Modeling UO₂ and UN Fuel Fission Gas Release Instances in BISON for Microreactor Applications

Abstract

Pelletized UO₂ and UN (mononitride) fuel concepts are currently under consideration for microreactor technology. To implement these fuel concepts, the performance of UO₂ and UN under microreactor irradiation conditions must be well understood. One key fuel performance phenomenon is fission gas release, where gaseous fission products are expelled from the fuel pellet to the plenum in the cladding. The fission gas release threshold plot of burnup vs. temperature at 1% release, first introduced by Vitanza and commonly coined the “Vitanza curve,” is of particular interest because it describes when fission gas release begins [1]. Thus, accurately modeling fission gas release is an active area of research. Though empirical models of UO₂ and UN fission gas release thresholds exist, like Vitanza and Wallenius et al., they fail to account for the low linear powers found in microreactor concepts [1, 2]. As a result, BISON, a MOOSE-based (Multiphysics Object Oriented Simulation Environment) fuel performance code, was used to evaluate fission gas release in UO₂ and UN fuels at power levels representative of microreactors. The fission gas release threshold curve was constructed from BISON results for both fuel types, validated against Vitanza and Wallenius et. al, and then extended to a third dimension to incorporate power dependency and create 3D surface threshold plots [1]. At both light water reactor and microreactor power levels for UO₂ and UN, BISON calculated the threshold curve as the expected exponential decay, within 100 K of the Vitanza and Wallenius curves, respectively. When fuel surface temperature was gradually increased at a constant low power level, the threshold curve decreased. This was as expected since higher temperatures drive faster gas atom diffusion. Faster gas atom diffusion causes fission bubbles to form, interconnect into “tunnels” and dispel fission gases to the plenum more rapidly [3]. Ultimately, this study demonstrates that the fission gas release threshold is not only influenced by temperature but also by power level. Low power levels associated with microreactor technology ultimately delay the onset of fission gas release. When combined with low-temperature operation, UN fuel may produce very minimal, if any, fission gas release. This may lead to enhanced reactor safety and potentially design and construction cost reductions.