Patent Number: 050376047
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to FIGS. 1-6. These Figures show a typical Westinghouse, 3-loop pressurized, water reactor (PWR) nuclear power plant. The actual arrangements of nuclear power plants, however, vary by specific plant. Also, 2-loop and 4-loop PWR plants have similar but different arrangements, as known in the art. The present invention can be readily adapted by those of ordinary skill in the art to all these different plant arrangements, number of loops, plus non-Westinghouse PWR plant designs. FIG. 1 is a perspective view of the containment building 10 with a cutaway in the front left of a forward concrete wall 11 to show the reactor vessel 12 therein. The reactor vessel 12 is an elongated, generally cylindrically shaped member of a familiar design for use in a nuclear reactor system. The reactor vessel 12 has the usual hemispherical bottom and a plurality of inlet and outlet primary system water nozzles (not shown). In FIG. 1 the reactor vessel 12 closure head and fuel (not shown) have already been removed and the radioactive lower and upper internals, 14 and 16, respectively, have been removed and stored. The front right portion of the wall 11 is also cutaway to show the internals 14, 16 in their respective storage racks, 15, 17, in a refueling canal 18. The refueling canal 18 is located above the reactor vessel 12, with an upper flange 22 of the vessel 12 being generally coplanar with the floor or bottom 24 of the refueling canal 18. The reactor vessel 12 as shown in FIG. 1 is under the refueling condition and is ready for, e.g. in situ annealing. At this time, water in the refueling canal 18 is at maximum level 64 shown in FIGS. 1 and 6. Before introducing an annealing apparatus into the reactor vessel 12, precautions must be taken to prevent radiation emitted by the stored internals 14, 16 from being introduced into the area at which the annealing apparatus will be installed and hooked up by human assistance for operation. In this regard, a coffer dam according to the present invention is used for temporary shielding. The structure of the coffer dam will now be described with reference to FIGS. 2-6, wherein the coffer dam is generally referred to by reference numeral 30. The coffer dam 30 is cylindrical and generally includes: a plurality of segments 40; first sealing means 50 positioned between fit up flanges or edges 42 of adjacent segments 40; first connecting means 70 for attaching together the edges 42 of adjacent segments 40; second sealing means 60 positioned between a bottom flange 32 of the completed coffer dam 30 and the reactor vessel upper flange 22; and second connecting means 76 for attaching the bottom flange 32 to the reactor vessel upper flange 22. FIGS. 2a and 2b are schematic views illustrating introduction of the segments 40 according to the present invention into the containment building 10 and assembly thereof into the completed coffer dam 30. Each segment 40 is pre-fabricated in a factory and is a one-piece, curved, metal member. Although only one segment 40 is shown in FIG. 2a, a plurality of segments are assembled to form the coffer dam 30, as 40 described below. Accordingly, each segment 40 may be referred to as 40a, 40b, etc. In the preferred embodiment of the coffer dam 30 shown in FIGS. 2a, 2b and 4, each segment 40 is an elongated vertical cylindrical section, i.e. each is an equal longitudinal curved section of the total cylindrical coffer dam 30. If four segments 40 are used, each segment 40 is curved 90 degrees; if three segments, 120 degrees; and so on. In an alternate embodiment shown in FIG. 3, each segment 40 is a horizontal cylindrical section, i.e. each is a cross sectional portion of the cylinder. Further, as shown in FIG. 6, if desired the coffer dam 30 can be made of a combination of vertical and horizontal sections 40 connected together. In any case, the segment 40 size is selected, most importantly, so as to fit through the equipment hatch 28 of the containment building 10 and yet still correspond to the reactor vessel 12 size. The choice of segment 40 size and quantity can also be varied to satisfy other manufacturing, transport and plant specific conditions. Referring again to the preferred embodiment of the present invention shown in FIG. 4, each of the four vertical segments 40 can include a side port 46a46b, etc. which is used to direct the annealing apparatus connections out of the coffer dam 30 to a control station (not shown). The alternate embodiments of the segments 40 shown in FIGS. 3 and 6 can also include side ports 46 in the uppermost segments 40. As shown in FIGS. 2a and 2b, and as described more fully below, each segment 40 is brought into the containment building 10 and preferably assembled by humans in a low radiation area of the operating floor 72. In this regard, each segment 40 includes vertical and horizontal fit up flanges or edges 42. Adjacent edges 42 are mated and connected by the first connecting means 70, such as bolt and nut combinations 71. The lowermost set of horizontal fit up flanges or edges 42 form the bottom flange 32 of the completed coffer dam 30, whereas the uppermost set of horizontal fit up flanges or edges 42 form the upper flange 82 of the coffer dam 30. Each of the segments 40 can be pre-fabricated to contain the sealing means described below. Alternatively, all or some of the sealing means could be installed when the segments 40 are being assembled on the operating floor 72. More particularly, the coffer dam 30 includes the first sealing means 50 between the edges 42 of adjacent segments 40. As the first sealing means 50 strip seals 44 can be used as shown in FIGS. 4-6. Because the