Patent Number: 048184799
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 there is shown a ductless core component 10 for use in a liquid cooled fast flux nuclear reactor in accordance with the invention, preferably a reactor cooled by a liquid metal such as sodium. The component 10 includes a plurality of coextending, parallel rods 12 each having therein nuclear material, such as fuel pellets, bundled together in a generally hexagonal open-lattice configuration. The term "open-lattice" refers to the absence of a duct structure laterally encasing the full length of the bundled rods. The rods are supported at one end, such as by an inlet nozzle 14, and are free to expand axially upward. The core component is further made up of an upper support plate 16, an upper grid member 18, a plurality of intermediate grid members, only two of which, 20 and 20aare shown for simplicity, a lower grid member 22, and a lower support plate 24. Rods 12 are supported at one end by lower support plate 24 and extend axially upward through lower grid member 22, intermediate grid members 20 and 20a, upper grid member 18 and upper support plate 16. As is well known in the art, the lower grid member 22, intermediate grid members 20 and 20a and upper grid member 18 allow axial expansion while maintaining lateral support for the rods. Core component 10 derives its axial and lateral strength from sodium tubes 26 which are attached to lower support plate 24 by nuts 28 which thread onto the portion of the sodium tube 26 extending through lower support plate 24. Sodium tubes 26 pass through lower grid member 22, intermediate grid members 20 and 20a, upper grid member 18, and are attached to upper support plate 16 by nuts 30. Spacer sleeves 32 are provided about each sodium rod between lower grid member 22 and the lowermost intermediate grid member 20a to space this intermediate grid member 20a from lower grid member 22. Spacer sleeves 32 are also provided around each sodium rod between each of the other intermediate grid members, not all of which are shown, and also between the uppermost grid member 20 and upper grid member 18 to maintain a predetermined distance between adjacent grid members. Each spacer sleeve is provided with a bow of a predetermined magnitude before it is slip fit about a sodium tube 26 in order to provide a snug fit of the spacer sleeve 32 about sodium tubes 26 so that flow induced vibration does not cause wear on sodium tubes 26. The ductless core component is also provided with orifice plates 34 which are used to regulate the flow of coolant through individual core components. In this manner, ductless and ducted core components can be used in the same reactor core with the flow of coolant through each being independently adjustable to complement that of the others. The ductless core component is further provided with a flow dispersion tube 35 fluidly and mechanically connecting the nozzle 14 to the lower support plate 24. FIG. 3 depicts a preferred embodiment of the nuclear reactor spacer grid 36 manufactured in accordance with the invention. FIG. 4 is an enlargement of one portion of FIG. 3. A grid structure 36 is formed by the interconnection of top grid strip members 38, middle grid strip members 40, and bottom grid strip members 42 as depicted in FIGS. 5, 6, 7A, 7B, and 7C. Tabs 44 along the outer edge of top grid strip member 38, middle grid strip member 40 and bottom grid strip member 42 are engaged by apertures 46 in outer grid strip members 48, which extend around the periphery of the grid structure 36. The resulting joint can be secured by welding or by brazing as depicted in FIGS. 7B and 7C respectively. Grid structure 36 is further provided with formed grid tube members 50 which accommodate sodium tubes 26 which extend through the grid structures 36. FIGS. 8, 9, 10, 11, and 12 depict the preferred manner in which grid tubes 50 are formed and attached to top grid strip members 38, middle grid strip members 40, and bottom grid strip members 42. As observable in FIGS. 9 and 10, a grid tube strap slot 52 is stamped in top grid strip member 38 at a first elevation between grid tube straps 53. As shown in FIGS. 11 and 12 respectively, a grid tube strap slot 52 is stamped in middle grid strip member 40 at a second elevation and a grid tube strap slot 52 is stamped in bottom grid strip member 42 at a third elevation to aid in forming grid tube straps 53. A grid tube 50 is then formed by the interconnection of top grid strap member 38, middle grid strip member 40, and bottom grid strip member 42 and comprises the six grid tube straps 53 previously described. Grid strip members 38, 40, and 42 are then welded and/or brazed at their interconnection as depicted in FIG. 8. Final structural grid tube 50 interlocking for grid strip members 38, 40 and 42 is achieved when sodium tubes 26 are extended through grid tubes 50 of grid structure 36. FIGS. 13 and 14 depict the method of