Patent Number: 042749760
Section: description

The present invention is further illustrated, by way of example only, in the following Examples. EXAMPLE 1 A mixture of oxides as set out in Column A of Table 4 above is selected to correspond to a desired mineral assemblage: perovskite CaTiO.sub.3, Ba felspar BaAl.sub.2 Si.sub.2 O.sub.8, hollandite BaAl.sub.2 Ti.sub.6 O.sub.16, kalsilite KAlSiO.sub.4, and zirconolite CaZrTi.sub.2 O.sub.7. Ninety percent by weight of this mixture is intimately mixed with 10 percent of HLW calcine (Table 1.) The combined mixture is then melted in a suitable furnace at about 1330.degree. C. under mildly reducing conditions and allowed to cool over a period of 2 hours to a temperature of 1100.degree. C., at which stage essentially complete solidification is achieved. The resultant product is found to be well-crystallized and composed mainly of the 5-phase mineral assemblage: perovskite-hollandite-Ba felsparzirconolite-kalsilite. However, because of the partial substitution of potassium for barium in the hollandite lattice, and the non-stoichiometry of the hollandite phase, crystallization occurs during cooling in such a direction that the residual liquids are enriched in potassium, barium and silica. From this residual liquid, a K-Ba-aluminosilicate possessing the leucite structure is observed to crystallize. Compositions of these phases as determined by electronprobe microanalyses are given in Table 5. The distribution of HLW elements among the major phases of the mineral assemblage of Example I has been determined by electronprobe microanalyses of coexisting phases. It is found that the rare earths and actinide elements dominantly enter the perovskite and zirconolite phases to form stable solid solutions, whilst molybdenum and ruthenium likewise enter the perovskite and hollandite phases replacing titanium providing that the synthetic rock composition is melted under appropriate redox conditions. Strontium is found to become preferentially incorporated in the perovskite phase, whilst barium enters the Ba felspar, and to a lesser degree, the hollandite phase. Rubidium mainly substitutes for potassium in the leucite phase, in the KAlSiO.sub.4 phase and also in the Ba felspar phase. Zirconium enters the zirconolite phase whilst palladium becomes reduced to the metallic state. During crystallization of the mineral assemblage, caesium tends to become enriched in the residual liquid, and finally becomes incorporated mainly in the leucite phase and/or in a (K,Cs)AlSiO.sub.4 solid solution which possesses the RbAlSiO.sub.4 structure. Some caesium is also found to occur in solid solution in Ba felspar. EXAMPLE 2 A mixture of oxides is selected so that when the mixture is heated, the oxides combine together to form a mineral assemblage consisting of BaAl.sub.2 Ti.sub.6 O.sub.16 hollandite (25%), CaZrTi.sub.2 O.sub.7 zirconolite (20%), BaAl.sub.2 Si.sub.2 O.sub.8 barium felspar (20%, CaTiO.sub.3 perovskite (15%) and KAlSi.sub.2 O.sub.6 leucite (20%). Ninety percent of this mixture is intimately mixed with 10 percent of HLW calcine (Table 1), and the combined mixture is then heat-treated under reducing conditions as described in Example 1. The resultant product is found to be well-crystallized and composed mainly of the 5-phase mineral assemblage: perovskite-hollandite-Ba felspar-zirconolite-leucite. The distribution of the HLW elements among coexisting phases is similar to Example 1 except that nearly all of the caesium is found in solid solution in the leucite-type phase as a KAlSi.sub.2 O.sub.6 -CsAlSi.sub.2 O.sub.6 solid solution. EXAMPLE 3 A mixture of oxides is selected so that when the mixture is heated, the oxides combine together to form a mineral assemblage consisting of BaAl.sub.2 Ti.sub.6 O.sub.16 hollandite (25%), CaZrTi.sub.2 O.sub.7 zirconolite (20%), BaAl.sub.2 Si.sub.2 O.sub.8 barium felspar (20%), CaTiO.sub.3 perovskite (15%) and NaAlSiO.sub.4 nepheline (20%). Ninety percent of this mixture is intimately mixed with 10 percent of HLW calcine (Table 1) and the mixture is then heat treated under reducing conditions as