Leak-before-break analyses of PWR and BWR piping concerning size effects

Abstract

Components of primary circuit piping of light water reactor (LWR) plants fulfil high safety requirements. Beside prevention against any damage, cracking, and leaking, pipes are especially designed to exclude double-ended ruptures, which may cause a high loss of coolant. One option in this strategy, supported by regulations and standards in several countries, is the leak-before-break (LBB) behavior: a crack-like leakage should be detected before it becomes a break.Evidently, many factors influence the LBB behavior of a specific piping section. Material properties, plant operation parameters, assumed accident scenarios and the local geometry determine the result of a LBB analysis as well as the associated safety margins. Therefore, an analysis is traditionally done case by case, and site by site, according to the applicable standard.In this article, typical components of pressure water reactor (PWR) and boiling water reactor (BWR) primary circuits are examined, and their LBB behavior is investigated. The German standard (KTA) and the US approach (U.S.NRC) are compared. The quantitative results are obtained with the GRS codes PROST and WinLeck, which include different methods for fracture assessment and leak rate calculation. A clear trend concerning the pipe size is found, resulting in large safety margins for large-diameter piping.