Materials for ALFRED and ELFR – selection and challenges

Materials are one of the challenging issues for all future GENIV reactors. Several EU funded projects, e.g. ELSY and LEADER, aimed to design Pb cooled nuclear systems including the European lead cooled reactor ELFR and the demonstrator ALFRED. The selection of the structural materials determines the design and the design has to consider the properties and the availability of the materials. Beside general requirements for material properties that are common for all fast reactor types specific issues arise from coolant compatibility. The high solubility of steel alloying elements in liquid Pb at reactor relevant temperatures is clearly detrimental. Therefore, all steels that are considered as structural materials have to be protected by dissolution barriers. The common barriers for all steels under consideration are oxide scales that form in situ during operation. However, increasing the temperature above 500 °C will result either in dissolution attack or in enhanced oxidation. For higher temperatures additional barriers like alumina forming surface alloys are discussed and investigated. Mechanical loads like creep stress and fretting will act on the steels. These mechanical loads will interact with the coolant and can increase the negative effects. For ALFRED austenitic steels (316 and 15-15Ti) are selected for most in core components. The 15-15Ti is the choice for the fuel cladding of ALFRED and the ferritic martensitic steel T91 was selected as fuel clad material of the ELFR. T91 is in both designs the material to be used for the heat exchanger. Surface alloying with alumina forming alloys is considered to assure material functionality at higher temperatures and is therefore selected for fuel cladding of the ELFR and the heat exchanger tubes. (author)