Patent ID: 8325872

Claim:
A working method for doing works under an annular shroud support plate arranged substantially horizontally between an inner surface of a nuclear reactor pressure vessel and an outer surface of a shroud, the method comprising: conveying a working equipment with a folding/unfolding mechanism, a ballast tank, a vertical thruster and a horizontal thruster to a working position with the folding/unfolding mechanism held in a folded state, the conveying step including: suspending the working equipment from above the nuclear reactor pressure vessel; injecting water into the ballast tank to reduce buoyancy of the ballast tank; injecting air into the ballast tank to increase buoyancy of the ballast tank; thrusting the working equipment vertically by the vertical thruster for going down lower than the shroud support plate; and thrusting the working equipment horizontally by the horizontal thruster for going below the shroud support plate; unfolding the folding/unfolding mechanism and setting up the working equipment under the shroud support plate after conveying the working equipment to the working position; moving the set up working equipment along a lower surface of the shroud support plate and positioning the working equipment, while pressing the working equipment against the lower surface of the shroud support plate; and doing a work on the shroud support plate with the moved and positioned working equipment.