Nickel Alloy Crack Growth Correlations in BWR Environment and Application to Core Support Structure Welds Evaluation

Abstract

An intergranular stress corrosion cracking (IGSCC) growth model for unirradiated nickel-base alloys (Alloys 82, 182 and 600) in boiling water reactor (BWR) environments has been developed by EPRI. This model has been used for assessment of the crack growth rates in BWR nickel base austenitic alloys with particular application to the BWR shroud support structure materials and welds, including attachments to the reactor pressure vessel fabricated from these alloys. However, the crack growth model can be used for other components with like materials in BWR environments provided that specific parameters such as stresses and stress intensity factor (KI) distributions are determined. The methodology involves development of crack growth disposition curves that can account for the variability of important IGSCC parameters to provide a conservative, yet realistic assessment of crack growth rate in BWR environments. An extensive nickel base alloy crack growth rate database was developed from data generated through the peer review process and includes both experimental data points and in-plant crack arrest verification system data. Most of the data in the database have reasonable definition of environmental conditions and other important crack growth parameters thus permitting a more realistic generic crack growth model to be developed. Although most of the data is for Alloy 182, it bounds the crack growth rate of Alloy 82 and Alloy 600. The database was used to derive crack growth disposition curves under normal water chemistry (NWC) and hydrogen water chemistry (HWC) conditions. The disposition curves have two stress intensity regimes; one for KI < 25 ksi√in where the crack growth is KI-dependent and one for KI > 25 ksi√in where the crack growth is KI-independent. The crack growth disposition curves were used together with a crack growth estimation methodology to determine the crack propagation of the BWR shroud support structure welds which are fabricated from Alloy 82/182. The steps involved in the development of the methodology include determination of residual stresses and operating stresses, development of stress intensity factor (KI) solutions for crack propagation in the through-thickness direction and estimation of crack growth rates. This methodology was applied specifically for crack growth in the through-thickness direction. Application of this crack growth model to BWR shroud support structure welds H8 and H9 indicates that there is an adequate time period between inspections before initial cracks of ≤10% through-wall thickness reaches the allowable flaw sizes, particularly for HWC conditions.