WO1995026558A1

WO1995026558A1 - Treating radioactive, toxic or other hazardous waste

Info

Publication number: WO1995026558A1
Application number: PCT/EP1995/001122
Authority: WO
Inventors: Graham Andrew Fairhall; Terence Hough; Brian Thompson; Arthur Robinson; John Guy Walker; Jarrod Richard Haworth
Original assignee: British Nuclear Fuels Plc
Priority date: 1994-03-25
Filing date: 1995-03-24
Publication date: 1995-10-05
Also published as: GB9405988D0

Abstract

Radioactive, toxic or hazardous waste, in particular intermediate level sludge waste containing metal items and containers, some of which include pyrophoric material, is treated by drying the waste in containers in at least one waste dryer (34) and reducing the volume of the containers in a compacter (32), the dryer and compacter both being operated in a protective sealed environnement (14-16) which confines to the sealed environment the radiation or contamination resulting from the waste. Remote controlled manipulators and the like (e.g. gantry manipulator (37)) are used for handling the waste and containers within the sealed environnement (14-16). Prior to drying, oversize pyrophoric material, identified by screen separation of the waste into oversize and undersize and subsequent inspection of the oversize, is introduced into containers with undersize (consisting mostly of sludge), ready for drying. This avoids any risk of spontaneous ignition of the pyrophoric material.

Description

TREATING RADIOACTIVE. TOXIC OR OTHER HAZARDOUS WASTE

The present invention is concerned with treating radioactive, toxic or other hazardous waste. The description hereinbelow relates principally to treating radioactive waste, because that is the application for which the invention was developed. However, the treatment method is equally applicable to treating toxic, biologically dangerous or other hazardous waste, i.e. that is potentially harmful to humans or creatures on exposure to radiation from, or on contact with, such waste.

In the nuclear industry, spent nuclear fuel is reprocessed because most of its content, e.g. uranium and plutoniu , are reusable. However, a proportion of radioactive waste is produced.

Radioactive waste divides into three categories, depending on the amount of radioactivity it contains - low, intermediate and high-level. Low level wastes are ones that typically need to be handled by personnel wearing protective clothing, but no physical barrier of substantial construction, e.g. concrete, is additionally required to shield operating personnel from radiation. High level waste, on the other hand, has a high enough radioactive content that it continues to generate heat, due to radioactive decay, in sufficient quantities that special cooling measures are needed to prevent the temperature of the waste from rising to unacceptably high levels. Intermediate level waste has radioactivity levels between these two extremes. Solid radioactive wastes are in fact categorised in the UK as follows (Radioactive Waste Management Advisory committee 5th Annual Report June 1984) :

High Level Waste (HLW) : significantly self heating wastes

Intermediate Level Waste : more than 4E9 Bq per te alpha or more than 12E9 Bq per te beta/gamma but not self heating.

Low Level Waste (LLW) : less than 4E9 Bq per te alpha and/or less than 12E9 Bq per te beta/gamma

Techniques for processing low level waste are well known and do not present major difficulties in handling, while ensuring adequate protection for operating personnel and humans and other living creatures seldom or regularly in the vicinity of the plant.

At the other extreme, high level waste, which in relative terms is produced in very small quantities indeed, requires special handling.

On the other hand, intermediate level waste is produced in relatively large quantities and although it is not as difficult to handle as high level waste it requires more careful and specialised handling than low level waste. There is therefore a need for treating intermediate level waste in a manner which is both safe and able to handle a sufficient throughput. For many years now, it has been commonplace to store intermediate level waste pending further treatment prior to final disposal. Since intermediate level waste is characterised solely by an approximate range of radioactivity, any waste materials of the requisite radioactivity are classified as intermediate level wastes. Therefore, such wastes, in addition to solid or liquid waste resulting from the reprocessing of nuclear fuel, also include contaminated components and materials. Over long periods of time, e.g. 20 to 30 years, some types of radioactive waste, e.g. fuel cladding, decompose to a sludge-like product including magnesium, magnesium hydroxide, uranium, uranium hydride, uranium oxide, dissolved hydrogen and water, while other types of waste remain largely unaffected. The waste product typically varies in moisture content (wet and runny to relatively dry and firm) , particle size distribution (coarse to colloidal) and components (e.g. % sludge, metallic items etc) . When the sludge is dry and firm, it resembles a clay-like material. Hereinafter, the term "sludge" will be understood to include a waste product resembling a clay.

While safety standards in the nuclear industry continue to rise and radioactive waste initially stored several years ago has decomposed to a sludge, the operators are under pressure from the regulators to deal with intermediate level radioactive waste in an improved way.

In the case of low level waste, it is known to load the waste into drum-like containers that are heated up in a special oven having radiant heaters at the bottom, so that the liquid content, principally water, of the radioactive sludge is removed by vapourisation. The dried waste product, which is a solid containing a multiplicity of minute internal voids distributed throughout its bulk where occluded liquid had previously been, is then removed in its drum from the oven and inserted into a compactor which applies an extremely large compressive force to the dry waste and its encasing drum, so as to compress them to a minimum volume by squashing the voids flat, as far as possible. In this way, the external volume of the compressed final product is considerably smaller than that of the wet waste sludge from which it was made and the final solid form is more suitable for further storage.

Although in principle the idea of drying a radioactive sludge and then compacting it is also applicable for intermediate level radioactive waste, special difficulties arise, because operating personnel have to be protected against exposure to radiation from the intermediate level waste, for which protective clothing above is insufficient. Therefore, a need exists for a plant for handling intermediate level waste by drying and compaction, providing protection for operating personnel.

In addition, as mentioned above, intermediate level waste sludges generally contain metal objects, such as old gearboxes, steel tubes, swarf (obtained by peeling off metal fins from spent fuel rods) and sealed metal containers of radioactive waste. These metal objects present additional difficulties for three reasons. Firstly, the sealed metal containers must be removed before the sludge is dried because otherwise there is a risk that a container may explode in the drying process. Secondly, metal components of more than a certain size are too large to insert into the sacrificial containers in which the sludge is to be dried. Lastly, however, it is not sufficient simply to separate all the metal components from the sludge because some of them include uranium or uranium compounds, which carries the risk of spontaneous ignition of uranium hydride, this being pyrophoric when dry and exposed to air.

According to the invention from a first aspect, there is provided apparatus for treating radioactive, toxic or hazardous waste, comprising a dryer for drying the waste in a container, a compacter for compacting the container and its dried waste to reduce the volume that they occupy, a protective cell in which the dryer and at least part of the compacter are disposed, for confining to the cell radiation and radioactive, toxic or hazardous contamination resulting from the waste, and means associated with said protective cell and operable from outside the cell for transporting said container from said dryer to said compacter.

Preferably, the protective cell includes a storage area for compacted containers and the container transporting means, which conveniently comprises a travelling gantry carrying at least one manipulator, is also operable for transporting compacted containers from said compacter to said storage area.

For handling a waste feed including material that is too large for handling in the container, or where it is necessary or desirable to remove material which otherwise could cause problems in the dryers e.g. sealed cans which are liable to explode when heated, a preferred embodiment further comprises a separator (e.g. a vibrating screen separator) for separating oversize from the feed to leave undersize, a conveyor, and means for loading the resulting undersize into empty containers on the conveyor, the conveyor being arranged to transport the containers to the container transporting means, and the separator, conveyor and undersize loading means all being located in the protective cell.

Where the oversize contains pyrophoric material, e.g. uranium hydride, the cell may further include a size reduction area in said cell and manipulator means associated with said size reduction area and operable from outside the cell for separating pyrophoric material from the oversize collected by the separator, collecting the pyrophoric material, reducing the size of collected pyrophoric material that is larger than a predetermined size, and transporting the size-reduced material to containers on the conveyor already containing undersize.

The waste treatment apparatus may further comprise a container lidding machine associated with the conveyor and arranged to secure lids to the containers before they arrive at the container transporting means. The lids help to prevent any waste spilling from the containers during transit to the dryer.

Preferably, the dryer comprises a drying chamber for at least one container of waste, a heater located outside the drying chamber for heating a fluid, and means for conveying the heated fluid to the drying chamber for heating the waste in the container.

