Integrated head assembly for a nuclear reactor

An integrated head assembly (100) is disclosed for a nuclear reactor. The preferred integrated head assembly includes a lift assembly (150) that supports the reactor vessel closure head (90) and integrated head assembly for removal, a separate support structure (202) supported by a ring beam (151) that sets atop the reactor vessel closure head, a shroud assembly (200), a seismic support system (300), a baffle assembly (500), a missile shield (400), and a CRDM cooling system. The CRDM cooling system draws cooling air into the baffle assembly, downwardly past the CRDMs (96), outwardly to upright air ducts (600), upwardly to an upper plenum (680), and out of the assembly through the air fans (190). In a second embodiment the integrated head assembly (1100) includes a missile shield (1400) and CRDM cooling system (1600) that permits access to individual CRDMs from above.

FIELD OF THE INVENTION

This invention relates to reactor vessel closure head assemblies and, in particular, to an integrated head assembly for a pressurized light water reactor.

BACKGROUND OF THE INVENTION

In a typical pressurized water reactor (PWR) power plant, various mechanical components and systems are installed on the reactor vessel closure head. These mechanical components and systems include, for example, a control rod drive mechanism (CRDM) cooling system, a reactor vessel closure head lift rig, CRDM seismic restraints, and a CRDM missile shield. Each of these components is typically designed and installed as a permanent fixture to perform designated functions during plant operation. However, during refueling of the reactor these components have to be disassembled in order to remove the reactor vessel closure head from the reactor vessel. These components are stored in designated storage areas, generally inside the reactor containment. Typically, in a PWR plant, a series of steps are followed before the reactor vessel closure head is removed from the reactor vessel. The operational steps that are performed prior to detensioning the reactor vessel closure head studs include some or all of the following:Remove and store heavy concrete missile shields.Remove and store the CRDM cooling ducts.Remove the seismic restraints.Disconnect and store the CRDM power and rod position indicator cables.Install the reactor head lifting rig tripod.Remove cable trays and cables running from the reactor head to the operating deck or walls.Disconnect heated junction thermocouples, nuclear steam supply system instrumentation, monitoring system cables, and reactor head vent lines.Install temporary lead shield blankets around the vessel closure head area.The procedure also requires that the nuts and washers be removed from the reactor vessel closure head and placed in storage racks during preparation for refueling. The storage racks are then removed from the refueling cavity and stored at convenient locations inside containment prior to reactor vessel closure head removal and refueling cavity flooding. The above steps are then reversed while reinstalling the reactor vessel closure head and the related reactor systems.

Each of these steps contributes significantly to the total cost associated with refueling the reactor. The total costs include costs associated with personnel man-hours required to perform the refueling, power plant down time and consequent loss of electricity production, radiation exposure to personnel, and potential human errors. In addition, the various components that must be removed for refueling activities require a large amount of the limited storage space available inside containment and raise the risk of inadvertent contamination of work and storage areas.

Concepts and designs for integrating some of the reactor vessel closure head systems into a modular integrated head design have been proposed. For example, in U.S. Pat. No. 4,678,623 to Malandra et al., a modular head assembly is disclosed wherein vertical lift rods are attached to the reactor vessel lifting lugs, and a missile shield, seismic support platform, CRDM cooling system, and lift rig are supported by the lift rods above the reactor vessel closure head. Because most or all of the modular head assembly taught by Malandra et al. is supported by the lift rods, however, very large loads are concentrated at the clevis connection at the reactor vessel closure head lifting lugs, which may cause damage to the lifting lugs and/or the body of the reactor vessel closure head. In addition, very heavy components such as the missile shield and the fans are supported at the distal ends of three relatively long lift rods, resulting in an unstable structure that may subject the lift rods to undesirable compressive, bending and torsional stresses. Malandra et al. also does not provide a structure for putting a shroud around the CRDMs.

In U.S. Pat. No. 4,830,814, Altman discloses an integrated head package having a missile shield that is slidably mounted near the distal end of three lift rods connecting to the reactor vessel closure head lifting lugs. A shroud is shown disposed about the CRDMs. Similar to the apparatus disclosed by Malandra et al., however, the heavy missile shield and lifting rig are installed at the distal end of three elongate lift rods that are connected at their proximal end to the reactor vessel closure head lifting lugs. The Altman apparatus, therefore, will also produce relatively high local loads in the reactor vessel lifting lugs and head. Altman also does not disclose any system for cooling the CRDMs.

