Patent Number: 053496140
Section: summary

FIELD OF THE INVENTION This invention generally relates to an apparatus for installing a plug in a line which communicates with a vessel. In particular, the invention is directed to an apparatus for installing a plug in the steam outlet nozzle of a boiling water reactor ("BWR"). BACKGROUND OF THE INVENTION During disassembly of a BWR, the steam outlet nozzles must be plugged to allow maintenance and testing of the main steam isolation valves in parallel with reactor refueling and servicing operations. The steam line plugs are used to seal the steam lines to prevent the flow of water from the reactor cavity during servicing of the safety valves, relief valves and main steam isolation valves. Historically, plug installation has been performed using an overhead crane and service poles manipulated by hand. This conventional operation is attended by problems. First, this current method of installing the plugs requires that personnel be located in the reactor cavity to position and maneuver the steam line plugs as they are being lowered by the overhead crane. Second, the current method of installing the plug requires that personnel push the plug into the steam outlet nozzle from the opposite side of the reactor vessel using a pole. Third, the steam line plug installation envelope should allow for unobstructed removal of the steam separator. Conventional installation tools interfere with or obstruct the removal of the separator and must be disengaged from the plug and removed after the plug has been installed. SUMMARY OF THE INVENTION The invention is a steam line plug installation tool which overcomes the foregoing disadvantages of the conventional apparatus. Using the installation tool of the invention, the steam line plug can be installed underwater by personnel standing on the refueling bridge. The tool inserts the steam line plug into the steam outlet nozzle using a scissors jack mechanism actuated by a lead screw operated using a grapple and a service pole. The installation tool is designed to position the plug at the proper elevation and azimuth using the reactor vessel flange and head closure stud as references. No personnel are required at the vessel flange to position the tool, thereby reducing the exposure of personnel to radiation. In addition, the plug installation tool envelope is such that the tool can remain in place after installation of the plug and still allow removal of the steam dryer and separator to their underwater storage pool. The steam line plug installation tool in accordance with the preferred embodiment of the invention comprises a strongback assembly on which the plug is securely mounted, a collapsible structure (e.g., a scissors jack assembly) which supports the strongback assembly and is movable between an extended state and a collapsed state, a hanging bracket assembly for hanging the collapsible structure inside the vessel, and a rotatable actuating screw coupled to the collapsible structure such that the latter moves between the extended and collapsed states in response to rotation of the actuating screw. During this operation, the plug is carried from a position outside and aligned with the vessel orifice to be plugged to a position inside the orifice. In accordance with the preferred embodiment, the plug installation tool further comprises a mechanism for adjusting the elevation of the collapsible structure relative to the hanging bracket assembly and a mechanism for adjusting the elevation of the strongback assembly to the collapsible structure. The latter mechanism is operated via a rotatable actuating screw. Both actuating screws are turned remotely using a conventional service pole. The plug installation tool also includes a guide plate which cooperates with the reactor head studs to place the tool at an azimuth corresponding to the azimuth of the orifice to be plugged. The tool is installed using a grapple which couples to a rotatable handling bracket assembly. The handling bracket is locked in a first angular position by a releasable pawl. After the plug has been installed in the orifice, the pawl is released remotely using a service pole. The handling bracket is then rotated to a position at which removal of the steam separator assembly from the reactor vessel will not be obstructed.