Patent Number: 050892215
Section: summary

This invention relates to spacers for use in nuclear fuel bundles. More particularly a spacer is disclosed which has an Inconel grid for maintaining side-by-side fuel rods within a fuel bundle in spaced apart relation with a surrounding Zircaloy band which band has relatively low neutron absorption. BACKGROUND OF THE INVENTION Spacers are utilized in nuclear fuel bundles in order to maintain the required spacing between vertically upstanding side-by-side fuel rods within the bundles. A review of the construction of nuclear fuel bundles can clarify the function of the spacers set forth herein. Simply stated, a nuclear fuel bundle contains many so-called fuel rods supported on a lower tie plate. This lower tie plate is for supporting and spacing in side-by-side relation the rods of the bundle. These supported fuel rods extend upwardly of the fuel bundle to an upper tie plate. The fuel bundle between the tie plates is surrounded by a fuel channel which channel is usually square in cross section. This channel surrounds the fuel rods and extends from the lower tie plate to and beyond the upper tie plate. Fluid flow occurs within the fuel bundle through the lower tie plate, between the fuel rods, and out through the upper tie plate. Water coolant enters through the lower tie plate and a mixture of steam and water exits through the upper tie plate. Fuel rods are typically of the order of 170" long, whereas the fuel rod diameter is typically between 0.35" and 0.50". This being the case, it is necessary to brace the fuel rods in between the tie plates with spacers. The primary function of a spacer is to support the fuel rods and hold them in their correct side-by-side relation. A secondary function of the spacers is to enhance the water film on the fuel rods and to improve the critical power performance. Simply stated, by maintaining a water film on the fuel rods, the individual rods are maintained in an optimum steam generating condition and have resistance to so-called "non-nucleate boiling" which can cause overheating and damage to the individual fuel rods. Spacers are commonly constructed with two constituent parts. The first constituent part includes an interior grid. In one type of spacer, this grid consists of an array of ferrules which ferrules surround each of the fuel rods and keep the fuel rods spaced apart one from another. The second constituent part of the spacer is an outer band. This band extends around the grid and locates the grid inside the channel. The band occupies some of the gap between the grid and the channel, and limits the coolant flow between the grid and the channel. Without the band, there would be excessive flow near the channel which does not contribute to cooling the fuel rods. In one spacer type, a ferrule surrounds each fuel rod. The outer band has flow tabs along its upper edge. The flow tabs are adjacent the channel wall and hence function to direct some of the steam water mixture onto those fuel rods at the edge. This redirected flow increases the thickness of the water film on the edge rods and increases the critical power for these rods. Spacers have two adverse effects on fuel bundle performance. First, each spacer is an obstruction to the coolant flow. Consequently, it increases the pressure drop through the fuel bundle. The increase of pressure drop within a fuel bundle is undesirable, especially with modern high density fuel rod arrays exceeding eight by eight and often including arrays ten by ten or denser. Consequently it is desirable to construct the grid portion of the spacers to produce as little pressure drop as possible. However, the thickness of the material from which the ferrule portion or grid portion of the spacer is constructed sets a lower limit on the minimum of flow area obstructed by the spacer. Secondly, the spacers absorb neutrons. Additional fuel is required to compensate for the reactivity loss resulting from the neutron absorption. SUMMARY OF THE PRIOR ART In current boiling water reactor fuel designs, two types of fuel rod spacers are used. In one type of fuel rod spacer the entire spacer construction is mainly of Zircaloy, a metal having a low neutron absorption. Unfortunately, Zircaloy is not suitable for use as a spring material. As a consequence, such spacers require separate springs, usually manufactured of Inconel. The springs are held in place in the spacer by elaborate construction techniques. The other type of known spacer uses Inconel in the entirety for both the structure of the grid, the springs and the surrounding band. The springs are fabricated as an integral part of the grid structure. Each type of spacer has its advantages and disadvantages. Spacers constructed of Zircaloy have the advantage of having a very low neutron cross section. Thus, those spacers have a small adverse effect on the so-called neutron economy necessary for the maintenance of an efficient nuclear reaction. However, the