GB2441794A - Method of deep borehole disposal of nuclear waste - Google Patents

Abstract

A method of deep borehole disposal of nuclear waste comprises ```the steps of: encasing the waste as a solid cylinder (30); ```arranging that the cylinder heats by nuclear reactions in the cylinder to a minimum temperature (preferably about 200 {C under the conditions of disposal in a deep borehole, but less than a critical temperature (about 1000 {C) at which the integrity of the cylinder is compromised; ```providing a deep borehole (10) (preferably 3-5 km), lining same with a liner (14) (preferably a perforated liner) and deploying at least one of cylinders in said borehole; and ```after deployment, releasing flowable solid metal beads above said cylinders so that the space (20,22) around the cylinder and between the cylinder and borehole is filled with said beads, wherein, ```the density of said metal is less than the density of the cylinder by no more than 50%; ```the melting temperature of said metal is less than said minimum temperature; and sufficient of the beads are released so that, when they melt and displace flowable material of lesser density than said metal in the space around the cylinder, at least three-quarters of the height of the cylinder is encased in said metal. For preference, the metal beads are an alloy of lead and tin and/or bismuth.

Description

2441794

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Nuclear waste borehole disposal arrangement and method

This invention relates to the disposal of radioactive waste and spent nuclear fuel in deep bore holes.

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BACKGROUND

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Deep boreholes are emerging as a potentially safe, secure, cost-effective and environmentally sound option for the geological disposal of high-level radioactive wastes. 10 The disposal of high-level radioactive wastes (HLW), including spent nuclear fuel, in deep boreholes offers many advantages over more conventional mined and engineered repositories". In particular, the greater depth (3-5 km as against 300-800 m) and better hydrogeologicai conditions provide a much more powerful geological barrier against the return to the biosphere of any radionuclides that might escape from the primary 15 containment. Consequently, deep borehole disposal places more reliance on the geological barrier and less on the engineered barriers, the performances of which are surrounded by uncertainties on the timescale necessary for the isolation of high level nuclear waste (105 - 106 years). In addition to greater safety, other potential benefits of deep boreholes include better cost-effectiveness, higher security (against terrorist or 20 accidental intervention), wider availability of geologically suitable sites and less environmental disruption2,4.

In the U.S.A. the MIT "think tank" report on the Future of Nuclear Power3 recommended to the government and nuclear industry that deep borehole disposal for SNF "merited a 25 significant R&D program". More recently in. the UK, the Committee on Radioactive Waste Management (CoRWM)5, in recommending geological disposal for all HLW to the government, specifically stated that decision making about the exact form of such disposal "should leave open the possibility that other long term management options" [than mined repositories] "(for example, borehole disposal) could emerge as practical 30 alternatives".

Among the different versions of deep borehole disposal currently under investigation2,6,7 is one designed specifically for spent nuclear fuel that we refer to4 as "low-temperature very deep disposal - variant 2" (LTVDD-2). In LTVDD-2, after removal from the reactor 35 and several years of cooling, the spent fuel rods or 'pins' are removed from the fuel assembly and packed into a cylindrical metal (e.g., stainless steel or copper) container.

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Depending on the height of the container, the fuel pins might or might not need to be cut down. The container is then heated and a suitable matrix material, such as molten borosilicate glass or lead, is poured in to fill the voids between the fuel pins. When cool the containers are sealed by welding on the lid and any surface contamination is 5 removed before storing or transporting to the disposal site. At the disposal site the containers are deployed, singly or in batches, into a borehole in, for example, granitic continental crust. When deployment is complete the borehole is sealed at intervals above the deployment zone in order to close off any flow path by which escaping radionuclides might return to the surface. Such sealing could be accomplished by a 10 number of methods and materials but perhaps the safest and most permanent would be 'rock welding' through the partial melting and recrystallization8,9 of the granite wall rock and backfill.

