Patent Number: 041938437
Section: summary

BACKGROUND OF THE INVENTION 1. Field of Invention The invention relates to a method and an apparatus for locating defective fuel rods within fuel assemblies of water cooled nuclear reactors, and the like. 2. Description of the Prior Art The core of a light water-cooled reactor, for instance, typically consists of about 40 to 50 thousand fuel rods which are usually arranged in groups of about 200 rods to form a fuel assembly. A fuel assembly for a reactor of this nature consists of two end fittings, control rod guide tubes and spacer grids for positioning the rods. The fuel rods consist of Zircaloy-4 cladding tubes which contain the fuel in oxide form and are closed at both ends with welded caps. During prolonged operation, several rods may develop leaks so that cooling water can seep in or radioactive material can escape. The coolant purification system of the nuclear reactor is capable of handling a certain amount of radioactive fission products. However, it is desirable to keep the radiation level as low as possible in order to protect the operating personnel. Accordingly, the fuel assemblies are usually subjected to a so-called "seepage test" during shutdowns of the reactor, e.g. during refueling. The fuel assembly is placed in a water filled storage tank for this test. The fuel rods and the water heat up by residual decay. If a fuel assembly contains a defective rod, the fission products escape during the heating into the water. Through sampling of the water it can be determined whether the fuel assembly contains defective rods. This method is a totalizing method which determines only whether the fuel assembly contains defective rods. It cannot, however, identify the position or the location of the defect. U.S. Pat. No. 3,983,741 suggests removing the upper end fitting from the fuel assembly, and slipping an immersion cask over the exposed fuel rod end caps while keeping the upper ends of the fuel rods above the water. Water seeping into defective rods evaporates due to the decay heat. Instrumentation can detect temperature differences of rods containing steam. It is known from U.S. Pat. No. 3,945,245 to remove the end fittings from the fuel assembly, to slip a heating element on the endcaps of the fuel rods and to detect the generation of steam or condensate in the rods containing water by ultrasonic means. Therefore, in accordance with these patents, it is possible to locate defective fuel rods. A disadvantage of these methods is that the end fitting of the fuel assembly must be removed and special provisions must be made to evaporate the ingressed water before the defective rods can be found by the instrumentation. A further disadvantage is that in many fuel assembly designs only the lower end fitting is removable. The lower fuel assembly end fitting is installed in the reactor at the bottom end of the fuel rods. Therefore, before this end fitting can be removed the fuel assembly must be turned 180 degrees under the water in the fuel storage pool, an additional time consuming operation. The seepage test, the disassembly of the end fitting and the evaporation of the leaked-in water, require a time consumption which is a loss in availability of the power plant. In addition, every operator of a nuclear power plant strives to keep the testing times of the fuel elements at a minimum to reduce the exposure time of the maintenance crew. When the allowable exposure limit is exceeded, a new crew must be employed. Thus, a need has arisen to find a simpler inspection method for locating defective fuel rods which could reduce the required time and minimize the radiation exposure of the maintenance crew. SUMMARY OF THE INVENTION This task has been solved, by insertion of ultrasonic transducer heads into the spaces between the individual fuel rods disposed to touch the fuel rods and to emit ultrasonic waves perpendicularly to their axes. The resulting difference in the resonance is an indication of leaked-in water. During the periodic fuel replacement outages about one-third of the fuel assemblies (batch I) are removed from the reactor core as fully spent fuel. The other two-thirds, which consist of two further batches having different U-235 enrichments are relocated according to a certain carefully planned scheme and placed in different positions in the core for optimum utilization. An advantageous provision of this invention is the ability to inspect (test) the fuel assemblies during the relocation in the water filled canal arranged above the reactor. It has become advantageous to test the fuel assemblies of "batch I" during the transport from the water filled canal to the fuel storage pool. The fuel assemblies of batch I which contain defective fuel rods are stored in separate storage tanks in order to prevent the escape of fuel or radioactive materials into the storage pool. The detector transducer heads are preferably applied in the region of the lower gas space of the fuel rods where, according to experience, the leaked-in water accumulates when the reactors are not under pressure. Normally, the testing of fuel rods is accomplished at that condition. The device used to perform the inspection method of this invention contains a carriage provided with comblike fingers. The carriage is arranged to slide along guides of a support plate. The fingers are fitted with ultrasonic heads which are pressed against the fuel rods by springs. The comb-like fingers are arranged on only one side of the carriage and the ultrasonic heads are attached at the free end of the fingers. An advantageous embodiment is obtained when the support plate is attached to the fuel-handling machine. The testing in this manner can be performed during the transportation of a fuel assembly. It has proven also beneficial to make the entire fingers from a material which conducts ultrasonic waves so that the ultrasonic heads do not have to be placed at the free ends of the fingers. The method of this invention has the advantage that testing can be accomplished immediately after removal of the fuel assembly from the reactor core at a significant time saving. The disassembly of the end fitting and the waiting for the evaporation in the defective fuel rods is not required. Thus, the expensive seepage test becomes superfluous, because the present simple and fast method does not require a pre-testing for fuel assemblies containing defective fuel rods. A considerable time saving is achieved in comparison with the previous methods. Refueling time can therefore be considerably shortened thereby improving the economics of the power plant's operation.