GB2158284A - Installation for producing sinterable granules of burnable nuclear fuel - Google Patents

Abstract

The installation, which more particularly enables UO2 granules to be produced, provides an apparatus for continuous production from powders. The apparatus (10) has, at its upper part, means (16, 18) for receiving a powder container (12) which are connected via progressive feed and metering means (22) to a powder-compacting press (20) discharging into a buffer vessel (29) receiving the pellets, a comminuting unit (30) being provided which is fed from the vessel by progressive feeding and metering means and which feeds granule-receiving means (46) via screening means (34, 40) adapted to retain those particles whose grain sizerched within the area of the dome- shaped spring element (1). In this way, the switching stroke is divided 2, not shown) which discharges the granules into a continuous drying kiln (106) via a buffer volume (104). <IMAGE>

Description

SPECIFICATION Installation for producing sinterable granules of burnable nuclear fuel The invention relates to an installation for producing sinterable granules of burnable nuclear fuel by coid compacting of powder of the material to form pellets, whereafter the pellets are comminuted into granules and a bonding agent is added to the granules. The invention applies more particularly to the production of sintered pellets on an enriched uranium dioxide base, such pellets being subsequently enclosed in cans to form fuel rods of the kind used more particularly in natural water nuclear reactors.

The uranium dioxide powder obtained from chemical conversion, for example, from uranium hexafluoride, must of course as a rule be subjected to a granulation operation to give it characteristics suitable for pelletting. Various granulation processes are known. As a rule these are discontinuous processes which operate on successive charges an in two cycles.

The first cycle is constituted by precompacting, the second cycle by granulation properly speaking. The various prior art processes mainly differ by the presence or absence of an added bonding agent (organic products or surface oxidation-hydration products) and by the precompacting pressure (which in some cases is very much higher than the subsequent moulding pressure of the raw pellets).

It is an object of the invention to provide an installation whch enables the necessary operations to be performed continuously and in which the various components are highly integrated in conditions ensuring very flexible operation and high metering precision, thus ensuring reproducable results and satisfactory product quality.

To this end the invention provides, more particularly an installation of the kind specified hereinbefore, comprising an apparatus for the continuous production of pellets from powder, has, at its upper part means (16,18) for receiving a powder container which are connected via progressive feed and metering means to a powder-compacting press (20) discharging into a buffer vessel receiving the pellets, a comminuting unit being provided which is fed from the vessel by progressive feeding and metering means and which feeds granule-receiving means via screening means adapted to retain those particles whose grain size is outside a predetermined range.

The comminuting unit can be formed by a wheel forcing the pellets to pass through a grid. The comminuting unit can also have a circuit for re-cycling over-sized particles retained by the screen to a point upstream of the unit. The unit can also comprise a circuit for taking the fines, with switching as required to a storage place or to an apparatus for mixing the fines with the particles of grain size lying within the pre-determined range.

As a rule the installation will also comprise an apparatus for adding bonding agent, thus ensuring final granulation. The apparatus can advantageously comprise a vessel which can rotate around its axis and has a spray bar for the application of a bonding agent and is contained in a casing which is substantially sealing-tight and has tilting means enabling the vessel to be emptied into means for progressively feeding granules leaving the vessel via a continuous drying kiln in the direction of the container-charging means.

The invention will be better understood from the following description of a non-limiting exemplary embodiment thereof, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a precompacting and comminuting tower forming a first apparatus of the installation, Figure 2, which is similar to Fig. 1, is a schematic diagram of a tower for granulation and impregnation with a bonding agent, associated with the installation and adapted to be fed with granules coming from the apparatus illustrated in Fig. 1, Figure 3 is a partially vertically sectioned elevation of a unit for impregnation with bonding agent associated with the apparatus illustrated in Fig. 2, and Figure 4 is a plan view of the unit shown in Fig. 3.

The apparatus (10) shown in Fig. 1, takes the form of a precompacting and crushing tower. It is adapted to receive containers 1 2 filled with burnable fuel powder for granulation. In general the material will be enriched uranium dioxide, possibly containing a poreforming agent and a small fraction of U308 in a homogeneous form.

Each container 1 2 has a closure valve 14 adapted to be connected to receiving means disposed at the top of the apparatus and comprising a hopper having a closure valve 1 6 at its base. The closure valve 16 discharges via a vertical feed conduit 18 into means for progressive feeding and metering to a rotary press 20 adapted to form pellets.

The progressive feeding means 22 are formed, just like the other means of the same kind, disposed at various places in the installation, by a vibrating conveyor in sealing but flexible connection with the feed conduit 18 and the rotary press 20.

