Multiphysics Design and Sensitivity Analysis of Nuclear Heated Critical Heat Flux Pool Boiling Test Devices in TREAT.

Following the events of the 2011 Fukushima Daiichi accident, there has been a drive to develop accident tolerant fuels (ATF) capable of enhancing safety margins provided by conventional light water reactor (LWR) materials, with a focus on the critical heat flux (CHF) behavior under fast transient heating irradiation conditions. Presented in this dissertation, is the modeling scope of a current effort aimed at elucidating the mechanisms of CHF under in-pile fast transient irradiation conditions using the Transient Reactor Test (TREAT) facility. A heater rodlet made from stainless steel type-304 with tailored natural boron content was held within experimental pool boiling capsules, to induce CHF in the surrounding coolant when submitted to a power pulse. The experimental aspect of this project is focused on studying the CHF impacts of radiation-induced surface activation (RISA), as well as rapid surface heating effects.

The initial unique contributions of the computational studies in this dissertation, depict the multiphysics design process of an experimental separate effects borated heater apparatus that was inserted into TREAT in December of 2019. Boron concentrations between 0.1-2.09 wt.% were considered. A self-shielding study determined that a borated tube could be used instead of a solid rod. Following, a thermal hydraulics study determined that the current borated tube configuration achieved a maximum CHF multiplier value of 7.8 using a 1400 MJ power pulse in TREAT.

Following, sensitivity studies analyzed the potential impacts of the CHF event on the heat transfer of more complex integral TREAT experiments under rapid heating conditions, utilizing the heat transfer time constant (HTTC) as the fundamental basis. The analysis showed the maximum fuel centerline temperature is independent of the CHF event, and the UO₂ volumetric heat capacity is the only significant HTTC parameter. For the peak outer cladding temperatures (POCTs), the occurrence of DNB was determined to be dominant on the heat transfer mechanisms of these experimental fuel designs. For the cases where the DNB event manifested, the HTTC was resolved to have significant impacts on the predictions of the POCTs. Furthermore, when studying the time occurrence of the CHF, the variations in the gap thickness was dominant.