Patent Number: 046817310
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like reference characters designate like or corresponding parts throughout the several views. Also in the following description, it is to be understood that such terms as "forward", "rearward", "left", "right", "upwardly", "downwardly" and the like, are words of convenience and are not to be construed as limiting terms. Prior art LMR with Top Suspended Reactor Vessel For a clearer and better understanding of the present invention, it is thought that it would be helpful to provide a brief description of the prior art liquid metal cooled nuclear reactor (LMR), generally identified by the numeral 10, having a top suspended reactor vessel 12. Toward this end, FIG. 1 of the drawings schematically illustrates the conventional plant construction of a typical LMR 10. The LMR 10 is of the type fully described in the EPRI report number NP-1016-SY, Project 620-26,27 dated March 1979 and entitled "Large Pool LMFBR Design, Executive Summary". Since the plant is an exceedingly complex structure, as can be appreciated by those skilled in the art of reactors, only a simplified version of the main components of the prior art plant, which are generally relevant to the improved support system of the present invention, are shown in FIG. 1. The LMR 10 is of the "pool" type which basically includes a cylindrical reactor vessel 12 which holds a large pool of coolant, such as liquid sodium, and houses a reactor core 14, a circulation pump 16 and a heat exchanger 18. The reactor vessel 12 has an open top end and is supported in suspended fashion at its top end from a transverse deck 20 which, in turn, is supported on its outer ring girder 22 by a reinforced concrete cylindrical side wall 24 that extends upwardly from a concrete base mat 26. Also supported on the base mat 26 are outer cylindrical vertical walls 28,30 and intermediate walls 32 that are intertied by various horizontal walls 34 to the side wall 24 in a honeycomb fashion to define a plurality of individual rooms or cells 36 for housing various equipment associated with the reactor. The LMR 10 also includes a guard tank 38 which surrounds the reactor vessel 12. While the sodium-filled reactor vessel 12 is suspended within the guard tank 38, the vessel 12 and tank 38 are spaced apart and supported independently of one another. On the one hand, the vessel 12 is attached at its open top end in any suitable manner, such as by a full penetration bimetallic weld, directly to the bottom of the deck 20. The deck 20 thus provides a seal or enclosure for the reactor vessel 12 for containment of reactor coolant, cover gas, fuel and other radioactive materials. On the other hand, the guard tank 38 is an open tank and has an upper flange 39 by which it is suspended in a reactor cavity 40 defined by the cylindrical concrete side wall 24, from a lower annular recessed ledge 42 formed in an upper portion of the cylindrical side wall 24. The tank flange 39 is bolted to the support ledge 42 so as to withstand vertical seismic loads. The guard tank 38 serves as a catch basin for reactor primary sodium that might escape from the reactor vessel 12 under faulted conditions. It also serves to insulate the heat generated by the reactor core 14 from the reactor cavity side wall 24 and base mat 26. The space between the reactor vessel 12 and the guard tank 38 is filled with nitrogen gas. Thus, while the reactor vessel 12 is attached directly to the deck 20, the guard tank 38 is not attached to the deck 20 at all. As seen in FIG. 1, the upper flange 39 is spaced outwardly from the perimeter of the deck 20 and below its outer ring girder 22 where the deck 20 is supported on an upper annular recessed ledge 44 also formed in the upper portion of the cylindrical side wall 24. Therefore, although the reactor vessel 12 and deck 20 provide a primary boundary or barrier between the contents of the reactor vessel 12 and the external atmosphere, the guard tank 38 in reality does not provide a true secondary boundary or barrier between the reactor vessel 12 and exterior atmosphere. Any sodium leaking into the tank 38 from the reactor vessel 12 could eventually contact and escape through the joint between the concrete side wall 24 and outer girder ring 22 of the deck 20 or the ledge 39 of the tank 38. Since regulatory requirements for nuclear reactors make the provision of a double boundary or barrier about the reactor mandatory, the concrete containment building 46 of the conventional LMR 10 which houses all of the above-mentioned parts includes an outer steel liner 48 which encompasses all of the parts. The liner 48 is exaggerated in cross-sectional thickness in FIG. 1 for purposes of illustration. Also, it should be understood that, while not shown in FIG. 1, in the upper dome 50 of the containment building 46, the liner 48 is spaced from the interior wall of the concrete structure of the building 46. Additionally, an inner steel liner 52 is provided adjacent the concrete side wall 24 and base mat 26 of the reactor cavity. While the liner 52 is also illustrated directly contacting the interior surfaces of the side wall 24 and base mat 26, it should be understood that a small gap is present between the liner on the one side and the wall and base mat on the other. The respective gaps between liner 48 and dome 50 and between liner 52 and wall/base mat 24,26 serve to impede the transfer of heat from within the dome 50 to the concrete structure of the building 46 and from within the reactor cavity 40 to the concrete base mat 26 and side wall 24. The problems associated with the top suspended reactor vessel 12 have been described supra in the background section of the application and need not be repeated here. Suffice it to say that the vessel 12 will react to seismic loads like a pendulum and develop high stresses therein near the top open end thereof. Improved LMR with Bottom Supported Reactor Vessel Turning now to FIG. 