The rotary press 20 is adapted to receive by gravity a powder fed via the vibrating conveyor. The press 20 is adapted to precompact the powder in the form of coherent pellets. In practice the press can be adapted to form at a pressure of the order of 1,000 bar tablets in the form of pellets 20mm in diameter and 3-5mm in height. A detector 26 is advantageously provided to measure the level of the powder in the receiving vessel 24 of the press and to put the press into operation.

The press 20 discharges onto a vibrating conveyor which regularises the flow of the pellets, simply compacted but having no appreciable cohesion, into a receiving hopper 29 forming a buffer. The granules are then introduced into a mill 30 having comminuting means formed by a wheel 32 which forces the pellets through a grid provided with openings of well-defined dimensions. An opening of 1200 microns can be considered representative. Disposed above the grid are separating means comprising a screen 34 adapted to retain the over-sized particles having, for example, a grain-size above 1600 microns.

The screen 34 represents a safety measure, in case the grid through which the pellets are forced is broken. The screen 34 is associated with a circuit for re-cycling the over-sized particles, comprising an elevator 36 and a vibrating conveyor 38 for reintroducing the over-sized material into the hopper upstream of the wheel 32. It can be seen how the oversized particles retained by the screen 34 slide towards the base of the elevator 36, which can be of any conventional kind.

The fines, having for example a grain-size less than 200 microns, pass through a second screen 40 and are sent to a circuit 42 for recovering the fines which will be described hereinafter.

The particles whose grain size lies within the range determined by the screens 34 and 40 and which are considered to be satisfactory flow to the inlet of a vibrating conveyor 44 which discharges into a buffer reservoir 46 having level detectors 48. The reservoir 46 discharges into a vibrating conveyor 50 whose operation is controlled by signals detected by the level dectectors 48. The vibrating conveyor 50 feeds a weighing hopper 52 which has downstream a sectioning valve 54 discharging into bellows 56 terminating in a second valve 58. The valve 58 is adapted to be connected in sealing-tight manner to the inlet of a container 60 identical with the container 1 2. The container 60 can be borne by scales 62 arranged to close the sectioning valve 54 when the container has received a pre-determined mass of granules.

The circuit 42 for recovering the fines (Fig.

1) comprises a switching system 11 5 controlled manually or automatically via an actuator 11 6. One of the branches of the switching system discharges into a conduit 11 8 feeding the second apparatus of the installation. The other branch discharges into a buffer hopper 120 having bellows 1 22 connecting it to a valve 124 connecting with the inlet of a container 1 26 identical with the containers 1 2 and 60. Scales 128 can also be provided for receiving the container. Since the pulverulent particles must be prevented from escaping and polluting the atmosphere, a sealing-tight connection must be ensured between the valve 1 24 and the inlet of the container.Such connection can be ensured by a cover 1 30 acted on by an actuator 1 32.

The apparatus just described is adaptable to be fed with oxide fuel powder of various origins. They may more particularly be powders obtained by the direct conversion of uranium hexafluoride in a furnace, which can be the furnace forming the subject of French Patent Application 83 20942, to which reference can be made.

The apparatus shown in Fig. 2 is adapted to receive containers 60 containing granules coming from the apparatus illustrated in Fig.

1. The apparatus shown in Fig. 2, which takes the form of a tower for impregnation and final granulation, comprise container-receiving means which will not be described, since they can be similar to those shown in Fig. 1. The granules coming from the container 50 are collected in a hopper 63 acting as a holding vessel for feed to an impregation unit 64, which is shown very diagrammatically in Fig. 2. The impregnation vessel comprises a tank 66 which can be connected via means adapted to limit dispersions into the atmosphere to the hopper 63. The rotary tank 66 is contained in a casing 68 bearig the motor 70 for rotating the tank. The tank is advantageously made from a material (for example a cross-linked polymer) which prevents sticking and the retention of substances which might be harmful to homogenization.

Extending through the casing 68 is a spray bar 72 for sprinkling a bonding agent coming from a vessel 74. The bonding agent can more particularly be an organic product of the polyvinyl alcohol kind diluted in water, more particularly, polyvinylpyrrolidone.

The casing 68 is mounted on a base 72 via a horizontal tilting pivot 74 enabling the tank 63 to be moved out of the position shown in Fig. 2 (with about 35' inclination) into a position for emptying by pouring, as will be seen hereinafter, using mechanism comprising a rack 76 and a drive motor 78.

A particular embodiment of the impregnation unit is shown in detail in Figs. 3 and 4.

