Mixing Device For Powdered Materials

Abstract

The mixing device for powdered material and especially fissile material which remains geometrically subcritical even when a large quantity is present comprises a stationary cylindrical tank having a horizontal axis and obturatable openings for the introduction and withdrawal of materials ; a rotary drum which is coaxial with the tank and forms the internal wall of a substantially leak-tight annular space whose external wall is constituted by the tank; a core of neutron-moderating material located within the drum and covered with material having a high neutron capture cross-section ; stirring arms mounted on the external wall of the drum and means for driving the drum in rotation.

Description

This invention relates to a mixing device for powdered materials and especially for fissile materials.

Fissile materials which are provided in the form of a powdered mass (this term being understood to designate flakes and granules as well as powders proper) are often required to undergo a mixing or stirring process in order to make the mass homogeneous.

The problem of criticality arises immediately in operations of this type. It is known that the critical mass of a given fissile material (e.g., plutonium oxide, oxides of uranium-235 or uranium-233, uranium carbide or plutonium carbide) is a function of two parameters, namely density and moisture content. The importance of the range of variation is immediately apparent if it is borne in mind that, in the case of plutonium oxide, for example, the critical mass varies between approximately 100 kilograms and a few kilograms. In fact, during a stirring operation, the density of a powdered material can increase to a very appreciable extent under the action of its own weight. Moreover, the moisture content which is usually of very low value can increase to a considerable extent as a result of an incident. In consequence, the mixers which are employed in conventional industrial applications are not suited to this purpose unless the capacities are limited to extremely low values which are in any case objectionable. In all these mixers, the powdered material to be treated is in fact collected together, with the result that a criticality accident is always possible if reliance is placed on maintenance of low values of density and/or moisture content in order to prevent the attainment of criticality.

The aim of the present invention is to remove these difficulties and more specifically to provide a mixing device for powdered material which remains geometrically subcritical even in the case of large capacities.

To this end, the invention proposes a device which comprises : a stationary tank of revolution having a horizontal axis and provided with obturatable openings for the introduction and withdrawal of materials; a drum which is coaxial with the tank and forms the internal wall of a substantially leak-tight annular space whose external wall is constituted by the tank, said drum being adapted to carry stirring arms ; and means for driving the drum in rotation about its axis.

It is apparent that the fissile material occupies a volume which has the shape of a circular ring : both the thickness and diameter of this ring can be chosen so as to ensure absolute safety in the case of all materials to be processed, such materials being usually limited to plutonium and uranium compounds. It is further apparent that a given device can be adapted to the treatment of a material having a critical mass of lower value than the material for which it was designed at the cost of simple modifications (reduction in thickness of the ring).

In a preferred embodiment of the device, the drum is occupied at least partially by a core of neutron-moderating material which is covered with material having a high neutron capture cross-section (such as cadmium, for example). The drum which is preferably cylindrical as well as the tank can be fixed on a driving shaft which is rotatably mounted in bearings and carried by a frame, the axial length of said drum being greater than that of the tank and sliding sealing means being interposed between the tank and the drum.

A better understanding of the invention will be obtained from the following description of a device which constitutes one particular embodiment as given by way of example without any limitation being implied. In this description, reference is made to the single accompanying FIGURE which is a sectional view along the vertical midplane of the device.

As illustrated in the FIGURE, the device essentially comprises a tank 10 and a rotary drum 12.

The tank consists of a circular shell which is constructed of stainless steel sheet, for example, and supported for rotation by a frame which is not shown in the drawing. Said tank is provided at the top with a large inspection opening having a length which represents a substantial fraction of the axial length of the tank. Said opening is normally closed by a door 14 which is secured in leak-tight manner around the periphery of the opening by means of detachable components such as screws (not shown). On each side of the inspection opening and in the axial direction, the tank is provided with two nozzles 16 for the admission of powdered material to be homogenized. Said nozzles terminate in flanges 18 for the connection of supply tubes.

Similarly, the bottom portion of the tank 14 is provided with a nozzle 20 for the discharge of homogenized material, said nozzle being provided with a coupling flange 22. The nozzle 20 communicates with the interior of the tank by means of an opening which is sealed-off by means of a gate 24 when stirring is in progress. Said gate can be withdrawn laterally so as to free the opening as a result of displacement in a direction parallel to the axis by means of a control jack 26 which is placed outside the tank. In order to permit said displacement of the gate 24, the nozzle 20 is provided with a lateral extension 28. The tank is further provided with a vent 29 for connecting the interior of the tank to atmospheric pressure.

