Liquid level monitoring and quenching front tracking for SMR rod bundle CHF tests under low pressure, low flow, high quality conditions

Abstract

In most Light Water Reactor (LWR) based Small Modular Reactors (SMRs), one of the critical factors that might affect Critical Heat Flux (CHF) and other thermal–hydraulic phenomena under low flow, low pressure, high-quality conditions is the existence of high water column over a relatively short fuel core in the majority of the integral design SMR reactor vessels. With the existence of this water column over the relatively short and high-power peaking fuel bundle, the potential of a reflood quenching effect is highly plausible under low pressure, low flow and high-quality conditions as the location of the CHF event will be much closer to the exit plenum, especially in the case of exit power peaking. The results from CFD modeling and flow visualization tests presented in this paper demonstrate the importance of liquid level as well as its impacts on rod bundle CHF. Such a critical factor has often been overlooked by the prevailing SMR designs as many of the rod bundle CHF tests performed for SMR, including those with the licensing data, have either neglected the effects of the exit quenching effect, or were performed with non- prototypical heater rod designs or surrogate fluid that may not include such exit quenching phenomena.In this paper, the phenomenon of exit quenching on SMR rod bundle CHF has been introduced with the development of the proprietary On-line, Real-time Liquid Level Monitoring System (OR_LLMS) and On-line, Real-time dynamic Quenching Front Tracking Systems (OR_QFTS), which allow accurate measurements of rod bundle CHF as a function of both liquid level and degree of downward quenching under various test conditions. The future application is also presented, which will aid in the development of rod bundle CHF correlations that account for and take advantage of different water levels over the reactor core.