Innovations in Thermal-Hydraulic Testing and Modeling of Boiling Water Reactor Fuel at Westinghouse

The TRITON11® fuel design is the latest advancement in boiling water reactor (BWR) fuel technology from Westinghouse, developed to improve fuel cycle economics, thermal performance, and reliability compared to previous designs. This paper provides a comprehensive overview of the thermal-hydraulic testing and modeling efforts supporting the qualification and licensing of the TRITON11 fuel, along with recent innovations related to BWR core thermal-hydraulics. Extensive experimental activities were conducted at the Westinghouse fuel thermal-hydraulic laboratory using the FRODE and FRIGG loops, which simulate prototypical BWR operating conditions. Key tests included critical boiling transition, pressure drop, void fraction, and hydraulic axial force measurements. Additionally, recent tests supporting the development of the new StrongHold® debris filter are discussed. In parallel, Westinghouse advanced core simulation and subchannel analysis tools (POLCA7/POLCA8 and MEFISTO-T) were further developed, allowing accurate predictions of complex core flow distribution, critical power performance, bundle lift force, and moderator density. The MEFISTO-T code integrates advanced two-phase flow models, in addition to a novel four-field model of annular two-phase flow, explicitly accounting for disturbance waves, along with a multifield transport model of reactor coolant impurities. These models have been validated against experimental data, demonstrating reliable predictive capabilities for critical power and newly observed localized impurity deposition under BWR core conditions. The combined results from these thermal-hydraulic testing and modeling efforts contribute to improved operational flexibility and reliability of Westinghouse BWR fuel designs and support the accurate evaluation of safety margins.