A Study on the Application of Burnup Credit Methodology on Fuels with Burnable Absorbers

Sammanfattning

The management of spent nuclear fuel (SNF) is a critical challenge for the safe and efficient operation of nuclear power systems. Traditional criticality safety analyses often assume fresh fuel conditions, introducing significant conservatism that limits storage and transport capacity. Burnup credit (BUC) methodology addresses this by accounting for the reduction in fuel reactivity during irradiation. However, current licensing practices are typically restricted to fuels that do not contain burnable absorbers (BAs), such as gadolinium, because of their complex reactivity profiles. This study investigates the impact of gadolinia-based (Gd2O3) integral burnable absorbers (IBAs) on the reactivity of VVER-1000 fuel assemblies and evaluates their implications for BUC methodology. Using the Monte Carlo N-Particle (MCNP) code, depletion simulations were performed for fuel assemblies (FAs) with varying uranium enrichment, gadolinia content, and IBA rod count, up to a burnup of 60 GWd/MTU (Gigawatt-days per metric tonne uranium). Results show that FAs using IBAs show consistently lower reactivity (Δkinf < 0) throughout burnup than reference cases without IBAs. Furthermore, increasing the number of IBA rods and or gadolinia content will only increase ∣Δkinf∣ showing that more simulations are unnecessary. The findings suggest that neglecting IBAs in licensing basis analyses provides a conservative and practical approach, reducing computational complexity while maintaining safety margin.