Because the heater is located outside the drying chamber, there is no risk of radioactive waste that may be spilled from the container coming into contact with the heater. Therefore there is no risk of fire caused in this way. Furthermore and because a fluid is used for transferring heat from the heater to the container, no hot spots can arise.

The fluid that is heated may be a liquid or a gas. However, it is less desirable for the fluid to be a liquid because liquids that can withstand high temperatures (up to about 300°C is required) are generally oils. There is a small possibility that the liquid in the closed circuit may become contaminated with radioactivity from the waste and, as is well known, it is very difficult to remove radioactive contamination from oils.

Therefore it is preferred that the heating fluid be a gas, which for convenience is air, since radioactive contamination, e.g. contaminated dust particles, can easily be removed by suitable filters, which are well known in the art.

For energy efficiency and to reduce the spread of contamination from the container to the heated fluid, it is preferred that the heated gas conveying means comprises a closed circuit including a fan operable for circulating air from the heater to the drying chamber and back to the heater, and at least one thermocouple for controlling the heat output of the heater.

The design of the enclosure should be such as to permit a container to be loaded with a quantity of radioactive sludge to be dried before the container is loaded into the drying chamber. Therefore, the enclosure may have a cover which is removable for loading the container into and from the drying chamber. The container will also have a lid with an opening in it for the passage of vapourised liquid produced by the wet radioactive waste in the container, and a gas line is provided for conveying the vapour from the container to the exterior of the enclosure.

It is particularly preferred that the dryer enclosure be able to accommodate a plurality of containers, so that a number of containers can be dried at the same time using only a single circulating gas heater system. Conveniently, the several containers are carried in a stillage which is itself removably held in the drying chamber. This means that the containers can firstly be positioned in the stillage, after which they are loaded at the same time, in the stillage, into the drying chamber. Conversely, they can be removed at the same time by lifting the stillage out through the top of the enclosure after previously removing its cover. Alternatively the containers can be loaded and unloaded individually without removing the stillage from the dryer.

Each container may be supported on a plurality of guide baffles mounted inside the stillage, for - 9 -

inducing spin in the flow of heated gas. This ensures uniform air and heat distribution and improves the heat transfer rate.

In accordance with a preferred embodiment of the invention, the compacter comprises a press table for supporting a container to be compacted, a ram mounted above the press table and extending generally in an upward direction, the ram having a replaceable ram plate on the lower end thereof, said ram plate being releasably held in position by first securing means, e.g. securing bolts, which is operable from an upper region of the ram for releasing the ram plate, hydraulic pressure means for urging the ram towards the press table to compact the container between the lower end of the ram and the press table and for withdrawing the ram from the container, and a mould that is positioned or positionable above and adjacent the press table for surrounding the container during compaction, the mould having a replaceable internal liner releasably secured in the mould by second securing means (e.g. securing bolts) which is operable from the top of the mould for releasing the mould liner.

Because the mould liner and the ram plate can be released from the top of the mould and an upper region (e.g. the top end) of the ram, respectively, which, with the compacter installed in a waste handling plant having a protective cell for the safe handling of the waste, would both be positioned outside the sealed environment in a safe handling area for operating personnel, the required maintenance i.e. replacement of the ram plate and mould liner when they have become worn, can be performed without any risk of exposing operating personnel to harmful radiation. According to a preferred embodiment, the hydraulic means comprises a single-acting, hydraulic ram cylinder and co-operating ram piston, one of which is connected to a plurality of upright support columns at or near an upper end thereof and the other of which is connected to said ram which is mounted generally parallel to the support columns and in a central location and is united with a cross-piece which is movably mounted on the support columns, so that hydraulic fluid supplied under pressure to said hydraulic cylinder causes said ram to move downwardly to compact the container against the press table, and at least two return hydraulic cylinders and respective co-operating pistons arranged on opposite sides of the ram cylinder, one of each return cylinder and co-operating piston being, near their upper ends and the other being connected to said cross-piece.

It is desirable for the compacter to be installed with the press plate located in a sealed environment and the support columns to extend upwardly through a transverse structure interconnecting the support columns and forming part of the upper boundary of the sealed environment, the mould extending downwardly through an opening in the further transverse structure which carries a seal which seals around the exterior of the mould. Even if any hydraulic fluid that has leaked from either double-acting cylinder has found its way down to the top surface of the roofing section where it is present as a few drops or a small pool, the leaked hydraulic fluid can reliably be prevented from entering the radioactive environment by the seal. According to the invention from another aspect, there is provided a method of treating radioactive, toxic or hazardous waste, comprising drying a container of radioactive, toxic or hazardous waste in a protective cell which confines to the cell radiation and radioactive, toxic or hazardous contamination resulting from the waste, and compacting the container and its dried waste in said protective cell to reduce the volume that they occupy.

According to the invention from a third aspect, there is provided a method of treating radioactive, toxic or other hazardous waste in the form of a sludge containing pyrophoric material, comprising the steps of:

(a) separating oversize from the sludge to leave undersize,

(b) separating the oversize into pyrophoric oversize and non-pyrophoric oversize,

(c) loading pyrophoric oversize and the undersize into a container, so that the undersize blankets the pyrophoric oversize to prevent spontaneous ignition thereof,

(d) drying the undersize and pyrophoric oversize in said container to form a dry product, and

(e) compacting the container and its dry product to reduce the volume that they occupy. In this way, spontaneous ignition of the pyrophoric material can be prevented and the pyrophoric material safely treated, for final disposal. Furthermore, step (b) can be used for removing oversize sealed containers in the sludge that, potentially, could otherwise explode in the drying step. Also, this step can be used for separating metal items that are too large to insert in the sacrificial waste container.

To reduce still further any risk of spontaneous ignition of pyrophoric material, preferably the oversize is kept dampened with water sprays between steps (a) and (b) and the pyrophoric oversize is kept dampened with water sprays between steps (b) and (c) .

Between steps (b) and (c) the pyrophoric oversize that is larger than a certain size is cropped to reduce its size. This ensures the pyrophoric material is of a manageable size for reintroduction into the container.

According to a convenient way of treating the non-pyrophoric oversize that is larger than a certain size, it is reduced in size and loaded into an empty container which is compacted to reduce its size without prior drying.

Preferably, a plurality of compacted containers with their dry product are introduced into an encapsulation drum, and grouting or cement introduced into the drum to encapsulate the containers and their dry products. The encapsulation drum can then be held in a long-term store for final disposal.

Usually, all the defined steps of the method are carried out within a protective cell which confines to the cell radiation and radioactive, toxic or other hazardous contamination resulting from the waste. In this way, operating personnel outside the cell are not at risk from radiation or contamination.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:-

Figure 1 is a flow diagram of a preferred treatment process for an intermediate level waste feed comprising sludge incorporating metal pieces and sealed containers of radioactive waste,

Figure 2 is a plan view of one floor of a waste treatment plant, in which the main steps of the process according to Figure 1 are carried out,

Figure 3 is a general perspective view in the region of a screening station used for separating oversize from the sludge,

Figures 5 and 6 are horizontal sectional views taken along the lines II-II and III-III, respectively in Figure 4 ;

Figure 7 is a vertical sectional view taken along the line IV-IV in Figure 4; Figure 8 is a side elevational view of a stillage in which four containers of radioactive sludge are positioned, only two of these containers being visible in Figure 8;

Figure 9 is a schematic view of a system for handling and treating the offgas produced from a number of dryers located in a common protective cell;

Figure 10 is a vertical sectional view through a preferred form of compacter which is installed in a protective cell that is horizontally divided to provide an upper working environment, an intermediate environment and a lower environment, in the last of which container compaction takes place;

Figure 11 is a vertical sectional view on an enlarged scale of the compacter of Figure 10, taken along the line IV-IV of Figure 13;

Figure 11A is an enlarged detail of part of Figure 10;

Figure 12 is a side elevational view of the compacter of Figure 11 as seen in the direction of arrow V in Figure 13, a double-acting cylinder/piston arrangement and its connection to a mould cross-piece being omitted for clarity; and

Figure 13 is a plan view of the compacter shown in Figure 11.

