Sensitivity analysis of in-pile CHF experiments in the TREAT facility: Characterization of impacts of fuel system thermal properties

Understanding fuel-to-coolant heat transfer mechanisms under transient heating irradiation conditions is an important aspect of pressurized-water reactor (PWR) safety. The work here aims to elucidate the potential impacts of critical heat flux (CHF) events on nuclear fuel systems, utilizing the heat transfer time constant (HTTC) as the fundamental basis. A set of thermophysical material and heat transfer coolant properties was established to represent the HTTCs of two Transient Reactor Test (TREAT) facility experimental fuel/cladding designs. Pool and flow boiling conditions were used to model these fuel systems within TREAT experimental capabilities, and a power pulse was used to represent a transient insertion of energy. Using Sobol sensitivity analysis and the Reactor Excursion and Leak Analysis Program (RELAP5-3D) code, an investigation was conducted to identify the most important HTTC parameters pertaining to the output of the peak outer cladding temperature (POCT), maximum fuel centerline temperature, and the time of CHF. The maximum fuel centerline temperature was shown to be highly dependent on the UO₂ volumetric heat capacity. Regarding the POCTs, DNB was determined to have a dominant effect on the heat transfer mechanisms of these fuel systems. Further, the HTTC significantly impacted POCT predictions with the UO₂ volumetric heat capacity, the film boiling heat transfer coefficient, and the CHF multiplier being most influential regardless of coolant flow conditions. Gap variations were shown to influence the POCT in cases that experienced the DNB event. In non-DNB cases, the HTTC had no significant impact on POCT. For the time of CHF, variations in gap thickness, and the CHF multiplier were overwhelmingly dominant.