Patent Number: 
Section: description

As illustrated in FIG. 1, a fuel assembly 10 comprises a plurality of laterally spaced fuel rods 12 supported between an upper tie plate 14 and a lower tie plate 16. 8xc3x978, 9xc3x979 or 10xc3x9710 arrays of fuel rods are typical but, for clarity of illustration, only some of the fuel rods 12 are shown. The fuel rods 12 pass through a plurality of vertically spaced fuel rod spacers 18 (one shown) which provide intermediate support to retain the elongated fuel rods 12 in spaced relation and to restrain the lateral vibration. Each of the fuel rods 12 is formed of an elongated tube containing a column of nuclear fuel 20. A plenum at the upper end of the fuel rod contains a spring 22 which maintains the column of fuel in position. The fuel rods 12 are sealed by upper and lower end plugs 24 and 26. The lower end plugs 26 are formed with a taper for registration and support within cavities 28 in the lower tie plate 16. Upper end plugs 24 are formed with extensions 30, the upper ends of which fit into support cavities in the upper tie plate 14. Several of the support cavities 28 in the lower tie plate 16 are formed with threads to receive the end plugs of certain fueled tie rods 12xe2x80x2 having threaded end plug shanks 32. Extensions 34 of the end plugs 24 of these same fueled tie rods are elongated to pass through the cavities in the upper tie plate 14 are formed with threads to receive retaining nuts 36. Fitted on the extensions 34 between the upper end plugs 24 and the upper tie plate 14 are expansion springs 38. In this manner, the upper and lower tie plates in the fuel rods are formed into a unitary structure. The fuel assembly 10 further includes an open-ended, thin walled, tubular flow channel 40, of substantially square cross section, sized to form a sliding fit over the peripheral surfaces of the upper and lower tie plates 14, 16 and the spacers 18 so that the channel 40 can be mounted and removed from the fuel bundle without difficulty. Fixed to the top corner of the channel 40 is a tab 42 by which the channel 40 is fastened to a standard 44 of the upper tie plate 14 by screw 46. Since the channel 40 is not fastened to the lower tie plate 24, the upper end of the channel 40 is free to move with respect to the lower tie plate 16, in the event of movement of the upper end of the fuel assembly 10. The lower tie plate 16 is formed with a downwardly extending nose piece 48 which is tapered to engage a fuel assembly support socket (not shown). The lower end of the nose piece 48 is formed with an opening 50 to receive pressurized water so that it flows upward among the fuel rods. To aid in equalizing neutron moderation, the fuel assembly 10 is fitted with at least one large water tube 52 for conveying relatively cool water upwardly through the central region of the fuel assembly. The water rod 52, like the fuel rods, extends between and is supported by the upper and lower tie plates 14, 16, respectively. In this prior art arrangement, the water tube is provided with a plurality of holes 54 at its lower end which provide an inlet for water into the tube, while the upper end of the water tube is provided with a plurality of holes 56, 58 which provide an exit for the water flowing therein near the upper end of the fuel column 20 within the fuel rods. With this background, the discussion below with respect to the schematic drawings shown in FIGS. 2-8, all of which relate to water rod configurations, will be readily understood by those skilled in the art. The schematic diagram in FIG. 2 represents a water rod similar to that illustrated in FIG. 1. More specifically, the water rod 60 has a narrowed lower end 62 with one or more inlet holes 64, while the upper end of the water rod also includes a narrowed portion 66 with one or more outlet holes 68. The placement of these holes at the top and bottom of the water rod imposes the full bundle pressure drop to drive flow through the water rod. When reactor flow reduces, the pressure difference driving liquid through the water rod is also reduced. This configuration, however, maintains very little vapor formation even for low flow conditions. The water rod configuration in FIG. 3 has been developed to have varying amounts of steam in the water rod at different reactor flow conditions. More specifically, the water rod 70 has a narrowed lower end portion 72 with one or more inlet holes or apertures 74. At the upper end of the water rod, however, there is added a small diameter downflow extension tube 76 offset from the uppermost end of the water rod 70 by a horizontal extension 71 and extending downwardly to a location proximate the narrowed lower portion 72 of the water rod (and hence at the bottom of the fuel column). One or more outlet apertures or openings 78 are provided at the lower end of the extension rod 76. In this arrangement, water flows upward through a large path and then downward through the small extension tube for essentially the full length of the fuel in the adjacent fuel rods before reaching the one or more exit holes 78. Note that in this configuration, there is only a short axial distance between the inlet holes 74 and the outlet holes 78, with resultant small imposed pressure differential across these holes. For low flow conditions, the downward flow tube 76 is predominantly filled with steam, and the fluid in the upward path is supported like a standpipe with a low pressure differential. The resultant liquid content in the water rod 70 is thus quite low, being proportional to the imposed pressure differential. For normal operation, the small downflow tube 76 and significant outlet flow restriction combine to severely limit water rod flow. Thus, this design results in significant steam formation in the water rod with associated unfavorable fuel efficiency, under normal operation conditions. Turning now to FIG. 4, there is illustrated another recent water rod design wherein the water rod 80 has a narrowed lower end 82 with one or more inlet holes 84 and a narrowed upper end portion 86 with one or more outlet holes 88. In this configuration, however, a central standpipe 90 extends from the inlet openings 84 upwardly to a location proximate the narrowed upper end portion 86. With flow restrictions typical of current designs, sufficient water rod flow is allowed at normal operating conditions to avoid steam formation. For low flow conditions, it was contemplated that the annular region outside of the standpipe 90 would fill with steam when the imposed pressure differential drops below that necessary to spill liquid over the top of the standpipe. Analyses have indicated, however, that under such conditions, liquid will flow backward through the upper outlet hole or holes 88 and refill the annular region 92 