Quenching of Metal Surfaces in a Narrow Annular Gap

Abstract

The quenching of overheated core debris and reactor components is the fundamental objective of emergency procedures and accident management guidelines. Knowing the reasons that (a) quenching can occur and (b) the approximate net steam generation are important elements for developing such guidelines. For example, the TMI-2 accident resulted in high temperature core debris draining into the RPV lower head (approximately 20 tons) with the resulting consequence that a significant part of the vessel lower head was heated to temperatures of approximately 1100°C. At this time, the lower plenum wall apparently cooled rapidly (10 to 100°C/min) and the vessel integrity was preserved. What was the reason that such cooling occurred? Is this specific to the TMI-2 design and accident sequence or is it more generic?

There is experimental evidence that a contact resistance, specifically a narrow gap, is formed when molten material contacts a metal surface which is submerged in water. If a narrow gap exists, what would be the cooling behaviour in this gap? Is such cooling important to preservation of the vessel integrity? To address these questions, fundamental experiments have been performed to assess the counter-current two-phase cooling rate in narrow annular gaps (1 to 3 mm) with an overlying water pool. These experiments were performed on concentric carbon steel pipes with initial temperatures of approximately 800°C; the cooling transient being monitored by thermocouples in the steel walls. The measured cooling transients show that significant cooling can occur in narrow gaps. Through such fundamental investigations, the importance of cooling in such narrow regions for reactor accident conditions can be assessed for different reactor designs and for various possible debris locations in these designs.