MXPA99001372A - Detection of nuclear fuel rod failure - Google Patents

Abstract

A method and apparatus for ultrasonic inspection of a nuclear fuel rod having an inner nucleus of nuclear fuel surrounded by a coating tube or shield having a wall thickness in which ultrasonic guided waves are transmitted which permeate or they leak into the fuel rod and from which a reflected ultrasonic wave is detected which is indicative of a failed fuel rod

Description

DETECTION rpg T? T.T.? np? VAPTT.T.? NUCLEAR MACHINE FIELD OF THE INVENTION The present invention relates to the inspection of nuclear reactor fuel rods having a gap through its coating or shielding wall and more particularly with a method for ultrasonic inspection to detect water or other compressible fluid in the fuel rod. that evidences a failure of the coating tube of the fuel rod.

ANTECFn'P ^ rFp.q OF THE INVENTION Most nuclear power reactors are water cooled and moderated reactors that use enriched uranium dioxide as fuel. The reactor core is formed by elongated fuel rods which are grouped into assemblies known as fuel assemblies which are generally rectangular in cross-section. The fuel rods have diameters that are usually in the range of 6.4-13 mm (1 / 4-1 / 2 inch) and are often larger than 3 m (10 feet). Each fuel rod is formed of a coating tube typically made of a zirconium alloy which surrounds an inner core of nuclear fuel, typically uranium dioxide. Most commonly, uranium dioxide is in the form of granules which are placed inside the coating. The fuel rods are maintained in a parallel arrangement and in fixed positions closely spaced from each other by sparring grids. The spacer grids typically have an egg shell shape and each sparger cell includes curls and / or springs to place a fuel rod so that fuel rod spacing is maintained relative to the fuel rod (i.e., spacing) between fuel rods). The springs and curls keep the fuel rods in their proper lateral positions. But, under the influence of radiation, the springs are susceptible to loosening which can lead to undesirable changes in the spacing of the fuel rods or can cause the gaps or spaces to develop between the fuel rods and the springs and curls. which increase the likelihood that the rods and / or spacer grids will vibrate due to the coolant flow. Such spaces, changes in fuel rod spacing and vibration can lead to wear of the fuel rods commonly known as corrosion of the fuel rods of the spacer grid interaction, and ultimately can lead a gap or a through hole. of the wall of the fuel lining tube which encloses the enriched uranium dioxide. Such a gap or hole results in a failure of the fuel rod. Abrasion occurs at the point of contact within the spacer between the spacer spring and the fuel rod for each spacer within the fuel assembly. Although all fuel rods in a mount are susceptible to this type of abrasion, damage is more common in the fuel rods in the lower spacer region. During operation of the reactor, holes may be developed in the coating due to stress, corrosion, debris, defective welding or from fuel rod corrosion typically caused by repeated relative movement of the spacer in contact with the fuel rod. If repeated wear from corrosion of the fuel rod or other cause eventually produces a hole through the wall of the fuel rod cover, the fission gases will escape through the hole in the coating tube into the water of reactor cooling and cooling water from the reactor will enter the fuel rod. Such a fuel rod is characterized as a failed fuel rod. The amount and level of water in the fuel rod after the operation depends on the size and location of the hole, and the background of operation energy of the rod. Most fuel rods fail in the lower portion of the rod, due to spacer corrosion, residue corrosion or failed lower end layer welders. Therefore, due to the location and gravity defect, the most likely location to detect water inside a fuel rod is just above the lowest spacer. After a rod assembly has been exposed in the reactor for a given period of time, it is typically extracted, verified to determine faults and defects, repaired if necessary, and returned to the reactor or sent to storage for final disposal. . If the fuel assembly is returned to the reactor, it is typically tested and verified to detect defective and failed fuel rods. These irradiated assemblies are highly radioactive and must be stored, handled and inspected under water in order to remove the heat caused by the decay of fission products and at the same time to provide protection to people working with them. Tests of a fuel assembly for detecting failed nuclear fuel rods are usually carried out by ultrasonic methods or by washing detection. In the wash detection, the method detects fission gases that are released from a failed fuel assembly. Since washing detection can not indicate which rod has failed, such determination must be made using stray current tests, ultrasonic equipment or visual inspection. In the ultrasonic test, the presence of water in a fuel rod indicates a breach or hole in the coating of the fuel rod and a failure of the fuel rod. In one method, water is detected by measuring the difference in the attenuation of the ultrasonic energy returning from the water (failed rod) or the gas interface (rod in good condition) of the inner wall of the fuel rod coating. An example of this approach is set forth in