Patent Number: 046506373
Section: summary

TECHNICAL FIELD This invention relates generally to nuclear reactors, and more particularly to a method and apparatus for identifying those fuel rods of multiple-rod nuclear reactor fuel assemblies which contain leaks. The method and apparatus of this invention can be used on existing nuclear fuel assemblies without requiring any modification of the fuel rods or to the composite fuel assembly formed by such fuel rods. BACKGROUND OF THE INVENTION Nuclear reactor cores used for power generation typically have a large number of fuel assemblies arranged in suitable configuration to heat water, thereby producing steam for turning electrical generators. Each of such fuel assemblies typically contains from 49 to as many as 300 fuel rods which contain nuclear fuel. A larger nuclear reactor may contain as many as 40,000 to 50,000 fuel rods. Each fuel rod is operatively independent of other fuel rods within a fuel assembly and is basically a metal tube approximately 0.5 inches in diameter that extends typically from 8 to 15 feet in length. Suitable fissionable material such as uranium oxide in the form of cylindrical fuel pellets is stacked within the fuel rod. The upper end of the tube is void of fuel pellets and forms a plenum, when sealed, for gas. A small clearance space is provided around the fuel pellets to accommodate expansion or swelling of the fuel. Since the nuclear fuel within the fuel rods becomes extremely radioactive during operation and is contained only by the metal tubes or sheaths of the rods, inspection methods and apparatus for verifying the seal integrity of the rods is of primary importance. During prolonged operation, the fuel rods may develop leaks as a result of cracks, pinholes or other defects, resulting in coolant seeping into the fuel rod and/or the escape of radioactive products from the fuel rod. A leakage of fission product gases and other radioactive materials from a fuel rod during reactor operation can enter the reactor coolant system. While the coolant purification system of a reactor facility is capable of handling a certain amount of radioactive fission products, it is desirable to keep the radiation level as low as possible. In early fuel assembly designs, the individual fuel rods were part of a unitized assembly wherein the fuel rods were welded together by fuel assembly guides in a manner making it very difficult to replace a failed fuel rod. More recent designs have constructed fuel assemblies in a manner such that a fuel rod that fails before its fissionable material is spent can be removed or replaced by a new rod. Such fuel assembly design, generally referred to as a reconstitutable fuel assembly, facilitates removal and replacement of fuel rods and saves considerable expense heretofore incurred as a result of premature discharge of an entire fuel assembly because of the failure of a single one of its fuel rods. A key step in the fuel assembly reconstitution process is the identification of the specific fuel rod or rods of the fuel assembly which have failed due to cracks, pinholes or other penetrations of the tube or cladding of the fuel rod. The first step in such process is to identify a fuel assembly containing one or more leaking fuel rods. This operation is typically performed during reactor refueling. During refueling operations, the reactor is shut down, and the reactor fuel assemblies of the reactor are usually subjected to a so-called "sipping test". The fuel assembly under test is placed in a water filled storage tank. The fuel rods of the assembly and the water in the storage tank heat up by residual decay. If a fuel assembly contains a defective rod, the fission products from that rod escape through the defect into the water. Through sampling of the water, it can be determined whether the fuel assembly contains any defective rods. Such sipping test is a totalizing method that only determines whether the entire fuel assembly contains any defective rods, and does not identify the position or location of the defect or the defective rod per se. While a number of methods and devices have been utilized in the past for detecting and locating single fuel rod failures, such techniques of the prior art have generally not provided the desired simplicity, reliability, speed, safety and cost effectiveness for such operation. A desired rod failure detection method should provide identification of the defective rod, without requiring any significant design changes to the fuel, to the rod itself or to the fuel assembly. In short, it is most desirable that the rod failure detection mechanism be capable of operation on existing fuel assembly and fuel rod designs. Further, the test apparatus and method used should not damage or increase the risk of further damage to the fuel rods or to the fuel assembly. The fuel rod detection method should provide a high probability of being able to readily distinguish between failed and good rods, should require little if any dismantling of the fuel assembly in order to perform the test, and should be capable of testing an entire fuel assembly in a reasonably short time, and preferably while such assembly is still immersed within the coolant fluid. This inventon is distinguished