Some commercial light water reactors—for example, pressurized water reactors produced by Babcock & Wilcox (B&W)—have a reactor vessel closure head having inverted L-shaped flanges that extend upwardly from the reactor vessel closure head. Many B&W reactors also employ a control rod design wherein the lead screw from each control rod must be decoupled from the control rod and parked before the reactor vessel closure head is removed from the reactor vessel. In order to decouple and park the control rod lead screw, a 15-foot tool is typically inserted from above into the CRDM housing. For these types of commercial reactors, therefore, significant overhead space, or headroom, is required above the reactor vessel to accommodate the control rod tool, prior to removing the reactor vessel closure head. To provide the necessary head room, various components disposed above the reactor may need to be disassembled, removed, and stored before the control rod lead screws can be decoupled and parked and the closure head removed.

There is a need, therefore, for an integrated head assembly for a pressurized water reactor that can be removed from the reactor vessel integrally with the reactor vessel closure head, and that does not introduce undue local stresses at the reactor vessel closure head and lifting lugs.

SUMMARY OF THE INVENTION

The present invention is directed to an apparatus and method that satisfies this need. The apparatus includes an integrated head assembly for a pressurized light water nuclear reactor having a lift assembly that engages the lifting lugs on the reactor vessel closure head. A support structure is provided above the reactor vessel closure head with a shroud assembly and a baffle structure attached thereto. At least one upwardly extending duct for a CRDM cooling system is also provided. The apparatus includes a seismic support system and a missile shield attached to the support structure and disposed generally above the control rod drive mechanisms. At least one cooling air fan is fluidly connected to the duct.

In an embodiment of the invention, the duct is cooperatively formed by the baffle and the shroud assemblies.

In an embodiment of the invention, the support structure includes a ring beam with a number of saddle members that are disposed atop the reactor vessel closure head. The ring beam may be formed from three annular segments that are joined end to end. The support structure may also include a cylindrical support grid that extends upwardly from the ring beam. The shroud assembly may also comprise multiple axial segments and provide air inlet port(s) for the air cooling system. In a disclosed embodiment, the air cooling system includes an upper plenum interconnecting three cooling fans and two vertical ducts.

An embodiment of a method for retrofitting a pressurized water nuclear reactor according to the present invention includes shutting down the nuclear reactor and removing the reactor vessel closure head from the reactor vessel and placing it on a reactor head stand. Lift rods are then attached to the lifting lugs on the reactor vessel closure head. An integrated head assembly module is then installed, the module including a ring beam that rests atop the reactor vessel closure head, a shroud assembly that sets atop the ring beam, and a baffle assembly attached to the shroud assembly. A seismic support system is then connected to the control rod drive mechanisms and a missile shield is installed above the CRDMs. A lifting assembly is then operatively attached to the lift rods above the missile shield, and the reactor vessel closure head is reinstalled on the reactor vessel.

In yet another embodiment of the present invention, an integrated head assembly includes a lower ring beam that is disposed atop the reactor vessel closure head, lift rods that attach to the vessel head lifting lugs, a shroud assembly with cooling air ducts that is supported by the ring beam, a seismic support assembly and missile shield assembly installed above the reactor vessel closure head, and fans connected to the cooling air ducts. An upper ring beam and lifting tripod may be provided at the upper end of the lift rods, wherein the upper ring beam acts as a spreader for the lifting tripod. The upper ring beam is annular, providing access to the upper portion of the integrated head assembly.

In a disclosed embodiment, the missile shield assembly includes an array of shield plates, each shield plate positioned above a control rod drive mechanism, the shield plates being removable such that individual control rod drive mechanisms can be accessed from above. The shield plates are slidably retained between grooved beams and a center shield plate in each row is removable, allowing adjacent shield plates to be slid to access the desired control rod drive mechanism.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the figures, an integrated head assembly100according to the present invention is shown atop a reactor vessel closure head90inFIG. 1. The reactor vessel closure head90is attachable to the top of a reactor vessel (not shown) and seals the reactor vessel, which contains the nuclear fuel (not shown). As seen more clearly inFIG. 2, the reactor vessel closure head90is a circular structure that typically includes a dome-shaped central portion92and an outer ring portion94having a plurality of stud mounting holes95. The dome portion92supports a number of control rod drive mechanisms (CRDMs)96that extend vertically above the reactor vessel closure head90and pass through the head into the reactor vessel. The CRDMs96are electrically operated devices that control the vertical position of the control rods (not shown) inside the reactor vessel. CRDMs96are well known in the art and are therefore depicted in the figures in functional form, without showing the structural detail. For example, CRDMs generally include upwardly extending guide tubes that, for clarity, are not shown inFIG. 2. The reactor vessel closure head90includes three integral lifting lugs98that are used to facilitate lifting the head for removal and replacement.

