Patent Number: 061012319
Section: description

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIG. 1, there is illustrated a nuclear storage pool, generally designated 10, having side walls 12, a bottom wall 14 and an upper opening along which a platform 16 is mounted. A fuel bundle 18 is illustrated in FIG. 1 in pool 10, it being appreciated that the fuel bundle comprises a plurality of nuclear fuel rods 19 in an ordered array with spacers S at axially spaced positions therealong maintaining the fuel rods in the array. The spacers at different elevations are indicated S1 . . . S8 starting adjacent the lower end of the bundle. Each spacer has a plurality of cells C, each cell containing a spring 23 (FIG. 3A) whose spring force is to be measured. The fuel bundle 18 is conventionally transported in the pool 10 and a suitable mechanism 22 may be coupled to the fuel bundle 18 for raising and lowering the bundle within pool 10. Referring to FIGS. 2 and 2A, the in-bundle spacer spring force measurement system of the present invention includes a draw rod standard 24. Standard 24 includes an elongated rod having a series of carefully calibrated fixed diameter segments along the length of the standard separated by transition areas having gradual tapers from smaller to larger outside diameters. For example, segment 26 has a diameter corresponding to the nominal diameter of the fuel rod. The next segment 28 has a diameter incrementally increased relative to the nominal diameter of segment 26, i.e., an increase of x beyond the nominal diameter. The third illustrated segment 30 has a nominal diameter 2x plus the nominal diameter of the segment 26, x being a fraction of the nominal diameter of segment 26. Additional segments may be provided as desirable. The segments are separated by transition sections 32 and 34 which have gradual tapers from the smaller to the larger diameter segments. The larger diameter segments are selected to permit slight compression of the spacer springs as the standard 24 is withdrawn through the cell in which the spring to be measured is located. The presence of different diameter segments enables comparison of the changes in the measured spring forces and an estimation of the spring constant. The upper end of the draw rod standard 24 has a key slot 37 for receiving a ball 39 at the end of a cable 36 whereby the standard 24 may be displaced upwardly and drawn through the spacer openings in sequence, as described below. The overall length of the draw rod standard is designed such that the draw rod standard can be withdrawn through a single spacer cell at a time without entering another spacer. Also, the lower end of the draw rod standard has a bullet nose to minimize or eliminate any damage to the spacer upon insertion or withdrawal of the draw rod standard relative to the fuel bundle. Referring back to FIG. 1, the cable 36 passes through a rigid insertion tube 38 which, in turn, is coupled at its upper end to a standard rigid extension tube 40. The cable 36 continues through tube 40 and is coupled to a load cell 42. The cable is wound around a constant speed drum cable winch 44. The load cell 42 is connected to a computer data acquisition system 46 having an operating system and data acquisition software for receiving signals from the load cell 42. The loading on the cable as described below is measured by the load cell 42 and typically thousands of data loading measurements per second are taken as the draw rod standard is drawn through each spacer opening. Those measurements are statistically analyzed to give the withdrawal forces applicable to each segment of the draw rod standard and the different results from the different segments of the standard are analyzed to determine the spring rate, i.e., the spring force constant. The measured spring rate and withdrawal forces are used to determine the spacer cell spring force at the bundle design's nominal fuel rod diameter. The insertion tube 38 is a hollow tube having the same outside diameter as the nominal diameter of the fuel rod for the bundle design being measured. A different insertion tube is therefore required for each different fuel design. As best illustrated in FIG. 2, the upper end of the standard 24 has a bullet nose 48 and the lower end of the insertion tube 38 is received over the bullet-shaped end 48 of the standard 24, as illustrated by the dashed lines. This enables the insertion tube and draw rod standard to be attached together for alignment and insertion into and withdrawal from the fuel bundle. However, the fit between the insertion tube 38 and standard 24 is not sufficient to lock the two components together, for reasons which will become apparent from the ensuing description. The extension tube 40 is a standard hollow aluminum stainless steel or Zircaloy tube which has threaded fittings for securement to the upper end of the