minimum thickness of Zircaloy is limited to about 0.020" because of corrosion and hydriding. This thickness sets a minimum cross-sectional area of the spacer and, therefore, sets a lower limit to the pressure drop of the spacer. Spacers constructed of Inconel have very good corrosion resistant properties. Further, such spacers are not subject to hydrogen embrittlement and have a relatively high yield strength. Therefore, they can be used in thicknesses as low as about 0.010". Therefore, the resultant pressure drop from the use of such spacers is low. However, Inconel has a very high neutron cross-section. An Inconel spacer will absorb more neutrons than a Zircaloy spacer, even though the volume of the Inconel spacer is considerably less than that of the Zircaloy spacer. In summary, spacers constructed entirely of Inconel have low pressure drop but high neutron absorption. Spacers having mainly Zircaloy with Inconel springs have low neutron absorption but much higher pressure drop. SUMMARY OF THE INVENTION An improved spacer is disclosed which contains an Inconel grid and a Zircaloy surrounding band. In the prior art Zircaloy spacers, most of the pressure drop of the spacer is caused by the grid, and the band contributes little to the pressure drop. Therefore, most of the pressure drop advantage of the Inconel spacer can be obtained by using an Inconel grid. A Zircaloy band gives a slightly greater pressure drop than an Inconel band, but the effect is small. A Zircaloy band provides good critical power performance for the outer fuel rods and has much less neutron absorption than an Inconel band. In prior art spacers, a Zircaloy band is welded to a Zircaloy grid, or an Inconel grid is welded to an Inconel band. In the disclosed spacer, the Inconel grid cannot be welded to the Zircaloy band. Therefore, the grid and band must be joined by other methods. When the band and grid are not welded together, the band is subject to vibration caused by flow forces. Therefore, the band must be made rigid, or must be attached to the grid in a manner which prevents vibration. At the corners of the spacer, the clearance between the spacer and the surrounding channel is small. This clearance becomes smaller as the fuel rod lattice changes from an 8.times.8 array to a 9.times.9 array and to a 10.times.10 array. If a thick-wall Zircaloy band is placed around and outside of a corner Inconel cell, interference between the band and the surrounding channel results. Therefore, provision must be made in the spacer design to avoid this problem. The Inconel grid can be fabricated from extremely thin and highly elastic spring metal utilizing a modification of a prior art cell construction including individual cells for holding and locating each fuel rod. Each cell includes paired inwardly bent vertical spring legs with cantilevered and rod encircling upper and lower arm pairs. The spring legs extend at spaced apart locations between the upper and lower arm pairs and have a medial rod contacting portion. This medial rod contacting portion biases the rods within each ferrule of the grid onto stops on the rod encircling arm pairs. The springs of each cell are provided with spring dimple stops at the upper and lower ends to prevent over stressing of the spring during handling of the fuel bundle into which the spacer is incorporated and during insertion of the fuel rods into the spacers. A single cell with its cantilevered arms is fragile and difficult to handle during assembly. Therefore, pairs of cells are spot welded together such that the spring pairs are at opposite ends of the cell pair. This provides a rigid and rugged structure which can be easily manipulated during assembly of the spacer. The cell pairs can in turn be arranged as a unit to define the necessary types of spacer grids required for any particular cell construction. A preferred cell construction for a ten by ten fuel rod matrix is disclosed including, a grid enabling the placement of water rods of varying diameter. In the lower part of the fuel bundle this grid provides for support of fuel rods at all lattice positions. In the upper part of the fuel bundle there are missing lattice positions overlying part length fuel rods which permit upward venting of steam and thus reduce the pressure drop. An all Zircaloy band is disclosed which encircles the Inconel grid. Two embodiments of this band are disclosed. In the first embodiment, slots are provided in the corners of the Zircaloy band. Portions of the corner Inconel cells project into these slots, thus capturing the Inconel grid. Each of the four sides of the band has a corrugation extending over most of the width of the side. This corrugation gives the band sufficient stiffness to prevent flow-induced vibration of the band. The second embodiment of the band does not have the corrugations but has notches in the band at the upper and lower edges. These notches are adjacent to sides of the Inconel grid cells. An Inconel strap is used