WO2015049521A1

WO2015049521A1 - Encapsulation of waste materials

Info

Publication number: WO2015049521A1
Application number: PCT/GB2014/052977
Authority: WO
Inventors: Robert Glyn Lewin; David Norcross; Paul Knight
Original assignee: National Nuclear Laboratory Limited
Priority date: 2013-10-02
Filing date: 2014-10-01
Publication date: 2015-04-09
Also published as: GB201317463D0; GB2522173A

Abstract

The invention provides a method for the in situ encapsulation of radioactive waste materials which comprises providing an empty containment vessel in a handling location containing radioactive waste material, introducing radioactive waste and encapsulant into the vessel, tumble mixing the vessel using remote handling equipment, allowing the mixture to cure, capping the mixture, placing a lid on the vessel and transferring the sealed vessel to a storage location. The method allows for the in situ treatment of waste materials and provides major benefits in terms of safety, due to the reduced handling and transport requirements, as well as economic advantages, in view of the fact that all operations can be carried out using a combination of relatively inexpensive equipment.

Description

ENCAPSULATION OF WASTE MATERIALS

Field of the Invention

[0001] This invention relates to a method for the safe and convenient encapsulation and disposal of waste materials and is particularly applicable to the management of radioactive waste materials.

Background to the Invention

[0002] The safe management of radioactive waste materials has long been a key issue for the nuclear industry and many methods have been developed for use in this context, so that the industry has, on a worldwide basis, a large range of process options to manage new and legacy waste materials for safe disposal. This waste may take the form of spent nuclear fuel, or it could arise as a result of spent fuel treatment for fuel recycling.

Alternatively, the waste may be associated with decommissioning and clean-up operations. The various waste materials have many different characteristics but can, nevertheless, be generically categorised as High Level Waste (HLW), Intermediate Level

Waste (ILW) or Low Level Waste (LLW).

[0003] The category of an individual waste stream is defined by the radionuclide content of the waste, and the subsequent disposal route, and final waste form for disposal is determined by this categorisation. In the United Kingdom, HLW is processed in accordance with very strict criteria so as to produce a well controlled vitrified waste product with a specification which is in accordance with agreed standards established by the UK nuclear regulatory bodies and relevant stakeholders. Intermediate Level Waste (ILW) is immobilised and encapsulated in bespoke cement matrices which, in turn, are held within a secondary - and sometimes a tertiary - stainless steel vessel. The final form, again, is according to standards agreed with the UK nuclear regulatory bodies and relevant stakeholders. Low Level Waste (LLW) is normally compacted in order to minimise volume, and therefore cost, and is then consigned to a designated storage vessel before final storage at a designated repository.

[0004] Untreated HLW and ILW materials are currently held in designated, radiation and contamination controlled, secure locations. The natural characteristics of these materials, involving highly radioactive and highly mobile species, necessitate access to these locations being restricted; consequently, management of these waste types has to be carried out either remotely, or with highly restricted human access. Within the nuclear Industry, there is a need and a desire to minimise personnel exposure to ionising radiation and, therefore, remote technology is a preferred option and is deployed whenever practicable or possible.

[0005] In the UK and other, non-UK, nuclear installations, sites and organisations, the encapsulation and immobilisation of waste using such processes is usually performed in dedicated encapsulation facilities. As a consequence, ILW has to be retrieved from its source, packaged for transport to an encapsulation facility, and managed through the dedicated facility so as to produce a final, solid, non-mobile, contamination-free waste form. Naturally, such processes require highly engineered retrieval equipment, sited at a particular waste location, and the equipment and processes are usually bespoke and, therefore, suited to one particular waste form only.

[0006] In addition, well managed and robust interfaces between retrieval and transport packages are required to prevent the spread of contamination and the exposure of personnel to radiation during transportation. Waste containers and, in particular, transport vessels are highly technical, well engineered units which are capable of performing dedicated and specific operations safely, albeit that they are designed to transport specific wastes in dedicated waste containers and, therefore, have limited flexibility of use. Transport vessels are necessarily robust and contain shielding designed to minimise emissions of radiation from transported materials; as a consequence they often have a mass in excess of several tens of tonnes and, hence, require an infrastructure capable of managing such large units.

[0007] Existing encapsulation plants have been designed to treat characterised wastes, and dedicated infrastructure similarly adapted to manage the safe retrieval, transport and encapsulation of waste has been established. These processes have some, albeit necessarily limited, flexibility with respect to accommodating different sources of waste; however, retrieval and transport infrastructure, and the management of plant interfaces, are still known to be major challenges in the industry, even with some of the more easily managed wastes.

[0008] Three main approaches are considered to be viable for treating legacy waste materials. These are:

■ Designing and building dedicated Local Encapsulation Plants (LEP);

^■ Designing and building Mobile Encapsulation Plants; and

^■ Designing and building flexible, centralised, encapsulation facilities capable of treating a wide range of waste products.

[0009] A local encapsulation facility comprises a plant dedicated to treating one specific waste at a location next to, or as close as practicable to, the source of a waste material. It is normally regarded as non-flexible, and any associated work programmes need to include contingency plans for post operation clean out (POCO), decommissioning and dismantling.

