Experimental Investigation of Thermal Stratification in Boiling Water Reactor Suppression Pools during Reactor Core Isolation Cooling System Operation and Reactor Safety Implications

Abstract

The primary safety concern at a nuclear reactor is to maintain water inventory inside the reactor core so that the nuclear fuel is adequately cooled. The Reactor Core Isolation Cooling system is designed to automatically provide make-up water to the reactor pressure vessel during isolation events by passing steam from the reactor pressure vessel through a turbine which drives a pump to inject water into the reactor pressure vessel. Exemplary performance of the RCIC system during the Fukushima Daiichi nuclear accident of 2011 showed the RCIC system’s potential utility as a safety device during Beyond Design Basis Accidents as well. This has led to increased interest in maximizing the performance of RCIC systems installed in about 25 US BWR nuclear power plants. RCIC system performance is dependent on conditions in the suppression pool which serves as the RCIC pump water source. To investigate the development of thermal stratification within the pool, a model facility was constructed at the Laboratory for Nuclear Heat Transfer Systems at Texas A&M University. The facility was modified for this work to produce the first high-resolution 3-D temperature measurements in a large water pool. Addition of a scaled-down Terry Turbine of the same design as those used in the RCIC system increases data fidelity to real-world RCIC system behavior. Development of thermal stratification in the suppression pool was investigated by changing two parameters: the steam flowrate and suppression pool pressure conditions. Thermal stratification was most strongly influenced by suppression chamber pressure, with a maximum thermal separation between the top and bottom of the pool of 40.9 °C observed under high pressure while the thermal separation under low pressure with the same steam flowrate was only 18.2 °C. Increased steam flowrate tended to improve mixing, which decreased the duration of thermal stratification. RCIC system performance was found to be strongly influenced by thermal stratification within the pool, showing an improvement in heat removal of up to 6% for thermally stratified pools, compared to well-mixed pools. Inducing thermal stratification in BWR suppression pools could maximize the RCIC system’s ability to safely cool the reactor core.