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Water Jet Peening
Water Jet Peening (WJP) is a mitigation technique for Stress Corrosion Cracking experienced in reactors caused by superposition of tensile stress, material, and environment. The WJP process offers a solution to this problem by improving the residual stress by generating cavitation at the boundary between high speed water flow from WJP nozzle and the surrounding water. This creates plastic deformation using shock pressurefrom the collapse of the cavitation bubbles to change surface condition to alleviate this stress.
The entire process can be conducted underwater during an outage and uses only water to relieve the stress. This is a significant benefit for the technology in that no foreign materials are introduced into the reactor and no heat is applied to the material. Also, because the easy-to-operate equipment can be controlled remotely under water, the occupational dose exposure is extremely low.
Mitsubishi has extensive successful field experience in WJP applications as well as extensive technical analysis resulting in a high level of confidence in the process’s reliability. The process has been applied in various configuration locations, such as the RPV Nozzle (large bore), SI Nozzle (small bore), BMI-ID (tiny pipe), and BMI J-weld (complicated). Additional locations, such as the Core Flood Nozzle, are also possible with this maneuverable technology.
The application of Mitsubishi’s WJP technology has gained approval for extended inspection intervals in Japan and is currently being studied for the same benefit by MNES and EPRI for the US. In high temperature environments, the effects of WJP will last at least 30 years, but the residual stress improvement has been studied to be sustainable for 60 years in high temperature, cyclic stress T-hot conditions.