In the unit 64 illustrated, the casing 68 comprises a vessel in which the tank 66 is mounted for rotation. The vessel has a cover in two parts. One 80 of the parts of the cover is permanently attached to the vessel. The other part 82 is mounted on a hinge 84, which is mounted at the top and enables it to tilt. The movable part 82 is made of a transparent plastics material, to enable production to be supervised visually. It has a suction mouth 86 adapted to extract the unagglomer ated fines before the contents of the tank are poured. The movable part of the cover also has a ring 88 enabling it to be lifted, a safety bolt 90 and a shock absorber 92 adapted to attenuate shocks at end of closure travel.

The fixed part 80, formed by a metal plate, bears a welded coupling sleeve 94 adapted to be applied against a rim of fibres 96 borne by the charging hopper 62, so as to limit dispersions during feeding.

The motor 70 for driving the tank 66 is borne by the casing and drives the tank via a step-down transmission 98 (Fig. 3).

The apparatus illustrated in Fig. 2 also comprises a chute 100 receiving the charge poured from the tank 66. The chute 100 feeds a vibrating distributing conveyor 102 discharging into a hopper 104 which forms a buffer and has a volume selected in dependence upon the capacity of the continuous drying kiln 106 and the unit charge of the tank 66. The drying kiln feeds a buffer outlet reservoir 108 which collects the dried granules. The reservoir 108 feeds a vibrating conveyor 110 discharging into a weighing hopper 11 2. This hopper is also connected to a vibrating conveor 11 4 connected to the fines-supplying conduit 11 8 (Fig. 1).The weighing hopper 11 2 therefore enables dried granules coming from the reservoir 118 to be mixed with fines, which improve the cohesion of the raw pellets subsequently shaped. The weighing hopper 112 discharges into an assembly similar to that shown by references 54, 56, 58, 60 and 62 in Fig. 1, thus making available containers 114 which contain a clearly determined mass of granules having precise features.

The apparatus illustrpted in Fig. 2 can be supplemented by extra elements, more particularly for the introduction of a lubricant, such as zinc stearate.

There is no need to describe here the operation of the apparatus illustrated in Fig. 1, since its operation can be derived directly from its make-up. As regards the apparatus illustrated in Fig. 2, it is sufficient to state that each charge is impregnated in the tank 66 with a predetermined quantity of bonding agent while the tank is in the orientation shown in Figs. 2 and 3, the tank being rotated by its motor 70. When impregnation has been completed, the motor 78 is so actuated as to move the casing 68 into the orientation shown in chain-dotted lines. The movable part 82 of the cover is then released to permit the granules to be poured.

Claims (8)

1. An installation for producing sinterable granules of nuclear fuel material in the form of oxide, by cold compacting of the pulverulent material to form pellets, comminuting the pellets into granules and adding a bonding agent to the granules, the installation comprising, at its upper part, means for receiving a powder container which are connected via progressive feed an metering means to a powder-compacting press discharging into a buffer vessel receiving the pellets; means for feeding a comminuting unit from the vessel; granule-receiving means connected to receive granules from said comminuting unit through progressive feeding and metering means and screening means adapted to retain those particles whose grain size is outside a predetermined range; and a unit for adding bonding agent to said granules, having means for receiving granules from said granule-receiving means and delivering it to a vessel, means for rotating said vessel about its axis, spray means for dispersing said bonding agent into the vessel and a substantially gas tight casing receiving said vessel and having tilting means for emptying the vessel into means for progressively feeding granules leaving the vessel, via a continuous drying kiln, to containercharging means.

2. An installation according to claim 1, characterized in that the comminuting unit has a circuit for re-cycling oversized particles retained by the screens to a point upstream of the unit.

3. An installation according to claims 1 or 2, characterized in that the compacting press is adapted to produce pellets in the form of flat pellets which can more particularly have a diameter of the order of 20 mm and a thickness of 3 to 5 mm.

4. An installation according to any of the preceding claims, characterized in that the screening means are adapted to retain oversized particles of grain size above 1 500 microns and fines of grain size smaller than 200 microns.

5. An installation according to any of the preceding claims, characterized in that the comminuting unit comprises a recovery circuit which takes the fines and has means for switching such fines as required to a place of storage or to an apparatus for mixing them with the particles of grain size lying within the predetermined range.

6. An installation according to any of the preceding claims, characterized in that the progressive feeding means are formed by vibrating conveyors.

7. An installation according to any of the preceding claims, further comprising scales for continuously weighing the granule-receiving container.

8. An installation according to claim 1, constructed and arranged substantially as shown in and as described with reference to the accompaying drawings.