Influence of boron carbide on core degradation during severe accidents in LWRs

• Summary of the current knowledge on boron carbide behavior during severe accidents.
• Main focus on the consequences for core degradation and hydrogen source term.
• Interactions of B4C with the surrounding stainless steel components cause rapid formation of eutectic melts.
• 250 g Hydrogen per 1 kg B4C are formed by oxidation in steam.
• 13 MJ heat is produced by oxidation of 1 kg B4C in steam.

Boron carbide (B4C) is widely used as neutron absorbing control rod material in light water reactors (LWRs). It was also applied in all units of the Fukushima Dai-ichi nuclear power plant. Although the melting temperature of B4C is 2450 °C, it initiates local, but significant melt formation in the core at temperatures around 1250 °C due to eutectic interactions with the surrounding steel structures. The B4C containing melt relocates and hence transports material and energy to lower parts of the fuel bundle. It is chemically aggressive and may attack other structure materials. Furthermore, the absorber melt is oxidized by steam very rapidly and thus contributes to the hydrogen source term in the early phase of a severe accident.After failure of the control rod cladding B4C reacts with the oxidizing atmosphere. This reaction produces CO, CO2, boron oxide and boric acids, as well as significant amount of hydrogen. It is strongly exothermic, thus causing considerable release of energy. No or only insignificant formation of methane was observed in all experiments with boron carbide.The paper will summarize the current knowledge on boron carbide behavior during severe accidents mainly based on experiments performed at KIT, and will try, also in the light of the Fukushima accidents, to draw some common conclusions on the behavior of B4C during severe accidents with the main focus on the consequences for core degradation and hydrogen source term.