[en] The electrochemical properties of ionic liquids (ILs) make them attractive for possible replacement of inorganic salts in high temperature molten salt electrochemical processing of nuclear fuel. To be a feasible replacement solvent, ILs need to be stable in moderate and high doses of radiation without adverse chemical and physical effects. Here, we exposed seven different ILs to a 1.2 MGy dose of gamma radiation to investigate their physical and chemical properties as they related to radiological stability. The azolium-based ILs experienced the greatest change in appearance, but these ILs were chemically more stable to gamma radiation than some of the other classes of ILs tested, due to the presence of aromatic electrons in the azolium ring. All the ILs exhibited a decrease in their conductivity and electrochemical window (at least 1.1 V), both of which could affect the utility of ILs in electrochemical processing. The concentration of the irradiation decomposition products was less than 3 mole %, with no impurities detectable using NMR techniques

[en] Using bremsstrahlung beams up to 50 MeV, (γ,xn) and (γ,γ') reaction cross sections of ⁹⁹Tc were determined for the first time, by means of gamma-spectrometry. Calculations were also carried out in the framework of a neutron evaporation and a statistical gamma-ray cascade model, respectively. As a result, no deviation from the widely accepted picture was experienced in the case of (γ,xn) reactions. The photoexcitation function for ^99mTc production, however, while exhibiting a maximum at neutron emission threshold, dropped monotonously from about 9 to 50 MeV, in sharp contrast with the long-standing belief in a two-humped excitation function based on earlier reports. Allowed β-transitions from photoexcited states of ⁹⁹Tc to ⁹⁹Ru levels may reduced dramatically the terrestrial half-life of ⁹⁹Tc in stellar environments. Significance of Tc abundance measurements in stars and shell-model calculations in determining the parameters of the s-process nucleosynthesis are highlighted. (orig.)

[en] Tc(III)-EDTA and Tc(III)-HEDTA complexes have been synthesized by the ligand substitution reactions of hexakis(thiourea)technetium(III) ion with EDTA and HEDTA. The compositions were identified as [Tc(III)(tu)Hedta]⁰, [Tc(III)(tu)₂Hhedta]⁺ and [Tc(III)(tu)hedta]⁰. The UV-Vis spectra of these complexes exhibited two absorption maxima at 368 and 470 nm, 377 and 493 nm, and 373 and 487 nm, respectively. The formation reaction proceeded predominantly in the following processes: Tc(tu)³⁺₆ + H₂edta → Tc(III)-EDTA (1) Tc(tu)³⁺₆ + Hhedta → Tc(III)-HEDTA (2) The rate constants for (1) and (2), determined at 25 C, were k = (2.1 ± 0.2) x 10^-1 M^-1s^-1 and k' = (3.2 ± 0.3) x 10^-1 M^-1s^-1, respectively. (orig.)

[en] The complexation of technetium with well characterized humic acid, which was extracted from one of the Gorleben groundwaters in northern Germany, was studied. It was found that technetium humate was produced as a brownish-black precipitate by addition of Sn²⁺ as a reductant in a mixture solution of pertechnetate and humic acid at pH 4 in 0.1 mol l^-1 NaClO₄. This precipitate could be distinguished from the TcO₂ with respect to the dissolution behavior at higher pH. The produced amount of the formed technetium humate linearly increases with the added amount of Sn²⁺ with a slope of ca. 0.5, showing that the valence state of technetium is 3+. With decrease of the concentration of humic acid in the solution, the formation of TcO₂ became prominent. (orig.)

[en] Several sampling campaigns were carried out in the Seine estuary in order to study the behaviour of plutonium isotopes versus salinity (²³⁸Pu and ^239,240Pu), in 0.45 μm filtered waters. During estuarine mixing, the plutonium has a non-conservative behaviour: plots of Pu activities versus salinity display an excess of dissolved ²³⁸Pu (RI ²³⁸Pu/^239,240Pu > 1) in low salinity waters (0.5 and 1.5 g l^-1) and a depletion (RI < 1) in the zone of intermediate salinities (8 to 15 g l^-1), with respect to conservative dilution. Laboratory experiments showed that Pu(V) present in seawater is reduced to the III, IV states and sorbed onto particles with a maximum in low salinity waters which could explain the depletion observed at intermediate salinity in the Seine estuary. Reduction occurs prior to sorption and is connected with the Seine river colloidal and dissolved species (humic acids). The Pu associated with estuarine sediments desorbs from the ''oxide-hydroxide'' fraction in contact with river water, thus partly explains the plutonium excess measured in low salinity filtered waters of the Seine. (orig.)

