Patent Number: 043303714
Section: description

Turning now to the drawings, wherein like components are designated by like reference numerals throughout the various figures, a nuclear reactor generally indicated by the reference numeral 10 is illustrated in FIG. 1. In a preferred embodiment, this reactor is a pool type liquid metal fast breeder reactor but may be of any other type compatible with the present invention to be described hereinafter. As seen in FIG. 1, reactor 10 includes a main vessel structure 12 which extends below a reactor cover assembly 14 and which contains a number of reactor components within a pool of hot sodium generally indicated at 16 and a pool of cold sodium generally indicated at 18. The reactor components within the vessel structure include an overall core assembly 20 comprised of a central core 22 containing both fuel and blanket assemblies (not shown) and a coolant plena 24 supported in a fixed position within vessel structure 12 on and by an overall structural arrangement 26 which is constructed in accordance with the present invention and which will be discussed in detail hereinafter. Other reactor components include one or more primary circulation pump assemblies 28 and associated discharge piping interconnecting the pump between the cold pool 18 and plena 24 for passing coolant from the cold pool to and through the core assembly for cooling the fuel and blanket assemblies and thereafter to hot pool 16. At least one intermediate heat exchanger 38, is provided to receive coolant from the hot pool for passage back to the cold pool after heat from the coolant has been transferred to an independent medium. An instrument tree 40 is mounted over the core. With the exception of structural support arrangement 26, all of these reactor components including ones which may or may not have been illustrated but which are not pertinent to the present invention are known and may be readily provided by those with ordinary skill in the art. Therefore, these latter components will not be discussed herein except where necessary for a better understanding of the present invention. In this latter regard, it should be noted that vessel structure 12 includes reactor vessel 42 having a bottommost, closed section 44 as illustrated best in FIG. 3. Referring specifically to FIGS. 2 to 6, attention is now directed to a detailed description of structural arrangement 26 which, as stated previously, is constructed in accordance with the present invention. As will be seen hereinafter, this arrangement includes a grillage of I-beams generally indicated at 46 in FIG. 2, a circumferential box girder 48, best illustrated in FIG. 3, and a circumferential support skirt 50, also best shown in FIG. 3, all of which are interconnected together in the manner to be described for achieving the objectives discussed previously. As best seen in FIG. 2, the grillage 46 of I-beams includes a first group of horizontally extending, spaced apart and substantially parallel I-beams 52 and a second group of horizontally, spaced apart and substantially parallel I-beams 54 extending transverse (preferably normal) to and across each of the I-beams 52 at specific crossing points or junctures 56. Each of the I-beams 52 includes a longitudinally extending intermediate section comprised of a vertically extending center web 58, an integrally formed, continuous lower horizontal flange 60 at the bottom of the web and integrally formed, upper horizontal flange sections 62 at the top of the web. The flange sections 62 define spaces 64 between one another and include upwardly chamfered welding surfaces 66 on opposite sides of each space. Finally, a fabrication access hole 68 is provided through web 58 a predetermined distance between each space 64 along with a vertical slot 70 extending upward from each access hole to the top edge of the web so as to open into the space. Each of the I-beams 54 is similar to I-beams 52 to the extent that it includes an intermediate section comprised of a center web 72 and longitudinally spaced fabrication access holes 74. However, the intermediate section of each I-beam 54 includes an integrally formed continuous flange 76 at the top edge of its web (rather than at its bottom) and discontinuous flange sections 78 at the bottom of its web (rather than at its top). Sections 78 define spaces 80 therebetween and include downwardly directed chamfered surfaces 82 in vertical alignment with associated access holes 74. Finally, each access hole and space 80 include a vertically extending slot 84 therebetween. Only the intermediate sections of the I-beams 52 and 54 have thus far been described. For reasons to be discussed hereinafter, each of these I-beams also includes opposite end sections 86 (in the case of I-beams 52) and 88 (in the case of I-beams 54) which are best shown in FIG. 2. As seen there and in FIG. 6, each of these end sections is flangeless, that is, it includes only web 58 or 72 without an associate flange and flange sections and, while it may include an access hole as shown in FIG. 3, it does not necessarily include an associated slot. As stated previously, the I-beams 52 and 54 cross one another at crossing points or junctures 56. Each I-beam 54 is welded to and mechanically interlocked with each I-beam 52 at each crossing point or juncture 56 in a particular way which is best illustrated in FIG. 4. One of the crossing points is shown therein in an exploded view such that a given access hole 74, its associated space 80 and slot 84 of I-beam 54 are vertically aligned above a given access hole 68, its associated spaces 64, and slot 70 of I-beam 52. The two I-beams illustrated in FIG. 4 are interconnected by lowering the top I-beam 54 into and past the slot 70 and access hole 68 of lower I-beam 52 such that this latter hole and the hole 74 are perpendicular to but otherwise aligned with one another with slot 70 above these holes and slot 84 below them. The space 64 between flange sections 62 is just wide enough to receive an aligned transverse section of flange 76 such that welding surfaces 66 engage the underside of this transverse flange section. In a similar manner, the space 80 is just wide enough to receive an aligned transverse section