[0010] Mobile plants are potentially more attractive and can, in theory, be designed to be taken close to the source of waste, where a fully flexible cement grouting and encapsulation system, capable of accepting a wide range of waste types, can be deployed. However, to date, such a fully flexible mobile unit has never been built. The secure, contamination free, management and transfer of waste in a suitably controlled radiation environment, and the handling, production and transfer of bespoke cementitious materials prior to encapsulation in a mobile form remain as major challenges which need to be overcome to the satisfaction of technology users, regulatory authorities and stakeholders.

[0011] New, large, central plants, designed to be flexible and well suited to a range of identified Intermediate Level Waste materials, are known to have been scoped, designed and built, and various encapsulation techniques have been proven for different waste forms. However, much work still remains to be done in order to optimise process conditions and throughput. Such facilities will, however, allow for the treatment of a range of waste forms, although significant further capacity will still be required in order to allow for the treatment of all known legacy and future waste streams.

[0012] The present application is directed to the treatment of ILW, or wastes in which encapsulation in a cementitious grout is, in the UK nuclear industry, the preferred technology for immobilisation, and various methodologies are used which include in-drum mixing, flood grouting and tumble mix and pour.

[0013] Such technologies are described, for example, in WO-A-2004/006268, which relates to a method for the encapsulation of a nuclear material which involves treating the material with an encapsulant comprising a cementitious material, such as Portland Cement, and curing the cementitious material. The method is particularly suitable for the treatment of nuclear fuel materials such as uranium metal, complete Magnox fuel elements, or fuel element debris, and provides products which remains stable and monolithic for many hundreds of years, thereby offering a safe and convenient alternative means of handling other than nuclear fuel reprocessing.

[0014] An alternative process is disclosed in WO-A-2005/101426, which envisages a method for the encapsulation of uranium metal which requires treatment of the metal with an encapsulant comprising a cementitious material and curing the cementitious material, the process additionally requiring the provision of means for the minimisation of the corrosion of the metal which typically comprises the provision of a source of oxygen within the cement matrix. This may be achieved, for example, by facilitating enhanced oxygen access from the atmosphere using air entraining agents or cenospheres, or by the inclusion of an independent source of oxygen, such as a peroxide. Alternatively, the means for the minimisation of corrosion may involve minimising the water content of the matrix, which may be conveniently achieved by the addition of superplasticisers. The method allows for the long term storage of uranium metal and provides significant benefits in terms of health, safety and the environment.

[0015] WO-A-2006/027554, on the other hand, teaches a cementitious composition including at least one sulphoaluminate cement which comprises a sulphoaluminate salt of an alkaline earth metal, wherein the cementitious composition is essentially free of other cementitious components. Preferably the sulphoaluminate salt comprises calcium sulphoaluminate and the cementitious composition additionally comprises at least one further salt of an alkaline earth metal, preferably calcium sulphate. The invention also provides a method for the encapsulation of materials with the composition, and the materials which are encapsulated typically comprise waste materials generated in the nuclear processing industry comprising amphoteric reactive metals.

[0016] A further encapsulation treatment may be gleaned from WO-A-2006/097696, which provides a method for the production of a stable monolith by the encapsulation of a waste material so as to promote chemical bond formation within the monolith, and also describes a method for the disposal and storage of waste materials which comprises the production of a stable monolith by this method. Waste materials which are particularly suitable for such treatment include various geopolymer precursors, most particularly ion exchange materials such as aluminosilicate materials, and the method is especially useful for the disposal and long term storage of radioactive waste materials.

[0017] ln-drum mixing is an elegant practice in which waste is mixed with cementitious powders inside a drum using a sacrificial mixing paddle. The drum also acts as the outer containment vessel, thereby minimising the transfer of materials, spread of contamination, and the need to clean the inside of the process equipment. The method however, has limitations in terms of the wastes which can be mixed; typically it is used for sludges or other 'pumpable' wastes, and large, bulky items cannot be processed using this methodology due to risk of damaging the mixing paddle or drum.

[0018] In a development of such processes, WO-A-2011/098817 discloses an apparatus for the encapsulation of waste materials which comprises a container and agitation means, rotatably mounted about an axis of rotation in the container, wherein a plurality of means for the aeration of the waste materials are located on the agitation means. Preferably, the agitation means comprises a lost paddle system and the plurality of aeration means comprises a plurality of projecting elements, typically comprising annular louvers. An encapsulation method is also provided and the apparatus and method are preferably used for the encapsulation of radioactive waste materials, typically comprising uranium and/or its compounds, and allow the formation of pyrophoric uranium hydride to be avoided.

[0019] Flood grouting involves the simple pouring or addition of cementitious grout on to a waste material held in a box or drum. The form of the waste allows the grout to flow through and around the material so as to establish full encapsulation. Large bulky items can be readily treated in this way; however, it is not suitable for the treatment of fine particulate material, due to the limited penetration and flow of the grout into associated narrow channels and cavities. Again, the drum acts as the outer containment vessel and the need to clean the inside of process equipment is limited to the non-active grouting equipment only.

