Patent Number: 048896845
Section: description

Referring to FIG. 1A, a fuel bundle S is shown in relation to a reactor core. The core includes two flow regions. One of these flow regions is interior of the fuel bundle, the remaining region is exterior of the fuel bundle and in core bypass region B. A simplified description of the fuel bundle can be presented. A lower tie-plate L is illustrated. The tie-plate supports a group of fuel rods R. Some of these rods are threaded at 16. These threaded rods extend between lower tie-plate L and upper tie-plate U. The threaded rods form connections between the respective tie-plates and maintain the remaining rods in side-by-side upstanding relation. Apertures in the region between the fuel rods permit flow of water into the fuel bundle. Extending between the two tie-plates L and U and on the outside of the fuel bundle is a channel C. Channel C forms a flow barrier between the inside of the fuel bundle F and the core bypass region B. At the bottom of the lower tie-plate are metering apertures A. The pressure difference between the inside of the fuel bundle and the bypass region causes water to flow through the metering apertures A into the bypass region B. As has been set forth, it is this pressure drop acting from the interior of the fuel bundle F to the core bypass region B that causes the channel C overlying the lower tie-plate L to bulge away from the sides. The prior art has prevented unreasonable flow rates in the interstices between the sides of the lower tie-plate L and the channel C by the use of fingers F. Fingers F are typically spring biased away from the sides of indentations 18 in the lower tie-plate. Such spring bias enables the fingers F to move into the volume between the lower tie-plate L and the channel C as the channel deflects away from the lower tie-plate. Unfortunately, such spring bias is an additional contributing factor to channel deflection. Referring to FIG. 1B, a section of the lower tie-plate L at the channel C is illustrated. This Figure shows how leakage flow occurs without finger springs, or other means for limiting leakage flow. Specifically, water under pressure enters aperture 14 flowing upwardly through a grid 20 for support of the individual fuel rods R. After passage through grid 20, some water finds its way into the interstitial flow volume 22 defined between the lower portion of the channel C and the overlapping portion of the channel tie-plate L. The pressure effect produced on this portion of the channel can be seen with respect to the coolant pressure diagram plotted with respect to the channel C. Remembering that a pressure P.sub.i is present on the interior of the fuel bundle F, it will be seen that channel C above the lower tie-plate L is under a complete pressure differential between the core bypass region B and the interior of the fuel bundle F. Water passing in the interstitial area 22 gradually loses its pressure until escape from the bottom of the channel at 25. Consequently, coolant pressure likewise falls gradually over the length of the channel as it is illustrated. Such pressure experiences an immediate drop at the entrance to the channel at 23 until an almost full drop is realized upon exit at 25. Referring to FIG. 2, an embodiment of this invention is illustrated. Lower tie-plate L is shown. The lower tie-plate includes first and second labyrinth seal rings 30, 32. These labyrinth seal rings define expanded flow areas 31, 33. Functions of labyrinth seals are well known. Specifically, the labyrinth seals provide expanded volumes for inefficient turbulent flow to occur immediately after each of the constricted apertures 30, 32. Such expanded and inefficient flow causes large and rapid pressure differentials across the labyrinth seal. Referring to the flow diagram, it can be seen that the labyrinth seal matrix of the respective seals 30, 32 produces a rapid and full pressure between the entrance to the seal 29 and the region below the seal 38. The portion 38 of the channel C underlying this rapid pressure drop area has no pressure load acting on it. The part of the channel which is not loaded can thereafter reinforce the overlying portions of the channel. Referring to FIG. 3, the reader can understand that it is possible for a labyrinth seal to be configured by both cooperative shaping of the lower tie-plate L and the channel C. According to this embodiment, channel C is provided in the vicinity of the lower tie-plate L with corrugations, which corrugations 40 define with respect to the lower tie-plate L successive contracted regions of flow 42, 44 followed by expanded regions of flow 43, 45. These respective regions of flow define a labyrinth seal along the length of the interstitial volume between the channel C and the lower tie-plate L. Referring to FIGS. 4A, 4B, and 4C, a preferred embodiment of this invention is illustrated. A lower tie-plate L and channel C are shown. The area of the improvement herein is shown by circular detail 50. Referring to FIG. 4B, a first detail is illustrated. Specifically, channel C