on each side of the band at the upper and lower edges, and overlies the notches in the band. At each notch, the strap bends inward to contact the grid, and is welded to the grid. The strap is designed to act as spring, and apply a spring force to the band at points between the notches. These spring forces hold the band against the grid and prevent vibration of the band. There results a spacer with an Inconel grid and surrounding Zircaloy band having minimum pressure drop and minimum neutron absorption useful with a high density fuel rod matrix required in modern fuel bundle design. Other Objects, Features and Advantages An object of this invention is to disclose a spacer having an internal grid constructed of minimum of spring material (such as Inconel) with a surrounding band comprised of material having a low neutron absorption cross section (such as Zircaloy). According to this aspect of the invention, a plurality of spring metal spacer cells are used. Each cell includes at least one spring leg having an inwardly deflected medial portion for contact with a fuel rod within the spacer cell. Each cell or ferrule includes at least two rod encircling arms affixed at remote ends of the spring legs. These rod encircling arms surround each fuel rod and define stops for abutting against the fuel rods. A unitary grid is fabricated from groups of the spacer cells. A grid encircling band is provided fabricated from material having a low neutron cross section such as Zircaloy. The band consists of two or four symmetric segments which are placed to encircle the grid. Mechanical means are provided to interlock the band with the grid. After placing the band segments around the grid, the segments are welded together. The firm fastening of the low neutron absorption band to spring metal grid results. An advantage of the disclosed band is that it can be formed from material having low neutron absorption. It has been found that continuous thick bands have superior hydraulic performance compared to thin spring metal bands containing formed apertures for the reduction of neutron absorption. Specifically, such bands cause more coolant flow from the inside of the fuel bundle channel wall to the outer fuel rods. A further object of this invention is to disclose spring metal spacer cell pairs for use in constructing spacers. According to this aspect of the invention, the two cells of a cell pair are confronted with their spring legs remote from one another. Fastening of the cell pairs by spot welding occurs at their rod encircling arms. As a result, a stable substructure is formed in which the two cells of a pair mutually reinforce one another. The resultant substructure can be a building block for various arrays necessary in spacer construction. An advantage of this aspect of the invention is that the paired cell arrays can be configured to receive all types of water rods as well as to define open regions overlying partial length rods for permitting the free passage of the steam/water mixture. A further object to this invention is to disclose an improved construction to the cells of this invention. According to this aspect of the invention, dimples are formed in the rod encircling arms, just above and below each spring. The dimple is dimensioned with respect to the rod contacting portion of the spring to prevent movement of the spring beyond the elastic limit responsive to rod loading on the spring. Accordingly, during assembly of the fuel bundle and during transport of the fuel bundle, overstressing of the spring legs in each of the cells is avoided. A further object of this invention is to disclose a band construction which provides sufficient clearance between the spacer corners and the surrounding channel, and which keys the grid into the band. According to this aspect of the invention, horizontal slots are provided in each corner of the band. The arms of the corner cells project into these slots, locking the grid in position. An advantage of this aspect of the invention is that the slots provide a means for locking the grid to the band, and allow the band to be moved inward at the corners, thus giving clearance between the spacer corner and the channel corner. A further object of this invention is to disclose a band construction which will not vibrate due to flow forces. According to one embodiment of this aspect of the invention, a long horizontal corrugation is formed in each side of the band. These corrugations create a very stiff band structure. According to the second embodiment of this aspect of the invention, an Inconel strap is used at the upper and lower portions of each side of the band. These straps fit into notches in the band and enclose the regions of the band between the notches. At the notches, the straps contact the arms of the Inconel cells and are welded to these arms. An advantage of this aspect of the invention is that the band acts as a very stiff structure, and is not subject to flow-induced vibration.