described in Example 1. The resultant product is found to be well-crystallized and composed mainly of the 5-phase mineral assemblage: perovskite-hollandite-Ba felspar-zirconolite-nepheline. The distribution of HLW elements among coexisting phases is similar to Example 1 except that nearly all of the caesium is found in the nepheline phase. EXAMPLES 4,5 and 6 Mixtures of oxides are selected as described in Examples 1, 2 and 3, respectively and 95 percent of each oxide mixture is intimately mixed with 5 percent of HLW calcine (Table 1). Each mixture is then heat treated under reducing conditions as described in Example 1. The products are found to correspond essentially to the mineral assemblage described in Examples 1, 2 and 3 respectively. EXAMPLES 7, 8 and 9 Mixtures of oxides are selected as described in Examples 1, 2 and 3, respectively, and 80 percent of each oxide mixture is intimately mixed with 20 percent of HLW calcine (Table 1). Each mixture is then heat treated under reducing conditions as described in Example 1. The products are found to correspond essentially to the mineral assemblages described in Examples 1, 2 and 3 respectively. EXAMPLE 10 A mixture of oxides as set out in Column B of Table 4 hereinbefore is selected so that when the mixture is heated, the oxides combine together to form a mineral assemblage consisting of BaAl.sub.2 Ti.sub.6 O.sub.16 hollandite, CaZrTi.sub.2 O.sub.7 zirconolite and CaTiO.sub.3 perovskite. Ninety percent of this mixture is intimately mixed with 10 percent of HLW calcine (Table 1). The combined mixture is then heated to about 1300.degree. C. for about half an hour in the presence of metallic nickel and simultaneously subjected to a confining pressure (e.g. 1000 atmospheres) using the conventional technique known as "hot-pressing". The resultant product is found to be a fine grained, mechanically strong assemblage of hollandite, zirconolite and perovskite possessing the above compositions. The distribution of HLW elements among the major phases of the mineral assemblage of Example 10 has been determined by electronprobe microanalyses of coexisting phases and is summarised in Table 6 hereinafter. It is found that caesium enters the hollandite phase as Cs.sub.2 Al.sub.2 Ti.sub.6 O.sub.16, strontium dominantly enters perovskite as SrTiO.sub.3 and the actinide elements dominantly enter the zirconolite phase, in each case, forming dilute solid solutions. Samples of the product of Example 10 have been subjected to leaching tests by pure water and by water--10% NaCl solution at high temperatures and pressures. It has been found that the mineral assemblage remains stable and caesium remains incorporated in hollandite when subjected to leaching at temperatures up to 900.degree. C., combined with pressure up to 5 kilobars over a period of 24 hours. For comparison, a representative selection of borosilicate glasses devitrified and disintegrated at temperatures above 350.degree. C. Moreover, the alternative crystalline waste form "Supercalcine" was found to exchange its caesium for sodium at temperatures above 400.degree. C. These experiments demonstrate the remarkable stability of the product of the present invention and its superiority over other immobilisation forms. TABLE 6 ______________________________________ "Hollandite" Zirconolite Perovskite ______________________________________ Cs.sup.+ Mo.sup.4+ U.sup.4+ Sr.sup.2+ Rb.sup.+ Ru.sup.4+ Th.sup.4+ REE.sup.3+ K.sup.+ Rh.sup.3+ Pu.sup.4+ Y.sup.3+ Na.sup.+ Fe.sup.3+ Cm.sup.4+ Am.sup.3+ Ba.sup.++ Cr.sup.3+ Am.sup.3+ U.sup.4+ Pb.sup.++ Ni.sup.2+ Y.sup.3+ Th.sup.4+ Fe.sup.2+ REE.sup.3+ Cm.sup.4+ Na.sup.