U.S. Patent No. 4,879,088 assigned to the same assignee of the present invention. According to the approach of U.S. Patent No. 4,897,088, a transducer is caused to traverse a nuclear fuel assembly during which a series of ultrasonic pulses is emitted from the transducer in the form of a beam. When the beam hits a fuel rod, it is reflected from the outer surface. If the beam is normal to the surface it will be reflected back into the transducer to a maximum extent which results in an electrical signal ("pulse-echo" technique). Part of the ultrasonic energy penetrates the coating but the normal rod does not pass ultrasonic energy into the fuel rod (the internal gas and uranium dioxide are not conductors) resulting in a "wall oscillation" between the walls interior and exterior of the fuel rod coating. The wall oscillation is recorded. If the rod is filled with water, there will be a transfer of ultrasonic energy from the pipe wall of the fuel rod coating to the water where it is effectively dispersed. This greatly reduces the wall oscillation. This will happen if uranium dioxide is present or not. The attenuation factor of the wall oscillation identifies a defective fuel rod. In another method, which is described in U.S. Patent No. 4,313,791, a transducer is placed that emits ultrasonic signals against a fuel rod and an ultrasonic beam is transmitted inside the rod by the transducer. The test is performed on the lower plenum of the fuel rod, a portion of the fuel rod which does not contain uranium dioxide. An analysis of the waves received by the pulse-echo system reveals whether this portion of the rod is filled with water or not. i. The North American patent number 4, 126,514 discloses a method for detecting defective fuel elements by insulating them from contact of the outer surface of a fuel element coating of the normally used cooling liquid and then performing a pulsed echo attenuation measurement to identify the presence of fuel pellets. swollen excessively The measurement of the presence of any coolant inside the element or a number of swollen fuel pellets is obtained by the proper interpretation of the echo pulses. U.S. Patent No. 3,350,271 discloses a transducer which contains a liquid such that heat transfer from the transducer heats the liquid. Under normal operating conditions, the pressure maintains the liquid as a liquid. The boiling of the liquid in the transducer, detected ultrasonically, can be used as an indication that a predetermined pressure is reached. There may be a leak in the fuel rod, there may be a decrease in pressure and therefore the liquid will vaporize and expand through a hole to indicate that a leak has occurred. U.S. Patent No. 4,009,616 is directed to an acoustic method for measuring gas pressure in a hermetically sealed enclosure. This is done by determining the change in velocity and attenuation of an ultrasonic signal caused by the internal gas pressure inside the fuel rod. This process requires that the signal is transmitted through the gas and that the effects of the signal received through the coating are minimized. Failed fuel detection systems of the prior art which use ultrasonic equipment to detect water in the fuel rod as an indication of a coating with a gap, measure the difference in the amplitude of an ultrasonic signal reflected from the inside diameter of the fuel. covering. The difference in acoustic impedance between the "air" and water inside causes the signal to be reduced in amplitude if water is present. The main difference between the methods of the prior art is the manner in which the signal is injected into the fuel rod. Some use the throw-receive system, the transmit and receive transducers are different, and the signal propagates circumferentially around the fuel rod in a narrow band. Some use pulse-echo, in which the transmission and reception transducers are the same and their signal propagates in a narrow band completely around the fuel rod. In any case, the inspected area is very small, usually no larger than the circumferential band around the fuel rod that extends 2.54 mm (0.1 inch) axially. Therefore, the prior art ultrasonic inspection techniques and the methods to inspect for the presence of water within the fuel rod to determine if it has failed by testing a very narrow area of the coating, from 0.39 to 0.64 cm2 (0.060-0.100 square inches). Due to such a limited volume of acoustic interrogation of the ultrasonic inspection methods of the prior art, repeated testing is not performed on the entire portion of the axial height of the fuel rod where water may be present. Accordingly, the detection capability as well as the precision of the ultrasonic methods of the prior art are poor, resulting in a high percentage of erroneous determinations of fuel rods that are approved. If there is no water in the specific position of the inspection, the ultrasonic method will not detect any fault in the failed fuel rod. Therefore, there is a need for an inspection method which overcomes the aforementioned disadvantages and which allows a fast and efficient inspection of fuel rod coating tubes of the fuel rods within a fuel assembly to identify if there has been a gap in any fuel rod, which often occurs at the contact points of the grid with respect to the fuel rod, without the need to disassemble the fuel assembly in order to have access to the fuel rods.

