Thermodynamic interpretation of neptunium coprecipitation in uranophane for application to the Yucca Mountain repository

Interpretation and modeling of recent experimental data [1] yield thermodynamic constants for the distribution of trace Np(V) between aqueous solutions and uranophane. These data indicate that neptunyl is relatively excluded from the uranyl mineral structure, but the interpretation depends on uncertain aqueous speciation and thermodynamic properties as a function of temperature. Despite Np exclusion, the low calculated solubility of uranophane at 25 °C under conditions relevant to the proposed nuclear waste repository at Yucca Mountain, Nevada, leads to Np concentrations at equilibrium with a Np-bearing uranophane solid solution that are low compared to concentrations invoked as solubility limits in Yucca Mountain performance assessments

Comparison of complexed species of Eu in alumina-bound and free polyacrylic acid: A spectroscopic study

International audienceThe speciation of Eu complexed with polyacrylic acid (PAA) and alumina-bound PAA (PAAads) was studied at pH 5 in 0.1 M NaClO4. Structural parameters were obtained from 7F0 → 5D0 excitation spectra measured by laser-induced fluorescence spectroscopy as well as from Eu LIII-edge extended X-ray absorption fine structure (EXAFS) spectra. The coordination mode was also investigated by infrared spectroscopy. To elucidate the nature of the complexed species, Eu–acetate complexes were used as references. The spectroscopic techniques show that two carboxylate groups with 2–3 (EuPAA) and 4–5 (EuPAAads) water molecules are coordinated to Eu in the first coordination sphere. For EuPAAads, the coordination between carboxylate groups and Eu appears to be bidendate. A similar coordination is probable for EuPAA but the EXAFS data indicate a slightly distorted coordination. The results show that the degree of freedom of carboxylate groups is not the same for free or adsorbed PAA. For PAA, the degree of freedom is constrained by the flexibility of the methylene chain. When PAA is adsorbed on alumina, the polymer chains cannot any more be treated as independent chains. One may rather assume formation of aggregates that form an organic layer at the mineral surface presenting a complex arrangement of carboxylate groups

Discrepancies in Thorium Oxide Solubility Values: a New Experimental Approach to Improve Understanding of Oxide Surface at Solid/Solution Interface

Accepted for publication in Radiochimica ActaThe solubility of ThO2(cr) was studied since many years but a large discrepancy in solubility values is noticed in the literature. The present work suggests that this discrepancy is related to differences in the surface properties of thorium oxide. To understand the role of surface properties on solubility values, we conducted experiments with ThO2(cr) spheres with reproducable surface properties. Batch dissolution experiments were conducted in 0.01 M NaCl solution at pH = 3.0 and 4.0 for periods of time up to 270 days. The solutions were spiked with 229Th to determine precipitation (sorption) rates of thorium, while dissolution rates were determined by measuring 232Th released from ThO2(cr) spheres. We assume that 229Th atoms are exchanged only with active sites involved in Th-dissolution. Using 229Th as local sensor of attachement and detachment processes at the ThO2(cr) surface under close-to-equilibrium conditions, allows to assess surface reactivity of the solid during solubility experiments

Human-Centric Process-Aware Information Systems (HC-PAIS)

Process-Aware Information Systems (PAIS) support organizations in managing and automating their processes. A full automation of processes is in particular industries, such as service-oriented markets, not practicable. The integration of humans in PAIS is necessary to manage and perform processes that require human capabilities, judgments and decisions. A challenge of interdisciplinary PAIS research is to provide concepts and solutions that support human integration in PAIS and human orientation of PAIS in a way that provably increase the PAIS users' satisfaction and motivation with working with the Human-Centric Process Aware Information System (HC-PAIS) and consequently influence users' performance of tasks. This work is an initial step of research that aims at providing a definition of Human-Centric Process Aware Information Systems (HC-PAIS) and future research challenges of HC-PAIS. Results of focus group research are presented.Comment: 8 page

