Matrices for Isolation of Actinide Wastes in a Deep Well Repository

Yudintsev, S. V.; Никольский, М. С.; Никонов, Б. С.; Mal’kovskii, V. I.

doi:10.1134/s1028334x18050203

Cited by 9 publications

(5 citation statements)

References 11 publications

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“…Compared to the ideal formula A 2 B 2 O 6 O’, there was a deficiency of cations in the A position and an absence of the O’ anion in the formula of the pyrochlore. Defective pyrochlores were found in the samples S3–S5, S8 ( Figure 9 ), obtained by melting–crystallization [ 144 , 147 , 148 ]. The difference between the formula of zirconolite and the ideal one with seven O 2− lies in the reduction of part of the Ti 4+ to Ti 3+ during the reaction of the melt within the carbon crucible: Ti 4+ O 2 + C = Ti 3+ + CO 2 .…”

Section: Ebsd Study Of Zirconolite Polytypes and Murataite Polysomesmentioning

confidence: 99%

“…These structural features contradict the data [ 143 ] suggesting the absence of zirconolite in the NdO 1.5 —TiO 2 —ZrO 2 system. In most of the samples studied by us, obtained in this system by sintering or melting [ 148 ], for example, S3–S5, S8 ( Figure 9 ), zirconolite was not found, and their phase composition mainly corresponded to the phase diagram ( Table 6 ). Zirconolite was previously found only in two samples [ 48 , 144 ].…”

Section: Ebsd Study Of Zirconolite Polytypes and Murataite Polysomesmentioning

confidence: 99%

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Zirconolite Polytypes and Murataite Polysomes in Matrices for the REE—Actinide Fraction of HLW

et al. 2022

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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.

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Section: Ebsd Study Of Zirconolite Polytypes and Murataite Polysomesmentioning

confidence: 99%

Section: Ebsd Study Of Zirconolite Polytypes and Murataite Polysomesmentioning

confidence: 99%

Zirconolite Polytypes and Murataite Polysomes in Matrices for the REE—Actinide Fraction of HLW

et al. 2022

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“…The preferred types of HLW forms for placement into deep wells [108][109][110][111][112][113][114][115] are waste with high heat release, including chlorides and fluorides of Cs and Sr; cesium-strontium fractions; actinide-containing waste in a stable matrix, including Pu-containing waste; and the REE-actinide fraction of HLW. The advantages of the borehole disposal of HLW over mines include the following: long-term safety due to the large burial depth and extremely low solubility of actinides under reducing conditions; economical access to rocks with high insulating properties; lower infrastructure requirements and significantly smaller surface area; shorter terms of construction and placement of HLW; significantly lower cost; the possibility of creating waste in close proximity to the place of waste generation; extremely low probability of unauthorized access to radioactive materials; minimal control after HLW placement and storage-facility closure.…”

Section: Assessment Of the Possibility For Deep-borehole Disposal Of ...mentioning

confidence: 99%

“…Promising matrices (waste forms) of REEs and MAs are considered to be the glasses of B-Al-Si, Ca-B-Al, Al-P, and Fe-P composition, as well as glass-ceramics with crystalline phases of zirconolite, britholite, and monazite 49,59,[112][113][114][115][116][117][118][119]. The content of the REE-MA fraction in such matrices can reach 30-50 wt.%, and very deep wells with placement of the HLW forms at a depth of 3 to 5 km are optimal for their disposal.…”

Section: Assessment Of the Possibility For Deep-borehole Disposal Of ...mentioning

confidence: 99%

Thermal Effects and Glass Crystallization in Composite Matrices for Immobilization of the Rare-Earth Element–Minor Actinide Fraction of High-Level Radioactive Waste

Yudintsev,

Ojovan,

Malkovsky

2024

J. Compos. Sci.

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The current policy of managing high-level waste (HLW) derived in the closed nuclear fuel cycle consists in their vitrification into B-Si or Al-P vitreous forms. These compounds have rather limited capacity with respect to the HLW (5–20 wt%), and their properties change over time due to devitrification of the glasses. Cardinal improvement in the management of HLW can be achieved by their separation onto groups of elements with similar properties, followed by their immobilization in robust waste forms (matrices) and emplacement in deep disposal facilities. One of the possible fractions contains trivalent rare-earth elements (REEs) and minor actinides (MAs = Am and Cm). REEs are the fission products of actinides, which are mainly represented by stable isotopes of elements from La to Gd as well as Y. This group also contains small amounts of short-lived radionuclides with half-lives (T1/2) from 284 days (144Ce) to 90 years (151Sm), including 147Pm (T1/2 = 2.6 years), 154Eu (T1/2 = 8.8 years), and 155Eu (T1/2 = 5 years). However, the main long-term environmental hazard of the REE–MA fraction is associated with Am and Cm, with half-lives from 18 years (244Cm) to 8500 years (245Cm), and their daughter products: 237Np (T1/2 = 2.14 × 106 years), 239Pu (T1/2 = 2.41 × 104 years), 240Pu (T1/2 = 6537 years), and 242Pu (T1/2 = 3.76 × 105 years), which should be immobilized into a durable waste form that prevents their release into the environment. Due to the heat generated by decaying radionuclides, the temperature of matrices with an REE–MA fraction will be increased by hundreds of centigrade above ambient. This process can be utilized by selecting a vitreous waste form that will crystallize to form durable crystalline phases with long-lived radionuclides. We estimated the thermal effects in a potential REE–MA glass composite material based on the size of the block, the content of waste, the time of storage before immobilization and after disposal, and showed that it is possible to select the waste loading, size of blocks, and storage time so that the temperature of the matrix during the first decades will reach 500–700 °C, which corresponds to the optimal range of glass crystallization. As a result, a glass–ceramic composite will be produced that contains monazite ((REE,MA)PO4) in phosphate glasses; britholite (Cax(REE,MA)10-x(SiO4)6O2) or zirconolite ((Ca,REE,MA)(Zr,REE,MA)(Ti,Al,Fe)2O7), in silicate systems. This possibility is confirmed by experimental data on the crystallization of glasses with REEs and actinides (Pu, Am). The prospect for the disposal of glasses with the REE–MA fraction in deep boreholes is briefly considered.

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“…The preservation of the structure with variations in composition, i.e., wide ranges of solid solution stability, is an important characteristic of matrices. The formation of compounds with higher solubility in water during the synthesis of We obtained data on the content of impurities in REE titanates and zirconates, their behavior upon irradiation, and interaction with solutions [66][67][68][69][70][71][72]. The 12: cuboctahedron (Fig.…”

Section: Introductionmentioning

confidence: 99%

Crystal Chemistry of Titanates and Zirconates of Rare Earths—Possible Matrices for Actinide Isolation

et al. 2022

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Titanates and zirconates of light rare earth elements (REE): REE2TiO5, REE2Ti2O7, REE4Ti9O24, and REE2Zr2O7, 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 2REE3+ ↔ Ca2+ + U4+, 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.

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Matrices for Isolation of Actinide Wastes in a Deep Well Repository

Cited by 9 publications

References 11 publications

Zirconolite Polytypes and Murataite Polysomes in Matrices for the REE—Actinide Fraction of HLW

Zirconolite Polytypes and Murataite Polysomes in Matrices for the REE—Actinide Fraction of HLW

Thermal Effects and Glass Crystallization in Composite Matrices for Immobilization of the Rare-Earth Element–Minor Actinide Fraction of High-Level Radioactive Waste

Crystal Chemistry of Titanates and Zirconates of Rare Earths—Possible Matrices for Actinide Isolation

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