The present invention relates to a hot pressing arrangement which may be used for immobilisation of high level radioactive waste into synthetic rock but may also be used for other applications including the immobilisation of toxic chemical materials into synthetic rock. Furthermore the invention may have application in the formation of ceramics in which particulate precursor material is placed in a container which is subjected to heat and pressure to form a block of ceramic material. Numerous prior proposals exist particularly in the field of immobilisation of high level radioactive waste in synthetic rock materials. It is essential to achieve uniform and reliable densification of the synthetic rock precursor since otherwise the exceedingly high levels of leach resistance required for safe disposal of the material cannot be assured. One known method is hot isostatic pressing (H.I.P.) in which a compressible container is filled with particulate material to be hot pressed, the container is evacuated and sealed and then placed in a hot isostatic press. This arrangement provides uniform pressure around the container so that uniform densification should be achieved. However, it is considered that this process, particularly when applied to immobilisation of radioactive waste, imposes serious potential problems because of the need for operations in an active cell, the difficulty of providing reliable evacuation and sealing of the containers for the waste, and the high cost of active cell space for accommodating very bulky and complex apparatus, namely a hot isostatic press. Such a press is massive and poses ultimate disposal problems when its working life is at a conclusion. Alternative proposals utilise hot uniaxial pressing (H.U.P.) of compressible containers, for example containers of cylindrical general form with a bellows-like sidewall. The present inventor's prior patent specifications disclose the use of an off-gas system to collect gas discharged from such a bellows container as it is hot pressed and hot upward pressing processes have been preferred. One such process

includes the use of a metal cylindrical container into which a bellows canister is inserted in an upward direction by a hydraulic ram and hot uniaxial pressing takes place, whereby during the densification process the small degree of radially outward expansion of the canister causes it to become firmly jammed into the container. A series of canisters are pressed one after the other into the container and the container then facilitates safe disposal.

However, with a view to identifying a system providing the highest potential reliability and serviceability, especially when radioactive waste is being processed in an active cell, the present invention has been conceived as a useful alternative to prior proposals.

According to a first aspect of the present invention, there is provided an apparatus for hot uniaxial processing of capsules containing material to be densified, the apparatus comprising a cylindrical tubular body along the axis of which the capsules are arranged to move during progressive uniaxial compression, heating means for heating the capsules as they pass along the tubular body, the capsules being of predetermined diameter and predetermined radial expansion during hot uniaxial pressing, the tubular body having a bore which in an intermediate region increases in diameter along the tubular body to match the radial expansion of the capsules, whereby the capsules can be pressed along the bore while having restraint to any excess radial expansion, means for introducing capsules to be processed at an upstream end of the tubular body, means for discharging compressed capsules at a downstream end of the body and means for mounting the tubular body for co-operation with pressure application means, the pressure application means comprising an upstream pressure applicator having a pressure pad adapted to abut a trailing end of the capsule at the upstream end of the tube and to move down the upstream end of the tube to advance capsules along the tube, and second pressure application means at the downstream end of the tube having a pressure pad to act as an abutment for the downstream capsule, and control means for controlling

operation of the first and second pressure application means to provide substantially continuous movement along the tubular body of the capsules while maintaining a predetermined axial pressure. In short, the present invention provides for a continuous hot pressing arrangement which provides significant process advantages over batch processes of the prior art.

The rate of increase in diameter along the tubular body may be constant or may vary. Preferably, the increase in bore diameter corresponds to an operating process having the following characteristics:

(a) The upstream end portion of the tubular body is arranged to provide steady heating of the capsule but with little axial compression taking place and accordingly only a low rate of increase in bore diameter is provided. Furthermore, the end capsule (which is in contact with the upstream pressure pad while being pressed steadily in a downstream direction) should be under temperature conditions such that at most a minor degree of heating occurs and the capsule is kept at a relatively low temperature such that little or no radial expansion takes place and accordingly the bore in this region can be parallel. Since the pressure pad is a clearance fit within the bore, a peripheral annular region on the trailing end face of the capsule will not be in contact with the pressure pad and, by adopting the preferred conditions, the risk of unacceptable deformation of the trailing end face of the capsule can be essentially avoided; in particular the risk of extrusion of the peripheral portion of the capsule around the edge of the pressure pad can be obviated. When a new capsule is introduced then the new capsule presses against the trailing end face of the previous capsule and it is fully supported and its temperature can be increased whereby small, εteady, radial expansion is possible.

(b) Typical proportions for a tubular body will be that the initial heating zone of the body accommodates three capsules or more and only a minor portion of the downstream

end of the heating zone provides temperatures where increased rate of radial expansion is experienced. In this zone a corresponding increase in bore diameter occurs.

(c) The central region of the body provides an elongated zone of active compression and will typically be similar in length to the heating zone but due to substantial axial compression will accommodate more capsules. A small steady increase in bore diameter occurs.