interconnecting top grid strip member 38, middle grid strip member 40 and bottom grid strip member 42 to form the cell members 54 depicted in FIG. 4. Top grid strip member 38 is provided with inverted single slots 56 along its bottom surface. Middle grid strip member 40 is double slotted, with slots 58 along its upper surface and inverted slots 60 along its bottom surface. Bottom grid strip member 42 is provided with slots 62 only along its upper surface. Slot 56 of top grid strip member 38 slidingly engages slot 58 along the top surface of middle grid strip member 40. Slot 60 in the lower edge of middle grid strip 40 slidably engages slot 62 along the upper edge of bottom grid strip member 42. The resulting intersection of the four slots is as depicted in FIG. 14A. The joint is then made permanent by welding or brazing as depicted in FIGS. 14B and 14C respectively. FIG. 15 depicts a prior art nuclear grid structure 64 for use in a liquid cooled, fast flux nuclear reactor. It is readily observable that the cell members 66 are generally hexagonal. Closer scrutiny of FIG. 15 reveals that half of the wall members 68 are formed from a portion of a single grid strip member while the other half 70 are formed from portions of two grid strip members. Since each wall member must be formed to the same tolerance, the double metal thickness of wall members 70 increases the difficulty in achieving this tolerance level by a factor of two. As seen in FIG. 4, all of the wall members 72 of cell members 54 are formed of a single thickness of metal from a portion of a single grid strip member. Referring to FIG. 4, nuclear material containing rods 12 extend through each of cell members 54 and must be laterally constrained therein. Towards this end, hardstop means such as hardstop members 76 are provided along two of the wall members 78 and 80 which comprise cell member 54. A spring means, such as spring member 82, is provided along the third wall member 84 which makes up cell member 54. Spring member 82 engages rod 12 and forces it firmly against hardstop members 76 to prevent damage caused by flow induced vibration. A novel spring structure for use in this grid structure is illustrated in FIGS. 16A, 16B, and 17. Dimple 86 contacts the rod. Spring member 82 is provided with an aperture 88 at its bottom edge. This aperture 88 enables coolant flowing in the direction of arrow 90 to enter a channel means such as channel 92 formed on the side of the spring member 82 opposite dimple 86 as observable in FIG. 17. The force with which spring member 82 pushes against the rod is dependent upon the rate of flow of coolant through channel 92, which flow forces spring member 82 and dimple 86 against the rod. The rate of flow of coolant through channel 92 is directly related to the rate of flow of coolant through the reactor and the fuel assemblies therein. Slot 94 provided in spring member 82 serves to create a predetermined spring force in spring member 82 based on a predetermined rate of coolant flow through the core component. Spring member 82 may be formed integral with grid strip member 84 or may be attached thereto by welding or otherwise. In either case, an aperture 96 is provided in grid strip member 84 on the side of spring member 82 opposite dimple 86 and proximate channel 92. The angle of inclination of spring member 82 with respect to the generally axial coolant flow also serves to regulate the spring force with which spring member 82 contacts rod 12. As depicted in FIG. 16A, the preferred angle of inclination with respect to the axis of the core component, which generally corresponds to the direction of coolant flow through the core component, is 9 degrees. This inclination of spring member 82 serves to further enhance mixing of the coolant by directing a portion of the coolant flow traveling in channel 92 through aperture 96 and into the adjacent cell member and by directing the balance of coolant flow traveling in channel 92 into flow with a radial component. As illustrated by arrows 97 in FIG. 4, preferred fuel assembly coolant mixing characteristics can be achieved by establishing a preferred orientation of spring members 82 in cell members 54. While the above description is of a spring for use in a liquid metal cooled nuclear reactor, this hydraulic spring is also suitable for use in a water cooled nuclear reactor. FIG. 18, which is a cross section through a hardstop member 76 on wall member 84, serves to illustrate a preferred method of manufacturing the hardstop members 76. Hardstop members 76 are preferably formed integrally with grid strip members 78 and 80 by stamping. Hardstop members 76 may alternatively be welded onto the grid strip members. FIGS. 4, 16A, 16B and 17 reveal that while wall member 84 contains the spring member 82 for one cell member 54, it also bears the hardstop member 76 for the adjacent cell member. In operation rods 12 may contain fuel, blanket, or absorber