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An application for mass computing in simulation of severe accident scenarios with variation of input parameters of best-estimate (BE) codes is developed. It can be used with a variety of BE codes and allows creating sets of input files with desired parameter distributions, managing code instances launch and performing a statistical analysis of the results in scope of an uncertainty assessment.The application named Varia (from "variation") was designed as a cross-platform solution and can be run both on Windows and UNIX-like systems. This opens up possibilities for its use on a wide range of platforms -from personal computers to large-scale clusters and grid systems.Varia in combination with Russian SOCRAT BE code was tested on the problem of core melt retention in the VVER-440 reactor. A statistical analysis of the influence of several varied parameters on the simulation results was performed. It showed that the maximum heat flux through the vessel is governed exclusively by the intensity of the decay heat, whereas the influence of the parameters of the convection model is negligible. However, the maximum temperature of the melt depends on the convective heat transfer in the oxide layer of the stratified core melt.A series of thermomechanical simulations was performed for the problem of molten core retention in BWR/3 reactor vessel. They showed that the vessel failure due to the creep tends to occur during the stage of vessel heating by debris. The mechanical vessel failure on the later stage of the melt retention process is scarcely probable.

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