[0020] The most flexible of the mixing systems is tumble mixing, which can accommodate most - if not all - waste types. In such systems, waste materials and grout, or cementitious powders, are mixed in a vessel according to a process which is similar to that of a conventional concrete mixer, and the resulting mixture is then poured into an intermediate or final steel containment vessel. In this method, however, process vessels do need to be cleaned and maintained, and washing process liquors have to be managed and/or disposed of, thereby adding complexity of operation in a remotely operated nuclear encapsulation plant.

[0021] In the operation of the majority of these prior art processes, the waste has to be retrieved, loaded into primary containment vessels, placed in transport flasks, and then transported to, and fed into, an encapsulation plant. Occasionally, it is possible to pump wastes through dedicated pipework directly to an encapsulation plant; more typically, however, the processes are not sufficiently flexible to allow for the treatment of all waste types, and do not necessarily provide the correct option from an economic perspective.

[0022] Outside the Nuclear Industry, a large variety of mixer technologies is available, ranging from agricultural, asymmetric, hand driven tumble units to typical industrial mobile cement mixers. Whilst interesting and technically elegant, such units - if deployed - can only form a small part of an integrated encapsulation process designed for nuclear applications.

[0023] Therefore, the present inventors have sought to develop an encapsulation system which, whilst retaining the benefits of efficiency, economic viability and high safety standards, additionally offers the flexibility and convenience which is absent from the processes of the prior art. In developing the process, the inventors have utilised the tumble mixing methodology of the prior art, but have refined and adapted such processes so as to allow for the safe, convenient and efficient disposal of a range of waste materials using a process which is applicable across a wide variety of different locations.

Summary of the Invention

[0024] Thus, according to the present invention there is provided a method for the in situ encapsulation of radioactive waste materials, said method comprising the steps of:

(a) providing an empty containment vessel;

(b) introducing said vessel into a handling location containing radioactive waste material, said location being provided with (i) remote mechanical handling equipment adapted for use with said vessel and (ii) means for the introduction of an encapsulant into said vessel, wherein said vessel is introduced via an import/export chamber;

(c) introducing a quantity of radioactive waste into said vessel;

(d) introducing a quantity of encapsulant into said vessel;

(e) placing a first, temporary, lid on said vessel and tumbling said vessel using said handling equipment in order to mix the contents;

(f) holding or placing the vessel in a vertical plane with the lid uppermost and removing said first lid;

(g) allowing the encapsulant/waste mixture to solidify (cure);

(h) introducing further encapsulant into said vessel to cap the mixture;

(i) placing a second, permanent, lid on said vessel;

(j) removing said sealed vessel from said location via said import/export chamber; and

(k) transferring said sealed vessel to a storage location.

[0025] In certain embodiments of the invention, interim storage of the encapsulated waste may be necessary or desirable prior to permanent removal and transfer of the sealed vessel. In such embodiments, said interim storage typically occurs following step (g) of the method of the invention.

[0026] The handling location may comprise any location that is suitably bunded, restricted or rigorously contained so as to isolate it from the outside environment. In particular embodiments of the invention, said location is a sealed location. [0027] In embodiments of the invention, the import/export chamber may comprise washing means and/or monitoring means. Said washing means typically comprises aqueous washing means, for example, pressure washing means such as spraying means, adapted to remove external contamination from said vessel. Said monitoring means typically comprises a measuring device adapted to detect external contamination which comprises radioactive waste material. Said import/export chamber is adapted so as to ensure that the handling location remains isolated from the outside environment, for example by the use of separate inner and outer doors or strategic ventilation.

[0028] Typically, said vessel is washed, monitored for contamination and, if necessary, re-washed prior to transfer to storage. It is envisaged that all washings may be recycled and subsequently used as make-up water in the preparation of future batches of encapsulant, so that the production of secondary waste is avoided.

[0029] In certain embodiments of the invention, the quantities of radioactive waste and encapsulant which are introduced into the vessel are predetermined with reference to the volume of waste solid and associated liquor and are typically introduced sequentially into the vessel in the handling location. The order of loading of the waste and encapsulant may vary according to the particular circumstances and materials involved.

[0030] In certain embodiments, the introduction of said materials into the vessel is most conveniently carried out using loading means. Said loading means typically comprises a device which is placed into the vessel prior to introduction of the radioactive waste and which facilitates loading of the waste material in a clean and efficient manner which avoids contamination of external surfaces or connecting parts of the vessel.

[0031] In embodiments of the invention, said loading means comprises covering means, adapted to cover and protect surfaces of said containment vessel from contamination, and transferring means which facilitates the introduction of the radioactive waste into the interior of the vessel. In typical embodiments, the dimensions of the transferring means are adapted to allow for the metering and control of waste volumes entering the vessel.

[0032] Most conveniently, the loading means may be provided in the form of a hollow conical device wherein the base of the cone comprises, as the covering means, a protruding collar and optional further attachments, adapted to protect the connecting parts of the containment vessel, such as vessel rims, threads and securing points, from contamination, and the internal surfaces of the cone provide the transferring means. Thus, for example, further attachments may include a central cylindrical portion, adapted to fit around a vertically mounted screw thread affixed to the base of the containment vessel and adapted to engage with a lid fitment. The use of such a device prevents waste deposits on the outer vessel surface, rims, threads and securing points from occurring during the loading of waste, and it is of primary importance that such parts of the vessel should be free from contamination in order to allow for subsequent lidding operations to be performed successfully.

