Development of Alumina Forming Materials for Corrosion Mitigation in Heavy Liquid Metal Cooled Nuclear Reactors

Liquid Pb and PbBi are promising coolants for nuclear reactors operating in fast neutron spectrum. Beside its excellent neutronic properties, Pb and PbBi offer improved safety features. But, deficient compatibility of structural materials with the heavy liquid metals (HLM) especially at higher temperatures is a major challenge to be solved for the use of Pb and PbBi as coolant or heat storage material. The major driving force for corrosion attack is the solubility of steel alloying elements like Ni, Fe and Cr in the HLM. Addition of oxygen to the HLM to provide in-situ formation of protective oxide scales is the mitigation mechanism of choice, but in a limited operating temperature window of up to 500°C. To enlarge the operation window of HLM cooled nuclear reactors advanced mitigation measures are under investigation since several years. Alloying of strong oxide formers like Al into steel surfaces can shift the operating temperature window to 600°C and even higher. FeCrAlY surface layers and bulk materials are one further option to increase the corrosion resistance in oxygen-containing liquid metals. Both methods rely on the in-situ oxidation capability of such material. Recently, new classes of bulk materials, alumina-forming austenitic steels (AFA) and alumina forming high entropy alloys(HEA), are investigated. The paper will briefly discuss the involved corrosion (dissolution and oxidation) mechanisms and will then focus on the advanced mitigation strategies that rely on in-situ alumina formation (surface alloying and mainly the new advanced alumina forming bulk materials (HEA and AFA) required for reliable long term operation at higher temperatures for HLM cooled fast reactors. (author)