Patent Number: 044029047
Section: summary

BACKGROUND OF THE INVENTION This invention relates to nuclear reactor fuel, and more particularly to the inspection for failure of cladded nuclear fuel rods. In modern light-water nuclear power reactors, the reactor core typically consists of over one hundred closely spaced fuel assemblies, each assembly containing an array of over one hundred individual fuel rods. The fuel rods are typically elongated, sealed tubes of Zircaloy containing a column of uranium dioxide pellets. Safe operation of the reactor requires that the integrity of the Zircaloy clad be maintained throughout the burnup history of each fuel rod. Occasionally, however, the clad is perforated during operation. Although each fuel assembly typically burns for a total of about three cycles, or about three years, the assemblies in the reactor core are usually rearranged annually during the refueling process. During refueling, the assemblies may be removed from the core and inspected for failed fuel. This inspection is typically very complicated and time consuming because the inspection must be performed remotely under water and because most of the rods in the assembly are not on the assembly periphery and thus not readily accessible. It would be far too costly to disassemble, inspect, and reconstitute every assembly suspected of containing one or more leaking fuel rods. What is needed is a method for quickly inspecting all fuel rods in an assembly to identify those in which cladding has been breached, permitting coolant water to enter the rod. SUMMARY OF THE INVENTION It is thus an object of the present invention to provide a method for quickly and simply identifying failed fuel rods during routine refueling operations. It is a further object that the method be compatible with existing fuel and fuel assembly designs, and not require significant materials or fabrication cost increases to existing fuel designs. According to the invention, a method for testing the clad integrity of a nuclear fuel rod is provided, comprising the first step of fabricating a sealed fuel rod with a wad of electrically conducting material mounted therein. The material is of a type that undergoes a permanent change in electrical conductivity when exposed to water. The next step is to establish an eddy current signal characteristic of the moisture-free rod. The rod is then loaded into the core as part of a fuel assembly. After producing power, the assembly is removed for inspection, and an eddy current signal is again obtained from the rod. The eddy current signals are compared to determine whether inleakage of moisture has oxidized or otherwise altered the conductivity of the wad enough to significantly change the characteristic signal. In the preferred embodiment a zirconium wool similar to that found in conventional photographic flash bulbs is secured to the upper end caps of each nuclear fuel rod and is exposed to the plenum region above the fuel pellets. Any inleakage of water oxidizes the wool and transforms it into a powder that in effect vanishes from the cap and plenum. The eddy current signal corresponding to this oxidized condition is dramatically different from the characteristic signal for a moisture-free rod containing the zirconium wool. Since the upper ends of the fuel rods in an assembly are typically accessible during refueling, and since the probe signal corresponding to a failed rod is readily identifiable to the probe operator, the inspection of each assembly can be simply and conveniently completed within a short time. It is not necessary to remove or left any rods and they can be inspected simultaneously in large quantity.