Integration and validation of models for thermal-mechanical behaviour of nuclear fuel rods in ATHLET

• Advancing Thermal-mechanical Modelling in ATHLET. • Bridging Design-Basis and Severe Accident Scenarios. • Validation Against QUENCH-L0 Experiment. • Comprehensive Benchmarking Against ATHLET-CD. The thermal–hydraulic system code ATHLET, developed by GRS as part of the AC 2 code package, is designed for the safety analysis of nuclear facilities under operational conditions, design-basis accidents, and design extension scenarios where core degradation does not occur. For severe accident conditions, ATHLET is extended with the core degradation module ATHLET-CD, for the simulation of critical phenomena such as oxidation, radiative heat transfer, and advanced thermal–mechanical fuel rod behaviour, including thermally driven cladding creep and rod burst. Ongoing GRS projects aim to integrate selected ATHLET-CD models directly into ATHLET, thereby expanding its predictive capabilities. Notable examples include power ramp scenarios and design extension conditions involving loss-of-coolant accidents or reactivity-initiated accidents. This integration is expected to enhance analytical accuracy and supports more comprehensive safety assessments. This work focuses specifically on the integration of ATHLET-CD’s thermal–mechanical model into ATHLET. The integrated model has been further improved by incorporating state-of-the-art methods from fuel performance codes, allowing for a detailed simulation of fuel rod behaviour. The implemented model accounts for cladding elastic deformation, secondary creep-induced plastic strain, rod ballooning, dynamic variations in internal pressure, and potential hydraulic channel blockage. The methodologies adopted, including those based on open literature, are described in detail. To validate the new capabilities, ATHLET was applied to the QUENCH-L0 experiment, designed to study rod ballooning and burst phenomena under accident-like conditions. Comparisons with both experimental data and ATHLET-CD simulations show that the enhanced ATHLET yields equivalent or, in some cases, improved predictions of cladding surface temperature, internal rod pressure, and burst timing.