The preferred embodiment of the integrated head assembly100includes a lift assembly150that provides support structure for lifting the reactor vessel closure head90, a cylindrical shroud assembly200that rests atop the reactor vessel closure head90, a seismic support system300(seeFIG. 12) that protects the CRDMs96and integrated head assembly100from seismically-induced loads, a missile shield400(seeFIGS. 11A and 11B) that provides protection in certain accident scenarios wherein the CRDMs96and/or control rods are ejected, a baffle assembly500(seeFIG. 8) for directing the flow of cooling air to the CRDMs96, and a CRDM cooling system including cooling air ducts600connected through an upper air plenum680to cooling fans190.

The primary components of the lift assembly150are shown inFIG. 2. The lift assembly150includes a bottom ring beam151that sets atop the reactor vessel closure head90. The ring beam151of the preferred embodiment has a short, cylindrical lower portion152and a flange portion153that extends outwardly from the top edge of the cylindrical portion152. A plurality of saddle members155is disposed peripherally around the ring beam151, the saddle members155having a lower surface that generally conforms with the shape of the reactor vessel closure head90, thereby distributing the weight of the integrated head assembly100over a larger portion of the reactor vessel closure head90. In the preferred embodiment, the ring beam151comprises three generally identical segments that are connected through the lift rod connecting members162, as described below.

Three lift rods160extend vertically upwardly from the ring beam151. Each lift rod160includes a connecting member162at one end having a clevis-type connector163that slidably engages one of the head lifting lugs98. The connecting member162is attached to the head-lifting lug98with a clevis pin166. A detail of the connecting member162of the preferred embodiment is shown inFIG. 3. The connecting member162is positioned between ring beam151segments, and includes oppositely disposed horizontal flanges164that connect to the ring beam151with bolts165, thereby interconnecting the ring beam segments and removably attaching the ring beam151to the reactor vessel closure head90. Although the preferred embodiment utilizes three ring beam segments, it will be appreciated that other configurations are possible and contemplated by the present invention, including, for example, a unitary ring beam having cut out portions to accommodate connecting members.

The upper end of the lift rods160are threaded and extend through orifices182in a circular fan support plate180(seeFIG. 10), where they are attached to the fan support plate180with the tripod base brackets172and/or other suitable connecting hardware. A lift tripod170is disposed above the fan support plate180and includes three rods171, each rod171pivotally connected at one end to a tripod base bracket172, and pivotally connected at the opposite end to a lift shackle174. It will be appreciated that the lift assembly150permits the integrated head assembly100and the reactor vessel closure head90to be lifted as a single unit by an appropriate lifting mechanism, such as a hoist (not shown), acting on the lift shackle174. It will be appreciated that the fan support plate180also functions as a spreader for the lift tripod170. The three large apertures184through the fan support plate180are the outlet air ports for the upper air plenum680fluidly connected to the cooling fans190as described below.

As seen most clearly inFIG. 4, a generally cylindrical support column assembly202is provided on top of the ring beam151. The support column assembly202includes six support columns204that extend upwardly from the ring beam151, each support column204preferably being positioned above one of the saddle members155. The support columns204are attached to the ring beam151with a clip angle bolted connection206. Curved transverse members208interconnect the support columns204at three vertically spaced locations. The support column assembly202provides a cylindrical grid support structure over the reactor vessel closure head90that supports the integrated head assembly components, and transfers the weight and dynamic loads from the integrated head assembly100to the ring beam151. Although the preferred support structure has been described, it will be apparent to one of skill in the art that many variations in the support structure may be made without departing from the present invention. For example, and not by way of limitation, more or fewer support columns204and/or transverse members208may be utilized, or the transverse members208may be replaced with hoop beams that encircle the support columns.