BRIEF DESCRIPTION OF THE INVENTION In one embodiment of the present invention, there is provided a method for ultrasonic inspection of a nuclear fuel rod in a nuclear fuel assembly having a separate parallel nuclear fuel rod arrangement, submerged in a compressible fluid, the nuclear fuel rod has a coating tube surrounding an inner nucleus of nuclear fuel, the coating tube has an inner wall, the method comprises the steps of transmitting ultrasonically guided waves in the coating tube and detecting an ultrasonic wave reflected from coating tube, wave reflected ultrasonic which is indicative of the presence of water in the fuel rod which represents a breach in the integrity of the coating tube wall.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the excitation of guided waves using a longitudinal wave transducer, obliquely incident, in a Plexiglas wedge; Figure 2 shows the phase velocity dispersion curve for zircaloy pipe (zirconium alloy); Figure 3 shows the group speed dispersion curve for zircaloy pipe; Figure 4A is a graphical representation of the phase velocity excitation zone when guided waves are induced by longitudinal wave excitation, obliquely incident; Figure 4B is a graphical representation of the phase velocity excitation zone when waves are induced guided by a periodic distribution of normal excitations with a spatial period equal to the wavelength of the excited mode, the "comb" method; Figure 5 shows the comb-like excitation and the relationship between the space period and the wavelength; - Figure 6 shows the experimental test results of a segment of coating tube that has water inside it; Figures 7-26 show experimental test results of a segment of other coating tubes having water therein; Y DESCRIPTION T rAT.T.?D? The present invention provides a method which allows the rapid inspection and testing of nuclear fuel rods within fuel assemblies BR (boiling water reactor) and PR (pressurized water vector) to detect failure of rod coating pipes. of nuclear fuel, indicated by the presence of water and caused by residues trapped inside a separator as well as any separator corrosion induced by the flow of the refrigerant. The present invention allows rapid testing of all fuel rods in an assembly without removing the rods for individual inspection. According to the present invention, there is provided a method for ultrasonic inspection of nuclear fuel rods in a nuclear fuel assembly having an array of parallel, separate nuclear fuel rods submerged in a compressible fluid, the nuclear fuel rod having a coating tube surrounding the inner nuclear fuel core, the coating tube has an inner wall, and the method comprises the steps of transmitting ultrasonic guided waves in the coating tube which leak from the inner wall of the coating tube to the inner core and detect an ultrasonic wave reflected from the coating tube, reflected ultrasonic wave which is indicative of the presence of water in the fuel rod and represents a breach in the integrity of the coating tube wall. A guided wave is an ultrasonic wave induced in a thin-walled object with properties such that it allows the wave to remain within the limits of the material with little or no leakage to the outer surface. Guided waves can be induced in thin plates as well as thin-walled tubing. The production of guided waves in an object is controlled by the appropriate selection of transducer frequency, transducer incidence angle, material velocity of test objects and material wall thickness. Since the wave is attenuated only by the impedance of the material, the waves can travel long distances with little attenuation. There are several ways to induce guided waves in an object. The two most commonly used are an oblique angle of incidence transducer and specially designed transducers such as a "comb" transducer. In the first case, a standard transducer is placed at a predetermined angle to test the object and an injected wave. Since the transducer element is not in contact with the test object due to the angle of incidence, a conductive means such as plastic or water must be added to transfer the sound from the transducer element to the test object. This greatly increases the thickness of the transducer and prevents its effective use within a fuel assembly due to the narrow spaces of rod to rod and rod to guide tube, which will be further discussed in the following. Referring to Figure 1, the generation of a guided wave at an oblique incidence in a thin-walled tube is shown using a longitudinal wave transducer or an acoustic coupling material such as plastic. The longitudinal wave is incident at some incident angle, T, and the velocity Vplexl. Due to Snell's law, the waves undergo mode conversion, reflection and refraction at the interfaces. At a certain distance away from the transducer, the waves will no longer be individually identifiable, but will be superimposed on a wave packet. For certain cases of incident angle, thickness and material properties, the constructive interference will take place, and the guided wave will propagate in the tube or plate. These conditions for constructive interference can be satisfied for many combinations of thickness, angle and material properties. The combinations which result in constructive interference are called modes. Each mode has its own characteristics of wave structure, propagation speed and voltage distribution. In particular, each mode has a displacement in plane and out of plane in the waveguide surface. The amount of off-plane displacement determines how "permeable" (leaking) the mode will be, since off-plane displacement causes the generation of longitudinal waves outside the material and which carry energy out of the waveguide. This feature is particularly important in the application where the fuel rod fails because the faulty fuel rod tube will have water in it. Therefore, for a fuel rod with faults or gaps, the selection of a permeable mode to the inner surface of the coating tube will allow the detection of such failure since the water will rapidly attenuate the sound field which is indicative of the presence of water. It is necessary to choose a non-permeable (non-leaking) mode for both the interior and exterior surface of the coating tube for the inspection of a fuel rod which does not have a coating tube with gaps. The elaboration of a theoretical model of guided wave propagation results in scattering curves and wave structure diagrams that describe wave propagation. The phase velocity dispersion curve shown in Figure 2 is typically used to generate criteria for guided waves. In the case of oblique incidence generation, the phase velocity is simply related to the angle of Sen Ti Sen90 incidence by Snell's law: The curve of V Plexi V "Fwase group velocity dispersion is shown in Figure 3 and provides the propagation speed of the generated guided wave modes, as stated in the above, by inducing waves guided by oblique incidence, the longitudinal wave excitation is accompanied by coupling of a longitudinal wave transducer to a fuel rod coating tube at an incident angle by the use of a plastic wedge. A specific phase velocity is generated which is governed by Snell's law for the plastic wedge. Therefore, modes can be generated which intercept the particular phase velocity excitation zone. Figure 4A shows the excitation zone for this method. A second way to induce guided waves in an object is the "comb" method. A comb transducer is a contact type transducer to produce guided waves and whose characteristics are determined by the separation and thickness of the comb elements. In this method, the excitation is carried out by a periodic