against lower tie-plate L defines a diffuser volume 54, diffuser volume 54 includes an initial venturi flow 53. Referring to the diagram configured opposite to FIG. 4A, the effect of this can be seen. Specifically, at inflow area 51, pressure exceeds that of the core bypass region P.sub.o. However, between the region above the aperture 51 and the beginning of the aperture 55 there is a large increase in flow velocity and a corresponding decrease in pressure, in accordance with Bernoulli's theorem. Between the beginning of the aperture 55 and the beginning of the diffuser 53 there is an additional pressure drop caused by friction losses over the length of the aperture. Thereafter, and due to the action of the diffuser, pressure slowly increases until it equalizes to the pressure of the core bypass region at P.sub.o. In accordance with this aspect of the invention, not only does the lower unloaded portion of the channel C reinforce the overlying loaded portion of the channel but additionally, a negative pressure acts inwardly in the vicinity of the lower portion of the channel C at the tie-plate. It is important to notice that this negative pressure will increase upon increasing deflection. That is to say, the very effect of pressure induced deflection, which it is the purpose of this invention to combat, causes increased flow with increased negative pressure differential. The reader will also understand that by configuration of the lower tie-plate at 60 on FIG. 4C, the same effect can be reached. Specifically, a pressure drop at the aperture 53 is formed. Moreover, a diffuser 54 is present. Thus, by either the configuration of the lower tie-plate L or the bottom of the channel C, the hydraulic forces on the channel C counteract the bulging forces heretofore set forth. Having set forth the invention as relates to the sides of the channel adjacent the lower tie-plate, attention can now be directed to an improvement at the corners of the polygon sectioned (square sectioned) channel. Corners are an unavoidable leakage path. Because of manufacturing considerations, a gap is intentionally designed at the corners and a snug fit between channel and lower tie-plate is designed over the sides. Because of difficulty in maintaining tight tolerances there is uncertainty in this leakage. Referring to FIG. 5, a diffuser 70 is shown placed at the corner of the lower tie-plate L between the channel C and the tie-plate. FIG. 5A shows a top view of one corner of the lower tie-plate, and FIG. 5B show a section through the corner. This diffuser, however, is reversed. It includes an aperture 72 disposed towards the core bypass region B and a diffuser aligned to and towards the interior of the fuel bundle F. The purpose of this reverse diffuser can be easily understood. First, reference will be made to the aperture A in the prior art of FIG. 1A. Thereafter, and with a brief explanation, a return to FIG. 5 will illustrate the principles involved. In the prior art, and for the purposes of reflood of the fuel bundles F, aperture A serves two purposes. During normal reactor operation, aperture A permits a small and metered amount of water to pass from the interior of the fuel bundle F to the core bypass region B. In such passage, a pressure drop was experienced, which pressure drop enabled metered flow under low pressure to the core bypass region B. Upon a loss of coolant accident, aperture A is intended to have a reverse flow. In such a reverse flow it enables water flooding the core bypass region B to pass interior of the fuel bundles F. In such passage, the fuel rods R remained flooded and are not subjected to the consequences of overheating. Turning to FIG. 5B, the function of the reverse diffuser can be set forth. During normal operation, flow will typically be down the diffuser section 73 to the aperture 72. Thereafter, flow will pass through in a constant metered flow the length of the aperture 74 until discharged to the core bypass B occurs. Such flow can be relatively constant and metered. Thus, the reverse diffuser configuration can serve the function of metering water from the interior of the fuel bundle F to the core bypass region B. Upon reflood, the water flow will be reversed as shown by arrows 75. As is well known in the hydraulic arts, by aligning the diffuser with respect to flow, an efficient expansion of flow occurs with an accelerated passage. Thus, with the reverse diffuser here disclosed, reflood of the fuel bundle F from the core bypass region B will occur with greater efficiency. The reverse diffuser has very little effect on the leakage flow during normal operation. Some of the pressure drop is friction as the flow passes through the gap, most is the loss in energy as the exiting jet dissipates its energy in the bypass region. During reflood the diffuser acts as a diffuser. There is very little pressure loss as the flow exits from the gap through the diffuser, into the fuel bundle. Thus the time to reflood is reduced. Thus, the only new feature is the enhancement of reflooding provided by the corner diffusers.