What is unique about MHI’s water jet peening system is that it can reach, and treat, areas inside the reactor vessel that are inaccessible to other mitigation methods, in particular bottom-mounted instrumentation nozzles (diameter: 10 mm) or the reactor vessel-main coolant pipe weld, underwater, with water.

The system directs a jet of high-pressure water to vulnerable or cracked surfaces. Manufacturer Mitsubishi Heavy Industries would not specify the exact pressure, but did say that the jet was ‘in the GPa’ range, so at least 145,000 psi. Cavitation bubbles are generated at the boundary between the high-pressure water and the surrounding water. The shock pressure caused by the collapse of these bubbles presses against the steel surface underneath. This cold work process tends to compress the surface, which makes it more resistant to crack formation. The system has the advantage of not needing to add any foreign materials to the cavity water.

In a statement, Rich Clemens, Wolf Creek’s vice president of strategic projects, said: "The water jet peening process provides an interior surface mitigation approach for reactor-vessel nozzle-dissimilar metal welds. This is a preferred approach due to our plant’s configuration and radiological conditions. Working together with Ameren Missouri is logical due to the common design of the Wolf Creek and Callaway plants. Additionally, we will be able to concurrently perform, within a reasonable outage schedule, WJP mitigation on the reactor vessel nozzles as well as on other less-susceptible materials and welds located in the lower region of the reactor vessel. We have confidence that the process will provide mitigation of potential stress corrosion cracking."

During an outage, following a pre-service inspection of the RV area, the rig is lowered from a temporary bridge structure to the site by an operating team of five to six workers. A remote-controlled positioning system places the nozzle in the target area with a certain required minimum stand-off distance from the surface. A water jet peening application on the inner and outer surface of for example a bottom-mounted instrumentation nozzle can take one or two hours. An external pump pressurizes the reactor cavity water, which is carried to the target through hoses (borated water works just as well). During the work, parameters such as flow rate, time, distance and angle are monitored. After the work is done, workers inspect the target to confirm there are no deformations.

MHI, the primary reactor vendor of Japanese PWRs, began development of the process in 1995, when primary water stress corrosion cracking in Alloy 600 started to be noticed. Since then, it has carried out 45 applications in 21 PWRs: Kansai’s Mihama 1-3, Takahama 1-4, Ohi 1-4; Kyushu’s Genkai 1-4 and Sendai 1-2; Hokkaido’s Tomari 1-3 and JAPC’s Tsuruga 2. (In fact the only Japanese PWRs not treated are Shikoku’s Ikata 1-3 reactors).

In the USA, the process will be managed by MHI Nuclear Energy Systems as prime contractor. It is MHI’s wholly-owned subsidiary that offers reactor services and is working toward licensing of MHI’s advanced pressurized water reactor (APWR) design, as prime contractor. MHI NES will also supply equipment and develop an implementation plan. Field work will be carried out by a consortium of services companies AZZ WSI and Structural Integrity Associates (SI). SI will also provide engineering and licensing support. Work is planned to be carried out in 2016.