distribution of normal excitations with a spatial period equal to the wavelength of the excited mode (Figure 5). Instead of a region of horizontal excitation as in the oblique incidence method (Figure 4A), the excitation is in the form of a straight line passing through the origin (Figure 4B). The equation of the line is of the formula y = mx, where y = vphase x = fd and m = s / d, and where Vphase = f? = fs = s / d fd, and s is the comb separation. Figure 4B is a graphical representation of the phase velocity excitation zone for this method. Although any generation method can be used for detection of defects such as in the detection of coating tube faults by a gap, or by corrosion, wear or defects, because the comb transducers can be manufactured with a profile design narrower, they provide a means to access the interior of the fuel assembly between the narrow spaces between the fuel rods to generate guided waves in the fuel rod coating tubes within a fuel assembly without the need to disassemble the assembly made out of fuel. The comb transducers can be manufactured with varying degrees of spacing and element thicknesses to generate the desired mode in a variety of thicknesses and material properties. An advantage of a type of transducer comb is that it can be manufactured relatively thin and still maintain its characteristics. Due to its small thickness, the comb transducer is the best practical method of introducing guided waves into fuel rods within an assembly. To test a fuel rod inside a mount, the transducer must pass between small spaces (1.8 mm or 0.072 inches of fuel rod to the guide tube) to test the inside rods. The comb transducer is designed to produce guided waves in the fuel rod coating that have a longitudinal phase velocity that is "non-permeable" for both the inner surface and the outer surface of the fuel rod cladding tube , that is, the guided wave remains inside the coating rod. This provides maximum sensitivity and minimum attenuation. The frequency of operation of the transducer is selected to produce circumferential sound field strengths that are maximum at the desired inspection points (normally at 135 ° with respect to the possible trajectories of presentation of the transducer). The parameters of the transducer (operating frequency, distance between the "teeth" of the comb transducer, etc.) can be modified to cause the sound field to be virtually any desired inspection point (ie, axial placement along of the fuel rod). In the tubes, both axisymmetric and non-axisymmetric guided waves can propagate. The axisymmetric guided waves refer to ways in which the particle movement is only in the longitudinal and radial directions and has uniform distributions of tension and movement of particles around the tube. This can be done by using a full-circle comb transducer. The non-axisymmetric guided waves have torsional components as well as longitudinal and radial components with respect to the movement of the particle and have a non-uniform distribution of tension and a particular movement around the tube. There are two inspection modes that use comb type transducers, "loaded" and "partially loaded". In a loaded application, the transducer completely surrounds the outer surface of the fuel rod (i.e., the outer surface) of the coating tube (or the inner diameter for applications such as in steam generating tubes). This is the ultimate test where the transducer generates a uniform sound field (axisymmetric) that travels in both directions (ie axially and circumferentially) in the tube and provides maximum sensitivity. In a partially loaded test, only one segment of the transducer makes contact with the test object. This is less than optimal insofar as it produces a non-axisymmetric sound field circumferentially and axially along the tube. This non-axisymmetric sound field has a limited inspection value due to the lack of uniformity of the sound field at any specific position. Due to the very limited access of the fuel rods within a nuclear fuel assembly, only partial charge sources such as a section of a comb transducer can be used which will produce non-axisymmetric waves. This is not a problem, due to the physical characteristics of the transducer (eg number of comb teeth, loading area, etc.), and the fundamental frequency parameters can be optimized to generate the maximum sound field in the desired position . Changes in the position and intensity of the sound field are more sensitive to changes in frequency. The method for inspecting nuclear fuel rods to detect faults or a gap in the fuel rod coating comprises using a comb transducer, activated in tone discharge, partially charged, which passes through the spaces from rod to rod. a fuel assembly. By selecting the comb spaces and thicknesses, the transducer produces a field strength at a specific axial and circumferential distance along the fuel rod. In a particular case, the circumferential distance can be selected to be 90 ° from the sound entry point to take into account the spring position in relation to the input path of the available probe. The sound field characteristics are highly dependent on the frequency, when the properties of the test object are constant, such as in the nuclear fuel rod coating tubes. To further increase the effectiveness of the inspection technique, the tone discharge excitation may be "swept" through a specific frequency band which causes the sound field to change in intensity circumferentially around the fuel rod which is inspected. The use of a discharge generator in the sweep frequency tone as a transducer excitation source allows the entire region of the fuel rod within the separator cell to be inspected from a position without the need to generate axisymmetric waves for a transducer of full circle comb. The use of such a full circle comb transducer will require the disassembly of the fuel assembly in order to place the fuel rod through the circle comb transducer. Defects within the fuel rod coating will reflect part of the guided wave signal which will be received by the comb transducer operating in the pulse-echo mode. The amplitude of the reflected signal is proportional to the facial area of the defect. Therefore, the use of the sweep frequency will allow the sound field to inspect the entire 360 ° of the fuel rod by changing the sound field according to the sweep frequency. In another method, the comb transducer can be inserted into the spaces between the fuel rods, a specified distance above or below a spacer. The transducer will be pulsed at a repetition rate. When the transducer is placed normal to the fuel rod, a maximum load will be obtained on the fuel rod, and the injected signal travels up and down the fuel rod, which produces sound field intensities in the fuel rod. desired point or inspection points, determined by the physical characteristics of the transducer. Because the guided wave produced in the fuel rod coating tube is non-permeable, it is more sensitive to wear, defects or other changes in only the coating wall. The portions of the sound wave are reflected, for example, by any change in wall-to-wall thickness such as would be produced by a corrosion mark. The reflected signal is received by the transmitting transducer and transmitted and processed by the ultrasonic instrument. Then the transducer will continue to be placed through the assembly while recovering data from each fuel rod. In this way, each fuel rod is inspected during assembly. Ultrasonic testers can be tone discharge or shock excitation (all tests must be performed with tone discharge.) The placement system can be a standard X-Y table for underwater use.

