Cea R and D programme on structural materials for future gas-cooled nuclear systems

CEA has launched feasibility studies of future nuclear advanced systems in a consistent series of gas-cooled reactors ranging from thermal systems (PMR, VHTR) for the mid-term to fast reactor (GFR) for the longer term. Since structural materials of these systems must operate at least up to 1000 deg C in normal conditions several key technologies must be mastered such as high temperature structural materials under new environments and constraints. The material technologies that have to be investigated are on the one hand, the technology of very high-temperature materials (up to 1650 deg C in accidental conditions) for core applications (graphites for thermal reactors and ceramic materials for GFR) and on the other hand, the technology of medium (∼450-650 deg C) and high-temperature (∼650-950 deg C) metallic materials for vessel structures, internals, primary circuit and turbine components. The first chapter of R and D to take up is related to out-of-core metallic structural materials. This set of tasks is important because it is a common R and D need for a large range of He-cooled thermal or fast neutron modular systems operating at high temperature. Ni-base alloys and superalloys have been chosen to cover the highest working temperatures of the un-irradiated components (hot gas systems and turbine components). This paper will give an overview of the selected material variants to be studied and the corresponding R and D programme (mechanical and corrosion properties characterisation) planned over the next six years. As concerns the reactor pressure vessel structures, the first candidate belongs to the class of 9-12Cr martensitic steels. The main R and D items addressed in this paper include basic metallurgical studies on optimisation of chemical composition and thermal treatments regarding possible evolutions of the material specification. The programme of out-of-pile characterizations of creep, fatigue, Charpy and fracture toughness properties as well as the mechanical analysis and codification studies applied to gas-cooled RPV are described. Finally, this paper gives elements of CEA R and D on ceramic materials for in-core applications, where the main effort will be directed toward the most promising ceramics to be chosen among micro- and nano-structured carbide (nitrides) materials (SiC, ZrC, TiC,...) or cermet composite materials. (author)