AU3921699A - High level nuclear waste disposal - Google Patents

Description

WO 99/60577 PCT/AU99/00376 -1 HIGH LEVEL NUCLEAR WASTE DISPOSAL This invention relates to the treatment and disposal of high level radioactive wastes (HLW) from nuclear reactors, and in particular relates to a mineral assemblage incorporating HLW, 5 to a process for producing such a mineral assemblage and to a process for immobilisation of HLW in a mineral assemblage which will retain dangerously radioactive isotopes in the waste for periods sufficient to ensure that they do not re-enter the biosphere prior to their effective decay. 10 HLW such as spent fuel from nuclear reactors, as are used in commercial power plants, contains a wide range of highly radioactive isotopes. These radioactive isotopes emit radiation which is dangerous to living organisms and must be disposed of in such a manner that they do not re-enter the biosphere during their effective decay periods. One group of these isotopes is formed by the fission of uranium (and plutonium). From the disposal point 15 of view the most important components formed by such fission are 137Cs, 90Sr and the radio-active isotopes of the actinide elements. The fission products 137Cs and 90Sr have half lives of about 30 years and must be contained for a period of about 600 years before they decay to safe levels. After 600 years, the dominant radioactive species in the waste are the actinide elements, principally isotopes of Pu, Am, Cm and Np which decay by the emission 20 of alpha particles. After about a million years, the activity of the waste becomes comparable to that of the original uranium which was mined to produce the nuclear fuel. This is usually taken to be a suitable time for containment. Spent fuel rods are generally reprocessed to recover plutonium and unused uranium. During 25 this reprocessing the spent fuel rods are placed in cooling ponds for several years to permit the decay of several highly radioactive, short-lived fission products. Subsequently, the rods are chopped into sections and dissolved in nitric acid. Plutonium and uranium are recovered from the solution, the remainder of which is a HLW solution.

WO 99/60577 PCT/AU99/00376 -2 In most cases these HLW solutions are transformed initially into a solid, insoluble form. This is accomplished in the first instance by evaporating the HLW solution to dryness and calcining the material to produce a fine-grained mixture of radioactive oxides-called "calcine". Calcine 5 is an unsatisfactory form for disposal because of its low density, low thermal conductivity and high solubility. Thus, further processing of this material is necessary for its safe disposal. One approach has been to incorporate the HLW calcine into a borosilicate glass. The glass is contained in thick stainless steel cylinders for burial in suitable geological environments. 10 The shortcomings of this technique are well recognised and include the thermodynamic instability of glasses which is likely to lead to leaching of the HLW elements over time. Another approach to the problem has involved the incorporation of HLW calcines into ceramic materials composed of crystalline phases. Whilst this approach offers some 15 advantages over the glass technique there are still a number of recognised disadvantages which result in the ceramic materials not being ideal for long term containment of HLW components. A number of the problems inherent in the above approaches have been addressed by the use 20 of a synthetic rock to retain the HLW elements. A synthetic rock known as SYNROC is described for example in United States Patent No. 4,274,976. The SYNROC materials are a mineral assemblage containing well formed crystals capable of providing lattice sites in which the elements of the HLW are securely bound. The crystals belong to or possess crystal structures closely related to at least two of the titanate mineral classes selected from the group 25 consisting of perovskite (CaTiO 3 ), zirconolite (CaZrTi 2

