In-core and Out-of-core Materials Selection for the HPLWR

In this report a state-of-the-art study was performed to investigate the operational conditions for in-core and ex-vessel materials in a future High Performance Light Water Reactor (HPLWR) and to evaluate the potential ofexisting structural materials for application in fuel elements, core structures and out-of-core components. In the conventional parts of such a novel plant the approved materials of supercritical fossil power plants (SCFPP) can be used for the given temperature (≤ 600°C) and pressure ( 250 bar). These are either ferritic/martensitic or austenitic stainless steels. The design data for the in-core components are, however, very ambitious in comparison with conventional Light Water Reactors, especially regarding the coolant which is under high pressure (≤ 250 bar) and will have a transition from sub- to supercritical state in the core, since the water temperature increases from 290 to 510°C outlet. The expected temperature in the cladding of the fuel elements can reach up to 650°C and the neutron exposure can accumulate up to 1.13.10 2 3 n/cm 2 or 60 displacements per atom (dpa) for an envisaged target of 70 GWd/t U burnup. Taking these novel operational conditions into account an assessment of available material data was made. It is based on existing creep-rupture data, an extensive analysis of the corrosion in conventional steam power plants and the material behavior under irradiation. Compatibility between fuel and cladding materials is also considered. The potential of the different material groups available for in-core application, to be used as cladding materials of fuel elements, was further investigated by quantitative assumptions on the stress development in claddings and by a determination of the maximum achievable temperatures in dependence of cladding dimensions and the above mentioned operational conditions. More qualitative arguments on stress corrosion susceptibility are also included. It was stated that for a maximum temperature of 650°C from a standpoint of creep-rupture strength and corrosion resistance not only Ni-alloys but also austenitic stainless steels would fulfill the requirements for application as cladding materials. Taking into account specific items like the neutron absorption, the sensitivity to irradiation-induced helium embrittlement and stress corrosion cracking, it was finally concluded that the austenitic stainless steels are the better choice. The assessment has also shown that the most uncertain areas in the present analysis are the corrosion behavior under supercritical water conditions, including the effects of water chemistry/radiolysis, and the influence of a high stress state on stress corrosion and deformation mechanisms which govern the creep-rupture and creep buckling properties under irradiation. Future R&D activities should, therefore, concentrate on these open questions.