from that body of prior art relating to fuel rod failure detection that requires the fuel rod itself to be constructed in a particular fashion or to include detectable structures or members that move or change their physical condition or parameters as a result of a leak in the fuel rod. Such art is exemplified, for example, by U.S. Pat. Nos. 3,666,625 and 4,217,173 to Nybo and Jabsen respectively. In the Nybo structure, the fuel rod is configured to include a magnetic device which moves within the fuel rod in the event of a rod failure. The Jabsen invention requires a specially designed fuel rod end cap having a hollow construction, in combination with a cellular end fitting for the fuel assembly, to enable radiation probes to be lowered into registry with the fuel rod end caps for measuring radioactivity emanating from the fuel rod, thereby providing an indication of the structural integrity of the rod. In contrast to the above-described art, the present invention can be used with fuel rods and fuel assemblies constructed therefrom of any standard configuration, as long as the elongate fuel rods are arranged in generally parallel, spaced-apart relationship within the fuel assembly. Others have devised apparatus and methods for locating leaking fuel rods in a fuel assembly, without requiring specially constructed fuel rods or significantly modified fuel assemblies. Such methods and apparatus, however, have not generally satisfied the above desired criteria for such a system. For example, U.S. Pat. No. 4,193,843 to Womack et al. describes a leak detector apparatus that applies ultrasonic waves to the fuel rods and measures the fuel rod resonance response thereto, which is indicative of the water that leaks into the failed fuel rod. The Womack method presumes that a leaking fuel rod will have an accumulation of water coolant at its lower end, and only performs its test of the fuel rod at the lower portion of the rod. Such test may be accurate for gross rod failures, but does not necessarily detect smaller rod failures which have not yet resulted in accumulation of significant amounts of coolant within the rod. U.S. Pat. Nos. 4,016,749 and 4,039,376 to Wachter describe apparatus and methods for detecting the emission of bubbles expelled from defective rods. The fuel assembly is placed within a liquid bath in a manner such that the fuel rods are differentially pressurized greater than the liquid bath. Sensing means are provided to detect bubbles emitted from the rods and to correlate the position of the bubble on the surface of the liquid bath, to that of the defective rod submerged within the bath. The accuracy of such system to identify the leaking rod depends upon the bubble traveling vertically through the liquid bath to the bath surface, and upon the accuracy of the correlation apparatus. Since the lateral spacing between adjacent fuel rods in a fuel assembly can be as small as 0.05 inches, the sensing accuracy of the bubble detection apparatus must be extremely accurate, and any deviations of the bubble from a straight-line path to the coolant surface can further lead to unreliability of this system. Another method known in the art for detecting one or more fuel rod failures within a fuel assembly is disclosed by U.S. Pat. No. 3,878,040 to Martucci. The Martucci apparatus delivers a stripping gas to the bottom of the fuel assembly which percolates up through the assembly entraining any gaseous fission products contained in the coolant. The stripping gas and entrained gases are then collected by a hood overlying the fuel assembly and measured for radioactivity content. While such apparatus can detect a leaking fuel assembly, it cannot determine which of the fuel rods in the assembly is defective. Another leak detection apparatus requiring specially configured fuel rods and fuel assembly is disclosed in U.S. Pat. No. 4,192,373 to Wolowodiuk. The Wolowodiuk structure includes physical leak-detection connections permanently provided to each of the fuel rods, which can be monitored periodically for leaking conditions. Such apparatus requires a duplex wall fuel rod construction as well as a relatively complicated communication channel of tubes and bores to complete the leak detection system, and does not readily lend itself to a reconstitutable fuel assembly construction. While such prior art structures and techniques for detecting fuel rod leaks have individually addressed various ones of the desired criteria for structures, no single device or method has simultaneously satisfied all the previously described criteria. In general, either the leak detection structures of the prior art have been unduly complex or required costly, expensive sensing or detection equipment, or have been unreliable or inaccurate, or too time consuming in performing the test function. The present invention effectively addresses and overcomes most of the above-mentioned deficiencies of prior art fuel rod leak detector and locating structures and methods. The detector apparatus and methods of this invention allow accurate location of defective fuel rods of a fuel assembly without requiring any design modification to existing fuel rods or fuel rod assemblies, and uses actual leak measurement techniques to