A numerical prediction model is developed for the safety analysis of molten sodium–sulfur battery. Under the assumption that a crack occurred in a solid electrolyte of a cell, a rapid increase in the temperature and pressure from a direct reaction between sulfur and sodium can be predicted by solving equations for flow, energy and the chemical reaction. The crack, the powder-type in-filler and the carbon felt in the sulfur electrode are modeled by porous media approximations. The results are compared to experimental results with extreme value probability statistics for the data assessment. The effects of the size of the cell and wick tube, the depth of discharge, and the filler material are summarized quantitatively.