Patent Number: 046506373
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the Drawing, wherein like characters represent like parts throughout the several views, there is generally illustrated in FIG. 1 a diagrammatic view of a portion of a typical nuclear reactor installation. The nuclear reactor vessel (not illustrated) is generally located in an adjacent building connected to the spent fuel storage pool, generally illustrated at 22, by a water-filled tunnel (not illustrated). The spent fuel storage pool, will be understood by those skilled in the art to be of any suitable or usual design, and will not be detailed herein. The pool basin 24 is empty during normal reactor operation, but during refueling operations, is filled with coolant up to a level such as indicated at 25 so that the fuel assemblies (hereinafter described) are kept immersed in the coolant. In the preferred embodiment, the coolant is water. It will be understood, however, that the principles of this invention apply equally well to the use of reactors using other than water for the coolant. During the refueling operation, fuel assemblies such as the one indicated at 30 are lifted from the reactor by suitable lifting means described in more detail hereinafter. The fuel assemblies are vertically lifted from the reactor vessel, are laid down and carried horizontally through a water-filled tunnel to the spent fuel storage basin 24 where they are restored to a vertical position by lifting means essentially identical to that used for lifting the fuel assemblies from the reactor vessel. The lifting means generally includes a telescoping grapple tool 26 suspended from a hoist or crane member 27 which moves the fuel assembly 30 in a vertical direction. The hoist structure 27 is mounted on tracks, generally illustrated at 28, for laterally positioning the grapple tool in vertical alignment with the fuel assembly 30 to be handled, and for laterally moving the fuel assemblies. It will be understood that while only one such track 28 is illustrated in FIG. 1 for moving the hoist apparatus 27 in a lateral direction, lying in the plane of the illustration, that other appropriate track means (not illustrated) are also provided for moving the hoist structure in the lateral direction perpendicular to the plane of the FIG. 1 illustration. A more detailed illustration of a typical fuel assembly 30 is illustrated in FIG. 2. Referring thereto, the fuel assembly 30 includes a plurality of fuel rods 32, assembled in parallel and held in spaced relationship to each other by a plurality of support grids 34, 35 and 36 spaced along the fuel assembly length. Only three of such support grids are illustrated in FIG. 2; however, it will be understood that more of such support grids may be required to adequately support the fuel rods 32 along their length. The fuel rods 32 are closely spaced by the support grids 34, 35 and 36 along their entire lengths, being separated by approximately only 0.12 to 0.20 inches. In a typical fuel assembly 30, the fuel rods 32 have an outside diameter of approximately 0.36 to 0.50 inches and a typical length ranging from 8 to 15 feet. Each fuel rod 32 contains suitable fissionable material, as will be described in more detail hereinafter. The illustration of FIG. 2 depicts only the forwardmost row of fuel rods 32, however, it will be understood that in a complete fuel assembly 30, there will typically be from 49 to as many as 300 fuel rods contained within the assembly, and typically arranged in matrix fashion when viewed in cross-section as in FIG. 4. A plurality of control rods, generally designated at 40, are reciprocally movable in control rod guide tubes 42 at predetermined positions in each selected fuel assembly within the reactor. The control rods are used to control the fission process. The control rod guide tubes are attached to the support grids 34, 35 and 36 and are interspersed (in cross-sectional view as illustrated in FIG. 4) among the fuel rods 32. With reference to FIG. 4, the control rods are diagrammatically illustrated as solid circles as compared to the open ring configuration of the standard fuel rods such as hereinafter described with reference to FIG. 3. It will be understood that the control rod depiction of FIG. 4 is for diagrammatic illustrative purposes only, and does not represent the physical nature of the control rod structure. The fuel assembly also includes a top nozzle 43 and a bottom nozzle 44 to which opposite ends of the control rod guide tubes 42 are attached, thereby forming a unitized fuel assembly which can be conveniently handled without risk of damage to the assembly contents. The guide tubes 42 typically include sleeve members for welding to the upper and lower support grids 34 and 36 and to the top and bottom nozzles 43 and 44. Since the control rod structure does not form a part of this invention, other than for the physical spacing of the