(d) The downstream end portion of the tubular body is a cooling zone in which the bore can be parallel and typically will hold at least twice the number of capsules as are accommodated in the heating zone.

(e) The dimensions of the tubular bore at working temperature relative to the capsules are chosen such that the capsules are supported but in non-binding engagement. A small clearance is provided. The surface of the capsule will be somewhat irregular although the bore of the tubular body can be an accurate cylinder. Thus, typically intermittent point-to-point contact will occur between the periphery of a capsule and the interior of the bore. Providing that the degree of restraint at any contact between the capsule and the bore is less than the force required for shear of the metal in which the capsule is made, constraint can be provided. With advantage the surface of the bore could be a ceramic thereby facilitating restraint of the capsule by the bore without adverse effects on the metal forming the capsule.

Preferably, the apparatus includes a computer control system including sensors and a microprocessor for controlling the pressure applied to the respective pressure application means so that the capsules move downstream at a selected rate.

Furthermore, it is very desirable to include a control system which monitors the pressures and detects the movement rate of the capsules and ^~ adjustment is made to the respective pressures to speed up the movement of the capsules along the bore if the measurements are consistent with build-up of frictional engagement between one or more

capsules and the wall of the bore.

It will be appreciated that an inherent and significant advantage of embodiments of the present invention is that the pressure pads associated with the upstream and downstream pressure application means are not in contact with capsules undergoing high temperature compression but instead are respectively in contact with much lower temperature capsules at the beginning of the preheating phase and at the end of the cool-down phase. This greatly facilitates the manufacture of pressure pads in desirable materials such as high temperature metals which can provide cost effectiveness, durability and resistance to thermal shock. This is a major advantage over some of the prior art proposals. The invention is especially advantageous when arranged in the form of a horizontally disposed tubular body. Of especially great advantage for processing of radioactive waste is an embodiment of the invention in which the pressure application means comprise hydraulic rams which are located outside an active cell and which apply their forces through operating rods passing through its respective sealed bearings through the active cell wall. This arrangement means that the press installations which are very bulky and require regular maintenance are fully accessible outside the cell and the equipment located in the active cell is very simple in design and can simply be replaced when worn out.

Where the tubular body is arranged horizontally, it can include mounting units in the intermediate portion of the tubular body and having connectors in the base to couple through the floor of the active cell to control equipment and a power supply for operating the heating means.

Especially when radioactive waste is being treated it is possible with the invention to incorporate an off-gas system since small amounts of volatile radioactive components may be released during the hot pressing operation. For this purpose, the pressure pad associated with the upstream pressure means can include a central

recess having means for coupling with a central tubular neck extending from the end of the capsule, the capsule having a ceramic filter for preventing the egress of particulate material as gaseous material is pressed out during densification.

Since the initial passage down the tubular body is a preheating phase and densification substantially occurs in the central region of the tubular body, the volatile components are driven off during the preheating phase and the remaining gas in the capsule can be safely discharged into an off-gas plant in the active cell during densification.

Preferably the tubular body is formed of a multiplicity of rigid interconnected sections. For example, the sections can be interconnected in a male to female connection and in a preferred embodiment the tubular body is of a high strength, thermally shock resistant ceramic material such as silicon nitride, silicon carbide or silicon carbide reinforced with silicon carbide whiskers. In a preferred embodiment the tubular body is formed with a spirally wound heating element for example of silicon carbide and it is preferred to surround the tube with a low mass - low thermal conductivity ceramic fibre insulation with a containment shell around the insulation to provide a composite structure which can be handled as a unit. This is most advantageous for applications to the densification of radioactive waste, since modules can be moved around within an active cell, and when a module requires replacement it can be lifted off its mounts, cut up for disposal and a replacement module placed in position.

For illustrative purposes only, an embodiment of the invention will now be described in more detail with reference to the accompanying drawings, of which:-

Fig. 1 illustrates the concept of the embodiment integrated into a synthetic rock-radioactive waste disposal plant;

Fig. 2 is a general arrangement side view of the continuous hot pressing facility in the system of Fig. 1;

Fig. 3 is a half, axial cross-section through a capsule for use in the method and incorporating a reducing gas coupling and, at the left hand end, an off-gas coupling; and Fig. 4 is a schematic diagram illustrating bore profile for an experimental embodiment of the invention.

Referring first to Fig. 1 there is schematically illustrated an impregnation plant 10 adapted to fill bellows-like capsules 11 which after sealing advance to a calcination plant 12 before being supplied to the continuous hot press 13.