O

7 ) and hollandite-type (BaAl 2 Ti60 1 6 ) mineral classes. The SYNROC materials have been extensively investigated and are predicted to provide stable immobilisation of HLW elements, allowing the assemblage to be safely buried in an appropriate geological environment. Consequently, SYNROC materials are in the process of commercial development for the storage of HLW wastes. 30 WO 99/60577 PCT/AU99/00376 -3 The extensive investigations into the SYNROC materials has led to the incorporation of rutile, and often with a small percentage of titanium metal. It has been reported that the preferred formulations of SYNROC have been designed to avoid destabilisation and incorporate an excess of rutile TiO 2 such that rutile is the major phase of the SYNROC assemblage. 5 Whilst SYNROC assemblages are preferably formulated with substantial amounts of rutile in order to improve stability, rutile plays little or no part in taking up the HLW. The present invention is directed to an alternate assemblage which does not contain significant quantities of rutile. Experiments associated with the present invention have identified that mineral 10 assemblages which include calzirtite provide at least a useful alternative to the previously disclosed SYNROC compositions. Accordingly, in one aspect this invention provides a mineral assemblage comprising crystals belonging to or possessing crystal structures closely related to both the perovskite and 15 zirconolite mineral classes and further comprising crystals belonging to or possessing crystal structures closely related to the calzirtite mineral class wherein the mineral assemblage incorporates high-level radioactive wastes immobilized therein. In the present specification, including the claims, the description of a mineral complex which 20 forms part of a mineral assemblage is generally qualified by the term "crystals belonging to or possessing crystal structures closely related to ". This term will be understood by those skilled in the art to refer not only to the mineral complex having an ideal crystal structure but also to mineral complexes incorporating additional elements therein, such as elements replacing one or more of the elements of the ideal crystal structure or additional elements 25 retained interstitially within the ideal crystal structure. The additional elements may give rise to a departure of the crystal structure from the ideal. Perovskite is a complex of calcium and titanium oxides having an ideal formulation CaTiO 3 . Perovskite-type structures are adopted by ABO 3 compounds such as CaTiO 3 perovskite. The 30 ideal CaTiO 3 component is of orthorhombic symmetry.

WO 99/60577 PCT/AU99/00376 -4 Zirconolite is a complex of calcium, titanium and zirconium oxides having an ideal formula CaZrTi 2 0 7 . Zirconolite may be more generally described as CaZrxTi 2 -xO 7 and is a generic term to encompass a group of closely related structural polytypes which occur in monoclinic, trigonal and orthorhombic polytypes. 5 Calzirtite is a complex of calcium, titanium and zirconium oxides having an ideal formula Ca 2 ZrTi 2

O

16 . Calzirtite may be more generally described as Ca2Zrs 5 xTiO 16 and is a generic term to encompass a group of closely related structural polytypes. Both calzirtite and zirconolite are anion deficient fluorite-related superstructure phases. In calzirtite, of ideal composition 10 Ca 2 ZrTi 2 0 1 6 , the cations occupy fluorite-type positions in the tetragonal cell. The mineral assemblages of the present invention preferably comprise crystals of mineral complexes which are relatively small in size, in order to maximise diffusion controlled uniformity of HLW incorporation into desired crystalline structures. Preferably the crystals of 15 the mineral assemblages are generally of up to two hundred microns in size. We have found that the presence of titanium in the crystal structures of the crystals of the mineral assemblages of the present invention where the titanium is of at least two different co-existing valency states permits a more stable immobilization of the radioactive isotopes of 20 the HLW within the mineral assemblage. The presence of Ti 3 " and Ti 4+ is particularly preferred. In the present invention, particularly where it is desired to incorporate Ti in a number of different valency states, Ti 2 0O 3 may be present in solid solution in the crystals of the mineral assemblage. 25 Mineral complexes incorporating other elements may advantageously be incorporated into the mineral assemblages of the present invention. It is preferred that mineral complexes which incorporate barium and/or aluminium be incorporated into the mineral assemblages. Hollandite of general composition AxByC,yOl 6 , ideal end member (BaAl 2 T60 1 6 ) is particularly preferred as a host phase for HLW elements together with perovskite, zirconolite and calzirtite. 30 Under certain conditions of production, other Ba, Al phases may also occur. The presence of these elements and crystal structures is particularly preferred for hosting certain HLW elements, WO 99/60577 PCT/AU99/00376 -5 which otherwise do not readily partition into the other phases. Other hollandite-type mineral complexes which may be included within the mineral assemblages include K and Sr replacing Ba. Preferably, the assemblage includes at least some of each of the calzirtite, zirconolite, 5 perovskite and hollandite. More preferably, the assemblage includes at least 10 weight percent of each of calzirtite, zirconolite, perovskite and hollandite. We have found that the mineral assemblages may include at least 20 weight percent HLW. 10 The present invention also provides a process for immobilizing high level radio-active waste comprising the steps of: a) mixing a high level radio-active waste calcine with selected oxides; 15 b) heating and cooling said mixture to form a mineral assemblage comprising crystals belonging to or possessing crystal structures closely related to both the perovskite and zirconolite mineral classes and further comprising crystals belonging to or possessing crystal structures closely related to the calzirtite mineral class wherein the mineral assemblage incorporates high-level radio-active wastes immobilized therein. 20 The high level radio-active waster calcine is generally formed from a solution of HLW such as may be produced from commercial nuclear power plants. The calcine may typically be formed by evaporating the solution of HLW to dryness and calcining the material to form a fine-grained mixture of radio-active oxides. The composition of a typical HLW calcine 25 resulting from the fission of uranium (and plutonium) is set our in table 1 below: WO 99/60577 PCT/AU99/00376 -6 TABLE 1 Typical composition of calcined high level nuclear reactor wastes 5 Mole per cent Rare earths (REE elements) 26.4 Zr 13.2 Mo 12.2 Ru 7.6 10 Cs Fission 7.0 Products Pd 4.1 Sr 3.5 Ba 3.5 Rb 1.3 15 U + Th 1.4 Actinides Am + Cm + Pu =Np 0.2 Fe 6.4

