves. Simulations suggest that the time window for recriticality may be substantial if the core periphery remains intact and coolant flow through a breached vessel is limited. In cases where recriticality occurred, fission power evolution was irregular, and even relatively low fission power accelerated containment heat-up and pressurization. These results demonstrate that the simplified model provides an efficient tool for investigating recriticality phenomena, their impact on severe accident progression, and the effectiveness of mitigation strategies under uncertainty and sensitivity analyses. • A simplified neutron kinetics model for recriticality analysis in severe accidents is developed. • The model is implemented in the MELCOR code using its built-in control functions. • Simulations indicate that late-stage recriticality may occur if the core periphery remains intact.