The shroud assembly200of the preferred embodiment includes bottom shroud220, a middle shroud240and an upper shroud260(seeFIG. 1). The bottom shroud220, shown inFIG. 5, is a cylindrical assembly that is installed at the lower end of the support column assembly202, extending upwardly from the ring beam151. The bottom shroud220includes an outer wall panel222that is preferably formed in multiple sections. The outer wall222includes access openings224that provide access to the interior of the shroud assembly200—for example, to monitor and/or service the CRDMs96. A plurality of doors226are attached at the access openings224, whereby the access openings224can be closed, for example, during operation of the reactor and when access to the interior of the shroud assembly200is not otherwise required. It will be appreciated that although hinged attachments are shown, any other suitable closure system could be used—for example, removable panels, sliding panels, and the like. The bottom shroud outer wall222and doors226may be made from any suitable material such as, for example, ASTM-A36 carbon steel. The thickness of the panel222and doors226are selected depending on the required level of radiation shielding that is desired. For example, in the preferred embodiment, the panel222and doors226are ¼ inch thick if radiation shielding is not an issue, and 1½ inches thick if radiation shielding is desired.

A lower baffle portion520extends through the bottom shroud220, comprising a left panel521, a right panel522, a forward panel523, and a rearward panel524. The baffle panels521,522,523, and524are oriented approximately parallel to and generally surround the CRDMs96. The lower baffle portion520defines a central airflow path for cooling airflow. The left and right panels521,522, cooperatively with a portion of the outer wall panel222, form a pair of longitudinal channels620near the periphery of the integrated head assembly100.

Referring now toFIG. 6, a middle shroud240is aligned with the bottom shroud220and extends upwardly from the bottom shroud220. Similar to the bottom shroud, the middle shroud240includes a multisection outer wall panel242that attaches to the support column assembly202. Air inlet ports244are provided on opposite sides of the middle shroud240that permit ambient air to enter the shroud assembly200for cooling the CRDMs96. A middle baffle portion540of the baffle assembly500extends vertically through the middle shroud240. The baffle middle portion540includes a left panel541and a right panel542that each attach to the shroud outer wall242, forming a pair of peripheral longitudinal channels640, aligned with and vertically continuing the channels620formed in the bottom shroud220. The baffle assembly middle portion540is preferably open at the oppositely disposed forward and rearward regions between the baffle left and right panels541,542, which openings are generally aligned with the shroud air inlet ports244. Horizontal plates248extend inwardly from the bottom of the middle shroud240from the air inlet ports244, such that air entering the air inlet ports244is directed to the interior of the baffle assembly500towards the CRDMs96.

An upper shroud260is shown inFIG. 7. The upper shroud260extends upwardly from the middle shroud240and includes an outer wall262that attaches to the support column assembly202. A baffle upper portion560of the baffle assembly500extends vertically through the upper shroud260, including a left panel561and a right panel562, aligned with the middle baffle portion540. The baffle upper portion560and upper shroud outer wall262cooperatively form a pair of longitudinal channels660aligned with and continuing the middle section channels640. The forward and rearward portions of the upper shroud260have apertures264to provide electric power and control access to the CRDMs96through a CRDM cable disconnect panel120(seeFIG. 13). It will be appreciated that the shroud channels620,640, and660cooperatively form longitudinal cooling ducts600that extend from near the reactor vessel closure head96upwardly substantially through the length of the shroud assembly200.

A view of the baffle assembly500disposed within the support column assembly202is shown inFIG. 8, with the shroud outer walls222,242,262removed for clarity. The baffle structure500extends upwardly from near the reactor vessel closure head90and provides a flow path for cooling air to the CRDMs96. A gap is provided between the reactor vessel closure head90and the baffle assembly500that functions as an air outlet port such that the cooling air flowing downwardly along the CRDMs96exits the baffle and flows outwardly toward the periphery of the integrated head assembly.

An upper air plenum680, shown inFIG. 9, is provided at the top of the integrated head assembly100. The upper air plenum680is a generally circular plenum that includes the fan support plate180having outlet ports184to the cooling air fans190. The fan support plate180, with three cooling air fans190installed, is shown inFIG. 10. The plenum lower panel comprising the missile shield400discussed in more detail below and a vertical peripheral wall682are provided between the fan support plate180and the missile shield400. The missile shield400includes left and right cutout portions420that are disposed over the cooling air ducts600and provide the inlet ports to the upper air plenum680. In the preferred embodiment, the cooling air fans190draw air upwardly through the upper air plenum680. In operation, therefore, the fans190draw air into the middle shroud inlet ports244, downwardly along the CRDMs96in the baffle assembly500, upwardly through the ducts600into the upper air plenum680, and out of the integrated head assembly100.

