Modelling and Assessment of Inert Gas Behaviour in UO2 Nuclear Fuel for Transient Analysis

This paper concerns the modelling and the assessment of inert gas behaviour (IGB) in UO2 nuclear fuel, with application to integral fuel performance codes (FPC) and emphasis on transient behaviour. We adopted a modelling strategy mixed between physics-based and semi-empirical, always aiming at obtaining a conveniently simple description of IGB, suitable for FPCs. The considered IGB phenomena covered by our models are: (1) the evolution of the intra-granular bubble population, which can be responsible of up to half of the gaseous swelling during transients and strongly interacts with the diffusion of single gas atoms towards the grain boundaries; (2) the burst release of gas occurring during temperature transients and ascribed to the micro-cracking of grain boundaries; and (3) the formation of the high burnup structure (HBS) with a combined description of the grain recrystallization and of the intra-granular gas depletion. All together, these models allow for representing the complete evolution of inert gas behaviour during transients. 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 potential impact on the fuel performance. We compared the results of each developed model with experimental data, according to a dedicated validation strategy (comprising uncertainty and sensitivity analyses), always showing an improvement with respect to available state-of-the-art models. Overall, the outcome of this work encompasses several different aspects in IGB modelling, i.e., both intra- and inter-granular behaviour, both regular fuel structure and HBS, both normal operating and transient conditions. All the models developed have been implemented in FPCs (BISON and/or TRANSURANUS). Moreover, all the developed models are available in the 0D stand-alone SCIANTIX code. The developed models provide an interface for scale bridging within multiscale modelling approaches. The new models are applicable with minor modifications to different fuel materials, since the main physical phenomena driving IGB are present in many fuel materials. Summarizing, the main outcomes of this work are: (1) the development and assessment of a physically-based IGB model (representing intra-granular, inter-granular and HBS IGB) suitable for transient analysis and its implementation in FPCs; (2) The development and verification of SCIANTIX, a 0D IGB code useful for testing, verification and validation of models and specifically designed for coupling with existing FPCs.