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 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, grain size, impurity levels, all of which can come to bear in its performance. This is not surprising when one considers the combination of the material alteration in the neutron flux, and the massive temperature gradients (up to 500°C/mm). Furthermore, reactor core designs have evolved, with particular effort based on thermal hydraulics and neutron physics to reduce such safety relevant parameters as void coef...
Fuel materials. 3.2.1 Fuel materials qualification and design rules
