Abstract:
A continuous decanter assembly suitable for processing nuclear materials comprises a mechanical assembly for driving a decanter bowl and an Archimedean screw. Sealing structure is provided to enable the decanter bowl and screw to be disconnected from the mechanical assembly and accessed by an operator without risk of nuclear contamination.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a centrifugal decanter with continuous evacuation of the solid, for the processing of nuclear products. 
     Machines of this type are known, which generally comprise a mechanical zone separated from the active zone via a bearing structure in which a removable slab for biological protection is provided. The machine is generally a pendulous decanter operating by cycles, which comprises a vessel fixed to the bearing structure and inside which a decanter bowl is driven in rotation. The machine conventionally comprises a charging tank and a pipe which projects a jet of liquid under pressure for extraction of the decanted solid. 
     The vessel is provided with an evacuation of the clarified liquid, a retractable supply of the suspension to be decanted, and with a drain for the humid solid after decantation. 
     Operation of the machine by cycles requires that the flow be periodically stopped, this involving storage of the substance to be decanted. The decanted solid is eliminated by means of a jet of liquid under pressure which transforms it into a slurry which risks blocking the evacuation conduit. Moreover, due to its evacuation in humid form, the decanted solid comprises a quantity of liquid which reduces the quality of decantation of the substance. 
     The bowl of the machine is of large diameter in order to be able to place the different elements such as the charging tank and the pipe. The large dimension of the bowl requires a low speed of rotation due to the mechanical stresses, which implies a poor decantation of the substances. Finally, the larger the diameter of the bowl, the greater is the volume of substances to be treated, which is contrary to the processing of nuclear products. 
     It is a more particular object of the present invention to overcome these drawbacks. 
     The continuous decanter according to the invention has for an object to reduce the threshold of criticality for the same mass of processed product modifying its geometrical form while improving its quality. 
     SUMMARY OF THE INVENTION 
     To that end, the continuous decanter comprises in the mechanical zone an assembly which allows drive in rotation of a decanter bowl and an Archimedean screw placed in the active zone, and means making it possible to disconnect the mechanical assembly of the bowl and the screw in order to be able to intervene thereon. 
     In addition, the continuous decanter comprises no bearing or roller bearing for axially guiding the decanter bowl and the Archimedean screw in the active zone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more readily understood on reading the following description with reference to the accompanying drawings, in which: 
     FIG. 1 is a longitudinal section illustrating the continuous decanter according to the present invention. 
     FIG. 2 is an enlarged section showing the device for coupling the bowl and the Archimedean screw with the mechanical assembly. 
     FIGS. 3 and 4 are views illustrating the different steps to be taken in order to withdraw the mechanical assembly totally or partially. 
     FIGS. 5 and 6 are sections representing the different steps, after total withdrawal of the mechanical assembly, of the extraction of the bowl and Archimedean screw from the active zone in a tight container or &#34;castle&#34;. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring now to the drawings, FIGS. 1 and 2 show a centrifugal decanter 1 with continuous evacuation of the processed solid. Decanter 1 comprises a mechanical zone M which is separated from an active zone A via a bearing structure 2 in which is provided a removable bushing 2a for biological protection. Bushing 2a defines a first bore 2b which communicates coaxially with a second bore 2c of smaller diameter traversing the rest of the bushing to form a passage between the two zones A and M. 
     The first bore 2b is separated from the second 2c by two successive shoulders 2d, 2e, for reasons which will be more apparent hereinafter. 
     In zone M is placed a metallic frame 1a which abuts on the bearing structure 2. Frame 1a supports in known manner a motor 1b which drives a reduction gear 1c in rotation via one or more belts. 
     On frame 1a and beneath the reduction gear 1c is welded along a vertical axis a tube 1d in which is provided a bearing 1e. Inside the bearing 1e are disposed ball or roller bearings for axially guiding a hollow sleeve 1f of which one of the ends cooperates by means of an intermediate piece 1i with the reduction gear 1c to drive it in rotation. Sleeve 1f presents a flange 1g which is placed coaxially inside the bore 2b of the biological slab 2a. Inside the bore 2m, the flange 1g is fixed by means of screws on an intermediate disc 3 which is fastened to a pin 4a traversing bore 2c. Pin 4a is secured to a decanter bowl 4 provided in the active zone A to drive it in rotation. Inside bowl 4 is disposed in known manner an Archimedean screw 5 fixed to a shaft 5a which traverses pin 4a of the bowl and which is guided axially inside sleeve 1f to cooperate with the reduction gear 1c. 
     The lower face of the intermediate disc 3 is provided just above the shoulder 2d of the bore 2b determining therewith an operational clearance X. Moreover, the shoulder 2d comprises an O-ring 2f whose outer diameter is slightly smaller than that of the intermediate disc 3. 
     In the active zone A and around the bowl 4 there is provided a vessel 6 which is fixed on the lower face of the bearing structure 2 so that it surrounds the removable bushing 2a. Vessel 6 comprises in its upper part an evacuation outlet 6a of the clarified liquid. In addition, in the lower part of vessel 6 is provided a fixed supply inlet 6b for introduction of the substance to be decanted. Vessel 6 presents beneath the supply inlet 6b an opening 6c in chute form for extraction of the decanted solid. 
