FLAW CRITICALITY ASSESSMENT FOR CORE SPRAY SPARGER SUPPORT BRACKET SHROUD CRACKING

摘要

Stress corrosion cracking (SCC) has been observed in the weld joining the core spray sparger bracket to the core shroud at a number of Boiling Water Reactors (BWRs). The cracking occurs preferentially on the shroud side of the weld and has typically been detected with visual inspections. Limited ultrasonic (UT) inspections have been performed at selected plants. EPRI BWRVIP investigations showed that the cracks are relatively shallow and oriented in the radial direction (with respect to the shroud). This report documents results of a project performed by LPI (Lucius Pitkin, Inc.) whose objective was to develop a generic fltness-for-service evaluation for the observed cracking Deterministic SCC crack growth calculations were performed associated with cracking observed in the field. The extent of crack growth was based on the 95 percentile K-dependent the crack growth rate (da/dt) relationship for the BWR shroud materials identified in BWRVIP 14A. The calculations also considered the changes in material properties and crack growth rates as the crack extends beyond the weld HAZ within the shroud thickness. The results demonstrate that the cracking is essentially benign and that extraordinary efforts to characterize fully the cracking are unwarranted. These results will be used as part of the technical basis for revised BWRVIP guidance for inspection of the sparger brackets.