Sergey V. Stefanovsky

Nuclear waste forms

Abstract This review describes nuclear waste forms for high-level waste (HLW), that is, glasses, ceramics, and glass-ceramics, as well as for low- and intermediate-level waste (LILW), that is, cement, bitumen, glass, glassy slags, and ceramics. Ceramic waste forms have the highest chemical durability and radiation resistance, and are recommended for HLW and actinide (ACT) immobilization. Most radiation-resistant materials are based on phases with a fluorite-related structure (cubic zirconia-based solid solutions, pyrochlore, zirconolite, murataite). Glass is also a suitable matrix for HLW containing fission and corrosion products, and process contaminants such as Na salts. Within the framework of the HLW partitioning concept providing separation of short-lived (Cs, Sr) and long-lived (rare earth element-ACT) fractions, glass may be used for immobilization of the Cs-Sr-bearing fraction, whereas the rare earth-ACT fraction may be incorporated in ceramics. Glass-based materials or clay-based ceramics are the most promising LILW forms, but cement and bitumen may also be applied as matrices for low-level wastes (LLW).

Synthetic minerals with the pyrochlore and garnet structures for immobilization of actinide-containing wastes

Complex oxides of the pyrochlore (space groups Fd3m, [8]A2[6]B2O7) and garnet (Ia3d, [8]A3[6]B2[4]T3O12) structures (“A” = Ca2+, Ln3+/4+, An3+/4+; “B” = (Ti, Sn, Hf, and Zr)4+ in pyrochlore, and Al3+, Ga3+, and Fe3+ in garnet alone; “T” = (Al3+, Ga3+, and Fe3+) are promising matrices for actinide-bearing wastes. In order to identify optimal compositions of these phases, their isomorphic capacity with respect to REE, actinides, and other components of wastes was examined. The long-term behavior of the matrix at a repository was predicted based on data obtained on the behavior of pyrochlores and garnets under ion irradiation and 244Cm decay and on the determined leaching rates of REE from the matrices because of their interaction with aqueous solutions, including that after amorphization. In order to propose efficient synthesis techniques, samples prepared with the use of various methods were studied. The possibility of incorporating long-lived decay products of 99Tc into the crystalline matrices was analyzed.

Ion-beam-induced amorphization and order-disorder transition in the murataite structure

Murataite (A3B6C2O22−x∕2,F4¯3m), a derivative of an anion-deficient fluorite structure, has been synthesized as different polytypes as a result of cation ordering. Ion-beam-induced amorphization has been investigated by 1-MeV Kr2+ ion irradiation with in situ transmission electron microscopy. The critical amorphization dose was determined as a function of temperature and the degree of structural disordering. A lower critical amorphization temperature (∼860K) was obtained for the disordered murataite as compared with that of the murataite superstructure (930to1060K). An ion-beam-induced ordered murataite to a disordered fluorite transition occurred in the murataite superstructure, similar to that observed in the closely related pyrochlore structure-type, A2B2O7. The ion-beam-induced defect fluorite structure is more energetically stable in the murataite structure with a higher degree of structural disordering, as compared with the murataite superstructure. This suggests that the degree of intrinsic structu...

Murataite-based ceramics for actinide waste immobilization

Studying the Synroc, doped with a simulated HLW, the authors have found, along with conventional Synroc phases (zirconolite, perovskite, hollandite), an extra phase with a stoichiometry (Ca,Mn,U,Tr){sub 4}(U,TR,Zr,Ti){sub 2}(Al,Ti){sub 7}O{sub 22}. XRD and TEM study has shown this phase is related to a very rare mineral murataite. In the present work a ceramic based on murataite is studied. The ceramic samples in the system: Ca-Mn-Ti-Zr-U-Ce-Al-Fe-O were produced and examined in details using XRD, SEM/EDS, TEM, and optical microscopy. Total amount of actinide (U) and rare earth (Ce, Gd) elements in the murataite exceeds 20 wt.%. Isomorphic substitution schemes in the structures of synthetic and natural murataites are discussed. High isomorphic capacity of the murataite structure towards actinides and REEs, flexibility of its composition, feasibility of synthesis by melting, including a cold crucible melting, and very high chemical durability under hydrothermal conditions make the murataite-based ceramics very promising for actinides and excess weapon Pu fixation.

Cold Crucible Vitrification of Defense Waste Surrogate and Vitrified Product Characterization

In the framework of the contract “Advanced Melter Technology Application to the Defense Waste Processing Facility (DWPF) –Cold Crucible Induction Heated Melter (CCIM)”, vitrification tests with Savannah River Site defense waste surrogate were performed at the SIA Radon facility. Cold crucible melters with inner diameter of 216 mm and 418 mm were used in the testing. Commercially available (USA) frits 200 and 320 were used as glass-forming additives. In three different test campaigns, waste additive mixtures were fed as slurries with ∼60 wt.%, ∼30 wt.%, and 45 wt.% water content. Maximum slurry capacity and glass productivity under steady-state conditions were 35.4 kg/h and 16.2 kg/h, respectively. Specific glass productivity reached up to ∼3000 kg/(m 2 ×day). The average melt process temperature was 1250- 1350 °C. Waste loadings in glass were 45 wt.% in tests 1 and 2 and 50 wt.% in test 3. The glasses produced were found to be homogeneous but contained a magnetite-type phase with the spinel structure due to high iron and manganese content in waste. Spinel was observed in the glassy matrix as individual regular crystals and their aggregates. All the waste uranium entered the vitreous phase. Infra-red spectra consist of strong absorption bands due to bridging Si-O-Si and non-bridging Si-O - bonds, some weak bands due to B-O bonds, and a number of narrow bands due to occurrence of the crystalline phase. The glassy products demonstrate high leach resistance. Normalized release of major glass elements (Na, Li, B, Si) is by 10 to 50 times lower than the values required for repository disposition by EPA.