(PO

4 ) Processing contaminants 3.2 20 Na - 1.0 HLW elements may be incorporated into this mineral assembly by adding an HLW calcine mixture prior to heating. The HLW calcine may make up, up to about 20% by weight of the mixture of oxides. 25 The oxides may be mixed by any convenient means and heated by processes as are known to those skilled in the art.

WO 99/60577 PCT/AU99/00376 -7 The oxides are selected, having regard to their composition and their relative proportions so as to form the desired mineral assemblage. The oxides and amounts will be in part dependent upon the processing conditions and it will be apparent to those skilled in the art how to make such selections in order to obtain the mineral assemblages hereinabove described. Oxides 5 which may be used in the process of the present invention include CaO, ZrOz, TiOz, TiOz Ti20 3 solution, A1 2 0 3 and BaO, oxides, carbonates, gels or glasses. The heating can be to either subsolidus or above solidus conditions. Heating to above solidus conditions may allow the mineral assemblage to be produced in less time. 10 In one form of the invention a mineral assemblage is preferably formed by heating the mixture of oxides to subsolidus conditions. The formation of the mineral assemblage under subsolidus conditions requires the mixture be maintained at an elevated temperature, say 1000 to 1600oC for a period sufficient for the mineral assemblage to achieve phase equilibrium and 15 have the crystals of the desired particle size. The preferred time of heating varies with temperature. It can be at a temperature of 1000 0 C for 36 hours or up to a temperature of 1600 0 C for one four as well as intermediate temperatures and heating times to produce desirable results. It is then allowed to cool to ambient temperature. 20 The assemblage of this invention can be formed under pressure of one atmosphere or by using hot isostatic pressing techniques. A reducing environment is preferably used for the incorporation Ti in a number of different valency states. This can be achieved by several methods under appropriate reducing 25 atmospheric conditions. One approach is to take TiO 2 and under appropriate atmosphere convert it into a Magnelli phase with the desired Ti 4 +-Ti solid solution. This is then added to the other oxides prior to synthesis. Another approach is to add Ti 4 directly to the other oxides and convert some of it into Ti 3 + prior to heating (with or without Ti metal). 30 WO 99/60577 PCT/AU99/00376 -8 Figure 1 shows a molecular proportion phase diagram which illustrates the mineral assemblages of the present invention. Mineral assemblages according to this invention are within the area bordered by the lines respectively joining points marked as calzirtite, zirconolite and perovskite. The assemblages may also include hollandite (Ba A1 2 Ti60 16 ). 5 Previous SYNROC compositions have fallen into the area bounded by the lines connecting zirconolite, rutile and perovskite. Several reported SYNROC compositions are labelled A, B, C, E, F and fall within the area bounded by the lines joining the perovskite, zirconolite and rutile. The mineral assemblages 10 of the present invention at or about phase equilibrium contain little or no rutile. Under ideal conditions at phase equilibrium the mineral assemblages can contain no rutile as rutile is not stable at these proportions of Ca, Ti and Zr. Figure 2 shows a molecular proportion phase diagram which incorporates Ti. As can be seen 15 in Figure 2 the mineral assemblages which include perovskite, zirconolite and calzirtite may also include Ti 2 03 in solid solution, in the absence of rutile. In the drawings the calzirtite and zirconolite proportions are shown in accordance with an ideal consideration. As will be appreciated by those skilled in the art these phases can show 20 significant solid solution formulation which has the practical effect of varying the molecular proportion co-ordinates. The mineral assemblages according to the present invention contain a number of coexisting titanate and hollandite-type phases, in which calzirtite, zirconolite, perovskite and preferably a 25 Ba-phase such as hollandite are prevalent. Other Ba-phases as well as baddeleyite, srilankite may occur under certain conditions. These phases, when synthesised under appropriate conditions, substitute HLW elements into their crystalline structures by a variety of substitution mechanisms. This results in significant departure from ideal end member compositions. 