GB2448346A

GB2448346A - Nuclear waste disposal

Info

Publication number: GB2448346A
Application number: GB0707037A
Authority: GB
Inventors: Fergus Gibb
Original assignee: University of Sheffield
Current assignee: University of Sheffield
Priority date: 2007-04-12
Filing date: 2007-04-12
Publication date: 2008-10-15
Also published as: GB0707037D0; WO2008125803A1

Abstract

An encapsulation process for actinides comprises immobilisation of the actinides in a mineral or ceramic crystalline structure; mixing said crystals with granulated granite at least partially melting said granite whereby said crystals are dispersed in a liquid phase of melted granite, and cooling the granite so that it recrystallises as a solid radioactive actinides storage package. The term granite is used to mean all igneous rocks which are aggregates of one or more minerals formed by the crystallisation of said minerals from hot silicate liquid. The preferred mineral or ceramic used for immobilizing the actinides is selected from cubic zirconia, urania, zircon, zirconolite, uranite, or a solid solution of uranium dioxide and plutonium dioxide (MOX). The encapsulated product is disposed in a borehole of at least 3km depth in granite.

Description

NUCLEAR WASTE DISPOSAL

This invention relates to a method of disposal of nuclear waste and to a nuclear waste disposal product.

BACKGROUND

Disposal of nuclear waste is obviously an important subject facing the world when demand for non-carbon based energy sources are required and nuclear energy again becomes a potentially environmentally-sound alternative. The present invention is concerned with perhaps one of the most problematic materials involved in current nuclear technology, namely spent nuclear fuel containing high levels of plutonium and other actinides such as americium and neptunium. Apart from the radioactivity of these materials with their long half-lives (in the i04 years regions) they are also among the most chemically toxic of materials. Furthermore, isotopes of plutonium are fissile and decay to uranium and other actinides, some of which are also fissile. The critical mass of such fissile components is of the order of only a few kg. Consequently there is also a security risk, not just from the waste becoming critical, but through the possibility of its being acquired by people who wish to convert it into weapons. Indeed, an important source of plutonium is from dismantling of existing weapons. The worldwide stockpile of plutonium is estimated at about 1800 tonnes.

Ewing [1], Lumpkin [2] and others suggest that plutonium and other actinides may be incorporated in the crystal structure of minerals or ceramics. Different structures have been studied according to different criteria, primarily physical and chemical durability over time spans of 2x105 years. For example, Ewing suggests that between 2 and 10 wt% of plutonium can be incorporated in zircon, but other minerals such as pyrochiore, zirconolite, apatite monazite and baddelyite have been proposed. Lumpkin describes both poly-phase and single-phase waste forms already in use or suggested, such as Synroc-C, -D, and -F which comprise different amounts of zirconolite, perovskite, hollandite, rutile, spinel, nepheline, pyrochiore and uraninite. He also describes other minerals and ceramics including cubic zirconia comprising fluorite-defect fluorite structures based on the general formula MO2 where M is calcium, lanthanides, actinides and other elements including impurities. It does, of course, include plutonium.

However, the perceived problem of all these wasteforms is the radioactivity of the entrapped actinid jufl4jçipI5Is to its metamictisation and transformation from crystalline to amorphous structure. In the latter there is weaker fixing of the actinide and leaching of the actinide out of the wasteform is a considered risk.

It is an object of the present invention, in a first aspect thereof, to provide a process where the potential effects of metamictisation of the crystalline structure in which actinides are locked is reduced. Indeed, in its second aspect, the invention proposes a potentially safe and effective storage structure. In a further development of the invention, in a third aspect thereof, there is provided a storage process further enhancing the potential safety of the process of the first aspect and employing the product of the second aspect.

BRIEF SUMMARY OF THE DISCLOSURE

In accordance with the first aspect of the present invention there is provided an encapsulation process for actinides comprising: immobilisation of the actinides in a mineral or ceramic crystalline structure; mixing said crystals with granulated granite; at least partially melting said granite whereby said crystals are dispersed in a liquid phase of melted granite; and cooling the granite so that it recrystallises as a solid radioactive actinides storage package.

Granite is an igneous rock and is an aggregate of one or more minerals (such as quartz, orthoclase, plagioclase and biotite) formed by the crystallization of these minerals from a hot silicate liquid on cooling (whether geologically or artificially). The term "granite" as used herein should be understood to cover all such igneous or metamorphic rocks. That is to say "granite (sensu Iato)', as intended in the present specification, covers granodiorite, monzonite etc, as well as many high grade metamorphic rocks such as gneiss, which would count as granitic, but are not granite, sensu stricto.