[0033] In typical embodiments, loading means comprising a hollow conical device is deployed in inverted form, wherein the collar at the base of the cone attaches to the rim of the containment vessel and the top of the cone extends to a point adjacent the base of the vessel. Such a design limits the volume that any loaded waste can occupy and the design may be modified in accordance with the selected waste to provide a variable and passive control option for metering waste volumes. Thus, the degree of tapering in the sides of the cone may be different according to the particular waste which is being handled, so that cones of different shapes may be employed in different situations.

[0034] In embodiments of the invention wherein loading means is deployed, the encapsulant is most conveniently introduced into the containment vessel prior to addition of the waste material. In this way, the loading means may be deployed in order to ensure that contact between waste and encapsulant does not occur until removal of the loading means from the containment vessel. Typically, the encapsulant may be introduced into the vessel in the import/export chamber, prior to introduction into the handling location.

[0035] In other embodiments of the invention, instead of introducing the radioactive waste and encapsulant sequentially into the vessel in the handling location, the waste material may be initially introduced into the vessel in the handling location, and the vessel may then be supplied with a lid and transferred to a treatment plant equipped with said remote mechanical handling equipment for tumbling said vessel and typically additionally comprising an encapsulant source, de-lidding and lidding means, and washing and monitoring means for completion of the process.

[0036] Particular embodiments of the invention envisage the use of an encapsulant which comprises a cementitious material such as a cementitious grout, examples of which include ground granulated blast furnace slag (GGBS) and/or Ordinary Portland Cement (CEM 1).

[0037] In embodiments of the invention, the encapsulant is added as an aqueous slurry to the containment vessel. In alternative, more typical, embodiments of the invention the encapsulant is added as a dry powder to the vessel prior to mixing. Certain embodiments of the invention envisage the addition of the dry powder to the vessel by adding the dry powder in rupturable containers such as bags. In said embodiments, the encapsulant is typically added to the containment vessel before introduction of the waste via loading means and, after removal of the loading means, rupture of the bags typically occurs during mixing of the components. In said embodiments, said loading means provides protection to such rupturable containers and facilitates the efficient addition of the waste material whilst ensuring that no premature mixing of materials or rupturing of the containers occurs. This is particularly the case when the loading means comprises an inverted hollow conical device which extends down to the base of the containment vessel and which, as a consequence, prevents contact between waste and encapsulant until such time as the device is removed from the vessel.

[0038] In certain alternative embodiments of the invention wherein said encapsulant is added to said containment vessel as a dry powder enclosed in one or more rupturable containers such as rupturable bags, it is envisaged that said rupturable container or containers may be located on the underside of a vessel lid and introduced into the vessel by means of the lid in order to assist in the distribution of encapsulant through the vessel. Either the first (temporary) lid or the second (permanent) lid may be used for this purpose. Thus, in the event that the rupturable container is located on the underside of the first lid, steps (d) and (e) of the method of the invention are essentially performed concurrently whilst, if the rupturable container is located on the underside of the second lid, steps (h) and (i) are essentially performed concurrently. In any event it will be appreciated that, in said embodiments, introduction of the waste material into the containment vessel is effected prior to the introduction of the encapsulant on the underside of the lid. The rupturable containers of encapsulant are pre-loaded onto the lids prior to their introduction to the handling environment and subsequent attachment to the containment vessels; typically, pre-loading is carried out in the import/export chamber.

[0039] In typical embodiments of the invention, tumbling of said vessel to achieve mixing of the waste and encapsulant may be achieved using a compact, flexible, mobile tumbler mixer device. Alternatively, said tumbling may be performed using mechanical handling equipment comprising a rotary tumble mixing device attachment associated with a conventional robotic device such as those supplied by Brokk^®. Motive force to these tumbling devices may be provided by electrical, pneumatic, hydraulic or other suitable means.

[0040] Tumble mixers may be used to mix a wide range of wastes with a very wide size distribution and solids content, and the present invention thus essentially comprises a remotely operated sealed in-drum tumble mixing system for mixing toxic and radioactive waste material with cementitious grout in a compact, controlled and confined unit designed to minimise the movement of material, the spread of contamination and the degree of human exposure to ionising radiation, wherein the drum forms the external containment vessel. Despite the potential environmental and economic benefits, no such system - wherein waste and encapsulant may be tumble mixed in a disposal container which also serves as the mixing vessel - is currently available in the nuclear industry.

[0041] Hence, the current invention seeks to provide a fully flexible encapsulation system which dramatically reduces the risk of contamination and human exposure to radiation, minimises transportation requirements and the number of plant interfaces, and provides the option to store encapsulated waste in situ. The invention thereby offers the potential to considerably improve operating processes while reducing the cost without detriment to the final product. As previously disclosed, the entire process is capable of being performed in the area in which waste is stored.

[0042] Typically, the second, permanent, lid which is placed on the containment vessel comprises a filter which is adapted to prevent any build-up of pressure within the sealed vessel due to gas generation therein. The use of a temporary lid - which does not include such a filter - during mixing operations is, therefore, a prudent course of action in order to prevent any blockage of the lid filter or contamination of the lid surface during tumbling.