30 WO 99/60577 PCT/AU99/00376 -9 Without wishing to be bound by theory, it is believed that the HLW elements are immobilized in a number of coexisting minerals which depart from ideal compositions due to their abilities to accommodate HLW elements into their crystalline structures. The mineral assemblages of the present invention is believed to incorporate significant HLW components through a variety of 5 substitutions for Ca, Zr, Ti, Ba, Al in the various crystal structure sites. Certain HLW elements partition differently into the different phases, based on crystal chemical principles. These substitutions range from simple replacements (one element for another) through to coupled substitutions such that several HLW elements replace several element on different crystallographic sites in the ideal crystal structures. 10 In the perovskite-type crystals, it is believed that both Ca and Ti may be substituted to varying amounts by certain HLW additive elements. This may be by either element-element replacements (for example Ca replaced by Sr) or else more coupled substitutions involving rare earth elements, U, Na) as well as Ti 3 . These substitutions can result in symmetries other than orthorhombic 15 (such as cubic, rhombohedral). In the calzirtite and zirconolite type crystals, it is believed that certain HILW elements such as rare earth and actinide elements may be accommodated within the structure. The present invention will now be described with reference to the following non-limiting 20 examples, which vary depending on solid solution within product phases as a function of synthesis conditions. EXAMPLES 1 The oxides listed below were mixed in the specified amounts by physical stirring and grinding 25 until an essentially homogeneous mixture was produced CaO 7.9 ZrO 2 26.1 TiO 2 48.0 30 A1 2 0 3 7.2 BaO 10.8 WO 99/60577 PCT/AU99/00376 - 10 The mixture was heated in a metal vessel at a temperature of 1300oC for a period of 6 hours and allowed to cool to ambient temperature. The resultant mineral assemblage had the following crystals present in the proportions listed below: 5 perovskite 4.8 zirconolite 12 calzirtite 31.2 hollandite 52 10 EXAMPLE 2 The oxides listed below were mixed in the specified amounts by physical stirring and grinding until an essentially homogeneous mixture was produced CaO 10.7 15 ZrO 2 35.2 TiO 2 41.9 A1 2 0 3 4.9 BaO 7.3 20 The mixture was heated in a metal vessel at a temperature of 1300oC for 6 hours and allowed to cool to ambient temperature. The resultant mineral assemblage had the following crystals present in the proportions listed below: perovskite 6.5 25 zirconolite 16.2 calzirlite 42.3 hollandite 35 WO 99/60577 PCT/AU99/00376 -11 EXAMPLE 3 The oxides listed below were mixed in the specified amounts by physical stirring and grinding until an essentially homogeneous mixture was produced 5 CaO 10.0 ZrO 2 25.6 TiO 2 40.2 A1 2 0 3 6.0 BaO 18.2 10 The mixture was heated in a metal vessel at a temperature of 1300oC for 6 hours and allowed to cool to ambient temperature. The resultant mineral assemblage had the following crystals present in the proportions listed below: 15 perovskite 8.2 zirconolite 20.5 calzirlite 26.8 hollandite 44.5 20 The foregoing examples illustrate describes only some of the possible variations in the present invention and modifications can be made without departing from the scope of the invention.

