Neutronic analysis of accident-tolerant fuel cladding materials for VVER-1200 reactors using the SERPENT code

Abstract

The failure of zirconium (Zr) fuel elements in Loss-of-Coolant Accidents (LOCAs) is primarily caused by the exothermic zirconium-steam reaction between the Zr fuel cladding and coolant water in VVER-type reactors, which generates explosive hydrogen gas and compromises cladding integrity. Enhancing fuel element safety can be achieved by replacing the Zr cladding with Accident Tolerant Fuel (ATF) materials that exhibit improved high-temperature oxidation resistance and reduced interaction with coolant under both normal and emergency conditions. LOCA is a design basis accident for VVER reactors, making the development of alternative cladding materials a priority for next-generation nuclear power plants. This study investigates the effective multiplication coefficient (Keff) of standard Zr-clad fuel rods compared to those with three candidate ATF cladding materials – FeCrAl alloy, SiC composite, and NiCr alloy–using detailed Monte Carlo simulations with the SERPENT code for the VVER-1200 reactor geometry. The results demonstrate significant differences among the materials: one candidate exhibits neutronic behavior very close to zirconium with minimal reactivity control, another shows a moderate but increasing penalty throughout the burnup cycle, while the third displays a substantial reactivity control that would necessitate major core redesign. These quantitative findings highlight the promise of select ATF materials as replacements for Zr, contributing to reduced oxidation risks, elimination of hydrogen generation, and decreased accident probabilities in nuclear reactors while maintaining acceptable neutronic performance. The calculations are specifically performed for the WWER-1200 nuclear power plant and provide essential data for material selection decisions and core design optimization.