Patent Number: 052788806
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A pressurizer tank 22 as shown in FIG. 1, is a vertically elongated tank supported on a substructure 30 in a reactor containment building. A compartment 32 is provided to substantially enclose the pressurizer tank 22, and the compartment 32 is shown in the drawings rather than the full containment building. The compartment 32 can be defined, for example, by interior concrete walls 34 in the containment. As shown in FIGS. 2, 5 and 6, an upper support 40 for the pressurizer tank 22 is provided at a space from the bottom 44 of the tank 22, preferably adjacent the top 46 of the tank. The upper support 40 includes a circular box girder 50 surrounding the pressurizer tank 22, attached to the walls 34 of the compartment 32 by sway struts 54. The sway struts 54 couple any lateral forces on the girder 50 to the compartment walls 34, such that such lateral forces are not transmitted through the pressurizer tank 22 to the substructure 30 on which the pressurizer tank 22 is supported vertically. The sway struts 54 are shown in detail in FIGS. 2-4, and the girder 50 in FIGS. 5 and 6. The girder 50 can rest vertically upon valve support bracket plates 118 protruding radially from the outer wall of the pressurizer tank 22, as shown in FIG. 7. The sway struts 54 define substantially inextensible coupling members which are attached between the pressurizer tank 22 and the compartment walls at tie points defining a horizontal pivot axis, as discussed more fully hereinafter. The pressurizer tank 22 is susceptible to lateral forces due to the structures carried at the top and above the pressurizer tank. A relief/discharge valve support frame 60 is disposed over the pressurizer tank 22 for mounting such structures. The support frame 60 includes vertical legs 62 and horizontal beams 64, the legs and beams being welded together to form a structural frame. The legs 62 are rested on and welded at their lower ends to the upper surface of the box girder 50. The relief/discharge support frame 60 carries discharge piping 72 and valve module elements 74, through which the pressurizer tank 22 can be vented to an in-containment refueling water storage tank (not shown). The piping 72 and valve modules 74 define a weight which is spaced from the substructure 30 of the containment interior concrete which supports the pressurizer tank 22. It is not advantageous to attempt to support the piping 72 and valves 74 independently of the pressurizer tank 22, because the piping 72 is coupled to the tank 22, and the tank is subject to dimensional variations due to changes in pressure and pressure in the reactor coolant system. Accordingly, it is necessary to ensure that the mounting of the pressurizer tank is sufficiently strong to hold the tank 22 and its supported appendages against lateral displacement in the event of a seismic shock or other lateral force. The box girder 50 is restrained against lateral displacement relative to the compartment 32 by a plurality of opposed sway struts 54 extending from the girder 50 to the walls 34 of the structural compartment 32 in which the pressurizer tank 22 is located. In the example shown, the compartment 32 is approximately square in plan view and the sway struts 54 extend from the girder 50 between the corners 82 of the compartment 32 and a respective attachment to the girder 50 approximately midway between the corners 82, in opposed pairs. The sway struts 54 can be conventional supporting struts (such as Grinnell FIG. 211 or the like). Attachment at the corners 82 of the compartment 32 provides maximum stiffness and load bearing capacity. Referring to FIGS. 2-4, the sway struts 54 comprise horizontal load bearing shafts 92 terminating in pivot fittings 94. The distal member of each pivot fitting 94 can be welded or similarly fixed to the girder 50 and to a member of the compartment 32 such as a vertical beam at a corner 82 as shown. The pivot fittings 94 comprise backplates 102 shaped substantially as U-brackets opening toward the shaft 92 of the respective sway strut 54. Connecting pins 104 extend through the backplates 102 and through an end of each sway strut shaft 92, the pins 104 each defining a horizontal pivot axis in the connection between the box girder 50 and the compartment wall 34. At the box girder 50, two vertically spaced attachment plates 108 having side edges 112 aligned substantially parallel to the respective sway strut pivot axis are welded to the outer periphery of the box girder 50 and provide a point of attachment for the backplates 102 of the sway strut fittings 94. The sway strut shafts 92 can include self-aligning bushings such that each end of the shafts remains aligned with the pivot axis defined by the connecting pin 104 extending through the backplate 102. FIGS. 5 and 6 show the box girder 50 in greater detail. The attachment plates 108 as shown can comprise two horizontal plates, spaced vertically and welded on the surface of the box girder 50. It is also possible to use a channel member or a solid block to define a point of attachment for the sway strut backplates 102. As also shown in FIG. 6, a plurality of coupling plates 114 are also welded or similarly attached to the girder 50. The coupling plates 114 protrude axially downwardly from girder 50 and are oriented along radii of the tank 22, for example at intervals of 45.degree.. These coupling plates 114 are positioned and dimensioned to extend between pairs of complementary coupling plates 118 which are welded onto the outer surface of the pressurizer tank 22. This form of connection is shown in FIG. 7. The coupling plates 114, 118 attached respectively to the girder 50 and to the