Patent Number: 048287822
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the inspection of fuel assemblies in nuclear power plants and, more specifically, to an arrangement for inspecting fuel rods of boiled water reactors using an ultrasonic transducer which does not require removal of the fuel channel slip encasing the fuel rods. 2. Description of the Related Art Two types of nuclear fuel reactors are primarily used in the nuclear power industry: pressurized water reactors (PWR) and boiling water reactors (BWR). The nuclear fuel for either of these type of reactors is housed in fuel assemblies containing an array of fuel rods, each fuel rod in turn containing the fissionable material used to power the reactor. The fuel assemblies are submerged in circulating coolant water during operation, and are designed to efficiently transfer the generated fission heat to the coolant water. The conventional technique for testing leakage from the fuel rods of both PWR and BWR reactors is to sip the water surrounding the fuel rods, and then test the water for radioactivity. Three different types of sipping methods exist: wet sipping, dry sipping and a more recent technique of vacuum sipping, to which U.S. Pat. No. 4,034,599 assigned to General Electric Co., is directed. In a typical vacuum sipping cycle, the fuel assembly is inserted in an isolation chamber, which is then sealed and flushed with either demineralized or reclaimed water. In preparation for testing, a gas space is established above the fuel assembly by releasing a small quantity of water from the chamber while injecting air at the top at a slightly higher than pool pressure. This gas is then evacuated by a vacuum pump and passed through an in-line beta scintillation detector. Although vacuum sipping is faster than either wet or dry sipping because sample acquisition and laboratory analysis steps are eliminated, all three sipping techniques merely detect the presence of fissionable material which has leaked into the coolant water. None of the sipping techniques isolate a problem to a particular fuel rod, and thus all require subsequent removal and testing of all fuel rods in the assembly. In view of the above drawback, a subsidiary related to the assignee of the present application developed a failed fuel rod detection system for PWR reactors employing an ultrasonic transducer in the form of a two-fingered probe which traverses horizontally through each row of the rod array. The probe transmits an ultrasonic pulse and can detect and localize the presence of minor quantities of water inside each individual fuel rod from the received signal. The presence of water implies a through-wall defect in the particular fuel rod, indicating a fuel rod failure. This ultrasonic apparatus and technique, covered by U.S. Pat. No. 4,193,843 to Womack et al., issued Mar. 18, 1980, is much more accurate than sipping, and provides to the user a real time plot of the PWR fuel assembly showing the actual location and analysis of each of the rods. Thus, no fuel assembly dismantling is necessary for inspection/detection. Although the above-described failed fuel rod detection system could conceivably be used to inspect all types of light water reactor assemblies (both PWR and BWR), the rod assemblies of BWR reactors are typically encased with a fuel channel slip, and thus inaccessible to a horizontal probe. It is possible to remove the fuel channel slip of BWR reactors to test the fuel rods, but such disassembly is highly undesirable. As shown in FIG. 1, the only entrance aperture for testing the fuel rods in BWR assemblies without removing the slip is at the bottom, through the nosepiece 2. However, the nosepiece has only about a 31/2 inch diameter opening, thus partially obscuring direct access to the outer fuel rods from beneath. An inverted tripod extending across this opening in the nosepiece makes access to the individual fuel rods even more difficult. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an arrangement for the inspection of BWR fuel rod assemblies in which the fuel rods are accessed through the nosepiece of the assembly. Another object of the invention is to provide an arrangement for inspecting BWR fuel rods which isolates a leakage problem to a particular fuel rod. A further object of the invention is to provide an arrangement for inspecting BWR fuel assemblies which is highly accurate and requires minimal re-inspection. These and other objects are achieved by the present invention, comprising an inspection arrangement for BWR fuel assemblies in which an ultrasonic transducer probe is inserted through the nosepiece in the shape of an inverted tripod located at the bottom of each fuel assembly. The fuel rods are supported in each assembly by upper and lower tie plates, in the form of grids. The grids have first apertures for supporting the ends of the fuel rods, and second apertures to allow coolant water to flow between the fuel rods during operation. The probe is inserted up through the second apertures of the lower tie plate grid. The probe may be successively inserted in each second apertures or, preferably, the probe may be inserted only in one of every four apertures and rotated to inspect each of the four surrounding fuel rods for the presence of water. In order to access the second apertures in the fuel assembly covered by the nosepiece, the probe is pivoted about a single pivot point, the pivot point being located in the trident of the nosepiece which is non-symmetric with respect to the array of fuel rods. The pivot point is preferably positioned centrally in the non-symmetric trident. However, the pivot point can also be positioned in any trident or below the tripod. Moreover, multiple single pivot points, one in each trident, can be provided to allow multiple inspections in parallel. In the preferred embodiment of the invention, the probe passes through a ball joint located at the single pivot point. In this arrangement, the lower end of the probe may be moved at its lower end by an x-y scanning bridge to cause the probe to pivot at the ball joint. In a second embodiment, the probe is seated in a two-stage goniometric cradle which, when rotated, cause the probe to pivot about the single pivot point. In a third embodiment, the probe is seated in a goniometric cradle disposed on a rotational table which, when rotated, causes the probe to pivot about the single pivot point. Other features and advantages of the invention are described below, with reference to the accompanying drawings.