Safety Performance of High Burnup and Extended Enrichment Pressurized Water Reactor Fuels

Efforts to introduce new cladding materials and fuel systems are being pursued by several major fuel vendors, with substantial interest from operating utilities. These designs have the potential to provide economic benefits, notably in the ability to extend the operating cycle length of pressurized water reactors from 18 to 24 months. To support this increase, these fuel forms must be rigorously shown to meet adequate safety criteria under long term irradiation and during design basis accidents. Current challenges in developing this technical basis include demonstrating the feasibility and safety of fuels with UO₂ enrichments greater than 5% and burnup levels greater than 62 GWd/MTU.

The first two studies presented in this work consider two proposed near-term high burnup core designs using differing burnable poison strategies to meet design constraints. The safety analysis simulation tools PARCS and RELAP5-3D were used to evaluate the designs based on their normal operating cycle performance and safety performance during reactivity-initiated accidents. Loss of coolant accident performance was also analyzed, including an assessment of fuel performance behavior with BISON. High soluble boron concentrations likely prevent the implementation of an ZrB₂ integral-fuel burnable poison design without significant design iteration. From a safety perspective, the two designs perform well for both control rod ejection and loss of coolant accidents. Future designs should aim for improved hydriding resistance and reduced rod internal pressures. Additional efforts should be made to reduce linear heat rates of rods at intermediate burnups to gain additional safety margin.

To further support model development and physical understanding of high burnup fuel behavior, a sensitivity analysis was performed to characterize the impacts of thermal property and two-phase boiling uncertainties on fuel safety predictions during reactivity-initiated accidents. These uncertainties were found to be highly significant for predictions of fuel behavior. Further work is needed to develop mechanistic cladding failure criteria appropriate for this accident type.

This work identifies high impact areas of need for model development relevant for fuel safety and makes recommendations for future high burnup core designs. These developments will be necessary to achieve the adoption of high burnup pressurized water reactor fuels.