Review of pool boiling critical heat flux (CHF) and heater rod design for CHF experiments in TREAT

Preventing the occurrence of a departure from nucleate boiling (DNB) event is an important aspect of nuclear safety in pressurized water reactors (PWRs). This phenomenon is partially governed by the cladding-to-coolant heat transfer under transient irradiation conditions, such as during a reactivity-initiated accident (RIA). Currently, there are large uncertainties about cladding-to-coolant heat transfer under these rapid transient conditions. This effort aims to elucidate the mechanisms of CHF under in-pile fast transient irradiation conditions using the Transient Reactor Test (TREAT) facility. These experiments will be carried out under pool boiling conditions, within experimental capsules that will be inserted into the TREAT reactor. A heater rodlet made from stainless steel 304 with tailored natural boron content will be usedin these experiment capsules to induce CHF when submitted to a TREAT power pulse. We will investigate the impacts of the presence of an oxide layer, radiation-induced surface activation (RISA), heat transfer time constant, and rapid surface heating effects on the CHF phenomenon. We review the parameters with significant effects governing the predictions of pool boiling CHF. Only CHF influencing parameters that are highly important, among them coolant subcooling and pressure, as well as oxide layer thickness, RISA, and rapid heating effects were included in this literature assessment. Preliminary neutronics and thermal hydraulic results from the design of the experimental apparatus are also presented in this paper. To aid in the modeling approach, the energy deposition and occurrence of CHF were identified as the most crucial key Figures of Merit (FoMs). In the heater rod design, boron concentrations between 0.1 and 2.09 wt% were considered. Further, a self-shielding study was performed to determine whether an instrumented borated tube could be used in place of a solid borated rod. This study determined that the inner region of the rod can be excluded or instrumented without heat generation penalties. Lastly, a thermal-hydraulics sensitivity study determined the lowest limiting boron concentration needed to induce CHF in water with different degrees of subcooling. Additionally, the value of CHF is known to increase during a rapid transient. Therefore, a CHF multiplier sensitivity study determined what multipliers would inhibit CHF for varying degrees of subcooling of two chosen power coupling factors (PCFs). The current borated tube rodlet geometry configuration achieved a maximum CHF multiplier value of 7.8 using a 1400 MJ power pulse in TREAT. Although this was the power pulse with the greatest energy deposition considered for this study, the TREAT facility is capable of pulses up to similar to 2500 MJ. This provides a significant margin in energy capacity that was not included within the scope of the calculations carried out to date. The application of the heater rod design was successfully demonstrated in initial experiments in December 2019. The results of these experiments will be explored in future publications.