Referring to the flow chart of the batch waste treating process according to Figure 1, feed 1 comprising intermediate level radioactive sludge including metal items and sealed containers is subjected to a physical separation process 2, using a vibrating screen, to separate oversize from the sludge. The screen mesh size is chosen so as to separate sealed containers and metal items of a size that potentially might present a safety concern if they were to be subjected to drying. The metal objects and containers are retained by the screen are termed "oversize".

In view of the presence of pyrophoric uranium hydride in the feed, dampening water is used to keep the feed wetted, to guarantee safe handling. The undersize is collected in an undersize collection vessel where most of the associated water is decanted, and then the remaining undersize is fed into sacrificial containers (step 3) to a level compatible with the downstream drying process (step 5).

The oversize material from the screening stage is inspected and sorted (step 9) , either manually or automatically. All sealed waste containers potentially containing uranium or uranium compounds are opened and, using a uranium monitor, any items containing uranium or uranium compounds, whether from the opened containers or not, are removed and routed to and loaded into the undersize sacrificial containers, the larger items being reduced in size by on-line cropping (step 10) . The dotted flow line in Figure 1 indicates that uranium and uranium compounds are routed to the undersize sacrificial containers only infrequently. Of the remaining oversize, any items that are unsuitable for compaction are placed in non-sacrificial containers, the remainder being introduced into sacrificial containers (step 12) , the larger oversize items being reduced in size (step 11) beforehand.

Water sprays keep wet the oversize separated by the vibrating screen and subjected to inspection and sorting, and also the items identified as containing uranium and uranium compounds, which are routed to the undersize sacrificial containers. Once these items are introduced into the sacrificial containers, however, and submerged in the sludge, the latter will generally serve as a blanket to exclude air from contact with uranium hydride, and thereby prevent spontaneous ignition and propagation of fire. If insufficient sludge is available, blanketing material, e.g. sand, may need to be added to the undersize containers to ensure that the uranium material is adequately blanketed. If the sludge is of a clay-like consistency which is unsuitable for blanketing the uranium material, the latter is stored under water until a container is available containing sludge of a suitable consistency.

Following step 3, lids are fitted to the undersize sacrificial containers in step 4 and then the containers are transported to drying station (step 5) , where they are loaded into an array of dryers, which are described in detail with reference to Figures 4 to 9. The containers are dried for a predetermined period to give a moisture content in a range satisfactory for subsequent compaction and long-term product stability.

Although the reason why the pyrophoric waste material, when treated in the manner described, is not liable to spontaneous ignition is not fully understood, it is believed that this may be due not only to the blanketing of the pyrophoric uranium material by the sludge but also partly to the uranium hydride being decomposed in the drying process. After drying is completed, the containers are removed from the dryers and transported to the compacting station (step 6) .

In step 4, lids are also fitted to the overize sacrificial containers which are transported directly to the compaction station as indicated in Figure 1 by flowline 13, since the moisture content of the oversize, being essentially metal parts, is very low, so that no drying step is needed before the compaction process.

In step 6, undersize and oversize sacrificial containers are compacted by a large applied force into compacted containers or "pucks" . Pucks are measured for height, weight and gamma radiation level before being sent to a puck buffer store. Pucks are selected from the store according to size and shape to give optimal loading into encapsulation drums, which entails stacking the combination of pucks which best fills the available volume in the drum (step 7) . The stacking of the drums includes selection also from the non-sacrificial containers containing oversize that was unsuitable for compaction.

Lastly, as indicated by step 8, the filled encapsulation drums are transported to a grouting station, for grouting or cementing the container contents in place. It may be necessary to cool the containers following removal from the dryers, under forced or natural cooling conditions, before they are transported to the compacting station, because otherwise the residual heat may affect the strength and integrity of the grouting in the encapsulation drums. Alternatively, cooling could take place after compaction. However, it is believed that cooling before compaction is preferable since then, ongoing corrosion of metal by residual moisture will mostly be complete before compaction takes place. Therefore distortion of the puck is less likely.

Referring now to Figure 2, there is shown a plan view of one floor of an intermediate level waste treatment plant for carrying out the treatment process as described with reference to Figure 1. The plant comprises a plurality of concrete walls 13, depicted as having a significant thickness (e.g. 1.2m) and defining three cells 14, 15, 16, which intercommunicate with each other and together define a single sealed and ventilated environment in which the radioactive waste is processed in a manner to be described, the walls 13 serving to shield operating and maintenance staff from radiation and radioactive contamination. Inspection windows 17 allow visual inspection of the operations taking place, in particular within the cells 14 and 16.

With reference to Figure 3 which is a perspective view of part of the interior of cell 14, waste to be treated is transported to the cell 14 in an open-topped skip 18 and introduced into the cell 14 through a transfer gate (not shown) , which enables the boundary of the cell 14 to be crossed without release of radiation or contamination through the transfer gate. Transfer gates are well known in the art and do not need to be described further herein.

The skip 18 travels on a rail and bogie system 19 to a skip tipping station at which a skip tipping device 20 empties waste from the skip onto vibrating screen 21 which is positioned over an undersize collection vessel 22. After the skip has been emptied, it is removed from the cell on the bogie which returns with the next skip load of waste to be treated.

The undersize (mainly sludge) that passes through the screen mesh is collected in collection vessel 22, from where it is loaded by grab 23, remotely controlled by operating personnel on the outside of the wall 13, into a succession of empty, open-topped, containers 24 at the beginning of a conveyor 25, which conveys the containers, sucessively to a lidding machine 26, which fits lids to the containers, and from there through cell 15, to a dryer station 27 (Figure 2) .

The screen 21 is movably mounted, e.g. on rails, above the collection vessel 22, so that after the undersize has passed through the screen, it can be displaced to the far end of the rails, so that the oversize on the screen 21 can be inspected and sorted, using remotely operated manipulators 28. Uranium and uranium compounds items, identified by a uranium detector (not shown) , are placed in a holding bath 29 containing water. Items from the uranium holding bath 29 are periodically transferred to a size reduction area 30, in which large items are cut up. Sufficiently small uranium-containing items from the holding bath and cut up items from the size reduction area are periodically transferred to partially-filled undersize containers before lids are fitted to them. Alternatively, they may be transferred to the empty containers 24 at the beginning of the conveyor 25, before the undersize is loaded into these containers at the loading station.

Suitable dampening sprays (not shown) keep the oversize dampened at all times, until all uranium-containing items have been identified and removed, and also keep the uranium-containing items themselves permanently dampened, until they have been safely loaded into the sacrificial undersize containers 24. Oversize identified in the inspection and sorting as suitable for oversize sacrificial containers 31, one of which is shown in Figure 3, is loaded directly into these containers, which are periodically transported by conveyor 25, for loading into compacter 32. All these manipulations can be carried out by the manipulators 28, either alone or jointly with other remote handling devices.

With reference to Figure 2, at the dryer station 27 is provided a plurality of dryers 35, shown by way of example in Figure 2 as twelve dryers arranged in two groups of six with each dryer being able to dry four containers at once. Undersize and oversize containers are transported to the end of the conveyor, where an overhead travelling gantry with at least one manipulator, diagrammatically shown at 37 and which can travel between the end of the conveyor and the far end of a compacted container (or "puck") store 33, is remotely operable to transfer undersize containers individually into the dryers 35 and oversize containers directly to a transfer carousel 34, with associated transport system, for loading and unloading containers into compacter 32.

Each dryer 35 preferably includes a stillage or basket (as will be described in detail hereinbelow) for carrying the containers in the dryer, and the gantry manipulator loads the undersize containers individually into the stillage. Alternatively, the empty containers 24 may themselves be loaded into stillages which are transported, through the waste tipping, container filling and container lidding stations, to the end of the conveyor, where the gantry manipulator loads the loaded stillages into the dryers. After drying is completed, the gantry manipulator 37 delivers the dry waste containers to the transfer carousel 34.

As described above in connection with Figure 1, it may be desirable or necessary to cool the dried-waste containers before they are to be compacted into pucks. In that case, a cooling area will also need to be provided in cell 16, which conveniently could be located between the dryer station 27 and the puck store 33. It may be convenient to employ a second gantry manipulator in place of the transfer carousel, in view of the additional container/puck handling capability necessitated by the cooling area.

