Patent Number: 061012319
Section: summary

TECHNICAL FIELD The present invention relates to a system for measuring the spring force of springs in the spacers of nuclear fuel bundles and particularly relates to a spacer spring force measuring system for measuring the force of springs in spacers in irradiated nuclear fuel bundles. BACKGROUND OF THE INVENTION In boiling water reactor (BWR) nuclear fuel bundles, fuel spacers are installed at fixed heights relative to a lower tie plate to maintain rod-to-rod spacing between the fuel rods from the top to the bottom of the bundle. That is, the spacers maintain the fuel rods in an ordered array within the fuel bundle, e.g., 9.times.9 or 10.times.10 arrays of fuel rods. A typical nuclear fuel bundle will contain seven or eight spacers held in place by tabs secured to one or more central water rods. Each spacer consists of a banded lattice of cells through which the fuel rods pass and which cells hold the fuel rods by contact between the fuel rod cladding and stops in the body of the cells and springs forming part of or assembled into the cells. These spacer springs impose a lateral force on each fuel rod, for example, in a range of 1.5 to 3.0 pounds for a newly-manufactured fuel bundle. After a full cycle of in-reactor burn-up, the spring forces are anticipated to be somewhat lower. During manufacture and prior to assembly of the spacers into a fuel bundle, it is common practice to measure the spring forces on all of the springs using a spacer spring force measurement device. Such device is a gauge in the form of a plunger assembly containing a miniature compression load cell whose signal is monitored and interpreted in a data collection system. The gauge plunger assembly is inserted into each cell of the spacer and the load cell is capable of giving a measurement of the cell's spring force accurate to a few tenths of a pound. The spacer spring force measurement devices used during manufacture of the spacers, however, cannot be made for measurements of spacer spring forces when the spacers are in the bundle or in irradiated fuel bundles due to the harsh radiation environment, thermal temperatures and water pressures that exist in a nuclear reactor core or storage pool. While in-bundle spacer spring measurements are desirable, formidable technical difficulties have remained. For example, spring force measurements for spacers in-bundle must be made with remote handling equipment due to the high radiation fields. The spacer springs are also not directly accessible in either irradiated or non-irradiated fuel bundles and the spring force measurement equipment for irradiated fuel must be designed to be remotely positioned, usually from the top of the bundle. Additionally, transducers such as load cells conventionally cannot withstand the harsh environment present in irradiated fuel bundles in fuel storage pools. Probes used in spring force measurements must also be designed so that they do not damage the spacers or the fuel bundle, e.g., misalignment of the probes can damage the spacers or spacer springs. Further, the accuracy of the spring force measurements must be comparable to that made for newly-manufactured spacers. BRIEF SUMMARY OF THE INVENTION In accordance with a preferred embodiment of the present invention, there is provided an in-bundle spacer spring force measurement system for measuring the spacer spring force in irradiated or non-irradiated nuclear fuel bundles. The spring force is determined by measuring the tension in a connecting cable used to withdraw a measuring or draw rod standard through the corresponding spacer cell, after the fuel rod has been removed from the cell. The spring force is measured for a single spacer cell at a time. A predetermined calibration curve of withdrawal force versus spacer spring force is used to compute the spacer spring force based on the following relationship: 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. The spring force is the force applied by the spacer spring to the draw rod standard in a lateral direction, i.e., perpendicular to the axes of the fuel rods of the fuel bundle. With the mass of the standard and cable, as well as the coefficients of friction being known, the withdrawal force can be measured with a load cell attached to the winch cable at a location above the fuel bundle, e.g., above a storage pool in the case of irradiated fuel bundles. The draw rod standard per se includes an integral rod having fixed diameter segments separated by transition sections having gradual tapers from smaller to large diameters. The smallest diameter of the draw rod standard corresponds to the nominal fuel rod diameter and the other diameter or diameters will be slightly larger to permit slight compression of the spacer springs. In certain cases, segments may also be used with diameters slightly less than the nominal value of the fuel rod diameter. The presence of segments with outside diameters different from the nominal fuel rod diameter will allow comparison of the changes in the measured spring forces and permit an estimate of the spring constant. The following is a brief description of a preferred embodiment of an in-bundle spring force measurement system hereof at a poolside storage area of an irradiated bundle, it being understood that the system can be applied to measure the spring forces of spacer springs in non-irradiated fuel bundles. The nuclear fuel bundle is brought into the storage pool and a fuel rod at the location of the spacer cells whose spring forces are to be measured is removed from the bundle. The draw rod standard is coupled to a cable which is inserted into an insertion tube having an outside diameter no greater than the nominal diameter of the fuel rods and which insertion tube is also connected to a standard extension tube. The cable exiting the extension tube is coupled to a load cell and is wound on a drum cable winch. With the extension and insertion tubes, as well as the draw rod standard forming essentially an elongated substantially rigid structure, and with the insertion tube and draw rod in the pool, the fuel bundle can be raised with the insertion tube/draw rod standard being received through the spacer cells vacated by the removed fuel rod until the standard is below the bottommost spacer whose spring force is to be measured. The insertion tube and draw rod standard are then disconnected by withdrawing the insertion tube, leaving the smaller diameter of the standard in the spacer cell whose spring force is to be measured. After disconnection, the insertion tube is elevated to the next higher spacer to avoid interference with the draw rod standard as it is withdrawn through the lower spacer by operation of the winch. As the standard is withdrawn through the spacer cell, withdrawal force data from the load cell is accumulated and analyzed by a computerized data acquisition system and the computed spring force and spring rate may be displayed upon completion of the measurement. After the measurement, the spring forces in higher spacer cells in the same cell lattice position may be measured. This is accomplished by indexing the fuel bundle in a downward direction so that the new spacer is at the same vertical location as the initial spacer. The spring force draw rod standard may then be positioned in this higher spacer similarly as previously described and drawn through the spacer cell with measurements being taken. After measuring the spring force of the cell in the highest spacer of the bundle, the standard is withdrawn from the bundle ready for new measurements to be taken. In a preferred embodiment according to the present invention, there is provided in a nuclear fuel bundle having a plurality of generally parallel, axially extending fuel rods and spacers spaced from one another and including discrete spacer cells for holding the fuel rods in an ordered array thereof, the spacers having spacer springs for biasing the fuel rods in directions generally normal to the axes of said fuel rods, a method of measuring the spring force of the spacer springs, comprising the steps of passing a measuring rod standard through a spacer cell in contact with the spring, the spacer cell being void of a fuel rod, measuring the force necessary to pass the measuring rod standard through the spacer cell and determining the spring force from the measured force. In a further preferred embodiment according to the present invention, there is provided in a nuclear fuel bundle having a plurality of generally parallel, axially extending fuel rods and spacers spaced from one another and including discrete spacer cells for holding the fuel rods in an ordered array thereof, the spacers having spacer springs for biasing the fuel rods in directions generally normal to the axes of said fuel rods, a method of measuring the spring force of the spacer springs, comprising the steps of suspending a measuring rod standard from a cable into the fuel bundle and below a selected spacer cell of a first spacer containing a spring whose spring force is to be measured, relatively moving the fuel bundle and the measuring rod standard to displace the measuring rod standard through the spacer cell thereby tensioning the cable, measuring the tension on the cable caused by displacement of the measuring rod standard through the cell and determining the spring force from the measured tension in the cable.