Numerical Analysis of Pressure Propagation and Energy Conversion Ratio in Sodium Vapor Explosions

A computer code PROVER-II is developed for the propagation phase of a sodium vapor explosion. A new thermal fragmentation model is proposed that includes three kinds of timescales for modeling the instant fragmentation, spontaneous nucleation fragmentation, and normal boiling fragmentation. The pressure wave propagation in a sodium vapor explosion is analyzed and compared with that in a steam explosion. The energy conversion ratio of an in-vessel sodium vapor explosion is calculated by using hydrodynamic and thermal fragmentation mechanisms, and sensitivity analyses are carried out for some parameters. The initial thermal conditions for energetic fuel-coolant interactions in a sodium system are examined. Results show that the high saturation temperature of sodium results in a much lower pressure peak in a sodium vapor explosion compared to a steam explosion, and the mechanical energy release is limited by the mass of melt participating in the explosion during the core disruptive accident in liquid-metal-cooled fast breeder reactors.