[en] The acid dissociation constants, pK_a, and the stability constants for NpO⁺₂ and UO²⁺₂ have been measured for certain organic ligands [acetate, α-hydroxyisobutyrate, lactate, ascorbate, oxalate, citrate, EDTA, 8-hydroxyquinoline, 1,10-phenanthroline, and thenoyltrifluoroacetone] in 5 m (NaCl) ionic strength solution. The pK_a values were determined by potentiometry or spectrometry. These methods, as well as solvent extraction with ²³³U and ²³⁷Np radiotracers, were used to measure the stability constants of the 1:1 and 1:2 complexes of dioxo cations. These constants were used to estimate the concentrations required to result in 10% competition with hydrolysis in the 5 m NaCl solution. Such estimates are of value in assessing the solubility from radioactive waste of AnO⁺₂ and AnO²⁺₂ in brine solutions in contact with nuclear waste in a salt bed repository. (orig.)

[en] The influence of silica and phosphate on schoepite weathering in calcium bearing systems has been examined. The presence of Si (10^-3 M) retarded the transformation of schoepite to becquerelite in 10^-2 and 10^-3 M Ca systems as compared to silica free systems. The formation of uranyl silicates was not observed. The presence of phosphate (10^-2 M) in a 10^-2 M Ca system led to the transformation of schoepite into autunite via several intermediate products, including becquerelite and one or more uranyl phosphates. These results indicate the importance of silica and phosphate in the transformation of secondary uranyl phases in natural systems. (orig.)

[en] The retention capacity of crystalline rock matrix induced by diffusion into the open space has been investigated in the present study. The characterization of this space is crucial since it acts as the potential pathway of access of groundwater into the interior of the rock matrix. To this end, two very different rock matrices have been studied. The structure of El Berrocal granite (Toledo, Spain) and that of Muscovite Granite (Haemeenlinna, Finland) were compared at scales ranging from cm to μm. The experimental procedure combines two complementary approaches to the rock matrix structure. The recently developed ¹⁴C-labelled polymethylmetacrylate (¹⁴C-PMMA) method allows the study of rock matrix porosity and routes of migration in cm to sub-mm scales (Laboratory of Radiochemistry, University of Helsinki, Finland). Using complementary microscopy techniques, the microstructural network of rock matrix, its geometry and connectivity have been evaluated in the Group of Petrophysics of the University of Oviedo, Spain. These observations are combined with laboratory scale diffusion experiments to achieve a better understanding of the characteristics controlling the retention capacity of crystalline rocks. (orig.)

[en] Non-destructive neutron activation analysis has been applied to the determination of 11 - 14 rare earth elements in samples of fluorites collected from 7 different deposits of Argentina. The adequate selection of conditions of irradiation, decay and measurement for the determination are described. The results obtained for analysis of 22 samples are presented and their interpretation are discussed. (orig.)

[en] The Pt-catalyzed interaction between formic and nitric acids has been studied in the presence of 1% Pt/SiO₂. Kinetic and stoichiometric measurements were carried out under the following conditions: [HNO₃]₀ = 0.7-8 M; [HCOOH]₀ = 0.55-4.5 M; T = 55-85 C; liquid-to-solid ratio S/L = 0.1-0.02 g/ml. It was shown that in the presence of platinum catalyst the interaction between HCOOH and HNO₃ starts with confidence at 60-80 C without any induction period. Complex nature of the interaction between HCOOH and HNO₃ in the presence of platinum catalyst is revealed. It was found that the reactions of heterogeneous catalytic reduction of nitric acid and homogeneous autocatalytic denitration proceed simultaneously. A contribution of each process depends on the acidity and the total nitrate concentration in the solution. The order of the reaction with respect to [HCOOH] is 1. The order of the reaction with respect to (HNO₃) is 0 under the conditions when an autocatalytic accumulation of HNO₂ does not take place. It was found that the catalytic interaction between HCOOH and HNO₃ takes place in the presence of antinitrite agents as urea and hydrazine, though the rate of this process is not high. At the same time, the reaction of HCOOH catalytic decomposition does not proceed in non-nitrate media (under the conditions of the experiments). A method of the catalytic denitration was proposed for the reprocessing of aqueous radioactive waste. (orig.)