[0043] In certain embodiments of the invention, however, said method may additionally comprise the provision of a sacrificial protective layer between the lid/filter and the contents of the vessel. Said sacrificial layer may be sealed between the lid and vessel edges when the lid is located on the vessel and after tumble mixing, the lid is removed and the protective layer is pushed into the waste/encapsulant mixture before curing. Optionally, this protective layer may also include, on its underside, at least one rupturable container of dry encapsulant powder. After curing, a capping layer can be added in the usual manner. The adoption of such a process allows for the protection of the lid filter and, therefore, removes the need for the use of a temporary lid and allows for the use of the permanent lid throughout the procedure, thus, the lid can simply be removed to allow for curing and capping, then replaced on the vessel.

[0044] In certain embodiments of the invention, said vessel may comprise a mould, wherein said radioactive waste and encapsulant are initially introduced into the vessel, mixed and allowed to cure, prior to storage. Said storage may be in situ or ex situ and, therefore, there may or may not be a requirement for transferral of the materials to a second vessel for final storage. In the event that a second vessel is required, the transference of the mixture would occur between steps (g) and (h) of the claimed method.

[0045] In embodiments of the invention wherein said vessel comprises a mould, said radioactive waste and encapsulant are initially introduced into said vessel, then mixed and allowed to cure in order to produce a well bonded monolithic structure. Said mould may be of sufficient dimensions to allow for the maximum possible mixing and waste loading. Typically, the vessel would include a sacrificial thin liner and framework, generally formed from metal, which could include a lifting aid, e.g. a hook or eye, facilitating easy removal, by lifting, of the cured monolith from the vessel and its placement in a second vessel - typically a metal container formed from a metal such as steel. Vacant space within the second vessel may then be filled by the addition of further encapsulant prior to capping and sealing with a lid. In alternative embodiments, the cured monolith may be stored in situ without being transferred to a second vessel. The monolith encapsulation process may, again, be performed in situ - or, alternatively, at a suitable treatment plant.

[0046] In typical embodiments of the invention, the vessels which are employed are of cylindrical construction. However, alternative embodiments of the invention envisage the use of vessels having a rectangular or square shape. In particular, embodiments wherein the vessel comprises a mould including a thin metal liner and lifting aid has the potential to simplify the entire process by effecting relatively simple primary encapsulation at the location at which waste is stored and subsequent final encapsulation, either at the same location or at an alternative location, possibly in a second vessel which has larger dimensions than the first vessel and allows for the insertion therein of further monoliths which can be close packed in a manner which is not possible with a multiplicity of cylindrical monoliths. In such a manner, storage may be achieved in a way which is much better suited to specific building access and layout than is possible according to the known procedures of the prior art.

[0047] In examples of such embodiments, square or rectangular units may be finally encapsulated in a second, square or rectangular, vessel, and the requirement for the application of capping encapsulant in the formation of each monolith would be removed, since this process could be included in the final stage of processing in the second, larger, vessel.

[0048] In certain embodiments of the invention, said containment vessel comprises internal furniture. It has been demonstrated that such internal vessel furniture can play an important role in facilitating efficient mixing during tumbling, when mechanically rotating the vessel. Typically, said internal furniture is permanently secured to the internal surface of the vessel and is adapted to effectively distribute encapsulant throughout the internal volume of the drum. Said internal furniture may comprise at least one linear or curved baffle plate or mesh extending the full length of the vessel, or part way towards the centre of the vessel; alternatively, in the case of a mesh, the furniture may extend across the full diameter or width of the vessel.

[0049] In alternative embodiments of the invention, internal furniture may take the form of helical designs or screw-shaped baffle plate arrangements, or may comprise mobile objects adapted to provide improved agitation and mixing to the system; such objects could, for example, include rabble bars or lengths of solid flexible material - such as, for example, a chain or other suitably shaped object.

[0050] Typical embodiments of the present invention comprise the in situ treatment of waste in a specified location but embodiments of the invention may be envisaged wherein the process of the invention may be carried out in an existing encapsulation plant in order to facilitate simple, safe and swift treatment of waste materials.

Brief Description of the Drawings

[0051] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

Figure 1 shows an embodiment of a typical cylindrical drum including internal furniture which may be employed as a containment vessel in the method of the invention;

Figure 2 depicts a monolith which has been obtained according to the method of the invention using a cylindrical vessel which does not include internal furniture;

Figure 3 is an illustration of a monolith which has been obtained according to the method of the invention using an asymmetric mixing action achieved in a cylindrical vessel which includes internal furniture in the form of a mesh;

Figure 4 provides a view of a monolith which has been obtained according to the method of the invention using a straight mixing action achieved in a cylindrical vessel which includes internal furniture in the form of a mesh, wherein the encapsulant was added as a powder.

Figure 5 offers a schematic illustration of a loading means in the form of a hollow inverted conical device intended to prevent contamination of lid seals and threaded components with waste product debris and encapsulant; and

Figure 6 is a schematic illustration of a compact, flexible tumble mixing unit for use in the method of the invention.

