Structural Integrity Associates, Inc. (SI) is nearing the end of a three year, successful effort funded by the Electric Power Research Institute (EPRI) to lead a diverse team of engineers and scientists from across the United States in performing metallurgical testing and evaluation of an irradiated material sample removed from an operating Boiling Water Reactor (BWR). Our team, composed of experts, from SI, BWXT NOG Technologies, Inc., Pacific Northwest National Laboratory and EPRI investigated the nature of off-axis cracking observed in the beltline region of the core shroud in a domestic BWR. It is a rare and expensive occurrence to remove and test in-vessel specimens. Consequently, selecting qualified vendors capable of performing work on highly irradiated materials, possessing appropriate knowledge of degradation mechanisms relevant to the BWR environmental conditions, and having the ability to interpret the significance of the data and relevance of the findings to the operation and management of the BWR fleet, is critical. Selection of the SI team as the most qualified team to perform this work is notable since numerous organizations from around the world, including original equipment manufacturers and scientific institutions, had submitted proposals for consideration.
The experimental program was performed to investigate the likely cause of the off-axis cracking observed in the domestic BWR core shroud. The cracking was considered off-axis since it exhibited visual characteristics considered unusual for intergranular stress corrosion cracking (IGSCC) commonly observed in the BWR core shroud weld heat affected zones (HAZs). Specifically, the off-axis cracking exhibited the following unusual characteristics:
■ Propagation transverse to the weld rather than parallel to the weld HAZ,
■ Propagation into the non-sensitized base material generally considered not to be susceptible to IGSCC since this material would not have been weld sensitized, and
■ Propagation through the weld material generally considered to be more resistant to IGSCC.
The fundamental question to answer was whether the cracking was attributable to irradiation-assisted stress corrosion cracking (IASCC) or whether it was sensitization related IGSCC. The significance of this question lies in the possible implications that IASCC initiation and growth into base material could have on the manner in which the industry manages aging of the core shroud assemblies.
To answer these and other questions, the SI team, with valuable contributions from EPRI and its individual consultants, initiated a multifaceted experimental program consisting of optical metallography, scanning electron microscopy (SEM), analytical transmission electron microscopy (ATEM), mechanical property measurement (tensile and fracture toughness testing), hardness testing, retrospective dosimetry measurements, and material composition confirmation using inductively coupled plasma mass spectrometry (ICP-MS). Specimens were machined at the BWXT facility in Virginia and tested both at BWXT and at the PNNL facility in Washington state.
Review of the body of data collected showed that while there was clear evidence of radiation damage to the material such as radiation-induced segregation (RIS) of nickel and chromium along the grain boundaries and elevated through-thickness hardness resulting in radiation-induced susceptibility, there was also strong evidence of elevated surface hardness. This could have been the result of a surface mechanical treatment such as grinding that would also have caused the material to be susceptible to IGSCC initiation due to cold work. This observation, taken with the clear evidence of intergranular cracking as shown in the photomicrographs, suggested that the cracking was SCC and that the initiation mechanism could not conclusively be distinguished between cold work-induced IGSCC that had initiated earlier in operation or IASCC initiated later in operation.
We considered it probable that the cracking initiated earlier in plant operation as a result of surface cold work, grew under the influence of locally high weld residual stress and propagated into the base material as the cumulative neutron fluence made the material microstructure susceptible to SCC as a result of radiation-induced segregation and hardening.
The experimental results of this study, when combined with the results of related work on the topic, supported the conclusion that the existing core shroud aging management guidelines documented in BWRVIP-76, and augmented by the off-axis cracking inspection and evaluation guidelines, published by EPRI in 2016, provide an effective basis for BWR core shroud aging management.