locate a leaking rod as opposed to making predictions or assumptions as to the manner or location of fuel rod failure. Further principles of this invention can be used to test a fuel assembly without requiring removal of the fuel assembly from the coolant bath of the reactor facility in which it is normally submersed. SUMMARY OF THE INVENTION The present invention provides a simple, reliable, safe, efficient and effective method and apparatus for locating leaking rods within a nuclear fuel assembly. The method and apparatus of this invention can be used with existing fuel assembly configurations and does not require any design changes to be made to the fuel, the fuel rod or to the fuel assembly under test. The method and apparatus of this invention can be used without increasing risk of further damage to either leaking or intact fuel rods of the assembly under test. This invention provides a reliable method for isolating leaking fuel rods from the good rods within an assembly without requiring dismantling of the fuel rod assembly to perform the testing function. The test probe apparatus of this invention is readily adaptable for use with existing structures used for moving fuel assemblies within a reactor facility. According to one embodiment of the invention, the method of locating a defective fuel rod within a nuclear fuel assembly that is leaking radioactive products into its surrounding coolant comprises the steps of: determining the approximate longitudinal segment of the fuel assembly at which radioactive products are being emitted from a fuel rod of the fuel assembly; and selectively sampling the coolant surrounding the fuel rods at such longitudinal segment, to isolate the fuel rod that is actually emitting the radioactive products into the surrounding coolant. The invention contemplates the use of one or more test probe members used in a succession of logical test steps in order to isolate the leaking fuel rod. While the test probe apparatus of this invention can be used to determine whether a fuel assembly in fact contains "any" fuel rods that are leaking fissionable products into their surrounding coolant, its preferred application presumes that tests have previously been performed, which have determined that the fuel assembly to be tested by this invention does contain one or more leaking fuel rods. When thus testing a fuel assembly known to contain a leaking fuel rod, the first step in practicing the method of this invention is to determine that longitudinal segment of the fuel assembly from which radioactive products are being emitted from the defective fuel rod or rods without the assembly. According to one embodiment of the invention, the longitudinal location of the leak is determined by sampling the coolant surrounding the fuel rods of the fuel assembly under test for the leaking radioactive products at a plurality of longitudinally aligned segments of the fuel assembly, and by determining that tested longitudinal segment from which the coolant under test contained the highest sampled radioactive product content. In one embodiment of the invention this is accomplished by placing a test probe member defining a volumetric test zone, in proximity with the fuel assembly such that a longitudinal segment of the fuel assembly passes through the volumetric test zone of the probe member. By then drawing a sample of the coolant from the volumetric test zone and by measuring the radioactive product content of the drawn sample, a determination can be made as to whether the fuel rod leak is present within that fuel assembly longitudinal segment which is under test. By moving the test probe longitudinally of the fuel assembly so as to successively test adjacent longitudinal segments of the fuel assembly, the longitudinal position(s) of the radioactive product leak(s) from the leaking fuel rod(s) of the fuel assembly is established. Once the longitudinal position of the fuel rod leak has been determined, the exact fuel rod or rods which are leaking are determined by selectively sampling the coolant at that longitudinal position, surrounding different ones of said fuel rods, to thereby determine the exact coordinates of the leaking fuel rod within the cross-sectional matrix of said fuel rods comprising the fuel assembly. According to one embodiment of the invention, the step of selectively sampling the coolant surrounding the fuel rods at the determined longitudinal segment of the fuel assembly at which the leak occurs comprises: dividing the fuel rod matrix into identifiable submatrices of such fuel rods; and sampling the fuel rods within the submatrices, in a logical manner, for radioactive products to logically isolate that fuel rod or rods that is leaking. According to one embodiment of the invention, the sample testing is performed so as to first isolate that quadrant of the fuel rod matrix which contains the leaking fuel rod, and to thereafter subdivide the identified quadrant into ordered groups of fuel rods and to systematically sample the coolant surrounding such ordered fuel rod groups so as to isolate the leaking fuel rod. According to one embodiment of the invention, the step of subdividing the fuel rod matrix into submatrices comprises selectively inserting one or