CFD Study of Thermal Stratification in a Scaled-Down, Toroidal Suppression Pool of Fukushima Daiichi Type BWR

During the 2011 nuclear catastrophe at Fukushima Daiichi, Unit 3 had a sharper increase in containment pressure than Unit 2, with thermal stratification of the suppression pool cited as one of the contributing factors. In the present work, the buoyancy-induced circulation consequent to steam condensation in a large, toroidal pool of water is studied using computational fluid dynamics (CFD) simulations with a view to understanding the role of important design parameters of the suppression pool system. The tunnelling phenomenon observed in the development of the thermal stratification process is delineated in terms of the establishment of a thermocline. The effects of the number of steam injection points and the cross-section of the pool on thermal stratification characteristics have been investigated through a number of case studies. In all the cases, the surface temperature, which is responsible for over-pressurization of the containment, is found to be significantly higher than the bulk pool temperature. Multiple injection points with the same overall steam flow rate are found to lead to higher surface temperatures due to a shortened circulation path. For the same volume of pool water, the simulations show that a deeper and narrower pool gives rise to significantly higher temperatures than a wider and shallower pool. This is attributed to the relatively deeper penetration of the buoyancy-induced circulation into the pool