FR2786116A1 - Powdered nuclear fuel processing comprises passing powder through reduction unit, retaining coarse particles and subjecting to continuous reduction - Google Patents

Abstract

Processing of nuclear fuel powder in an oxide form, e.g. UO2, comprises passing the powder through a reduction unit, retaining the coarse particles above a predetermined or critical size and subjecting to continuous reduction before mixing with the flow of fine particles. An Independent claim is also included for the apparatus for processing nuclear fuel, in which a particle reduction unit (24) incorporates a coarse particle retainer (44) with outlets (86) which allow the particles to pass through once they have been reduced below the critical size by a rotary conveyor unit (52) with cutters (58B) passing across grinder blades.

Description

 The present invention relates to a method and an installation for treating a nuclear fuel powder.

 The invention relates more particularly to the treatment of such a powder, of the oxide type, such as UO2, obtained downstream of the step aimed at transforming into this oxide a compound, such as UF6. The powder having undergone this conversion step is then subjected to a phase of reduction of the average size of the particles which constitute it.

 UO2 can be obtained in different ways.

A first mode, called wet, leads to the production of coarse aggregates, the average diameter of which is around 1 mm. In this case, the step of reducing the average size of the particles consists in shifting the particle size spectrum as a whole, without modifying the general appearance of this spectrum.

This is obtained, in the usual way, by grinding by means of mechanical installations, such as for example ball, hammer or air jet mills. All of the particles forming the combustible powder are subjected to the action of these mechanical mills.

 This known solution has certain drawbacks, however. Thus, ball mills do not allow a significant flow and induce contamination problems related to their unloading. They are also noisy and do not solve the problem of re-agglomeration of the powder. Hammer mills, apart from their noisy nature, do not guarantee obtaining powder of satisfactory homogeneity.

 Finally, air jet mills cause clogging problems and induce significant overheating, which means working with a low flow rate.

It is also necessary to add a gas reprocessing installation to such mills.

 There are also processes for converting UF6 into UO2 using a so-called dry route and leading to the production of a finer powder than that corresponding to the wet route, formed of aggregated particles whose average size is approximately 100 micrometers. However, this powder contains fluorinated particles of the UO2F2 type, which have a larger size than the constituent particles of the rest of the powder. When these fluorinated particles exceed a critical size, of around 300 micrometers, they are the cause of problems in particular related to sintering, so that it is necessary to eliminate them.

 To this end, it is known to implement a sieving intended to retain the particles whose size is greater than the critical size previously mentioned. The average size of the particles flowing through this sieve is therefore less than that of the particles admitted upstream of the latter.

Contrary to the techniques aiming to shift the granulometric spectrum as a whole, one proceeds here to a capping, which contributes to modify the general shape of this spectrum.

 The particles of large size, retained by the mesh of the sieve, must be subjected to operations aimed at recycling and deflowering them with a view to their subsequent mixing with finer particles.

 This solution using sieving also has certain drawbacks. In fact, the particles which do not flow through the sieve form what is called "sieve rejection" and must be regularly removed, which causes frequent stoppages in manufacturing. These evacuation operations are, most of the time, carried out manually and therefore prove to be particularly contaminating.

 Due to their mechanical structure, the screens have limited strength and are subject to breakage. This then involves reprocessing all of the powder which has flowed through the screen in question. This fragility also implies that it is necessary to frequently change these sieves.

 Damaged screens can be repaired in order to limit costs, but these repairs have the effect of reducing the filtering surface.

 In the case of powders having poor flowability, the use of sieves induces a clogging problem, due to the fact that the powder agglomerates and clogs the sieve. It is then necessary to implement unclogging, for example by ultrasound, or even by percussion, which then causes mechanical degradation of the screen.

 The present invention proposes to implement a method for treating nuclear fuel powder making it possible to overcome all of the drawbacks inherent in the various methods of the prior art mentioned above.

