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Immobilization of High Level Waste: Analysis of Appropriate Synthetic Waste Forms
Pages 208-224

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From page 208...
... This article presents the results of a selection of crystalline matrices for the immobilization of actinide wastes. Pyrochloretype phases could be used for the fixation of the actinide-zirconium-rare earth fraction of high-level wastes, while ferrites with a garnet structure could be used for the immobilization of complex wastes, including corrosion products, for example, Fe, Al, and Ga.
From page 209...
... Radionuclide T1/2, in years Radionuclide T1/2, in years 90Sr 29 237Np 2.1 × 106 93Zr 1.5 × 106 238Pu 89.9 99Tc 2.1 × 105 239Pu 2.4 × 104 126Sn 105 240Pu 6.5 ×103 129I 1.7 × 107 241Pu 14 137Cs 30 242Pu 3.78 × 105 147Pm 2.6 241Am 433 151Sm 93 242Am 152 154Eu 16 243Am 7.3 × 103 Activated corrosion products 243Cm 28 59Ni 7.5 × 104 244Cm 17.9 60Co 5.3 245Cm 8.5 × 103 63Ni 96 246Cm 4.76 × 103 with the study of various vitreous and crystalline materials based on silicates, phosphates, and titanates.3 For industrial-scale operations for this purpose, glasses are currently used -- borosilicate glasses abroad4 and aluminophosphate glasses in Russia.5 The shortcoming of glasses is their limited capacity to incorporate actinides (especially plutonium) and their low chemical stability.6 The interaction of glass-like matrices with underground water is accompanied by the formation of colloidal particles7 in which radionuclides could migrate over great distances.
From page 210...
... They may be used for the immobilization of both excess plutonium and more complex wastes, for example, actinide-zirconium-rare earth element fractions of highlevel wastes, actinides and fission products, such as 93Zr, 99Tc, and 126Sn, and wastes from the conversion of plutonium into nuclear fuel. The process of seeking high-level waste matrices may be optimized by doing a preliminary analysis of the characteristics of the crystal structure of promising phases.
From page 211...
... .22 Titanate pyrochlore is the fundamental phase in matrices for wastes with high actinide content, for example, spent nuclear fuel.23 By analogy with other materials, Synroc-F was named as another promising ceramic. Matrices with 80­90 percent pyrochlore content are considered in the United States to be promising forms for the fixation of excess plutonium.
From page 212...
... The symbol ± indicates Sm, Eu, and Gd zirconates, which have a structure that is both fluorite and pyrochlore in nature. REE indicates rare earth elements.
From page 213...
... It is possible that part of the Zr4+ could also be in positions with coordination numbers equal to 8. It is preferable that the positions with this coordination number be filled with bivalent calcium ions instead of quadrivalent actinides.
From page 214...
... The data obtained on the correlation of composition and structure must be taken into account in selecting matrices for complex actinide wastes, for example, actinide-zirconium-rare earth element fractions of high-level wastes. The octahedral ions of the pyrochlore phases of rare earth elements and trivalent actinides could vary in size from 0.55 to 0.75 Å.
From page 215...
... The way the polyhedrons are joined in the crystalline lattice determines the relation between the sizes of the ions in the various structural positions. For rare earth garnets the effect of the structural factor on the chemical composition of the compounds is obvious.32 These compounds include phases with aluminate, gallate, and ferrite composition (REE3B2X3O12)
From page 216...
... NOTE: + means garnet structure stable; ­ means garnet structure unstable; ? means no data available.
From page 217...
... These studies were later continued, which led to the creation of a matrix with a garnet content of more than 80 percent.35 The content of oxides of rare earth elements and zirconium in the garnet exceeds 20 percent by mass, but the possibility of including actinides has not yet been studied. Specialists from the Radium Institute are studying garnet ceramics as potential actinide matrices.36 They have focused on a rare earth (Y, Gd)
From page 218...
... in garnet has also been established for plutonium.37 Thus, rare earth Al-Ga garnets have a low capacity for incorporating uranium, plutonium, and cerium, and evidently cannot be recommended for immobilizing high-level wastes with high quadrivalent actinide content. These results do not mean that garnet ceramics should be ruled out as actinide matrices.
From page 219...
... attest to the significant capacity of the ferrite garnets to incorporate the rare earth elements (Gd, Ce) , Zr, and the actinides.
From page 220...
... In calculating the formulas it is conditionally accepted that all iron in the sample is in trivalent form and all uranium and cerium are in quadrivalent form. various charges and sizes creates conditions for the inclusion in them of various waste components, including actinides.
From page 221...
... . It has been experimentally established that the actinide content in garnet varies from 0.6­0.8 percent by mass to 16­18 percent by mass depending on its composition.
From page 222...
... A phase with a garnet structure represents a more universal matrix. This material may be used for immobilizing both actinide and actinide-zirconium-rare earth fractions and actinide wastes with a more complex composition containing high levels of corrosion products (Al, Ga, Fe, Cr, Ni, Mn, Co)
From page 223...
... Fundamental and technological aspects of actinide oxide pyrochlores: relevance for immobilization matrices. Proceedings of the Symposium on the Scientific Basis of Nuclear Waste Management XXII 556(1999)
From page 224...
... Uranium accommodation into garnet host (Isomprphic capacity and radiation stability of the garnet structured actinide host)


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