     A seal 1h with double bearing is placed, at one end, on the periphery of the bore 2b and, on the other end, on the outer face of the bearing 1e, to constitute a tight separation between the active zone A and the mechanical zone M during operation of decanter 1. 
     It will be noted that bowl 4 and screw 5 are not supported axially in their lower, conical parts by bearings or roller bearings in order to facilitate, on the one hand, withdrawal thereof and, on the other hand, intervention on the mechanical assembly if a breakdown occurs, as will be seen hereinbelow. 
     The continuous decanter 1 described hereinabove operates in accordance with the same principle as all other continuous decanters. 
     Motor 1b, via reduction gear 1c, drives the decanter bowl 4 and the Archimedean screw 5 at different speeds. 
     The radioactive substance to be processed in the decanter bowl 4 is supplied via the fixed tube 6b so that the substance arrives in the middle of the thread of screw 5. The difference in speeds between bowl 4 and screw 5 makes it possible to direct the decanted solid in the lower or conical part of the decanter to be extracted via orifice 6c of vessel 6. On the contrary, the clarified liquid takes the reverse path, to be evacuated by overflow by means of tube 6a. 
     An essential particularity of the continuous decanter 1 described above lies in the fact that, if a breakdown occurs in the mechanical assembly located in zone M, an operator can intervene without risk of being contaminated. 
     In fact, the whole mechanical assembly, i.e. for example frame 1a, motor 1b up to reduction gear 1c, and even bearing 1e and sleeve 1f, may be withdrawn without risk of transferring radioactive particles between the two zones. 
     FIG. 3 shows the first step that the operator must take before any intervention in order to render zones A and M totally tight with respect to each other. To that end, the operator raises the static seal 1h so as to slide it on the outer wall of bearing 1e. The operator then removes the screws which retain flange 1g of sleeve 1f on the intermediate disc 3 fixed to pin 4a of bowl 4. When disconnection is effected, it is ascertained that the lower face of disc 3 comes into abutment on shoulder 2d of bore 2b in order to crush seal 2f to constitute a perfect seal between the two zones. It is ascertained that clearance X provided between the lower face of disc 3 and shoulder 2d is zero, being given that bowl 4 has descended vertically inside vessel 6 without risking abutting on the periphery of the upper tube 6a. Finally, the operator dismantles reduction gear 1c in order to release shaft 5a of screw 5. It is ascertained that, when shaft 5a is axially free, screw 5 descends vertically to abut via its thread on the conical part of bowl 4. 
     When these different steps are effected, the operator may then freely dismantle the whole mechanical assembly such as bearing 1e and sleeve 1f with a view to replacing the different roller bearings if necessary, as shown in FIG. 3. Similarly, the operator may change reduction gear 1c and motor 1b if desired. 
     The operator effects the reverse procedure to remount the mechanical assembly of the continuous decanter 1, tightness between zone M and zone A remaining perfect as disc 3 is in abutment against shoulder 2d. 
     FIGS. 5 and 6 show the continuous decanter 1 without its frame 1a, i.e. when the operator has proceeded with total withdrawal thereof, as described above. Withdrawal of frame 1a enables the operator to intervene on the decanter bowl 4 and the Archimedean screw 5 if they are defective. Such intervention is effected in known manner in a tight container or &#34;castle&#34;, the latter being shown in dashed and dotted lines. 
     Lifting means (not shown) are fixed on the removable bushing 2a to lift the assembly composed of the bowl and the screw inside the tight &#34;castle&#34;. It will be noted that withdrawal is possible, being given that the intermediate disc 3 is in abutment on shoulder 2d of bore 2b and that the screw is against the conical part of the bowl. 
     It will be noted that vessel 6 remains fast with bushing 2 as well as supply 6b. Withdrawal of bowl 4 and of screw 5 is effected without human intervention in active zone A. Before withdrawing the &#34;castle&#34; shown in dashed and dotted lines, a tight slide closes the opening of the bearing structure 2 in which bushing 2a is placed so that no communication is possible between the two zones. Positioning of another bowl provided with another screw is effected under a tight &#34;castle&#34;. The operator then proceeds with mounting the mechanical assembly described hereinabove until the bowl and screw are connected to the reduction gear 1c. 
     The continuous decanter 1 makes it possible to extract the solid dry, which considerably improves the quality of decantation of the substance. In addition, continuous operation increases yield of the machine and avoids storing substance to be decanted. 
     It will be noted that reduction of the diameter of screw 5 with respect to the conventional machines for an equivalent flowrate makes it possible to increase considerably the speed of rotation thereof. This high speed of rotation of the continuous centrifugal decanter 1 according to the invention and the extraction of the decanted solid dry, considerably improves the quality of decantation of the substance. 
     In addition, the elongated form of screw 5 modifies the geometry of the product processed which, for the same mass of product, enables more radio-active substances to be processed without ever attaining the threshold of criticality. 
     It must, moreover, be understood that the foregoing description has been given only by way of example and that it in no way limits the domain of the invention which would not be exceeded by replacing the details of execution described by any other equivalents.