Corrosiveness of Li2TiO3 ceramic tritium breeder to structural RAFM steel: Effects on microstructure and thermal conductivity

Abstract

Structural steel will be in long-term contact with ceramic tritium breeder at a high temperature in a solid tritium breeding blanket. The maximum temperature in the tritium breeding zone would reach nearly 1173 K. At this high temperature, a large amount of lithium oxide would evaporate and deposit on the structural materials, causing the overall corrosion of the structural materials. Corrosiveness of Li2TiO3 ceramic tritium breeder to reduced activation ferritic-martensitic steel CLF-1 was investigated in this study. The experiment was carried out at 1123 K under He + 0.1 vol% H2 atmosphere for up to 400 h. The formation of non-uniform corrosion layers with a Fe-rich outer layer and a Cr-rich inner layer were found on the surface of CLF-1 steel. Secondary ion mass spectrometry (SIMS) test at contact and non-contact areas were performed to demonstrate evaporation of lithium compound. The corrosion layers were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. The non-contact area with tritium breeding pebbles were corroded indicating a large amount of lithium oxide on the non-contact zone, resulting in serious overall corrosion of structural materials. LiFe5O8 was found as the main phase on the surface of the corroded samples by X-ray diffraction (XRD). The thermal conductivity of LiFe5O8 (∼0.8 W/m/K, 300 K) calculated by first principles phonon Boltzmann equation (PBE) is significantly lower than the original CLF-1 (∼23 W/m/K, 300 K). Almost no change of thermal conductivity for experimental results was found after corrosion, which is due to non-uniform corrosion layer presenting thinner layer at some areas, as well as the hole, crack and peeling of corrosion layer indicated by SEM results. However, in the long-term service of blanket structural material, it is predicted that a much thicker corrosion layer may form on the surface of RAFM steels thus bring a risk to the heat transfer of blanket.