Evaluation of boron evaporation kinetics from stainless-steel–B4C alloy during steam oxidation at high temperatures

• Boron evaporation from SS–B 4 C alloy under severe accident condition with steam starvation. • Low steam supply caused Fe–O layer formation embedding Fe–B–O and Fe–Cr–O on surface. • The rate constant of boron evaporation was obtained as Arrhenius type function. • Decreasing oxygen partial pressure reduced Fe–O layer, enhancing B evaporation. To understand the core degradation process at the Fukushima Daiichi Nuclear Power Station, the oxidation of boron carbide–stainless steel alloy under steam starvation condition was studied at temperatures in the range of 1,288–1,573 K. Low steam supply led to swift Fe–O layer formation, embedding Fe–B–O and Fe–Cr–O, and boron evaporation mainly as oxides was observed through the Fe–B–O phase precipitated in the Fe–O layer. The rate constant of boron evaporation k B was derived from the measured data as k B = 0.0157 exp (–79.8 × 10 3 / RT ) for T ≥ 1,423 K and k B = 8.69 × 10 −5 exp (–44.4 × 10 3 / RT ) for T < 1,423 K where R and T are the gas constant and temperature, respectively. The obtained constant was comparable to the reaction rate of B 4 C oxidation. In addition, a test with an even more decreased steam supply was conducted to examine the impact of steam quantity on the boron evaporation kinetics. Consequently, it was confirmed that decreasing the oxygen supply resulted in a slowdown of outer Fe–O layer formation, which enhances the outwards diffusion of B and allows greater evaporation of B oxides.