Modelling of the Behaviour of Inert Gases In Oxide and Mixed Oxide Nuclear Fuel under Design Basis Accident Conditions

This final report summarizes the work performed by Politecnico di Milano (POLIMI) in the FUMAC Project. The objective is the development, implementation in fuel performance codes (FPCs), and validation of models for the behaviour of inert gas in UO2 nuclear fuel during transients, together with their fundamental parameters. The introduction of these modelling capabilities intends overcoming current limitations of FPCs for the simulation of transients, such as (but not limited to) design basis accidents. The activity of POLIMI focused on developing/improving three aspects of fission gas behaviour modelling in fuel performance codes, namely: the modelling of inter-granular gas behaviour, by the addition of a burst release model; the modelling of helium behaviour, by the proposal of improved correlations for its diffusivity; and the modelling of intra-granular fission gas behaviour, by the development of a new model describing intra-granular bubble evolution. Each of these models embodies peculiar physical phenomena (e.g., intra-granular bubble nucleation, grain-boundary micro-cracking) which all together participate in the proper representation of the complete evolution of inert gas behaviour (IGB) during transients and design basis accidents (e.g., LOCAs). This represents a significant step forward with respect to the state of art, in which some of these phenomena are represented by fully empirical correlations (with none or very limited transient capabilities) and others are completely not represented despite their huge impact on the fuel performance. The models have been implemented, tested, and verified in SCIANTIX (0D stand-alone code, developed by POLIMI), as well as in the fuel performance codes TRANSURANUS [1] (grain-boundary micro-cracking model) and BISON [2] (grain-boundary micro-cracking, intra-granular bubble evolution). The results were compared against available separate effect experiments and against integral irradiation experiments, showing a better predictive capability of the developed models compared to state-of-the-art ones. As far as inert gas in oxide fuels is concerned, the extension of the work on helium is crucial (e.g., introducing in FPCs correlations for helium solubility in oxide fuel) due to the important role played by this gas in MOX fuels and in storage conditions. More in general, the following items are of interest, in perspective: the extension to other fuel materials (e.g., MOX) and other reactor conditions (i.e., fast reactors), the extension to include other non-inert fission products (e.g., caesium and iodine), and the inclusion of the description of collateral physical phenomena (e.g., restructuring of MOX fuels in fast reactor conditions).