Literature review on experiments and models associated with degradation and oxidation of boron carbide control material during severe accidents

Abstract

Boron carbide (B{sub 4}C) is one kind of neutron absorbing control rod/blade materials used in light water reactors (LWRs). In the Fukushima Daiichi nuclear power station (NPS), all units used B{sub 4}C as absorber material. The degradation of control rod/blade will affect the early phase of in-vessel core melt progression. Furthermore, the release of carbon compound gases including carbon dioxide (CO{sub 2}) as well as boron compounds due to the oxidation of B{sub 4}C with steam is possible to affect source terms of radioactive materials. Past experiments related to B{sub 4}C degradation and oxidation and numerical modeling in severe accidents codes are investigated in the current report with a main view to apply the acquired knowledge into the modification of THALES-2 developed at JAEA as an integral severe accident analysis code. The eutectic interactions of B{sub 4}C with other materials such as stainless steel and Zircaloy will lower the melting point of control rod/blade. The Nagase's correlations for eutectic interaction are selected as one of candidates to be applied into THALES-2. The oxidation reaction of B{sub 4}C with steam will release considerable amount of thermal energy, and form CO{sub 2}, boric acids and boron oxide, which could make an impact onto source terms by changing the pH value of a water pool where those dissolve. The correlation determined by Institut de Radioprotection et de Surete Nucleaire (Radiation and Nuclear Safety Institute, IRSN) is chosen to be used in the modeling of oxidation reaction in THLAES-2. (author)