The possibilities of scanning electron microscopy and electron backscatter diffraction (EBSD) in the study of matrices for immobilization of high-level radioactive wastes are shown from the example of two samples with Th (a Pu imitator) and Nd (an imitator of the REE–actinide fraction). The samples were produced by melting-crystallization and are composed of murataite and zirconolite (sample with Th) or pyrochlore, zirconolite, and rutile (sample with Nd). Murataite is a polysome 8C, and zirconolite includes polytypes 3T (sample with Th) or 4M (sample with Nd). The formation of zirconolite in a sample with Nd is probably caused by partial reduction of Ti 4+ up to Ti 3+ upon the reaction of melt with a glassy carbon crucible. It is concluded with the efficiency of EBSD in a study of crystalline matrices for radioactive wastes.

Titanates and zirconates of light rare earth elements (REE): REE 2 TiO 5 , REE 2 Ti 2 O 7 , REE 4 Ti 9 O 24 , and REE 2 Zr 2 O 7 , are of interest as matrices for isolating the REE actinide fraction of high-level waste from the reprocessing of irradiated nuclear fuel. Data on the incorporation of impurities (Zr, U, Ca) into Nd and La titanates are examined. They display limited isomorphism toward these elements, including by the reaction 2REE 3+ ↔ Ca 2+ + U 4+ , which is common for minerals and their synthetic analogues. The reasons for the low solubility of Zr and U in Nd titanates and the role of the crystal chemical factor in the choice of crystalline matrices for the immobilization of the REE actinide fraction are considered.

Electron backscatter diffraction (EBSD) has been used for more than 30 years for analyzing the structure of minerals and artificial substances. In recent times, EBSD has been widely applied for investigation of irradiated nuclear fuel and matrices for the immobilization of radioactive waste. The combination of EBSD and scanning electron microscopy (SEM/EDS) methods allows researchers to obtain simultaneously data on a specimen’s local composition and structure. The article discusses the abilities of SEM/EDS and EBSD techniques to identify zirconolite polytype modifications and members of the polysomatic murataite–pyrochlore series in polyphase ceramic matrices, with simulations of Pu (Th) and the REE-actinide fraction (Nd) of high-level radioactive waste.

Light rare earth (REE) titanates, such as REE2TiO5, REE2Ti2O7, and REE4Ti9O24, are potential matrices for the REE-actinide fraction of high-level waste from the reprocessing of spent nuclear fuel. The data on the “solubility” of impurity elements (zirconium, uranium, and calcium) in these phases are summarized. The structures considered demonstrate limited isomorphism with respect to these elements, according to the reaction 2REE3+ = Ca2+ + U4+, which is common for natural minerals and their synthetic analogues. The reasons for the low “solubility” of these impurities in the REE titanates are considered. The role of the crystal-chemical factor in the selection of matrices for the immobilization of the REE-actinide fraction is analyzed.

Matrices with simulators of actinides (Sm, Nd) and technetium (Re) are produced by self-propagating high-temperature synthesis (SHS). The samples are composed of rare-earth aluminate with a garnet structure, glass, and alloys based on Fe, Re, Ni, and Cr. SEM/EDS analysis in combination with electron backscatter diffraction (EBSD) is used for their characterization. Alloy segregations have a complex structure, which is revealed during their element mapping. According to EBSD data, they are composed of phases of the following structural types: σ, ε, bcc (main) and fcc, A13, and P-phase (minor). The reasons for their formation are the heterogeneity of the sample and its local enrichment in different elements, as well as the fast occurrence and nonequilibrium of the SHS process.

Crystalline phases are promising materials (matrices) for isolating nuclear waste with long-lived radionuclides, actinides first of all. Melting in an induction-heated cold crucible was used to produce a potential matrix with the following nominal composition: 50 wt % TiO2, 10 wt % MnO, 10 wt % CaO, 5 wt % Al2O3, 5 wt % Fe2O3, 10 wt % ZrO2, and 10 wt % CeO2 (actinide simulator). The target murataite phase is the predominant phase in the sample; crichtonite and glass are present as well. The emergence of glass is due to contamination of the melt by the refractory coating of the crucible. As in other samples produced by melting, murataite is represented by zonal crystals, the center of which is enriched in heavy elements (Zr and Ce). Murataite accounts for up to 80% of simulated radioactive waste. Irradiation with a dose of 22 million Gray resulted in partial amorphization of crichtonite. According to the results of a dynamic test (single pass flow test, SPFT), the rate of Ce leaching (powder, water, 70°C) is 10–4–10–5 g/(m2 day).