COMPARATIVE STUDY OF SYNROC-C CERAMICS PRODUCED BY HOT-PRESSING AND INDUCTIVE MELTING

Three Synroc-C samples, containing simulated high level waste were studied. One was produced by the conventional hot-pressing method at ANSTO, Australia, and the others were obtained using cold crucible technology at Radon, Russia. One of the melted samples was prepared using the Australian sol-gel precursor and the second one was obtained from an oxidenitrate mixture. It was established that the specimens have closely similar mineral compositions, with major hollandite, perovskite, zirconolite, and rutile. Small amounts of hibonite were also found. Unlike the hot-pressed Synroc containing metallic alloy particles, melted Synrocs contain molybdates. An investigation of mineral compositions and elemental distribution in the samples was carried out. Features of hot-pressed and melted ceramics were compared. Unit cell parameters of the Synroc phases were determined and preliminary results on durability of the melted Synroc are presented.

Murataite–Pyrochlore Series: A Family of Complex Oxides with Nanoscale Pyrochlore Clusters

identified murataite in a Synroc matrixwith imitators of HLRWs from PO Mayak, a radiochemicalfacility for production and reprocessing of nuclear fuel in theRussian Federation. Five volume percent of this phaseaccumulated about 40% of the total uranium present in thesample,andthisledtodetailedinvestigationsofthechemistryand properties of murataite, in particular, its chemicaldurability and radiation resistance.

Thermally induced phase decomposition and nanocrystal formation in murataite ceramics

Synthetic murataite, an isometric derivative of the fluorite structure, has been proposed as a host phase for the immobilization of lanthanides and actinides. In this study, murataite polytypes have been synthesized in the system Ca–Ti–U–Mn–Al–Zr–Fe–Ce–O by melting oxide mixtures at 1400–1600 °C followed by crystallization under slightly oxidizing conditions. The phase and structural compositions of the synthetic murataite ceramics were characterized by energy dispersive spectroscopy and transmission electron microscopy (TEM). The thermal stability was studied by in-situ heat treatment under TEM observation. The influence of radiation damage as a result of alpha-decay events of the incorporated actinides on the thermal stability of the synthetic murataite ceramics was simulated by 1 MeV Kr+ irradiation. A thermally induced, Fe-rich phase decomposition and nanocrystal formation was observed in murataite ceramics under vacuum above 573 K. Ce was closely associated with the Fe-rich phase in the Ce-incorporated murataite ceramics. Ion beam irradiation-induced amorphization inhibited the process of phase decomposition. The effects of thermally induced phase decomposition on the radiation resistance of murataite ceramics utilized as potential nuclear waste forms for the immobilization of actinides and, particularly plutonium, were also analyzed.

Corrosion Study of Actinide Waste Forms with Garnet-Type Structure

Garnet-based ceramics doped with Gd, Th, and Ce as waste surrogates and compositions: (Ca 1.5 GdTh 0.5 )ZrFeFe 3 O 12 (G3) and Ca 2.5 Ce 0.5 Zr 2 Fe 3 O 12 (G21) were prepared at 1300 °C in air. Corrosion rates in deionized water measured using an MCC-2 procedure were found to be (in g × m -2 × day -1 ): 10 -6 for Ce (G21), 3×10-5 for Gd and 10 -6 for Th (G3). Corrosion rates of Ce, Gd, and Th in an acid solution (0.01 M HCl) were 2 × 10 -1 (G21), 4 × 10 -3 and 10 -3 (G3), respectively. Chemical durability of the garnet ceramics is comparable to durability of titanate zirconolite and pyrochlore samples under similar conditions. Scanning electron microscopy has revealed no alteration of the sample surfaces after their contact with water, and the presence of newly formed iron-enriched phases of 1 to 3 micrometers in size, after interaction of the G21 sample with acid solution for 30 days. High leaching of the elements from the Ce-loaded garnet may be due to reduction of Ce in acid solution to Ce(III) or may be explained by lower stability of the structure of garnets (Ca 2.5 An 0.5 )Zr 2 Fe 3 O 12 compared to (Ca 1.5 GdAn 0.5 )(ZrFe)Fe 3 O 12 , An = Ce 4+ and Th 4+ . It was found that the (Ca 2.5 Ce 0.5 )Zr 2 Fe 3 O 12 and (Ca 2.5 Th 0.5 )Zr 2 Fe 3 O 12 garnets have distorted the elementary unit cell with decreasing lattice symmetry to tetragonal.

Aluminophosphate glasses with high sulfate content

To immobilize a high sulfate radioactive wastes a system Na{sub 2}O-Al{sub 2}O{sub 3}-P{sub 2}O{sub 5}-SO{sub 3} has been chosen as one where glasses have a relatively low melting points and good chemical durability. Glasses within partial system 44 Na{sub 2}O, 20 Al{sub 2}O{sub 3}, (36-x)P{sub 2}O{sub 5}, x SO{sub 3} have been prepared at 1,000 C. A possibility of assimilation up to 12 mole % of SO{sub 3} has been established. The basic properties of sulfate-containing glasses as density, microhardness, thermal expansion coefficient, transformation and deformation temperatures, viscosity, electric resistivity, leach rate of ions and diffusion coefficients of {sup 22}Na, {sup 35}S, {sup 90}Sr and {sup 137}Cs have been measured. Glass structure by infrared and EPR spectroscopies has been investigated.