Boron effects on fission product behavior under severe accident conditions

This paper addresses basic studies for the evaluation of boron (B) release from the degraded BWR control rod and its effect on cesium (Cs) and iodine (I) chemistry during a severe accident. Possible enlargement of gaseous iodine release induced by released B was predicted under various atmospheres by chemical equilibrium calculations. Results showed that the change in atmosphere affected B release fraction from the control rod and generation of volatile iodine species with the formation of CsBO₂. Under a steam-starvation atmosphere, B release fraction could be reduced by the formation of stable iron (Fe)-B alloys in the molten control rod materials and, therefore, the generation of gaseous iodine as well as CsBO₂ is considered to be suppressed. The calculations results show finally that the B release kinetics as well as the atmosphere play an important role for the Cs and I vapor species. For the validation of these results, basic experiments for the evaluation of B release kinetics, thermochemical properties and phase states of Cs-B-O complex compounds, and reproduction tests focused on FP release and transport are underway.