Detection < * Faulty fuel rod using guided waves The use of guided waves (described above) greatly improves the probability that the ultrasonic technique will detect water in the fuel rod. In the embodiment mentioned before the present invention, the frequency is chosen purposely to ensure that the sound is not "permeable" to the outer or inner surfaces of the coating tube, as the water rapidly attenuates the sound field. In this embodiment, the frequency is chosen so that the wave is permeable mainly to the inner wall of the coating tube. In this way, any amount of water will rapidly attenuate the sound field and indicate the presence of water while the "air" does not (differences in acoustic impedances). The technique of using guided waves for fuel rod inspection to detect wear, defects or other changes in the wall of a fuel rod coating tube can be extended, in accordance with the present invention, to the detection of failed fuel. The operating characteristics of the comb transducer are changed so that the injected signal is "permeable" (easily attenuated) on the inner wall of the coating tube by a coating-water interface, since the water in the fuel rod is a fault indicator. Secondly, you can use a receiver transducer (comb or comb) in addition, which can be located on any or the same side of the fuel rod as the transmitting transducer, or on the opposite side . The amplitude of the received signal is indicative of the good condition of the fuel rod. A signal of high amplitude indicates that there is no water in the fuel rod. Alternatively, a single probe transducer system is used to evaluate the amplitude of the wave reflection guided from the lower end cap of the fuel rod. A high amplitude of the reflected wave indicates that the fuel rod has no faults. The advantage of the present invention with respect to the prior art ultrasonic techniques for determining whether a fuel rod coating tube has faults, is that a much larger area of the inner wall of the fuel pipe can be interrogated detect the presence of water which makes it more likely to detect faults. The ultrasonic methods of the prior art at best only test a very small portion of the fuel rod, from 0.39 to 0.64 cm2 (0.060-0.100 square inches) (i.e., 2.54 mm (0.1 inch) in height). axial). In prior art techniques, only a single point of 0.19-0.77 cm2 (0.030-0.12 square inches) is interrogated for a complete circumferential examination. The present invention allows the interior wall area of the fuel coating tube to be tested on an axial height of 15.2 cm (6 inches) from the fuel rod. This greatly increases the possibility of positively detecting water in a faulty fuel rod. Because the comb transducer is only partially loaded producing a non-axisymmetric sound field that travels on the fuel rod from the position of the transducer to the end cap of the fuel rod, the area of the inner wall of the tube of coating that is interrogated increases by at least an order of magnitude. According to another method of the present invention, a comb transducer is placed in the center of the extension above the lowermost spacer of a fuel rod and which transmits ultrasonic guided waves which travel downward and are reflected out of the welded bottom end cap of the fuel rod being inspected; and the reflected ultrasonic signal is received by the comb transducer wherein the amplitude of the reflected signal is an indication of water in the fuel rod. In accordance with another aspect of the present invention, an apparatus for the detection of nuclear fuel rod wear or fuel rod failure is provided, which comprises a "comb" style ultrasonic transducer to produce guided waves, a tester ultrasonic; and a transducer placement system. More specifically, the comb transducer is mounted on a delivery system, an X-Y table places the transducers within the fuel assembly, and an ultrasonic defect detector receives and displays the ultrasonic signal received from the fuel rod. A sweep frequency generator drives the transducer and a power amplifier amplifies the output of the output frequency generator.

