Material degradation processes and relevant properties Sample Clauses

Material degradation processes and relevant properties. As stressed in the previous section, GenIV concepts will involve very harsh conditions in terms of temperature, irradiation levels and interaction with the coolant for all materials. Here we focus on structural and fuel materials. Coolant operation temperatures between 400°C and 600°C in the case of liquid metal-cooled systems, and targeting 850°C in gas-cooled systems, which will lead to temperatures around 2200°C at the centre of the fuel in normal conditions and exceeding 1000°C in structural materials in off-normal conditions, coupled to tremendous temperature gradients, inflict severe thermomechanical stresses on the fuel and plant components. These trigger the degradation of both structural and functional materials, through several simultaneously occurring mechanisms. Moreover, cooling fluids represent invariably a chemically hostile environment, the exposure to which also affects severely the performance of materials in contact with them. Finally, nuclear materials are exposed to varying levels of irradiation, the number of displacements per atoms (dpa) reaching 1 in the fuel in less than 1 day in reactor, exceeding 100 in the cladding during its stay in the reactor, though being less than 2 in the vessel during the whole reactor life. These levels of irradiation invariably affect negatively the performance of the materials. The mechanical degradation mechanisms that are known to operate at high temperature are creep, thermomechanical fatigue, creep-fatigue, ratcheting and thermal ageing. Thermochemical mechanisms due to interaction between fuel, structural materials and cooling fluids are corrosion (oxidation), dissolution, erosion. In some cases exposure to fluids may affect also the mechanical properties (e.g. liquid metal embrittlement). Irradiation affects the materials by changing substantially and continuously their chemical composition, microstructure and microchemistry. Importantly, these mechanisms act simultaneously, thereby introducing a high complexity in terms of synergic effects that cannot be simply “linearly superposed”. For fuel, the main parameter that needs to be maintained against these mechanisms is the temperature at the centre of the material, which needs to be less than its melting temperature, itself strongly affected by the modifications undergone by the fuel in reactor. For materials with structural function the properties that need to be preserved are: (i) strength and resistance to creep deformation, with rupture beyond...