outside the standpipe 90. Since this region 92 has no bottom drain, it can potentially collect even more liquid than current water rod designs under similar conditions. The configurations illustrated in FIGS. 3 and 4 highlight the difficulty in designing water rods that achieve negligible vapor content at normal reactor operating conditions, while providing sufficient vapor content at low reactor flow rates. In connection with the present invention, it has been determined that the locations of the inlet and outlet holes, as well as the flow areas and hydraulic characteristics of the upflow and downflow paths are most important. The imposed pressure differential across an SWR is in fact determined by the placement of the inlet and outlet holes relative to the fuel bundle. Designs with higher imposed pressure differential will cause the SWR transitions to occur at lower reactor flow rates. One method for changing the imposed pressure differential is by locating the inlet holes above or below the lower tie plate (LTP). The latter configuration adds the LTP pressure drop to the imposed pressure differential on the SWR. Another method for changing the imposed pressure differential is by varying the elevation of the outlet holes. The imposed pressure differential increases as the outlet holes are moved further up the fuel bundle. However, since this also shortens the length of the downflow region (which is steam filled during standpipe mode of operation) it results in somewhat higher liquid content in the SWR during standpipe mode of operation. Ultimately, raising the outlet hole elevation sufficiently will result in SWR designs that have no significant improvement over current designs. Conversely, lowering the outlet holes too near to the inlet can result in unfavorable designs that are unable to transition back from standpipe mode to siphon mode, even at full reactor flow rates. Preferred locations for the downflow outlet holes fall in the range 35% to 65% of the fuel column height. Within that range, however, outlet holes should be located just above the fuel bundle spacers (element 18 in FIG. 1). Since pressure changes between spacers are relatively small compared to local spacer losses, placing outlet holes just above spacers provides added downflow length for a small penalty in imposed pressure differential. Turning now to FIGS. 5-8, specific exemplary siphon water rods in accordance with this invention are illustrated. In the first exemplary embodiment shown in FIG. 5, the water rod 94 has a narrowed lower end portion 96 with one or more inlet apertures or openings 98 which are located adjacent and above the lower tie plate. At the upper end of the water rod 94, a return or downward flow tube 100 extends downwardly from the uppermost end of the water rod, similar to the extension 76 shown in FIG. 3. In this arrangement, however, the downward extension 100 is of significantly larger diameter and also terminates approximately midway along the length of the water rod 94 (and approximately midway along the length of the fuel columns in the fuel rods) with flow exiting one or more holes or exit openings 102. Raising outlet hole elevation increases SWR liquid content somewhat during standpipe mode and causes operating mode transitions to occur at lower reactor flows. As a practical matter, the return path or extension 100 could be contained within the cross sectional area of a single water rod. Such an arrangement is shown, for example, in FIG. 6 where the downward return tube 104 lies within the water rod 106 with one or more outlet openings or apertures 108 located approximately midway along the water rod 106. The extension 104 has an open top portion 110 so that fluid flowing upwardly through the water rod 106 can spill into the downward extension 104 and exit through the one or more apertures or openings 108. As in the previously described embodiments, the water rod 106 has a narrowed lower end portion 112 with one or more inlet holes or openings 114. Turning now to FIG. 7, another siphon water rod similar to that shown in FIGS. 5 and 6 is illustrated but wherein the return tube takes the form of an outer annulus. More specifically, the siphon water rod 116 of FIG. 7 includes a narrow lower end portion 118 with one or more inlet openings 120. Internally of the water rod 116, there is a narrowed upper end portion 122 which terminates at an open upper end 124. Surrounding the narrowed upper portion 122, is a substantially closed annular region 126 with one or more outlet exits or apertures 128 located substantially midway along the length of the water rod 116, just above a radial shoulder 130 where the narrowed upper portion 122 commences. FIG. 8 illustrates a siphon water rod configuration similar to that shown in FIG. 5. The shortened downflow tube of this invention permits usage in a fuel bundle assembly which incorporates part length fuel rods (PLR""s) in the region below the downflow tube. Thus, the arrangement in FIG. 8 includes a water rod 132 with a narrowed lower end portion 134 with one or more inlet holes or openings 136. The downflow path is formed by a substantially similar diameter extension tube 138 having one or more exit openings or holes 140 at the lower end of the downflow path. Note that the exit hole or openings 140 are again located substantially midway along the length of the main water rod 132 (and approximately half way along the fuel columns within the fuel rods). In this arrangement, the fuel assembly includes one or more conventional partial length fuel rods 142 which terminate at a location proximate the outlet openings or holes 140. As indicated, the disclosed siphon water rods in accordance with this invention have bimodal states. The water rod operates in xe2x80x9csiphon modexe2x80x9d when reactor flows are high enough for the imposed pressure differential to maintain the water rod filled with liquid. The water rod operates in xe2x80x9cstandpipe modexe2x80x9d when the imposed pressure differential decreases sufficiently to allow steam generation in the water rod to break the siphon effect. When the imposed pressure differential cycles from high to low and back to high (i.e., as reactor flow cycles down and up again), there is some hysteresis in the transitions between these bimodal states. Changing from the standpipe mode requires the imposed pressure differential to be greater than the density head of the upward path completely filled with liquid. Beyond that transition point, the siphon effect will cause the water rod flow to increase rapidly. However, once the siphon effect is operative, the siphon mode can be maintained even though the imposed differential is decreased somewhat below the prior transition point. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.