The actinides with which the present invention is concerned include uranium, neptunium, plutonium, americium and curium, of which plutonium is the most important.

Preferably, the radioactive actinides storage package so formed is arranged to have a surface layer devoid of said crystals. Said layer may be provided by partial remelting of granite granules a be provided by pouring said mixture of crystals and granulated granite into a mould in which a surface layer of granite crystals is formed. Indeed, the mould may comprise a solid granite cup.

Preferably, said ceramic is cubic zirconia or urania. Preferably said mineral is zircon, zirconolite or uraninite.

In any event, said ceramic or mineral will be of the type containing large ions (such as zirconium, niobium, titanitium, tantalum, uranium or thorium), which ions can be selectively replaced by plutonium and other actinides.

A wide range of plutonium-bearing phases can be synthesised by a variety of processes known to those skilled in the art and not discussed further herein. Indeed, processes yet to be developed are included, as R&D effort is going on world wide to create and study them.

A particularly promising ceramic is a solid solution of uranium dioxide (the uranium here being depleted, used, or un-enriched -in any event with too low 235uranium content to burn as fuel) and plutonium dioxide known as "Low-spec MOX", which could be made (in existing facilities) using the same (existing) technology as mixed oxide fuel, but in this case for storage and disposal.

Although these phases are stable, there are serious concerns that if and when they end up in a shallow, mined geological repository where groundwater will eventually get at them, the plutonium will leach out and return to the biosphere in a lot less than the million years or so of isolation required. Again, a significant international research effort is going on to try to quantify this leaching behaviour -with mixed, but generally unsatisfactory, results. The situation is further complicated by the damage done to the crystal structure over time by alpha radiation from the plutonium, causing loss of crystallinity (metamictisation or amorphisation), expansion and increased susceptibility to aqueous leaching. All of this uncertainty casts serious doubts on the long-term suitability of these immobilising materials.

However, these problems are overcome by the present invention in incorporating the mineral/ceramic plutonium-bearing crystals into solid granite formed by partial melting and recrystallisation Copied from 0707628 on 09-05-2007 Furthermore, such ceramics and minerals can be dispersed throughout a recrystallised granite or other igneous rock and be in stable, or metastable, equilibrium therewith.

Consequently there will be no tendency for the plutonium-bearing phase(s) to react with the encapsulating granite under any likely geological conditions and for millions of years.

That is to say, the plutonium does not penetrate the granite and, being enclosed in solid granite, water cannot get at the plutonium-beanng phase to leach the plutonium.

Thus, in accordance with the second aspect of the present invention there is provided a radioactive actinides storage package comprising crystals of a ceramic or mineral in which said actinides are immobilised by incorporation into the crystal structure of the ceramic or mineral, said crystals being dispersed in a solid granite matrix.

Preferably, said package is cylindrical, preferably right cylindrical Said crystals may be from 0.1 mm to 10 mm in diameter, preferably from 0.5 mm to 5 mm, more preferably between 1 and 2 mm. Diameter" here means a screen size of x mm by x mm through which the crystals pass, or not, as the case may be, where x is the diameter quoted.

Attrill & Gibb [3] and [4] describe the partial melting and recrystallisation of granite in the context of radioactive waste being disposed in deep bore holes and melting surrounding rock to encase the waste in a granite sarcophagus. They demonstrate the feasibility of creating a monolithic mass of recyrstallised granite after partial melting.

The present invention proposes such a scheme, but not in the context of forming a sarcophagus for a nuclear waste package in a bore hole, but of forming such a package in the first instance. Given that granite is known to survive unchanged for millions of years, the structure should provide a secure storage package for the actinides. Even if the actinide containing crystals become metamict, this will represent merely amorphous blobs within the granite structure. Of course, numerous granites already contain radioactive material and while there are said amorphous parts, they are encompassed by remaining crystalline granite. There is therefore no escape route for the actinides.

Granite is effectively impervious, and consequently leaching cannot happen.

In one respect, such packages can be stored in mined or other nuclear waste repositories. However, the third aspect of the present invention provides a method of Copied from 0707628 on 09-05-2007 disposal comprising the step of disposing a nuclear waste package of the first and second aspects of the present invention in a deep bore hole greater than 3 km depth in granite whose interstitial aqueous component is in stable or metastable equilibrium with said recrystallised granite. That is to say, the granulated granite of the package and the granite of the site at which the deep bore hole is sunk are matched so that the interstitial aqueous component of the host rock does not react with the granite of the package.