outside 78 of the coffer dam 30 (see FIG. 6) is in contact with refueling canal water, temperatures are reduced, allowing the first sealing means 50 to be, e.g. rubber composition or metal. Such sealing means 50 helps resolve a significant feasibility issue by allowing a plurality of segments 40 to be passed through the hatch 28 and to form the complete coffer dam 30. Once the segments 40 are connected together to form the completed coffer dam 30, the coffer dam 30 is moved and attached to the reactor vessel flange 22. As shown in FIG. 5, a seating surface 33 of the bottom flange 32 mates with a closure head seating surface 34 of the reactor vessel flange 22 with the second sealing means 60 therebetween. The type of second sealing means 60 used in this area depends on the thermal design requirements of the reactor vessel flange 22 during the annealing operation. Where high temperatures in the reactor vessel upper flange 22 area are required to minimize thermal gradients and residual stresses, the second sealing means 60 will be a thermal insulator gasket-type seal in combination with metallic and non-metallic O-rings. For those applications where low temperatures at the reactor vessel upper flange 22 can be tolerated, the second sealing means 60 can be merely low temperature O-rings. The pressure differential across the second sealing means 60 is low (max 15 PSI), thus allowing a wide range of allowable second sealing means 60, flange 32 and second connecting means 76 combinations. The bottom flange 32 of the coffer dam 30 is connected to the reactor vessel flange 22 via the second connecting means 76. Such connecting means 76 can be, e.g. a threaded bolt 38 arrangement. More particularly, the bottom flange 32 of the coffer dam 30 has sufficient holes 36 to allow the completed coffer dam 30 to be bolted to the threaded holes 37 formed in the reactor vessel 12 for receiving the closure head. This bolt down arrangement prevents a catastrophic seal failure as the flanges 32, 22 can be in intimate contact. This arrangement also resolves a significant feasibility problem of the conventional coffer dam described in the "Description of the Prior Art" section, suora, which has no bolt down feature. That is, the coffer dam was merely seated on the bottom of the refueling canal outside of the reactor vessel and employed the weight of the coffer dam to create the seal clamping force. Long handled tools (not shown) can be used from the operating floor 72 to bolt the coffer dam 30 to the reactor vessel flange 22. In this way, the lowest possible personnel exposure to radiation is obtained. If personnel were to work around the reactor vessel flange 22 to connect the coffer dam 30 directly to the flange 22, with only the reactor vessel 12 filled with water, radiation exposure to the personnel coming from the stored internals 14, 16 would be high. Nevertheless, if radiation from the stored internals 14, 16 were adequately shielded by other means, the coffer dam segments 40 can be assembled directly on the reactor vessel upper flange 22 or personnel can be used at the reactor vessel upper flange 22 to connect the bolts 38, if desired. With the coffer dam 30 installed on the reactor vessel 12, the refueling canal 18 is flooded to shield the radiation emitting from the stored internals 14, 16. As shown in FIG. 3, a conventional seal leak detection means 90, with passage to the dry side of the first and second sealing means 50, 60 can be incorporated for monitoring seal effectiveness. As shown in FIGS. 3, 5 and 6, an annealing apparatus 100 is then inserted in the reactor vessel 12, with a lower seating surface 106 of a top plate 102 thereof seated on the reactor vessel internals seating ledge 104. This structure frees up the reactor vessel closure head seating surface 34 to allow the seating surface 33 of the coffer dam 30 to be seated thereon. This change eliminates the need for sealing the top plate 102 of the annealing apparatus 100 to the vessel 12. Workmen (not shown) in the area above the annealing apparatus 100 hook up the various heater power leads 108, thermocouples 110, etc. known in the art. The power leads 108 and thermocouples 110 are led out of the coffer dam 30 through the side ports 46. These ports 46 are then closed by sealant plugs 48. The side ports 46 project above the water level 64 in the refueling canal 18 so water cannot enter the coffer dam 30. These ports 46 and the access cover plate 80, discussed below, are sealed to prevent any potential airborne radiation particles that might be released during heat up and cool down from being released to the containment atmosphere. That is, the annealing process incorporates an inward air movement concept to control airborne radiation particles. A vacuum pulls on the internal vessel volume and the exhaust is passed through an external filter. Any air leakage which might occur around seals for the thermocouples 110 and power cables 108 and the access cover plate 80 are inward. The vacuum is required only to be slightly under atmospheric pressure as there is only a slight pressure differential. The access cover plate 80 is also introduced through the equipment hatch 28. The access cover plate 80 is secured to the upper flange 82 of the coffer dam 30 by third connecting means 83 and sealed via third sealing means 88, such as an O-ring. Sealing at this point above the water line 64, significantly removed from the heat sources, allows for use of low temperature seals. The access cover plate 80 can be made of two, semi-circular sections joined to each other by fourth connecting means 84 such a hinges, pins or bolts. Of course, the access cover plate 80 could be made in more sections than two