Physics and modelling of the core bubble interface in post-disassembly phase analysis

For a large pool-type LMFBR and initial core temperatures in the neighbourhood of 5000 K the core bubble which forms above the subassembly tops consists of a mixture of fuel droplets and of sodium vapour generated from coolant engulfed in the early stages of the expansion. The bubble pressure and hence the effective mechanical energy release are determined by the balance between continued evaporation and condensation upon droplets freshly entrained at the bubble interface. The SIMMER-II code which is widely employed in post-disassembly studies does not model entrainment adequately, and a technique is suggested for incorporating the effect of Taylor instability entrainment upon the condensation process in a simple way. Estimates of its consequences suggest that better modelling of entrainment should have a significant effect upon the mechanical energy release as calculated by SIMMER