The challenge of nuclear fuels

Nuclear fuels are generally ceramic materials which are submitted to extreme conditions. Indeed, materials are submitted to the higher temperatures in reactor cores because in reactor cores, heat is produced. Temperatures at the middle of the pellets are of about 1000 C in PWR reactors but they are above 2000 C in fast neutrons reactors. The temperature gradients along the radius of a fuel pellet reach several thousands of degrees Celsius per cm. In such conditions, pellets crack and a first difficulty consists to simulate the thermo-mechanical behaviour of a cracked oxide from which it has to be possible to make the cracks lattice evolve during the simulation. As the other reactor core materials, fuel is submitted to irradiation damage, but in fuel, damage is produced essentially by fission peaks which induce a lot of more point defects than neutrons. The irradiation rates in fuel can easily reach 1000 dpa. At low temperature, at the circumference of the pellets, the irradiation defects can not be annealed and the fuel ceramics resists remarkably well to this aggression by a deep modifying of its microstructure, creating in particular sub-micron crystal grains. Being able to understand the physics underlying to these transformations and to anticipate the behaviour of these areas at very high combustion rates and in all the cases is a fantastic challenge. Besides the heat, each fission of an uranium or a plutonium atom produces two new atoms: the fission products. At the end of the stay time in the reactor, at high combustion rates, an important proportion (typically 20%) of atoms present in the fuel did not exist at the beginning of the irradiation. These new atoms modify the different physical properties of the fuel material. According to their nature and chemical state, they could take different forms inside the fuel material and induce different type of phenomena potentially limiting. Thus rare gases could release and increase the internal pressure in the rod, or on the contrary stay in the fuel and make it swell. Some volatile products could migrate, accumulate, and potentially induce corrosion at cans. All these mechanisms have to be understood, modelled and controlled by the fuel elements designer to be able to guarantee the resistance of the first confinement barrier in all the running situations. (O.M.)