Indeed, one way of ensuring this as far as possible is to employ rock taken from the host site in the construction of the package.

Preferably, said borehole is greater than 6 km depth.

Since plutonium waste is relatively rare, there being presently only approximately 1800 tonnes worldwide, said package may be quite small in diameter and yet permit significant quantities to be buried at extreme depth. Thus preferably, the diameter of the package is less than 300 mm.

Such a method has the advantage that water that penetrates along any cracks in the granite is at chemical equilibrium with the surrounding granite so that, when it comes into contact with the package there will be no dissolution of the granite of the package.

Indeed, this is what is meant by the term "granite of the same type" as used above. That is to say, the granite of the package is either the same rock as, or mineralogically and chemically extremely similar to, the host rock in the deployment zone of the deep borehole, such that it is already in chemical equilibrium with the surrounding aqueous brine that fills the hole.

Should the granite package crack at any time, then the water/brine will penetrate the crack and come into contact with actinide containing crystals, but only those on the surface of the crack will suffer any contact with the water and potentially be leached out.

Such potential contamination is minor.

Deep bore hole storage of nuclear waste is a known proposal but is especially attractive for such dangerous waste as plutonium waste since it appears safer, more secure, less environmentally disruptive and more cost-effective.

Once the granite cylinders are sealed in at the bottom of the hole, where they would be surrounded and supported by a backfill of crushed granite, they will eventually become Copied from 0707628 on 09-05-2007 immersed in aqueous fluids seeping very slowly into the borehole from the host rock.

These fluids in deep bore hole conditions several km down in the granitic basement of the continental crust, are likely to be sodium-and calcium-rich brines, which could only pose a leaching threat to the plutonium-immobilising phase(s) if they could penetrate the granite cylinder by some form of dissolution or chemical reaction. Since these fluids would have equilibrated with the host rock (granite) over geological time they would be in equilibrium with it -and hence with the (ideally same) granite used to encapsulate the plutonium-bearing phase(s). Consequently, there would be no tendency to react with the granite cylinders and the plutonium would remain isolated from the borehole environment by three stable or metastable thermodynamic equilibria: (a) within the crystal structure(s) of the immobilising phase(s); (b) between these phases and the encapsulating granite; and (c) between the granite and groundwater. The plutonium would be effectively removed from the biosphere for ever on a human timescale.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments of the invention will now be described with reference to the accompanying drawings in which: FIGURE 1 shows a radioactive actinides storage package according to a first embodiment of the invention in (a) perspective view and (b) cross-sectional view; and FIGURE 2 shows a radioactive actinides storage package according to a variation of the first embodiment in (a) perspective view and (b) cross-sectional view.

DETAILED DESCRIPTION

In a first aspect of the invention a radioactive actinides storage package 10 (FIG. 1) in the form of a cylinder of granite 12 is formed by mixing millimetre to centimetre sized pieces of the waste form 30 with crushed granite 20 and H20 in a suitable cylindrical container of appropriate dimensions for deep bore disposal.

The mixture is then held at over 750 C for -30 days under the appropriate conditions (Pressure = -150 MPa; H20 content -5%; f02 = close to the Ni/NiO buffer) before cooling to under 550 C at less than 0.1 C per hour. From 550 C the now solid granite Copied from 0707628 on 09-05-2007 may be cooled fairly quickly and the cylinder 12 extracted for disposal. The entire process takes about 120 days.

According to the first embodiment the granite cylinder 12 is in the form of a dispersion of pieces of the waste form 30 in a matrix of granite 20. Some pieces of the waste form 30 are exposed at peripheral edges of the package 10 and are therefore likely to come into contact with liquids within the borehole in which the package 10 is to be stored.

However, since the pieces 30 have been formed to a relatively small size, at worst the amount of waste material that can be leached from the package is limited to the small amounts of waste form 30 present at the peripheral edges.

Following manufacture, cooling and interim storage, if required, the granite cylinder 10 is disposed by inserting it into a borehole to a depth of -6 km. According to the present embodiment the borehole is fully cased.

Once deployment of the cylinders is complete, the casing may be withdrawn; however in some embodiments of the invention the casing is not withdrawn.

