Evaluation of the Energy Conversion Ratio of Vapor Explosions for the Assessment of Nuclear Reactor Safety

Abstract

The calculation of energy conversion ratio of vapor explosion is an important problem for evaluating the integrity of reactor containment and pressure vessel in severe accident. The propagation and expansion stages of vapor explosion are numerically simulated based on both hydrodynamic and thermal fragmentation mechanisms. The thermal fragmentation model gives much higher pressure peak and higher energy conversion ratio than the hydrodynamic fragmentation. A low energy conversion ratio is expected in a large-scale vapor explosion because the hydrodynamic fragmentation is dominant when the pressure wave becomes strong. So the application of energy conversion ratio measured by the experiments into the assessment of reactor safety is conservative. The non-equilibrium explosion model gives the reasonable result that the vapor explosion is weak in larger vapor volume fraction conditions, which is consistent with fuel-coolant interaction experiments. The energetics of sodium vapor explosion is also examined. The high saturation temperature of sodium results in a much lower pressure peak. A coolable fuel debris bed can be formed during the core disruptive accident.