Patent Number: 050283804
Section: description

DESCRIPTION OF PREFERRED EMBODIMENTS The method for the identification the leakiness of a neutron-capturing pencil of a nuclear reactor, according to the invention, shall be described firstly in relation to FIG. 2. As shown schematically in the above-mentioned figure, the pencil, or control rod, marked Cr is placed in an impervious chamber marked 1. This chamber is filled with an aggressive solution called an analyzing solution. The aggressive solution may be formed by a solution of an acid taken from one of the following groups: nitric acid, sulphuric acid, hydrochloric acid. The concentration of this solution ranges between N/100 and 10.sup.N. As a non-exhaustive example, it may be N/30 where N designates the normality of the solution. The above-mentioned analyzing solution is then put under pressure, marked HP, in order to make this solution penetrate the pencil Cr through the leakiness of this presumably defective pencil. The pressure is then relaxed and the solution is brought to a low pressure level (marked LP) so that the solution, which has penetrated the defective radius, can go out of it and return to the impervious chamber 1. The solution is then analyzed in order to show up, in this solution, metallic salts of metals forming the core of the pencil. For a pencil Cr, with a core formed by an alloy of cadmium Cd, indium In and silver Ag, the metallic salts are silver, indium and cadmium salts. In addition to showing up nitrocompounds, the method according to the invention may advantageously consist in detecting, in the solution contained in the chamber 1 after the pressure is relaxed, a radioactive isotope of the alloy, more especially that of silver, marked Ag 110 m, with reference to a threshold value of concentration of this isotope. Advantageously, the radioactive isotope of silver Ag 110 m is detected by gamma spectrometry. In order to improve the detection of the above-mentioned radioactive isotope of silver Ag 110 m, prior to the step in which the pencil Cr is placed in the impervious chamber 1, filled with acid solution, this solution then forming the starting solution, the method according to the invention may also consist in determining, by spectrometry, the concentration in radioactive isotopes, including Ag 110 m, of the starting solution mentioned above. The concentration of the starting solution in radioactive isotopes, including Ag 110 m, is then chosen as the threshold value of concentration to measure the concentration in radioactive isotopes, notably silver. The chemical concentration of silver in water may also be measured. According to a particularly advantageous aspect of the method according to the invention, the pressure (HP) applied to the analyzing solution in order to make this solution penetrate the pencil Cr is maintained for a period of about 10 minutes at least. This pressure may advantageously be equal to several bars. Furthermore, in order to accelerate the chemical reaction of the solution on the metals forming the core of the pencil Cr, the solution may be subjected, before or after the introduction of the pencil in the impervious chamber 1, to heating. Furthermore, the method according to the invention may include a step consisting in the measurement of the chemical concentration of silver, indium and cadmium in the water before the pencil is placed in the chamber, said step being repeated after the pressurizing and heating operations. A more detailed description of a device enabling the implementing of the method according to the invention shall be given with reference to FIG. 2. According to the above-mentioned figure, the device includes the above-mentioned impervious chamber, marked 1, capable of receiving at least one pencil Cr to be analyzed. Of course, it will be understood that the device according to the invention can be advantageously implemented so that the impervious chamber 1 is capable of taking not one pencil Cr, but an entire cluster in order to make checks on it. The impervious chamber 1 is provided with a circuit 2 for the supply of analyzing solution. The circuit for the supply of analyzing solution has a high pressure conduit provided with a valve V1 and a low pressure conduit provided with a valve V2. It will be understood, of course, that the actuation of the valves V1 and V2, with the HP conduit being connected, for example, to a compressor system, makes it possible, firstly, to pressure the analyzing solution in the impervious chamber 1 and to keep it under pressure by opening the valve V1, the valve V2 being closed or, on the contrary, by the closing of the valve V1 and the opening of the valve V2, the valve V2 being installed on the low pressure conduit, to bring the pressure down to a sufficiently low value, to enable the solution that has penetrated the pencil Cr, after chemical attack on the constituent elements of its core, to come out in the solution contained in the impervious chamber 1. The high pressure circuit