pressurizer tank 22 have alignable holes 122 for receiving locking pins 124, thereby engaging the box girder 50 with its attached supporting frame 60 vertically on the pressurizer tank 22. The support frame carrying 60 the relief/discharge valve and conduit system is welded at the top of the box girder 50 encircling the pressurizer tank 22. As shown in FIG. 7, the relief/discharge valves 74 and conduits 72 can comprise a multi-tiered arrangement, for example enabling staged depressurization of the depressurizer tank 22 through conduits 72 of different diameters, each having one or more valves 74 which open a flow path to a discharge, for example, in an in-containment refueling water supply. FIG. 9 generally illustrates the flowpath couplings according to such an arrangement, with a nuclear reactor vessel 162 coupled in a coolant circuit with a steam generator 164, and also a high pressure makeup tank 166 and an atmospheric pressure refueling water supply tank 168. Pressurizer 22 controls the pressure of the coolant circuit through check valve 172, and also is coupled via depressurization valves 74 to discharge into the refueling water supply 168. The respective valves 74 can be operated successively to obtain staged depressurization. At least a first level 132 of the relief/discharge support frame 60 is insulated in order to reduce thermal stresses due to differences in temperature between the relief/discharge support frame 60 and the piping 72 associated with the relief/discharge system. During automatic staged depressurization of the reactor coolant system, different valves 74 are opened in sequence, thereby subjecting selected subsets of the conduits 72 to thermal loading. The girder 50 is not insulated in order to preclude high thermal loading on the sway struts 54. As mentioned above, the girder 50 is preferably attached to the tank 22 vertically via mating coupling plates 114, 118 attached respectively to the pressurizer tank 22 and the girder 50, held together by pins 124. Valve supporting brackets are welded on the pressurizer tank surface, and a bracket matching such a valve support bracket can be welded to the bottom of girder 50 to mate with the valve supporting brackets. Either a slot in the girder bracket 114 is custom machined at the correct location, or an oversized bracket 114 with a slot already provided is custom fit and welded in place to properly fit the pressurizer valve support bracket in that case. Of course, it is readily possible to use two brackets on either of the girder 50 and the pressurizer 22 to mate with one bracket on the other, or to attach single plates to each of the girder 50 and the tank 22, e.g., using bolts. It is desirable to leave a small gap between the girder brackets 114 and the valve brackets 118, which helps to prevent weak axis bending loads on the brackets 112, 114. Strip shims 136 are located between the pressurizer outer surface and the girder 50 with an appropriate hot gap 138 (e.g., 1/32 inch or 0.8 mm). Strip shims 136 provide the necessary lateral load transfer between the girder 50 and the pressurizer tank 22 while minimizing the heat transfer path. Since the relief/discharge support frame 60 is welded to the girder 50 and the girder is fastened to the building structure by the sway struts 54, all relief/discharge system lateral loads are borne directly by the compartment walls 34 rather than passing wholly or partly through the pressurizer 22 and then to the compartment 32 or to the substructure 30. The girder 50 can accommodate upward and downward displacement relative to the compartment walls 34 as the tank 22 enlarges and contracts during plant heatup and cooldown. The pivotal couplings of the sway struts 54, and their self aligning bushings, permit vertical pivoting of the sway struts 54 around their connecting pins 104, which allows some vertical displacement of the girder 50 (and tank 22) relative to the compartment walls 34. Insofar as the girder 50 moves up or down, the sway bars 54 still can accommodate the relative displacement while bearing any lateral forces which may occur. The sway struts 54 as shown are arranged in pairs aligned substantially along tangents to the wall of tank 22, which is cylindrical. Although four pairs of sway struts 54 are preferred at 90.degree. intervals, it would be possible to use a larger or smaller number to restrain the pressurizer tank 22 and the upper support 40 thereon. For example, a 120.degree. arrangement using three pairs of struts 54 could be employed to obtain a triangular configuration in which the struts 54 would still be substantially tangential to the girder 50 and arranged in opposed sets to bear lateral loading in any direction. The pressurizer tank 22 has a top dome 142, attached to a tubular body 144 at a weld 146. Preferably the girder 50 is disposed below the weld 146 in order to facilitate inspection of the weld, e.g. about 9 inches (23 cm) below the weld 146. The support according to the invention provides an improved restraint for the pressurizer tank 22, and eliminates the need for seismic lugs, for example of the type shown by lugs 152 in FIG. 10 and identified as prior art. All lateral forces according to the invention are coupled to the compartment walls 34, preferably at the corners 82, for maximum stiffness and load bearing capacity. The invention having been disclosed, certain variations within the scope of the inventive concept will now be apparent to persons skilled in the art. The invention is intended to encompass a reasonable range of equivalents, and accordingly, reference should be made to the appended claims rather than the foregoing specification in order to assess the scope of exclusive rights in the invention claimed.