Patent Number: 052290689
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, 2A and 2B, the prior art construction of a fuel bundle can be understood insofar as it is relevant here. A fuel bundle B is illustrated, having a channel C with an upper tie plate U and a lower tie plate L. A plurality of fuel rods R are supported on the lower tie plate L, and extend upwardly to and toward the upper tie plate U. In the embodiment here shown, a large central water rod W is utilized. Operation of the fuel bundle as part of a core in a large boiling water reactor (not shown) can be understood. Water enters through lower tie plate L. The water passes through upwardly and about the rods R. During this passage, steam is generated. Finally, a steam and water mixture passes outwardly up and through the upper tie plate U. During the steam generation, channel C isolates the core bypass volume from the flow interior of the fuel bundle. As shown in FIG. 1, and 3A, seven spacers S.sub.1 -S.sub.7 are normally utilized. These spacers are shown respectively in FIGS. 2A and 2B. With respect to FIG. 2A, spacers S.sub.5 through S.sub.1 occupying positions in the lower portion of the fuel bundle B are illustrated for a 9 by 9 fuel rod matrix. These spacers surround the large water rod W and maintain individual rods at their discrete elevations in the proper alignment. Referring to FIG. 2B, the upper spacers S.sub.6 and S.sub.7 are illustrated. These respective spacers raise the rod matrix above the termination of the so-called "part length rod." As of this writing, the preferred embodiment of this invention includes the 9 by 9 array of FIGS. 1, 2A and 2B. With respect to FIG. 2C, spacers S.sub.5 through S.sub.1 occupying positions in the lower portion of a fuel bundle similar to fuel bundle B are illustrated for a 10 by 10 fuel rod matrix. These spacers surround the large water rod W and maintain individual rods at their discrete elevations in the proper alignment. Referring to FIG. 2D, the upper spacers S.sub.6 and S.sub.7 are illustrated. These respective spacers raise the rod matrix above the termination of the so-called "part length rod." Referring to Dix et al., U.S. Pat. No. 5,112,570, issued May 12, 1992 entitled TWO-PHASE PRESSURE DROP REDUCTION BWR ASSEMBLY U.S. patent application Ser. No. 07/176,975 filed Apr. 4, 1988) In that application, it was disclosed to place within the fuel bundle a group of part length rods P. Simply stated, and above spacer S.sub.5 and before spacers S.sub.6 and S.sub.7, part length rods P were utilized. The part length rods were supported on the lower tie plate L. They extended up to and through spacer 5. They terminated a short distance above spacer 5. From their point of termination above spacer 5, the part length rods P define voids in the upper two-phase region of the fuel bundle. Benefits are realized from this construction. These have been set forth above. As the reader undoubtedly further understands, we have, through extensive testing, discovered that critical power is less than anticipated in the upper two-phase region of the illustrated fuel bundle compared with all full length rods in the bundle. That is to say, above the part length rods and through the end of the active fuel of the full length fuel rods R, critical power conditions may be met prematurely at the fuel rods. This being the case, the entire bundle must be limited so that at no individual point on any individual fuel rod R, the critical power limitations are exceeded. Referring to FIGS. 3A through 3F, the invention herein is schematically illustrated. FIG. 3A, only, represents prior art. Specifically, only two elements are illustrated. First, there is a bar graph 40. Bar graph 40 shows seven spacers S.sub.1 -S.sub.7, all on 20 inch centers. Secondly, there is a partial length rod P illustrated. Partial length rod P is shown being approximately 102" in length, and terminating just above spacer S.sub.5. This bar graph illustrates the construction set forth in FIG. 1. FIG. 3B is our preferred embodiment. Specifically, it constitutes a configuration on which actual tests have been run. Partial length rod P is 102" in length. Spacer distribution from spacer S.sub.1 through spacer S.sub.5 is the same as it has been before. Spacers S.sub.6, S.sub.7 and S.sub.8 are on respective 13.3" centers. As the reader will understand, an additional spacer S.sub.8 has been added. In all of the embodiments that follow, the reader will understand that the additional spacers are more than that number required to maintain the rods against rod bow and in their design side by side relation. Further, the spacers are ferrule-type spacers, utilizing a relatively