+ Pu.sup.4+ ______________________________________ Table 6 is a summary of observed preferential distributions of HLW elements in solid solution in phases of the mineral assemblage of the composition given in Column B, Table 4, produced in accordance with Example 10. The quadrivalent actinides are more strongly partitioned into the zirconolite phase than into perovskite. Trivalent actinides preferentially enter zirconolite; however, in the presence of somewhat higher Al.sub.2 O.sub.3 concentrations than shown in Table 4, Column B, the trivalent actinides may instead preferentially enter the perovskite phase. EXAMPLE 11 The procedure of Example 10 is repeated except that the proportion of mixed oxide additives to HLW calcine is 80 to 20 by weight. The product is a mineral assemblage essentially similar to the product of Example 10. EXAMPLE 12 The procedure of Example 10 is repeated except that the proportion of mixed oxide additives to HLW calcine is 95 to 5 by weight. Again, the product is a mineral assemblage essentially similar to the product of Example 10. EXAMPLE 13 A mixture of oxides is selected so that when the mixture is heated, the oxides combine together to form a mineral assemblage consisting of BaAl.sub.2 Ti.sub.6 O.sub.16 hollandite (50%) and CaZrTi.sub.2 O.sub.7 zirconolite (50%), the actual composition of the minerals resembling those in Table 5, Columns G and I. From 5 to 20 percent of HLW calcine is then intimately mixed with 95 to 80 percent of the above oxide mixture and the combined mixture heat-treated as in Example 10. It is found that nearly all actinide elements in the HLW enter the zirconolite whilst strontium becomes partitioned between hollandite and zirconolite, mostly entering zirconolite. Other HLW elements including caesium enter the hollandite as in Example 10. EXAMPLE 14 A mixture of oxides is selected so that when the mixture is heated, the oxides combine together to form a mineral assemblage consisting of BaAl.sub.2 Ti.sub.6 O.sub.16 hollandite (50%) and CaTiO.sub.3 perovskite (50%), the actual compositions of these minerals resembling those in Table 5, Columns G and H. From 5 to 20 percent of HLW calcine is then intimately mixed with 95 to 80 percent of the above oxide mixture and the combined mixture heat-treated as in Example 10. It is found that the actinide elements and strontium in the HLW enter the perovskite, whilst caesium and the other elements of the HLW continue to enter the hollandite as in Example 10. EXAMPLE 15 The procedures of Examples 10-12 and 14 are repeated except that CaTiO.sub.3 perovskite is replaced by SrTiO.sub.3 perovskite. EXAMPLE 16 The procedures of Examples 10-14 are repeated except that CaTiO.sub.3 perovskite is replaced where present by SrTiO.sub.3 perovskite, and BaAl.sub.2 Ti.sub.6 O.sub.16 holandite is replaced by SrAl.sub.2 Ti.sub.6 O.sub.16 hollandite. The above Examples 1 to 16 demonstrate how the HLW elements in HLW calcine can be firmly incorporated in stable solid solutions within the minerals of an appropriately selected assemblage. The product of each Example containing the immobilized HLW elements can be safely buried in an appropriate geological-geochemical environment. The results obtained from investigation of mineral assemblages produced in accordance with this invention demonstrate that when HLW products are treated by the processes described herein, they can safely be confined for periods of millions of years. By such means, the biosphere can be protected from the radiologic hazards posed by high level wastes from nuclear reactors. The compositions of two other crystalline ceramic waste forms proposed for nuclear waste immobilisation have been given above in Table 4, Columns C and D. It is seen that the compositions and mineralogies of these ceramic waste forms differ drastically from those of the mineral assemblages comprising the synthetic rock described in this invention. It should also be noted that in the waste forms designated in columns C and D, caesium is present as the mineral pollucite. This mineral readily loses its caesium when subjected to the action of aqueous solutions containing sodium at temperatures above 300.degree. C. In comparison, caesium remains firmly incorporated in hollandite-type mineral phases at temperatures up to 900.degree. C. under otherwise similar conditions. It will be appreciated by persons skilled in this art that many modifications and variations may be made to the specific embodiments described herein without departing from the spirit and scope of the present invention as broadly described herein.