Claims (16)

1. A mineral assemblage comprising crystals belonging to or possessing crystal structures closely related to both the perovskite and zirconolite mineral classes and further comprising 5 crystals belonging to or possessing crystal structures closely related to the calzirtite mineral class wherein the mineral assemblage incorporates high-level radioactive wastes immobilized therein.

2. A mineral assemblage according to claim 1 wherein the mineral assemblage comprises 10 titanium in the crystals of the mineral assemblages is of at least two different co-existing valency states.

3. A mineral assemblage according to claim 2 wherein the titanium of at least two co existing valency states is Ti 3+ and Ti 4 + . 15

4. A mineral assemblage according to any one of claims 1 to 3 wherein the mineral assemblage comprise Ti 2 03 in solid solution.

5. A mineral assemblage according to any one of claims 1 to 4 wherein the mineral 20 assemblage further comprises mineral complexes which incorporate barium and/or aluminium.

6. A mineral assemblage according to claim 5 wherein the mineral assemblage further comprises crystals belonging to or possessing crystal structures closely related to the 25 hollandite mineral class.

7. A mineral assemblage according to any one of claims 1 to 6 wherein the mineral assemblage comprises at least 10% by weight of crystals of each of the calzirtite, zirconolite, perovskite and hollandite mineral classes. 30 WO 99/60577 PCT/AU99/00376 -13

8. A mineral assemblage according any one of claims 1 to 7 wherein the crystals of the mineral assemblages are of up to two hundred microns in size.

9. A mineral assemblage according to any one of claims 1 to 8 wherein the mineral 5 assemblages includes at least 20 weight percent HLW.

10. A mineral assemblage according to any one of claims 1 to 9 wherein the mineral assemblage is substantially free of rutile. 10

11. A process for immobilizing high level radio-active waste comprising the steps of: a) mixing a high level radio-active waste calcine with selected oxides; b) heating and cooling said mixture to form a mineral assemblage comprising crystals 15 belonging to or possessing crystal structures closely related to both the perovskite and zirconolite mineral classes and further comprising crystals belonging to or possessing crystal structures closely related to the calzirtite mineral class wherein the mineral assemblage incorporates high-level radio-active wastes immobilized therein. 20

12. A process according to claim 11 wherein the selected oxides comprise CaO, ZrO z , TiO 2 , TiO 2 -Ti z O 3 solution, A1 2 0 3 and BaO, oxides, carbonates, gels or glasses.

13. A process according to claim 12 wherein the mixture is heated to a temperature in the range of from 1000oC to 1600oC for a period sufficient for the mineral assemblage to achieve 25 phase equilibrium and have the crystals of the desired particle size.

14. A process according to either claim 12 or 13 wherein a reducing environment is preferably used for the incorporation Ti in a number of different valency states. 30 WO 99/60577 PCT/AU99/00376 -14

15. A mineral assemblage substantially as hereinbefore described with reference to the drawings and/or examples.

16. A process for immobilizing high-level radioactive waste substantially as hereinbefore 5 described with reference to the drawings and/or examples.