Advanced INLAY System for Inlet/Outlet Nozzles of RV for Preventive Maintenance Against Alloy 600 PWSCC in Japanese PWR Plants

Abstract

Since PWSCC has been observed in Alloy 600 used for butt welds between the low alloy steel RV-nozzles and stainless steel pipes in recent years, MHI has developed the inlay technology for outlet/inlet nozzles as a preventive maintenance and repair method. The inner surface, even if it has PWSCC, is exchanged from Alloy 600 to Alloy 690 which has excellent PWSCC resistance, by applying inlay. This paper introduces the experiences of the MHI inlay System (RV-INLAY) and describes the merits as a counter measure against PWSCC. MHI has adopted the Cylindrical container for RV-INLAY. The Cylindrical container is installed into RV when the cavity is filled with water, then we can secure an atmospheric working space inside outlet/inlet nozzles. 3 or 4 manipulators are installed on the bottom of the container and carry out precise works like ambient temperature temper bead welding, UT, ECT, dimensional or visual inspection exchanging many kinds of tools (end-effectors) on the tip of the manipulators remotely. Heavy devices for machining, blast-decontamination or nozzle-plug handling are carried down into the container and set remotely using the special handling crane. Manipulators and other heavy devices move remotely to reduce the radiation exposure and simultaneously in 3 or 4 nozzles to shorten the working period. By using the Cylindrical container, we can directly access the inside outlet/inlet nozzles. This is different from the other preventive maintenance methods which need to access from outside of outlet/inlet nozzles. We can apply RV-INLAY from inside the nozzles without considering the difficulty in access due to the tight space and high radiation outside the nozzles. Furthermore when a RV is manufactured in a factory, we apply PT for the groove face before welding and the surface after welding. During RV-INLAY operation, we apply PT in the same manner because we can secure an atmospheric working space inside outlet/inlet nozzles. As a result, we can ensure the quality of welding same as manufacturing process in a shop. In addition, we don’t have to consider the crack propagation after RV-INLAY and we can perform ISI without preparing repair equipment against PWSCC. The Cylindrical container has a seal function same as the seal-plate used in usual outages, and it can separate the water inside the cavity from RCS. While the Cylindrical container is installed into RV, the RCS is drained and RCS piping works can be performed in parallel. In fact, we have shortened the outage duration by implementing RV-INLAY and RCS piping works in parallel. This is one reason why MHI has not selected under water welding but RV-INLAY.