Patent Number: 053944488
Section: description

DETAILED DESCRIPTION FIG. 1 shows the entire installation 1 making it possible to carry out the separate grinding and disposal of the constituents of fuel element jackets of a graphite/gas nuclear reactor. The installation 1 comprises a feed hopper 2 whose output end is placed above an inclined conveyor 3. Fuel element jackets 4 made of graphite containing stainless steel wires are discharged into the hopper 2 which feeds them to the input end of the conveyor 3, the output end of which communicates with a chute for feeding an impeller-disk mill 6. The fuel elements 4 enter the chute 5 which ensures regular feeding of the impeller-disk mill 6 which includes two rollers 7 turning in opposite directions and fitted with grinding cutters. The grinding mill 6 separates the jackets 4 into fragments which consist either of pieces of graphite with a size of a few tens of millimeters, or shredded fragments of the stainless steel wires. At the output of the grinding mill 6, a reception chute 8 receives a heterogeneous mixture of fragments consisting either of pieces of ground graphite or of segments of stainless steel wires. The chute 8 removes the mixture of heterogeneous fragments onto a conveyer 10, of the same type as 3, whose output is situated above the input opening of a two-stage magnetic separator 12. The separator 12 comprises an upper stage 12a and a lower stage 12b, which are equipped with magnetic separation means consisting of assemblies with belts and magnetic rollers, 13a and 13b respectively, of high intensity, of the type comprising rare-earth permanent magnets and which will be referred to below as magnetic rollers 13a and 13b. Each of the separator stages 12a and 12b comprises two outputs, respectively 15a and 16a and 15b and 16b. The outputs 15a and 15b, termed the first outputs, are intended mainly to receive the first constituent which is the magnetic constituent, i.e., in the case of fuel element jackets, the stainless steel wire. The second outputs 16a and 16b are intended mainly to receive the second non-magnetic constituent, i.e., in the case of jackets for fuel elements. Each of the outputs 15a, 16a, 15b, 16b can be equipped with remotely controlled closure means. When the heterogeneous mixture discharged by the conveyer 10 into the input opening of the separator 12 arrives in contact with the magnetic roller 13a, the first magnetic constituent, i.e., the stainless steel wires are retained and carried by the roller 13a which discharges them into the hopper of the output 15a of the separation stage 12a. The fragments consisting mainly of graphite are discharged, without being diverted by the roller 13a, into the second output 16a. The stainless steel wires discharged into the hopper of the output 15a pass into the stage 12b of the separator to be discharged directly into the first output 15b of the stage 12b. The mixture consisting mainly of graphite which is discharged onto the second magnetic roller 13b undergoes a separation, the stainless steel wires which may remain mixed with the pieces of graphite being retained and transported by the magnetic roller 13b to be discharged into the first output 15b of the stage 12b. The fragments reaching the first output 15b of the stage 12b, which mainly consist of stainless steel, are discharged into a hopper 18 comprising a closure hatch 19. Below the hatch 19 is arranged a chute or a storage container 20, as will be explained below with reference to FIGS. 2A, 2B and 2C. It is unimportant that a few fragments of graphite are still present mixed with the segments of stainless steel wires reaching the first output 15b of the separator, since these graphite fragments have much lower activity than that of the stainless steel. The conditions of packaging or disposing of the stainless steel wire may make it possible to process a few residual fragments of graphite. On the other hand, the object of the separation is to obtain, at the second output 16b of the second stage 12b of the separator, only pieces of graphite which constitute the majority of the bulk of the elements processed with a view to their disposal and which have low or moderate activity. The second output 16b of the separator stage 12b is arranged above the input end of a continuous handling means 21 which may consist for example of a belt conveyor. The belt conveyor 21 has drive means which make it possible to run it in either direction, as schematically represented by the double arrows 22. The input end of the conveyor 21 is situated above the hopper 18 for receiving the stainless steel wire. The output end of the conveyor 21 is placed inside a hopper 24 comprising a closure hatch 25 at its lower part, making it possible to isolate the hopper 24 or make it communicate with an output chute or a packaging container 26 as will be explained with reference to FIGS. 2A, 2B and 2C. A radioactivity detector 27, such as a scintillation detector, is arranged above the belt of the conveyor 21. The detector 27 is adjusted to a sensitivity level such that it makes it possible to detect any segment of stainless steel wire inside the graphite fragments supported by the conveyor 21 in the first direction of motion of the conveyor going from the input end, below the output 16b of the separator, to its output end, inside the hopper 24. The detector 27 is connected to a unit 28 for controlling the motorization of the continuous conveyor 21 and of the input carrier 3. In the event that the detector 27 detects the presence of at least one stainless steel wire in the graphite fragments transported by the conveyor 21, the control unit 28 stops the conveyor 21, closes the output 16b of the separator, then operates the continuous conveyor 21 in its second direction of motion, going from the hopper 24 to the hopper 18. The fragments of graphite containing one or more segments of stainless steel wire are then discarded