Referring now toFIGS. 11A and 11B, the missile shield400is provided above the CRDMs96near the top of the baffle assembly500. The primary purpose of the missile shield400is to protect against the possible ejection of the CRDMs96or fuel rods in certain accident scenarios. The missile shield400may be made from any suitably strong material and is preferably a steel panel having circular forward and rearward portions410and cutout left and right portions420that are shaped to accommodate the cooling air ducts600as discussed above. The missile shield400is supported by the support columns204and includes outwardly extending tab portions430to facilitate attachment to the support columns204.FIG. 11Bshows a plan view of the missile shield400installed in the integrated head assembly100(with some structural detail removed for clarity).

A seismic support system300for the integrated head assembly100is shown inFIG. 12. The seismic support system300is designed to support the CRDMs96in a seismic event to ensure that the control rods are able to drop down into the reactor if it is necessary to shut the reactor down. The seismic support system300includes an array of seismic cap plates310of various shapes (310a,310b,310c, and310d), each seismic cap plate attached to the upper end of a CRDM96. The seismic cap plates310include a generally flat portion311with a notched-out section312to accommodate electrical power and control cables. A hat-shaped recess or cavity313is formed at an intermediate portion of the seismic cap plate310, and sized to accommodate the end of a CRDM96. The CRDM96extends into the cavity313and is attached to the respective seismic cap plate310. As shown inFIG. 12, the flat portions311of the cap plates310are approximately adjacent neighboring cap plates310, such that the cap plates310cooperatively form a lateral support panel across the CRDMs96.

A baffle stiffener ring beam320surrounds the seismic cap plate310array, and preferably a plurality of adjustable engagement mechanisms (not shown) are provided between the cap plate310array and the baffle stiffener ring beam320, whereby only a slight gap is provided therebetween. A seismic ring beam340, comprising a generally circular beam, surrounds the baffle stiffener ring beam320and is connected to the ring beam320with forward and rearward seismic stiffener plates330and left and right seismic stiffener beams335. In the preferred embodiment, a bolt tensioner rail350is provided on the outer perimeter of the seismic ring beam340to accommodate a bolt tensioning/detensioning apparatus (not shown). A plurality of seismic restraints360connects the seismic ring beam340to a relatively stable anchor such as the reactor containment walls (not shown).

FIG. 13shows the CRDM cable disconnect panel120discussed above, which is preferably installed in the upper shroud260. The cable disconnect panel120provides an array of electrical connectors122providing a central location to disconnect the CRDMs96from their electric power and control systems prior to removal of the reactor vessel closure head90. More than one cable disconnect panel120may be provided.

The integrated head assembly100of the present invention simplifies the removal and replacement of the reactor vessel closure head90for standard maintenance procedures, as well as for unscheduled outages, by integrating the lifting support, CRDM cooling system, missile shield, and seismic support into a single assembly that may be removed as a unit from the reactor vessel. In practice, to remove the integrated head assembly a polar crane hook or other appropriate lifting and moving mechanism is attached to the tripod assembly lift shackle174, the CRDM cables are disconnected at the cable disconnect panel120, the seismic restraints360are disconnected, and the reactor vessel closure head studs are loosened and removed. Additional site-specific steps well known in the art and not important to understanding of the present invention may also be required, such as moving one or more cable bridges away from the lift path of the head. The reactor vessel closure head can then be removed from the reactor vessel to permit the necessary maintenance procedures to be performed.

Although the preferred embodiment has been described in some detail, it will be readily apparent to one of skill in the art that many variations in the present invention may be made without departing from the present invention.

It will be appreciated that the present invention is amenable to retrofitting of existing nuclear power plants. No modifications to the reactor vessel closure head90would be required.