New Methods for HTR Fuel Waste Management

International audienceConsidering the need to reduce waste production and greenhouse emissions by still keeping high energy efficiency, various 4th generation nuclear energy systems have been proposed. As far as graphite moderated reactors are concerned, one of the key issues is the large volumes of irradiated graphite encountered (1770 m3 for fuel elements and 840 m3 for reflector elements during the lifetime (60 years) of a single reactor module [1]). With the objective to reduce volume of waste in the HTR concept, it is very important to be able to separate the fuel from low level activity graphite. This requires to separate TRISO particles from the graphite matrix with the sine qua non condition to not break TRISO particles in case of future embedding of particles in a matrix for disposal. According to National Regulatory Systems, in case of limited graphite waste production or of short duration HTR projects (e.g. in Germany), direct disposal without separation is acceptable. Nevertheless, in case of large scale deployment of HTR technology, such approach is not economical and sustainable. Previous attempts in graphite management (furnace, fluidised bed and laser incinerations and encapsulation matrices) dealt with graphite matrix only. These are the reasons why we studied the management of irradiated compact-type fuel element. We simulated the presence of fuel in the particles by using ZrO2 kernels. Compacts with ZrO2 TRISO particles were manufactured by AREVA NP. Two original methods have been studied. First, we tested high pressure jet to erode graphite and clean TRISO particles. Best erosion rate reached about 0.18 kg/h for a single nose ending. Examination of treated graphite showed a mixture of undamaged TRISO particles, particles that have lost the outer pyrolytic carbon layer and ZrO2 kernels. Secondly, we studied the thermal shock method by immerging successively graphite into liquid nitrogen and hot water to cause fracturing of the compact. This produced particles and graphite fragments with diameter ranging from several centimetres to less than 500 µm. This relatively simple and economic method may potentially be considered as a pre-treatment step and be coupled with other method(s) before reprocessing and recycling for example

Study of the Interaction of Ni2+^{2+} and Cs+^+ on MX-80 Bentonite; Effect of Compaction Using the "Capillary Method"

The goal of the paper is to assess the applicability of sorption models to describe the retention of contaminants on clay materials, both in dispersed and compacted states. A batch method is used to characterize the sorption equilibria between Cs, Ni, and MX-80 bentonite for solid-to-liquid ratios varying from 0.5 to 4200 kg/m3. For compacted bentonite (dry density of 1100 kg/m3), a new method is presented where the material compaction is performed in PEEK capillaries. Sorption edges and isotherms were measured in the presence of a synthetic groundwater. A model considering cation exchange reactions with interlayer cations and surface complexation reactions with edge sites was used for the dispersed state. Montmorillonite was shown to be the dominant interacting phase in MX-80 bentonite. The applicability of the model to compacted bentonite was tested. The results indicate that under conditions where the cation exchange mechanism is dominant, there is no difference between the dispersed and compacted states. For the degree of compaction studied, all exchange sites are available for sorption. For Ni, when surface complexation is the dominant sorption mechanism, a decrease of the Kd values by a factor of about 3 was observed (pH 7-8, trace concentrations). This could be explained quantitatively by a diminution of the conditional interaction constant between Ni and the edge surface site in the compacted state. One consequence of this decrease is that the contribution of the organic matter content of MX-80 bentonite to the total sorption becomes significant

Surface site density, silicic acid retention and transport properties of compacted magnetite powder

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Modeling the complexation properties of mineral-bound organic polyelectrolyte: An attempt at comprehension using the model system alumina/polyacrylic acid/M (M = Eu, Cm, Gd)

International audienceThis paper contributes to the comprehension of kinetic and equilibrium phenomena governing metal ion sorption on organic-matter-coated mineral particles. Sorption and desorption experiments were carried out with Eu ion and polyacrylic acid (PAA)-coated alumina colloids at pH 5 in 0.1 M NaClO4 as a function of the metal ion loading. Under these conditions, M interaction with the solid is governed by sorbed PAA (PAAads). The results were compared with spectroscopic data obtained by time-resolved laser-induced fluorescence spectroscopy (TRLFS) with Cm and Gd. The interaction between M and PAAads was characterized by a kinetically controlled process: after rapid metal adsorption within less than 1 min, the speciation of complexed M changed at the particle surface till an equilibrium was reached after about 4 days. At equilibrium, one part of complexed M was shown to be not exchangeable. This process was strongly dependent on the ligand-to-metal ratio. Two models were tested to explain the data. In model 1, the kinetically controlled process was described through successive kinetically controlled reactions that follow the rapid metal ion adsorption. In model 2, the organic layer was considered as a porous medium: the kinetic process was explained by the diffusion of M from the surface into the organic layer. Model 1 allowed a very good description of equilibrium and kinetic experimental data. Model 2 could describe the data at equilibrium but could not explain the kinetic data accurately. In spite of this disagreement, model 2 appeared more realistic considering the results of the TRLFS measurements