Reproducibility of the uptake of U(VI) onto degraded cement pastes and calcium silicate hydrate phases

http://www.radiochimacta.deInternational audienceThe U(VI) uptake in degraded cement pastes was undertaken in the laboratories of CEA/L3MR and SUBATECH in order to check the reproducibility of the study. Two well hydrated cement pastes, CEM I (Ordinary Portland Cement, OPC) and CEM V (blast furnace slag (BFS) and fly ash added to OPC) were degraded using similar protocols. Equilibrium solutions and solid materials were characterised for three degradation states for each paste. All samples are free of portlandite and the pH of the equilibrated cement solutions vary in the range 9.8–12.2. Three calcium silicate hydrate phases (C-S-H) were synthesised in order to compare the sorption properties of degraded cement pastes and of hydrate phases in similar pH conditions. In order to avoid precipitation processes, the operational solubility limit was evaluated before batch experiments. These solubility values vary significantly in the pH range [9–13] with a 2.4×10−7 mol/L minimum at pH close to 10.5. In batch sorption experiments, the distribution ratio Rd values are high: 30000–150000 mL/g. The uptake of U(VI) increases when comparing the least and the most degraded cement pastes whereas the initial composition of cement has relatively insensitive effect. Sorption isotherms, expressed as a log [U(VI)solid]/ log [U(VI)solution] plots are linear. A slope of 1 is calculated indicating the predominance of sorption processes. As sorption and desorption values are close, the uptake mechanism seems reversible. The Rd values measured in C-S-H suspensions are in good agreement with Rd values of degraded cement pastes, and C-S-H materials could be one of the cementitious phases which control U(VI) uptake in cement pastes

CORI Task 4 : Radionuclide-Organic-Cement Interactions -CNRS/Subatech contribution

International audienc

Extraction du molybdene hexavalent de solutions aqueuses acides par le di-n-butyl 2,2 N-hydroxyhexanamide(acide tri-n-butylacetohydroxamique)

SIGLEINIST T 74808 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Uranium Lability in soils downstream a former mine (France) using a coupled DGT/DET approach

International audienc

Interactions et transfert de macromolécules organiques et de colloïdes dans l'argile nanoporeuse du Callovo-Oxfordien

La roche argileuse du Callovo-Oxfordien, peu perméable, est envisagée comme roche hôte d un stockage de longue durée des déchets nucléaires. Les radionucléides présentent une mobilité très faible au sein de cette roche mais leur complexation avec des espèces colloïdales ou des macromolécules organiques, naturellement présentes dans l eau de pore ou induites par le stockage, pourraient être un vecteur de leur transport. Afin de comprendre dans quelles mesures ces espèces colloïdales et organiques sont mobiles dans l argilite, si elles interagissent chimiquement et/ou physiquement (filtration) avec la roche, des expériences d adsorption en milieu dispersé et de transport en roche intacte (expériences de percolation), ont été menées sur des colloïdes de Se(0) et des acides organiques : acide succinique (118Da), acide citrique (192Da) et acide polymaléique (1900Da). Les études d adsorption sur les colloïdes de Se(0) mettent en évidence la rétention de cette espèce sur la roche, ses propriétés étant écrantées par la présence nécessaire de protéine pour stabiliser la suspension. Les études d adsorption sur les acides organiques montrent une rétention de ces espèces (entre 2,2.10-3 et 4,20.10-2 mol/kg de roche), fortement irréversible et dépendante de la teneur en solution en ion Ca2+ et des propriétés de complexation des acides organiques avec cet ion. Les expériences de percolation sur les acides succinique et polymaléique mettent en évidence la mobilité de ces espèces au sein de la roche intacte. Ces molécules sont soumises à des interactions similaires à celles identifiées en milieu dispersé et ne subissent aucune filtration par le réseau poral de la rocheLow permeability Callovo-Oxfordian clay rock is under consideration as a possible host rock for long-term disposal of radioactive waste in a deep geological repository. Radionuclides move slowly in the Callovo-Oxfordian rock but complexation with colloidal species or organic macromolecules, naturally present in the pore solution or generated by storage, may enhance their migration. These colloidal and organic species can be retained by the clay through chemical interactions or physical immobilization (filtration). To increase the understanding of their mobility, batch and transport experiments (percolation) were performed on crushed and whole rock samples using Se(0) colloids and the following organic acids: succinic acid (118Da), citric acid (192Da) and polymaleic acid (1900Da). Adsorption studies on Se(0) colloids show the retention of this species on the clay. However, the properties of these colloids are masked by the presence of the protein required for the stabilization of the suspension. Adsorption studies on organic acids highlight the retention of these species (between 2.2.10-3 and 4.20.10-2 mol/kg of rock) which is strongly irreversible and which depends on the Ca2+ ion concentrations in solution and on the capacity of organic acids to complex with this ion. Percolation experiments performed on succinic and polymaleic acids show that despite their size, these molecules are mobile in the pore water of the compact rock. Filtration was not observed. Interactions rock/organic molecule were similar to those identified on a crushed rockNANTES-BU Sciences (441092104) / SudocSudocFranceF