Test Results Experiments were carried out using a tone discharge system and a variable angle transducer to generate non-axisymmetric guided waves in fuel rod tube samples. The tone discharge system provides a sine-wave, high-voltage gate pulse that activates the transducer with a narrow frequency spectrum. The narrow spectrum is particularly important insofar as it allows the selection of unique modes from the scattering curves. The frequency of the transducer can be changed so that different portions of the scattering curve can be excited. In this test, segments of nuclear fuel rod coating tubes that have defects, surface scratches or wear (without nuclear fuel) were plugged at one or both ends and immersed in a water bath. Modes were chosen by means of an experimental and theoretical basis from the dispersion curves. A sensitivity mode was determined by these tests for all samples with defects and with an adequate signal to noise ratio. Initially, a non-axisymmetric third-order mode was found, at a frequency of 5.18 MHz with adequate sensitivity and signal-to-noise ratio, to perform the inspection. The experimental results with the transducer at 7.6 cm (3") and 0 ° with respect to the position of the defect are shown in figure 6, in which the defect was detected, in figure 6, for a defect of 0.64 mm ( 0.025"), the coating tube is charged with water internally as well as externally. Figures 7-26 show additional results which demonstrate the effect of non-axisymmetric modes on detection sensitivity. These results are for a generation angle of 37 ° and 4.67 MHz. All waveforms are for a 0.25 mm (0.10") defect. Figures 7-21 show the variation in sensitivity as the transducer increases longitudinally ( that is, axially) away from the defect Figures 22-26 show the variation in sensitivity as the transducer increases circumferentially around the tube and a single longitudinal position.