insertion tube 38. A single design for the extension tube may be used for all bundle types since the extension tube does not enter the fuel bundle. At the top of the extension tube 40 is a clamping mechanism which enables the tube to be locked in place relative to the cable 36. This enables the insertion tube 38 and spring force draw rod standard 24 to be held together while they are moved up and down in the bundle. That is, the clamping device holds the insertion tube and draw rod together since the upper end of the insertion tube 38 is provided with a threaded end plug which screws into the threaded coupling on the lower end of the extension tube 40. As noted previously, the spring force of the spring on the spacer in a particular opening is measured by measuring the tension in the cable employed to withdraw the standard through the spacer cell. The spring force can be computed from the following equation: EQU Withdrawal Force=T.sub.o +.mu..times.Spring Force, where T.sub.o is the tension in the cable connected to the spring force draw rod standard needed to support the mass of the draw rod and cable, and PA1 .mu. is the coefficient of friction between the spacer spring projection and the draw rod standard. Thus, with the withdrawal force being measured by the load cell 42, the weight of the cable and standard being known and the coefficient of friction likewise being known, the spring force can be ascertained. To employ the in-bundle spacer spring force measurement system hereof, the bundle 18 is moved into the pool 10 and a fuel rod at the location of the cells whose spring forces are to be measured is removed. As illustrated in FIG. 1, the elongated extension tube, insertion tube and draw rod standard combination are located over the cells vacated by the fuel rod and in which cells the spring forces of the springs are to be measured. The fuel bundle is then elevated relative to the force measurement system such that the draw rod standard is received within the bundle with the first segment 26 located within the lowermost spacer S1 having a spring whose spring force is to be measured as illustrated in FIGS. 3 and 3A. Suitable mechanisms, not shown, maintain the assembly of extension and insertion tubes and draw rod standard substantially fixed against vertical and lateral movement to enable reception of the insertion tube and draw rod standard within the bundle. Alternatively, the relative movements of the bundle and the assembly can be accomplished by moving one or the other of the assembly or bundle or both. As illustrated in FIGS. 4 and 4A, the extension tube and insertion tube are then drawn back leaving segment 26 of the standard located within the cell. The insertion tube 38 and extension tube 40 are drawn back to locate the lower end of the insertion tube 38 adjacent the next higher spacer to avoid interference with the standard as the standard is drawn through the lower spacer. The drum 44 is then activated at constant speed and the draw rod standard is slowly raised through the cell of the lowermost spacer into the space between spacers S1 and S2 as illustrated in FIGS. 5 and 5A. Withdrawal force data from the load cell is accumulated and analyzed by the system 46 and the computed spring force and spring rate may be displayed upon completion of the measurement. After the first measurement is taken, the spring force of the spring of the next higher spacer cell in the bundle can be measured. This is accomplished by again engaging the bullet nose 48 of the draw rod standard within the lower end of the insertion tube 38. The fuel bundle and the force measurement system are relatively displaced, i.e., the fuel bundle is preferably lowered and the draw rod standard, insertion tube and extension tube combination is lowered to a lesser extent, to locate the upper end of the standard in the cell of the next higher spacer S2. The insertion tube and extension tube are then raised to the further higher spacer S3 to permit the draw rod to be withdrawn through the cell of spacer S2 with measurements being taken during withdrawal as described with respect to spacer S1. It will be appreciated that for additional measurements of the spring forces for springs in higher spacers, the method steps noted above are repeated with the bundle being indexed downwardly by the handling machine 22 so that each new spacer is disposed at essentially the same vertical location in the pool 12 as the lowermost spacer S1 was during the initial force measurements. When all of the measurements for the same fuel rod cell lattice location have been made, FIGS. 6 and 6A illustrating the final measurements being taken for spacer S8, the fuel rod originally removed can be reinstalled. The above-mentioned steps may then be repeated to determine spring forces in other cell lattice locations as desired. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.