Development of remote-controlled lapping machine for main steam isolation valve

In the BWR power plants in operation in Japan, two isolation valves connected in series in each main steam line are used to contain radioactive material within reactor containment and pressure vessels at the time of a major accident. To minimize leakage from closed valve seats, lapping of the valves and seats is performed regularly. Since the valve stems incline with respect to the steam line, there are a number of problems in the maintenance of the valves. To perform the lapping smoothly, a remotely-controlled lapping machine has been developed, which consists of a lapping mechanism, a control panel and cables and working-gas hoses connecting them. It will actually be used shortly in a BWR plant. The following matters are described: the present state of repairing works, fundamental conditions in the design of the lapping machine, the course taken in its development, the lapping machine developed, and simulation test. (Mori, K.)