2, there is shown the preferred embodiment of the improved LMR of the present invention, generally designated 54, incorporated a bottom supported reactor vessel 56. The improved LMR 54 per se includes generally some of the same basic components as found in the prior art LMR 10 of FIG. 1, for example, a nuclear reactor core 58, one or more circulation pumps 60 and one or more heat exchangers 62. Also similar per se in function to the prior art LMR 10, the improved LMR 54 includes the reactor vessel 56 for holding the large pool 64 of low pressure liquid coolant, for instance liquid sodium, and for housing the reactor core 58 in the coolant pool 64. In the preferred embodiment, the circulation pump 60 and heat exchanger 62 also extend into the coolant pool 64. While the improved LMR 54 of the present invention is shown in the form of a pool-type reactor, the principles of the present invention are readily adaptable for employment in a loop-type reactor. The improved LMR 54 basically includes the nuclear reactor core 58, the generally cylindrical reactor vessel 56, a generally cylindrical concrete containment structure 66, a core central support pedestal 68, a core annular support structure 70, a reactor vessel bottom structural support means 72, a bed of insulating material 74, a top deck 76 and a serially connected series of bellows 78. As mentioned, the reactor vessel 56 holds a large pool 64 of low pressure liquid metal coolant, such as liquid sodium, and houses the core within the pool. The reactor vessel 56 has an open top end 80, a closed flat bottom end wall 82 and a continuous cylindrical closed side wall 86 interconnecting the top end 80 and the bottom end wall 82. The concrete containment structure 66 defines a cavity in which the reactor vessel 56 is inserted such that containment structure 66 surrounds the reactor vessel 56 and is generally spaced in concentric relationship. Particularly, the containment structure 66 has a cylindrical side wall 88 spaced outwardly from the reactor vessel side wall 86 and a flat base mat 90 spaced below the reactor vessel bottom end wall 82 and peripherally merged with the containment structure side wall 88. For neutralizing any leakage of liquid coolant from the reactor vessel 56 into the containment structure 66, an inert gas, such as nitrogen, is contained within the space between the two. The vessel and structure 56,55 respectively provide the primary and secondary barriers between the coolant 64 and the exterior atmosphere. The reactor vessel 56 is supported at its bottom end wall 82 upon the base mat 90 of the containment structure 66 by the central support pedestal 68, the structural support means 72 and the insulating material bed 74. In one embodiment, the pedestal 68 is anchored to the containment structure base mat 90 by being buried therein and extends upwardly therefrom. A lower portion 92 of the pedestal 68 being disposed below the reactor vessel bottom end wall 82 has a larger cross-sectional size than that of an upper portion 94 thereof which extends upwardly above the end wall 82. An upwardly-facing annular shoulder 96 is formed on the pedestal 68 at the transition between the lower and upper pedestal portions 92,94 and supports the vessel bottom end wall 82 at the central region thereof. The upper pedestal portion 94 also extends upwardly therefrom through the core inlet plenum 98 to the core 58 so as to support the core at a lower end thereof in spaced apart relationship above the reactor vessel bottom end wall 82. The structural support means 72 includes an annular support ring 100 having a plurality of inward radially extending linear members 102. The support ring and members 100,102 are disposed between the containment structure base mat 90 and the cylindrical wall 86 of the reactor vessel 56. The ring 100 is also connected to the reactor vessel 56 at its bottom end wall 82 so as to support and transmit the weight of the vessel 56 and its contents down to the containment structure base mat 90. The combined configuration of the ring and radial members 100, 102 allows the reactor vessel to expand radially but substantially prevents any lateral motions that might be imposed on it by the occurrence of a seismic event. The bed 74 of insulating material which supports the bottom end wall 82 of the reactor vessel 56 is in sand-like granular form, preferably composed of high density magnesium oxide particles or beads. The bed 74 is disposed and distributed between the containment structure base mat 90 and the reactor vessel bottom end wall 82 so as to uniformly support the bottom end wall and the weight of the liquid sodium coolant in the vessel 56 on the base mat. The bed 74 is preferably about twenty inches deep and it insulates the reactor vessel bottom end wall 82 from the containment structure base mat 90. The granular nature of the bed 74 allows the reactor vessel bottom end wall 82 to freely expand as it heats up, while providing continuous support thereof. The high density magnesium oxide beads are compatible with sodium if it should leak. The beads are a proven high temperature insulation material, and will easily shear at the interface with the vessel bottom end wall 82 to accommodate the radial thermal expansion. The reactor core 58 is also supported in the reactor vessel 56 by the annular reinforced support structure 70. The support structure 70 takes the form of a cylindrical skirt 104 and an annular platform 106, being located outboard of the core inlet plenum 98, which are disposed in the reactor vessel 56 on its bottom end wall 82 and extend about the lower end of the core 58 so as to support not only the core at its periphery, but also any other internal structures, such as the circulation pump 60 and the heat exchanger 62. The support structure 70 also has a system of radial keys and keyways which engage the reactor vessel wall 86 to center the core 58 and support structure 70 concentrically in the reactor vessel 