 To this end, it relates to a process for treating a nuclear combustible powder, of the oxide type, such as qutUO2, said process being carried out in particular after a phase of conversion into said oxide of a compound, such as UF6, said process comprising a step of passing the powder through a unit for reducing the average size of the particles of the powder, inducing a modification of the general shape of the spectrum of the particles, step in which it is retained in the reduction unit , particles, called coarse, whose size is greater than a predetermined size, or critical size, while particles, called fine, which are smaller than said critical size, are allowed to flow freely, characterized in that '' the size of the coarse particles is continuously reduced in the reduction unit so as to form particles of size smaller than the critical size, or reduced particles, and one distr ibue reduced particles in the free flow of fine particles.

According to other characteristics of the invention:
the size of the coarse particles is reduced at the same time as the passage of these coarse particles is prevented;
-we direct along a common path, fine particles and reduced particles;
the common path comprises at least one path defined by opposite faces of coarse particle reduction elements, the or each path having dimensions adapted so that the fine particles can flow therein freely and so that the coarse particles do not are not allowed in this path;
- said coarse particles are sheared against at least one face of the reduction elements defining the or each path.

 The invention also relates to an installation for treating a powder made of a nuclear combustible material, of the type containing an oxide, such as UO2, said installation being arranged in particular downstream of a unit for converting into said oxide. a compound, such as UF6, said installation comprising a unit for reducing the average size of the particles forming the powder, inducing a modification of the general appearance of the spectrum of particles, said unit comprising means for retaining so-called coarse particles, whose size is greater than a predetermined size, or critical size, said retaining means being adapted to allow free flow of so-called fine particles, of size less than the critical size, characterized in that said reduction unit also comprises means for continuously reducing coarse particles, so as to form so-called reduced particles of size smaller than the critical size, as well as reduced particle distribution means in the free flow of fine particles.

According to other characteristics of the invention:
the means for retaining coarse particles and the means for reducing coarse particles are combined;
the retaining and reducing means comprise at least a pair of reducing elements, of which opposite faces define a path, said reducing elements being arranged to allow fine particles to flow freely along the path and to reduce the size of the coarse particles;
the reduction elements are arranged on an annular reduction member receiving, in its internal volume, a rotary conveying means capable of directing the powder, by centrifugal effect, towards the reduction elements.

said rotary conveyor means is provided with means for shearing coarse particles against the reduction elements;
the reduction elements are grinding blades;
-each blade has one or two side faces in clearance with respect to a longitudinal axis of the blade, at an angle between 0 and 10 and
the longitudinal axis of each blade is inclined, with respect to a diametrical axis of the annular reduction member, at an angle between 0 and 10.

 The invention makes it possible to achieve the objectives mentioned above. Indeed, the fact of reducing the particles whose size is greater than a critical size, in order to disperse them among the particles flowing freely allows, while not allowing the passage of a precise fraction of the powder, to get rid of the operation of evacuation of this fraction considered, as was the case in the prior art using sieves. The process according to the invention therefore proves to be significantly less contaminating than those using such sieves. The process according to the invention is also capable of operating continuously, without appreciable flow break and possibly by means of successive powders of different natures. It also allows the treatment of powders whose flow rates are able to vary significantly.

 The process according to the invention has remarkable efficiency, in that all the particles flowing downstream of the reduction means actually have a size less than the predetermined critical size.

 The process according to the invention guarantees, to a large extent, an absence of pollution, since it does not involve either re-treatment of products which may have been eliminated, or reprocessing of any fluids used for reducing the size. particles.

 The nuclear fuel powder processing installation according to the invention is very flexible. Indeed, it is likely to treat products whose enrichment, even the nature, are different. Since the installation according to the invention has no powder retention zone, it is not necessary to carry out any cleaning when switching from one product to another.

 The installation according to the invention makes it possible to achieve high flow rates, which can for example be five to ten times greater than those authorized by means of a treatment installation using sieves. The installation of the invention generates reduced operating costs compared to the prior art. It also guarantees a very satisfactory integrity of the treated powder, since it makes it possible to avoid, to a large extent, all overheating and therefore does not induce any oxidation phenomenon.

Since this installation does not use any lubricant, the powder, during its passage through the installation, is not subject to contamination due to its contact with such an external product.