more baffle members between adjacent rows of the fuel rods within the matrix so as to isolate the external environment of the fuel rods within one submatrix from that of an adjacent submatrix. The invention includes not only the method of locating the position of the leaking fuel rod within the fuel assembly, but also the test probe apparatus for performing the testing function. According to one embodiment of the invention, such test probe apparatus comprises a pair of opposed baffles longitudinally extending between first and second ends and connected at their first ends by means of a collector in a manner such that the volume defined between the baffles and the collector forms the volumetric test zone. The collector is configured to collect liquid coolant drawn from the volumetric test zone. The baffle and collector probe arrangement is configured for operative alignment with a nuclear fuel rod assembly such that the baffles laterally project on opposite sides of one or more fuel rods of the assembly such that a longitudinal segment of one or more fuel rods of the assembly can be positioned selectively within the volumetric test zone of the probe apparatus. The test probe apparatus may also include means for drawing the liquid coolant from the volumetric test zone and through the collector, as well as means for measuring the radioactive content of such coolant sample drawn through the collector. The test probe apparatus may also include means operatively connected with the test probe apparatus for moving the test probe so as to longitudinally and laterally move the volumetric test zone relative to the fuel rods of the fuel assembly so as to select those fuel rods which are to be disposed within the volumetric test zone at any instant of time. According to a preferred application of the present invention, a plurality of test probes are used to perform the logical testing operation which determines the location of the leaking fuel rod of the fuel assembly. The actual number of such test probes utilized in any particular application will depend upon the configuration of the fuel assembly being tested, upon the number of fuel rods within such fuel assembly, and upon the logical testing sequence employed to determine the exact position of the leaking fuel rod within the fuel rod assembly matrix. In the preferred embodiment, three such probe members are used; however, it will be understood that any number of such probe members could be used within the scope of this invention. In the preferred embodiment of the invention, the first probe member is sized and configured so as to encompass all of the fuel rods within its volumetric test zone, and is used to determine the longitudinal segment of the fuel assembly at which the leak actually occurs, or is noticeably detectable. The second test probe in the preferred embodiment described has a volumetric test zone approximately equal to one-half that of the first test probe, and is used in a manner so as to subdivide the fuel rod matrix into four quadrants for identifying which quadrant contains the leaking fuel rod. The third test probe member is used in the preferred embodiment to further subdivide the identified leaking quadrant into smaller test submatrices, until the exact location of the leaking fuel rod is determined. According to a preferred construction of the test probe members of this invention, the baffle members are constructed of thin, semi-rigid sheet material having a thickness sized to readily slide between adjacent rows of fuel rods within the fuel rod assembly matrix so as to isolate such adjacent fuel rods from one another when drawing coolant from the volumetric test zone defined by the test probe during a sampling operation. It will be readily appreciated from a more detailed description of preferred embodiments of the invention, that use of the methods and apparatus of the invention enable rapid and reliable detection of those fuel rods within a fuel assembly which are leaking, so that decisions regarding the repair of the defective fuel assembly can be reliably made. It will be appreciated that while particular sequential methods and test probe apparatus will be disclosed in describing the preferred embodiments of the invention, the invention is not limited to such specific techniques or configurations. For example, while the test probes of the preferred embodiment will be described relative to particular dimensions or materials, the invention is not limited to such specific materials or dimensions. Further, while the invention will be described in an application wherein a rectangular fuel assembly is being tested, the invention is not limited to use with fuel assemblies with rectangular cross-sectional fuel assembly matrices. Further, while the test probe of the preferred embodiments has a "U-shaped" baffle and collector configuration, the invention is not limited to such configuration or to the use of test probes having baffles or collector members configured as illustrated and described with respect to the preferred embodiments. These and other variations and modifications of the invention will become apparent to those skilled in the art in light of the following description of preferred embodiments of the invention.