control rods with relation to the fuel rods as depicted in FIG. 4, further discussion thereof will not be detailed herein, it being understood by those skilled in the art that the fuel assembly 30 is constructed in a typical manner well known by those skilled in the art suitable for appropriate use in a nuclear reactor. Similarly, the top and bottom nozzle structures 43 and 44 respectively, will not be described in detail herein, as they do not form a part of the invention. An enlarged view of one of the fuel rods 32, as typically constructed, is illustrated in FIG. 3. Referring thereto, each fuel rod is cylindrically shaped and has an outer jacket or casing 50 extending between upper and lower ends 50a and 50b respectively and defining an internal cavity. The lower end 50b of the casing 50 is sealed by a lower end plug or cap 51. A plurality of cylindrical fuel pellets 55 are stacked within the internal cavity defined by the casing 50, and have an outside diameter slightly less than the inside diameter of the casing 50 so as to define a fuel-clad gap 56 therebetween. The uppermost fuel pellet 55 is spaced back from an upper end plug or cap 52 which seals the upper open end 50a of the cladding 50, so as to define a space or plenum 57 therebetween. A spring 58 is compressed between the lower surface of the upper end plug 52 and the upper surface of the top fuel pellet to retain the ordered position of the fuel pellets within the casing cavity. Typically, the casing 50 and the upper and lower end plugs 51 and 52 are constructed of a zirconium alloy material. The upper and lower plugs 51 and 52 are typically welded to the upper and lower ends 50a and 50b respectively of the casing 50. The compression spring 58 also is typically of a material such as 304 stainless steel. The fuel pellets 55 are of appropriate fissionable material, and typically comprise uranium oxide. The fuel-clad gap 56 provides for radial swelling or expansion of the pellets 55 during operation. The plenum 57 is often pre-pressurized with an inert gas such as helium such that during operation of the fuel rod, the differential pressure between the fluids within the internal cavity of the fuel rod and the outer coolant bath is minimal. The above-described fuel rod structure contains no special apparatus for detecting a breach of the cladding jacket 50 that would permit the surrounding coolant (typically water) to leak into the internal cavity of the fuel rod, and conversely for the radioactive material contained within the fuel rod from leaking out into the coolant. While the present invention provides simple apparatus and method for locating a cladding breach of such a fuel rod, it will be understood that the invention would apply equally well to the detection of cladding failures of fuel rods having internal structural configurations, cladding and coolant materials other than that above-described, as long as the general matrix fuel assembly structure such as that generally illustrated in FIG. 4 is preserved. The present invention includes both test probe apparatus and a method for utilizing the probe apparatus in a manner so as to systematically test the fuel assembly with the probe apparatus so as to eliminate sound fuel rods and to successively narrow down the number of fuel rods being tested, until the defective fuel rod or rods are located. The test probe apparatus is generally indicated at 60 in FIG. 1. As illustrated in FIGS. 1 and 11, the probe apparatus 60 includes three probe heads 62, 63 and 64 attached to a support mast 66 by means of a common coupling 65. It will be understood that while three probe heads are illustrated for use in testing a 17 by 17 fuel rod matrix, that other sized fuel assemblies might require more or fewer probe heads. The tri-probe head arrangement is illustrated in more detail in FIG. 11. While a particular tri-probe head configuration will be described in the preferred embodiment, it will be understood from a more detailed description of the invention, that other appropriate probe head and mounting configurations could be employed, and that it is not necessary for the multiple probes to be commonly attached to a single support mast or coupling as illustrated. The arrangement illustrated, however, provides a simple technique for indexing and positioning the multiple probe heads throughout the testing procedure, with minimum interference from those probe heads not in current use in any particular portion of the test procedure. The support mast 66 is operatively mounted to a Probe Position Control Module, generally functionally illustrated at 67. While a separate such Control Module 67 has been illustrated in FIG. 1, it will be understood that various functions of such Control Module could be performed by other means. The Control Module includes appropriate apparatus, well-known and/or capable of implementation by those skilled in the art, for moving the support