Neutronic evaluation of coating and cladding materials for accident tolerant fuels

• Neutronic impact of protecting LWR cladding with ceramic coatings or replacing zircaloy with alternative materials is evaluated.
• Adding thin ceramic coatings, the corresponding reactivity penalties are small and proportional to the coating thickness.
• Substituting FeCrAl, TZM, and Alloy 33 for zirconium-alloy cladding generates considerably shorter fuel cycles.
• SiC cladding can be up to ∼25% thicker than zirconium-alloy cladding with no impact on cycle length.

In severe accident conditions with loss of active cooling in the core, zirconium alloys, used as fuel cladding materials for current light water reactors (LWR), undergo a rapid oxidation by high temperature steam with consequent hydrogen generation. Novel fuel technologies, named accident tolerant fuels (ATF), seek to improve the endurance of severe accident conditions in LWRs by eliminating or at least mitigating such detrimental steam-cladding interaction. Most ATF concepts are expected to work within the design framework of current and future light water reactors, and for that reason they must match or exceed the performance of conventional fuel in normal conditions. This study analyzed the neutronic performance of ATF when employed in both pressurized and boiling water reactors. Two concepts were evaluated: (1) coating the exterior of zirconium-alloy cladding with thin ceramics to limit the zirconium available for reaction with high-temperature steam; (2) replacing zirconium alloys with alternative materials possessing slower oxidation kinetics and reduced hydrogen production. Findings show that ceramic coatings should remain 10–30 μm thick to limit the neutronic penalty. Alternative cladding materials, with the exception of SiC, enhance neutron loss compared to zirconium-alloys. An extensive parametric analysis concluded that reference performance metrics can be met by employing 300-μm or less thick cladding or increasing fuel enrichment by up to 1.74% depending on material and geometry.