Sorption and phase distribution of activation products (65Zn, 54Mn, 59Fe, 60Со) in systems consisting of Pb–Bi melt and a porous material were studied under laboratory conditions. Radionuclides of elements with minimal oxygen affinity (Ru, Te, and Sb) remain predominantly in the coolant melt and are weakly sorbed by the solid phase. Radionuclides with higher oxygen affinity are well sorbed by structural solid sorbents. The maximum of the 60Со sorption onto porous steel is reached at 853 K.

Sorption and phase distribution of 54 Mn, 59 Fe, 60 Со, 106 Ru, 125 Sb, 137 Cs, 144 Се, 154,155 Eu, and 235,238 U radionuclides in contact of the Pb–Bi melt with the steel surface were studied. Radionuclides of the elements with the minimal oxygen affinity (Ru, Te, and Sb) remain mainly in the coolant melt and are weakly sorbed onto the solid phase. The other fission product radionuclides having higher oxygen affinity are sorbed onto the surface of steel structural materials. The sorption increases at 350–400°С.

A method of high-level waste (HLW) radionuclide immobilization in a long-life matrix based on Y–Al garnet, a material highly chemically resistant to natural environments, has been developed for the ultimate HLW isolation from the environment. Model systems containing Ce, Nd, Sm, Zr, Mo, 238U, and 241Am were used in the study as simulators of HLW radionuclides. An energy-saving technology of self-propagating high-temperature synthesis (SHS) was employed to synthesize the matrix material with fixation of HLW radionuclide simulator elements in the Y–Al garnet structure. The results of an X-ray phase analysis for the synthesized materials have shown that the simulator elements, as well as uranium and americium are incorporated strongly in the structure of Y–Al garnet predominantly forming the matrix's major neoformation fit for environmentally safe disposal. The produced synthetic mineral-like matrices feature high water resistance, the property confirmed by a very low rate (10−9–10−10g/cm2⋅day) of americium leaching into water. Besides high strength of the americium fixation in the structure of Y–Al garnet, the latter's carryover is small at high temperatures due to a short duration of the process. The technically, economically and environmentally attractive novel HLW immobilization technique proposed herein may form the basis for the closing process of the spent nuclear fuel reprocessing.

The possibility of preparing by self-propagating high-temperature synthesis (SHS) metal-ceramic (cermet) matrices with simulated wastes of REE-actinide fraction and Tc was examined. The specimens consist of oxide crystalline phases, glass, and melts. In the aluminate composite, the component (Sm) simulating the REE-actinide fraction is in the garnet and glass phases, and in the titanate composite, in the pyrochlore, titanosilicate of perrierite structure, and glass phases. Rhenium (Tc simulator) is incorporated in alloy phases. To evaluate the prospects for radioactive waste immobilization by SHS, it is necessary to synthesize matrices containing actinide isotopes (Am) and Tc and to study their structure and isolation properties.

Изучено строение матрицы с имитаторами отходов пирохимической переработки облученного ядерного топлива. Состав имитаторов определен с учетом свойств оксидного высокоактивного остатка, полученного в ОАО “ГНЦ НИИАР” при переработке облученного нитридного и металлического топлива. Муратаит выбран в качестве матрицы благодаря возможности его структуры изоморфно включать различные элементы отходов. Он доминирует в образце и содержит основную часть актинидов и РЗЭ. Другие фазы матрицы - титанаты (кричтонит, цирконолит, перовскит, рутил) и щелочно-земельные молибдаты. Последние растворимы в воде, поэтому в течение 300 лет, необходимых для распада Sr - 90, контакт матрицы с водой должен быть исключен за счет использования коррозионно - устойчивых контейнеров и водонепроницаемого глинистого буфера. Альтернативный способ изоляции таких отходов - синтез матрицы в инертной среде для стабилизации молибдена (IV) и его вхождения в титанатные фазы. Это позволит предотвратить образование растворимых молибдатов щелочных земель и щелочных металлов.

Immobilization of the intermediate-level radioactive waste (RAW) in a new type of binding agents, geoconcrete monoliths, was studied. The experiments were performed using liquid radioactive wastes of various salinities formed in the course of operation of nuclear power plants and various sorbents saturated with radionuclides, in particular, with 137 Cs. The prolonged operation life of geoconcretes is determined by the chemical and mineralogical composition of the hydrated neogenic compounds, which are analogs of water-resistant rock-forming silicate materials. A procedure for synthesis of a geoconcrete matrix with immobilization of biologically important radionuclides was developed, and the properties of geoconcrete monolith (GCM) were studied. The pilot experiments with real RAW were performed. The strength of the GCM samples exceeds by a factor of 2–4 the level prescribed by the regulations, and the leaching rate of 137 Cs is lower than the prescribed level by 2–3 orders of magnitude.