thin zircaloy metal (in the thicknesses generally about 20/1000ths of an inch). It has been found that improved critical power results. Once the configuration of FIG. 3B is understood, other possible configurations suggest themselves. They will be briefly addressed below. With respect to FIG. 3C, bar graph 40 shows a part length rod approximately 115" long, extending up to, through and including spacer S.sub.6. The spacer separation is the same as FIG. 3B. Referring to FIG. 3D, a part length rod 113" is utilized. The spacing of the spacers differs only above spacer S.sub.4. From spacer S.sub.4 through spacer S.sub.8 the spacers are on 15" centers. Regarding FIG. 3E, it will be understood that the spacing of the spacers remains the same as in FIG. 3D. However, the part length rod is 97" long, and thus is braced at spacer S.sub.5. Regarding FIG. 3F, the design there appears to have potential even exceeding our preferred embodiment which we illustrate in FIG. 3B. As of the writing of this patent application, this configuration has not been specifically tested. We therefore do not claim it as our preferred embodiment, but do call to the attention of the reader the fact that this design may be beneficial. Simply stated, and above spacer S.sub.4, the pitch of the spacers S.sub.5, S.sub.6, S.sub.7 and S.sub.8 gradually decreases. The partial length rod used with the design is 116" in length, and extends through spacer S.sub.6. Specifically, between spacer S.sub.4 and S.sub.5 an 18" separation is utilized. Between spacer S.sub.5 and S.sub.6, a 16" separation is utilized. Between spacers S.sub.6 and S.sub.7, a 14" spacing is utilized. Finally, between spacers S.sub.6 and S.sub.8, a 12" spacing is utilized. The reader will realize that in this latter design, decreasing spacer pitch occurs at that portion of the fuel bundle wherein the void fraction increases. It has been found in addition to the increased spacer pitch, that spacers incorporating swirl vane constructions in the upper two phase region of the fuel bundle in conjunction with partial length rods have the same overall beneficial effect. Specifically, critical power is increased even though the insertion of the spacers having the swirl vanes tends to restore some--if not all--of the improved pressure drop in the upper two phase region of the fuel bundle. Accordingly, the following constructions are exemplary of spacers which when left on a regular pitch through the incorporation of swirl vanes produce an increased critical power phenomenon. Referring to FIG. 4A, an I shaped tab 109 having tabs 110 at the upper portion and tabs 112 is shown in the planar mode before twisting. FIG. 4B shows this construction in the twisted configuration. FIG. 4C shows the swirl vane incorporated to ferrules at their respective upper and lower ends. In this configuration, the main portion of the tab 109 deflects water towards the rods of the spacer while vapor is allowed to continue upwardly. More importantly, this spacer when incorporated to spacers S7, S6 and S5 of FIG. 3A or spacers S8, S7, S6 or S5 of FIGS. 3B-3F enables fuel bundles having part length rods to realize improved critical power. It is important to note a distinction. FIG. 3A insofar as it discloses ordinary spacers in combination with part length rods is prior art. However, when spacer having swirl vanes are added in addition to a fuel bundle having part length rods, the improved critical power limitations of this invention are realized. It is not necessary that the swirl vanes extend the entire length of the spacer. Specifically FIG. 5A illustrates a swirl vane end tab 132 before twisting. This swirl vane end tab 132 is show twisted and attached to the ferrule spacer construction illustrated in side elevation and plan respectively in FIGS. 5B and 5C. Attachment occurs at tabs 132 to the sides of the ferrules F so that turbulence imparting protrusion occurs above the spacer S. Other constructions can be utilized. Referring to FIG. 6A, a tab 140 having depending arms 142 is shown before twisting. In respective FIGS. 6B and 6C, attachment of the tabs occurs to the ferrules F with the full length of the arms 142 effecting secure fastening of the arms to the ferrules F. Finally, referring to FIG. 7A, a swirl vane 139 is shown in the untwisted state. Referring to FIG. 7B, the respective swirl vanes 139 have all been twisted. Tabs 140 are then ready for attachment to a ferrule spacer. Referring to FIGS. 7C and 7D attachment to the respective ferrules can be seen. Referring to FIG. 7C, an important detail can be noted. It is important that continuous web 142 not interfere with the spacing or pitch at the bottom of the spacer. Such