b'In-pile Investigation ofImproved cladding-to-coolant heat transfer understanding can increase safety Transient Boiling in theand design margins in both the current commercial fleet and advanced light Transient Reactor Test Facility water reactors.C ladding-to-coolant heat transfer during transient irradiation conditions remains a critical area of uncertainty in understanding nuclear reactor safety. This uncertainty impacts analytical predictions of both accident progression and fuel performance behavior. This uncertainty manifests itself in the use of extremely conservative models that widely bound the onset and duration of TOTAL APPROVED AMOUNT:transition phenomena. Accurate and reliable descriptions of cladding-to-coolant $2,650,000 over 3 years heat transfer may also provide the primary building blocks to allow for a change in the current fuel safety criteria and enable safe operation at higher powers, which PROJECT NUMBER:results in a substantial increase in the performance of current and future reactors. 18P37-001 The complexity of the phenomena and the variety of thermal-hydraulic scenarios of PRINCIPAL INVESTIGATOR:interest requires establishing a mechanistic description of the behavior and a body of Colby Jensen dedicated experimental data to validate the model.CO-INVESTIGATORS: This project developed an in-pile experiment for the TREAT facility to answer key Nicolas Woolstenhulme, INL questions regarding critical heat flux under prototypic irradiation with specific Nicholas Brown,interest in transient conditions. The modeling scope of this project extended to University of Tennessee, Knoxville the thermodynamic conditions in prototypic light water reactors. Experimental Richard Christensen,development and testing were focused on ambient coolant conditions. The research University of Idaho measured critical heat flux in the TREAT facility showing an approximate 24x increase from state-of-the-art predictions. To accomplish this goal, two unique-in-the-world capabilities were designed, constructed, and commissioned: (1) an in-pile experiment platform to study transient boiling in reactor; and (2) a high-power transient boiling experimental setup capable of running to relevant light water reactor conditions. Experiment design was guided by extensive modeling and simulation. The in-pile experiment platform was used in three separate assemblies for fourteen different experiments and demonstrated excellent performance. New machine learning analytical methodologies improved data extraction from the experiments (reduce uncertainties); these methods will benefit other experiments. A novel fission thermometer to improve understanding of energy deposited into the heater rod was developed, tested, and validated the model. The laboratory-based transient boiling setup was commissioned to show full functionality incorporating advanced diagnostics such as high-speed imaging and to experimentally challenging pressurized water reactor conditions.15'