1. Field of the Invention
The invention pertains to a method and an apparatus for locating defective fuel rods in water cooled nuclear reactors without removal of the fuel rods from the fuel assembly, whereby the fuel assembly standing in an inspection container is covered by a bell jacket (enclosure) and stored in a water pool.
2. Description of the Prior Art
The core of a light water reactor consists of about 40 to 50 thousands of fuel rods, which usually are mechanically joined together in groups of about 200 pieces into, so called, "fuel assemblies." A fuel assembly consists of two end fittings, of control rod guide tubes, of spacer grids and of the fuel rods. The fuel rods consist of Zircaloy -4 cladding tubes, which contain nuclear fuel in oxide form and are closed at both ends with end cups.
During operation, local leaks can sometimes occur in a fuel rod which permits escape of gaseous fission products into the cooling medium and increases its radioactivity. A certain amount of radioactivity in the coolant system can be tolerated. However, it is desirable to keep the radiation level around the reactor system as low as possible.
Because of the above reason the fuel assemblies are subjected to a so-called "seeping test" during each refueling. For this test the fuel assembly is transported under water into the fuel storage pool. The fuel rods and the water are heating up due to the decay heat. If a fuel assembly contains defective rods, the radioactive fission products escape into the water due to the heating. Through sampling of the water and measuring the radioactivity, it is possible to determine whether a fuel assembly contains defective fuel rods. This method is an integral method, which means, that it determines only whether a fuel assembly contains defective fuel rod(s); but it does not provide an answer about the location. Thus, to restore the fuel assembly to its operational readiness it is necessary to locate the defective fuel rods, to pull them out, and to insert into the empty spots new fuel rods, or dummy rods. With the presently known method of locating the defective rods(s), it is necessary to pull all the fuel rods partially or fully and to examine each of them with eddy current or ultrasonic testing, in order to locate the damaged spot(s). The good fuel rods are inserted again into the fuel assembly and the defective ones are replaced by new fuel rods.
This method is very costly and requires a lot of time. It is also the disadvantage that the fuel rods must be pulled out fully or partially. During the pulling and the reinsertion of the fuel rods, the spacer grids leave scratches or grooves on the outer surface of the cladding tube, which can become a starting point for future fuel rod defects.
The object of this invention is to determine the defective fuel rod within the fuel assembly without the necessity of removing any of the fuel rods from the fuel assembly or moving them within the fuel assembly.