mast 66 and attached probe assembly so as to move the probes in the desired three orthogonal directions of movement (i.e. in the "x", "y" and "z" orthogonal axis directions) so as to align and move the probe head assembly 60 relative to the fuel rod assembly 30. The Probe Position Control Module 67 also includes a control feature (generally illustrated at 67a in FIG. 1) which controls rotational indexing of the support mast 66 and the attached probe assembly 60 about the vertical or "z" axis, to position the desired probe head (62, 63 or 64) in operative alignment with the fuel rods 32 of the fuel assembly 30, as hereinafter described in more detail. It will be understood, that while a separate Probe Position Control Module 67 has been illustrated for performing the movement operations on the probe assembly 60, that the described manner of performing such probe position control is not unique, and that other movement control techniques could be devised by those skilled in the art. For example, certain of the motion control functions could be and generally are associated directly with the movement control functions of the hoist apparatus 27 or with the telescoping grapple tool 26. The probe members 62, 63 and 64, are illustrated in more detail in FIGS. 5 through 10. Probe members 62, 63 and 64 are similar to each other in construction and function, but differ in their relative sizes and material dimensions, and are used at different times in the testing sequence for locating the leaking fuel rod or rods of a fuel assembly. A preferred construction of the probe assembly can be described with reference to probe member 62, illustrated in FIGS. 5 and 6, it being understood that except for dimensions and material parameters, probes 63 and 64 contain identical functional elements as probe 62. Referring to FIGS. 5 and 6, probe 62 contains a pair of opposed containment or baffle members 62.1 and 62.2 of semi-rigid sheet-like material, connected at one end by a diffuser plate, generally illustrated at 62.3. The diffuser plate 62.3 mounts the baffle members 62.1 and 62.2 in generally parallel spaced-apart relationship, separated by a lateral spacing distance of "S1". The respective materials of the containment plates 62.1 and 62.2 and the diffuser member 62.3 are selected such that their interconnection will provide a semi-rigid structure wherein the containment plates 62.1 and 62.2 extend in cantilevered manner from their fixed end to their distal ends thereof, denoted at "A" in the figures, so as to substantially maintain the interplate spacing (S1) therebetween at their distal ends "A". In the preferred embodiment illustrated, the containment plates 62.1 and 62.2 are constructed of stainless steel sheet material; however, it will be understood that any appropriate material having the desired rigidity and resistance to radioactive contamination properties could be used. Further, while the containment plates 62.1 and 62.2 are illustrated as being generally rectangular in the preferred embodiment, it will be understood that other appropriate shapes could be employed within the spirit and scope of this invention. The diffuser plate member 62.3 is perforated so as to allow fairly free flow of fluid therethrough, while providing the structural support required for maintaining the containment baffles 62.1 and 62.2 in generally parallel spaced-apart relationship. The diffuser plate member 62.3 forms one wall of a collector member, generally illustrated at 62.4. The collector member 62.4 comprises one or more surfaces integrally connected with the diffuser plate 62.3 and/or the baffle members 62.1 and 62.2 so as to define a collection chamber for fluid flowing through the perforations in the diffuser plate 62.3. The walls or surfaces of the collector member 62.4 terminate at an output port 62.5, defining an orifice opening into the inner chamber or cavity defined by the collector walls 62.4 and providing a continuous path therewith through the perforations of the diffuser plate 62.3. The outlet port 62.5 is suitable for connection to a "sample line", as hereinafter described. The construction of probe members 63 and 64 is functionally identical to that of probe 62, wherein like members of probes 63 and 64 are identified by similar decimal reference numerals following the primary probe identifier number 63 or 64. For ease of later identification, however, the lateral spacing between the baffle members 63.1 and 63.2 of probe 63 is designated as "S2", and the lateral spacing between baffle members 64.1 and 64.2 of probe 64 is denoted by the label "S3". In further reference to FIGS. 5 through 10, and particularly with regard to FIGS. 5, 6, 9 and 10, it should be noted that the probe members 62 and 64 are not illustrated as drawn to the same scale. As illustrated in FIGS. 5 and 6, the probe 62 is drawn to approximately one-half of the scale of probe 64 illustrated in FIGS. 9 and 