if desired. The access cover plate 80/side ports 46 combination: allows for man entry to the top of the annealing apparatus 100 for initial installation and hook-up of power leads 108, thermocouples 110, other instrumentation, and piping while the refueling canal 18 is filled; allows man entry during operation for maintenance, inspection and dismantling; no disconnections of thermocouples 110 or power leads 108 are required for man entry into the coffer dam 30; allows for easy exit of the power leads 108 and thermocouples 110; allows for easy installation of the annealing assembly 100; and allows thermocouple 110 actuation and adjustment during annealing. The coffer dam 30 according to the present invention also provides for effective control of air flow to the internal volume of the reactor vessel 12 which is maintained at a negative pressure during the annealing process to control airborne contamination. A vacuum system 112 is used for maintaining the reactor vessel 12 at a slight negative pressure. The system 112 enters the coffer dam 30 at a side port 52 with piping 54 going along the interior wall 56 of the coffer dam 30 and entering the reactor vessel 12 through a connection 58 in the annealing apparatus top plate 102. Similar routing is made for a vent line 62 to the interior of the reactor vessel 12. In contrast to the temporary shielding discussed in the above referenced co-pending application entitled "Water Filled Tanks...," wherein vertical and horizontal water tanks are used between the reactor vessel and stored internals, the coffer dam 30 of the present invention allows for water shielding of the stored internals 14, 16 even in reactors where the refueling canal 18 is small and the internals 14, 16 are installed in close proximity to the reactor vessel 12. In some instances, however, it may be desired to use both supplementary lead or steel shielding hanging on the coffer dam 30 at the closest point of the coffer dam 30 relative to the stored internals, if the radiation reading merits same. The method of assembling the coffer dam segments 40 according to the present invention will now be described in greater detail with reference to FIGS. 2-6. As seen in FIG. 2a, each pre-fabricated segment 40 can be shipped from the factory in, e.g. a strong back reusable shipping frame 66, to the reactor site. At the site the shipping frame 66 and a segment 40a are transferred to the interior of the containment building 10 through the equipment hatch 28. Once inside containment, the segment 40a is released from the shipping frame 66 and upended using a containment crane 68 in a low radiation area of the operating floor 72. This upending operation is enhanced by a shipping frame 66 which has included upending features such as pivot pins and internal rails or rollers to permit upending vertically under the natural crane hook positions without loss of control or undue loading of the segments 40 or the shipping frame 6. As suggested above, each segment 40 can be pre-fabricated at the factory to include the first, second and third sealing means, where necessary, or the sealing means can be installed when the coffer dam segments 40 are assembled on the operating floor. Then, with the segments 40 assembled and the seals captured in place, the coffer dam 30 is lowered onto the reactor vessel flange 22 and connected in sealing relation to the reactor vessel 12. The next step in the annealing process is to introduce the annealing apparatus 100 into the reactor vessel 12 and pump the remaining water from the reactor vessel 12 and coffer dam. A suitable annealing apparatus is described in the above-referenced application entitled "Modular Annealing Apparatus For In Situ Reactor Vessel Annealing And Related Method of Assembly." Once the annealing apparatus 100 is inserted into the reactor vessel 12, the power leads 108, thermocouples 110, etc. are led by the workmen out the side ports 46 to the control station. The access cover plate 80 is then introduced into the containment building 10 and connected in sealing relation as described above via the bolts 86. FIGS. 3 and 4 illustrate the annealing apparatus 100 in the reactor vessel 12 and the complete coffer dam 30 installed above the annealing apparatus 100. In these embodiments, side ports 46 and an access cover plate 80 are used. In contrast, FIG. 6 is a perspective view of an alternate embodiment, wherein the side ports 46 and cover 80 are not used; the connections (not shown) for the annealing apparatus 100 are led directly out the top of the coffer dam 30. Once the annealing apparatus 100 is inserted and hooked up, annealing of the reactor vessel is performed. As would be understood by one having ordinary skill in the art, after annealing is performed, the coffer dam 30 can be disassembled and removed from the containment building 10 by merely reversing the steps described above. In this way, the coffer dam 30 can be reused by transporting same to other reactors. Further, should the coffer dam 30 require repairs, the entire coffer dam 30, a segment 40 thereof or the access cover plate 80 can be transported back to the factory where the repair can be performed. The foregoing is considered illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described. For example, although the present invention is described as particularly suitable to annealing operations, the invention is also equally applicable to other situations where the internals are stored and some work must be performed in the reactor vessel such as scheduled inspections or weld repairs. Accordingly, all suitable modifications and equivalents may be resorted to that fall within the scope of the invention and the appended claims.