Water Jet Peening (WJP) is an approved mechanical mitigation technique that has been widely applied in Japanese nuclear power plants since 1999. WJP suppresses stress corrosion cracking (SCC) initiation in SCC susceptible materials which have been widely used in the fabrication of reactor internals .
Laboratory data shows that WJP is effective at mitigating or suppressing shallow crack propagation if the post application compression depth layer is deeper than the original crack depth and that WJP has no adverse effect on pre-existing cracks, such as crack propagation or acceleration of the crack propagation during and after WJP . In addition, it has also been reported that WJP has no adverse effect on Ultrasonic testing (UT) sizing capability for pre-existing cracks in 304S.S. and Alloy 182 .
Based on these evaluation test results, Hitachi-GE Nuclear Energy, Ltd (HGNE) successfully applied WJP on the cracked surfaces of reactor internals in an operating Japanese nuclear power plant in 2011 .
Recently, to evaluate the applicability of this process in a PWR power plant, HGNE has performed an evaluation of Ultrasonic testing (UT) and Eddy Current Testing (ECT) sizing capabilities for surface cracks on a dissimilar metal weld coupon that had been mitigated utilizing the WJP process. No significant differences in their sizing capabilities were observed between the before and after WJP treatment test coupons. During this evaluation, it was also confirmed that surface residual stress on the coupon was improved from tensile to compressive (from max approximately 800MPa to min approximately −600MPa).
As a result of this evaluation, it was determined that WJP has no adverse effect on UT and ECT sizing capabilities. In addition, HGNE has taken this knowledge and applied it to the development of a WJP tool for PWR Bottom Mounted Instrumentation (BMI) nozzles, which accommodates all the parameters used for this testing and enables HGNE to reduce the total application time as a result of the inclusion of advanced technological improvements.