The Effects of Sodium Entrainment and Heat Transfer with Two-Phase UO₂ During a Hypothetical Core Disruptive Accident

A major portion of the safety analysis effort for the liquid-metal fast breeder reactor is involved in assessing the consequences of a hypothetical core disruptive accident. A postulated loss-of-flow transient without scram may produce a two-phase fuel source at high pressures. The heat transfer process between the fuel and the sodium coolant as it is ejected into the upper plenum is described in this study. One mechanism that can cause the coolant to become entrained in the two-phase fuel is Taylor instabilities. The characteristic size of the entrained coolant droplets is considered to be equal to the critical wavelength of a Taylor instability. Analysis of full-scale reactor conditions indicates that the dominant heat transfer mechanism is radiation. Also, if noncondensible gases are absent, fuel vapor condensation on the sodium coolant droplets is controlled by mass diffusion, hence the subsequent rate of coolant vaporization is small. The net effect of the heat transfer is to reduce the fuel vapor pressure and reduce the expansion work by a factor of 1.2 to 2.5. Small-scale simulant experiments utilizing refrigerants could confirm the fuel condensation/sodium vaporization behavior, while reactor material tests must be done to investigate the radiation heat transfer mechanism.