Unfortunately, this option cannot be used with maximum efficiency for spent nuclear fuel 15 because containers Tilled to more than a fraction of capacity with fuel pins would be very dense. Simple stacking of such containers on top of each other at the bottom of a borehole would create load stresses that could threaten the mechanical integrity of the containers near the bottom of the stack. Given the (justifiable) sensitivities surrounding disposal of nuclear fuel, boreholes have to be cased (i.e. lined) because there is no 20 possibility of guaranteeing that there will not be wall-rock cave-ins into the borehole that could prevent proper deployment of containers being lowered into the hole. The casing necessarily creates a space between itself and the borehole wall. Furthermore, given that boreholes are never absolutely straight (indeed, one would not want them to be straight in every situation so that full advantage can be taken of favourable geological 25 conditions) there must be clearance between the container and the casing. Consequently the loading on the containers is substantial. Should they rupture, or there be a risk of their rupturing, this is a significant undermining of the method, compromising its safety by relying mainly on the low hydraulic conductivity of the geological barrier and on the engineered barriers to prevent migration back to the surface through the 30 borehole.

The problem could be avoided by disposing of complete fuel assemblies enclosed in borosilicate glass thus greatly reducing the density of the packages. However, this would be very inefficient use of borehole space and significantly increase costs and the 35 number of holes needed for disposal of any given spent nuclear fuel inventory.

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Another solution might be to deploy the containers singly and fill the gaps with cementitious grout, which is allowed to set before emplacement of the next container. Alternatively, the containers could be deployed in small batches separated by bridging plugs to spread the load of the overlying containers on to the borehole walls. However 5 the former would greatly impair the efficiency and simplicity of borehole disposal, while the latter would present considerable engineering difficulties. Both could subsequently fail as a result of the heat output from the waste.

it is an object of the present invention to provide a system in which this problem is at 10 least mitigated if not cured.

BRIEF SUMMARY OF THE DISCLOSURE

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In accordance with the broadest aspect of the present invention there is provided a 15 method of deep borehole disposal of nuclear waste comprising the steps of:

encasing the waste as a solid cylinder;

arranging that the cylinder heats by nuclear reactions in the cylinder to a minimum temperature required by the conditions of disposal in a deep borehole, but less than a critical temperature at which the integrity of the cylinder could be 20 compromised;

providing a deep borehole, lining same with casing and deploying said cylinder through the casing in said borehole; and after deployment, releasing flowable solid metal beads above said cylinder so that the space around, and for a distance above, the cylinder is filled with said beads, 25 wherein,

the density of said metal is not less than 50% of the density of the cylinder; the melting temperature of said metal is less than said minimum temperature;

sufficient of the beads are released so that, when they melt and displace flowable 30 material of lesser density than said metal in the space around the cylinder, the whole height of the cylinder is encased in said metal.

Preferably, said casing has perforations and said beads at least partly fill the space between the casing and wall of the borehole when they are released above the 35 cylinder and after percolating through said perforations, wherein said space is completely filled after the said metal has melted.

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Preferably, said metal is an alloy of lead and tin or bismuth. The beads preferably are spherical. They may have a maximum dimension of 5 mm, preferably 3 mm.

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Preferably said metal has a density of less than the density of the cylinder and more than 75%, and ideally more than 90%, of the density of the cylinder.

Preferably, sufficient beads are released for the melted metal to remain entirely 10 covering the cylinder. Preferably sufficient are released that unmelted beads remain above melted beads, at least until such time as further cylinders are deployed.

Preferably, said cylinder is a container of said waste, preferably of stainless steel, with spaces between the waste comprising a solid filter. The filler may be glass or lead. 15 In the case of glass, the filler may be borosilicate.

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Preferably the specific gravity of the cylinder is arranged to be between 8 and 12, preferably between 9 and 11.

20 Preferably a plurality of cylinders are stacked in a batch one upon the other in the borehole before releasing said beads above the topmost cylinder of the batch.

Preferably, waste is stored in cooling ponds until such time as it can be processed into said cylinders with guarantee that the cylinder will reach the requisite 25 temperature in the borehole without exceeding the critical temperature. The critical temperature, where the integrity of the cylinder could be compromised, means a design temperature above which unintentional melting, weakening or destruction of the cylinder occurs such that its structural/mechanical integrity is lost and containment of the waste within the confines of the cylinder, with or without any deformation thereof, cannot be 30 guaranteed. This does not mean that the cylinder might not itself include or contain material that is melted at said minimum temperature, which may well be the case, of course, if the filler, where such is present, comprises lead.

