Patent Number: 048308145
Section: summary

In the refueling of a nuclear reactor, the primary time consuming procedure is the removal of the upper structure or head package of the reactor. In conventional reactors, the head package includes the pressure vessel head which seals the reactor vessel, control rod drive mechanisms which are used to raise and lower control rods in the core of the reactor, a seismic platform adjacent the upper ends of the control rod drive mechanisms, which laterally restrains the same, and various cables for operation of the control rod drive mechanisms. A missile shield, in the form of a concrete slab, is positionable above the head package to protect the containment housing and associated equipment from penetration by any of the control rod drive mechanisms in the event of a major break. The problems associated with such conventional head packages are further described in U.S. Pat. No. 4,678,623, and assigned to the assignee of the present invention, which patent is incorporated by reference herein. In such conventional plants, the large concrete slabs installed above the reactor vessel to act as a missile shield must be removed and stored prior to head disassembly and refueling of the reactor, and then must be replaced after the refueling and head reassembly. Such operations effect overall refueling time and radiation exposure and require space in the containment area for placement of the missile shield slabs when removed from the position above the reactor vessel. In order to reduce refueling time, personnel exposure and space requirements, an improved system, designated as an integral head package plant has been developed which incorporates an integral missile shield and head lift rig. The missile shield is in the form of a perforated circular plate which is directly attached to a head lift rig. Such an integral head package (IHP) system is described in British Pat. No. 2100496 issued to the assignee of the present invention, and published Dec. 22, 1982, which application is incorporated by reference herein. As described therein, and illustrated in FIG. 1 of the present drawings, an integral head package 1 includes a three-legged head lifting rig 3 that is pin connected at 5, by lift lugs 7, to a missile shield assembly 9. The perforated circular plate 11 that forms the missile shield acts as a spreader for the head lift load, and as a seismic support for the tops of the control rod drive mechanisms 13, with rod travel housings extensions 15 of the mechanisms protruding through apertures 17 in the circular plate 11. The missile shield 11 interfaces with the tops of the rod travel housings 15 which limits the overall vertical travel (and impact force) of a missile before it impacts the shield. The impact load of a missile against the underside of the perforated plate 11 is transmitted to head lift rods 19, through vessel head lift lugs 21 secured to the vessel head 23, and closure studs 25 to the vessel head 23, and ultimately to the vessel supports. A cooling shroud 27 surrounds the drive rod mechanisms 13, while electric cabling 29 is routed from the top of the control rod drive mechanisms 13 to a connector plate 31 and thence along a cable tray 33 to respective cable terminations. Cooling fans 35 circulate air within the shroud 27 to transfer waste heat from the control rod drive mechanism 13. Hoist supports 37, and trolleys 39 on hoist assemblies 40 are used to position stud tensioner tools and stud removal tools during refueling operations. While the integrated head package is a marked improvement over the conventional head package designs, and adaptable for retrofitting existing reactors or incorporation into new reactor designs, additional problems arise in connection with advanced pressurized water reactor (APWR) systems. In advanced pressurized water reactor systems, displacer rods are interspersed throughout the control rods in the reactor core and displacer rod drive mechanisms, as well as the control rod drive mechanisms, are required above the reactor vessel head. As an example, an advanced pressurized water reactor plant would use a moderator control core which requires the use of 185 drive mechanisms (97 control rod drive mechanisms and 88 displacer rod drive mechanisms). This results in a much more congested upper head area (the area above the pressure vessel head), whereas conventional and integrated head package earlier designs used approximately 50 to 60 control rod drive mechanisms only. Increased seismic requirements, along with the design of the displacer rod drive mechanisms which is not compatible with current seismic sleeve designs, would require a much stronger (i.e. larger size and/or higher strength material) seismic sleeve design. This, coupled with the increased number of drive mechanisms would only tend to increase the space limitations which already exist. In addition, the vertical missile travel distance (before impact) for an advanced pressurized water reactor is much larger (35 to 37 inches) whereas other integral head package designs have a much shorter missile travel distance (about 5 inches). This increased missile travel distance results in an increased missile load, which, if transferred directly to the lift rods, to which a missile shield is attached, would require a substantial increase in the lift rod diameter. It is an object of the present invention to provide an improved integrated head package for a nuclear reactor that does not transfer an impact load from a missile, striking the underside of a missile shield plate, to the lift rods and ultimately to the head lugs, closure studs and vessel supports of the reactor system. It is another object of the present invention to provide an improved integrated head package for a nuclear reactor that can be used as a retrofit on existing reactors or incorporated into new reactors such as advanced pressurized water reactors. It is a further object of the present invention to provide a nuclear reactor having an integrated head package that eliminates the need to increase the size of lift rods to absorb the impact of missiles and eliminate the need for seismic sleeves about drive rod mechanisms of a reactor system. SUMMARY OF THE INVENTION An integrated head package for a nuclear reactor has a missile shield plate that is vertically slidably retained on lift rods that are secured to the reactor pressure vessel head. The package includes a pressure vessel closure head that seals the reactor vessel, and control rod drive mechanisms, and displacer rod drive mechanisms for advanced pressurized water reactors, that are enclosed in a shroud, with the lift rods extending vertically from the pressure vessel closure head and having spaced stop members adjacent the upper ends of the lift rods The missile shield plate extends between the lift rods, above the drive rod mechanisms, and is vertically slidably retained between the spaced stop members, and has a lift rig secured thereto. The stop members on the lift rods preferably comprise a lower flanged member that is fixedly secured to the lift rod and an upper nut that is threadedly secured to the upper portion of the lift rod. A recess may be provided in the underside of the missile shield plate for seating therein of the lower stop member. The missile shield plate has thereon support blocks to which a support system, such as a tripod support system having a lift ring at the upper end thereof is secured by a clevis and pin securement. A collar, formed as a plate, is provided on each of the control rod drive mechanisms and displacer rod drive mechanisms, the collars adjacent the upper ends thereof and lying in a common horizontal plane so as to provide a seismic plate for the drive rod mechanisms.