The borehole is sealed at intervals above the deployment zone. Sealing, which could be performed using a variety of materials and methods including rock welding [5], is intended to deny the disposal zone fluids access to the surface. Eventually, the spaces around the granite cylinders will be invaded by the intra-rock fluids seeping slowly from the enclosing host rock. These fluids will be dense saline brines which have equilibrated with their granitic host over many millions of years [6] and hence will also be in thermodynamic equilibrium with the cylinders of recrystallized granite. There will therefore be no tendency for reaction or mineralogical alteration of the cylinders that might allow the fluids access to the plutonium-bearing waste forms 30. It is noteworthy in this context that natural zircons, monazites and uraninites in granites and similar rocks survive for thousands of millions of years under such conditions without any significant loss of their actinides (uranium and thorium), even when completely metamict.

An important aspect of the present invention is that dissolution of the waste form, reaction with the silicate melt or diffusion of the actinides out of the waste form does not occur during fabrication of the storage packages. Investigations have confirmed that this is the case.

Copied from 0707628 on 09-05-200 7 According to one example, the same procedures as described by Attrill & Gibb [3] were used to enclose zircon, U02 and Ce-doped cubic zirconia in crushed granite, which was then partially melted and held at high temperature and pressure for several months before quenching.

According to this example, a cylindrical pellet (0.19 g) of depleted U02 was sandwiched between two pieces of natural zircon (containing 1.3% Hf) with the ends of the pellet against flat faces of the zircon. The sandwich' was then placed in a gold capsule surrounded by 0. 79 g of crushed granite E93/7 [3] to which 0.02 g of H20 was added (total H20 3.44%). The sealed capsule was then held at 760 C and 150 MPa for 6.6 months, generating -60% melting [3], before quenching. Polished thin sections cut from the sample showed the zircon/U02/zircon sandwich' enclosed in partially melted granite.

The contacts between the quenched silicate liquid and both the zircon and U02 were sharp with no obvious signs of reaction or corrosion. All three types of contact (granite/zircon, granite/U02 and zircon/U02) were investigated by EPMA. A series of analyses across the granite/U02 interface showed no detectable uranium in the granitic melt adjacent to the contact and no detectable silicon, aluminium, sodium or potassium in the U02 close to the granite. Similarly, for the granite/zircon contact, EPMA detected no silicon, aluminium, sodium or potassium in the margins of the zircon crystal and no zirconium or hafnium in the granitic melt adjacent to the contact It is evident from the appearance of the contacts and the EPMA analyses that no dissolution of either the zircon or U02 in the silicate liquid occurred. Nor was there any significant diffusive transfer of elements across the interfaces despite the zircon and U02 having been in contact with granitic melt at 760 C for over 6 months.

To investigate the behaviour of a ceramic-based waste form, a gem quality single crystal of (20%) yttria-stabilised cubic zirconia doped with 0.3% CeO2 was used to simulate tetravalent actinides such as plutonium and neptunium. The 2.5 mm edge cube, weighing 0.1 g, was placed in a sealed gold capsule with 0.73 g of powdered granite and 0.02 g of H20 (total H20 = 3.43%) The capsule was held at 780 C and 150 MPa for 4 months, generating -70% melting [3], before quenching. Optical examination of sections through the sample and SEM imaging revealed a perfectly sharp junction between the zirconia crystal and the granitic melt with no evidence of corrosion or reaction between the zirconia and the silicate liquid. Electron microprobe analyses of the glass immediately adjacent to the interface found no zirconium, yttrium or cerium Copied from 0707628 on 09-05-2007 above the detection limits, indicating that no material had diffused out of the crystal.

Similarly analyses of the edges of the zirconia crystal revealed no silicon, aluminium, sodium or potassium had migrated in from the granitic melt. Laser-ablation ICP-MS analysis along traverses across the interface confirmed the absence of any reaction or diffusion of elements between the zirconia crystal and the silicate liquid during the experiment.

The results of these experiments demonstrate that mineral and ceramic waste forms proposed for the immobilisation of plutonium and other actinides, such as zircon, zirconia and U02 (including low-spec MOX), will not react with, or release their actinides to, granitic melts during the partial melting and recrystallization process required for their encapsulation in granite. Under the conditions of the proposed encapsulation they are either so refractory that the kinetics of any reaction are too slow for any effects to be observed or they are effectively in equilibrium with the granite. It therefore follows that, under the much lower temperatures involved in deep bore hole disposal, these phases, like their natural analogues, will survive and retain their actinides for as long as they are enclosed in the granitic rock and protected from aqueous leaching.

