Volume 105, Issue 7

Weak complexes between pentavalent and hexavalent actinyl cations have been reported to exist in acidic, non-complexing high ionic strength aqueous media. Such “cation-cation complexes” were first identified in the context of actinide-actinide redox reactions in acidic aqueous media relevant to solvent extraction-based separation systems, hence their characterization is of potential interest for advanced nuclear fuel reprocessing. This chemistry could be relevant to efforts to develop advanced actinide separations based on the upper oxidation states of americium, which are of current interest. In the present study, the chemical behavior of pentavalent uranyl was examined in non-aqueous, aprotic polar organic solvents (propylene carbonate and acetonitrile) to determine whether UO 2 + cations generated at the reducing working electrode surface would interact with the UO 2 2+ cations in the bulk phase to form cation-cation complexes in such media. In magnesium perchlorate media, the electrolyte adsorbed onto the working electrode surface and interfered with the uranyl reduction/diffusion process through an ECE (electron transfer/chemical reaction/electron transfer) mechanism. In parallel studies of uranyl redox behavior in tetrabutylammonium hexafluorophosphate solutions, an EC (electron transfer/chemical reaction) mechanism was observed in the cyclic voltammograms. Ultimately, no conclusive electrochemical evidence demonstrated uranyl cation-cation interactions in the non-aqueous, aprotic polar organic solvent solutions, though the results reported do not completely rule out the presence of UO 2 + ·UO 2 2+ complexes.

It was shown that the dissolution of UN with metallic conduction follows an electrochemical mechanism when it proceeds in contact with an electrically conductive medium (HNO 3 solution). According to this mechanism, the oxidation of UN (at the anode) passes an electron into the UN matrix, which is a conductor, and can then reduce nitric acid in a parallel reaction a short distance away at another exposed surface of the UN (at the cathode). As a result, the reduction of HNO 3 affords NO and NO 2 , while oxidation of uranium mononitride affords NH 3 , N 2 , and N 2 O. The occurrence of these two separate processes accounts for the composition and yields of the products formed from UN and HNO 3 as well as for the nitrogen isotope distribution between them when UN and HNO 3 were labeled with 14 N or 15 N. A mathematical equation describing the dependence of N 2 and N 2 O yields on HNO 3 concentration was derived. It was also shown that the calculated value of standard electromotive force of the galvanic pair formed on the UN surface during its dissolution in HNO 3 is high enough to initiate and support the electrochemical mechanism of its dissolution in nitric acid.

Two thorium organic frameworks, Th(BDC) 2 and Th(OH)(BCPBA) have been hydrothermally synthesized using 1,4-benzenedicarboxylic acid (H 2 BDC) and 3,5-bi(4-carboxyphenoxy)benzoic acid (H 3 BCPBA), respectively. The obtained two compounds were determined by single-crystal XRD, and they exhibited two new topologies. Th(BDC) 2 shows a 3-dimensional (4,4,8)-connected framework with the Schläfli symbol of (4 14 ·6 12 ·8 2 )(4 2 ·6 3 ·8)(4 4 ·6 2 ), and it is a mononuclear thorium(IV) complex. Th(OH)(BCPBA) possesses a (4,6)-connected topology with the Schläfli symbol of (4 15 ) 2 (4 6 ) 3 , and it has a dinuclear thorium(IV) asymmetric unit with the shortest Th–Th distances. Viewing along suitable directions, channels with different shapes can be found in the obtained two frameworks. Based on calculation with PLATON, the amount of void space is 21.9% and 13.5% in Th(BDC) 2 and Th(OH)(BCPBA), respectively. Density functional theory (DFT) studies revealed that the metal-ligand interactions were mainly of ionic character in both compounds and the hydroxyl ions might play an important role in the stability of dinuclear thorium(IV) of Th(OH)(BCPBA).

At present, amidoxime-based adsorbents are considered to be the most promising materials for extraction of uranium from seawater. However, the high concentrations of transition metals especially vanadium strongly compete with uranium in the sequestration process, which is extremely limited the commercial use of amidoxime-based adsorbents. In this work, the coordination modes, bonding nature, and stabilities of possible vanadium(IV) (VO 2+ ) and (V) (VO 2 + , VO 3+ , V 5+ ) complexes with amidoximate (AO − ), carboxyl (Ac − ), glutarimidedioximate (HA − ) and deprotonated glutarimidedioximate (A 2− ) on single and double alkyl chains (R=C 13 H 26 ) are systematically explored by quantum chemical calculations. Different from the uranyl (UO 2 2+ ) complexes, the AO − groups of the vanadium(IV) and (V) complexes prefer to coordinate as monodentate and chelate ligands, while few species with AO − groups in η 2 -binding mode have been observed in the vanadium complexes. Besides, the vanadium complexes are predicted to have obvious covalent metal-ligand bonds. According to thermodynamic stability analysis, all the vanadium complexes with AO − , Ac − , HA − and A 2− ligands on double alkyl chains are found to be more stable than corresponding complexes with ligands on a single chain. The synergistic effect of the amidoxime and carboxyl groups can be observed in most of VO 2 + and VO 3+ complexes with mixed ligands (AO − /Ac − ). The vanadium(IV) and (V) complexes are more stable than the corresponding uranyl complexes, and the adsorption capability of the amidoxime-based adsorbents toward vanadium(V) ions decrease in the order of VO 2 + >VO 3+ > V 5+ . The dioxovanadium cation VO 2 + is predicted to form multinuclear vanadium complex in the sequestration process, possibly resulting in higher stable VO 2 + complexes. Therefore, the higher complexation ability of the amidoxime-based adsorbents toward vanadium over uranium is probably due to the differences in the coordination modes and bonding nature. The current results might provide important clues for rational design of efficient ligands in sequestration of uranium from seawater.