In a preferred method of retrofitting an existing plant, it is contemplated that the design, fabrication, and installation effort for the integrated head assembly100of the present invention would be performed over a period of approximately 24 calendar months. The integrated head assembly100installation would preferably be performed during a refueling outage of the plant, such as are typically scheduled every 18 months. Accordingly, the design/fabrication/installation process needs to be scheduled based on the plant refueling schedule. The integrated head assembly shroud assembly200and associated components may be fabricated and shipped in three modules. The first module comprises the bottom ring beam151, the bottom shroud220, the baffle lower portion520, and other appurtenances associated with the bottom shroud220. The second module would comprise the middle shroud240, the baffle middle portion540, including the cooling air inlets, and other associated appurtenances. The third module would include the upper shroud260, baffle upper portion560, partial air inlet, partial assembly of the CRDM96seismic support system300, and related head area cable support systems and wires in pre-routed condition, the cable disconnect panel(s)120, the missile shield400, the cooling fans190, and other associated appurtenances. It is contemplated, although clearly not critical to the present invention, that the three lift rods160and the lift tripod170would be shipped as separate items.

The assembly of these components would preferably be accomplished while the reactor vessel closure head90is resting on a reactor head stand inside the containment. In a typical installation, the existing rig assembly would first be disassembled from the reactor vessel closure head90. The three lift rods160are then attached to the three lift lugs98on the reactor vessel closure head90. Temporary supports are preferably provided at the top of the lift rods160to hold them in place. Assembly of integrated head assembly components is accomplished starting from the bottom of the integrated head assembly (i.e., near the reactor vessel closure head90) and continuing on in upward direction. The first module is inserted through three lift rods160and the bottom ring beam151is attached to the connecting members162of the lift rods160. Once the lower shroud220is in place, the second module is lowered through the lift rods160and bolted to the bottom shroud220at the transverse members (i.e., ring angles)208and at the support columns204. For accessibility for bolted connections, some or all of the outer wall panel242of the middle shroud240may be removed from the shroud.

It is possible that the elevation of the top of the second module is very close to the elevation of the CRDM seismic cap plates310. In such cases, install all CRDM seismic cap plates310on all CRDMs96prior to lowering the third module over the lift rods160. In the next step of this preferred method, lower the third module through three lift rods160and attach it to the top of the middle shroud240by bolts at the transverse members208as well as at the support columns204. Again for accessibility for bolted connections, some or all of the outer wall panel262of the upper shroud260may be removed from the shroud. The installation of the CRDM seismic support system300may be completed at this time, excepting attachment of the seismic restraints360. The seismic restraints360are preferably installed when the integrated head assembly is in place atop the reactor vessel. After the third module is assembled and installed, the missile shield400may be installed along with the cooling fan support plate180including the rest of the upper air plenum680, the cooling fans190, and the lift tripod170.

After the cooling fan base is installed, the refueling disconnect panels may be installed near the bottom surface of the cooling fan support plate180. The retrofit is completed with the assembly of miscellaneous non-structural elements. After the assembly is complete, the whole integrated head assembly100with the reactor vessel closure head90is lifted and held in a lifted position for some time by the containment polar crane and then put back on the head stand. At this time all component connections are checked once again for their effectiveness. When it is ready to install the reactor vessel closure head90back on the reactor vessel, the entire integrated head assembly100, with the reactor vessel closure head90, is lifted from the head stand and moved over the reactor vessel and lowered slowly until the head is properly aligned and resting on the reactor vessel, and the assembly is attached to the reactor vessel. After the reactor vessel closure head studs are properly torqued, the seismic restraints360are attached to the integrated head assembly100on one side and to the refueling walls on the other side. After the integrated head assembly is installed it is contemplated that airflow test would be performed to ensure proper operation of the cooling fans190and the entire CRDM cooling system.

It will be apparent to one of skill in the art that other assembly methods are possible although less preferred, including, for example, installing or partially installing the integrated head assembly to the reactor vessel closure head while it is attached to the reactor vessel, or installing the integrated head assembly to the reactor vessel closure head utilizing more smaller modules, or fewer larger modules. In particular, it is contemplated that the integrated head assembly100could be substantially completely assembled prior to installing it on the reactor vessel closure head.

As discussed above, some commercial nuclear reactors require that the lead screw from each control rod be decoupled from the control rod and parked prior to removal of the reactor vessel closure head. Such reactors typically require significant headroom over the reactor vessel in order to decouple and park the control rod lead screws. A second embodiment of the present invention that provides substantial reactor vessel headroom, is shown inFIGS. 14 to 20.

