OPENING AND CLOSING DEVICE OF ACCESS AREA OF NUCLEAR REACTOR VESSEL, AND MAINTENANCE METHOD OF SEAL MEMBER IN ACCESS AREA OF NUCLEAR REACTOR VESSEL

PROBLEM TO BE SOLVED: To provide means in which the sealing effect reduced by adhesion of foreign matter onto the surface of a seal member for sealing between a lid body to open and close an access area and the inner peripheral face of a frame is restored to the original state in a short time.SOLUTION: An opening/closing device includes: a lid body 16 capable of opening and closing an access hole 12; a ring member 25 detachably fitted to a fitting recess 29 formed around the access hole 12 of a frame 8; an O-ring 26 having large diameter end face and an O-ring 27 having small diameter end face which are provided at the ring member 25 and seal between the ring member 25 and the lid body 16; and a peripheral face O-ring 28 which is provided at one of the ring member 25 or the fitting recess 29, and seals between the ring member 25 and the fitting recess 29.