GB2155366A - Pendular centrifuge for decanting - Google Patents

Abstract

The invention relates to a pendular centrifuge for decanting a liquid containing solid particles. The centrifuge has a cylindrical top part 8a and a bottom part 8b having an inclined side internal wall 20. The two parts are connected by a conical surface 22. The inclination of the bottom part of the vessel and the conicity of the connecting surface are such that when the vessel rotates at a certain speed, the liquid and particles initially in the bottom part rise into the annular "clarification" space 28. The invention has particular application in the reprocessing of nuclear fuel. <IMAGE>

Description

SPECIFICATION Pendular centrifuge for decanting The invention relates to a pendular decantation centrifuge.

More specifically, the invention relates to a machine for decanting small particles in suspension in a liquid by rotating the liquid in a centrifuge vessel, the vessel being secured to the bottom end of a driving shaft.

It is known that when nuclear reactor fuel assemblies are reprocessing, a number of operations are required for reprocessing, i.e. recovering any actinides remaining in the spent fuel assemblies from the reactor core. Firstly, the fuel assemblies are broken into pieces, e.g. by cutting them into short portions. Each short portion, therefore, contains part of the irradiated fissile material, solid fission products and part of the can (usually of zircalloy) protecting the fuel assembly. Secondly, the portions are dissolved. However, the sheaths and some particles do not dissolve but remain in suspension, e.g. in the case of small portions of the can which are converted into "filings" and can vary in size from a few microns to a few millimetres. There are also insoluble solid fission products which can vary in size from less than a micron to several microns.The second kind of particles remaining in suspension is, of course, highly radioactive.

It is known that after dissolution, the remaining fissile and other products are extracted from the solution by chemical methods. This constitutes the processing. However, the presence of the two kinds of insoluble products makes the extraction operation very difficult. For example, the insoluble products may accumulate and block the extraction devices. Among these solid particles, finely-divided zircalloy presents a danger of spontaneous combustion. Finally, the insoluble fission products may produce hot spots in the machines.

Accordingly, a "clarification" step is introduced between the dissolution step and the extraction step in order to extract the maximum amount of particles remaining in suspension in the solution.

Three kinds of methods have already been proposed. The first - static decantation - is of little importance since the particles are so small that the operation would remove only the largest if a reasonable time was taken over it. Another method is filtration. The main disadvantage of this method is that the filter cartridges rapidly clog up.

There is a third method, which consists in using a pendular decanting centrifuge. In this method, the solution containing insolubles is placed in a vessel having an overflow passage at its top edge.

In view of the speed of rotation, the solid particles are pressed along the side wall of the vessel to form a "cake" and the Iqiuid leaves the vessel through the overflow passages. However, the disadvantage of prior-art decanting centrifuges consists in the subsequent treatment of the insoluble "cake". After the centrifuging operation, the cake still retains a certain fraction of the solution of fissile or other material to be recovered. Accordingly, the cake is rinsed with nitric acid. This rinsing, however, presents certain problems. Considerable volumes of rinsing liquid are required, thus producing additional amounts of liquid effluents, since the operation has to be repeated a number of times. It is performed under substantially laminar conditions and therefore has low efficiency. Consequently, considerable volumes of solution are required.Furthermore, the efficiency of the rinsing process increases if the density of the rinsing solutions is equal to or greater than that of the retaining liquid which is to be driven off. Consequently the choice of rinsing solutions is restricted.

Furthermore, in prior-art centrifuges there is no reliable nuclear geometry, i.e. if the outlet orifices become accidentally stopped up, the configuration of the fissile material present may become critical.

The invention relates specifically to a pendular decantation centrifuge which obviates the aforementioned disadvantages. More particularly, the centrifuge according to the invention can decant with very high efficiency. In addition, the centrifuge vessel is such as to ensure sub-critical geometry.

Finally, the cake-rinsing operation is performed under much more favourable conditions and thus ensures better extraction of the fessile materials remaining in the cake, while requiring very small amounts of rinsing solution.