The influence exerted by the composition of ceramics with imitators of radioactive wastes (Nd, U) on their stability against leaching in water at 90 and 150°C has been studied. Raising the content of TiO2 in a ceramic with Nd led to an increase in the fraction of perovskite and decrease in the amount of murataite. Samples with perovskite have the lowest stability in solution, especially at 150°C. Making smaller the fraction of perovskite lowers the rate of Nd leaching by a factor of 2 (90°C) and 5 (150°C). The weaker influence exerted by temperature on the U leaching from the samples is observed because more stable pyrochlore or zirconolite appear in the samples instead of perovskite.

The phase composition, structure, and rhenium (technetium surrogate) speciation in sodium aluminum(iron) phosphate glasses synthesized under oxidizing and reducing conditions were examined using X-ray diffraction, scanning electron microscopy, Fourier transform infrared, Raman, and X-ray absorption near edge structure spectroscopic methods. In the sodium aluminophosphate glasses, Re is present in both metallic Re(0) form and Re(VII). In the Sodium aluminum-iron phosphate glasses, regardless of the conditions of their production, all the Re occurred as Re(VII). Elemental leach rates from the Re-containing glasses were found to be lower than the standard values.

Speciation of sulphate ions in sodium alumino(iron)phosphate glasses was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared and Raman spectroscopy. The high-sulphate glasses are subject to devitrification segregating minor sodium sulphates. As followed from SEM data iron-bearing glasses were higher homogeneous than the iron-free glasses. Vibrational (IR and Raman) spectra demonstrated occurrence of some bands typical of vibrations of S-O bonds in the sulphate ions: 620-640 cm(-1) (v(4) SO4 in IR spectra) and similar to 460-470 (v(2)), similar to 620-640 (v(4)), 967-1008 (v(1)) and similar to 1105-1130 cm(-1) (v(3)) in the Raman spectra. This exhibits that the sulphate ions have been shown to maintain their identity being independent in the aluminophosphate glass network. Sulfur leach rate is associated with that for sodium due to formation of a sodium sulphate enriched areas in the glasses.

Phase formation at synthesis of potential waste form for actinide immobilization was studied. Oxide mixture with composition (wt.%): 55 TiO2, 10 MnO, 10 CaO, 5 Al2O3, 5 Fe2O3, 5 ZrO2, and 10 Gd2O3 (trivalent actinide and neutron poison surrogate) was compacted under 200 MPa at room temperature and sintered for 1-6 h at temperatures from 1200 to 1350 degrees C. As follows from X-ray diffraction and electron microscopy studies, the ceramics obtained is composed of two structural varieties of murataite and crichtonite-type titanate. The results were compared to ceramic waste forms produced by melting and crystallization. (C) 2019 Elsevier B.V. All rights reserved.

Simulation of 137 Cs radioactive decay to 137 Ba by an equiatomic substitution of Cs with Ba in a 30 Na 2 O, 10 Cs 2 O, 10 Al 2 O 3 , 10 Fe 2 O 3 , 40 P 2 O 5 (mol%) glass was studied by X-ray diffraction, scanning electron microscopy, Fourier Transform Infrared spectroscopy, Mössbauer spectroscopy, and measurement of hydrolytic durability. Gradual Ba substitution for Cs yielded minor changes in the structural network but did not offer appreciable effect on phase composition and hydrolytic durability of the glasses.

Radioactive wastes with long-lived actinides derived in the nuclear fuel cycle can be disposed in very deep boreholes (depth up to 5 km) in titanate and zirconate matrices. During their interaction with a hot chloride brine very low release rate of the waste imitator (Nd) is shown. The elements in solution occur in soluble form, role of colloids is insignificant. The data allow selection of matrices for actinide waste immobilization.