HP may be connected, for example, to a compressor delivering pressure ranging from 1 to 15 bars. As a non-restrictive example, this pressure may be in the region of 12 bars. The low pressure circuit LP may, on the contrary, be connected to a circuit used to set up, in the impervious chamber 1, pressure close to that of the environment of this chamber or equal to a few bars. As will be further seen in FIG. 2, the device according to the invention also has an analysis circuit 3 connected to the impervious chamber 1. This analysis circuit 3 has at least one circulation pump 32 for the analyzing solution and a counting vessel 30 for the counting of radioactive particles, including the silver isotope Ag 110 m. Means 31 for the counting of the radioactive particles are also planned, these means being associated with the counting vessel 30. Advantageously, the counting vessel 30 and the counting means 31 may be formed by a system, normally available in the market, for the counting of gamma rays. Of course, the counting vessel 30 and the counting means 31 are advantageously complemented by a display system 33 enabling the display of the result of the above-mentioned counting measurements. Furthermore, the analysis circuit 3 also has a valve V3 for taking samples of the analyzing solution. With reference to FIG. 2, the sequence of operations enabling the application of the method is as follows: According to a particularly advantageous mode, and prior to the pressurizing of the impervious chamber 1 and before the introduction of the pencils Cr to be tested in the impervious chamber 1, a sample of the analyzing solution is taken by means of the valve V3 in order to measure the residual content of the solution in metallic salts, notably, silver salts. In the same way, a count is done, using the means 31 for the counting of radioactive particles in this starting solution, in order to determine the threshold value corresponding to the presence of the radioactive isotopes, including the above-mentioned Ag 110 m. After the pencil Cr to be tested is introduced into the impervious chamber 1, the opening of the valve V1 enables the pressurizing of the chamber, and this pressurizing is done at a value ranging from 1 to 15 bars. This pressure is maintained for at least ten minutes, then the valve V1 is again closed and the valve V2 is opened, thus enabling a depressurization to be done. The display means 33 can be used to follow the development of the on-line count signal delivered by the counting means 31. A sampling of the analyzing solution through the valve V3 then makes it possible to make an evaluation of the material in the fluid and a quantitative analysis of the silver, indium or cadmium ions or their radioactive isotopes therein. The comparison of the values measured, in nitrocompounds, of the constituent elements of the core of the pencil with their value in the starting solution, taken as a threshold value, then makes it possible to identify the pencil Cr or the leaky cluster. As is further shown in FIG. 2, the analysis circuit 3 also has, connected as a bypass to this analysis circuit, a circuit 4 to heat the solution. Advantageously, the solution heating circuit 4 may be formed by any circuit for the electrical heating of a thermostat controlled chamber used to carry the analyzing solution, put into circulation by the circulation pump 32, to a temperature ranging from 30.degree. to 90.degree. C. As a non-restrictive example, this temperature may be 75.degree. C. FIG. 3 shows the diagram of a complete installation of a device according to the invention. Of course, the pencil or pencils and the corresponding cluster have a non-negligible degree of radioactivity, and the manipulation of the latter can be contemplated only in immersion in the water of the pool of the corresponding nuclear reactor. To this end, and although the tools used to manipulate the pencil or pencils and the cluster have not been shown in FIG. 3, the device according to the invention, and especially the impervious chamber 1, is immersed in the pool P of the nuclear reactor. Of course, the set of circuits formed by the high pressure circuit HP, the low pressure circuit LP, the impervious chamber 1 and the analysis circuit 3 is impervious to the water of the pool. FIG. 3 also shows the valve V3 in a chamber shielded against ionizing radiation, it being possible to perform the analysis of chemical derivatives of the constituent elements of the core of each pencil in the above-mentioned chamber. We have thus described a method and device enabling the detection or identification the leakiness of one or more neutron-capturing pencils of a nuclear reactor. The method and device according to the invention are particularly advantageous inasmuch as they enable systematic checking of parts essential to the working of a pressurized-water nuclear reactor, namely essential parts such as neutron-capturing pencils and the corresponding clusters for the checking of the working of the reactor.