56. These keys and keyways allow the support structure to freely expand in a radial and axial direction with respect to the reactor vessel 56; and they transmit the seismic loads imposed on the core 58 to the vessel 56, and thence to the containment structure base mat 90. At the upper end of the improved LMR 54 is positioned the top deck 76. The deck 76 is supported upon a top edge of the containment structure side wall 88 and extends at a reduced diameter lower portion 108 into the side wall 88 above an annular recessed shoulder 110 spaced below the upper end of the containment side wall 88. The top deck 76 is spaced a short distance above the top open end 80 of the reactor vessel. In such manner, the deck 76 is independently supported from the containment structure 66, and carries the weights of the circulation pump 60 and heat exchanger 62 which extend through the deck. The primary barrier of the LMR 54 formed by the reactor vessel 56 is maintained continuous between the top end 80 of the reactor vessel 56 and the top deck 76 and thermal growth of the reactor vessel side wall 86 is accommodated by the use of coupling means in the form of the plurality of serially connected extendible and retractable annular bellows 78, as seen in cross-section in FIG. 3. The annular bellows 78 extend between the deck 76 and the top end 80 of the reactor vessel 56 so as to flexibly and sealably interconnect them together. When the reactor is constructed, the bellows 78 are extended to a predetermined length which develops acceptable stresses. As the reactor is heated to operating temperature, the vessel wall 86 expands upward closing the bellows and relaxing the initial stresses. Thus, at normal operating conditions of the LMR 54, the bellows 78 are essentially unstressed. As an example, the bellows 78 can be a series of twelve rings each ten inches wide and 0.05 inch thick, stacked together. As seen in FIG. 3, alternate rings are welded to the rings above them at the inside diameter of the stack, and to the rings below them at the outside diameter of the stack. It is proposed that when the vessel 56 is constructed, the rings will be stretched to a deflection of four inches at ambient room temperature. Then, as the vessel reaches normal operating temperature, the rings will be compressed, relaxing the initial stresses. Throughout their lifetime, the rings will experience a very low level of bending stress and will be limited to the differences in vessel expansion incurred by cooling down to refueling temperature. Since an annular sodium shield 111 is used to contain the hot sodium pool 64, the variation in vessel side wall temperature can be controlled more easily than if the wall were in contact with the hot sodium. The rings 78 are located in a relatively cold zone and are expected to have a maximum operating temperature of less than 300 degrees F. They are not subject to contact with the sodium and are accessable and can be continuously monitored for leakage of cover gas. Although they may be up to seventy feet in diameter, fabrication of the individual bellows rings 78 would be similar to a stiffening ring or flange for a vessel of this size. The occurrence of any lateral seismic event could cause the top end 80 of the reactor vessel 56 to move, thus imposing a shear load on the annular bellows 78. Therefore, an annular guide ring 112 is attached on the containment structure 66 and extends between its side wall 88 and the top open end 80 of the reactor vessel 56 for providing lateral support of the reactor vessel top open end 80 so as to limit allowable lateral deflection of the vessel 56 while allowing free axial expansion to occur. This prevents imposition of lateral loads on the annular bellows 78 should a lateral seismic event occur. The improved LMR 54 also has a generally cylindrical guard wall 114 disposed between the reactor and containment structure side walls 86,88. The guard wall 114 is connected at its lower end to the reactor vessel's structural support 72 adjacent the bottom end wall 82 so as to surround the vessel side wall 86 in an outwardly spaced relationship. Preferably, the vessel side wall 86 and the guard wall 114 have concentric cylindrical configurations and are separated by an approximately three inch wide annulus. The LMR 54 also includes a flat bottom liner 116 disposed between the base mat 90 of the containment structure 66 and the bottom end wall 82 of the reactor vessel 56 below the bed 74 of insulating material. This liner 116 has a cylindrical skirt at the periphery which is connected at its top end to the inside of the annular support ring 100. Finally, the LMR 54 can include cooling means 118 disposed in the base mat 90 of the containment structure 66 for removing heat from the containment structure and the bed 74 of insulating material. In a preferred form seen in FIG. 2, the cooling means 118 is a plurality of radial cooling pipes 119 embedded in the base mat 90 of the containment structure 66. In FIG. 4, an alternate arrangement is shown in which cooling tubes 120 are provided in a gas cavity 122 between the guard bottom liner 116 and the containment structure base mat 90. The cavity 122 would contain nitrogen gas, whereas, as in the case of the pipes 119, water would flow in the cooling tubes 120. It will be readily understood that, in view of the bottom supported reactor vessel design incorporated by the improved LMR 54, when the axial growth of the vessel 56 due to thermal expansion occurs in the direction upward toward the deck 76, the deck supported control rods (not shown) insert further into the core 58 and provide and inherent decrease in reactivity. This is just the opposite of what occurs when the reactor vessel is suspended from the top deck and expands downward. It is thought that the invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, construction and arrangement thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred or exemplary embodiment thereof.