Description of the Invention

[0052] The method of the present invention may be deployed for the immobilisation of toxic and hazardous waste or materials which are found as legacy wastes in the nuclear industry, and thereby finds particular application in the treatment of Intermediate Level Waste (ILW). [0053] The claimed method is especially useful for the encapsulation of materials which are known to be largely passive and non-reactive with the encapsulation material/grout. However, other materials which are highly reactive also frequently require encapsulation, but such materials have the potential to react with cementitious encapsulants. In this case, however, it is possible to apply the method of the invention whilst using alternative encapsulating materials which are compatible with the waste to be immobilised, such that it may subsequently be safely stored. Such encapsulants may, for example, include organic resins, polymers or bonding materials with suitable physicochemical properties. Curing of these systems may be effected by chemical agents or others activation systems, such as UV light.

[0054] Whilst the method of the invention finds particular application in the processing of ILW, the method offers the potential for assignment of the derived in-drum tumble mixed packages as ILW or LLW (Low Level Waste) after mixing. Thus, the claimed method will benefit from the use of vessels which typically comprise relatively cheap and/or off-the- shelf drums as containment vessels and, after filling, these vessels will typically be subsequently over-packed into an ILW container. The method also, however, provides the possibility that, after mixing, each such vessel could be monitored and assigned as either ILW or LLW, which may thereby determine the disposal route. Consequently, there is the potential that waste which is borderline ILW/LLW can be assigned as LLW, thus significantly reducing lifetime storage costs, which are significantly lower for LLW when compared with ILW.

[0055] Hence, using the method and system hereinbefore discussed, it is viable to treat and immobilise highly hazardous waste either in situ or at a location close to the location of the waste source by the use of remote and compact in-drum tumble mixing technology. The claimed method is applicable and well suited to the treatment of many diverse waste sources, which may be in the form of solids, liquids or sludges, and finds particular application in respect of radioactive waste materials.

[0056] The method utilises equipment which is compact and mobile and can, therefore, be deployed in a wide variety of different locations. The typical use of the method for in situ applications minimises the materials transfer requirements and removes the need for new plant, specialist plant interfaces and modification of existing plant when feeds are not compatible with the original plant design criteria. Furthermore, there are major positive impacts in respect of safety, due to the reduced handling and transport requirements, and economic benefits also accrue since all operations can be carried out using a novel and inventive combination of relatively inexpensive existing or slightly modified equipment. [0057] The possibility of utilising vessels having square or rectangular geometry also offers potential for providing packaged waste which is compatible with existing boxes which qualify for containment of highly dangerous wastes, such as radioactive wastes. Thus, relatively small, square or rectangular waste packages may be placed into such existing qualified boxes, so that further validation of the smaller waste packages in their own right would not be required. In addition, these vessels having square or rectangular geometry also provide the opportunity for combination in a densely packed product, which offers significant savings in terms of storage space and, therefore, facilitates major cost savings when compared with conventional round drum packages.

[0058] The provision of an in situ method as herein described also eliminates the requirement for the provision of new buildings and plants, together with the attendant infrastructure, for the processing of waste, and the installation of such in-drum tumble mixing technology into existing encapsulation facilities improves plant flexibility for the treatment of different waste streams and offers the potential for early deployment and accelerated decommissioning at an economically viable cost.

[0059] It is also envisaged that the disclosed method may find application outside the nuclear industry, in the treatment of materials and wastes that are highly toxic and dangerous to the environment, for example heavy metals which, according to current regulations, require dedicated and specialised treatment and storage. Whilst procedures which are presently in use, for the time being, allow for the satisfactory management of these materials, it is possible that a more robust strategy may be required in the future in order to achieve immobilisation and storage, and the method of the present invention offers a suitable means for addressing such issues.

[0060] The present invention essentially relates to a system and method for immobilising and encapsulating waste in situ, or locally, using a tumble mixing system. However, the deployment of remote material handling equipment to load waste into vessels which comprise drums that have been designed for in-drum mixing using a stirrer paddle, or drums designed for flood grout applications, is also a potentially viable option, and the use of these technologies in some such in situ or local niche applications may be envisaged. Generally, the use of remote loading and lidding technology, and other process operations as hereinbefore described for in-drum tumble mixing is also applicable to other forms of in- drum mixing. However, the flexibility associated with in-drum tumble mixing would be removed, and the economic advantages which accrue with the claimed method may be significantly diminished when compared to the fully flexible sealed in-drum mixing method of the invention. [0061] The invention will now be further illustrated with reference to the accompanying Figures and various experimental trials which have been conducted. Thus, in Figure 1 a vessel is illustrated which comprises the typical sort of drum configuration which is suitable for use in the method of the invention. The illustrated cylindrical container (1) includes internal furniture in the form of linear baffle plates (2) which extend through the length of the vessel. The actual design of the internal furniture can be tailored to the requirements of the particular application.

[0062] Experimentally, a set of nine mixing trials was undertaken to illustrate the applicability of the claimed method for a target waste loading of 50% v/v. The performance in each mixing trial was assessed using a test material consisting of coarse sand, 10-20 mm pea gravel and water, which occupied at least 50% of the volume of a 115 litre drum. Typically, 25 kg of sand was placed in the base of the drum, and pea gravel was added to fill half the drum volume. Water was added to just flood the sand and gravel i.e. the top of the water was at the same level as the top of the solids. These waste materials were selected to represent granular material which has previously been shown to cause mixing difficulties with a conventional mixing in-drum (paddle) mixer.

