Reaction between B4C and austenitic stainless steel in oxidizing atmosphere at temperatures below 1673Â K

The control rod of a light water nuclear reactor is constructed of a pole comprising stainless steel filled with a boron carbide (B4C) core. To appraise the stability of this control rod in the event of a severe accident, the reactions of the system of B4C and grade 304 austenitic stainless steel (SS) were observed at 1473 K in Ar, air, and a mixture of both. To clarify the reaction mechanism and the influence of the oxygen partial pressure, the weight change ratio was monitored and differential thermal analysis was performed at the temperature range from room temperature to 1673 K to monitor the reaction under controlled oxygen partial pressure. The results showed that there was no direct reaction between B4C and SS. When the temperature was higher than the melting point of B2O3 (743 K), the molten B2O3 formed by oxidation of B4C covered the surface of SS by spreading wetting. This B2O3 layer functioned to transport oxygen from the atmosphere to SS, leading to accelerated oxidation of SS. As a result, a Fe-Cr-Ni-B-O oxide phase covered the surface of SS. Oxygen continuously entered the oxide phase with prolonged reaction time, and oxides such as Fe2O3, Fe3O4, and FeOx-Cr2O3 were found on the outer layer. Therefore, in the presence of B2O3 formed by oxidation of B4C, the oxidation of SS was accelerated below the eutectic temperature of the Fe-C system.