interference could seriously alter the side-by-side spacing of the ferrules F. Accordingly, the tabs 139 have a length so as to dispose the continuous web 142 below the side-by-side ferrules F. As has been set forth above, other expedients associated with the spacers can be utilized to realize increased pressure drop. For example, in FIG. 8, a spacer of increased vertical height is utilized. Additionally, spacers having metallic constructions from thicker metallic sheets may be utilized. All that is required is to recapture at least some of the pressure drop achieved by the insertion of the part length rods. Regarding the extent of this recapture of pressure drop, we prefer to recapture less than all of the pressure drop realized. Accordingly, this leaves the upper two phase region of the fuel bundle with less pressure drop than the same bundle would have had with only full length rods. It is important to note that we use the increased spacer pitch or the swirl vanes attached to the spacer in combination with the two phase flow at the top of the fuel bundle. We rely on the effect of the spacer co-acting with the flow after it has passed through the spacer. This "downstream flow" occurs upwardly from the spacers after the two phase flow has passed over one of the spacers. This effect is important with respect to spacers S7 (FIGS. 3B-3F), spacer S.sub.6 and spacer S5. The top most spacer, spacer S7 in FIG. 3A and spacer S8 in FIGS. 3B-3F is an exception to this flow principle. It is not required that the top most spacer S7 in FIG. 3A or spacer S8 in FIG. 3B-3F be either a ferrule type spacer or have swirl vanes attached. In most fuel loadings, the kilowatt output per foot above the top most spacer is not at a level where transition boiling leading to adverse critical power ratios can occur. Consequently, an inconel spacer having low pressure drop with higher neutron absorption can be successfully used at this location. This upper spacer need not incorporate the increased spacer pitch or the disclosed swirl vanes. It will be apparent that this invention will admit of modification. It further will be appreciated that the decreased spacer pitch above the termination point of the partial length rods, is a major characteristic of this invention. Referring to FIG. 9, a fuel bundle B is illustrated. Bundle B includes a lower tie plate L, an upper tie plate U and a 9-by-9 matrix of discrete fuel rods F. A channel C surrounds the respective fuel rods and extends from the lower tie plate L to the upper tie plate U. Lower tie plate L supports the fuel rods F in a side-by-side matrix; upper tie plate U assures that the fuel rods are maintained in vertically upstanding relation. The fuel rods extend over a distance of approximately 160 inches and are flexible. This being the case, a group of spacers (typically in the order of 7) maintain the side-by-side relationship of the fuel rods F. In FIG. 9 spacers S.sub.1, S.sub.2 B and S.sub.5 illustrate three of the normally seven evenly placed spacers extending along the length of the fuel bundle B. Operation of the fuel bundle can be summarized. Typically, water moderator enters through lower tie plate L at defined apertures between the matrix of fuel rods F. The water flow is confined by channel C to flow outwardly through upper tie plate U. As the water moderator passes upwardly through the fuel bundle, steam is generated in increasingly higher fractions. Finally, at the top of the fuel bundle and up and through upper tie plate U, the discharge of water and steam occurs. Fuel bundle B contains part-length rods P. Such part-length rods P are disclosed and claimed in Dix, et al. U.S. Pat. No. 5,112,570 entitled TWO PHASE PRESSURE DROP REDUCTION BWR ASSEMBLY DESIGN issued May 12, 1992, (formerly U.S. patent application Ser. No. 07/176,975 filed Apr. 4, 1988). It will be noted that the two partial length rods P illustrated are spaced apart. Additionally, and overlying the part-length rods, there is defined an open spatial interval in the fuel bundle which interval is designated 114. As set forth in this original disclosure, these disbursed flow channels realize the natural tendency of the vapor phase of the two-phase mixture to migrate or drift towards the low resistance flow paths formed at the void volumes 114. It has been found that such disbursed flow paths are favorable to provide an improved fuel to moderator ratio in the upper two phase region of the bundle as well as to provide a low pressure drop path for the venting of steam which imparts combined nuclear, stability and thermal hydraulic advantages. At the end of each partial length rod P, I illustrate a separation device