Patent Number: 042253888
Section: summary

The invention relates to a crack detection cell for checking the leakproofness of fuel element units in the deactivation swimming-pool of a nuclear reactor, and the corresponding process of checking. When the core of a nuclear reactor is recharged with fuel element units which have been in the reactor for a certain period, it is necessary to check the leakproofness of these units in order to avoid introducing into the reactor units suffering from leakages due to cracks in the sheathing material of the fuel elements. Equally, if used units are transferred to the retreatment factory, it is necessary to know the level of .gamma.-activity released by these units so as to take the necessary precautions to avoid contamination. It is thus necessary to carry out a rapid detection of crack faults, on irradiated fuel units, on site at the time of discharging, and at the time of charging, the core of the reactor. One of the processes most used to carry out this rapid detection is a method of crack detection which consists of measuring the activity of fluids which have been in contact with the fuel unit during a rise in temperature of the fuel unit. During a rise in temperature of the fuel unit, the fission gases contained in the fuel elements expand and their speed of release through cracks in the sheaths increases so that volatile or water-soluble fission products increasingly contaminate the medium surrounding the unit. In this process, crack detection cells which are generally located in the deactivation swimming pool of the reactor are used. The fuel unit is located in a pocket, containing water, inside the crack detection cell. When the temperature of the fuel unit rises, the fission products are released into the water in which the fuel unit is immersed. Taking samples and measuring the .gamma.-activity of this water thus makes it possible to check the leakproofness of the fuel units. In order to achieve a rise in temperature of the fuel unit, a heat barrier is generally created between the pocket in which the fuel unit is located, and the water of the deactivation swimming pool, which is then no longer able to absorb the heat evolved by the fuel unit. To produce this heat barrier, the pocket intended to receive the fuel unit is generally located inside a double-walled chamber which rests vertically on the bottom of the deactivation swimming pool, and which has, in its external wall, orifices by which the gap between the walls of the chamber is connected with the swimming pool. This gap between the two walls of the chamber is connected to a feed line for inert gas under pressure from which gas under pressure passes into the gap and forces out the water of the deactivation swimming pool, to create the heat barrier. This system is particularly suitable in the case of fuel units which have to be checked shortly after discharge from the reactor core, the latter having functioned for a normal period of time, that is to say it is suitable for units which have a high residual power. In contrast, in the case of units of low residual power, that is to say fuel units which are checked long after discharge from the reactor, or following premature stoppage of the reactor, the rise in temperature of the unit becomes very slow so that the utilisation times increase considerably, which can make the system completely inapplicable. The above described process is in effect based on a comparison of the measurements of the .gamma.-activity in the water surrounding the fuel unit before and after the rise in temperature of the unit. The difference in activity is thus a function of the extent of the temperature rise of the unit. In order to treat a larger number of fuel units within a given time, it has been proposed to use a large number of crack detection cells simultaneously, but it is then necessary to provide the corresponding space in the deactivation swimming pool. It has also been proposed to empty the water from the pocket so as to allow a rapid rise in temperature of the fuel unit. However, this technique is difficult to master and suffers from certain dangers. On the other hand, in all the known processes the activity is measured on samples of fluids treated manually, which further increases the duration of use. According to one aspect of the invention there is provided a crack detection cell for checking the leakproofness of a fuel element unit in the deactivation swimming pool of a nuclear reactor, by measuring the .gamma.-activity of fluids in contact with the fuel unit, said cell comprising: a double-walled chamber for resting vertically on the bottom of the deactivation swimming pool, the external wall of said chamber defining orifices which connect the gap between said walls of said chamber with the swimming pool; means for connecting said gap to a source of gas under pressure for creating a heat barrier between the interior of said chamber and the deactivation swimming pool; a leakproof pocket for receiving a fuel unit in a vertical position, said pocket being located inside said chamber and comprising an upper part for serving as a zone for introducing said unit and a leakproof cover for closing said pocket; means for introducing water into, and for removing water from, said pocket; and heating means located at the lower part of said pocket and forming part of the internal surface of said pocket and for heating water contained in said pocket by direct contact to cause a rise in temperature of a fuel element when in said pocket. Preferably, the crack detection cell according to the invention includes means for circulating an inert gas in the pocket, and means, included in said circulation means, for continuously measuring the .gamma.-activity of the gas which has passed through the water contained in said pocket, in which the fuel element is immersed. According to another aspect of the invention there is provided a process for checking the leakproofness of a fuel element unit, using a crack detection cell as described above, comprising: causing circulation of inert gas in said pocket containing water in which said fuel unit is immersed, so as to entrain any fission products released by said fuel unit; and continuously measuring the activity of said circulating gas; thermally isolating said pocket containing said unit and energising said heating plates surrounding said pocket for a part of the time for which said gas is circulated; and checking the leakproofness of said unit by comparing the measurements of the activity of said circulating gas when said unit is not thermally isolated and heated and when said unit is both thermally isolated and heated.