Development of the ENIGMA fuel performance code for LWR applications with chromium-coated cladding

Zirconium-alloy cladding with a chromium coating is the most advanced of the near-term concepts amongst Accident Tolerant Fuel (ATF) materials for Light Water Reactor (LWR) applications. The ENIGMA fuel performance code has been updated to model the thermo-mechanical behaviour of such cladding in both normal and off-normal operating conditions. The focus was on accurately simulating the behaviour in Loss Of Coolant Accident (LOCA) conditions to evaluate the increase in coping time during design-basis accidents. New low-temperature and high-temperature models were incorporated for cladding oxidation and hydriding and cladding creep which take into account the impact of the chromium coating on the overall cladding behaviour. Furthermore, the consumption of the chromium coating due to high-temperature diffusion of chromium into the cladding base alloy’s β-Zr phase is simulated. The new models have been validated using measurements on chromium-coated cladding from irradiated rods, high-temperature annealing experiments and semi-integral LOCA tests. The validation showed good agreement between ENIGMA’s predictions and the experimental data; thereby demonstrating the applicability of the new models for simulating the performance of LWR fuel rods with chromium-coated cladding in both normal operation and accident conditions.