Experiments in the ATALANTE facility can lead to high activity samples (for example the dissolution of hulls and spent fuels), essentially coming from the presence of 137 Cs. Usually, these samples are handled in a shielded cell. The removal of this radionuclide from the sample would make it possible to handle it in glove boxes without having to perform an important dilution in the shielded cells beforehand. It would allow to analyze samples using techniques usually implemented in glove boxes (such as ICP, α spectrometry …) and to reach lower detection and quantification limits. To do so, a separation by extraction chromatography using a Triskem AMP-PAN column was developed. A cesium decontamination factor higher than 5×10 4 and detection limits improvement up to a factor 100 were obtained.

Adsorption behavior of 99 Mo(VI) and 99m Tc(VII) was studied on ceric tungstate (CeW) and compared with the adsorption on the conventional cation and anion exchangers Dowex-50X8 and AG-2X8, respectively. The studies were carried out under static and dynamic conditions. The effect of contact time and pH on the adsorption was investigated under static conditions. High K d -values for sorption of 99 Mo(VI) on (CeW) were obtained over the investigated pH range. 99m Tc was adsorbed with much lower K d -values. The K d -values were pH dependent. K d -values of 99 Mo-adsorption on AG-2X8 were lower than those on (CeW) and vice versa for 99m Tc. Ionic species of both elements were not adsorbed on Dowex-50X8, indicating the absence of cationic species and the adsorption of both elements on AG-2X8 and (CeW) as anionic species. The loading and elution behaviors of 99 Mo and 99m Tc on (CeW) were studied using different eluents. 99 Mo remained strongly adsorbed under all conditions whereas 99m Tc was easily eluted. Adsorption of some fission products, i.e. 95 Zr(IV) and 95 Nb(V), in addition to 123m Te(IV) and 75 Se(IV), as representatives of their corresponding fission product isotopes, as well as 181 Hf, as probable radioactive contaminant was also studied. Solutions of the ionic species of those metals were loaded in mixtures together with 99 Mo on (CeW) columns. Strong adsorption of those ionic species and 99 Mo on (CeW) was found whereas 99m Tc was easily eluted. Different eluents were investigated for eluting 99m Tc from 99 Mo-adsorbed on (CeW).

In a series of crystallization experiments, the directional solidification of silicon was investigated as a low cost path for the production of silicon wafers for solar cells. Instrumental neutron activation analysis was employed to measure the influence of different crystallization parameters on the distribution of 3d-metal impurities of the produced ingots. A theoretical model describing the involved diffusion and segregation processes during the solidification and cooling of the ingots could be verified by the experimental results. By successive etching of the samples after the irradiation, it could be shown that a layer of at least 60 μm of the samples has to be removed to get real bulk concentrations.

In this experimental work, the effect of gamma irradiation on the capacitance and impedance of some commercial capacitors namely electrolytic, MKP, and MKT capacitors in different radiation doses up to 120 kGy and a wide range of frequencies between 42 Hz and 5 MHz were studied. Results showed that the capacitances of the electrolytic capacitors exhibited a linear decrease by increasing the radiation dose and frequencies, which can be used for high dosimetry purposes, but non-ceramic capacitors as MKP and MKT showed much higher radiation resistance, particularly for the frequencies less than ~1 MHz.

Strontium (Sr) and Cesium (Cs) are two important nuclear fission products which are present in the radioactive wastewater resulting from nuclear power plants. They should be treated by considering environmental and economic aspects. In this study, artificial neural network (ANN) was implemented to evaluate the optimal experimental conditions in continuous electrodeionization method in order to achieve the highest removal percentage of Sr and Ce from aqueous solutions. Three control factors at three levels were tested in experiments for Sr and Cs: Feed concentration (10, 50 and 100 mg/L), flow rate (2.5, 3.75 and 5 mL/min) and voltage (5, 7.5 and 10 V). The obtained data from the experiments were used to train two ANNs. The three control factors were utilized as the inputs of ANNs and two quality responses were used as the outputs, separately (each ANN for one quality response). After training the ANNs, 1024 different control factor levels with various quality responses were predicted and finally the optimum control factor levels were obtained. Results demonstrated that the optimum levels of the control factors for maximum removing of Sr (97.6%) had an applied voltage of 10 V, a flow rate of 2.5 mL/min and a feed concentration of 10 mg/L. As for Cs (67.8%) they were 10 V, 2.55 mL/min and 50 mg/L, respectively.