Modelling of molten fuel relocation phenomena in a degraded BWR core

A model of fuel melting and relocation has been developed for the analysis of core meltdown accidents of light water nuclear reactors. Both the description of the model and the results of extensive numerical testing and assessment are presented in this paper. The numerical results shown include the test runs for the stand-alone model, as well as the calculations performed following the implementation of this model into the MELRPI code. The latter results are compared with those obtained for the previous version of MELRPI, showing the effect of the new mechanistic model for molten fuel relocation on the accuracy of predicting the timing of core and reactor pressure vessel failure