10. Accordingly, dimensional comparisons between the probes 62 and 64 as illustrated in FIGS. 5, 6, 9, and 10 respectively should not be inferred from such figures, but should only be made with respect to the later-described FIGS. 13 through 15. Referring to FIGS. 1 and 11, the output ports 62.5, 63.5 and 64.5 of probes 62, 63 and 64 respectively are mounted to the coupling 65 and are respectively continuously connected with sample lines 68A, 68B and 68C respectively. The sample lines 68 are mounted within the support mast 66 and operatively extend therethrough up to a pump assembly, generally referred to at 69, illustrated as mounted on the hoist apparatus 27 although the pump 69 and its associate sensor apparatus 70 generally would be located at a more convenient location on the walkways around the spent fuel storage pool 22. Appropriate valve or sample line selector means (not illustrated) associated with the pump 69 or configured into the support mast 66 or coupling 65 assembly selects that one of the sample lines 68A through 68C which is operatively connected to the pump 69 at any instant of time. A sensor apparatus, generally designated at 70, is also mounted on the hoist apparatus 27 and is cooperatively connected with the sample line 68 to continuously monitor and sense the liquid flowing therethrough for radioactive fission products such as iodine-131, xenon-133, cesium-134, cesium-137, or the like, as hereinafter described in more detail. A functional block diagram representation of the operative elements related to the probe assembly is illustrated in FIG. 12. Referring thereto, the probe head assembly 60 is illustrated as a functional block, the physical manipulation of which is controlled by the probe position control module 67. The probe head assembly 60 is operatively connected to the pump 69 by means of the sample line 68. When the pump 69 is energized, a coolant sample is drawn through the probe head assembly 60. The coolant sample passes through the appropriate sample line 68(A-C), through the pump 69 and is returned back to the coolant bath by an appropriate return line illustrated at 71 in FIG. 12. The sensor 70 is operatively connected to continuously monitor the coolant sample flowing through the sample line 68, and may be any appropriate multichannel analyzer for radioactive fission products. While not illustrated in the other figures, it will be understood that appropriate control means 72 are provided to correlate the operation of the probe position control module 67, the pump 69, the sensor 70, and to handle and manipulate the data and information received as a result of the tests. Each of the probes 62, 63 and 64 defines a volumetric test zone defined by that volume contained between the confinement baffle plates and the diffuser plate and the open ends "A" of the baffle plates. In the preferred configuration of the probe members illustrated in the Drawing, such volumetric test zone is rectangular in shape, and is schematically illustrated at "T" in FIG. 5. The volumetric test zone "T" of a test probe is that region or zone through which the probe will draw its coolant sample when operatively connected with the pump 69. When the probe is positioned around one or more fuel rods such that the fuel rods are aligned generally parallel with the probe's opposed baffle plates, the probe, when operatively connected with the pump 69, will draw coolant from around those segments of the fuel rods which lie within the probe's volumetric test zone "T". The coolant will pass through the probe's diffuser member, through its collector and the output port thereof, into the sample line 68 for analysis by the sensor 70. If that segment of a fuel rod contained within the volumetric test zone "T" of the probe contains a leak such that fissionable material is being expelled through the leak and into the coolant surrounding the fuel rod, such fissionable material will be drawn through the diffuser plate collector of the test probe and be detected by the sensor 70. Accordingly, by selectively positioning the probe members into the fuel assembly, between the fuel rods in the manner herein described, the location of leaking fuel rods can be readily and accurately determined. The dimensions of the respective probe members depend upon the desired size of the volumetric test zone "T" for each probe, which in turn depends upon the dimensions of the particular fuel assembly with which the test probes will be used. To simplify further description of the invention, the probes 62, 63 and 64 will hereinafter be described with respect to their applicability to testing a typical fuel assembly 30 having a cross-sectional fuel rod matrix of 17 by 17 (i.e. 289) fuel rods, generally as illustrated in FIG. 4. It will be understood, however, that the principles of the invention apply equally well to the testing of fuel assemblies containing any number of fuel rods, whether