[0063] The cementitious grout consisted of batches of a blended mixture of ground granulated blast furnace slag (GGBS) and Ordinary Portland Cement (CEM I), which was mixed with water to a specific recipe. Typically, the grout consisted of 30 litre batches of a 3: 1 mixture of ground granulated blast furnace slag (GGBS)/Ordinary Portland Cement (CEM 1), with a watercement ratio of 0.45 (w/w). The grout was prepared in a Belle Mini Mix and added to the drum. This incorporated the water and cement at low shear as further mixing was provided within the drum when the grout was mixed with the test material. The grout was a standard formulation, and this could be tailored to meet the specific requirements of particular waste treatments.

[0064] The mixing performance was assessed by removing the drum skin from the cement/waste product following curing, allowing for visual observations of the homogeneity of the product to be made, with particular reference to the distribution of grout and waste materials.

[0065] In order to provide a mixing action, the drum was secured inside the mixing bowl of a conventional concrete mixer, a Belle 200 XT, with all trials being carried out at a mixing angle which was 20° above the horizontal.

[0066] The experimental work studied a range of variables, which included waste loading, mixing time, internal furniture, asymmetric and non-asymmetric (straight) mixing action by off-setting the drum, and grout addition as a powder. [0067] The results which were obtained showed that simple rotation of the basic drum without the incorporation of any internal furniture produced a poorly mixed product, albeit that grout penetration was seen down to the base of the drum. This can be seen from the views of the monolithic product which are illustrated in Figure 2.

[0068] The inclusion of internal furniture in the form of an internal mesh, however, provided enhanced mixing performance, as can be seen from the illustration of a monolith presented in Figure 3. In this case, although the use of an asymmetric tumbling action delivered variable results, a reasonably well distributed cementitious product was obtained.

[0069] Figure 4 shows the final product obtained when adding dry cementitious powders to the vessel in preference to pre-mixed grout; this technique was observed to produce a thicker mix. Mixing was carried out using a straight mixing action in a vessel which included internal mesh furniture, and the results indicated that the increased viscosity achieved using the powdered encapsulant enhanced the mixing of the simulated waste and cement. It can be seen that no layering or distributed uneven mixing zones are evident in the final product, which exhibits a smooth even surface.

[0070] In Figure 5, there is depicted a loading means which comprises a hollow inverted conical device comprising conical body (3) which serves as the transferring means, together with covering means comprising rim collar (4) and thread guard (5) which are adapted to prevent contamination of lid seals and threaded components with waste product debris and encapsulant. Furthermore, the loading means also provides passive metering of the loaded waste and prevents overfilling of the containment vessel.

[0071] Finally, Figure 6 provides a schematic illustration of a compact, flexible tumble mixing unit for use in the method of the invention, the unit essentially comprising base section (6) drive rod (7) including rollers (8) which cause rotation of containment vessel (9) containing the waste and encapsulant.

[0072] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0073] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0074] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

1. A method for the in situ encapsulation of radioactive waste materials, said method comprising the steps of:

(a) providing an empty containment vessel;

(c) introducing a quantity of radioactive waste into said vessel;

(d) introducing a quantity of encapsulant into said vessel;

(g) allowing the encapsulant/waste mixture to solidify (cure);

(h) introducing further encapsulant into said vessel to cap the mixture;

(i) placing a second, permanent, lid on said vessel;

(k) transferring said sealed vessel to a storage location.

2. A method as claimed in claim 1 wherein said handling location comprises a sealed location.

3. A method as claimed in claim 1 or 2 wherein the import/export chamber is provided with washing means and/or monitoring means.

4. A method as claimed in claim 3 wherein said washing means comprises aqueous pressure washing means.

5. A method as claimed in claim 3 or 4 wherein said monitoring means comprises a measuring device adapted to detect contamination from radioactive waste material.

6. A method as claimed in any one of claims 1 to 5 wherein said import/export chamber is adapted so that the handling location remains isolated from the outside environment by the use of separate inner and outer doors.

7. A method as claimed in any preceding claim wherein said vessel is washed, monitored for contamination, and optionally re-washed prior to transfer to storage.

8. A method as claimed in claim 7 wherein washings are recycled and subsequently used as make-up water in the preparation of further batches of encapsulant.

9. A method as claimed in any preceding claim wherein the radioactive waste material and encapsulant are introduced sequentially into the vessel in the handling location.

10. A method as claimed in any one of claims 1 to 8 wherein the waste material is introduced into the vessel in the handling location, the vessel is supplied with a lid and transferred to a treatment plant equipped with said remote mechanical handling equipment for tumbling said vessel for completion of the encapsulation process.

11. A method as claimed in any one of claims 1 to 9 wherein said vessel comprises a mould, wherein said radioactive waste and encapsulant are initially introduced into said vessel, mixed and allowed to cure, prior to transferring to a second vessel for final storage.

12. A method as claimed in claim 11 wherein said vessel additionally comprises a sacrificial thin liner and framework formed from metal.

