Effects of Multiple Parallel Flaws on Crack Opening Area

Abstract

Cracking in boiling water reactor (BWR) core shroud welds has been identified in operating nuclear plants worldwide. The Boiling Water Reactor Vessel and Internals Project (BWRVIP) has published several reports providing inspection and evaluation (I&E) guidance for intergranular stress corrosion cracking (IGSCC) in the core shroud of BWRs. This guidance is predominately focused on evaluating crack stability. Calculating through-wall leakage was not previously a focus of the existing BWRVIP I&E guidelines for the core shroud; however, there is some guidance in the current documentation. In recent years there has been some evidence of through-wall indications in the core shroud where the through-wall indications were aligned in an array of parallel, short, flaws. BWRVIP-158-A contains rules for treating parallel flaws with respect to calculation of structural margin for both net section collapse (limit load) and brittle fracture (linear elastic fracture mechanics (LEFM)) failure modes. There is currently no BWRVIP document or other open literature, to the authors’ knowledge, that provides insight into whether the crack opening displacements (CODs) for an array of parallel through-wall cracks are larger than that calculated for a single through-wall crack. Developing an understanding of the effect of parallel cracks on the CODs and subsequent crack opening areas (COAs) of each crack is important in augmenting the existing guidance on how to appropriately disposition through-wall cracking in reactor internal components. Specifically, it is important to know if multiple parallel cracks can lead to individual COAs that are larger than for a single crack of the same length, in order to perform accurate leakage rate calculations. This paper documents linear-elastic finite element analyses (FEA) performed to study the effect of a parallel crack configuration on the resulting COA for the set of cracks compared to the COA calculated if each crack was treated as an individual crack, without adjacent cracking present. Various separation distances, number of crack cases and crack lengths are considered. While the object of this work is to provide criteria for the evaluation of reactor internals, the results can be applied to evaluate COD and COA in any component for which the cracking configuration and inherent assumptions of LEFM are applicable.