or not such assemblies have a square, or even a rectangular cross-sectional configuration. Referring to FIG. 4, in a typical 17 by 17 fuel rod assembly, the control rod guide tubes will be positioned throughout the matrix in an ordered manner, as dictated by the configuration of the rod cluster control assembly 45 and its interconnecting control rod arm structure 47 (see FIG. 2). For the typical 17 by 17 matrix configuration with which this invention will be described, the control rods 40 and associated guide tubes 42 will be positioned throughout the matrix as illustrated in FIG. 4. Referring thereto, it will be noted that certain adjacent rows of the fuel rod matrix will not contain any control rods or associated guide tubes. For simplicity in this description, the control rods and associated guide tubes will collectively be referred to as merely "control rods". In the FIG. 4 arrangement, the example, there are no control rods in either the seventh or eighth rows of fuel rods, as counted inward from any of the corners. Those divisions between the respective seventh and eighth rows have been denoted by "X1", "X2", "Y1" and "Y2". The significance of this fact is that since the diameter of the control rod guide tubes is slightly larger than that of a standard fuel rod, the inter-rod spacing between adjacent rows is narrower in those rows containing control rods, than in those without control rods. Accordingly, the inter-rod spacing in those positions marked "X1", "X2", "Y1" and "Y2" in FIG. 4 will be slightly wider than the spacing between a fuel rod row and any other adjacent row which contains a control rod. Therefore, in designing the probe members, and in particular the baffle or containment wall member portions thereof which are to slide between adjacent rows of the fuel rods and control rods, it is desirable to know not only the inter-rod spacing, but also those adjacent rows of the matrix which do not contain any control rods. This consideration will be appreciated more following further description of the invention, and in particular following a description of the method in which the probe members 62, 63 and 64 are to be used. Probe member 62 is the largest of the three probe members, and is configured to have a volumetric test zone with parameter "S1" suitable for completely encompassing an entire side (all 17 fuel rods) of a fuel assembly within its volumetric test zone "T". This situation PG,25 is diagrammatically illustrated in FIG. 13a, wherein the fuel assembly 30 is illustrated in top plan. The length of the volumetric test zone "T" (as measured from the diffuser plate member 62.3 to the distal end "A" thereof) of the opposed baffle members 62.1 and 62.2 is sufficiently long so as to extend slightly beyond the outermost row of fuel rods of the fuel assembly when the probe 62 is positioned so as to enclose the cross-sectional area of the fuel assembly as illustrated in FIG. 13a. The "height" dimension of the baffle plates 62.1 and 62.2 and of the collector diffuser member 62.3 may vary, and is preferably of approximately the same or greater dimension as the "length" of the baffle plates 62.1 and 62.2. The height dimension should be adequate to insure that the coolant sample being drawn into the collector 62.4 is primarily being drawn from the central area of the volumetric test zone "T" and is not primarily coming from the open upper and lower ends of the test probe. Since the baffle members 62.1 and 62.2 of the probe 62 extend outside of the outer rows of fuel rods of the fuel assembly 30, and are not configured to be placed between adjacent rows of fuel rods of the assembly, there are no particular limitations placed upon the maximum wall thickness of the baffle members 62.1 or 62.2. The apparatus of this invention is primarily intended for isolating the leaking fuel rod or rods of a fuel assembly 30 that has already been determined to contain a leaking rod. However, the principles of this invention, and particularly the use of probe 62 could be used in the first instance to determine whether a fuel assembly 30 contains any leaking fuel rods. The purpose of using probe member 62 is to determine the longitudinal (i.e. "vertical" or in the "z" direction) position of a leak or defect in one or more fuel rods of the fuel assembly that are ejecting fissionable products into the coolant. Probe member 62 is used as illustrated in FIG. 13 by moving the probe member 62 into cooperative alignment with the fuel assembly 30 such that the entire cross-sectional area of a vertical segment of the fuel assembly is contained within the volumetric test zone "T" of the probe 62. Preferably, the probe is initially lowered in the "z" direction to begin testing at the lower end of the fuel assembly 30 as it is suspended in the coolant channel by the hoist 27, as illustrated in FIG. 1. When so positioned, the pump 69 is energized so as to draw coolant through those