In the compacter 32, underize and oversize containers and their contents are compacted, and then the resulting pucks are off-loaded by the transfer carousel 35 from the compacter 32 to the puck store 33. The gantry manipulator 37 is used for latching stacks of selected pucks, each stack representing an optimal filling for an empty encapsulation drum, into a cradle and loading the cradle into an empty drum on a transfer bogie at one end of a transfer tunnel extending below the level of the floor of cells 14, 15, 16 below an export transfer gate 36 in the floor of cell 16 adjacent the puck store 33. The export transfer gate is used for exporting the loaded cradle from the sealed and ventilated environment of the cells 14, 15, 16 into the different environment, also sealed and ventilated, maintained within the tunnel.

The encapsulation drum is transported through the tunnel to an encapsulation station, at which grout or cement is introduced into the drum to encapsulate the pucks. Finally, the drums are transported to a suitable long-term store.

The dryers 35 at the drying station, which form the subject of our co-pending UK Patent application No. 9405924.3, will now be described in detail. Referring firstly to Figures 4 to 6, each dryer 35 comprises an enclosure 130 having a bottom wall 131 upright side walls 132 and a removable cover 133 which together define a drying chamber 160. In order to minimise heat loss from the enclosure, its bottom wall, side walls and cover are all of double-walled construction, comprising spaced apart outer and inner walls, e.g. of stainless steel, with heat insulating material 134 between them.

The dryer is designed to carry four sludge containers, though it will be appreciated that the number of containers for which the enclosure 132 is designed can be less than or greater than four. At the lower extreme, the enclosure can accommodate only a single container. To distinguish between the empty containers 24 in cell 14 (Figure 3) and these containers when loaded with sludge (and pyrophoric uranium material) , the latter are denoted by reference numeral 135.

Each container 135 is in the form of a vertically disposed, cylindrical drum which is closed at both ends, its upper end or lid 136 being formed with a central hole 137 through which passes evaporated liquid or "offgas", generated as the radioactive sludge is heated in the dryer.

As best shown with reference to Figures 4, 5 and 6, the four containers are removably carried on respective baffle assemblies, each comprising a plurality of guide baffles 141 whose function will be described hereinbelow, inside a stillage or crate 138 which fits snugly within the drying chamber 160, a rim 139 formed around the top peripheral edge of the stillage resting on an annular shoulder 140 formed on the inside of the side walls 134. The stillage has its bottom formed with a high proportion of openings (not specifically referenced) for the passage of hot air through them.

A central conduit 142 extends vertically from a lower end 143 positioned just below the bottom of the stillage 138 to an upper end comprising an end plate 144 blanking off the upper end and four inlet apertures 145 equally spaced around the peripheral surface of the central conduit 142 adjacent to the end plate 144. The conduit forms part of a closed loop, heated air, circulation system which is now to be described.

As best shown in Figures 4, 5 and 6, the enclosure 130 is extended to one side (i.e. the left side) to form an open-topped housing 146 in which is located a removable unit 147 which comprises a body 148 in which is mounted a conduit 149, comprising conduit sections 149a, 149b, 149c, an air heater unit 150 in the conduit section 149c, a fan unit 151 for maintaining air circulation through conduit 149 and temperature measuring thermocouples 152 which sense the temperature in the conduit section 149b. As in the case of the enclosure walls, bottom and cover, the walls of the housing 146 and the removable unit 147 are of double-skin construction with insulation filling the space between the walls.

The electric motor section of the fan unit 151 extends above the upper surface of the removable unit so that all electrical connections to the fan unit are readily accessible from the exterior. The particular means by which the removable unit is located in the housing 146 is not material, though it needs to be designed for easy replacement by a robot or remote control. Suitably, a plug-in fit can be provided between the removable unit 147 and the housing 146.

With reference to Figures 4, 6 and 7, air is drawn in to conduit section 149a by fan unit 151 from a conduit section 153 which is located in the bottom section of the enclosure 130 and is approximately U-shaped, having an upwardly angled inlet end 154, into which the lower, outlet end, of the central conduit 142 of the stillage 138 is received, an upwardly angled outlet end 155 which connects with the inlet end of conduit section 149a, and a central section 156 which connects the inlet and outlet angled sections 154, 155.

Similarly, air circulated by the fan 151 through conduit section 149b and heated by heater unit 50 passes through conduit section 149c into a short angled conduit section 157, whose inlet end is of the same shape and size as the outlet end of conduit section 149c. This short angled conduit section 157 (Figure 6) then leads into an air delivery chamber 158 which is defined within the bottom section of the enclosure 132, is positioned immediately beneath the drying chamber 160 and is of a similar shape and size, as seen in plan view, as the horizontal cross-sectional shape of both the drying chamber 160 and the stillage 138.

The heated air is circulated into the air delivery chamber 158 from the short, angled conduit section 157 and passes upwardly from the air delivery chamber through the openings in the bottom and side walls of the stillage 138, the curved guide baffles 141 serving to induce spin in the air flow to produce shearing and turbulence, which ensures good air distribution and enhanced heat transfer. The air flow then passes upwardly around the side walls of the containers 135, across the top surface of the container lids, through a small space 159 provided between the underside of the enclosure cover 133 and the top surface of each container lid 136 and then passes through the inlet apertures 145 of the central conduit 142. If desired, it may be appropriate to reverse the air flow direction of the heated air. It will be appreciated, therefore, that the drying chamber 160, the central conduit 142, the conduit sections 153, 149, angled conduit section 157 and air delivery chamber 158 together provide a closed-loop circuit through which air is circulated by the fan unit 151 through the heater unit 150 to the four containers 135 and back to the heater unit 150 again. The thermocouples 152 are connected to inputs of a controller (not shown) which controls the heat output of the heater unit 150, so that the temperature of the air in conduit section 149b is maintained at a predetermined value.

Heat from the hot air circulated through the drying chamber is conducted through the walls of the containers 35 and serves to heat the radioactive waste in the containers. This causes the liquid, principally water, contained in the waste to evaporate as offgas, containing principally water vapour and hydrogen, and the offgas passes through the hole 137 in the lid of each container under the action of suction applied to the offgas treatment system to be described below with reference to Figure 9. The hydrogen is produced primarily as a consequence of the corrosion of magnesium in water and hydrogen, which is greatly accelerated by the drying temperature.

It will be appreciated that, apart from small heat losses through the insulated walls of the enclosure 130, all the heat input to the circulated air from the heater unit 150 is transferred to the waste in the drums 135. Therefore, the dryer operates with high heating efficiency. As shown in Figures 4 and 5, enclosure cover 133 comprises a top wall, an underneath wall and connecting peripheral walls, which together define an internal space in which is mounted a manifold 61 having respective, downwardly angled, inlet pipes 62 which are each aligned at their lower end with the central opening 137 of the corresponding container 135.

The manner in which the cover 133 is removably fitted to the enclosure 130 is not material, but should be such that it can readily be released by a robot or by remote control. Suitable guidance and location means (not shown in the drawings) are provided for ensuring the correction location and orientation of the cover. A seal is provided between the cover and enclosure, a water seal 166 being preferred because it requires no maintenance and its integrity can readily be confirmed.

In order to provide segregation between the air circulating through the drying chamber 160 and the offgas generated from the containers 135 for minimising the spread of radioactive contamination, a metal sealing collar 163 is fitted around the outside of the lower end of inlet pipe 162 and is vertically slidable on the inlet pipe. The sealing collar rests, under the effect of its own weight, in metal-to-metal sealing contact with, and located surrounding the hole in, the container lid 136. If greater sealing contact is required, then a biasing spring can be used for urging the sealing collar in the downward direction. With reference to Figure 4, the upper end of the sealing collar is formed with an annular flange 164 for holding each sealing collar captive on the enclosure lid.