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Thus the essence of the procedure is that cylinders (containers) of the waste material are deployed when it is known that, in the absence of external cooling beyond the convection and conduction that will result in the conditions of the borehole (ie when

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surrounded by molten metal such as lead or a lead/tin or like alloy), the temperature of the cylinder will rise over a period of time, probably weeks and months, to above, say, 130°C (which may only be a few degrees higher than the resident temperature at that depth), and probably somewhat higher in the region of 250SC, but not reach 5 temperatures in the order of more than 1000°C, which may be regarded as the critical temperature. The specific gravity of the cylinder is arranged to be of the order of 10, possibly by the inclusion of dense uranium metal based spent fuel as a component. The specific gravity of the metal beads would be arranged to be about 9 and to have a melting (solidus) point of a minimum of 120°C, and probably about 200°C. When first 10 deployed, the beads simply flow around the containers, perhaps five of them stacked one upon the other over a period of a few days. Sufficient beads are deployed to overfill the space around the cylinders to a specific height above them. It is unlikely that the beads will fail to fill the volume of space between the containers and the liner, or fail to spill through the perforations in the liner, of which there would be many and of 15 sufficiently large dimensions not to inhibit flow therethrough of the beads.

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After a time (some weeks or months) the beads start to melt and the level of the beads above the cylinders fall as they further compact, filling any remaining voids (e.g., between the casing and borehole wall), but primarily as they melt and displace upwardly 20 the aqueous fluids that will almost certainly be at the bottom of the hole filling the spaces not occupied by the beads, cylinder and casing. Indeed, as the melting progresses, the melted metal may begin to penetrate the cracks (if any) in the surrounding rock formation although as it gets further from the borehole the temperature will drop and the furthest reaches of the melt will form an impenetrable barrier to intra-rock fluids 25 accessing the borehole. After a period of many months and possibly some years, the waste will have cooled until the temperature drops below the melting point of the metal. At this time the waste is encased in a solid high-density support matrix extending into the surrounding geological formation sealing all gaps.

30 After initial deployment of the beads further cylinders can be lowered onto the floor of the borehole (now formed by beads above cylinders below). It is not anticipated that many iterations of the stacking procedure will be effected while the beads around the bottommost cylinder are still solid. Otherwise cylinder integrity would be relying only on the solid beads to support the cylinders. Instead, it is intended that the beads around 35 the lowermost cylinders will have melted by the time a significant number of cylinders are stacked above. That is to say, before such number of cylinders has been added that

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could otherwise, in the absence of substantial support of the cylinders, lead to mechanical rupture of the cylinders. However, when the metal beads melt, two effects are experienced by the cylinders. Firstly, they have complete support from all directions by the metal contacting all surfaces of the cylinder. Secondly, they ideally have only 5 marginal (preferably as little as 10% or less) negative buoyancy, in the metal matrix. Of course, if the metal was denser than the cylinders, then the cylinders would tend to float in the metal, and arrangements would then be necessary to hold the cylinders down and prevent them from floating above the melted metal. However, this is not very practical, and it is preferred that the density of the metal is less than the cylinders. In that event, 10 the cylinders will sink into the metal and any weight on a cylinder below is only the difference between the densities of the cylinder and matrix, multiplied by the volume of the cylinders above. Consequently many more cylinders can be stacked one upon the other. Indeed, as the beads below begin to melt, the cylinders above will sink, but there is no reason to believe that this will be a sudden process; rather the cylinders will more 15 likely settle gently onto one another as the beads melt and molten metal and less dense aqueous fluid are displaced upward between the container and the casing.

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Preferably, said cylinders are deployed over the bottom 500 to 1500 m of the borehole which is preferably 3000 to 5000 m deep. Preferably the diameter of the borehole is up 20 to 1.0 m. More specifically, the bottom 1000 m may be employed and the borehole may be one up to about 0.8m in diameter.

Thus, not only does the invention provide a method of disposing of a substantial quantity of nuclear waste in a single borehole, but it does so in a particularly safe 25 manner. In due time, the waste becomes encapsulated in an impermeable matrix of high density material that not only prevents physical leakage of the hazardous material, but also reduces radioactive emissions. The full advantages of deep borehole storage can now be realised, wherein the risk that ground water that may ultimately come to the earth's surface contacting the waste, and becoming contaminated, is almost entirely 30 eliminated.

In addition, in the nuclear industry, a large volume of lead is employed, mainly as radiation shielding that ultimately becomes contaminated, that also has to be disposed of. Conversion into lead shot or beads for use in the present invention both provides a 35 suitable application of such lead as well as disposal thereof.

