Patent Number: 051820765
Section: description

DESCRIPTION OF PREFERRED EMBODIMENT FIG. 1 shows a cask 2 for the transfer of a nuclear reactor fuel assembly. The cask 1 comprises a cask body 2 of a cylindrical shape incorporating in its central part and along its axis a channel 3 passing through the cask body over its whole length. The cask body consists of a solid piece of steel which is very thick and very long. In the case of a fast neutron reactor fuel assembly whose length is greater than 4 m and which has a hexagonal cross-section with a side of length 125 mm, the cask body 2 has an outer diameter of 1.30 m, a central channel with a diameter of 0.30 m and a length of 6 m. The cask body thus has a wall thickness of 0.50 m; its total weight is about 55 tons. In FIG. 1, the fuel assembly has been represented in its transport position inside the central channel 3 of the cask body. The assembly 4 occupies only a part of the length of the channel 3; the grab 7 of the assembly hoist 6 is placed in the upper part of the channel 3. The lower part of the cask body 2 is connected by a flexible bellows joint system to the support 8 of a motor-driven cask valve 9 enabling the end of the channel 3 to be opened or tightly closed at its lower part which emerges into the end of the body 2. The support 8 of the cask valve 9 is capable of coming to bear, by means of an abutment surface 8a, on a corresponding abutment surface 10a of the support 10 of a well valve 11 making it possible to open or tightly close a traversing well, e.g., a passage passing through a slab covering the reactor vessel containing liquid sodium and enclosing the reactor core. A sealing device comprising O-rings 13 is interposed between the abutment surfaces 8a and 10a in the coupled position of the transfer cask 1 represented in FIG. 1. The systems for closing the valves 9 and 11 are located inside the zone bounded by the O-ring 13a forming the inner part of the sealing device. When the valves 9 and 11 are superposed, after the emplacement of the cask, and are open, the fuel assembly 4 can be made to pass from a position in which it is immersed in the liquid sodium filling the reactor vessel to its transfer position inside the cask 1 by using the hoist 6. A part of the support 8 of the transport cask valve and of the support 10 of the well valve can be seen on a larger scale in FIG. 2. The annular supports 8 and 10 have inner cylindrical surfaces, 14 and 15 respectively, directed towards the inside of the cask and the well, and outer surfaces, 16 and 17 respectively, in contact with the outside atmosphere. Three O-rings 13a, 13b and 13c are disposed in corresponding seatings in the valve support 8, the said seatings having a depth which is less than the nominal diameter of the sealing rings 13a, 13b and 13c. In this way, when the cask valve is not in a position where it bears on the surface 10a of the support 10, a part of the sealing rings 13a, 13b and 13c protrudes with respect to the corresponding seating. The sealing ring 13a, which is smaller than the sealing rings 13b and 13c, is located towards the inside and the sealing ring 13c, which is larger than the sealing rings 13a and 13b, is located towards the outside of the supports 8 and 10. The sealing ring 13b is located in an intermediate position. The sealing rings 13a, 13b and 13c are arranged concentrically in corresponding seatings in the support 8. When the cask has been placed on the fixed abutment structure, as represented in FIG. 2, the sealing rings 13a, 13b and 13c are compressed so as to produce a leakproof seal between the surfaces 8a and 10a. Moreover, the sealing rings 13 keep the surfaces 8a and 10a slightly separated from each other, so that there remains a first enclosed annular space 18 and a second enclosed space 19 bounded by the surfaces 8a and 10a that are slightly separated from each other, the space 18 being situated between the sealing rings 13a and 13b and the space 19 between the sealing rings 13b and 13c. The support 8 is machined internally to form a first channel 20 emerging on the surface 8a between the seatings for the sealing rings 13a and 13b, and a second channel 21 emerging on the surface 8a between the seatings for the sealing rings 13b and 13c. When the cask is positioned in such a way as to allow its valve to come to bear on the abutment surface 10a of the support 10, the channel 20 emerges into the inner space 18 and the channel 21 into the outer space 19. The channel 20 and the inner space 18 are connected by means of piping 22 to a manometer 23. The channel 21 is connected by means of a duct 24 to a vacuum pump 25 and a manometer 25'. During the emplacement of the cask on the abutment structure formed by the support 10, the heavy cask valve produces compression of the sealing rings 13a, 13b and 13c, this compression generally amounting to between 20 and 40% of the diameter of the sealing rings. Such a compression of the sealing rings produces a reduction in the volume of the spaces 18 and 19 located between the sealing rings during the emplacement of the cask. The air contained in the spaces located between the sealing rings is compressed, so that its pressure increases from atmospheric pressure to a pressure depending on the amount of compression of the sealing rings. During the emplacement of the cask on the support 10, the pressure in the inner space 18 is measured continuously by the manometer 23. The pressure rises very rapidly during the emplacement of the cask, to reach a maximum when the sealing rings 13 are in their maximum state of compression. The pressure is then maintained at the maximum value reached or at a value close to it, depending on the degree of sealing produced by the sealing rings 13a and 13b. Prior to the use and the emplacement of the cask, the maximum gas pressure in the space 18 for a correct emplacement of the cask on a perfectly plane abutment surface is determined by calculation and/or calibration. Also determined is the variation of pressure with time over a very short period following the emplacement of the cask, still with the assumption of a correct emplacement of the cask. The maximum pressure and the variations of this pressure over a very short period of the order of 30 seconds is compared with the corresponding value or predetermined variations. In cases where the maximum pressure produced during an effective emplacement of the cask is substantially identical to the predetermined maximum pressure, it can be deduced that the cask valve has been correctly positioned, the measurement and the monitoring being almost instantaneous. A check is made over a period of 30 seconds that the pressure in the space 18 is maintained at a level close to the maximum value or decreases very slightly according to the predetermined law of variation. Confirmation is obtained in this way that the sealing at the connection between the cask and the fixed structure is tight. It is also possible to reduce the pressure in the space 19 by using the vacuum pump 25 after the emplacement of the cask. A means of correcting faults is thus made available in the case of unintended leakage or breakage of the seal during a handling operation. The vacuum pump 25 makes it possible to evacuate any trace of gas capable of entering the space 19 and capable of coming from inside the cask and the vertical well, thus avoiding any external contamination. The method according to the invention therefore makes it possible to verify almost instantaneously, or within a very short period of time, of the order of 30 seconds, the correct emplacement of the cask on an abutment surface. Such monitoring can be carried out by using very simple means which are fixed, for example, on the support of the lower valve of the cask. The method according to the invention therefore makes it possible, in particular, to reduce the time required to carry out the operations involved in replacing fuel assemblies of a fast neutron nuclear reactor cooled by liquid sodium. In addition, the design is such that the cask valve can slide slightly, in a horizontal direction, over the valve of the fixed structure without breaking the sealing. This arrangement is useful in the event of earth tremors. It is possible to use a measuring apparatus of any type to measure the pressure in the space located between the sealing rings during the emplacement of the cask. Such apparatus for measuring pressure can be connected to a recording device or to a means of display or of alarm making it possible to carry out rapid and accurate monitoring of the conditions for the emplacement of the cask on an abutment surface. Finally, the method according to the invention can be used to monitor the emplacement of transportable elements other than a cask for handling fuel assemblies, such transportable elements incorporating at least two sealing rings coming into contact with an abutment surface of a fixed structure during the emplacement.