The impregnation plant comprises a horizontally extending vessel 14 having at its downstream end an inlet 15 for synthetic rock precursor powder at its intermediate portion an off-gas port 16 leading to an off-gas system and at the downstream end an inlet 17 for high level radioactive waste (HLLW) . The upper region of the vessel contains a horizontally extending, cranked mixing blades 18 driven by a motor 19 and the lower portion of the vessel has a screw conveyor 20 driven by a motor 21 for advancing the powder material in a downstream direction. A heater not shown in the drawing is provided so that mixing and drying of the liquid high level waste and the synthetic rock powder occurs. The impregnated powder advances at the downstream end to a vertical discharge tube 22 having a downwardly directed screw conveyor 23 driven by a motor 24 for metering impregnated powder into a series of bellows-like capsules 11. The bottom of each capsule has a reducing gas coupling 25 described later with reference to Fig. 3 and when the upper end of the capsule is sealed with a closure at sealing station 26 an end wall with an off-gas installation as described below in more detail with reference to Fig. 4 is installed. After closure the capsule is advanced to the calcination station 12 where it is placed inside and heating furnace 28 and using quarter turn bayonet fit couplings, the upper end of the capsule is connected through connector 29 to an off-gas system and the lower end is connected to a reducing gas coupling 30. During the calcination process the temperature is increased to around 800°C while reducing

gas is supplied under pressure through coupling 30 to provide the correct process conditions. The gas discharged at the upper end is processed in an off-gas system to remove radioactive volatile components. In the continuous hot press 13, a capsule undergoes radial expansion due to compression exerted at the ends of the capsule but the major change in dimension is in the overall length of the capsule which in a typical case reduces to approximately one third its original size. Thus the continuous hot press 13 operates such that three fully compacted and compressed capsules are removed prior to the insertion of one new capsule. Fig. 1 illustrates the hot press only schematically and it will be seen that a practical arrangement as illustrated in Fig. 2 is adapted to contain a much greater number than three capsules.

The hot press 13 comprises a rigid densification tube 31 incorporating a heating element controlled such that the capsule is heated after initial insertion at the left-hand end of the press, undergoes higher temperature densification in the central region and undergoes a cool down e.g. to

900°C at the downstream right-hand end before removal from the press. Uniaxial pressing is achieved by the use of upstream and downstream hydraulically controlled rams 32 and 33 which are computer controlled so as to maintain the required pressures and to position the group of capsules in the densification tube in accordance with the preferred processing. Fig. 1 also shows an off-gas duct 34 incorporated in the upstream ram unit whereby it is connected to the trailing end of each capsule 11 so that during the preheat phase and while the capsule is the extreme downstream capsule, its end is connected so that volatile components are removed in the off-gas system. It has been found that once the working temperature of the capsule has been reached and compression and densification has commenced, no further volatile components requiring off-gas treatment arise and off-gas connection is not required when the capsule has moved downstream into the higher temperature densification region and a fresh capsule

is introduced into the upstream position.

Turning now to Fig. 2, the hot press of Fig. 1 is shown in general arrangement as applied to processing of radioactive waste and located in an active cell having a floor 40 and end walls 41 and 42. In Fig. 2 like parts have been given like reference numerals.

The densification tube 31 is fabricated from a series of sections interconnected with bell & spigot connections 31A and 31B. The tube is preferably fabricated from a high strength, thermally shock resistant ceramic material and incorporates ceramic heating elements arranged to provide a control of temperature gradient for the capsules. The densification tube is surrounded by low mass insulation 31C and a containment shell 31D surrounds the insulation and provides a structure to facilitate handling of the densification tube. At the two bell and spigot connections shown in the drawing, the densification tube is supported and mounted on sliding cradles 43 having in their respective base portions connectors 44 for interconnecting the power and thermal control connectors located in mounting blocks 45. Thus all connections to the operating system can be through the floor of the cell 40 and being outside the cell can readily be maintained. Furthermore, the sliding base portions of the cradles 43 are mounted on hydraulically driven actuators 43A which through an auxiliary mechanical or hydraulic drive adjust vertically the height of the cradles to adjust for thermal expansion characteristics. Thus the axis of the tube can be maintained in accurate alignment with the axis of the pressure pad system. The location of the densification tube is controlled by two hydraulic rams 43B and 43C and these rams accommodate thermal expansion of the densification tube.

At its left hand end, the densification tube 31 is coupled to an inlet port section 46 having an inlet port 46A for receiving downwardly a preheated, calcined but uncompressed capsule 11. The downstream right hand end of the tube 31 is coupled to a discharge tube section 47 having discharge port 48 from which are discharged compressed

capsules 11A. Although the schematic drawing of Fig. 2 for convenience shows the inlet port 46A and discharge port 48 located uppermost, in practice it may be preferred to locate both of these ports in a different location e.g. on the side of the tube.