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BRIEF DESCRIPTION OF THE DRAWING

Embodiments of the invention are further described hereinafter, by way of example, with reference to the accompanying drawings, in which 5 Figure 1 is a schematic representation of the bottom one of a stack of radioactive waste containers in a borehole; and

Figures 2a and b give the binary phase diagrams10 for Pb-Sn and Pb-Bi alloys.

DETAILED DESCRIPTION

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In Figure 1 of the drawings a borehole 10 is drilled into the earth's crust at a geologically stable location, preferably through granite rock 12. The borehole is about 4 km deep and may be one of several adjacent boreholes. Indeed, it may be one of several branches from a single bore hole. In any event, at the bottom 16, and throughout its 15 depth from the surface 18, it is lined with a steel casing 14. The diameter of the hole 10 is about 0.8 m. The casing is perforated, at least over that length of it that is intended, in use, to receive nuclear waste containers. The perforations may be round or elongate but should be arranged so that metal beads, described further below, will pass through. The skilled person can determine what size, shape, frequency and distribution of 20 perforation is required, given the objectives discussed further below.

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By the necessity of ensuring reliable insertion of the casing 14 in the borehole 10, there is a space 20 between the casing and the borehole 10. The casing ensures that rock fragments breaking from the bore wall do not block insertion of the containers. Indeed, it 25 is primarily for this reason that the casing is employed. Any failure to deploy the casing simply means that the casing has to be withdrawn and the hole reamed or redrilled. Since that is a considerable waste of effort and resource, it is not until after many attempts have been made to drive the casing past an obstruction that this retreat occurs. By then, however, it may be impossible to withdraw the casing, and the entire borehole 30 may have to be abandoned, or the casing fished out. In any event, the consequences of such an event are merely inconvenient and costly. If the same problem occurred with a container of nuclear waste (in the absence of casing), the consequences could be more serious . Thus, providing the borehole with a smooth sleeve down which containers of waste can reliably be slid without risk of jamming is fundamental to the process, 35 although not to the underlying principles of the present invention.

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When a sleeve 14 is employed, it is preferably perforated. Without wishing to be limited to any particular format of perforations, they may cover between 5% and 30% of the surface area of the casing wall. The maximum diameter of a circle fitting in the perforations is preferably between 10 and 150 mm, and preferably about 50 mm, so that the risk of rock fragments protruding through a perforation and substantially intruding into the casing interior is remote. In this respect, slotted perforations may be preferred. The perforations must permit circles of a minimum diameter of 5 mm to be fitted in them, and preferably circles greater than 10 mm. This is to permit relatively unimpeded penetration of the lead shot (metal beads) when it is deployed.

Also, to avoid any risk of jamming, especially since boreholes are seldom absolutely straight and containers likely to be employed may be 3 to 5 metres in length (for example, if they contain uncut nuclear fuel rods, which generally are in that order of length) there is also plenty of clearance provided between containers 30 and the inner 15 wall of the casing 14. in the drawing a container 30 is shown deployed at the bottom of a borehole and further containers 30a stacked above. Clearance space 22 is provided between the container 30 and casing 14. Typically, in a borehole of 0.8 m diameter, the casing has an outside diameter of 0.73 m and an inside diameter of 0.7 m. The containers will have a maximum diameter of about 0.63 m giving a clearance of 0.035 m 20 all around. The containers typically have a wall thickness between 0.035 and 0.05 m and may be of 3.75 to 4.5 m height.

If no further support for the containers is provided, the integrity of the lowermost containers could not be guaranteed, with the weight of those containers above being 25 taken alone by the top of the containers beneath. Accordingly lead shot is deployed or released above the topmost container of a batch of containers already deployed. The batch number may be one, or it may be five or more. Given the role that the lead shot plays, it can in fact be desirable to arrange that the clearance between the container and casing is generous. In any event, both the clearance and the perforations need to be 30 such that when the lead shot is deployed, there is no question but that it will percolate past containers and through the perforations and fill the spaces between the container 30 and casing 14 and between the casing 14 and borehole 10.

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Although also referred to as lead shot, the shot likely comprises beads of a lead/tin/bismuth/zinc alloy of such composition to provide both the most desirable density and desirable solidus and liquidus temperature. The beads are about 3 mm in diameter

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although this is not critical. Indeed, the beads could be anywhere from 0.1 mm in diameter up to about 5 mm, and possibly even outside this range. The person skilled in the art is able to make the selection of size and constituency in dependence upon the circumstances.