longitudinal segments of the fuel rods enclosed within the volumetric test zone "T" of probe 62 and into the collector 62.4. The coolant sample is transmitted through the sample line 68 to the sensor 70 for detection of fissionable products therein. The vertical ("z" direction) position of the probe member 62 is longitudinally moved along the length of the fuel assembly 30 until the sensor detects a comparative increase in the amount of radiation in the coolant, which indicates the presence of a leaking fuel rod. The probe motion is fixed at that vertical (i.e. longitudinal) location with respect to the fuel assembly 30 and the process is ready for use of the next probe 63. Alternatively, in case the fuel assembly may contain more than one defective fuel rod, the entire longitudinal length of the fuel assembly may be tested by probe 62, and those longitudinal positions at which leaks are detected are each recorded, for subsequent testing at each of such positions by probes 63 and 64. The second probe member 63 is sized to localize the particular "quadrant" in which the leaking rod whose vertical or longitudinal position was determined by probe 62, is positioned. Accordingly, the "S2" dimension of probe member 63 is sized to approximately one-half of the width dimension of the fuel rod matrix (approximately one-half of the "S1" dimension). The thickness of the baffle members 63.1 and 63.2 is sized so as to permit free sliding movement of the baffle members between adjacent rows of the fuel rods of the fuel assembly. As previously discussed with reference to FIG. 4, if the fuel assembly with which the probe members are to be used has particular adjacent rows close to the center of the matrix which do not contain control rods, then it is desirable to slide the baffle wall members 63.1 or 63.2 through those inter-row positions, since the baffle members 63.1 and 63.2 can be constructed of somewhat thicker material than would be possible if they were required to traverse inter-rod spacings between rows which contain control rods. As an example, in a preferred construction of probe member 63, which is designed to be slid into the 17 by 17 matrix of FIG. 4 along the "X1", "X2", "Y1" and "Y2" paths, the baffle walls 63.1 and 63.2 can be formed from sheet material as thick as 0.10 inches, providing increased rigidity to the probe structure than would be the case if the baffle members were required to be thinner so as to traverse narrower inter-rod spaces. Accordingly, in a preferred construction of the probe member 63, as applied to the 17 by 17 matrix configuration illustrated in FIG. 4, the inter-baffle spacing dimension "S2" is sized to accommodate ten fuel rods within the volumetric test zone "T". The length of the baffle plates 63.1 and 63.2 is generally the same as that of the baffle plates 62.1, 62.2 of the probe member 62, such that when inserted between fuel rods of the fuel assembly as illustrated in FIG. 14, the distal end "A" of the baffle plates 63.1 and 63.2 extends slightly beyond the outermost row of fuel rods. Referring to FIG. 14a, a probe 63 is used to locate that quadrant of the fuel assembly in which the leaking rod is positioned. The quadrant is determined by inserting probe 63 four or less times into the fuel assembly matrix 30 in a manner so as to isolate that quadrant (Q1-Q4) containing the leaking rod. All insertions of the probe member 63 are performed at that vertical position determined by probe 62 to be the vertical (longitudinal) location of a leak. In practice, such vertical leak position determined by probe 62 may actually be slightly above that segment of the defective fuel rod at which the actual leak is positioned. This situation can result from the leaking fission products "rising" through the coolant after leaving the defective fuel rod. Testing of the coolant sample drawn through probe 63 at that position of probe 63 as illustrated in FIG. 14a will determine whether a leaking rod exists in quadrants "Q3" or "Q4". By moving the probe 63 in the "x" direction so as to disengage baffle member 63.1 from the fuel assembly 30, probe 63 can next be shifted in the "y" direction so as to insert baffle member 63.2 into the fuel assembly such that quadrants "Q1" and "Q2" of the assembly are encompassed within the volumetric test zone "T" of probe 63. By sampling the coolant drawn from quadrants "Q1" and "Q2", it can be determined whether the leaking fuel rod is present in those quadrants. Similarly, probe 63 can then be removed from the fuel assembly 30, and the fuel assembly 30 rotated by means of the rotation control 29 b so that probe 63 can be reinserted into the fuel assembly to successively test in similar manner quadrants (Q2, Q4) and (Q1, Q3) respectively. By simple, logical deduction as to those of the four tests in which leaks were detected, the particular quadrant in which the leaking rod is present is determined. An example of