The inlet and outlet tubular sections 46 and 47 accommodate respective cylindrical ceramic or metal pressure pads 49 and 50, the upstream pad incorporating the off-gas discharge duct 34 leading from a cylindrical recess 51 in the central face of the pressure pad. Each of the pressure pads is mounted on a respective pusher rod 52 which extends through seals in an aperture in the cell wall for coupling to respective hydraulic actuators of the hydraulic ram systems 32 and 33. Since the pressure pads are not in contact with the hottest capsules undergoing axial compression but instead are at a relatively low temperature, a wide choice of materials is available. Preferably the pressure pads are of nimonic metal and this is considered to be most cost effective providing a material which is resistant to thermal shock, durable and machineable and therefore contributes to the provision of a long life structure for use in a hot cell.

The densification tube 31 has a predetermined diameter at its left hand end such that the capsules are a sliding fit. In the region of densification, i.e. at the location around the left hand bell and spigot connection, the bore of the tube tapers as shown at 53 so that the capsule remains a sliding fit despite radial expansion under the heat and pressure which is applied. Before describing operation of the hot press of Fig. 2, a detailed description will be given of the capsule with reference to Fig. 3. In Fig. 1 there is illustrated the calcining and preheating of the capsule and during this process it is necessary to maintain a flow of reducing gas through the capsule. Accordingly, one end of the capsule (which is shown lowermost in Fig. 1 and to the right hand end in Fig. 3) has a reducing gas port 65 whereas the opposite end of the capsule has a similarly shaped off-gas

port 70. The ports are similar and are used with bayonet connectors. The capsule 60 has a thin wall of metal which is highly resistant to corrosion and suitable for use at the temperatures needed to densify synthetic rock. In the central region of each end wall, outwardly extending tubular necks 61 are provided to form the ports 65 and 70, each neck having an outwardly extending rib arrangement 63 for bayonet coupling to the system connectors. A shouldered inner cap 68 is welded around its periphery to the inner face of each end wall 60A and 60B of the capsule and each cap is filled with a ceramic fibre 69 to form a filter.

So that collection of volatile components into an off-gas system can occur during the continuous hot pressing operation, the port 65 which will be at the leading end of the capsule needs to be sealed. Although it is possible that impact of the neck 61 on the trailing end of the previous capsule may crush the neck to form a seal, this may be unreliable and it is preferable to crimp the neck 61 closed before the capsule is supplied to the continuous hot press. The process conditions are controlled such that volatile components requiring off-gas treatment arise in the capsule at the extreme upstream end of the tube and when a further capsule is to be inserted, the pressure application pad and the associated off-gas coupling are retracted as shown in Fig. 2 and a new capsule inserted. The leading end of this capsule then is pressed against the trailing neck 61 on the previous capsule and crushes it. It will be appreciated that instead of the calcining and preheating conducted in the capsule, calcining of the powder material can take place in a separate vessel such as a rotary calciner under reducing gas conditions and then the capsule is simply filled with the powder material; with this process no gas coupling is needed at the end of the capsule which will be leading. Referring back to Fig. 2, the sequence of operations is that when sufficient space exists in the densification tube for receiving a further capsule 11, the left hand press 32 is withdrawn to permit a new capsule to be inserted and the

pressure pad 49 is then moved forwardly to press the capsule into the pre-heat zone in the densification tube 31. The leading end of the capsule comprises end wall 60A with the associated reducing gas port 65 which on the application of pressure is crushed against the trailing end of the previous capsule, (unless the option of crimping the neck 61 has been taken) .

The trailing end of the capsule 11 has its off-gas port 70 engaging within the recess 51 in the pressure pad 34 and thus is connected to an off-gas system. Gaseous components within the capsule pass through the perforated end cap 68, through the ceramic fibre filter 69 and out into the off-gas system as the pressure pad is applied to exert uniaxial pressure on the end wall 71. When the capsule has been preheated and downstream capsules further compressed sufficiently, and the downstream hydraulic ram 33 withdrawn, capsules 11 can be ejected through the discharge port 48 and then the downstream pressure pad 50 is advanced by the hydraulic drive system 33 to engage the leading edge of the last remaining capsule within the densification tube. The upstream ram 32 can withdraw the pressure pad 34 from the tube 31 to the fully retracted position shown in full lines in Fig. 2 and a new capsule 11 inserted. A new capsule is pushed to the right and the gas reduction port 65 engages against the port 70 of the previous capsule and the respective necks forming the ports crush down on the application of pressure. Thus this system eliminates crimping and total sealing of capsules which is required in the HIP process. Referring now to Fig.4 there is schematically illustrated the profile of a densification tube. Typical dimensions are given for an experimental unit with a 54mm nominal diameter capsule and operating on a modified synthetic rock precursor which will densify at temperatures lower than those experienced for synthetic rock precursors suitable for immobilising radioactive waste. Similar experiments can be conducted to determine dimensions for other embodiments such as commercial scale embodiments for

radioactive waste and synthetic rock which typically would utilise capsules of the order of 300mm diameter.