Thermal Hydraulic and Economic Analysis of Grid-Supported Hydride and Oxide Fueled PWRs

Abstract

This report advances the Hydride Fuels Project, a collaborative effort between UC Berkeley and MIT aimed at investigating the potential benefits of hydride fuel use in light water reactors (LWRs). This effort involves implementing an appropriate methodology for design and optimization of hydride and oxide fueled cores. Core design is accomplished for a range of geometries via steady-state and transient thermal hydraulic analyses, which yield the maximum power, and fuel performance and neutronics studies, which provide the achievable discharge burnup. The final optimization integrates the outputs from these separate studies into an economics model to identify geometries offering the lowest cost of electricity, and provide a fair basis for comparing the performance of hydride and oxide fuels. This report builds on the considerable work which has already been accomplished on the project. More specifically, it focuses on the steady-state and transient thermal hydraulic and economic analyses for pressurized water reactor (PWR) cores utilizing UZrH[subscript 1.6] and UO[subscript 2]. A previous MIT study established the steady-state thermal hydraulic design methodology for determining maximum power from square array PWR core designs. In lieu of a detailed vibrations analysis, the steady-state thermal hydraulic analysis imposed a single design limit on the axial flow velocity. The wide range of core geometries considered and the large power increases reported by the study makes it prudent to refine this single limit approach. This work accomplishes this by developing and incorporating additional design limits into the thermal hydraulic analysis to prevent excessive rod vibration and wear. The vibrations and wear mechanisms considered are: vortex-induced vibration, fluid-elastic instability, turbulence-induced vibration, fretting wear, and sliding wear. Further, the transients investigated are an overpower transient, a large break loss of coolant accident (LBLOCA), and a complete loss of flow accident. In parallel with this work, students at UC Berkeley and MIT have undertaken the neutronics and fuel performance studies. With these results, and the output from the steady-state thermal hydraulic analysis with vibrations and wear imposed design limits, as well as transient thermal hydraulic analysis, an economics model is employed to determine the optimal geometries for incorporation into existing PWRs. The model also provides a basis for comparing the performance of UZrH[subscript 1.6] to UO[subscript 2] for a range of core geometries. Though this analysis focuses only on these fuels, the methodology can easily be extended to additional hydride and oxide fuel types, and will be in the future. Results presented herein do not show significant cost savings for UZrH[subscript 1.6], primarily because the power and energy generation per core loading for both fuels with square arrays supported by grid spacers are similar. Furthermore, the most economic geometries typically do not occur where power increases are reported by the thermal hydraulics. However, preliminary analysis with the lower pressure drop characteristics of wire wraps compared to grids suggest that hexagonal array cores with wire wraps will allow tight ( P over D ≺ 1.25) packing which yield significantly better power performance. This should allow hydride fuel to outperform oxide fuel since this tight core region is not accessible to oxide cores, because of neutronic constraints.