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The density of the alloy should ideally be about 90% of the density of the container. Of course, the construction of the container is also manageable to arrive at a particular density, most likely in the region of a specific gravity of between 10 and 12 (but up to 14 if the waste spent fuel is U metal based, as in the UK's Magnox reactors). The container 10 30 is generally a stainless steel container of sufficient thickness and construction to withstand the rigours of transportation. It is generally filled with waste and then heated to a temperature at which molten glass, or lead, can be poured into the container to fill all the spaces between the waste with relatively incompressible material. However, the container could conceivably be omitted since it is provided primarily only for the 15 transportation protection role that it has. Once deployed, it potentially serves no further function in the present invention. Thus the waste could conceivably simply be moulded into a cylindrical shape with lead or glass or a suitable alloy. The consequences of not retaining the integrity of the package of waste in the event of the filler melting would, of course, need to be assessed and provided for. For example, in the case of fuel rods, 20 these could simply be bound with steel strapping into bundles. However, providing a container that does not melt is possibly the securest and simplest option. In any event, the cylinder is arranged to have a particular known density and a particular known temperature to which it will heat when deposited at the base of the bore hole. The metal beads are likewise arranged to have an appropriate density (probably 90-98% that of the 25 cylinder) and an appropriate melting point temperature (if pure metal) or solidus and liquidus temperatures (if an alloy) (probably in the region of 50-90% of the expected maximum temperature to be reached by the cylinder).

The metal shot would itself function as a dense support matrix but, shortly after 30 emplacement, the heat from the radioactive decay of the waste would melt the metal allowing it to flow freely into any remaining voids. It would also displace upwards the aqueous borehole fluid from between the grains of shot. It should be noted that this fluid remains as a liquid at temperatures well in excess of those likely to be generated by the waste as a result of the confining pressure, which can exceed 150 MPa at such depths 35 once the borehole is sealed6,7. Over the space of a few months to a few years the

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molten metal will cool and solidify to encase the waste packages in a base metal lined coffin or matrix. Put another way, the containers are soldered into the borehole.

The exact composition of the Pb-based alloy used is tailored to have the correct 5 (beginning of) melting point and liquid specific gravity for the disposal scenario. This specific gravity should be such that the containers 30 have only a slight negative buoyancy, thus reducing the effective weight of the stack so that the load on the bottom containers is minimised and the container can easily withstand deformation until long after the matrix has solidified around it. The melting point (or the beginnings of melting 10 in the case of alloys) is less critical as long as the temperatures generated by the decay of the waste are sufficient to melt the beads but, in cases where the heat output is low, this may become an important factor.

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A wide range of suitable alloys is available in the system Pb-Sn-Bi (see phase diagrams 15 in Figures 2a and b) but Zn, Cu, Sb or other metals could replace Sn or Bi or be added to make ternary or quaternary alloys. Specific gravity rises with Pb content, while increasing the amount of Sn or Bi is the simplest way of lowering the solidus temperature. At one atmosphere Pb-Sn alloys begin to melt at 183°C and Pb-Bi alloys at 126°C. As far as the inventor is aware, these beginnings of melting temperatures 20 have not been determined for 150MPa but, since this increase in pressure only raises the melting points of pure Pb, Zn and Sn by 10°C, 6°C and 4°C respectively11,12 the increases in beginning of melting temperatures for binary and ternary alloys should be similarly small. In cases where the specific gravity of the waste packages is greater than 11.3, Pb could be used on its own as a matrix material, with significant benefits. Simple 25 binary alloys of Pb and Sn offer a range of specific gravities between 7.27 and 11.3 and one atmosphere melting (solidus) points as low as 183°C.

Once a particular alloy composition has been selected there are two approaches to its manufacture and use. The alloy can be manufactured as shot and emplaced down the 30 drill string or deployment string of the borehole, releasing it a short distance ( 5 to 20 metres) above the most recently inserted container. Alternatively, the components of the target alloy can be supplied as pure metal shots and mixed in the required proportions before deployment. The decay heat of the waste then creates the alloy in situ by melting the mixture. The latter has obvious benefits of economics and flexibility but could run 35 into difficulties if heat outputs from the waste were barely adequate to melt the mixture.