D. Generally stated, the purpose of the separation devices D as set forth in this specification is to separate water from the volumes 114 which may either be entered into or be entrained into the upwardly venting steam within volumes 114. This enhances the natural tendency for steam to flow in these volumes such that even better steam venting benefits can be achieved without the part length rods being spaced apart. Summarizing the remainder of the specification is beneficial at this juncture. Specifically, FIGS. 10, 11, and 12 disclose various separation devices which may be placed at the end of individual fuel rods. FIGS. 13, 14, 15, 16, and 17 disclose separation devices mounted to spacers. FIGS. 18, 19, and 20 disclose separation devices which can also be mounted to spacers, but preferably pass through the spacers and are suspended form the upper tie plate. FIG. 13 shows a combination of a separation device mounted at the end of a part-length fuel rod as well as a separation device attached to a spacer. FIG. 18 discloses a separation device combined with and extending continuously from the end of a part length rod through overlying spacers. it will be understood that more than one such combined device can be placed in the fuel assembly. FIGS. 16, 17, 19, 20, 21, 22, and 23 disclose arrays of adjacent part length rods with overlying separation devices. Such part length rod arrays can be distributed in various arrangements within the fuel assembly. FIGS. 19, 20, 21, and 22 disclose these separation devices extending through two or more spacers. These extended devices can pass through and be suspended from the upper tie plate to maximize steam venting and allow for top removal of the devices. FIGS. 22, 23, and 24 disclose devices to incorporate water rods within or adjacent to the steam vent volume to improve moderator distribution within the fuel assembly. With reference to FIGS. 10, 11, and 12, the reader will understand that the partial length rod P is the only such rod shown. It will be understood that any one or more of the partial length rods P could be placed in the rod array as illustrated in FIG. 13. In FIG. 10 the partial length rod P has at the end thereof an outwardly flaring bell-shaped cone 116. The purpose of the cone 116 is to divert upwardly flowing water outwardly and away from volume 114 overlying the part-length rod P; such deflection is illustrated at arrow 118. At the same time, steam 120, having a lighter mass, can divert its flow into the volume 114 overlying the part-length rod. Referring to FIG. 11, part-length rod P has attached to the end thereof a swirl vane 120 the twist here being oriented over 180.degree. in the counterclockwise direction as the band 120 extends upwardly from the end of the part-length rod 121 into the volume 114 overlying the part-length rod. The function of band 120 is easily understood. It imparts to dense water particles an outward centrifugal vector; steam being of lighter mass continues into the upward volume 114. Finally, and referring to FIG. 12, part-length rod P is shown with an array of outwardly deflecting tabs 125. Outwardly deflecting tabs 125 have the function of deflecting outwardly the dense water and permitting steam to continue upwardly in an uninterrupted path. At this point the reader should understand that many other separation devices at the end of the part-length rods can be utilized. All that is required is that the devices be capable of deflecting outwardly the denser water flow while permitting steam to continue vertically upwardly. Referring to the view of FIG. 13, a part-length rod P termination shortly above a spacer S.sub.5 is shown at a section of a fuel bundle similar to that illustrated in FIG. 1. The particular separation device D utilized is similar to those separation devices illustrated in FIGS. 9 and 11. FIG. 13 shows an additional aspect of this invention. Specifically, spacer S.sub.6 is shown supporting a second separation device D', device D' taking the form of a downwardly disposed cone 30. Referring to cone 130, it can be seen that the apex of the cone is disposed towards the partial length rod P; the truncated base of the cone is mounted upwardly into spacer S.sub.6 which is the spacer immediately overlying the part-length rod. The function of the cone is easy to understand. Heavier liquid particles are directed outwardly to the adjacent fuel rods F. Steam continues upwardly in the volume 114 overlying the part-length rod P. Referring to FIGS. 14 and 15, the disposition of an alternate separation device D' at the spacer S.sub.6 is illustrated. In FIG. 6 the matrix defined by the spacer S.sub.6 maintains a swirl vane 140. Like the