 The installation in accordance with the invention makes it possible to overcome clogging problems, thanks to the drive effect of the turbine which equips it and which causes a suction of the powder during its passage through the particle reduction unit.

 The installation according to the invention is also very flexible in terms of adjustments since the critical size, above which particles are reduced, can be modulated very simply by modifying one of the parameters of the installation involved in determining this critical size. In addition, once this critical size has been determined, no further adjustment is necessary.

The invention will be described below, with reference to the accompanying drawings, given only by way of nonlimiting examples and in which:
FIG. 1 is a schematic view of an installation for processing nuclear fuel powder in accordance with the invention;
FIG. 2 is an exploded perspective view, with parts broken away, of the various constituent elements of the particle reduction unit of the installation of FIG. 1;
FIG. 3 is a view in partial diametral section of the reduction unit of FIG. 2; and
FIG. 4 is a cross section from above, showing a turbine and cutting blades belonging to the particle reduction unit illustrated in FIGS. 2 and 3.

 Figure 1 shows, schematically, a nuclear fuel powder processing installation.

This installation is provided, at its upper end, or upstream, with a flange or crown 2 allowing the docking of a container 4, the bottom of which is provided with a valve not shown, for example of the guillotine type. This container is loaded with a nuclear fuel powder obtained by conversion, for example in a dry process, intended to transform an initial compound such as UF6 into an oxide, such as UO2.

 The crown 2 surmounts a pipe 6 extending, at its lower end, by a conical hopper 8 with a double envelope. Line 6 is provided, at its upper end, with a detection probe 12 suitable for detecting the presence of powder and being in relation to an automatic valve 14 intended to close the line in the absence of powder. A flow regulator, not shown, is provided at the downstream end of the pipe 6, at the connection with the hopper 8.

 This hopper 8 is connected, at its downstream end, to a metal particle detector of known type, designated as a whole by the reference 18. This detector, which is provided with a bypass line 20 suitable for collecting metallic impurities is connected, via an intermediate tube 22, to a unit 24 for reducing powder particles. A nitrogen supply nozzle 25 opens out inside the reduction unit 24, at the downstream end of the latter.

 This reduction unit 24, which will be described in more detail below, is connected, at its downstream end, to an outlet conduit 26 of the installation, in which the treated nuclear fuel powder circulates, which is directed towards a storage unit 27. This outlet duct 26 is provided with a valve 28 intended to selectively block the flow thereof. A double pipe 29 (or recirculation) makes it possible to balance the pressure between upstream and downstream of the unit 24 for reducing powder particles.

 Figures 2 and 3 illustrate the particle reduction unit 24, shown on a larger scale than in Figure 1.

 This unit 24 comprises a hopper 30 for admitting powder to be treated, placed in communication with the downstream end of the tube 22 shown in FIG. 1. This hopper 30 is provided, at its lower end, with a collar 32 which is flush, as shown in particular in FIG. 3, the upstream end of a chamber 34, in which are housed all of the elements arranged downstream of the flange 32.

The latter is supported, by means of an annular flange 36, against the upper end of a micro-cutting head designated as a whole by the reference 38.

 As will be described more precisely with reference to FIG. 4, this micro-cutting head 38 comprises two annular flanges respectively upper 40 and lower 42, between which blades are arranged 44. The upper flange 40 bears against the underside of the flange 32 and against a shoulder 46 which is provided with an upper edge 48 of the chamber 34. The lower flange 42 is secured, for example by screwing, to a support 50, the lower end of which rests on a non-chassis pictured from the installation.

 The annular head 38 receives, in its internal volume, a circular turbine 52, comprising two flanges respectively upper 54 and lower 56, between which fins 58 are fixed, which will be shown more precisely in FIG. 4. The upper flange 54 has a frustoconical upper end 60 defining an upstream orifice 62 of the turbine 52, placed in communication with the downstream end of the intake funnel 30.

 The lower flange 56 has a bottom 64 extended by an upper flange 66. This bottom 64 is pierced with a central orifice intended for the passage of a stud 68, cooperating with a washer 70 and a nut 72 for the purpose of fixing the turbine 52 to a shaft 74 for driving the turbine, which is connected to a motor not shown and is free to rotate inside the support 50. A connecting piece 75, surrounded by an O-ring 75A, is interposed between the bottom 64 and the shaft 74.

