Vol 58, No 4 (2016)

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.

Samples of uranium-containing liquid luminophores activated with Dy³⁺ were prepared, and their properties were studied experimentally. Optical excitation of aqueous DyW₁₀O₃₆^9– solutions leads to the transfer of the excitation energy from the decatungstate ligand to Dy³⁺, followed by sensitized luminescence of Dy³⁺.

Databases on the coextraction of Th or Pu with Tc into 30% TBP in hydrocarbon diluent at 25°С are presented. The coextraction leads to mutual enhancement of the extraction of all the elements. The interactions in the system are described within the framework of a new approach to modeling, using the previously suggested and additional equations of concurrently occurring chemical reactions. The apparent equilibrium constants of these reactions are calculated.

The extraction of microamounts of U(VI), Th(IV), and REE(III) from HNO₃ and NH₄NO₃ solutions with solutions of diaryl(dialkylcarbamoylalkyl)phosphine oxides containing a dialkylcarbamoylmethyl substituent in the alkylene bridge was studied. The stoichiometry of the complexes extracted from nitric acid solutions with N,N,N',N'-tetrabutyl-2-(di-p-anisylphosphinyl)butanedioic diamide I was determined. The influence of the extractant structure and aqueous phase composition on the efficiency and selectivity of the extraction of U(VI), Th(IV), and REE(III) into the organic phase was examined. Introduction of the–CH₂C(O)NAlk₂ fragment into the methylene bridge of the diaryl(dialkylcarbamoylmethyl)phosphine oxide molecule considerably enhances the extraction of REE(III) from neutral nitrate solutions. Such modification of the extractant molecule only slightly influences the extraction of REE(III) from nitric acid solutions, but leads to a considerable increase in the U(VI) extraction and to a decrease in the Th(IV) extraction. The selectivity of the extraction of U(VI) and REE(III) is thus considerably increased.

Layered double hydroxides of Mg and Al, containing CО₃^2– ions in the interlayer space (LDH-Mg-Al-CО₃), and layered double oxides of Mg and Al (LDO-Mg-Al) were prepared using microwave radiation (MWR). The use of MWR allows not only acceleration of the synthesis of both LDH and LDO, but also preparation of compounds with high kinetic characteristics of the U(VI) sorption. The degree of U(VI) sorption (α) from 10^–2 M aqueous U(VI) solutions at a sorption time of 4 h and V/m = 50 mL g^–1 exceeds 99.0%. In sorption from more concentrated (10^–1 M) aqueous U(VI) solutions under similar conditions, α on all the samples does not exceed 37.5%.

Zirconium [⁹⁹Mo]molybdate(VI) gel (ZrMo) was prepared from dilute mixed solutions of zirconium oxychloride and [⁹⁹Mo]molybdate at pH 4.7. Paramolybdate(VI) anions were attracted to the positively charged surface of zirconia(IV) particulates (PZC at pH 7.2). The postulated reaction mechanism and gel formula were verified by radiometric analysis, X-ray diffraction, scanning electron microscopy, TGA, DTA, IR spectroscopy, and X-ray fluorescence analysis. ^99mTc elution from ZrMo dried at 50 and 200°C was investigated. Reproducibly high ^99mTc elution yield with high levels of radionuclidic, radiochemical, and chemical purity was achieved from the gel dried at 50°C. Correlation between structural properties of ZrMo and ^99mTc elution performance was discussed.

The behavior of the two-phase system consisting of a 30% solution of tri-n-butyl phosphate (TBP) in a paraffin diluent (C13) and a 12 M aqueous HNO₃ solution was studied in the temperature interval from 70 to 110°С. The parameters of thermolysis of TBP solutions in C13 in the reaction with HNO₃ in open vessels were determined with the aim of evaluating the explosion hazard of the system in the course of extraction reprocessing of high-level radioactive materials. Solutions of TBP in C13 in the two-phase systems are characterized in the examined temperature interval by more intense gas evolution compared to that from the singlephase systems.

The distribution of residual tritium in organs of Carassius gibelio (Prussian carp) freshwater rayfinned fish as a representative of the Carassius phytophagous genus of Cyprinidae family was studied for determining the ³Н accumulation in the aquatic plants–fishes food chain in water bodies contaminated with radionuclides. In the first step, tritium was accumulated by 3000 g (wet weight) of the aquatic plant biomass in aquaria of 100 L volume with the specific water activity of 100 kBq L^–1. The aquatic plants were shown to actively take up the introduced tritium, accumulating up to 70% of the radionuclide. The content of tritium bound to organics in the biomass increased to 21% of the whole accumulated tritium. After that, the plants (Ceratophyllum, Lemna) were dried and mixed with dry fish fodder (1: 1), and the mixture was pressed in granules. The activity of each granule was up to 50 Bq. These granules were used for forced feeding of fishes, so that each individual took up in the course of the 550-day experiment up to 330 kBq of tritium. According to the analysis results, about 75% of the whole amount of the accumulated tritium was present in muscle tissues. Determination of the tritium content of proteins and lipids in the experimental systems revealed no differences in the use of Ceratophyllum and Lemna as fodder.