To this end, the invention relates to a pendular centrifuge for decanting a liquid containing solid particules, the centrifuge being of the kind comprising a plug forming a biological shield under which a stationary external containment is suspended and holds a rotary vessel secured at its end to the bottom end of a vertical driving shaft, the shaft extending through the plug, the centrifuge being characterised in that the vessel has a top cylindrical part having a cylindrical edge forming an overflow edge for the liquid, and also has a bottom part having an inner wall inclined relative to the driving shaft and connected to the bottom of the vessel and also connected to the top part of the vessel by a very flared conical connecting surface; the plug has a cylindrical prolongation extending into the top part and co-operating with the cylindrical part to form an annular "clarification" space connected by a passage of revolution to the bottom part, the device comprising at least a first insertion pipe system extending through the plug and adpated to introduce the liquid into. the passage, a second pipe system comprising nozzles adapted to convey a liquid to the cylindrical wall of the top part of the vessel, means for collecting "clarified" liquid in the containment after the liquid has crossed the overflow edge, siphon means for extacting the liquid from the bottom part of the vessel and means secured to the plug for agitating the liquid in the aforementioned bottom part of the vessel when the vessel is rotated, the conicity of the bottom part of the vessel and the conicity of the connecting surface being such that when the vessel rotates at a certain speed, the liquid and particles initially present in the bottom part rise into the annular "clarification" space.

Preferably, the inner wall of the bottom part of the vessel has a conical top portion.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a view in vertical section of the centrifuge assembly in a first diametrical vertical plane, and Figure 2 is a view in vertical section in a second diametrical plane substantially perpendicular to the first.

The drawings show an embodiment of the centrifuge. A set of active centrifuge parts are disposed below a biological shield 2, e.g. of steel.

Above the centrifuge proper, the shield comprises a removable plug 4. The centrifuge is enclosed in a substantially cylindrical containment 6 suspended from shield 2 and having a vent tube 96. The centrifuge vessel (general reference 8) is secured at its end to a driving shaft 10 which extends through a vertical bore 12 in plug 4. At its top end 10a, shaft 10 is held by a bearing 12 giving a certain ball and socket effect. Above its top end 10a, shaft 10 is connected to the shaft 14 of an electric driving motor 15. Sealing means are provided, e.g. by scavenging gas introduced through a pipe system 18.

The rotary vessel 8 has a top decantation part 8a and a bottom part 8b having an end forming a connecting region. The decantation region 8a comprises a cylindrical wall having a diameter D1, whereas the inner wall 20 of the bottom or collecting part 8b is conical. More specifically, the wall is at an angle of about 15 with the vertical, i.e. the direction of the axis of rotation 10. More generally, the half-angle is between 10 and 20 and is preferably very near 15 .

A slightly conical annular connecting portion 22 connects the bottom part 8b to the top part 8a.

As previously stated, the inner wall of the bottom part 8b need not be conical, provided the top part of the wall is inclined to the vertical near the connecting portion 22. For example, the wall portion can be a paraboloid, in which case the planes at a tangent to the paraboloid surface will satisfy the aforementioned conditions regarding angles.

An annular plate 24 is disposed at the top edge of the top part 8a.

Plug 4 has a bottom prolongation 4a inside the top part 8a of the rotary vessel. Wall 8a has e.g.

four radial fins 9. The side wall 26 of prolongation 4a co-operates with the side wall 8a to define an annular clarification space 28. The annular space 28 is connected by a top annular passage 30 to the interior of containment 6 and its bottom is connected by an annular passage 32 to the collecting region 34 defined by the vessel bottom part 8b. A vessel 35 secured to containment 6 divides it into an outer overflow space 36 communicating with passage 30 and a collecting space 38 inside baffle 35. A pipe system 40 opens into the bottom end of the annular space 36. A discharge pipe system 41 opens into the bottom of space 38. The collecting region 34 contains a cylindrical baffle 52 which cooperates with the inner wall 54 of part 8b to define an overflow region 56. Region 56 has a passage 58 extending through the end of part 8b, thus connecting the overflow space 56 to region 38.

A pipe system 42 extends through the bottom part 4a of plug 4 and opens into the bottom surface 4b of plug 4. Pipe system 42 is prolonged by a deflector 44 whereby the liquid travelling along duct 42 is directed towards the annular passage 32.

At its other end, pipe system 42 is connected to a retractable pipe system 46 for supplying liquid to be decanted.

The centrifuge also contains a duct 60 whose free end is near the bottom of part 8b. Pipe system 60 extends through plug 4a and forms one arm of a siphon 62 having a second arm 64. The siphon will be described in greater detail hereinafter.

In practice, operation is made reliable by providing two siphon deivces. Two "reserve" siphons can be provided, As the drawings show, the centrifuge also comprises a pipe system 70 for unclogging the cake.