Samples of thorium-bearing ceramic with a target composition (wt%) 5 Al2O3, 10 CaO, 55 TiO2, 10 MnO, 5 Fe2O3, 5 ZrO2, 10 ThO2 were produced by melting in glassy carbon crucibles in a resistive furnace and by cold crucible inductive melting (CCIM) at a vibration power of 10 kW and operation frequency of 5.28 MHz. All the samples contained 85–95 vol% murataite polytypes with 5- (5C), 8- (8C), and 3-fold (3C) elementary fluorite unit cell composing core, intermediate zone and rim of the grains, respectively, and minor crichtonite, perovskite, pyrochlore, rutile, etc. A feature of the ceramics obtained by melting in glassy carbon crucibles is formation of Fe (II) titanate whereas the inductive-melted ceramics contained traces of vitreous phase due to melt contamination with a cold crucible putty material. Melting rate in the cold crucible of up to 350 kg/(m2 × h) has been achieved. The ceramics obtained have excellent chemical durability.

Radioactive nuclear waste containing long-lived actinides (Np, Pu, Am, and Cm) is proposed to be placed in well repositories with a depth of up to 5 km. The optimum form of such wastes is crystalline phases that are capacious in relation to radionuclides and stable in mineralized groundwater (brines) heated due to heat generation in the waste and due to the geothermal gradient. In order to find possible phases, we have studied samples of the Nd–Ti–Zr–O system, where Nd 3+ acts as an imitator of the rare-earth-actinide fraction of highly radioactive wastes of reprocessed nuclear fuel. The samples were obtained by induction melting in a cold crucible with subsequent melt crystallization. It has been concluded that the Nd 2– x (Ti,Zr) 2 O 7–1.5 х and Nd 4 Ti 9 O 24 phases are promising as potential matrices for these wastes. The structure of NdO 1.5 –TiO 2 –ZrO 2 system at a high temperature has been revealed.

Sodium aluminum (iron) phosphate glass ceramics containing of up to 20 wt.% rare earth (RE) oxides simulating pyroprocessing waste were produced by melting at 1250 degrees C followed by either quenching or slow cooling to room temperature. The iron-free glass-ceramics were composed of major glass and minor phosphotridymite and monazite. The iron-bearing glass-ceramics were composed of major glass and minor monazite and Na-Al-Fe orthophosphate at low waste loadings (5-10 wt.%) and major orthophosphate and minor monazite as well as interstitial glass at high waste loadings (15-20 wt.%). Slowly cooled samples contained higher amount of crystalline phases than quenched ones. Monazite is major phase for REs. Leach rates from the materials of major elements (Na, Al, Fe, P) are 10(-5)-10(-7) g cm(-2) d(-1), RE elements - lower than 10(-5) g cm(-2) d(-1). (c) 2017 Elsevier B.V. All rights reserved.

La-Ce-Gd titanate-zirconate pyrochlore-based ceramics was synthesized at the lab-scale inductive melting unit with a 56 mm inner diameter cold crucible. Batch feeding rate and melting ratio were 3 kg/h and 3 kW h/kg respectively. The ceramics is composed of 85-90 vol% zoned grains of the pyrochlore structure phase, the sample has excellent chemical durability in hot water and may be considered as a promising matrix for actinide - rare earth fraction of high level waste.

Sodium-aluminum-(iron)-phosphate glasses containing lanthanum, cerium, europium, and gadolinium (Ln) oxides were examined by X-ray diffraction, infrared spectroscopy, and X-ray photoelectron spectroscopy. Phase composition of the quenched and annealed materials was determined. It has been shown that introduction of up to ~5wt% Ln oxides to sodium-aluminum-phosphate (SAP) and sodium-aluminum-iron-phosphate (SAIP) baseline compositions did not cause their devitrification at quenching (except the La-bearing glass) and did not offer significant impact on their structure and hydrolytic durability. All the Lns studied are present in a trivalent form. After annealing the SAP-based glasses were partly devitrified with segregation of aluminophosphate, sodium-aluminophosphate, Ln- (monazite) and Na/Ln phosphate phases while in the Ln-bearing SAIP glasses sodium-iron orthophosphate and monazite were found. Devitrification at annealing reduced hydrolytic durability of glasses by factors of 5 to 10 as compared to the quenched samples (glasses).