Effects of Multiple Co-Linear Flaws on Crack Opening Area

Cracking in boiling water reactor (BWR) core shroud welds has been identified in operating nuclear plants worldwide. The nuclear industry has taken extensive efforts to disposition and evaluate core shroud cracking, most notably within the BWR Vessel and Internals Project (BWRVIP) where many industry guidance documents have been published regarding core shroud integrity [1, 2, 3, 4]. This guidance is predominately focused on evaluating crack stability. Calculating through-wall leakage was not previously a focus of the existing BWRVIP inspection and evaluation (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 co-linear, short, flaws. 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 co-linear, through-wall cracks are larger than that calculated for a single through-wall crack. Developing an understanding of the effect of co-linear 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 co-linear 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. The purpose of the study documented in this paper is to characterize the COA for axial co-linear crack distributions compared to the COA of an individual crack. Cracks that are aligned in series with an uncracked ligament between them are considered to be co-linear. To better understand how these crack distributions behave, an evaluation is conducted to analyze axial co-linear flaw configurations in core shrouds using traditional linear-elastic fracture mechanics (LEFM) and finite element analysis (FEA) techniques. Through FEA, the COAs and displacements of various co-linear flaw configurations are calculated and compared to the COAs for single flaw configurations. These flaw geometries are useful for the purpose of determining the potential core leakage associated with through-wall co-linear cracks. Co-linear crack configurations for a range of crack sizes and geometries are parametrically evaluated based on the ligament length between the co-linear cracks. Results show that crack openings of co-linear flaw configurations compared to a single flaw can vary substantially depending the crack size and ligament length. Trends of these crack openings are summarized within this report. 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.