CHF enhancement in downward-facing boiling surface using shrouds for calandria vessel during severe accident in PHWRs

Abstract

The orientation of the heated surface significantly affects the boiling process. Boiling on a downward-facing surface is particularly challenging because bubble detachment is hindered, leading to longer bubble residence times and unique interactions than on vertical or inclined surfaces. This study investigates boiling on a large downward-facing flat surface (100 × 400 mm), focusing on critical heat flux (CHF) phenomenon. During the postulated severe accident, the situation arises in Pressurised Heavy Water Reactors (PHWRs) due to multiple failures of cooling systems and safety systems. The pressure tubes and calandria tubes have the potential to break, resulting in hot debris that falls to the bottom of the calandria vessel. The calandria vessel has a large curvature due to its larger diameter, and the bottommost portion is like a flat plate. To contain the hot debris or molten corium inside the vessel and maintain the integrity of the calandria vessel at a higher temperature is crucial to arrest the progress of a severe accident. The cooling of the vessel from outside without occurring CHF at the bottom location is important. Historically, downward-facing boiling has received limited attention, as it is normally not used in industrial applications owing to lower heat transfer and CHF values due to adverse buoyancy. Nonetheless, it is important to investigate because of the severe accident situation in PHWRs. Incorporating a simple technique of shrouds surrounding the calandria vessel can enhance the CHF by enhancing the buoyancy. This paper investigates the potential enhancement of CHF through the use of shrouds.