of flange 60 so that welding surfaces 82 engage the top side of this latter transverse flange section. Each welding surface is welded to its engaging flange surface to provide a welded joint which is maintained in a state of compression. At the same time, the I-beams are mechanically interlocked such that the lower I-beam 52 supports the upper I-beam 54 even if the welded joint should fail. The foregoing has been a description of the way in which a given pair of I-beams 52 and 54 are interconnected at a particular crossing point or juncture 56. It is to be understood that the I-beams are interconnected in this way at each and every crossing point in a preferred embodiment. Referring specifically to FIGS. 2 and 3 in conjunction with FIGS. 5 and 6, attention is now directed to circumferential box girder 48. While not shown, this girder extends entirely around the outer periphery of I-beam grillage 46 in a circular fashion. As seen best in FIG. 3, the box girder includes an inner circumferential, vertical support plate 88 and an outer circumferential support plate 90 which also extend vertically except for a circumferentially extending, outward and downwardly facing step or shoulder 92. The two support plates are interconnected to one another to form a boxed cross section by top and bottom interconnecting plates 94 and 96, respectively (illustrated only in FIG. 2). All of these plates are interconnected together in a suitable manner, preferably by welded joints, to provide the cross-sectional configuration illustrated in FIG. 3. Reinforcement webs 98 may be suitably connected within and made part of the overall box girder at spaced-apart radial positions within the box girder, as seen in FIG. 2. In addition, for reasons to be discussed below, the inner circumferential support plate 88 includes a plurality of circumferentially spaced apart, vertical through slots 100, one of which is best seen in FIG. 6. There is one through slot 100 for each I-beam end section 86 and 88 and each slot is positioned to receive therethrough an associated I-beam end section as illustrated in FIG. 6. As shown there, an end section 86 extends through the slot 100 and engages the inner surface of outer circumferential plate 90. In this regard, in a preferred embodiment, the end of each I-beam end section is preferably stepped in a way which conforms with the stepped configuration of the outer circumferential support plate so as to engage the inner surface of the latter along its entire vertical extent. As seen in FIG. 6, end section 86 is not only mechanically interlocked with the box girder as a result of passing through slot 100 and sitting on step 92, but it is also welded to both plates as indicated by the various weld joints 102 in FIGS. 5 and 6. From the foregoing, it should be apparent that the grillage 46 of I-beams 52 and 54 and the circumferential box girder 48 are welded and mechanically interlocked to form an integral unit. This overall unit is supported in a horizontally extending position on top of the top circumferential edge of previously mentioned support skirt 50. As best seen in FIG. 3, the bottommost circumferential edge of the support skirt rests against and is connected to the inner surface of vessel 42 across the bottom section 44 thereof, preferably by means of a welded joint. The top circumferential edge of the skirt engages circumferential shoulder 92 and is welded thereat by a weld joint maintained in a state of compression. However, it should be apparent that even if the welded joint should fail, the mechanical interlocking configuration between the stepped support plate 90 and the circumferential skirt is such that the latter will continue to support the entire box girder and its associated grillage of I-beams. With structural support arrangement 96 constructed in the foregoing manner, should all the weld joints fail, the various components making up the arrangement are nevertheless mechanically interlocked to provide sufficient structural integrity to support core assembly 20 in a limited downwardly deflected position. The amount of deflection which actually results from a total failure is detectable so that appropriate action may be immediately taken. In an actual working embodiment where the overall structural arrangement includes the dimensions illustrated in the drawing and where the weight being supported is about 2000 tons, the amount of deflection or sag in the structure resulting from a total failure in its welded joints is approximately 2 inches while the deflection in individual joints due to individual failures there have been found to be much less than 0.25 inches (a limit set by the reactivity insertion due to the relative control rod withdrawl of that amount). The advantage of a relatively large total deflection, for example, one as large as 2 inches, before the entire core assembly collapes is that such a deflection is detectable under normal operation of the reactor, specifically by means of the control rod position indicators which would show an increased insertion from the same radioactivity/power level. This means that additional deflection monitors are not necessary. Another advantage of the present arrangement resides in the ability of the grillage of I-beams to be readily assembled in the field. The mechanical interlocks not only serve as a mechanical backup to the welded joints should the latter fail as discussed previously, but also simplifies and improves field fabrication by simplifying alignment of the I-beams and minimizing distortion during welding. The box girder stiffens the overall structure against deflections during seismic events and reduces stresses in the support skirt. The latter is also sufficiently flexible to absorb differential expansion and rotation of the box girder. In a preferred embodiment, the I-beams are constructed of type 304 stainless steel, the plates making up the box girder are constructed of type 304 stainless steel as is the support skirt. However, it is to be understood that the type of material selected as well as the size, shape and strength requirements generally of the individual components making up arrangement 26 will be dictated and readily provided by the overall reactor design.