System 70 comprises nozzles 72 directed towards the side wall of the cylindrical part 8a. There is also a duct 74 having an end which terminates near the end of part 8b. A pipe system 76 also extends through plug 4 and is used for washing the overflow region 56.

By way of example, the top part 8a has a diameter D, of 900 mm. The vessel is made of titanium, thus reducing the density and obtaining a largerdiameter vessel at a given rated speed of rotation.

This increase in diameter compared with prior-art centrifuges gives improved stopping power, i.e.

more efficient decantation, other things being equal.

The centrifuge operates as follows. During the actual decantation operation, the solution is sent through pipe system 42 after rotating the vessel at its rated operating speed. As a result of rotation and the presence of deflector 44, the liquid enters the annular decantation region 28. The suspended particules (indicated by crosses P) are pressed against wall 8a and form the cake. As a result of the centrifugal action, the liquid comes out through passage 30 and overflows into region 36.

Thus, the solution is clarified, i.e. contains hardly any remaining particles in suspension. The clarified solution comes out through pipe system 40. After a volume of solution corresponding to a fixed tonnage has been clarified, vessel 8 is stopped by natural deceleration. The cake is still stuck to the side wall 8a. The liquids fall into the collecting space 34. The solution, which may contain fragments of "cake", is discharged by siphon upstream of the clarification assembly, so that it can be subsequently recycled.

As previously stated, the "cake" which has formed on the side wall 8b must be rinsed in order to extract any solution remaining therein. This is performed by means of the following operations.

By means of pipe system 70, nitric solution is injected through nozzles 72 in order to unclog the cake. In the embodiment shown, the voluem of solution required for this purpose is smaller than the volume of the clarification ring.

As a result of the unclogging, the sludge falls into the bottom region 34 and is there held in suspension by preferably alternating low-speed rotation of vessel 8, brought about by actuating motor 16. The bottom ends of the siphon pipes 60 act as agitators, resulting in very efficient rinsing. After the mixing operation, the vessel is progressively rotated up to its rated speed. The liquids and solids rise along the conical wall 20 of the bottom part 8a until they are in the annular clarification space 28. The rise occurs at a speed considerably below the rated speed. After the cake has reformed, the centrifuge is stopped, preferably by natural deceleration. The rinsing solution falls to the bottom of part 8b and is recycled upstream of the clarification assembly. The rinsing operations are repeated a number of times.

It can be seen that, as a result of the device and the cake-rinsing process, the number of rinsing operations can be limited while ensuring higher efficiency than with the prior-art devices. Since the number of rinsing operations is limited, it can be seen that the resulting amount of effluent is also limited.

After rinsing, the particles are in the collecting region 34, forming a suspension highly concentrated in particles. Region 34 is emptied by supplying more rinsing solution to the vessel and continuing to agitate the solution by slowly rotating the vessel. The charged solution overflows into the overflow region 58 and thence into region 38.

When the concentration of the suspension has sufficiently fallen, a second siphon identical with si- phon 62 is used for discharging the solution.

Siphons 62 are started by a piston 80. The piston is disposed in a chamber 82 formed in plug 4 and blocked by a small plug 84. Piston chamber 80a is secured to plug 84. The piston proper comprises a bellows 80b secured at one end to plug 84 amd at the other end to a circular plate 80c. A control rod 80d is secured to plate 84. It extends through a small plug 84 and is driven by a motor 86. Piston chamber 80b is connected to siphon 62 by a passage 80f. It can be seen that when rod 80d is raised, a negative pressure is produced in chamber 80e, thus starting the siphon. Of course, other conventional starting devices could be used, but the present device has considerable advantages.

Firstly, bellows 80b ensures very good sealingtightness since the seals are all static. Secondly, if the siphon is damaged, the siphon assembly can be extracted by raising the small plug 84.

As Figure 1 shows, the second siphon arm 64 extends into an intermediate chamber 80 having an end 90a at the starting vessel. A duct 92 is used for supplying vessel 90 with sufficient inactive solution to start the siphon. A second siphon 94 is used for discharging the solution.

As the preceding description shows, the centrifuge according to the invention has numerous advantages over prior-art centrifuges. Its geometry is sub-critical, mainly because the central space is filled by the plug prolongation 4a. It also ensures more efficient decantation. Finally, the cake-rinsing operations require only a reduced volume of solution, which also reduces the resulting amount of liquid effluents. The operations can be performed with dilute nitric solutions, i.e. which are not likely to ignite particles of zircalloy.

