Fuel materials. Nuclear fuel is a consumable and removable component at the heart of the reactor. It is the seat of the fissions of nuclei, which produce the energy to be ultimately used to produce electricity. It remains several years in the reactor until the maximum burn-up admissible has been reached. It is exposed to the harshest irradiation and temperature conditions. Together with the cladding, however, it is the only component whose performance can be significantly improved during the lifetime of a reactor. The fuel materials are combined with cladding in the fuel elements. By conception, nuclear fuels and fuel elements must in particular • Provide the power expected during their whole stay in reactor. • Use the fissile elements as best as possible to reduce the cost of energy production • Confine the fission products inside the fuel elements in all operating and accidental conditions Nuclear fuels and fuel elements differ widely from reactor to reactor, in geometrical configuration, fuel composition and cladding. Fissile atoms used in nuclear reactors, however, are mainly uranium and plutonium. Fuel materials are thus compounds of these elements, either refractory ceramics such as oxides, carbides, nitrides or silicides, or stable metallic alloys. More complex systems include ceramic/ceramic or ceramic/metal composites, or molten salt fuels. As to geometries, fuel rods, fuel plates and pellets have been developed. Actinide oxides (U and mixed U-Pu oxides, MOX) are the most industrially used fuel materials and have been used in power reactors since the 1960s. MOX will be the fuel for the first cores of ALL ESNII fast reactor prototypes. Variations of this fuel were used in previous European fast reactor programmes: Phenix, Superphenix, and Dounreay fast reactors. MOX fuel is Europe’s only major knowledge and competence base for fast reactors, even if all European fast reactors are now closed. It is thus natural that this fuel is the first choice for the future. MOX fuel crystallises in the cubic fluorite structure, so isotropic behaviour can be expected. It has high melting point, though only moderate thermal conductivity, resulting in typical operating temperatures at 80% of the melting point. Its manufacturing technology is known and proven, although it needs new validation when a new plant is deployed. Though widely studied in the past, the MOX product can bear important intrinsic hallmarks linked to the fabrication methods used, e.g. porosity distribution, grai...
Fuel materials. 3.2.1 Fuel materials qualification and design rules This section addresses the development of materials data and simulation tools in support of the design and licencing of fuels for ESNII prototypes. Although the processes occurring in pile in nuclear fuels are interconnected, the fuel material properties and processes governing the behaviour of nuclear fuel under irradiation can be grouped in five main categories of issues, as follows.