separation device of FIG. 3, swirl vane 140 is twisted over 180.degree. and serves to centrifugally separate water from the volume 114 overlying part-length rod P. It will be understood that separation device D' is effective in separating out water that may be entrained into the steam vent of volume between the part-length rod P and spacer S.sub.6. The construction of FIG. 15 is similar, the only exception being that the swirl vane 142 is of a larger width occupying substantially the full volume within the spacer S.sub.6 between the fuel rods F. FIG. 16 illustrates a similar construction, with an array of adjacent part length rods P terminating below spacer S. Spacer S supports an array of overlying swirl vanes 150 at a distance above the ends of part length rods P. As the reader will understand, the disclosed side elevation only illustrates three part length rods and associated swirl vanes. More could be used. For example, a 3 by 3 matrix adjacent of part length rods could be used. FIG. 17 illustrates a similar construction to FIG. 16. Here a larger diameter swirl vane 152 attached to spacers overlies an array of adjacent part length rods P. The part length rods P are configured typically in a 3 by 3 square pattern. More could be used. FIG. 18 illustrates an extended swirl vane 160 attached to the end of a part length rod. Part length rod P and swirl vane 160 form a unitary structure. This rod P and swirl vane 160 mount in the same manner as the side-by-side full length fuel rod F. Consequently, part length rod P can be removed by grasping swirl vane 160 or any fixture attached to swirl vane 160 for that purpose. The swirl vane can also be constructed with two crossed (cruciform sectioned) metal bands to increase strength for the swirl vane of this design The part length rod and swirl vane are here shown extending between two spacers S.sub.1 and S.sub.2. FIG. 19 illustrates a single large swirl vane 162 overlying a number of adjacent part length rods (for example a 3 by 3 matrix of part length rods P). This single large swirl vane 162 is attached between spacers S.sub.1 and upper tie plate U. Alternately, provision can be made for attachment of the large swirl vane between two adjacent spacers (See FIG. 18). Provision of an opening for this device to pass through the upper tie plate U maximizes the steam venting effectiveness of this design. Mounting the device to the upper tie plate allows for removal of the device from the top, thus providing access to the part length rods underlying the device. The construction of FIG. 20 is similar, except the single large swirl vane device is replaced with a unitized matrix of smaller swirl vanes. The matrix of swirl vanes here shown and here illustrated is 3 by 3. This unitized matrix is illustrated with surrounding bands 168 to provide positioning at the fuel assembly spacers S.sub.1. This device preferably passes through and is suspended from the upper tie plate U. The construction of FIG. 21 is similar, except the underlying part length rods P are of unequal height. Consequently, the overlying unitized matrix of swirl vanes 165, 165' is of unequal length. The large steam vent volume may reduce local neutron moderation. Therefore, it may be necessary to improve moderator distribution by incorporating additional water into the central portion of the fuel assembly. FIGS. 22, 23, and 24 disclose devices wherein water rods are incorporated to the swirl vane structure of this invention. FIG. 22 illustrates an alternate construction for the large swirl vane of FIG. 11, wherein a central water rod W is placed integral with the swirl vane 170. The underlying central part length rod is removed to allow for downward extension of the water rod W. The water rod W is shown the same diameter of the fuel rods F and part length rods P; other so-called large water rods W may be used where the diameter exceeds the diameter of the fuel rods F. FIG. 23 shows similar construction for a water rod integral within a unitized swirl vane matrix 172. FIG. 24 illustrates a representative fuel assembly configuration using a swirl vane matrix 176 with individual swirl vanes 174 such as illustrated in FIG. 20 or FIG. 21. Water rods W are placed adjacent to the removable swirl vane matrix 176. Such placement of water rods also allows for standard axial positioning of the fuel assembly spacers S.sub.1 and S.sub.2 (See FIG. 23). The reader will understand that in my description of separation devices D', I contemplate any type of separation device overlying the part-length rod, this separation device acting to eject either entering or entrained water from the void volume overlying the end of the part-length rod.