Claims (9)

1. A pendular centrifuge for decanting a liquid containing solid particles, the centrifuge being of the kind comprising a plug forming a biological shield under which a stationary external containment is suspended and holds a rotary vessel secured at its end to the bottom end of a vertical driving shaft, the shaft extending through the plug, the centrifuge being characterised in that the vessel has a top cylindrical part having a cylindrical edge forming an overflow edge for the liquid, and also has a bottom part having an inner wall inclined relative to the axis of rotation and connected to the bottom of the vessel and also connected to the top part of the vessel by a very flared conical connecting surface; the plug has a cylindrical prolongation extending into the top part and co-operating with the cylindrical part to form an annular "clarification" space connected by an annular passage to the bottom part, and the device comprises at least a first insertion pipe system extending through the plug and adapted to introduce the liquid into the passage, a second pipe system comprising rollers adpated to convey a liquid to the cylindrical wall of the top part of the vessel, means for collecting "clarified" liquid in the containment after the liquid has crossed the overflow edge, at least two sets of siphon means for extracting the liquid from the bottom part of the vessel and means secured to the plug for agitating the liquid in the aforementioned bottom part of the vessel when the vessel is rotated, the inclination of the bottom part of the vessel and the conicity of the connecting surface being such that when the vessel rotates at a certain speed, the liquid and particles initially present in the bottom part rise into the annular "clarification" space.

2. A centrifuge according to Claim 1, characterised in that the inner wall of the bottom part of the vessel is conical.

3. A centrifuge according to Claim 2, characterised in that the half-angle at the apex of the conical wall is between 10 and 20 .

4. A centrifuge according to any of Claims 1 to 3, characterised in that the vessel is made of titanium.

5. A centrifuge according to any of Claims 1 to 4, characterised in that the bottom part of the vessel has an internal cylindrical baffle having a free top edge and surrounding the shaft and internally defining an overflow space for the bottom part.

6. A centrifuge according to any of Claims 1 to 5, characterised in that the siphon means comprise a first siphon arm extending into the bottom part of the vessel and partially extending through the plug, a second arm connected to the first by a T and opening into a vessel outside the containment, and starting means.

7. A centrifuge according to Claim 6, characterised in that the starting means comprise a piston comprising a chamber connected to the siphon T, the chamber containing a deformable metal bellows having an edge secured to the chamber wall, whereas the other edge is secured to the periphery of a plate connected to means for moving the plate along the chamber axis.

8. A centrifuge according to Claim 7, characterised in that the piston is disposed in a cavity formed in the plug and closed by a small biological shielding plug, the piston chamber being suspended from the small plug.

9. A centrifuge according to any of the preceding claims, wherein the means secured to the plug for agitating the liquid in the bottom part of the vessel comprises the siphon means.

9. A centrifuge substantially as described and as shown in the accompanying drawings.

New claims or amendments to claims filed on 11/ 12/81.

Superseded claims 1 New or amended claims:- 1 and 9

1. A pendular centrifuge for decanting a liquid containing solid particles, the centrifuge being of the kind comprising a plug forming a biological shield under which a stationary external containment is suspended and holds a rotary vessel secured at its end to the bottom end of a vertical driving shaft, the shaft extending through the plug, the centrifuge being characterised in that the vessel has a top cylindrical part having an inwardly inclined edge forming an overflow edge for the liquid, and also has a bottom part having an inner wall inclined relative to the axis of rotation and connected to the bottom of the vessel and also connected to the top part of the vessel by a very flared conical connecting surface; the plug has a cylindrical prolongation extending into the top part and co-operating with the cylindrical part to form an annular "clarification" space connected by an annular passage to the bottom part, and the device comprises at least a first insertion pipe system extending through the plug and adapted to introduce the liquid into the passage, a second pipe system comprising nozzles adapted to convey a liquid to the cylindrical wall of the top part of the vessel, means for collecting "clarified" liquid in the containment after the liquid has crossed the overflow edge, at least two sets of siphon means for extracting the liquid from the bottom part of the vessel and means secured to the plug for agitating the liquid in the aforementioned bottom part of the vessel when the vessel is rotated, the inclination of the bottom part of the vessel and the conicity of the connecting surface being such that when the vessel rotates at a certain speed, the liquid and particles initially present in the bottom part rise into the annular clarification" space.