COMSOL MULTIPHYSICS SIMULATION OF THE 1958 Y-12 CRITICALITY ACCIDENT

摘要

This paper presents the simulation of the 1958 Y-12 criticality excursion using the commercial multiphysics code COMSOL Multiphysics. ICNC 2011 reported the initial effort to simulate the Y-12 accident power history using a fully coupled neutronic and computational fluid dynamic (CFD) approach with the Imperial College London FETCH code. That simulation included the initial spike and a subsequent power history 30 seconds afterwards. A time-dependent lumped parameter excursion model was later developed at Imperial in November 2014 and taken to 20 min. The COMSOL simulation is viewed as a hybrid treatment of these efforts. The use of COMSOL as a platform for criticality excursion simulation had been previously demonstrated with SILENE step reactivity insertion benchmarks. The Y-12 accident represented a unique time dependent reactivity insertion mechanism by dilution of the fissile concentration over time that facilitated the excursion termination. The COMSOL model includes a moving mesh vertical boundary, compartment radiolytic gas model, coupled turbulent fluid flow and conjugate heat transfer. The results are compared with both FETCH and lumped parameter simulations. The COMSOL power history results are in agreement with the reported and reconstructed initial spike fissions, power and total fissions over time. The simulated power trace shows explicit subsequent spikes approximately 10-20 seconds apart from the initial spike, over the first three minutes of the excursion. This result qualitatively confirms the radiation dose rate trace from a far field monitor located well beyond the affected facility. Additionally, the COMSOL model shows the onset of boiling after 3 minutes, as postulated in the literature. The combination of boiling and fissile solution dilution contributed to the shutdown mechanism.