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

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 × 10³/RT) for T ≥ 1,423 K and k_B = 8.69 × 10⁻⁵ exp (–44.4 × 10³/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 B₄C 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.