Absorber material cladding chemical interaction in vented fast breeder reactor absorber pins. B4C/stainless steel chemical interaction in sodium environment and effect of metallic Nb and Cr4 layers

Out-of-pile tests were performed to study the chemical interaction between B₄C and cladding material (stainless steel) in sodium environment as components of a “Fast Breeder” absorber pin. The results of isothermal capsule tests (873–1073 K and 750–8600 h) show a chemical attack on the cladding. The penetration depth amounted to 50 microns within one year at 873 K and increased parabolically in the course of time. The test results indicate that diffusion of boron through the reaction layer is determinative for the kinetics of chemical interaction. Carbon diffusion is inhibited by the formation of metal boride layers. Metallic niobium or chromium layers on the inner surface of the cladding material reduce definitely the process of boronation. The tests show that chemical interaction in the B₄C-Na-stainless steel system is sufficient low at conditions foreseen for high absorber pins in SNR-300 reloads and SNR-2.