Boiling and Dryout Behavior in a Liquid-Metal Fast Breeder Reactor Subassembly Bundle Under Low Heat Flux and Low Flow Conditions

For certain postulated severe accident conditions such as a loss of piping integrity and a loss of heat sink in connection with liquid-metal fast breeder reactor safety analysis, the process of decay heat removal can lead to coolant boiling. For such low-heat-flux/low-flow conditions, a dryout or critical heat flux criterion is required in order to assess the potential for fuel pin failure and melting. Computer codes and full-scale experimental data are not available to completely address this problem at this time. Based on the interpretation of available experimental data and new analyses, it is concluded that a typical subassembly can be safely cooled (avoid dryout) under natural convection conditions for heat fluxes below ∼8 to 10% of the average nominal power; i.e., decay heat power levels can be safely accommodated in the natural convective regime. Furthermore, since this coolability limit is predicted to be rather insensitive to the subcooling value, it follows that the safety case relative to decay heat removal for an intact core geometry also becomes essentially independent of detailed accident conditions such as the potential for temporary stagnated flow or inlet flow reversal conditions.