material. The core components formed with fuel, blanket, and absorber rods are identical in mechanical construction and are mechanically interchangeable in any core fuel assembly location. Each core component is designed to maintain its structural integrity during reactor heat up, cool down, shutdown, and power operation including the most adverse operation conditions expected through its lifetime during operation of the reactor. Reference to FIGS. 1 and 2, liquid coolant, preferably sodium, is pumped by a reactor coolant pump (not shown) into the bottom of the nozzle 14 and flows upwardly through orifice plates 34 to lower support plate 24. At lower support plate 24, a portion of the coolant flow is directed upwardly through sodium tubes 26, which are opened throughout their lengths, while the balance of coolant flow is directed through sodium flow holes 98 in lower support plate 24 then through cell members 54 in lower grid member 22 and into contact with the exterior of rods 12. While most of the coolant flow continues on through the core component via the intermediate spacer grid members 20a, 20, and the others not shown, then through upper spacer grid member 18 and to upper support plate 16, a portion of the coolant flow is directed into adjacent core components (not shown). Within lower grid member 22, intermediate spacer grid members 20a and 20, and upper spacer grid member 18, the coolant flows through each cell member 54 as depicted in FIG. 4, passing in close proximity to rods 12. Also a portion of the coolant flow is directed through aperture 88 into channel 92. Within channel 92, the coolant flow pushes against the back of inclined spring member 82 and forces dimple 86 into contact with rod 12, forcing rod 12 against hardstop members 76. This continuous spring force derived from coolant flow prevents damage to rods 12 by flow induced vibration. While a portion of the coolant flowing through channel 92 is directed into the adjacent cell member 54 through aperture 96 to promote intermixing and thermal balance, the remainder of the coolant flowing in channel 92 is directed with a radial component out of channel 92. For a given coolant flow velocity, the spring force is regulated by the angle of inclination of spring member 82 with respect to the direction of coolant flow as indicated by arrow 90 as well as by a slot 94 provided in spring member 82. The force with which spring member 82 holds rod 12 against hardstop members 76 is not affected by prolonged exposure to radiation and high temperatures, as is the force of a spring which depends upon material resilience entirely for its spring force. Coolant flowing through sodium tubes 26 and through the core assembly about rods 12 continues upwardly through upper support plate 16 and out of the core component through handling socket 100 (see FIG. 1). In the unusual event of a severe accident, the sodium tubes 26 provide a passage through the core. Cooling through the core by pumping of the coolant through the sodium tubes can be maintained at all times, which serves to keep down the temperature of the core. Alternatively, the sodium tubes 26 can be blocked at each end and filled with fuel, blanket material, or absorber material to increase the packing density of the core component. Many variations of the hereinbefore described grid fabrication process are contemplated for design versatility. One alternative embodiment of grid structure 36 is illustrated in FIGS. 19 and 20. This grid structure 36 is an egg-crate type hex grid latticed into a single rod/cell grid. Various packing density effects can be achieved by, for example, forming triple rod/cell grid sections, providing rods at locations other than central to cell members, and installing honeycomb grids internal to a single rod/cell grid, thereby changing the packing density and improving the honeycomb grid strength. As observable in FIG. 20, the alternative embodiment presented utilizes two hydraulic spring members 82 and two hardstop members 76 to constrain each rod 12. Arrows 102 in FIG. 20 illustrate preferred coolant mixing patterns obtained by proper orientation of spring members 82. FIG. 21 depicts the method of interconnecting grid strip members 104, each of which is provided with a plurality of slot members 106 facilitating assembly, and FIG. 22 depicts a portion of the assembled grid structure 36. While the majority of wall members 108 are of a single thickness of metal, wall members 110 are formed of two thicknesses of metal in this embodiment. Therefore, the invention provides a grid structure with versatility in packing density equipped with a novel hydraulic rod constraining, spring and readily assemblable into a ductless core component. While preferred embodiments of the invention have been disclosed herein, many modifications thereof are possible. This invention should not be restricted except insofar as is necessitate by the spirit of the prior art