Most of the nuclear power plants(NPPs) now in service in Japan are based on either of two reactor technologies — PWR and BWR. While the PWR plant requires steam generators(SG), the BWR plant needs no separate SG because of its capability to produce steam directly. On one hand, the need for installation of SG makes the size of the PWR plant larger than that of the BWR plant, while on the other, separate SG installation offers an advantage in that the controlled area in the PWR plant can be limited, resulting in easier plant maintenance, compared with the BWR plant. Accordingly SG maintenance to preclude radioactive substances from flowing out into the secondary system is one of the most important tasks in operating PWR plants.