To avoid any risk of fire due to the presence of flammable gas, in particular hydrogen, in the offgas collected in the manifold 161, dilution air is added to the offgas. One way in which this can be done is to introduce dilution air directly into each container, for example, through an air inlet pipe (not shown) surrounding and arranged coaxially with the inlet end of each offgas inlet pipe 162. However, according to the arrangement shown in Figure 4 , a short pipe 163 passes through the removable cover 133 and opens into each inlet pipe 162. In this way, the offgas in the inlet pipe 162 passing the lower end of air pipe 163 draws in dilution air from the atmosphere of the environment in which the dryer is located. The site and geometry of the offgas inlet pipe 162 and air pipe 163, together with the design flow rates of offgas and air, are such as to ensure that more than sufficient dilution air is mixed with the off gas to avoid any possible risk of a fire occurring. In particular, it is preferred that the concentration of the flammable gas in the air-diluted offgas is kept at or below 25% of the lower flammable level.

As shown in Figure 5, the manifold 161 has a single outlet pipe 164 which leads to near one corner of the enclosure cover 33 where it is angled downwardly, leading into a vertical section 164a. The lower end of the vertical section 164a is removably connected to the upwardly angled inlet end of an off gas line 165, which is secured to the enclosure and passes through one of its upright side walls 132. Therefore, the air-diluted offgas collected in manifold 161 passes through outlet pipe 164, vertical section 164a and offgas line 165 to the exterior of the dryer enclosure.

In use of the dryer, containers of wet radioactive waste are loaded into an empty stillage 138 and, with the enclosure cover 133 removed from the enclosure, the stillage and its four containers are then inserted into the dryer enclosure, its rim 139 seating on the annular shoulder 140. As the enclosure lid is lowered onto the enclosure, the sealing collars 163 come into contact with the container lids and the enclosure cover can be finally located on the enclosure with the vertical pipe section 164a reconnected with the offgas line 165. The enclosure cover is then secured in position.

The fan and heater units are then switched on and heated air at a controlled temperature is circulated through the drying chamber 160 to heat up the radioactive sludge and drive off, as vapour, the liquid contained in the sludge. Since uranium hydride, which is typically present in radioactive waste, can ignite at temperatures as low as 350°C, the dryer temperature should be restricted to a maximum of about 300°C. The offgas is collected in the manifold 161 and ducted away in the offgas line 165. The drying process continues for a predetermined period of time (for example about 23 hours) that is found to be sufficient to produce a solid mass of a sufficient dryness. Lastly, the containers are unloaded in the stillage from the dryer ready for another set of drums containing wet radioactive waste for drying. Figure 9 is a diagrammatic view of the complete drying plant at the dryer station. Although the dryers 35 at the dryer station are arranged as two groups of six dryers as shown in Figure 2, for simplicity of description each group is depicted as consisting of four dryers in Figure 9. The eight dryers shown are mounted within the single sealed and ventilated cell which comprises the intercommunicating environments 14,15,16, as described with reference to Figure 2, and which serves for shielding operating personnel from radiation and contamination by radioactive waste. The sealed and ventilated all is shown diagrammatically in Figure 9 by reference numeral 170. The design and construction of such a cell is very well known in the art and will therefore not be further described.

In a manner that is also well known per se, the ventilation of the cell maintains its pressure below the atmospheric pressure of the environment adjoining this cell, in particular above, in which operating personnel are working by a predetermined amount (e.g. -200Pa) . Therefore, in the most unlikely event of there being any defect in the cell wall, the pressure differential will cause an air flow from the outer environment where the personnel are working to the inner environment maintained within the cell 170. This avoids any risk of the personnel becoming contaminated. The respective offgas lines 165 (offgas temperatures up to 140°C) from the eight dryers 35 are combined and connected to a single, common gas line 171 leading to an inlet of a scrubber unit 172. In the scrubber unit in per se known manner, the hot offgas is intimately contacted with cold water sprays that remove radioactive dust down to a size of about 1 micron and also serve to condense water vapour in the offgas. The resulting contaminated water collecting in the bottom of the scrubber is led away by pipework (not shown) to waste water storage tanks 173 for further treatment, while the cleaned offgas leaves overhead. The tanks 173 are vented to a common vent line 174 so that vapours released from these tanks are sent to another inlet of the scrubber unit 172.

The compaction process performed by the compacter 32 and described above with reference to Figures 1 and 2 and which serves to reduce the volume of the dry waste to a minimum by squeezing air voids from within its bulk, releases quantities of air which, as indicated in Figure 9, are vented from the compactor and introduced into the scrubber unit 172 along vent line 176, so that radioactive dust in this vent air can be removed.

The cleaned gas leaving the scrubber unit 172 overhead still contains a small quantity of tiny radioactive particles, typically of a size of one micron and less. In order to remove these particles, the cleaned gas is passed through a two-stage high efficiency filtration system comprising two high efficiency particulate air (HEPA) filters 177 and the clean air leaving these filters is drawn by the inlet suction of an exhaust fan 178, which maintains the space above the waste in each drum and the entire offgas treating system under reduced pressure, and exhausted through a stack into the atmosphere.

Although the scrubber unit 172 removes a high proportion of the moisture content of the gases introduced into it, the remaining moisture in the cleaned gas fed to the two-stage HEPA filters could damage these filters (whose filtering medium is made essentially of paper) if the moisture content is high enough. Accordingly, a monitoring device 179 monitors the relative humidity of the clean gas passing to the two-stage filtration unit and activates a heated air source 180 if the monitored relative humidity rises above a predetermined level. In this way, a flow of dry air is introduced into the gas flowing from the scrubber unit 172 to the two-stage filtration unit so as to maintain its relative humidity at an acceptably low level for the HEPA filters.

A differential pressure controller 181 monitors the pressure differential between the pressure of the environment maintained within the sealed cell 170 and the (lower) pressure of the offgas in the common gas line 171. Depending on the monitored pressure differential, the pressure controller controls the opening of a damper 182, which adjusts the gas flow into the exhaust fan 178 so as to maintain the pressure in common gas line 171 at a predetermined amount below that of the cell environment.

RECTIFIED SHEET (RULE 91) ISA/EP The dryer plant is designed such that any one dryer 35 can be opened at any one time in order to enable the dried waste to be removed and further wet-sludge containing containers to be loaded into that dryer. When the enclosure cover of the dryer is removed, the pressure differential maintained between the space in the containers above the sludge and the cell environment, which pressure differential is monitored for each dryer by pressure measuring device 184, is lost and the air flow through the respective offgas line 165 increases. However, the increased air flow is restricted by a respective restriction orifice 183 in the associated offgas line 165 and the design of the pipework from the offgas lines 165 to the exhaust fan 178 as well as the inlet suction of exhaust fan 178 are such that adequate offgas flows are maintained through the remaining seven offgas lines 165 and sufficient dilution air is drawn in to the dryers from the atmosphere maintained within cell 170.

Therefore the important advantage is achieved that the design allows any one dryer to be opened at any one time without affecting the continued reliable and safe evacuation of offgas from the remaining seven dryers and, very importantly, without using any flow control valves or other serviceable parts for isolating the one dryer which is open. When any one dryer is open, the differential pressure controller 181, sampling the pressure differential between the interior of cell 170 and the offgas in gas line 171, adjusts the setting of damper 182 to maintain the required pressure differential. In order that the offgas evacuation system functions in the same way irrespective of which one dryer is opened, the geometry and pipe runs of the offgas lines 165 are made the same, to the extent that is practicable, and by preadjustment of the restriction orifices 183 in the offgas lines 165, the flow rates along the offgas lines 165 can be balanced.

It will be appreciated that a zero pressure differential monitored by each pressure measuring device 184 indicates that the respective dryer is open, a "high" reading corresponding to the pressure differential when the drier cover is in place indicates that it is properly closed, and an intermediate differential pressure indicates that the dryer cover is not fully closed and sealing. Each pressure measuring device 84 includes an alarm unit which generates an audible and/or visible alarm when the pressure measuring device detects an intermediate differential pressure value, indicating that the corresponding dryer cover is not fully in place.

Further, the pressure measuring device 184 generates output signals indicating which of the three possible statuses each dryer cover has. These outputs signals are received by a control circuit (not shown) which is arranged to shut off the heater and fan unit of the corresponding dryers if two or more dryer covers have been removed.