13. A method as claimed in claim 12 wherein said framework includes a lifting aid, optionally a hook or eye, facilitating easy removal by lifting of the cured monolith from the vessel and its placement in a second vessel.

14. A method as claimed in claim 13 wherein vacant space within said second vessel is filled by the addition of further encapsulant.

15. A method as claimed in any preceding claim wherein interim storage of the encapsulated waste occurs prior to permanent storage.

16. A method as claimed in claim 15 wherein said interim storage occurs following step (g) of said method.

17. A method as claimed in any preceding claim wherein the introduction of materials into said containment vessel is carried out using loading means.

18. A method as claimed in claim 17 wherein said loading means comprises a device which is placed into said vessel prior to introduction of radioactive waste and facilitates loading of waste material whilst avoiding contamination of external surfaces or connecting parts of the vessel.

19. A method as claimed in claim 17 or 18 wherein said loading means comprises covering means and transferring means.

20. A method as claimed in claim 19 wherein said transferring means allows for the metering and control of waste volumes entering the vessel.

21. A method as claimed in any one of claims 17 to 20 wherein said loading means comprises a hollow conical device wherein the base of the cone comprises covering means comprising a protruding collar and transferring means comprising the internal surfaces of the cone.

22. A method as claimed in claim 21 wherein said covering means comprises further attachments adapted to protect the connecting parts of the containment vessel.

23. A method as claimed in claim 21 or 22 wherein said loading means comprising a hollow conical device is deployed in inverted form, wherein the collar at the base of the cone attaches to the rim of the containment vessel and the top of the cone extends to a point adjacent the base of the vessel.

24. A method as claimed in any preceding claim wherein tumbling of said vessel to achieve mixing of the waste and encapsulant is achieved using mechanical handling equipment comprising a rotary tumble mixing device.

25. A method as claimed in any preceding claim wherein said encapsulant comprises a cementitious material.

26. A method as claimed in claim 25 wherein said cementitious material comprises a cementitious grout, optionally granulated blast furnace slag (GGBS) and/or Ordinary Portland Cement (CEM I).

27. A method as claimed in any one of claims 1 to 24 wherein said encapsulant is selected from organic resins, polymers or bonding materials.

28. A method as claimed in claim 27 wherein curing is effected by means of chemical agents or UV light.

29. A method as claimed in any preceding claim wherein said encapsulant is added as an aqueous slurry to the containment vessel.

30. A method as claimed in any one of claims 1 to 28 wherein said encapsulant is added as a dry powder to the vessel prior to mixing.

31. A method as claimed in claim 30 wherein said dry powder is added in rupturable containers.

32. A method as claimed in claim 31 wherein said rupturable containers are added to the vessel prior to introduction of waste via said loading means.

33. A method as claimed in claim 31 wherein said rupturable containers are located on the underside of a vessel lid and introduced into the vessel by means of the lid after the introduction of waste.

34. A method as claimed in claim 33 wherein said lid comprises the first (temporary) lid or the second (permanent) lid.

35. A method as claimed in any preceding claim wherein said containment vessel comprises a drum formed from a metal, optionally steel.

36. A method as claimed in any preceding claim wherein the second, permanent, lid which is placed on the containment vessel comprises a filter which is adapted to prevent build-up of pressure within the sealed vessel due to gas generation therein.

37. A method as claimed in any preceding claim which additionally comprises the provision of a sacrificial protective layer between the lid/filter and the contents of the vessel, wherein said lid comprises a permanent lid comprising a filter.

38. A method as claimed in claim 37 wherein said sacrificial protective layer additionally comprises, on its underside, at least one rupturable container of dry powdered encapsulant.

39. A method as claimed in claim 37 or 38 wherein the lid is removed after mixing and the protective layer is pushed into the waste/encapsulant mixture before curing.

40. A method as claimed in any preceding claim wherein said vessel is of cylindrical construction.

41. A method as claimed in any one of claims 1 to 39 wherein said vessel has a rectangular or square shape.

42. A method as claimed in claim 41 wherein produced monoliths are close packed in a second storage vessel.

43. A method as claimed in any preceding claim wherein said containment vessel comprises internal furniture.

44. A method as claimed in claim 43 wherein said internal furniture is permanently secured to the internal surface of the vessel and is adapted to effectively distribute encapsulant throughout the internal volume of the drum.

45. A method as claimed in claim 43 or 44 wherein said internal furniture comprises at least one linear or curved baffle plate or mesh extending the full length of the vessel or part way towards the centre of the vessel.

46. A method as claimed in claim 43 or 44 wherein said internal furniture comprises a mesh which extends across the full diameter or width of the vessel.

47. A method as claimed in claim 43 or 44 wherein said internal furniture comprises a helical design or screw-shaped baffle plate arrangement.

48. A method as claimed in claim 43 wherein said internal furniture comprises one or more mobile objects adapted to provide improved agitation and mixing to the system.

49. A method as claimed in claim 48 wherein said or more mobile objects are selected from rabble bars or lengths of solid flexible material, optionally chains.

50. A method as claimed in any preceding claim which is applied to the immobilisation of Intermediate Level Waste (ILW).

51. A method as claimed in any preceding claim wherein said waste materials are in the form of solids, liquids or sludges.