Referring now toFIG. 14, a perspective view of a second embodiment of an integrated head assembly1100according to the present invention is shown. The integrated head assembly1100includes three lift rods160that attach to lifting lugs1098on a reactor vessel closure head1090—for example, in a manner similar to that shown inFIG. 3. Although it is not essential to the present invention, in the disclosed example the reactor vessel closure head1090includes an upwardly-extending flange1093, generally in the shape of an inverted “L,” as seen most clearly inFIG. 15. Such flanges are a common feature of certain existing commercial light water reactor designs.

The integrated head assembly1100includes a ring beam1151having an L-shaped lower portion1152that is adapted to rest atop the reactor vessel closure head flange1093. It will be appreciated that this configuration distributes the weight of the integrated head assembly1100over a large portion of the reactor vessel closure head1090. As on the first embodiment disclosed above, the ring beam1151also includes a ring-shaped horizontal flange portion1153. The ring beam1151may further be attached to the reactor vessel closure head flange1093—for example, with nuts and bolts or other clamps (not shown) as are well known in the art. In the preferred embodiment, the ring beam1151is formed in three approximately 120° segments, although more or fewer segments that cooperatively form a ring beam are also contemplated by the present invention.

The integrated head assembly1100includes a lift assembly having three lift rods160(two shown inFIG. 14) that connect at the lower end to the lifting lugs1098on the reactor vessel closure head1090(for example, with clevis-type connecting members162) and at the upper end to a lift tripod170(for example, with tripod base brackets172). The lift assembly is generally the same as that shown inFIG. 2, except that an upper ring beam1180acts as a spreader for the lift tripod170(rather than the fan support plate180). The upper ring beam1180is an annular beam, thereby providing access to the interior of the integrated head assembly1100from above. The upper end of the lift rods160are threaded and extend through orifices in the upper ring beam1180, where they are attached to the upper ring beam1180with the tripod base brackets172and/or other suitable connecting hardware. The lift tripod170is disposed above the upper ring beam1180, and includes three rods171, each rod171pivotally connected at one end to one of the tripod base brackets172and pivotally connected at the opposite end to a lift shackle174. It will be appreciated that the lift assembly170permits the integrated head assembly1100and the reactor vessel closure head1090to be lifted as a single unit by an appropriate lifting mechanism, such as a hoist (not shown), acting on the lift shackle174.

A generally cylindrical shroud assembly1200extends upwardly from the ring beam1151, preferably including a bottom shroud1220, a middle shroud1240, and an upper shroud1260. Access doors226are provided in the bottom shroud1220over access openings224. A baffle assembly1500extends upwardly from the reactor vessel closure head1090, the baffle assembly being attached to the shroud assembly1200, and cooperatively with the shroud assembly1200forming a plurality of vertical cooling air ducts1600that extend upwardly for a substantial portion of the integrated head assembly height. In the preferred embodiment three cooling air ducts1600are provided, circumferentially spaced around the integrated head assembly1100, as seen most clearly inFIG. 16.

Referring again toFIG. 14, three cooling fans190are installed in the vertical wall of the upper shroud1260. The cooling fans190are directed outwardly, and each fan190is fluidly connected to one of the vertical cooling air ducts1600, to draw air upwardly through the air duct1600. Inlet ports1244through the middle shroud1240and the baffle assembly1500provide a flow path for cooling air to enter the integrated head assembly1100for cooling the control rod drive mechanisms96. Horizontal plates1248are provided at the inlet ports1244between the shroud wall and the baffle assembly1500. It will now be appreciated that the cooling fans190operate to draw ambient air into the integrated head assembly1100through the inlet ports1244. The air flows into the baffle assembly1500and downwardly over the CRDMs96convectively cooling the CRDMs96, and into the inlet disposed at the bottom of the cooling air ducts1600, where the air flows upwardly and out of the assembly1100through the cooling fans190. Although the upper end of the cooling air ducts1600of the preferred embodiment are not fluidly interconnected at the top end, it is contemplated that an annular upper air plenum (not shown) could be provided to fluidly connect the cooling air ducts1600. An upper air plenum would improve airflow over the control rods96if one or two of the fans190fail or are otherwise not operational.