The drum 12 which is rotatably mounted within the tank 10 is made up of a rotary cylinder 30 of sheet steel, for example, which is attached to two end-plates 32. Said two end-plates are keyed on a driving shaft 34 which is rotatably mounted in bearings 36 of a stationary supporting frame. A reduction-gear motor (not shown) serves to drive the shaft at a suitable speed. The drum has an axial length which is greater than that of the tank. In order to prevent leakage in the connection zones of large diameter, the tank is adapted to carry stuffing-boxes 38 which maintain leak-tight packings applied against the end portions of the drum 12. The drum is provided with helical stirring arms 40 which serve both to bring the powdered material towards the center after this latter has been poured through the nozzles 16 and to assist the mixing process.

The interior of the drum 12 is occupied by a core which is intended to absorb the neutrons. Said core 42 is constituted by a mass of light material which is intended to slow-down the neutrons and by a sheet of material having a high neutron capture cross-section which covers said mass. It is possible in particular to employ a cadmium sheet having a thickness varying between a few tenths of a millimeter and one millimeter on a mass of polethylene : the fast neutrons emitted by the fissile material are slowed-down by the mass of light material and are thus much more strongly absorbed by the sheet of cadmium.

Instead of a neutron-moderating core covered with cadmium, it is possible to employ a mixture of moderator and absorbent material such as boron-containing polyethylene associated with plaster.

The following data relating to a homogenization device which has actually been constructed for the treatment of plutonium oxide can be mentioned by way of example : the cylindrical tank had an internal diameter of 620 mm and an internal length of 800 mm. The drum had an external diameter of 500 mm so that the annular space for receiving plutonium dioxide had a thickness of 60 mm. The interior of the core around the shaft was occupied by polyethylene and this latter was surrounded by a sheet of cadmium over a distance of 820 mm (the core accordingly extended over a distance of 10 mm beyond the annular mixing space at each end of this latter).

The operation of the device can readily be understood from the foregoing description : the fissile material (plutonium dioxide, for example) is introduced through one of the nozzles 16 and the mass employed can correspond to 50 kg of plutonium metal if the dimensions are those given above.

It is apparent that the device offers an advantage only on condition that the level of fissile material at rest is located above bottom portion of the drum : in practice, the useful capacity is approximately one-half of the annular volume.

The drum is driven in rotation at a speed of the order of 20 revolutions per minute. At the end of a time interval which is variable according to the state of the material which is introduced and which in practice is always less than one hour, the discharge gate 24 is opened by means of the jack 26 and the fissile material is poured off while the drum is maintained in rotation so that the arms 40 bring the product back towards the central plane of the device and the gate.

Claims

1. A mixing device for powdered materials comprising: a stationary tank of revolution having a horizontal axis and provided with obturatable openings for the introduction and withdrawal of materials; a drum which is coaxial with the tank and forms the internal wall of a substantially leak-tight annular space whose external wall is constituted by said tank, said drum being adapted to carry stirring arms; and means for driving the drum in rotation about a horizontal axis and a core of neutron-moderating material covered with material having a high neutron capture cross-section in the drum.

2. A device according to claim 1, wherein the neutron-moderating material is constituted by a polyethylene cylinder covered with a sheet of cadmium.

3. A device according to claim 1, wherein the drum is occupied at least partially by a core of homogeneous neutron-moderating and neutron-absorbing material.

4. A device according to claim 1, wherein the drum is fixed on a driving shaft carried by a frame and has an axial length which is greater than that of the tank, sliding sealing means being interposed between the tank and the drum.

5. A device according to claim 1, wherein provision is made at the top of the tank for an inspection opening whose axial length represents a substantial fraction of the total length of the tank, said opening being intended to be closed by means of a leak-tight door.

6. A device according to claim 1, wherein the discharge opening for powdered materials which is placed at the bottom portion of the tank terminates in a transfer duct and can be sealed-off by a gate which is displaceable within said duct in a direction parallel to the axis.