S. V. Stefanovsky

Phase composition, structure, and hydrolytic durability of glasses in the Na2O-Al2O3-(Fe2O3)-P2O5 system at replacement of Al2O3 by Fe2O3

Samples of sodium aluminum iron phosphate glasses of the composition (mol %) 40 Na2O, (20 − x) Al2O3, x Fe2O3, 40 P2O5 (series I) and 35 Na2O, (20 − x) Al2O3, x Fe2O3, 45 P2O5 (series II) were synthesized. The phase composition and structure of the samples obtained were determined by X-ray diffraction and IR spectroscopy. At equimolar replacement of Al2O3 by Fe2O3, the structure of the quenched glasses of series I does not change appreciably, in contrast to glasses of series II. Annealing of the glasses leads to their partial devitrification with segregation of crystalline aluminum iron phosphate phases. Glasses of series I with up to 10 mol % Al2O3 replaced by Fe2O3 exhibit the highest hydrolytic durability: The leach rates of Na, Al, Fe, and P from the samples are within (4–10) × 10−8 g cm−2 day−1, meeting the requirements of GOST (State Standard) R 50 926-96. Thus, glasses with approximately equal molar concentrations of Al2O3 and Fe2O3 are the most resistant to crystallization and hydrolysis.

Characterization of glassy materials for immobilization of radioactive waste with a high iron oxide content

The influence of the content of oxides in the simulated high-level wastes on the phase composition, the structure, and the water resistance of borosilicate-based glassy materials for immobilization is investigated. An increase in the waste oxide content from 45 to 65 wt % leads to an increase in the fraction of the crystalline phase of the magnetite-type spinel from 3–5 to 20–22 vol %. The glassy materials are characterized by a low leaching rate of the main waste components in water. A considerable increase in the leaching rate of sodium ions and, to a lesser extent, aluminum and uranium ions is observed for the glassy materials containing waste oxides at a content of 55 wt % and more due to the depolymerization of the structural glass network. Under the same conditions, the leaching rate of iron does not increase noticeably because of the high resistance of the iron-containing spinel to water.

Oxidation state and coordination environment of uranium in sodium iron aluminophosphate glasses

An analysis of the X-ray absorption near edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) of uranium determined the oxidation state and coordination environment of uranium atoms in glasses containing 40 mol % Na2O, 10 mol % Al2O3, 10 mol % Fe2O3, and 40 mol % P2O5 to which uranium oxides were added to a concentration of 50 wt % (above 100%). If the added amount of UO2 was small, uranium occurred as U(IV) in a near-octahedral oxygen environment with an average U–O distance in the first coordination sphere of 2.25 Å. At higher concentrations of uranium oxides introduced both as UO2 and as UO3, uranium occurred as U(V) and U(VI); the first coordination sphere is split; shorter (~1.7–1.8 Å) and longer (2.2–2.3 Å) distances were observed, which corresponded to the axial and equatorial U–O bonds in uranyl ions, respectively; and the redox equilibrium shifted toward U(VI). The glass with the maximal (~33 wt %) UO3 concentration contained mainly U(VI). The existence of low-valence uranium species can be related to the presence of Fe(II) in glasses. The second coordination sphere of uranium manifests itself only at high concentrations of uranium oxides.

X-ray photoelectron study of lanthanide borosilicate glass

The elemental and ionic quantitative analyses of the synthetic lanthanide borosilicate glass (Al-B-Gd-Hf-La-Nd-Pu-Si-Sr-O) are performed using the characteristics of the X-ray photoelectron spectra of the outer-shell and inner-shell electrons in the binding energy range 0–1000 eV. The oxidation states of the metal ions in this glass are determined and correspond to the Al3+, La3+, Nd3+, Gd3+, Hf4+, Pu4+, Si4+, and Sr2+ ions. Taking into account the binding energies of the O 1s electrons for the glass sample under investigation, the average lengths of metal-oxygen bonds on the surface of the sample are estimated to be 0.191 and 0.176 nm, which correspond to oxygen binding energies of 531.3 and 532,3 eV, respectively.

Valence state and speciation of uranium ions in borosilicate glasses with a high iron and aluminum content

The valence state and the local environment of uranium ions in borosilicate glasses intended for immobilizing high-level wastes with high concentrations of iron and aluminum ions are investigated using X-ray absorption (XANES, EXAFS) spectroscopy. It is demonstrated that, in glasses predominantly containing iron oxides, at least 80% of the total uranium exists in a hexavalent form as uranyl ions. In high-alumina glasses, uranium is in hexavalent and pentavalent states; in this case, the fraction of the latter form increases with an increase of the uranium concentration and its local environment is similar to the configuration of an axially distorted tetrahedron.

