Patent Number: 043137911
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to nuclear reactors and, in particular, to a method for locating defective fuel elements. 2. Description of the Prior Art In water cooled heterogeneous reactors, a multiplicity of elongated fuel elements and control element guide tubes are arranged, as a closely spaced array, in a unified structure called a fuel assembly. The reactor core is generally comprised of a lattice of vertically disposed fuel assemblies. Each of the elongated fuel elements, which are alternatively characterized as fuel rods, tubes or pins, contain nuclear fuel encapsulated by a thin cladding, plugged at its ends, that prevents erosion of the fuel and the release of fission products into the reactor coolant. Aluminum or its alloys, the stainless steels and zirconium are typical clad materials. Plenum chambers and clearances are provided within the fuel elements to accommodate fission product gas released from the fuel, differential thermal expansion between the cladding and the fuel, and fuel density changes during burnup. The plenums are generally located at the ends of the fuel element and contain plenum springs which maintain the nuclear fuel in a fixed relationship. In some cases, the fuel elements are initially pressurized with a gas, typically helium, to minimize clad creep during prolonged periods of operation at high reactor coolant system pressures. The fuel element cladding is designed to withstand the effects of the reactor operating environment including those due to coolant hydraulics, reactor temperature and pressure, fission gas pressure, fuel expansion, and irradiation growth. Some cladding defects, which permit the escape of radioactive fission products into the fluid coolant or moderator, however, may be expected to occur during the operating life of the reactor. Although purification systems are designed to remove the maximum amount of radioactivity expected to occur due to cladding defects, it may be desirable or necessary to detect and replace defective or "failed" fuel elements. Hence, it is important to have reliable means for locating defective fuel elements. On one hand, locating a defective fuel element within a fuel assembly is extremely difficult since an assembly is radioactive and may contain hundreds of closely spaced fuel elements and guide tubes. On the other hand, disassembly and reassembly of irradiated fuel assemblies is time consuming and may, in itself, result in fuel element damage. In reactors utilizing liquid coolants, a number of devices and techniques have been proposed for locating individual defective elements within the fuel assembly based upon detection and analysis of vibration, temperature differentials or ultrasonic phenomena. These prior art detection devices and techniques have depended, in general, upon at least partial disassembly of a fuel assembly. Moreover, great dependence has been placed, in the prior art, on the dynamics of thermodynamic changes of state of the fluid which has leaked into the defective fuel element, typically boiling or condensation or both. In order to facilitate the location of failed fuel elements within a fuel assembly, the development of a reliable method and apparatus which neither requires disassembly of a fuel assembly nor is dependent upon boiling or condensation of fluid within the fuel elements has been desired. SUMMARY OF THE INVENTION According to the present invention, in a fuel assembly of the type described above, a method and apparatus are provided for detecting defective fuel elements. An ultrasonic search unit, made in accordance with the invention, is inserted into the spacing between components of the fuel assembly. The smallest spacing between fuel assembly components into which the transducer assembly must be inserted is on the order of two millimeters. In this respect, the search unit is comprised of a transducer element supported by a carrier which is capable of traversing the restrictive spacing. The transducer is aligned with the lower plenum of the fuel element to be tested. It is known that ultrasound in the megahertz frequency range on the order of 5 to 15 megahertz is readily propagated through water. In contrast, at frequencies in this megahertz range, ultrasound attenuation is high in air or other gases. An ultrasonic pulse, at a frequency of a few megahertz, is transversely introduced into the wall of the fuel element. If the fuel element is not defective, then only gas will be in the lower plenum. The high reflection coefficient at the integral metal-gas interface will prevent significant propagation of the pulse past the inner surface of the cladding. If, in contrast, the fuel element has failed so that the lower plenum contains water, the reflection coefficient of the metal-liquid interface at the inner surface of the cladding will be reduced below that produced by a metal-gas interface. Thus, significant portions of the pulse will propagate through the water to the opposite wall, be reflected, and return to the transducer. Detection of the reflection from the opposite wall indicates that water has seeped into the fuel element. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawing and descriptive matter in which there is illustrated and described a preferred embodiment of the invention .