Patent Number: 048760633
Section: summary

BACKGROUND OF THE INVENTION This invention relates to fuel bundles. More particularly, this invention relates to an improved water rod for placement interior of the fuel assembly, the fuel assembly having a preferred 9 by 9 array of rods. SUMMARY OF THE PRIOR ART Fuel assemblies are known. Typically, such fuel assemblies include a lower tie-plate, an upper tie-plate, and a surrounding channel therebetween. The channel usually has a square cross-section. In the square cross-section are placed fuel rods. Preferably these fuel rods are placed in rows and columns. In boiling water nuclear reactors, water serves as a moderator. That is to say the water converts fast neutrons produced by an atomic reaction to slow neutrons. These slow neutrons in turn produce the desired atomic reaction. Typically, the process of the production of the atomic reaction, the production of the fast neutrons, the moderation of the neutrons with following production of atomic reaction endlessly reoccurs. The desired heating reaction results. Interior of fuel bundles it is desired to increase the amount of moderator present. Specifically, in modern fuel bundle assemblies it has been found desirable to place along the interior of the fuel bundle larger volumes of water. These larger volumes of water typically displace fuel rods. The fuel rods when displaced have their volume occupied by water. The presence of the increased volume of water provides optimum moderation of the reaction within the fuel rods. In order to provide water in a single phase and separate the provided water from the two phase steam water mixture, it is common to introduce such water in so-called "water rods". This invention relates to a water rod. SUMMARY OF RELATED DISCLOSURES (NOT PRIOR ART) In certain nuclear fuel bundle designs, the use of a 9 by 9 fuel rod array is optimal. However, the placement of volumes of moderating water within water rods has not been without difficulty. The cross-sectional area made available by removing the fuel rods has an irregular shape. For nuclear and thermal/hydraulic design efficiency, the water rod (or rods) should occupy as much of this area as possible. However, for minimum cost a simple cross-sectional shape is desirable; preferably circular or nearly circular. The desirability for such a 9 by 9 array of rods can be seen in U.S. patent application No. 176,975, filed Apr. 4, 1988 entitled Two-Phase Pressure Drop Reduction BWR Assembly Design, assigned to the Assignee herein. SUMMARY OF THE INVENTION In a nuclear fuel bundle having a lower tie-plate, an upper tie-plate and a surrounding channel therebetween, an improved water rod is disclosed for preferable use when fuel rods held between the tie-plates are placed in a 9 by 9 array. Typically, seven fuel rods are omitted centrally of the 9 by 9 array with the middle or fifth row having three rods removed and paired rods being removed in the 4th and 6th row with displacement of the removed pair towards opposite corners. Into the volume created by the removed rods, tbere are placed two "D" sectioned rods, the "D" rods each being cylindrical except for a truncating chord eccentrically located beyond the diameter of the rod, this truncating chord defining the straight back of each "D". In the preferred embodiment one of the "D" water rods is provided with spacer tabs for maintaining spacers separating the fuel rod at their correct elevations. This rod is inserted with an alignment that permits the tabs to pass through the spacers. When the rod is fully inserted, it is rotated to a locking alignment so that the tabs capture the spacers. In this locked alignment, the straight back of the "D" is aligned to confront the straight back of the confronting "D" on the second water rod. When the second "D" rod is inserted with the backs confronted mutual, locking of both "D" sectioned water rods occurs. Provision is made for narrowing of the upper and lower sections of the water rods. The upper and lower portions of the water rods are cylinders of reduced diameter which are joined to the central "D" section by transition pieces. Upper and lower end plugs are used for locating the complete water rod in the upper and lower tie plates, respectively. There results a pair of water rods which provides for efficient utilization of the available space, has low manufacturing costs, and provides for spacer capture enabling ease of assembly and disassembly of the fuel bundle. OTHER OBJECTS, FEATURES AND ADVANTAGES An object of this invention is to disclose placement of water rods in a 9 by 9 array of fuel rods contained within a square section channel. According to this aspect of the invention, the central row of fuel rods has three fuel rods displaced. On the periphery of the displaced central fuel rods and to and towards opposing corners, paired rods are removed. A total of seven rods are removed. Into this region there are placed two D-sectioned rods, the rods being circular except for a truncating chord wall. This truncating cord defines the straight back that gives the disclosed water rod a "D" profile. Two "D" section rods are placed with their backs confronting one another. An advantage of this aspect of the invention is that the "D" section rods are both easy to fabricate and conveniently occupy a large fraction of the volume of the displaced seven fuel rods from the 9 by 9 array. An additional aspect of this invention is to arrange for the support of the fuel element spacers. According to this aspect of the invention, one of the "D" section rods is provided with protruding spacer support tabs. These tabs are sized and aligned with respect to one "D" sectioned water rod to pass through spacers in one angular orientation and to lock to the spacers in another angular orientation of the rod with respect to the spacers. The water rod with the tabs is inserted at an alignment which permits the tabs to pass through the spacers. When the rod is fully inserted, it is rotated to a tab locking orientation so that the tabs capture the spacers. In this locking alignment, the straight back of the "D" is aligned to confront the straight back of the "D" on the second water rod. When the second water rod is inserted, with the straight back for the insert "D" rod confronted to the back of the first "D" sectioned rod, the two "D" water rods lock in rotational and supporting alignment. An advantage of this aspect of the invention is that both the assembly and disassembly of fuel bundles utilizing the water rods in this invention can easily occur. At the same time, the water rods cooperate in mutually locking relationship in maintaining spacer distribution within the fuel bundle.