Patent Number: 043538630
Section: summary

This invention relates to a method for localizing a leaking rod in a nuclear fuel assembly, notably an irradiated assembly. Nuclear fuel is generally in the shape of sealed tubes enclosing a pellet stacking, and one or a plurality of plenums for collecting the gaseous fission products which are released from the pellets during the nuclear reactor operation. Said tubes which are called "rods" in this description, are assembled in clusters according to a regular array with a square or triangular geometry for instance, the whole unit forming a rod assembly. Under producing conditions, the nuclear reactor is stopped regularly to unload part of the assemblies, the most used ones, and replace same by green assemblies. For various reasons some rods may lose the tightness thereof during the reactor operation. Said rods then release radio-active fission products the accumulation of which in the primary cooling circuit of the reactor would cause the access thereto to be more and more difficult. This is one reason why the users of nuclear power plants perform an overhauling of said assemblies as they are removed from the reactors, notably when the radio-activity of the primary circuit thereof exceeds some predetermined standards. During such overhauling the leaking assemblies are separated from the safe assemblies. By "leaking assemblies" is thus meant such assemblies as defined above at least one rod of which does not have the required tightness. So-called "wet sipping" or "dry sipping" sweating methods are generally used to perform such an overhauling. Such methods are well known, notably from French Pat. Nos. 2,389,202 and 4,147,587. The assembly to be overhauled is arranged under such conditions that the contaminating by the radio-active gaseous fission products, of that water or gas which has directly contacted said rods, may be measured either by having said water or air pass a radiation detector, or by drawing gas or water samples in which the radio-activity is measured. As a rule the leaking assemblies even as they are not completely spent, are not loaded back in the nuclear reactor. Consequently the presence of a leaking rod may penalize a complete assembly, that is about 300 rods in a PWR-type reactor ("pressurized water reactor"), about 50 rods in a BWR-type reactor ("boiling water reactor"), and about 250 rods in a FBR-type reactor ("fast breeder reactor"). The presence of such leaking rods in some assemblies has thus a substantial financial influence on the operating costs of a nuclear power plant. Another problem which is perhaps of less financial importance but which will cause heavier and heavier safety restraints in the future, is the conveying of leaking assemblies and the receiving thereof in retreatment works. One essential object of this invention is to provide a method allowing to obviate the drawbacks which have been sketched above. For this purpose according to the invention, for each rod in an assembly, the radio-activity in at least two discrete rod rows in which said rod lies is measured and a leaking rod is localized by sensing a varying of the radio-activity in the tested rows where said rod lies relative to the radio-activity in an identical row of non-leaking rods. In a preferred embodiment of the invention, the radio-activity of the gaseous fission products accumulated in the rod plenum(s) is measured. Advantageously the .gamma.-radiation generated by said fission products is measured, but according to the invention, the method may also be applied when adding gaseous radio-active tracers, and in such a case the .gamma.-radiation generated by such tracers is measured. Said latter method is generally applied for a single rod (U.K. Pat. No. 1,499,113 and Belgian Pat. No. 640,220). The invention also pertains to an equipment for the working of the above method which comprises at least one radio-activity sensor arranged behind such a collimator that only the rod plenums lie in the viewing area of the sensor. In a particular embodiment according to the invention, the sensor and the assembly are so mounted as to be movable relative to one another along a direction substantially at right angle to the rod axis.