Patent Number: 050283804
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention pertains to a method and device for the identification of leakiness flaws in a neutron capturing (or absorbing) pencil (or rod) of a nuclear reactor. 2. Description of the Prior Art In pressurized-water nuclear reactors used in nuclear power plants for the production of electrical power, the core of the nuclear reactor is formed by an assembly of enriched uranium rods between which neutron-capturing pencils are inserted in order to control the activity of the reactor. As a rule, as shown in FIG. 1a, the pencils are formed by hollow tubes made of stainless steel, the inside of each tube being filled with a metallic alloy formed by 80% silver, 50% indium and 5% cadmium. The tubes, in the case of a 900 MW power reactor for example, have a length of the order of 3.60 m. and are assembled in a cluster by means of a spider-support formed by a metallic part enabling the neutron-capturing pencils to be fastened by one of their ends. Thus, as shown schematically in FIG. 1b, in order to check the activity of the nuclear reactor, the neutron-capturing pencils, set up in a cluster, are introduced into the passage tubes made in the core of the reactor. The degree of penetration of the neutron-capturing pencils determines the instantaneous power of the nuclear reactor. The total penetration of the pencils corresponds to the shutdown of the reactor. During the working of the reactor, depending on the electrical power called up by the electrical power distributing network, the neutron-capturing pencils forming the cluster are, on the contrary, moved out of the passage tubes along about three-quarters of their length. The core of the reactor and the neutron-capturing pencils are then subjected to a very great flow of pressurized water, of the order of 3000 m3/s, in order to heat this water in contact with the core of the reactor. Owing to the flow of fluid to which they are subjected, the neutron-absorbing pencils should be held by guiding plates on their unoccupied part which is not engaged in the guiding tubes. The many swirls of pressurized water subject the neutron-capturing pencils, notably at the guiding plates and guiding tubes, to repeated shocks which, in the long run, cause a phenomenon of local wearing out of the corresponding pencils with the risk of loss of imperviousness of the steel tube forming each neutron-capturing pencil. The silver contained in the tubes forming the neutron-capturing pencils may become radioactive by neutron capture, notably at the lower part of these neutron-capturing pencils where the flow of neutrons is never negligible, and the risks of loss of imperviousness of the steel tubes forming each neutron-capturing pencil entails major risks of contamination of the environment and of people during the handling of the clusters outside the core of the reactor. Furthermore, the risk of total breakage of the neutron-capturing pencils at their lower part is not negligible, and there is then the risk that these lower parts will sink to the bottom of the guidance tube. The effect of this, owing to the risk of jamming, is to create an additional risk of preventing the shutdown of the nuclear reactor. Systematic studies have shown that a rate of total breakage of 50% of at least of a neutron-capturing pencil is reached after a lifetime of four to five years. SUMMARY OF THE INVENTION An object of the method and device for the identification of the leakiness of a neutron-capturing pencil according to the invention is enabling the identification of the untimely leakiness of the neutron-capturing pencils of a nuclear reactor. A noteworthy feature of the method used to identify the leakiness of a neutron-capturing pencil of a nuclear reactor according to the invention is that the pencil is placed in an impervious chamber filled with an aggressive solution called an analyzing solution. The analyzing solution is put under pressure in order to make it penetrate the pencil through the leakiness of the presumably defective pencil, the pressure being then relaxed in order to enable the solution to come out of the presumably defective pencil. An analysis of the solution is then done in order to make it possible to show up, in the solution, chemical derivatives of the constituent elements of the core of the pencil. The device for identifying the leakiness of a neutron-capturing pencil of a nuclear reactor according to the invention is noteworthy in that it comprises an impervious chamber capable of receiving at least one neutron-capturing pencil to be analyzed. The impervious chamber is provided with a circuit for the supply of analyzing solutions and an analysis circuit connected to the impervious chamber, comprising an analyzing solution circulating pump, a radioactive particle counting vessel, radioactive particle counting means and a valve for taking samples of the analyzing solution. The method and the device according to the invention can be applied to making checks on the imperviousness of neutron-capturing pencils of nuclear reactors pressurized-water nuclear power plants.