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Although described and defined herein as a cylinder, a perfect geometric cylindrical shape is not intended or required. Indeed, while it would not appear to have any advantages, the waste could be formed into a spherical or other non-cylindrical shape. However, in most practical applications, the cylinder will be of constant cross-section 5 over most of its length. This cross section is conveniently but not necessarily circular. Moreover, the cylinder may be shaped at its ends to ensure that each cylinder seats squarely on the one below. Thus the base of a container may be domed, while the top is dished, or vice-versa. However, it should be understood that this is not essential. Neither is it essential that the containers are central in the casing; they can be displaced 10 radially so that one edge contacts the casing wall. In this event, more of the beads fall to one side, but once they melt the liquid metal will flow into all crevices displacing lower density liquids such as the aqueous fluids that will inevitably be present. Indeed, in this respect, it is pointed out that while the lead does shield the environment against penetration by radioactive emissions from the waste, this is not an important role since, 15 at 3 km depth, such emissions are not consequential. On the other hand, provided all spaces into which the cylinder might distort by the pressures exerted from cylinders above have been filled, then how close the cylinder is to the casing wall is irrelevant. In any event, the provision of fins on the cylinder wall or other centralising mechanisms (provided they do not provide a jamming risk) is not excluded by the present invention.

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Incidentally, before the first cylinder is deployed, it may be desirable to release a first quantity of the metal beads to line the base of the borehole and provide a seat for the first cylinder.

25 As mentioned above, the casing is preferably perforated. One fundamental reason for this is that a steel casing with solid walls is simply too heavy to be handled with current technology. Of course, in principle, it would be possible to construct the casing so strongly that it alone can support the loading of a high density column of metal beads and containers. In that event, no perforations would be provided. However, this misses 30 two opportunities. The first is the possibility of disposing of more radioactively-contaminated lead. The second is in avoiding the need to provide such a very strong casing. Indeed, with the present invention, the casing need only be strong enough to withstand the rigours of its deployment in the borehole, and of receiving containers. A third problem with this approach is simply the substantial weight of such a solid casing 35 that would in most practical circumstances preclude such an arrangement altogether. Nevertheless, such a strong, unperforated casing could possibly be constructed using

lighter materials, eg carbon fibre reinforced plastics. However, whether or not such a solid casing is strong enough, the arrangement employing it is still less preferred; it leaves the gap between the casing and bore-hole wall unfilled, and that gives the opportunity for liquids to circulate around the casing and over time possibly bring radioactive material to the surface should the casing ever be compromised. In that respect, the borehole wall does nothing to support the casing. Consequently, while still being a theoretical possibility, a perforated casing as described above is preferred.

Finally, once the bottom 1000 m of a borehole has been filled with containers 30, the remaining hole is back filled with crushed granite, concrete or like material 24. Indeed, rock welding may be effected at some height above the topmost cylinder if additional security against possible leaching of radioactive material up the borehole is desired. Preferably, the top most cylinder is more than 3000 m below the earth's surface.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does 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.

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.

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.

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,

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may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract 5 and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

10 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.

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REFERENCES

1. Chapman, N. & Gibb, F. A truly final waste management solution. Radwaste Solutions 10/4, 26-37 (2003).

2. Gibb, F. Very deep borehole disposal of high-level nuclear waste. Imperial

20 Engineer (Royal School of Mines, London) 3,11-13 (2005).

3. M.l.T. The Future of Nuclear Power An Interdisciplinary MIT Study. Massachusetts Institute of Technology, Cambridge. 170 pages. (2003).

4. Gibb, F.G.F., Travis, K.P., McTaggart, N.A., Burley, D. & Hesketh, K.W. Modelling temperature distribution around very deep borehole disposals of HLW. Nuclear

25 Technology (In press)

5. CoRWM. Managing Our Radioactive Waste Safely: CoRWM's Recommendations to Government. Committee on Radioactive Waste Management, London. 192 pages. (2006).

6. Gibb, F.G.F. A new scheme for the very deep geological disposal of high-level

30 radioactive waste. J. Geol. Soc. London 157, 27-36 (2000).

7. Gibb, F.G.F. & Attrill, P.G. Granite recrystallization: the key to the nuclear waste problem. Geology, 31,657-660 (2003).

8. Attrill, P.G. & Gibb, F.G.F. Partial melting and recrystallization of granite and their application to deep disposal of radioactive waste. Part 1 - rationale and partial

35 melting. Lithos, 67,103-117 (2003).

