Application of BISON to UO[formula omitted] MiniFuel fission gas release analysis

Abstract

There has been a recent push to accelerate nuclear fuel qualification by combining advanced modeling and simulation with accelerated separate effects irradiation testing. One separate effects irradiation testing capability is the MiniFuel vehicle designed to accelerate burnup accumulation in “mini” fuel samples under isothermal temperature conditions. These steady-state MiniFuel irradiations effectively decouple the fuel temperature from the fission rate (i.e., power) by minimizing the fuel volume and relying on gamma heating in the surrounding components for temperature control. This provides experimenters a flexible means for targeting and evaluating specific fuel microstructures for a wide range of fuel types and operating conditions. However, the accelerated fuel qualification process must be informed by modeling and simulation to properly evaluate the most impactful fuel performance parameters and to identify modeling and data gaps. A test matrix can then be designed to fill those gaps and, ultimately, refine and validate the fuel performance models. This work uses the BISON fuel performance code to conduct an assessment and sensitivity study on calculated fission gas release (FGR) from UO2 MiniFuel disks during isothermal irradiation and temperature transients. Qualitatively, the existing fission gas behavior model in BISON reproduces the effects of temperature and burnup as expected. However, when quantitatively compared with FGR data from irradiated (103 MWd/kgU) UO2 disks under thermal annealing representative of LOCA conditions, the model shows a less satisfactory predictive capability. As a result, development needed to model the specific mechanisms of transient FGR during LOCAs, including fuel fragmentation and the role of the high burnup structure (HBS), are identified. Finally, a UO2 MiniFuel test matrix is proposed to extend the temperature and burnup ranges covered by previous experiments and provide new model validation data for fission gas behavior under high burnup and transient conditions.