Patent Number: 041742559
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates an ultrasonic search unit 20. The search unit 20 includes an ultrasonic transducer element 21 and a strip carrier 22 that, as is best shown in FIG. 2, has mutually opposing faces 23, 24, and an aperture in which the transducer element 21 is suitably mounted. The transducer element 21, which is a polarized ferroelectric ceramic having an electrode deposited or fired on two of its surfaces, is aligned within the aperture so that one surface 25 is flush with face 23 of the strip carrier 22. The opposing surface of the transducer element is recessed within the aperture and faced by a sonic damping material 26. A decoupling isolating material 30 is placed between the perimeter of the aperture and the respective opposing surfaces of the element 21. The element 21 is secured within the aperture by an electrically non-conducting cement 31. The surface 25 of the transducer element 21 which is flush with face 23 of the strip carrier is grounded to the carrier. Grounding is accomplished by spot welding several conductors 32, or by other suitable means. The decoupling material 30 is disposed between the transducer element and carrier in order to minimize ultrasonic coupling therebetween. A coaxial cable 33, having an inner conductor 34 and an outer conductor 35, is attached to an edge 36 of the strip carrier. The inner conductor 34 is attached to the transducer element 21. The outer conductor 35 is attached to the strip carrier 22. The search unit 20 must be capable of freely traversing the limited clearances between the fuel elements or between a fuel element and a control element guide tube of a fuel assembly which may be spaced to within two millimeters of each other. Hence, the search unit 20, as well as its individual components, must be selected to satisfy specific dimensional requirements without compromising the ultrasonic characteristics needed to apply the principles of the detection technique. A specific example of a search unit constructed in accordance with the principles of the invention includes a transducer element fabricated from lead zirconate titanate, measuring approximately 2.5 millimeters wide, 12.5 millimeters long and 0.3 millimeters thick, mounted in an aluminum carrier. The transducer element is isolated from the perimeter of the aperture by a layer of cork. The front and back surfaces of the transducer element are coated with fired silver electrodes, and the surface that is flush with one face of the carrier is grounded to the adjacent aluminum at several points through small copper wires tack welded to both the aluminum and the silver electrode. A layer of the conducting epoxy resin may be spread over the copper wires and face of the transducer element at face of the carrier in order to present a smooth surface for insertion into the fuel assembly. The damping material 26 is composed of two grades of tungsten powder mixed in a low molecular weight polysulfide polymer. A specific damping material includes a mixture of a tungsten powder of an average particle size of 4.5 microns with a tungsten powder of an average particle size of 1.33 microns mixed with a low molecular weight polysulfide polymer called Thiokol LP-3, manufactured by the Thiokol Chemical Corporation, Trenton, New Jersey. A non-conducting epoxy resin is used to secure the transducer element within the aperture. The recessed surface of the ceramic is connected to the inner conductor of a coaxial cable which is disposed along the edge of the carrier. Other arrangements, shapes and materials can be used for the transducer element as long as the search unit is insertable between the components of the fuel assembly. In an alternate embodiment, for example, a hollow tubular carrier within which the coaxial cable is contained might be used. FIG. 3 shows, as a section of a fuel assembly, a schematic planar representation of a search probe 20 transversely aligned with the lower plenum of a fuel element 40. The transducer element, coupled to the fuel element 40 for transmitting ultrasonic energy into the fuel element, is energized by a pulser (not shown) to emit pulses at a predetermined rate and frequency. The sweep of an oscilloscope is synchronized to display the transmitted and reflected pulses. The reflected waves are received by the scope via the transducer. If the fuel element has not failed, then gas will be the only fluid present in the lower plenum. A high reflection coefficient at the metal-gas interface will prevent significant propagation of the ultrasound past the inner surface of the cladding. The response displayed on a conventional pulse echo instrument for a gas filled fuel element is shown as an oscillogram in FIG. 4 with time (t) plotted as the abscissa. The oscillogram of FIG. 4, and also FIGS. 6 and 7, is representative of the resulting display generated at a frequency of approximately seven megahertz wherein each division of the time scale is approximately three microseconds and the fuel element outside diameter is slightly below 0.5 inches. In FIG. 4, the transmitted signal is substantially mixed with the received signal reflected from the first or front gas-metal interface due to the low coefficient of transmission of the gas. If, in contrast, the fuel element has failed so that the lower plenum contains water, the reflection coefficient at the front interface will be significantly diminished. Thus, as schematically shown in FIG. 5, significant portions of the ultrasonic pulse will propagate through the liquid and be reflected at the back liquid-metal interface within the fuel element 40. Hence, a reflected signal of a relatively pronounced magnitude separated from the transmitted signal on the time scale will be displayed. The response displayed on a coventional pulse echo instrument for a defective, water filled fuel element is shown as an oscillogram in FIG. 6. A significant response occurs at approximately fifteen microseconds on the abscissa--this represents the echo received from the back wall. The lower plenum of a fuel element generally contains a helical spring member which may restrict the free passage of the ultrasound. This does not, however, present an insurmountable difficulty. If the width of the piezoelectric element, measured along the longitudinal axis of the fuel element is greater than the pitch of the helical spring, then sound will propagate to the far wall and return. FIG. 7 shows the typical response of a water filled element containing a spring. Conventional ultrasound instruments contain gating circuits that allow the extraction of signals during a selected period of time relative to an initial pulse. In addition, circuitry can be provided to produce an alarm signal only when the ultrasonic signal amplitude in the gated period exceeds a preset threshold level. If the gate is set to pass signals between twelve and fifteen microseconds on the abscissa, and if the amplitude threshold is set at line 1 of the ordinate, then the presence of water is detectable in a fuel element with or without springs. In operation, the search unit is inserted into the spacing between adjacent components of the fuel assembly. Irradiated fuel assemblies are generally maintained under water, for cooling and shielding purposes, during removal from a reactor and initially are stored in a spent fuel pool. Hence, it will be understood that the inspection of the fuel elements is effected under water. The transducer element is transversely aligned with the longitudinal axis of the fuel element to be examined. A pulse is then emitted from the transducer into the fuel element. A fuel assembly can be tested by insertion of the search unit into the bundle of fuel elements without any component disassembly. Hence, the assembly need only be removed from the reactor for inspection purposes. The technique can be expanded to use multiplexed transducers to examine all the fuel elements of a fuel assembly automatically and rapidly.