Patent Number: 052356248
Section: summary

FIELD OF THE INVENTION The invention relates to a method and to a device for detecting leaks from a fuel element of an assembly for a nuclear reactor. BACKGROUND OF THE INVENTION Water-cooled nuclear reactors, such as pressurized water nuclear reactors, comprise fuel assemblies which are placed in a juxtaposed disposition inside the reactor vessel, in order to form the core which is the center of nuclear reactions producing the energy which is transmitted to the coolant of the reactor in the form of heat. The fuel assemblies are formed by bundles of lengthy fuel elements or rods of small diameter. The fuel rods themselves are formed by a metal cladding in which pellets of fuel material such as uranium oxide are stacked, and which is closed in a sealed manner at its ends by plugs. The cladding of the fuel elements is constructed from an alloy resistant to high temperature corrosion by the water cooling the reactor and of low neutron absorption. The cladding of the fuel element makes it possible to isolate the pellets made of fuel material from the cooling water and to prevent fission products formed by nuclear reaction in the fuel material from being entrained by the water cooling the reactor. After being present in the operating reactor for some period of time, certain assemblies of the core may exhibit sealing faults which are manifested by a release of radioactive fission products into the coolant. The level of radioactivity of the cooling water makes it possible to determine the presence of assemblies exhibiting leaks in the reactor core. These assemblies must be detected in order that their repair or their replacement can be effected, during a shutdown phase for maintenance of the reactor, so as to prevent contamination of the primary cooling system in which the cooling water flows. The detection of the defective fuel assemblies may be effected under water in the pool, by acoustic, ultrasonic or eddy current methods of measurement. It is also possible to use cells called sipping test containers in which the release of radioactive products by the fuel assembly is promoted, these radioactive products being subsequently carried away by the water or a gas into a unit enabling them to be detected. In the case where such radioactive products have been detected, it is necessary to effect the replacement or the repair of the fuel assembly before reloading it into the reactor core. Leaks of radioactive product originating from an assembly arise from a sealing fault in at least one rod of the assembly whose cladding is cracked. Certain methods for monitoring the fuel assemblies make it possible to locate the defective rod(s) but these methods are generally complex and do not make it possible to determine completely reliably the location of the defective rods. Refurbishing defective assemblies, effected by replacing the rods which are likely to leak by new rods, is therefore not always effected under satisfactory conditions. Furthermore, in the case of studies carried out in order to research the causes of the presence of faults in fuel elements, until now use has been made of eddy current test methods or macroscopic visual test methods carried out individually on each of the rods of the fuel assemblies subjected to the tests. Until now no method and device making it possible to determine extremely reliably the presence of leaks from a fuel element of an assembly for a nuclear reactor, such as a fuel rod, has been known. SUMMARY OF THE INVENTION The object of the invention therefore is to propose a method for detecting leaks from a fuel element of an assembly for a nuclear reactor which is extremely reliable, relatively simple and rapid to implement, and which may be carried out on individual fuel elements of variable length. For this purpose: the fuel element is inserted into a cell, under water, in a pool, PA1 compressed air is injected into the cell in such a manner as to drain it of the water which it contains, PA1 a pressure lower than atmospheric pressure is established in the cell, in such a manner as to promote the escape of fission products originating from the fuel element in the event of a leak, PA1 scavenging of the gases contained in the cell is performed by a stream of scavenging air, and these gases are collected in a fission product detecting unit, PA1 water is introduced into the cell, PA1 a sampling is carried out of water in the cell, PA1 a detection of fission products in the water sample is carried out, and PA1 the fuel element is extracted from the cell, after the possible presence of a leak from this fuel element has been determined by detection of fission products. The invention also relates to a device enabling the method according to the invention to be employed.