This piece 75 is fixed to the bottom by pins 75B.

 A lower retaining ring 76, or wear ring, to which a flange 78 of corresponding profile is attached, with the interposition of a seal of rectangular section 80, is disposed between the walls opposite the lower flange 42 of the head 38 and from the bottom 64 of the lower flange 56 of the turbine 52. Such an arrangement makes it possible to maintain the ring and to avoid the passage of unground powder.

 FIG. 4 illustrates, on a larger scale than FIGS. 2 and 3, part of the blades 44 equipping the micro-cutting head 38, as well as one of the fins 58 equipping the turbine 52. The blades 44 are for example provided at number of 180, at the periphery of the micro-cutting head 38.

Each blade 44 has, in top view, a generally rectangular profile and has two faces 82,84 in relief, extending symmetrically with respect to the main axis A of this blade. The cross section of the blade increases towards the inside of the cutting head. These faces 82, 84 have clearance angles a, ', with respect to the axis
A, which are between 0 and 10. These angles a and a can be identical or different.

 The distance d separating two adjacent blades 44, at their flared inner end, is for example between 0.03 and 3 mm and, preferably, between 0.1 and 0.3 mm.

 The axis A of each blade is inclined, with respect to the diameter D of the cutting head, at an angle> of between 0 and 10, preferably between 1 and 5. This inclination can be adjusted in a known manner.

 The fin 58, which extends over the outer periphery of the turbine 52, has a curved shape and comprises a body 58A made for example of stainless steel and terminated by an end plate 58B made of tungsten carbide. The opposite faces of the inner end of the blades 44 and of the plate 58B of the fin 58 are separated by a distance between 0.3 and 0.4 mm, depending on the wear of the blades and of the turbine. In service, each fin 58 is rotated, according to arrow F, at a speed of between 6,000 and 12,000 revolutions per minute.

 We will now describe the implementation of the nuclear fuel powder treatment process according to the invention, with reference to all of FIGS. 1 to 4.

 We first dock the container 4 on the crown 2.

One then proceeds to the successive openings of the valve of the container 4, of the valve 28, and of a valve not shown controlling the supply of the nozzle 25 with nitrogen. Then, the particle detector 18 is started up and the upper automatic valve 14 is opened, so that the powder flows from the container 4 towards the reduction unit 24.

 The powder is admitted into the reduction unit 24 via the hopper 30, then flows towards the upstream orifice 62 of the turbine. It then pours onto the upper surface of the lower flange 56 of the turbine, and is driven centrifugally due to the high speed rotation to which this turbine is subjected. These particles are therefore directed towards the blades 44 with which the micro-cutting head 38 is provided.

 Two neighboring blades define, by their opposite lateral faces, a path 86 along which a particle larger than a particle which cannot be progressed cannot be advanced, which is also known as critical size.

Each pair of adjacent blades 44 therefore constitutes a means for retaining coarse particles. The critical size is a function of the distance d separating the facing faces of two adjacent blades, the angles of inclination a and a ′ of these faces, the angle of inclination ss of each blade by the diametrical axis D of the head 38, as well as of the speed of rotation of the turbine 52.

 Only the powder particles whose size is less than this critical size flow freely through the path 86 defined by the assembly of the blades 44. By free flow is meant a flow in which the particles are not affected by the 'action of the blades 44. On the other hand, the particles whose size is greater than this critical size are subjected to reduction by grinding. The latter is generated by the action of the end plates 58B of the fins 58, which, during their rotational movement, continuously shear the coarse particles against the active faces of the blades 44.

 The particles reduced to a size less than the critical size then flow along the paths provided by each pair of adjacent blades and are dispersed in the flow of particles of size below the critical size, which are not subjected to the action of the blades 44.

The set of paths 86, the width of which increases towards the outside of the head 38, due to the clearance of the blades 44, forms a common path for the fine particles and the reduced particles.