During any period of dryer maintenance, the dryer concerned is isolated by removing the dryer cover and inserting a plug (not shown) into the open upper end of the offgas line 165. Then, while maintenance is being carried out on that one dryer, another dryer can be opened without a malfunction condition being detected and the respective fan and heating units being switched off.

From the foregoing description, it will be appreciated that the described individual dryer and the dryer plant both exhibit many advantages. In particular, there is no risk of hot spots or fire resulting from spilled radioactive sludge from the drums coming into contact with any heating elements, since the heater unit is mounted to one side of the dryer and circulating air serves to transfer heat from the heater unit to the drying chamber. Furthermore, the sealing collars provide a simple and effective means for isolating the circulating hot air from the offgas produced from the waste-containing drums. The side location for the removable unit, including the fan and heater unit, facilitates repair and maintenance.

Furthermore, the dryer plant is intrinsically failsafe, because of the cascading of pressure differentials maintained between the operating environment for the operating personnel and the cell interior and between the cell interior and the drying chambers of the dryers. The described offgas pipework and filtration system continues to operate safely and satisfactorily without the use of any moving or serviceable parts (apart from the exhaust fan) , such as isolating control valves, even when one of the dryers is open. Additional safety measures are provided by generating an alarm whenever the monitored dryer differential pressure relative to that of the cell environment indicates that one of the enclosure covers is not fitting properly. If any attempt is made to open two or more covers, then the corresponding fan and heater units are shut off and the operation of the gantry manipulator 37 (Figure 2) is inhibited.

The compacter 32, which forms the subject of our co-pending UK patent applications Nos.9405989.6 and 94059 87.0, will now be described in detail. Referring to Figures 10 and 11, the compacter comprises a base structure 202 on which is mounted a press table 203 and, adjacent the four corners thereof in a symmetrical arrangement, four upright support columns 204, which are interconnected at a lower region by a first transverse structure 205 and at their top ends by a second transverse structure 206, the support columns 204 and the lower and upper transverse structures 205,206 together forming a rigid frame 250.

As best shown in Figures 11, 12 and 13, a ram cylinder 207 is mounted on the upper transverse structure 206 and extends upwardly from it. Mounted centrally within the frame 250 is a compaction ram 251 extending generally parallel to the upright supports 204 and comprising an upper ram piston 208, which is slidably mounted inside the ram cylinder 207, a connecting piece 210, and a main ram member 209 formed integrally with a cross-piece 211, which is slidably mounted on the four support columns 204. Hydraulic fluid admitted under pressure to the ram cylinder 207 urges the compaction ram 251 downwardly, the upright support columns 204 serving to guide the compaction ram 209 in a stable manner. A cylindrical mould 213 is formed integrally with a cross-piece 214 which is also slidably mounted on the four upright support columns 204 for displacing the mould downwardly towards, and upwardly from, the press table 203 and which is positioned beneath the ram cross-piece 211. The function of the cylindrical mould 213 is to prevent radial distortion of the cylindrical drum when the ram is applied under pressure to the container 135, the mould, when positioned around the container 135, leaving only a nominal clearance between them. Figures 10 to 12 show the compaction ram 251 and the mould 213 in their fully raised positions in which adequate clearance is provided for a cylindrical container 135 containing dried sludge to be transported by the transport system associated with the transfer carousel 35 described with reference to Figure 2, to a predetermined central position on top of the press table 203.

Referring to Figures 11 to 13, the upper transverse structure 206 is approximately square-shaped in plan view (Figure 13) and is secured to the four support columns 204 at its four corners. Two ram return cylinders 215 are mounted at the mid-points of two opposite sides of the transverse structure 206 and project upwardly therefrom. Ram pistons 216, respectively mounted in the hydraulic cylinders 215, are connected by connecting rods 217 to the ram cross-piece 211, the cross-pieces carrying trunnions 218 which provide pivotal connections between the lower ends of the connecting rods 217 and the ram cross piece 211, so as to accommodate small changes in the relative orientations of the connecting rods and the ram cross-piece when the ram is lowered and raised. When hydraulic fluid is admitted under pressure to the hydraulic cylinders 215, the pistons 216 are urged upwardly to effect the return stroke of the ram 209 to its raised position. Although the return cylinders 215 need only be single-acting, it is preferred that they be double-acting, in order that the cross-piece 221 can be raised and lowered when the ram cylinder 207 and its ram piston 208 are taken out of service for maintenance.

Similarly, two double-acting hydraulic cylinders 219 are mounted at the mid-points of the other pair of opposite sides of the upper transverse structure 206 and project upwardly from the transverse structure. A piston 220 disposed in each double-acting hydraulic cylinder 219 is connected by a connecting rod 249, which passes freely through an oversize bore 221 in ram cross-piece 211, to mould cross-piece 214, a trunnion 222 providing a pivotal connection between the lower end of connecting rod 249 and mould cross-piece 214 to accommodate small changes in the relative orientations of the connecting rod 249 and mould cross-piece 214 as the mould 213 is urged up and down the support columns 204 by the action of the double-acting piston/cylinder arrangement 219, 220.

Referring to Figure 11A in particular, the lower end of the ram member 209 is fitted with a replaceable ram plate 223 of hardened steel, designed to withstand as far as possible the very high loading and localised pressure points resulting when the container is being compacted. Locating means, e.g. locating lugs received in correspondingly shaped bores formed in the underside of the ram member 209, serve to locate the ram plate correctly with respect to the ram member 209. In order to secure the ram plate in position, a plurality of bolts 224, for example four bolts, extend through registering holes or passageways formed in the ram piston 208, ram connecting piece 210 and ram member 209 and are received in complementary screw-threaded bores 255 formed in the top face of the ram plate 223. The bolt heads are tightened against the bottom end surface of an axial blind passage 225 formed centrally from the top face of the ram piston 208 and extending close to the bottom of the ram piston. In this way, the ram plate 223 is releasably secured to the bottom end of the ram 209.

This securing arrangement for the ram plate 223 is especially advantageous in that it provides ready access to the bolt heads from the top of the compacter, merely by removing a removable top cover 226 from the ram cylinder. In this way, when the ram plate becomes worn, it can be released from the ram 209 without entering the radioactive environment in which the container is compacted. Furthermore, access to the bolt heads through the open upper end of the ram cylinder 207 is from a working environment for operating personnel as will be described below in more detail. Another advantage is that the bolt heads cannot become clogged with debris and radioactive waste in view of their distancing from the compaction region. The released ram plate falls into the radioactive environment and is taken by for example a conveyor system and/or remote manipulators to a decontamination and maintenance facility. A replacement ram plate is supplied and fitting by essentially the reverse process and is not further described herein.

In view of the considerable loading, both general and localised, that can be exerted on the inner surface of the mould 213 by the container 135 during compaction, the mould is provided on this inner surface with a liner 227, again of hardened steel. The liner is preferably of single-part construction and is received in a complementary recess formed in the lower section of the mould. As shown in Figure 11, the height of the mould liner is substantially the same as that of the container.

Since the function of the liner is to protect the body of the mould from damage while the function of the mould body is to withstand the stresses exerted on the mould when the drum is compacted by the ram, the mould liner is made to be an interference fit within the mould so that it can transmit stresses to the mould without itself being distorted. To facilitate engagement of the liner 227 with the mould 213 and also removal of the mould liner from the mould, the liner is preferably a taper-fit, so that it can be freely inserted into the mould from below (when the mould is in its raised position) until it becomes an interference fit within the mould body as it approaches its final location within the mould and so that the interference fit is released after the liner has been displaced only a short distance downwardly.

As best shown in Figure 11A, the liner is removably secured in place by a plurality of bolts 229 that extend through oversize axial bores 230 formed through the height of the mould and are received in corresponding screw-threaded bores 254 formed in the wall of liner 227. Consequently, the liner wall thickness needs to be sufficient to accommodate the lower ends of the bolts 229. To ensure the liner is located in the correct angular orientation, locating dowels or the like can be used.