Etat de la réglementation et des pratiques sur les contrôles radiologiques des déchets ménagers dans différents états membres de la communauté européenne

Programme RECORD 02-0127/1A, Rapport fina

Détermination par expériences de percolation du seuil de filtration de l'argilite du Callovo-Oxfordien

International audienc

Nuclear waste disposal: I. Laboratory simulation of repository properties

After more than 30 years of research and development, there is a broad technical consensus that geologic disposal will provide for the safety of humankind, now and far into the future. Safety analyses have demonstrated that the risk, measured as the exposure to radiation, will be of little consequence. Still, there is not yet an operating geologic repository for highly radioactive waste, and there remains substantial public concern about the long-term safety of geologic disposal. In the two linked papers we argue for a stronger connection between the scientific data (this paper I) and the safety analysis, particularly in the context of societal expectations (paper II). In the present paper I, we use new experimental data on the properties of clay formations simulating geological disposal conditions to illustrate how one can understand the ability of clay to isolate radionuclides. The data include percolation tests on various intact clay–rock cores with different calcite contents. For the first time, hydrodynamic parameters (anion and cation accessible porosities, permeability, dispersion and diffusion coefficients), as well as retention parameters (sorption behavior of iodine, cesium) and materials interaction parameters (glass dissolution rates, etc.) have been obtained for a series of clay–rock samples of varying mineralogy. Increased calcite content leads to lower permeability and porosity, but the difference between anion and cation accessible porosity diminished. The data confirm very slow radionuclide migration, and a direct extrapolation to repository geometry yields isolation times, for a 70 m clay–rock formation, of many hundreds of thousands of years, even for the most mobile radionuclides such as iodine-129 and chlorine 36 and complete retention for the more radiotoxic, less mobile radionuclides such as the actinides or cesium-137.In order to assess the meaning of the technical results and derived models for long-term safety, paper II addresses model validity and credibility not only from a technical perspective, but in a much broader historical, epistemological and societal context. Safety analysis is treated in its social and temporal dimensions. This approach provides new insights into the societal dimension of scenarios and risk, and it shows that there is certainly no direct link between increased scientific understanding and a public position for or against different strategies of nuclear waste disposal

Leaching behaviour of unirradiated high temperature reactor (HTR) UO2_2–ThO2_2 mixed oxides fuel particles

The dissolution of different mixed oxide (U, Th)O$_2$ particles under reducing conditions has been studied using a continuous flow-through reactor. The U/Th ratio seems to have no or little influence on the normalised leaching rate of thorium or uranium, The release rate of uranium from the outer surface of a Th rich matrix seems to follow the behaviour of pure UO2 even though U is a minor component in these phases and the dissolution rate of Th is much lower. After long time U concentrations will become depleted at the solids surface and it will be expected that U release rates will become controlled by the release rates of thorium (rates at neutral pH < 10$^{−6}$ g m^{−2}$d$^{−1}$). Under reducing conditions, the matrix of HTR fuel particles presents significant intrinsic radionuclide confinement properties