Lelio Luzzi

Accurate prediction of fission gas release is essential to support fuel performance assessments across several plant states. This work presents an integral-scale assessment of the TRANSURANUS-SCIANTIX coupling using the updated modelling framework developed in the past two years. The assessment is performed through both code-to-data comparison against fuel central temperature, end-of-life fission gas release, rod internal pressure and cladding profilometry, and a supplementary code-to-code comparison against the previous TRANSURANUS-SCIANTIX (2024) version. In addition to model development, also the validation matrix has been extended with respect to the latest validation effort; it comprises water-reactor fuel rods under normal operation, anticipated operational occurrences, and design-basis-accident conditions represented by reactivity-initiated accident tests. In normal operation conditions, the model well reproduces the measured fission gas release, also providing improved kinetics thanks to athermal FGR modelling. Under accidental conditions, the physics-based model dedicated to grain-boundary micro-cracking reduces excessive activation of burst release at each power perturbation, as it is demonstrated via additional instrumented cases. When rod internal pressure data are available, the comparison highlights that reproducing pressure transients requires not only adequate release kinetics but also modelling gas transport toward the plenum. In general, the code predicts satisfactorily moderate releases, while high release conditions can be challenging. Cladding profilometry comparisons show a systematic tendency to overestimate the final cladding diameter, motivating future refinements in fuel swelling, relocation and recovery, as well as creep response. Overall, the code demonstrates a satisfactory performance of the fission-gas-related quantities.

This paper presents the results of the second phase of the benchmark exercise in the Plutonium Management for More Agility project (PuMMA), which aims to enhance the reliability of fuel performance codes (FPCs) for Mixed Oxide (MOX) fuels with high plutonium content. This phase involved the simulation of the CAPRIX irradiation (45% of Pu) carried out in the PHENIX reactor in France. Thirteen organizations from nine countries participated, using eight different fuel performance codes. In this phase, each code then improved its modelling by optimizing parameters and/or improving the formulation of some models involved in the computation. A comparative analysis was then conducted between the codes and the post-irradiation experimental (PIE) results, with the objective of identifying the improvements made, the discrepancies still observed, and to define future axes for improvement and development. The present paper demonstrates the results for the CAPRIX pin.The main results despite the discrepancies noted between the codes and/or with the experimental data, the work carried out through this Benchmark has demonstrated the capacity of the codes to optimise/improve their simulation in order to approximate the PIE. Moreover, the methodology employed yielded a comprehensive and precise analysis, thus facilitating the suggestion of approaches to the development of specific models. This finding thus demonstrates the potential of fuel performance codes modelling pins with high Pu contents.

Fission gas release modelling is a key aspect of fuel performance assessment, and quantifying the associated uncertainties remains a challenge. This work performs two sensitivity analyses to build confidence in recently developed athermal and burst fission gas release models. The computational framework adopted for fuel performance simulations couples the fission gas behaviour module of SCIANTIX with the TRANSURANUS fuel performance code. This system is interfaced with Dakota software to perform a global sensitivity analysis based on variance decomposition. Five parameters are investigated for the athermal release in the US PWR 16x16 base-irradiation case, i.e.: fabrication porosity, grain-edge length, burn-up, temperature, and fission rate. Results highlight the dominant role of fabrication porosity, suggesting the need for future modelling efforts on fuel densification. For the burst gas release from fuel microcracks five parameters are studied in the HATAC C2 rod irradiation experiment, i.e.: grain-boundary energy, bubble radius and internal pressure, temperature, and hydrostatic stress. Grain-boundary energy, which controls the microcracking threshold, emerges as the most relevant parameter. Furthermore, the analyses highlight the need for enhanced lower-scale physical modelling to further reduce empirical assumptions. The outcome of this study is twofold: an operational range of parameter values resulting in a calculated fission gas release within a factor two from the experimental value, and a methodology for testing and interpreting model behaviour within the TRANSURANUS–SCIANTIX framework prior to engineering-scale validation.