a typical test procedure is illustrated in the following table. In the table, an "X" designates those quadrants under test in which leaks are detected, and an "O" indicates those tested quadrants where no leak is detected. A dash indicates that no leak test was performed on that particular quadrant during a respective probe insertion step. In Test 1, (wherein quadrants Q1 and Q2 were included within the volumetric test zone of probe 63) a leak was detected in quandrants "Q1" and "Q2". Tests 2 and 4 indicated no detected leaks. Test 3 detected a leak in quadrants Q1 and Q3. Since the only quadrant common to Tests 1 and 3 was Q1, Q1 is deduced to be that quadrant containing the leaking tube. ______________________________________ Quadrant Q1 Q2 Q3 Q4 ______________________________________ Test Insertion Test 1 X X -- -- Test 2 -- -- O O Test 3 X -- X -- Test 4 -- O -- O Result: Leak No Leak No Leak No Leak ______________________________________ Key: "--" indicates quadrant not involved in test; "X" indicates leak present in quadrants tested; "O" indicates no leak in quadrants tested. Once the quadrant in which the leaking fuel rod is present has been identified, the third probe 64 can be utilized in successive tests in a manner similar to that above-described with respect to probe 63 to further subdivide such quadrant, until the exact location of the leaking rod is determined. In the preferred embodiment, the third probe 64 is configured with an inter-baffle spacing "S3" sized to accommodate a row of two fuel rods within the width of its volumetric test zone "T". The baffle walls 64.1 and 64.2 have lengths sized to slightly extend beyond the length of the quadrant under test (see FIG. 15). Since the walls of the baffle plates 64.1 and 64.2 must be inserted between adjacent rows of fuel rods and control rods within the fuel assembly, which do not necessarily have the wider spacing afforded by the wider tracks such as "X1" and "Y1" of FIG. 4, the thickness dimension of such plates must be appropriately scaled down so as to slidably fit between the fuel rod and control rod row separations. In a preferred embodiment, the baffle plates 64.1 and 64.2 are constructed of stainless steel sheets of approximately 0.05 inches thick, and are configured to "reach" ten rods deep into a 17 by 17 assembly, as illustrated in FIG. 15. In a manner similar to that previously described with respect to use of the second probe 63, the third probe 64 is sequentially inserted into the fuel assembly so as to isolate two rows at a time of the fuel rods within the quadrant known to have the defective fuel rod, until the leak has been tracked down to a 2 by 10 matrix of fuel rods. The fuel assembly is then rotated 90.degree. so that probe 64 can be inserted into the test quadrant to further subdivide the 2 by 10 matrix into 2 by 2 fuel rod segments. Once the leak has been narrowed to a 2 by 2 matrix, the test probe 64 can be shifted so as to subdivide the 2 by 2 matrix so as to isolate the exact rod containing the leak. Obviously, other variations of the testing sequence performed by probe 64 can be configured within the scope of this invntion. For example, once a leaking condition is sensed by probe 64 in a 2 by 10 row matrix, the probe can be shifted by one row to immediately narrow the possibly leaking fuel rods to a 1 by 10 row matrix. While specific examples and configurations of the apparatus and method of practicing the invention have been disclosed herein, it will be readily apparent to those skilled in the art that other appropriate probe configurations and methods for using same can be envisioned by those skilled in the art. The invention is not limited to a particular probe configuration, material or method of mounting or using multiple probes in practicing the method of this invention. As an example, while semirigid baffle or containment member walls of the probe members have been disclosed, it is possible that such baffle members could be constructed of somewhat flexible material for enabling the probe members to bend around fuel and control rod members when being inserted within a fuel assembly. Similarly, while the probe assembly and method has been illustrated in FIG. 1 as being used to test a fuel assembly 30 in the spent fuel storage area, it will be understood that the test operation could be performed in other locations of the reactor facility as well. Other modifications of the invention will be apparent to those skilled in the art in light of the foregoing description. This description is intended to provide specific examples of apparatus and methods which clearly disclose the invention. Accordingly, the invention is not limited to the described embodiments, or to the use of specific elements or materials described therein. Alternative modifications and variations of the present invention which fall within the spirit and broad scope of the appended claims are covered.