For the purpose of reduction of the risk of spread of transuranic actinides, carbon-14 and other radionuclides contained in the NPP HLW, the Rosenergoatom concern decided to develop a technology of self-propagating high-temperature synthesis (SHS) to obtain a ceramic matrix suitable for long-term and ecologically safe deep geological disposal of the high-level radwaste of the nuclear industry. The proposed graphite HLW treatment method to immobilise the radionuclides into the thermally, chemically and radiation stable carbide-corundum matrix is based on a SHS-process according to the chemical reaction in the system C+Al+TiO2, where the component C is irradiated graphite of the RBMK reactor core moderators. This paper reports the results of the R&D activities for optimising the SHS technology on a pilot plant using non-irradiated graphite and fuel spill simulators (HLW).

Having applied the method of the self-propagating high-temperature synthesis, we produced various crystal materials, including ones to be composed of fluorite and pyrochlore-type phases. The given paper describes new results obtained in the course of characterising the matrices using XRD and SEM/EDS analyses.

Preparation of analogs of minerals pollucite and sphene by metallothermal self-propagating high-temperature synthesis (SHS) was studied with model systems. These analogs are intended for immobilizing silica gel fixed cesium and strontium radionuclides, respectively. The optimal conditions for SHS were found, and the phase compositions of the reaction products were studied. In synthesis of pollucite, the energy-forming components are Al and Fe2O3, and in synthesis of sphene, Ti and MoO3. The mineral analogs synthesized are distinguished by high hardness and chemical stability. Virtually no cesium is lost by evaporation.

Host matrices for actinides prepared by self-propagating high-temperature synthesis are studied. The matrices consist of a pyrochlore or fluorite phase and metallic molybdenum. The factor determining the structural type of the crystal lattice of the target phase is the ionic radius ratio. When the difference in the ionic radii is insignificant, as in the case of Y3+ (r 0.102 nm) and Zr4+ (r 0.084 nm), the oxide Zr1-xYxO2-0.5x with a fluorite structure is formed, in which the cations occupy the eight-coordinate sites. This structure permits incorporation of heavy lanthanides and tetravalent actinides: U4+ (r 0.10 nm), Np4+ (r 0.098 nm), and Pu4+ (r 0.096 nm). When the difference in the ionic radii is more considerable, as in the case of Y3+ and Ti4+ (r 0.061 nm), a pyrochlore-related structure is realized. In this case the cations occupy different (eight- or six-coordinate) sites. The pyrochlore structure is preserved if the radii of ions occupying different structural sites change in parallel. This structure is typical of zirconates of trivalent actinides and light REEs. The decision on the major host phase for actinides is determined by the waste composition. At low content of light REEs and americium an oxide with a fluorite-related structure shows promise. At high content of these elements, zirconates and titanates with the pyrochlore structure are more stable.

The minerals which are most resistant to natural corrosive media and possess perovskite, zirconolite, fergusonite, and Y-Al garnet type structures are selected on the basis of an analysis of the mineralogical data. The minerals contain elements that are isomorphic to radionuclides appearing in high-level wastes.

Alternative vitrification technologies are being developed in the world for the immobilization of high radioactive waste in materials with improved thermodynamic stability, as well as improved chemical and thermal stability and stability to radiation. Oxides, synthesized in the form of analogs to rock-forming minerals and ceramics, are among those materials that have highly stable properties and are compatible with the environment. In choosing the appropriate material, we need to be guided by its geometric stability, the minimal number of cations in the structure of the material and the presence of structural elements in the mineral that are isomorphs of uranium and thorium, actinoids found in nature. Rare earth elements, yttrium, zirconium and calcium are therefore suitable. The minerals listed in the table (with the exception of the zircon) are pegatites by origin, i.e. they are formed towards the end of the magma crystallization of silicates form the residual melt, enriched with Ta, Nb, Ti, Zr, Ce, Y, U and Th. Uranium and thorium in the form of isomorphic admixtures form part of the lattice of the mineral. These minerals, which are rather simple in composition and structure and are formed under high temperatures, may be viewed as natural physio-chemical systemsmore » that are stable and long-lived in natural environments. The similarity of the properties of actinoids and lanthanoids plays an important role in the geochemistry of uranium and thorium; however, uranium (IV) is closer to the {open_quotes}heavy{close_quotes} group of lanthanoids (the yttrium group) while thorium (IV) is closer to the {open_quotes}light{close_quotes} group (the cerium group). That is why rare earth minerals contain uranium and thorium in the form of isomorphic admixtures.« less