Characterization of the glass-ceramic material prepared upon vitrification of an iron-containing surrogate of high-level wastes in a cold crucible

Vitreous materials are prepared by cold crucible induction melting of a surrogate of high-level wastes (from the Savannah River Site, United States) and a borosilicate glass frit taken in mass ratios from 45: 55 to 60: 40. According to the X-ray diffraction and electron microscopic data, the vitreous materials thus produced consist of a glass matrix and a magnetite-type spinel enriched in transition elements. The degree of crystallinity of the materials increases with an increase in the waste oxide content from 6 to 18–20 vol %. The vitreous materials are characterized by a high chemical durability, which decreases only at high contents of the waste oxides (55 wt % and higher) due to the formation of an additional nepheline phase.

Phase composition and structure of molybdenum-, copper-, and cesium-containing sodium aluminophosphate glassy materials for immobilization of high level wastes of nuclear reactors

Molybdenum-, copper-, and cesium-containing glass based on aluminophosphate designed to immobilize high-active wastes (HAWs), which are formed at the reprocessing of spent nuclear fuel (SNF) from Atom Peaceful Big (AMB) nuclear reactor, have been synthesized and studied by means of the X-ray diffraction and electron-microscopy methods. After quenching the melts, glass, which partially crystallizes at annealing, is formed. The introduction of magnesium oxide in aluminophosphate glass increases its crystallization resistance; and molybdenum oxide, decreases. The samples crystallize after heat treatment with the formation of aluminum and sodium-aluminum orthophosphates and solid solution of the (Na,Cs)3−3xAlxPO4 composition.

Brannerite, UTi2O6: Crystal chemistry, synthesis, properties, and use for actinide waste immobilization

The host materials suggested for immobilization of actinide waste of military or civil origin often contain the secondary (U,Pu)Ti2O6 phase of brannerite structure. For example, the materials for incorporation of excess plutonium, mainly consisting of pyrochlore, contain up to 30% brannerite. This is a usual phase in titanate host materials for isolating spent nuclear fuel (SNF) and products of its reprocessing, including waste from production of 99mТс for medical purposes and other kinds of waste with high U and Pu content. Despite simple ideal stoichiometry, brannerite can contain large amounts of rare earths. This feature is due to the presence of uranium not only in the 4+ oxidation state, but also in the 5+ and 6+ states, which favors the exchange of rare earth elements (REE), e.g., in accordance with the scheme 2U4+ ↔ U5+ + REE3+. The REE amount in brannerite reaches 0.5–0.7 atom per formula unit. Therefore, brannerite is of interest as a host material for the rare earth–actinide fraction of high-level waste (HLW). To evaluate the prospects for such use of brannerite, data on the radiation resistance of brannerite and its behavior in aqueous solutions are analyzed. In these properties, brannerite is inferior to pyrochlore and zirconolite. The rate of actinide leaching from brannerite is higher by an order of magnitude than from these phases, but lower by 3–4 orders of magnitude than from glass host materials. Natural brannerite is stable in media with weakly alkaline and reducing waters. Therefore, brannerite seems suitable for immobilization of rare earth–actinide waste. This host material can be synthesized by sintering or cold crucible induction melting followed by crystallization.

Chemical-technological and mineralogical-geochemical aspects of the radioactive waste management

This paper considers various matrices that are able to incorporate components of radioactive wastes (RAW) of different origin. It is noted that attempts to develop the single phase crystalline matrix to immobilize all RAW components failed. The only single phase matrix brought to the industrial application is glass, which is able to accumulate practically all RAW components but in limited concentrations. Prospects are related with some types of ceramics for immobilization of narrow fractions of RAW or individual radionuclides (for instance, minor actinides), as well as some types of low-temperature matrices (iron-phosphate, magnesium–potassium–phosphate, and geopolymers). Approaches to choosing the technology of waste form synthesis are considered. Perspectives of application of both high-temperature (cold-crucible induction melting, self-propagating high-temperature synthesis) methods and modified cementation technologies are demonstrated. It is noted that the final isolation of RAW from the biosphere suggests their disposal in underground repositories. The most difficult technical problem is the disposal of RAW containing long-lived radionuclides. It is shown that the quantitative assessment of repository safety with allowance for their characteristics and all possible processes and phenomena is required to substantiate the safe disposal of long-lived radionuclides.

Matrices for immobilization of the rare earth–actinide waste fraction, synthesized by cold crucible induction melting

The structure of eight samples containing simulated rare earth–actinide fraction of high-level waste was studied. Samples of weight from 0.2 to 6 kg were prepared by cold crucible induction melting followed by crystallization of the melt. The target phases (britholite, pyrochlore, zirconolite, rhombic and monoclinic rare earth titanates) prevail in all the matrices; glass, zirconolite, and rutile were detected as impurities, sometimes in significant amounts. These phases do not contain waste components (rutile) or are stable in solutions (zirconolite); therefore, their presence should not impair the properties of the matrix. The possibility of controlling the phase composition of the matrix by introducing zirconium or aluminum oxide into the charge was demonstrated.