Effect of B₄C absorber material on melt progression and chemical forms of iodine or cesium under severe accident conditions

Boron carbide (B₄C) used for BWR or EPR absorbers could cause phenomena that never occur in PWR with silver-indium-cadmium absorbers during a severe accident. B₄C would undergo a eutectic interaction with stainless steel and enhance core melt relocation. Boron oxidation could increase H₂ generation, and the change of liberated carbon to CH₄ could enhance the generation of organic iodide (CH₃I). HBO₂ generated during B₄C oxidation could be changed to cesium borate (CsBO₂) by combining it with cesium. This may increase cesium deposition into the reactor coolant system. There could be differences in the configuration, surface area, and stainless-steel to B₄C weight ratio between the B₄C powder absorber and pellet absorber. The present task is to clarify the effect of these differences on melt progression, B₄C oxidation, and the iodine or cesium source term. Advancement of this research field could contribute to further sophistication of prediction tools for melt progression and source terms of the Fukushima accident, and the treatment of organic iodide formation in safety evaluation. (author)