Root Cause Analysis of Cracking in Alloy 182 BWR Core Shroud Support Leg Cracks

Cracks were found in the Alloy 182 weld and buttering of two core shroud support legs in the Forsmark BWR Unit 1. This paper presents the root cause analysis based on mechanical and metallurgical analysis by Microscopy, Electron Backscatter Diffraction, and micro-and nano-indentation analysis along the crack path. The High Resolution Analytical Transmission Electron Microscopy examinations on the oxides are presented in detail in another paper at this conference. The morphology of the cracks, local hardness, crystallographic orientations and compositions along the crack path and crack tip regions were examined. The crack penetrated the boat keel surface into the A182 Low Alloy Steel mixing zone. Cracking was identified as IGSCC and signs of grinding and elevated micro-hardness near the boat top surface region were detected. Plastic deformation was observed in a crack tip region in a sample section with the largest crack depth, while not at the crack periphery.