Patent Number: 059784300
Section: summary

TECHNICAL FIELD The present invention relates to the measurement of water rod lengths in a nuclear fuel bundle and particularly relates to apparatus for and methods of measuring the lengths of adjustable length water rods during assembly of the fuel bundle. BACKGROUND OF THE INVENTION In a new fuel bundle designed by the assignee of the present invention, mechanical support for the bundle is provided by a pair of central water rods connected to the lower tie plate rather than by conventional tie rods. Each of the water rods comprises a pair of separate assemblies. The lower assembly in each rod comprises a central tube while the upper assembly includes a tie bar. The central tube and the tie bar are adjustably secured to one another by a mechanism not forming part of the present invention but which results in water rods having adjustable lengths. The adjustment is provided by relative axial movement of the central tube and the tie bar which, when adjusted, provide a water rod of fixed length. In this design, and because the fuel bundle is supported at the top by the pair of water rods rather than by tie rods, the overall lengths of the two water rods must be closely matched to prevent tilting or excessive stress on the tie bars. The lengths of each rod from the top face of the lower tie plate to the opposite end of the rod must lie within a very narrow range of tolerances relative to one another, for example on the order of 0.020 inches and must correspond substantially exactly to a predetermined length of the water rod e.g. within 0.040 inches. It will be appreciated that fuel bundles are typically assembled with the bundle lying in a horizontal position which, after assembly, is lifted to a vertical position. That movement as well as other assembly procedures cause stresses in the fuel bundle which can alter the length of the water rods after they have been initially adjusted in length. It is essential that the water rods lie within specific design length limits when the fuel bundle is shipped to the operational site. BRIEF SUMMARY OF THE INVENTION According to the present invention, there is provided a length gauge for in situ measurement of water rod lengths during and after final assembly of the nuclear fuel bundle. The length gauge includes an indicator location block, a gauge for mounting on the indicator location block and carrying a movable element, a standards rod and an indicator set block. The indicator location block preferably includes a block body have arcuate recesses opening along opposite sides of the body and carrying a boss for mounting an indicator gauge on the block body with the movable element projecting below a first reference surface on the block. The tip of the movable element constitutes a second reference on the block. The standards rod includes a reduced diameter lower end for insertion in an opening in the lower tie plate. The opposite end comprises a flat surface for engagement with the second reference surface of the element. The indicator set block includes a pair of known length cylindrical projections essentially straddling a reduced length cylindrical projection. The upper surfaces of the long projections constitute first datum surfaces and the upper surface of the smaller projection constitutes a second datum surface. To use the gauge, the gauge is first set to a predetermined indication, typically a zeroed-out position on the gauge's digital indicator, for a predetermined extension of the movable gauge element. To accomplish this, a long projection on the indicator set block is disposed in one of the recesses of the indicator location block with the first datum surface bearing against a third reference surface on the location block. The movable element of the gauge engages the second datum surface and the gauge is zeroed to the known distance between the first and second datum surfaces. To assemble the fuel bundle, the pair of water rods are secured to the lower tie plate. Using the length gauge, the length of the water rods are set. For example, one end of the standards rod is inserted into an opening in the lower tie plate. The location block is disposed below a reference seat at the upper end of a tie bar of one of the water rods. By engaging the opposite end of the standards rod against the movable element, the gauge indicates any deviation in the length of the water rod from a designed length corresponding to the length of the standards rod plus a known predetermined distance between the first reference on the location block and the tip of the movable element. By measuring each water rod, the rods may then be adjusted in length to the design length. After adjustment of the water rod lengths, the gauge and the standards rod are removed and the spacers are secured to the water rods at selected axial positions. Subsequently, the standards rod is inserted through the spacers. To accomplish this, a shaped end e.g. a bullet nose is disposed on one end of the standards rod to facilitate insertion of the standards rod through the openings in the spacers. Once the standards rod with the shaped end emerges into the region between the lowermost spacer and the lower tie plate, the shaped end is removed from the end of the standards rod and the standards rod is inserted into the opening in the lower tie plate. The fuel rods are then assembled into the spacers with the standards rod in place. Once the bundle has been finally assembled and displaced to an upright position, a second measurement using the length gauge is effected. This is accomplished in the same manner as previously described to ensure that the lengths of the water rods lie within design limits. The standards rods can then be finally removed by withdrawing it axially from the fuel bundle. It will be appreciated that the standards rod when inserted into the spacers occupies a lattice position corresponding to a part length fuel rod. Thus the final fuel rod, i.e., the PLR, may then be inserted into the fuel bundle through aligned openings in the spacers and secured to the lower tie plate to complete the assembly. In a preferred embodiment according to the present invention, there is provided in a nuclear fuel bundle having a water rod connected to a lower tie plate, apparatus for measuring the length of the water rod comprising a standards rod engageable at one end with the lower tie plate, an indicator location block, the block defining a first reference surface for bearing against a seat on the water rod, a gauge having a movable measuring element mounted on the block with the movable element engageable with an opposite end of the standards rod, the element defining a second reference surface a predetermined distance from the first reference surface at a predetermined gauge indication, whereby the gauge indicates any deviation of the length of the water rod from a designed length corresponding to the length of the standards rod and the predetermined distance. In a further preferred embodiment according to the present invention, there is provided a method of measuring the length of a water rod in situ in a nuclear fuel bundle, the water rod being connected to a lower tie plate of the bundle comprising the steps of disposing a standards rod in generally parallel side-by-side relation with the water rod, engaging one end of the standards rod against the lower tie plate, aligning a first reference surface on a location block and a reference mark on the water rod, providing a gauge on the block carrying a movable measuring element defining a second reference surface a predetermined distance from the first reference surface at a predetermined gauge indication and engaging the element with the standards rod at an opposite end of the water rod, whereby the gauge indicates any deviation of the length of the water rod from a designed length corresponding to the length of the standards rod and the predetermined distance. Accordingly, it is a primary object of the present invention to provide novel apparatus and methods for measuring the lengths of water rods in situ in a nuclear fuel bundle.