Although the above description and drawings represent the preferred embodiments of the present invention, it will be apparent to those familiar with the art that various changes and modifications may be made therein without departing from the true spirit and scope of the present invention.

Claims (17)

1. A method for ultrasonic inspection of a nuclear fuel rod in a nuclear fuel assembly having an arrangement of parallel nuclear fuel rods, separated from each other, in a compressible fluid, the nuclear fuel rod has a coating tube surrounding a inner nucleus of nuclear fuel, the coating tube has an inner wall, the method is characterized in that it comprises the steps of: transmitting ultrasonically guided waves in the coating tube; detecting an ultrasonic wave reflected from the coating tube, a reflected ultrasonic guided wave which is indicative of the presence of water in the fuel rod and represents a breach in the integrity of the coating tube wall.

2. The method according to claim 1, characterized in that the step of transmitting ultrasonically guided waves is from a comb transducer.

3. The method according to claim 2, characterized in that the step of detecting an ultrasonically guided wave reflected from the coating tube is by means of a receiver transducer.

4. The method according to claim 2, characterized in that the ultrasonic guided waves are transmitted by the comb transducer and are permeable (with leaks) with respect to the inner wall of the coating tube.

5. The method according to claim 4, characterized in that the ultrasonic guided waves are non-axisymmetric.

6. The method according to claim 5, characterized in that the ultrasonic guided waves are transmitted by a partially loaded comb transducer.

7. The method according to claim 6, characterized in that the ultrasonic guided waves are transmitted by the comb transducer from a sweep frequency tone discharge signal.

8. The method according to claim 7, characterized in that the step of detecting the ultrasonic guided wave reflected from the coating tube is performed by an ultrasonic defect detector.

9. A method for ultrasonic inspection of a nuclear fuel rod in a nuclear fuel assembly having an arrangement of parallel nuclear fuel rods, separated from each other, submerged in a compressible fluid, the nuclear fuel rod has a surrounding coating tube an inner nucleus of nuclear fuel, the coating tube has an inner wall, the method is characterized in that it comprises the steps of: transmitting ultrasonic guided waves in the coating tube which is permeable to the ultrasonic guided waves from the inner wall towards the inner core; detecting an ultrasonic wave reflected from the coating tube, a reflected ultrasonic guided wave which is indicative of the presence of water in the fuel rod and represents a breach in the integrity of the coating tube wall.

10. The method according to claim 9, characterized in that the step of transmitting ultrasonically guided waves is from a comb transducer.

11. The method according to claim 10, characterized in that the step of detecting an ultrasonically guided wave reflected from the coating tube is by means of a receiver transducer.

12. The method according to claim 10, characterized in that the ultrasonic guided waves are non-axisymmetric.

13. The method according to claim 12, characterized in that the ultrasonic guided waves are transmitted by a partially loaded comb transducer.

14. The method according to claim 13, characterized in that the ultrasonic guided waves are transmitted by the comb transducer from a sweep frequency tone discharge signal.

15. The method according to claim 14, characterized in that the step of detecting the ultrasonic guided wave reflected from the coating tube is performed by an ultrasonic defect detector.

16. An apparatus for ultrasonic inspection of a nuclear fuel rod in a nuclear fuel assembly having an array of parallel nuclear fuel rods, separated from each other, submerged in a compressible fluid, the nuclear fuel rod has a surrounding coating tube an inner nuclear fuel core, the coating tube has an inner wall, comprising: an ultrasonic comb transducer for transmitting ultrasonic guided waves in the coating tube; an ultrasonic tester for receiving the ultrasonic wave reflected from the coating tube, reflected ultrasonic guided wave which is indicative of the presence of water in the fuel rod; and a transducer positioning member to place the transducer within the fuel assembly.

17. A method for ultrasonic inspection of a nuclear fuel rod in a nuclear fuel assembly having a plurality of spacer grids through which fuel rods pass in a parallel arrangement of parallel nuclear fuel rods submerged in a fluid compressible, the nuclear fuel rod has a coating tube surrounding the inner core of the nuclear fuel, the coating tube has an inner wall, the method is characterized in that it comprises the steps of: (a) placing a comb transducer in the interior of the extension between a lowermost spacer and an adjacent spacer and above the lowermost spacer; (b) transmitting ultrasonic guided waves in the coating tube, guided waves which travel downward in the coating tube and are reflected in the lower end cap welding of the fuel rod; (c) detecting an ultrasonic wave reflected from the lower end cap of the fuel rod wherein the amplitude of the reflected ultrasonic wave is indicative of the presence of water in the fuel rod.