Heterogeneity of bwr control blade degradation under steam-starved conditions

This paper presents recent results on high-temperature control blade degradation at the very beginning phase of a severe accident in BWRs. The large-scale experiment has been performed in JAEA/CLADS laboratory using Large-scale Equipment for Investigation of Severe Accidents in Nuclear reactors (LEISAN). A prototypic sequence of Fukushima Dai-Ichi Unit 2 has been taken. It has been shown that due to specific conditions happened at Unit 2 the lack of available oxygen allowed more flexibility during metallic melt relocation and heterogeneous redistribution of melt components due to specially recreated temperature gradient. Phase composition of remained B4C control blade claddings at different elevations, and phase composition of melt has been investigated by complementary methods and have shown significant difference in elevations together with stratification of metallic components with origination of high-melting-point layers due to redistribution of steel components and involvement of B and C. It allowed absorber blade residuals with B4C inside being severely damaged by melting to survive at 1477 °C and protect B4C from direct contact with steam.