Discussions on loop seal behavior during cold leg small break LOCAs of a PWR

Abstract

This paper analyzes loop seal clearing behavior during small break LOCA transients to point out a possible uncertainty in the process. Further it also discusses core mixture level behavior during the small break transients. Energy balances in the broken and intact loop identify a difference in the mixture level drop velocities between the broken and intact loop. A hypothesis in the velocity fluctuation intensity as a function of a break size is proposed. This leads to a conclusion that, for certain break sizes, it is not certain which loop seal will pass steam to the break first during the transient. This fact has a very important implication in interpreting test data or numerical simulations with a lumped intact loop. The amount of water delivered to the core/vessel region after the loop seal clearing may differ significantly depending on whether the water is from the single broken loop or the lumped intact loop.