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

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

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

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

Comportement des déchets graphite en situation de stockage (Relâchement et répartition des espèces organiques et inogarniques du carbone 14 et du tritium en milieu alcalin)

23000 tonnes de déchets graphites seront générés lors du démantèlement de la première filière de réacteurs en France (9 réacteurs Uranium Naturel Graphite Gas, UNGG). Ces déchets radioactifs sont classés dans la catégorie Faible Activité Vie Longue (FAVL). Dans le cadre de la loi, l agence nationale pour la gestion des déchets radioactifs (Andra) étudie un concept de stockage à faible profondeur. Cette étude s intéresse plus particulièrement au carbone 14, qui est un des principaux radionucléides à vie longue (5730 ans) dans les déchets graphite, mais aussi au tritium qui est l un des principaux contributeurs de la radioactivité à court terme. Ces deux radionucléides ont la particularité d exister sous différentes formes, aussi bien en phase gaz (14CO2, HT, ) qu en phase liquide (14CO32-, HTO, ). Leur spéciation va influencer leur migration du stockage vers l environnement. Des expériences de lixiviation en milieu alcalin (NaOH 0,1mol.L-1, simulant les conditions de stockage), ont été réalisées sur des échantillons de graphites irradiés provenant de deux réacteurs : SLA2 et G2, afin de quantifier leur relâchement et de définir leur spéciation. Les études montrent que le carbone se trouve aussi bien en phase gaz qu en phase liquide. Dans la phase gaz, le relâchement est faible (< 0,1%), et correspond à des formes oxydables. Le carbone 14 est relâché majoritairement en phase liquide : 65% de la fraction d inventaire relâchée est sous forme de carbone 14 inorganique, et 35% de carbone 14 organique. Deux formes de tritium ont été identifiées dans la phase gaz : HTO et HT/Tritium Organiquement Lié. Plus de 90% du tritium en phase gaz se trouve sous forme HT/TOL, mais ce relâchement est faible (<0,1%). Majoritairement le tritium est en phase liquide sous forme HTO.23000 tons of graphite wastes will be generated during dismantling of the first generation of French reactors (9 gas cooled reactors). These wastes are classified as Long Lived Low Level wastes (LLW-LL). As requested by the law, the French National Radioactive Waste Management Agency (Andra) is studying concepts of low-depth disposals.In this work we focus on carbon 14, the main long-lived radionuclide in graphite waste (5730y), but also on tritium, which is the main contributor to the radioactivity in the short term. Carbon 14 and tritium may be released from graphite waste in many forms in gaseous phase (14CO2, HT ) or in solution (14CO32-, HTO ). Their speciation will strongly affect their migration from the disposal site to the environment. Leaching experiments, in alkaline solution (0.1 M NaOH simulating repository conditions) have been performed on irradiated graphite, from Saint-Laurent A2 and G2 reactors, in order to quantify their release and characterize their speciation. The studies show that carbon 14 exists in both gaseous and aqueous phases. In the gaseous phase, release is weak (<0.1%) and corresponds to oxidizable species. Carbon 14 is mainly released into liquid phase, as both inorganic and organic species. 65% of released fraction is inorganic and 35% organic carbon. Two tritiated species have been identified in gaseous phase: HTO and HT/Organically Bond Tritium. More than 90% of tritium in that phase corresponds to HT/OBT. But release is weak (<0.1%). HTO is mainly in the liquid phase.NANTES-ENS Mines (441092314) / SudocSudocFranceF

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

International audienc

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

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

Porosities accessible to HTO and iodide on water-saturated compacted clay materials and relation with the forms of water: A low field proton NMR study

International audienceThe aim of the present work was to quantify accessible porosities for iodide and for a water tracer (HTO) on water-saturated compacted clay samples (illite, montmorillonite and MX-80 bentonite) and to relate these macroscopic values to the forms of water in these porosities (surface/bulk water, external/internal water). Low field proton NMR was used to characterize and quantify the forms of water. This enabled the three different populations (structural OH, external surface and internal surface water) to be differentiated on hydrated clays by considering the difference in proton mobility. An accurate description of the water forms within the different populations did not appear possible when water molecules of these populations were in contact because of the occurrence of rapid exchange reactions. For this reason, it was not possible to use the low resolution NMR method to quantify external surface and bulk water in fully water-saturated compacted clay media at room temperature. This latter information could however be estimated when analyzing the samples at -25°C. At this temperature, a distinction based on the difference in mobility could be made since surface water remained in a semi-liquid state whereas bulk water froze. In parallel, accessible porosities for anions and HTO were determined by an isotopic dilution method using capillaries to confine the materials. HTO was shown to probe the whole pore volume (i.e. the space made of surface and bulk water). When the surface water volume was mainly composed of interlayer water (case of montmorillonite and bentonite), iodide was shown to be located in the pore space made of bulk water. When the interlayer water was not present (case of illite), the results showed that iodide could access a small fraction of the surface water volume localized at the external surface of the clay particles