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9. Attrill, P.G. & Gibb, F.G.F. Partial melting and recrystallization of granite and their application to deep disposal of radioactive waste. Part 2 - recrystallization. Lithos, 67,119-133 (2003).

10. ASM Handbook, Vol.3, Alloy Phase Diagrams (Ed. H.Baker), The Materials Information Society, Materials Park, Ohio.

11. Akella, J., Gangully, J., Grover, R. & Kennedy, G. Melting of lead and zinc to 60 kbar. J. Phys. Chem. Solids. 34,631-636 (1973).

12. Bamett, J.D., Bennion, R.B. & Hall, H.T. X-ray diffraction studies on tin at high pressure and high temperature. Science, 141,1041-1042 (1963).

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Claims (22)

CLAIMS • •• • • • • ••• • « • ••• «••• • • • • • • • •• • • • • • • • •• • • • • • • • ••

1. A method of deep borehole disposal of nuclear waste comprising the steps of:

5 encasing the waste as a solid cylinder;

arranging that the cylinder heats by nuclear reactions in the cylinder to a minimum temperature under the conditions of disposal in a deep borehole, but less than a critical temperature at which the integrity of the cylinder is compromised;

providing a deep borehole, lining same with casing and deploying said cylinder 10 through the casing in said borehole; and after deployment, releasing flowable solid metal beads above said cylinder so that the space around the cylinder is filled with said beads, wherein,

the density of said metal is not less than 50% of the density of the cylinder; the melting (solidus) temperature of said metal is less than said minimum 15 temperature; and sufficient of the beads are released so that, when they melt and displace flowable material of lesser density than said metal in the space around the cylinder, at least the full height of the cylinder is encased in said metal.

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2. A method as claimed in claim 1, in which said metal is an alloy.

3. A method as claimed in claim 2 in which the metals of the alloy are added as separate beads of component metal which, when melted and mixed together, form the alloy of desired composition.

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4. A method as claimed in claim 1 or 2, in which the alloy is of lead and tin and/or bismuth.

5. A method as claimed in any preceding claim, in which the beads are 30 spherical.

6. A method as claimed in any preceding claim, in which the beads have a maximum dimension of 5 mm, preferably 3 mm.

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7. A method as claimed in any preceding claim, in which said metal has a density of less than the density of the cylinder and more than 75%, preferably more than 90%, of the density of the cylinder.

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8. A method as claimed in any preceding claim, in which sufficient beads are released for the melted metal to remain entirely covering the cylinder.

9. A method as claimed in claim 8, in which sufficient beads are released that unmelted beads remain above melted beads, at least until such time as further

10 cylinders are deployed.

10. A method as claimed in any preceding claim, in which said cylinder is a container of said waste.

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11. A method as claimed in claim 10, in which the container is stainless steel.

12. A method as claimed in claim 10 or 11, in which spaces between the waste in the container comprises a solid filler.

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13. A method as claimed in claim 12, in which the filler is glass or lead.

14. A method as claimed in claim 13, in which the filler is borosilicate glass.

15. A method as claimed in any preceding claim, in which the specific gravity

25 of the cylinder is arranged to be between 8 and 12, preferably between 9 and 11.

16. A method as claimed in any preceding claim, in which a plurality of cylinders, preferably between 3 and 7 of them, are stacked in a batch one upon the other in the borehole before releasing said beads above the topmost cylinder in the batch.

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17. A method as claimed in any preceding claim, in which the waste is stored in cooling ponds until such time as it can be processed into said cylinders with guarantee that the cylinder will reach the requisite temperature in the borehole without exceeding the critical temperature.

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17

18. A method as claimed in any preceding claim, in which said cylinders are deployed in the bottom 500 to 1500 metres of the borehole, which is preferably 3000 to 5000 m deep.

5

19. A method as claimed in any preceding claim, in which the diameter of the borehole is up to 1 m.

20. A method as claimed in any preceding claim, in which said liner has perforations and said beads fill the space between the liner and wall of the borehole

10 when they are released above the cylinder and after percolating through said perforations.

21. A method as claimed in any preceding claim, in which the cylinder is arranged so that it reaches a maximum temperature between 150°C and 600°C over a

15 period of months after deployment in the borehole and the melting point/soiidus temperature of the metal beads is between 130°C and 350°C.

22. A method of storing nuclear waste substantially as hereinbefore described with reference to the accompanying drawings.

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