Beneath each bolt head is a split washer 228, in the form of two C-shaped elements (not shown) butted one against the other. When a damaged liner needs to be replaced, the bolts 229 are slackened off slightly so that the split washers can be removed and then the bolts are tightened again until their ends abut with the ends of the bores 254 formed in the mould liner. The lengths of these screwthreaded bores and the bolts are such that the bolt heads are spaced above the top surface of the mould by a short distance. Then, the two (C-shaped) halves of a shim

(not shown) are fitted around the central ram member

209 and rest on the protruding bolt heads. By applying hydraulic pressure to the ram cylinder 226 while holding the mould in its raised position, the ram moves downwardly, the ram cross-piece 211 pressing against the top surface of the shim and, in turn urging the mould liner downwardly too. The spacing between the bolt heads and mould is sufficient that before the bolt heads have been forced back into contact with the top surface of the mould, the taper interference fit between the liner and mould will have been released. Then, by raising the ram to its uppermost position again and lowering the mould until the liner sits on the press table

203, access can be had to the bolt heads, so that the bolts 229 can be unscrewed and removed, so as to release the mould liner from the mould. After the mould has been raised again to its uppermost position, the separated liner can be transported to a remote decontamination and maintenance facility for handling in ways which are well known in the art and which therefore do not need to be described in any detail.

Insertion of a replacement liner is essentially a reversal of the dismantling process, and involves transporting the replacement liner to the compacter and raising it into position inside the mould, using a combination of conveyor systems or the like and remote controlled manipulators. Locating means, such as the locating dowels mentioned above are preferably used to locate the mould liner in the correct angular orientation. Then, replacement bolts 229 are inserted and tightened up to secure the replacement liner in position in the mould.

Referring to the detailed view of Figure 11A, an annular space 252 exists between the inner face of the mould and the outside of the ram member 209. An annular seal 231 carried by the mould seals against the outside surface of ram member 209. An annular clearance 253 of smaller width than that of annular space 252, is provided between the ram member 209 and the surrounding mould liner 227. There is also an even narrower clearance between the ram plate 223 and the mould liner. An air inlet line 235, including a non-return valve 234 and filter (not shown) , communicates with an annular space 252 just below the seal 231 while an air exhaust line 232, also communicating with the space 252 just below the seal

231, includes a non-return valve 233 and filter (not shown) and passes down through the lower transverse structure 205 and terminates in the environment in which compaction takes place. The upward and downward movement of the mould is accommodated by a telescopic-type, sliding connection 246 in the exhaust line 232.

For compacting a container, firstly the mould is lowered around the container until its lower end contacts the press table 203. As the mould is lowered, a small quantity of air is exhausted through exhaust line 232 due to the relative movement between the mould and the ram plate 223. Then the ram is driven downwardly inside the mould, compacting the container to form a so-called puck and causing air in the mould to be compressed and pass upwardly through the annular space 252 and out through exhaust line

232. Since drums can fracture during compaction and since the space within the mould is filled with air from the radioactive environment in which the compaction is carried out before the ram is lowered, the expelled air contains radioactive contamination which is removed by the filter in air exhaust line 232.

After container compaction is completed, the compacter ram is held in its lower position while the mould is raised clear of the top of the puck. This guarantees the separation of the mould from the puck, which can get very tightly lodged in the mould during compaction. During raising the mould, non-return valve 233 shuts and non-return valve 234 opens to admit a small quantity of air into the annular space 252. Finally, the ram is raised to its raised position and the puck is removed, for example by a conveyor system and/or automatic manipulators (which can also be used for removing and replacing worn mould liners and ram plates, periodically) . Then the next container to be compacted is brought to the press table 203 and the cycle repeated.

Referring now to Figure 10, the compacter is installed in a protective, thick-walled (e.g. 1.2m thick) , concrete cell 236. The interior space within the protective cell is horizontally sub-divided by lower flooring 238 and upper flooring 239 to provide a first environment 240 in which compaction takes place and which communicates with, and is maintained at the same pressure as, the ventilated environments 14, 15, 16, a second ventilated environment 241, to which operating personnel can have access providing appropriate protective measures are taken (e.g. the wearing of suitable protective clothing) and a ventilated upper environment 242 for maintenance of the hydraulic equipment, this environment requiring a lower level of protection for operating personnel. Furthermore, the middle environment 241 is vertically segregated by vertical walls 243 on all four sides of the compacter to define a fourth environment 244.

The flooring 238 is made of concrete and contains the radiation from the radioactive waste within the bottom environment 240. The function of the upper flooring 239 and the vertical walls 243, however, is not to provide this shielding function because the radiation from the waste is absorbed by the concrete walls and floor of the cell and the concrete flooring 238. Rather, the upper flooring 239 and the vertical walls 243 serve to segregate and define the different environments 241, 242, 244 which are used to control the spread of radioactive contamination, which is present in successively smaller quantities in environments 244, 242 and 241, respectively. In particular, in a manner known per se, cascaded pressures are maintained in the three environments such that the pressure successively increases from the upper environment through the intermediate environment to the lower environment. For example, the bottom environment 240 would be held at an atmospheric depression of say 200 Pa with respect to the middle environment 241, which itself is held at a similar depression to the top environment 242. In this way, if there should be any fault which could give rise to air leakage from one environment to another, then the flow direction will always be into an environment of potentially higher level of radioactive contamination. A suction path for maintaining reduced pressure in the environment 244 is shown at 253.

In order to seal the environments 240, 241 and 242 from one another, the lower transverse structure

205 is designed to form a section of the lower flooring 238, to which it is connected in fluid-tight manner. Similarly, the upper transverse structure

206 forms a section of the upper flooring 239 and is connected to it in fluid-type manner. A lip seal 247 on the lower transverse structure 205 seals against the outside of the mould and another lip seal 248, carried on the underside of the upper transverse structure 206 seals against the outside of the ram connecting piece 210. Inspection windows, such as shown at 245, formed in the cell wall enable operating personnel to view operations taking place within the radioactive, bottom, environment 240, without danger of personnel being exposed to harmful radiation.

According to a modification (not shown) , instead of using bolts 224, as described above, for securing the ram plate 223 in position on the bottom face of the ram 251, the ram plate may be held in position by a securing rod extending in a central axial bore in the ram member 209, the rod being connected at its lower end to the ram plate by a bayonet connection and bolted, by means of a flange at its upper end, to the ram member 209 (or alternatively held in position by removable locating dowels) . Relative annular displacement between the ram member 209 and ram plate 223 is prevented by dowel pins. To release the ram plate, the ram connecting piece 210 is removed from the compacter, the securing rod flange released from the ram member 209 and turned from the top to release the bayonet connection, and, if need be, the rod pushed down against the ram plate to disengage the dowel pins locating the ram plate relative to the ram member.

The disclosed compacter is particularly advantageous in that the replaceable mould liner and ram plate can both be released when they are to be replaced, by untightening the respective two sets of securing bolts, whose bolt heads are accessible from the top of the mould and the top of the ram, respectively. Furthermore, there is no risk of the bolt heads becoming clogged with debris and radioactive material, in view of their remote location from the location of the container. Another advantage is that there is effectively no real risk of hydraulic fluid that has leaked from any of the hydraulic cylinders running down into the radioactive environment 240, which would be highly undesirable and result in effluent disposal problems. In the case of ram cylinder 207, hydraulic pressure is exerted only on the upper face of piston 208 and any leakage down the outside of the ram cylinder will be contained by the flooring 238 and the transverse structure 206. In the case of the ram return cylinders 215 and the double-acting cylinders 219, any hydraulic fluid, applied to the underside of the respective pistons to effect the upward stroke, that has leaked out of the cylinders would nevertheless be contained within the environment 244 by the lower transverse structure 205, the lip seal 247 guaranteeing that any hydraulic fluid on the upper surface of transverse structure 205 cannot pass into the radioactive environment 240. By these measures, hydraulic fluid is prevented from reaching containers in the compacter awaiting compaction or compacted containers (pucks) , which may still be hot from the dryer if they were not cooled before loading into the compacter.

Another advantage of the disclosed compacter is that even though the compacter ram has to have a significant length since it extends from the upper environment 242 down to the radioactive environment 240, the slidable mounting of the ram cross-piece on the four support columns ensures that, despite its length, the ram is stably guided throughout its downward and upward strokes. This is important, in order to minimise wear on the lip seal 248. Similarly, the mould is also guided in a stable manner by the support columns 204, which minimises wear on the lip seals 247. It is also advantageous that most routine maintenance procedures can be carried out without the need to gain access to the hostile environment 240 in which the container compaction takes place.