into the hopper 18 intended to receive the segments of stainless steel wires. When the detector 27 no longer detects a high level of radioactivity corresponding to the presence of stainless steel wires on the continuous handling means 21, the latter is operated in its first direction of motion, going from the output 16b of the separator to the hopper 24, the element for blocking the output 16b of the separator being placed in the open position. In the event that the detector 27 detects a very high level of radioactivity corresponding to a large proportion of stainless steel wires in the graphite particles transported by the conveyor 21, the control unit 28 stops the conveyor 3 feeding the grinding mill. In effect, in the event that the proportion of stainless steel wires is high, it can be concluded that the separator 12 is malfunctioning and it is then preferable to shut down the entire grinding and separation installation. The method and the device according to the invention, as described above, therefore make it possible to carry out efficient and very reliable separation of a first constituent and a second constituent having different magnetic properties and activation levels, in activated elements. The operation of the device according to the invention is completely automatic and does not require the presence of operators near the installation. Furthermore, the discharge devices for the materials at the output of the installation, which will be described with reference to FIGS. 2A, 2B and 2C, make it possible to discharge, package and dispose of the fragments separated by the separation, without contaminating the environment and under conditions which ensure effective biological protection. These means are particularly useful as regards removing materials containing mainly stainless steel wire through the first output of the installation comprising the hopper 18, the leaktight closure hatch 19 and the chute or the container 20. The description of the emptying device represented in FIGS. 2A, 2B and 2C will be given with reference to the elements 18, 19 and 20 of the first output of the installation 1 represented in FIG. 1. However, it is clear that the second output comprising the hopper 24, the hatch 25 and the chute or container 26 can also be produced in the manner described in FIGS. 2A, 2B and 2C. As can be seen in FIGS. 2A, 2B and 2C, the output hopper 18 of the installation comprises at its lower part a hatch 19 comprising a hatch body 30 and a sliding closure element 31. The lower part of the hatch body 30 has an opening 30a, at which a container 20 can be placed, this container being closed by a leaktight closure element 32, as represented in FIG. 2A. The container 20 intended to collect the highly radioactive fragments contained in the hopper 18 and consisting mainly of segments of stainless steel wires is a high-integrity container having a large wall thickness. The container 20 can be placed in the position represented in FIG. 2A using a carriage (now shown) which moves it over the floor 33, as schematically represented by the arrow 36. As can be seen in FIG. 2B, a lifting means combined with the carriage for supporting the container 20 makes it possible to lift the container 20, so as to insert its upper part and the leaktight closure element 32 into the opening 30a of the hatch body 30, as represented by the arrow 34. At the end of the upwards movement of the container 20, the elements 31 and 32 are in contact with each other. As represented in FIG. 2C, and represented by the arrow 35, it is then possible to move the two closure elements 31 and 32 simultaneously, so as to place the hopper 18 into communication with the container 20. The radioactive fragments contained in the hopper 18 are then discharged into the container 20 which is filled. The closure elements 31 and 32 are then moved in the direction opposite to the arrow 35 in order to return them into the position of closing the hopper 18 and the container 20. The container 20 can then be lowered into the transport position, for removal to a storage site or a processing plant. It is clear that the removal of the particles of graphite received in the hopper 24 can be carried out by using a container, as in the case of the fragments of stainless steel wires. However, because of the low radioactivity of the graphite particles, it is also possible to discharge the graphite particles via a chute into a continuous transport means feeding a graphite disposal unit, such as an incineration unit. The entire installation 1 represented in FIG. 1 can be placed in a containment unit held under reduced pressure using a conventional ventilation system, which need not be described. In order to operate continuously and entirely automatically, the installation may comprise various detection and control means. In particular, the grinding mill 6 may comprise level detection means. These detection means may be arranged at the grinding mill itself, in order to monitor the feeding of the grinding mill by the carrier 3, or in the outward chute, in order to make it possible to regulate the feed rate of the magnetic separator. It is possible to use a grinding mill other than an impeller-disk mill and a magnetic separator of a type other than a high-intensity roller separator with permanent magnets based on rare earths. It is possible to use any type of radioactivity detector for detecting stainless steel wires, or more generally a constituent with strong radioactivity on the continuous handling means. Finally, the method and the device of the invention can be used for processing activated elements other than jackets of fuel elements of a reactor of the graphite/gas type. The method and the device according to the invention can be used for processing, before disposal, any radioactive element comprising at least two constituents having substantially different magnetic characteristics and activity levels.