 The particles initially admitted inside the turbine 52 flow, possibly after being reduced, towards the outside of the cutting head 38 and are admitted into the chamber 34. The average particle size, downstream of the blades 44, is less than that of the particles upstream of these blades, due to the grinding operation to which some of these particles are subjected. The reduction unit 24 therefore constitutes a unit for lowering the average size of the particles, inducing a modification of the general allule of the spectrum of these particles. The flow of the particles is directed by gravity towards the outlet conduit 26, so as to be stored in a container, not shown. When the probe 12 detects the absence of powder, the upper valve 14 is closed and an additional container, similar to that 4 shown in FIG. 1, can be docked at the level of the upper crown 2.

 In the examples described and shown, only a generally vertical treatment installation has been mentioned, in which the particles progress by gravity. It is also possible to convey these particles by other routes, in particular by pneumatic means.

Claims (13)

 1. Method for treating a nuclear combustible powder, of the oxide type, such as UO2, said method being carried out in particular after a phase of conversion into said oxide of a compound, such as UF6, said method comprising a step of passage of the powder through a reduction unit of the average size of the particles of the powder inducing a modification of the general shape of the spectrum of the particles, step in which one retains, in the reduction unit, of the particles known as coarse , whose size is greater than a predetermined size, or critical size, while allowing particles to flow freely (at 86), said fine, whose size is less than said critical size, characterized in that continuously reduces (at 44) in the reduction unit (24), the size of the coarse particles, so as to form particles of size smaller than the critical size, or reduced particles, and the pa reduced particles in the free flow (at 86) of fine particles.  2. Treatment process according to claim 1, characterized in that the size of the coarse particles is reduced (at 44) while the passage of these coarse particles is prevented.  3. Treatment method according to claim 1 or 2, characterized in that directs along a common path (86), the fine particles and the reduced particles.  4. Treatment method according to claim 3, characterized in that the common path comprises at least one path (86) defined by opposite faces of reduction elements (44) of coarse particles, the or each path (86) having dimensions adapted so that fine particles can flow there freely and so that coarse particles are not admitted in this path (86).  5. Treatment method according to claim 4, characterized in that shears (by 58B) said coarse particles against at least one face of the reduction elements (44) defining the or each path (86).  6. Installation for processing a nuclear combustible powder, of the oxide type, such as UO2, said installation being arranged in particular downstream of a unit for converting into said oxide a compound, such as UF6, said installation comprising a unit (24) for reducing the average size of the particles forming the powder, inducing a modification of the general shape of the spectrum of the particles, said unit comprising means (44) for retaining so-called coarse particles, the size of which is greater at a predetermined size, or critical size, said retaining means (44) being adapted to allow free flow of so-called fine particles, of size less than the critical size, characterized in that said reduction unit (24) also comprises means for continuously reducing (44) coarse particles, so as to form so-called reduced particles, of size smaller than the critical size, as well as means for distribution of reduced particles in the free flow of fine particles.  7. Treatment installation according to claim 6, characterized in that the means for retaining (44) coarse particles and the means for reducing coarse particles are combined.  8. Treatment installation according to claim 7, characterized in that the retaining and reduction means comprise at least a pair of reduction elements (44), of which opposite faces define a path (86), said elements of reduction being arranged to allow fine particles to flow freely along the path (86) and to reduce the size of coarse particles.  9. Treatment installation according to claim 8, characterized in that the reduction elements (44) are arranged on an annular reduction member (38) receiving, in its internal volume, a conveying means (52) suitable for direct rotation the powder, by centrifugal effect, towards the reduction elements (44).  10. Treatment installation according to claim 9, characterized in that said rotary conveying means (52) is provided with means for shearing (58B) the coarse particles against the reduction elements (44).  11. Treatment installation according to one of claims 8 to 10, characterized in that the reduction elements are grinding blades (44).  12. Treatment installation according to claim 11, characterized in that each blade (44) has one or two side faces in relief with respect to a longitudinal axis (A) of the blade, at an angle between 0 and 10.  13. Treatment installation according to claim 12, characterized in that the longitudinal axis (A) of each blade (44) is inclined, relative to a diametrical axis (D) of the annular reduction member (38), at an angle between 0 and 10.