The waste treatment plant and method disclosed hereinabove may be used for treating toxic or other hazardous waste containing pyrophoric material in its oversize components. If no pyrophoric material is present, then all the oversize can be size reduced, if need be, and transported directly to the compacter.

Claims

CLAIMS :

1. Apparatus for treating radioactive, toxic or other hazardous waste, comprising a dryer for drying the waste in a container, a compacter for compacting the container and its dried waste to reduce the volume that they occupy, a protective cell in which the dryer and at least part of the compacter are disposed, for confining to the cell radiation and radioactive, toxic, or hazardous contamination resulting from the waste, and means associated with said protective cell and operable from outside the cell for transporting said container from said dryer to said compacter.

2. Apparatus according to claim 1, wherein said protective cell includes a storage area for compacted containers and said container transporting means is also operable for transporting compacted containers from said compacter to said storage area.

3. Apparatus according to claim 1 or 2, wherein said container transporting means comprises a travelling gantry carrying at least one manipulator.

4. Apparatus according to any preceding claim, further comprising a separator for separating oversize from a feed of radioactive, toxic or other hazardous waste to leave undersize, a conveyor, and means for loading the resulting undersize into empty containers on said conveyor, said conveyor being arranged to transport said containers to said container transporting means, and said separator, conveyor and undersize loading means all being located in said protective cell.

5. Apparatus according to claim 4, wherein said separator is a vibratory screen separator.

6. Apparatus according to claim 4 or 5, wherein said cell further includes a size reduction area in said cell and manipulator means associated with said size reduction area and operable from outside the cell for reducing the size of collected pyrophoric material that is larger than a predetermined size, and transporting the size-reduced material to containers on said conveyor already containing undersize.

7. Apparatus according to claim 4, 5 or 6, further comprising a container lidding machine associated with the conveyor and arranged to secure lids to said containers before they arrive at said container transporting means.

8. Apparatus according to any preceding claim, wherein the dryer comprises a drying chamber for at least one container of waste, a heater located outside the drying chamber for heating a fluid, and means for conveying the heated fluid to the drying chamber for heating the waste in the container.

9. Apparatus according to claim 8, wherein the heated fluid conveying means comprises a closed circuit including a fan operable for circulating air from the heater to the drying chamber, and back to the heater.

10. Apparatus according to claim 8 or 9, wherein the drying chamber is defined within an enclosure with a cover which is openable for loading said container into and from the drying chamber, the container has a lid with an opening therein for the passage of vapourised liquid produced by heating the wet radioactive waste in the container, and a gas line is provided for conveying the vapour from the container to the exterior of the enclosure.

11. Apparatus according to any one of claims 8 to 10, wherein the dryer further comprises a stillage for containers which is removably held in said drying chamber, and wherein a plurality of guide baffles mounted inside the stillage are arranged for supporting said containers in said stillage, for inducing spin in the flow of heated fluid entering the drying chamber.

12. Apparatus according to any preceding claim, whereing the compacter comprises a press table for supporting a container to be compacted, a ram mounted above the press table and extending generally in an upward direction, the ram having a replaceable ram plate on the lower end thereof, said ram plate being releasably held in position by first securing means, which is operable from an upper region of the ram for releasing the ram plate, hydraulic pressure means for urging the ram towards the press table to compact the container between the lower end of the ram and the press table and for withdrawing the ram from the container, and a mould that is positioned or positionable above and adjacent the press table for surrounding the container during compaction, the mould having a replaceable internal liner releasably secured in the mould by second securing means which is operable from the top of the mould for releasing the mould liner.

13. Apparatus according to claim 12, wherein said first securing means comprises a plurality of first bolts passing through respective oversize holes extending through the mould from an upper end face thereof, said first bolts being received in respective first screwthreaded bores in the upper end of the wall of the liner, and wherein said second securing means comprises a plurality of second bolts passing through respective oversize holes extending through the ram from an upper face thereof to its underside, said second bolts being received in respective second screwthreaded bores formed in the ram plate.

14. Apparatus according to claim 12 or 13, wherein the hydraulic pressure means comprises: a single-acting, hydraulic ram cylinder and co-operating ram piston, one of which is connected to a plurality of upright support columns at or near an upper end thereof and the other of which is connected to said ram which is mounted generally parallel to the support columns and in a central location and is united with a cross-piece which is movably mounted on the support columns, so that hydraulic fluid supplied under pressure to said hydraulic cylinder causes said ram to move downwardly to compact the container against the press table, and at least two return hydraulic cylinders and respective co-operating pistons, one of which is connected to said support columns at or near their upper ends and the other of which is connected to said cross-piece.

15. Apparatus according to claim 12, 13, or 14, further comprising a sealed environment in which the container is to be compacted and which intercommunicates with said protective cell , the press table being disposed in said sealed environment and said support columns extending upwardly through a transverse structure interconnecting the support columns and forming part of the upper boundary of said sealed environment, said mould extending downwardly through an opening in said further transverse structure which carries a seal which seals around the exterior of the mould.

16. Apparatus according to any one of claims 1 to 12, wherein the compacter comprises a press plate for supporting a container to be compacted, a plurality of upright support columns, a ram mounted generally parallel to and centrally relative to the support columns and having a cross-piece which is movably mounted on the support columns, a single-acting, hydraulic ram cylinder and co-operating ram piston, one of which is connected to the columns at or near an upper end thereof and the other of which is connected to said ram so that hydraulic pressure supplied under pressure to said hydraulic cylinder causes said ram to move downwardly to compact the container against the press table, at least two hydraulic return cylinders and respective co-operating pistons arranged on opposite sides of the ram, one of each return cylinder and co-operating piston being connected to said support columns at or near their upper ends and the other being connected to said cross-piece for raising said ram, and a mould that is positioned or positionable above and adjacent the press plate for surrounding the container during compaction.

17. Apparatus for treating radioactive, toxic or other hazardous waste, substantially as hereinbefore described with reference to the accompanying drawings.

18. A method of treating radioactive, toxic or hazardous waste, comprising drying a container of radioactive, toxic or hazardous waste in a protective cell which confines to the cell radiation and radioactive, toxic or hazardous contamination resulting from the waste, and compacting the container and its dried waste in said protective cell to reduce the volume that they occupy.

19. A method according to claim 18, wherein the container of dried waste is transported to a compacting station, at which the compacter is compacted, by handling operated from outside said protective cell.

20. A method of treating radioactive, toxic or other hazardous waste in the form of a sludge containing pyrophoric material, comprising the steps of:

(a) separating oversize from the sludge to leave undersize,

(c) loading the pyrophoric oversize and the undersize into a container so that the undersize blankets the pyrophoric oversize to prevent spontaneous ignition thereof,

(f) compacting the container and its dry product to reduce the volume that they occupy.

21. A method according to claim 20, wherein the oversize is kept dampened with water sprays between steps (a) and (b) and the pyrophoric oversize is kept dampened with water sprays between steps (b) and (c) .

22. A method according to claim 20 or 21, wherein, between steps (b) and (c) the pyrophoric oversize that is larger than a certain size is cropped to reduce its size.

23. A method according to claim 20, 21 or 22, wherein the non-pyrophoric oversize that is larger than a certain size is reduced in size and loaded into an empty container which is compacted to reduce its size without prior drying.

24. A method according to any one of claims 20 to

23, wherein a plurality of compacted containers with their dry products are introduced into an encapsulation drum, and grouting or cement introduced into the drum to encapsulate the containers and their dry products.

25. A method according to any one of claims 20 to

24, wherein all the defined steps of the method are carried out within a protective cell which confines to the cell radiation and radioactive, toxic or other hazardous contamination resulting from the waste.

26. A method according to any one of claims 20 to

25, wherein said waste comprises intermediate level waste in the form of a sludge containing metal items and containers.

27. A method of treating radioactive, toxic or hazardous waste, substantially as hereinbefore described with reference to the accompanying drawings.