Thermodynamic equilibrium state calculations for oxidation and corrosion reactions of B4C and oxide-based neutron absorber compounds in reactor control rods

Abstract

The challenges associated with conventional boron carbide (B4C) control rod materials, including helium gas accumulation and susceptibility to oxidation and corrosion in various environments, have been thoroughly explored. To address these issues, a comprehensive investigation into the potential of oxide-based neutron absorber compounds for control rods has been undertaken. Thermodynamic equilibrium state calculations have been conducted to assess the oxidation and corrosion reactions of various neutron absorber materials (Gd, Hf, Sm, Er, Eu, Dy-based oxide), including B4C, in various environments such as oxygen, steam, air, and air–steam mixtures. The results have unveiled vulnerabilities of B4C in these environments, notably the generation of gases such as H2, CO, and boron compounds. Furthermore, neutron absorber oxide materials have exhibited potential susceptibility to oxidation and corrosion in steam environments. Consequently, the potential of oxide-based neutron absorber compounds, formed by combining neutron absorber oxide materials with highly oxidation-resistant substances (ZrO2, TiO2), has been explored. Thermodynamic equilibrium state calculations indicate that these compounds maintain robust resistance to oxidation and corrosion across various environments. This paper demonstrates the superiority of oxide-based neutron absorber compounds as alternatives to existing boron carbide neutron absorber materials. Additionally, the oxide-based neutron absorber compounds are expected to extend the lifetime of the LWR control rod as well as improve stability and resistance to oxidation and corrosion.