General Formulation of an HCDA Bubble Rising in a Sodium Pool and the Effect of Nonequilibrium on Fuel Transport

Consideration is given to a fuel-dominated bubble, which is assumed to have just penetrated into the sodium pool in a spherical form subsequent to a hypothetical core disruptive accident. The two-phase bubble mixture is formulated as it rises through the sodium pool to the cover-gas region. The formulation takes into account the effects of the nonequilibrium mass transfer at the interfaces and of the radiative cooling of the bubble as well as the kinematic, dynamic, and thermal effects of the surrounding fields. The results of calculation for the amount of the fuel vapor condensed before the bubble reaches the cover-gas region are presented over a wide possible range of the evaporation coefficient as well as the liquid sodium-bubble interface absorbtivity. It is shown that the effects of nonequilibrium mass transfer become more meaningful at the later stage of bubble rise where the temperature difference between the liquid fuel and the gaseous mixture has been increased. The thermal radiative cooling is found to be very effective in attenuating the fuel content of the bubble; depending on the value of the liquid sodium-bubble absorbtivity, a great reduction of fuel vapor can result. Consequently, if the condensed fuel falls out of the bubble, the thermal radiation, which condenses out most of the fuel vapor, can effectively prevent and eliminate most of the fuel leakage from the reactor vessel.