Hence, the plutonium and/or other actinides would be contained in a stable (equilibrium) crystalline structure, which in turn, would be in stable or metastable equilibrium with the granite in which it is encapsulated. After disposal deep in the granitic continental crust, the granite cylinders would be in equilibrium with their host rock and its fluids. This "triple equilibrium" should guarantee isolation of the radionuclides from their environment until the physical destruction of the enclosing crust by geological processes. By even the most conservative estimate this would take many millions, possibly billions, of years.

In a variation of the first embodiment of FIG. 1, as shown in FIG 2, a package 50 is in the form of a granite cylinder 52 in which the plutonium-bearing waste forms 30 are absent from the outer margins 54 of the package. The package is formed by placing the pieces 30 of the waste form with the crushed granite in a granite cup before heating to partially melt the granite. An advantage of such a package 50 is that fluid in the bore hole will not come into contact with any plutonium-bearing waste forms 30.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means Copied from 0707628 on 09-05-2007 "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, 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 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.

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.

Copied from 0707628 on 09-05-2007 References [1] Ewing, Rodney C., Nuclear waste forms of actinides, Proc. Nat!. Acad. Sci. USA Vol 96, pp. 3432-3439, March 1999.

(2] Lumpkin, Gregory R., Ceramic waste forms for Actinides, Elements, Vol. 2, pp. 365-372, December 2006.

[3] Attrill, P.G., Gibb, F.G.F., Partial melting and recrystallisation of granite and their application to deep disposal of radioactive waste: Part 1 -Rationale and partial melting, Lithos, 67 (1-2), pp.103-107, 2003.

[4] Attrill, P.G., Gibb, F.G.F., Partial melting and recrystallisation of granite and their application to deep disposal of radioactive waste: Part 2 -Recrystallisation, Lithos, 67 (1-2), pp.119-133, 2003.

[5] 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. Nuc!.

Tech. (In press).

[6] MoIler, P. et a!. Paleofluids and Recent fluids in the upper continental crust: results from the German Continental Deep Drilling Program (KTB), J. Geophys. Res. 102, 18233-18254.

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Claims

1. An encapsulation process for actinides comprising: immobilisation of the actinides in a mineral or ceramic crystalline structure; mixing said crystals with granulated granite; at least partially melting said granite whereby said crystals are dispersed in a liquid phase of melted granite; and cooling the granite so that it recrystallises as a solid radioactive actinides storage package.

2. A process as claimed in claim 1 in which the radioactive actinides storage package so formed is arranged to have a surface layer devoid of said crystals.

3. A process as claimed in claim 2, in which said layer is provided by partial remelting of granite granules around said package.

4. A process as claimed in claim 2, in which said layer is provided by poring said mixture of crystals and granulated granite into a mould in which a surface layer of granite crystals is formed.

5. A process as claimed in claim 4, in which the mould comprises a solid granite cup forming said layer.

6. A process as claimed in any preceding claim, in which said ceramic is cubic zirconia or urania.

7. A process as claimed in any of claims 1 to 5, in which said mineral is zircon, zirconolite or uraninite.

8. A process as claimed in any of claims 1 to 5, in which said ceramic is a solid solution of uranium dioxide and plutonium dioxide.

9. A process as claimed in any preceding claim in which said crystals are from 0.1 mm to 20 mm in diameter, preferably from 0.5 mm to 5 mm, more preferably between 1 and 2 mm.

Copied from 0707628 on 09-05-2007

10. A radioactive actinides storage package comprising crystals of a ceramic or mineral in which said actinides are immobilised by incorporation into the crystal structure of the ceramic or mineral, said crystals being dispersed in a solid granite matrix.

11. A package as claimed in claim 10, made by a process as claimed in any of claims 1 to 9.

12. A package as claimed in claim 10 or 11, in which said package is cylindrical.

13. A package as claimed in claim 12, in which said package is right cylindrical.

14. A package as claimed in claim 13, in which said package is between 150 mm and 300 mm in diameter.

15. A method of disposal of waste actirüdes comprising the step of disposing a solid radioactive actinides storage package formed by process of any of claims I to 9 or as claimed in any of claims 10 to 14 in a deep bore hole greater than 3 km depth in granite whose interstitial aqueous component is in stable or metastable equilibrium with said recrystallised granite.

16. A method as claimed in claim 15, in which said borehole is greater than 6 km depth.

17. A method as claimed in claim 15 or 16 in which the granite from which the package is formed is from the same granite stock in which said deep bore hole is sunk.

Copied from 0707628 on 09-05-2007