A seismic support system1300for the integrated head assembly1100is shown inFIG. 17. The seismic support system stabilizes the CRDMs96in a seismic event to ensure that the control rods are able to drop down into the reactor if it is necessary to shut the reactor down. In this embodiment, the seismic support system1300includes an array of seismic support plates1310having an aperture1313that is sized to slidably receive the end of a CRDM96. The seismic support plates1310include a second aperture1312, which may overlap the first aperture, to accommodate electrical power and control cables and piping for the CRDM stator cooling water. A seismic support plate1310is provided for each CRDM96, forming an array of plates as shown inFIG. 17, and the seismic support plate1310clamps to the CRDM96. As shown inFIGS. 17 and 18, the seismic support plates1310are approximately adjacent to neighboring seismic support plates1310, such that the seismic support plates1310cooperatively form a lateral support panel across the CRDMs96. Although the seismic support plates are shown fixedly attached to each CRDM96, is it also contemplated that the seismic support plates1310may alternatively loosely engage the CRDMs96and be attached to a rectangular grid frame structure (not shown) that holds the seismic support plates1310in proper alignment.

A baffle stiffener ring beam1320surrounds the seismic cap plate1310array. Adjustable engagement mechanisms (not shown) may be provided between the cap plate1310array and the baffle stiffener ring beam1320, to adjustably maintain a slight gap therebetween. A seismic ring beam1340, comprising a generally circular beam, surrounds the baffle stiffener ring beam1320, and is connected to the ring beam1320with a plurality of seismic stiffener plates1330. In the preferred embodiment, a bolt tensioner rail1350is provided on the outer perimeter of the seismic ring beam1340to accommodate a bolt tensioning/detensioning apparatus (not shown). In this embodiment the seismic support system does not include any seismic restrains for connection to the containment wall. The seismic forces are therefore transmitted to the reactor vessel closure head1090through the integrated head assembly1100structure.

A missile shield assembly1400is disposed above the seismic support system1300, as shown in more detail inFIG. 19. The missile shield assembly1400includes a support structure1410including three work platforms1412that are circumferentially spaced around the missile shield assembly. A sliding frame structure comprising a number of parallel slotted beams1420is disposed generally between the work platforms1412. A plurality of missile shield plates1422are slidably inserted between adjacent slotted beams1420, as shown inFIG. 19. The missile shield plates1422are arranged in an array between adjacent slotted beams1420substantially filling the area between the work platforms1412over the CRDMs96. A central portion of each slotted beam1420is provided with a removable frame member1424that is secured to the slotted beam1420, for example, with bolts1426, such that removal of adjacent frame members1424will permit the missile shield plate1422disposed therebetween to be lifted out. It will be apparent fromFIG. 19that if a shield plate1422in any row of shield plates is removed, adjacent shield plates1422in the same row can be slidably moved to provide access therebelow. In the preferred embodiment, the missile shield plates1422are each provided with a pair of handles1428to facilitate sliding the shield plates1422within the slotted tracks. In the embodiment shown inFIG. 19, the shield plates1422are sized such that one shield plate1422is disposed directly above each one of the CRDMs96. It is also preferred that the missile shield assembly1400be made from a suitable material to provide radiation shielding to workers above the missile shield assembly1400, and that the assembly be sturdy enough to safely support such workers.

It should now be appreciated that the missile shield assembly1400disclosed above provides a work platform directly over the CRDMs96, whereby the 15-foot tool can be inserted into the CRDM casing to decouple and park the lead screw in each CRDM96so that the reactor vessel closure head1090can be removed. It will also be appreciated that by removing only a single shield plate1422at a time and sliding adjacent shield plates1422to access the desired CRDM96, the workers' radiation exposure will be minimal when performing this task.

As seen most clearly inFIGS. 14 and 20, in the disclosed embodiment the integrated head assembly1100is provided with a retractable cable bridge1700. The retractable cable bridge1700provides a platform for supporting cables that provide electric power and control signals (i.e., rod position indicator cables) to the CRDMs96. The cables are preferably removably connected to the CRDMs96through a CRDM cable disconnect panel120such as that described above and shown inFIG. 13. The cable bridge1700includes a support platform1710that is pivotally connected at an inner end1720to a work platform1412of the missile shield assembly1400. A pair of cables1724pivotally connects an outer end1722of the support platform1710to the upper ring beam1180through an attachment bracket1730. A pair of motorized pulleys1732is provided to retract and extend the cable bridge1700, as desired.