INVITED: Thermodynamic modelling of nuclear materials

In Light Water Reactor (LWR) and Sodium cooled Fast Reactors (SFR), different kinds of materials compose the nuclear reactor: the nuclear fuel (UO₂ or (U,Pu)O₂), the cladding (Zirconium alloy or stainless steel), the neutron absorber (B₄C) and other materials : the vessel (steel) and the basement (concrete). Moreover the nuclear reactions lead to the formation of numerous fission products (up to 60 elements) within the fuel whose chemical composition changes with time. Thus, in an operating reactor, the nuclear fuel is a high-temperature system that is continuously changing as fission products are created and actinides consumed and is also experiencing temperature and composition gradients while simultaneously subjected to a severe radiation field. For nuclear materials, computational thermodynamics is a crucial tool for many applications: the basic understanding and control of the fuel and cladding composition and microstructure, the manufacturing and design of the fuel and cladding materials, the prediction of the thermal stability of fuel and cladding materials in their environment (under different conditions of temperature, pressure, atmosphere, ...), the understanding and prediction of the fuel fission product chemistry and of the chemical interaction between fuel and structural materials during a severe accident. For such complex chemical systems, computational thermodynamics is an essential tool to calculate the equilibrium state. The approach used to model thermodynamics of nuclear materials will be described and examples of application calculations will be presented.