Pierre Toulhoat

Europium speciation by time-resolved laser-induced fluorescence

Abstract Time-resolved laser-induced fluorescence has been used to investigate Eu complexes formed with a few main ligands encountered in natural waters: hydroxide, carbonate and humic substances. By varying pH and concentrations of ligands at fixed europium concentration and ionic strength, it was possible, together with free europium Eu3+, to identify spectrally and temporally carbonate complexes, namely Eu(CO3)+, Eu(CO3)2− and Eu(CO3)33− and humate complexes EuHA. For hydroxide complexes, no differences were found in terms of fluorescence spectra and lifetimes. A spectral interpretation is described by using deconvolution for all systems.

Regional fluid characterisation and modelling of water–rock equilibria in the Boom clay Formation and in the Rupelian aquifer at Mol, Belgium

Abstract Interstitial water from the Boom clay Formation around the HADES (high activity disposal experimental site, an underground research facility belonging to the Nuclear research centre, SCK·CEN, at Mol-Dessel, Belgium.) underground research facility at Mol, Belgium, and waters sampled at different locations in the Rupelian aquifer, underlying the clay formation, have been studied. The Boom clay is a Tertiary mudrock containing about 55 % clay and the Rupelian aquifer is located in a silty layer. Special care was taken to adapt or improve measurement, sampling, conservation and analysis methods to get a reliable regional and local database, for the purpose of testing a general model describing the regulation and acquisition of the composition of these fluids. The isotopic and chemical composition of the waters allows them to be assigned to a common origin, namely mixing between a marine endmember and the clay interstitial water, followed by reequilibration with the host rock through dissolution–precipitation reactions involving identified secondary minerals. Oxidation–reduction state and trace element behavior are also discussed and the limits of the model are outlined.

INFLUENCE OF ORGANIC DEGRADATION PRODUCTS ON THE SOLUBILISATION OF RADIONUCLIDES IN INTERMEDIATE AND LOW LEVEL RADIOACTIVE WASTES

In highly alkaline cement pore waters, the degradation of cellulose and similar organic substances generates a large range of compounds. Some of them have strong complexing properties and might thus affect the confinement of radionuclides. We have identified and quantified the production of these degradation products with temperature and time. We have used conventional Potentiometrie titrations, HPLC, GC-MS and Capillary Electrophoresis. Among these products, we confirm the importance of isosaccharinic acid (ISA), but many other hydroxyacids, and possibly diketones could also play a role in the complexation of metals. We have experimentally studied the complexation properties of a chemical analogue of ISA, the tetrahydroxypentanoic acid (ΤΗΡΑ) towards transition elements (Cu, Co) and rare earth elements (Sm and Eu), in order to have a basis to derive the behaviour of some long-lived fission products and actinides. We determined the stoichiometrics of the complexes and the stability constants at zero ionic strength. In parallel, we have conducted solubility experiments on real cellulose degradation solutions. The solubility of transition metal cations increases by a factor of 10 to 10, and 10 to 10 for rare earth elements. These results impose a thorough evaluation of possible failure of the confinement in cement-based waste forms.

Effect of aqueous acetic, oxalic and carbonic acids on the adsorption of americium onto α-alumina

Summary The prediction of the migration for radionuclides in geologic media requires a quantitative knowledge of retardation phenomena. For this purpose, the sorption of Am(III) onto a model mineral — α-alumina — is studied here, including the effects of groundwater chemistry: pH and concentrations of small organic ligands (acetate, oxalate and carbonate anions). This work presents some experimental evidences for the synergic sorption mechanism of americium–ligand cationic complexes onto the alumina. As its anionic complexes were not sorbed, Am(III) cations were desorbed as a result of the formation of anionic complexes in the aqueous phase. By using the ion-exchange theory, and a corresponding restricted set of parameters — exchange capacities and thermodynamic equilibrium constants — the whole set of sorption experiments of Am(III) cationic species onto the α-alumina was modelled in various chemical conditions.

Applications of NAA, PIXE and XPS for the quantification and characterization of the humic substances/iodine association

This paper illustrates the coupling of Neutron Activation Analysis (NAA), Proton Induced X-ray Emission (PIXE) and X-ray Photoelectron Spectroscopy (XPS) to quantify iodine contents in natural humic substances from different geochemical origins and to determine its chemical environment. Non negligible contents of iodine from some hundreds of μg/g to more than one weight per cent have been quantified in the humic substances, and iodine seems to be attached to the organic matter by covalent bonds.

Preliminary studies of groundwater flow and migration of uranium isotopes around the Oklo natural reactors (Gabon)

In specific zones of the Oklo uranium deposit, criticality was naturally reached 2 Ga ago. This site thus provides a unique opportunity to show whether the stable nuclear reaction end-products have remained or not in the vicinity of the reactor zones after the termination of nuclear reactions. In addition, the evaluation of the stability of the uraninite matrix over very long periods of time provides information on the possible long-term stability of waste forms such as spent fuel. The Commission of the European Communities initiated in 1991 the Oklo Natural Analogue Programme, a part of which is devoted to present-day migration studies. The Swedish Nuclear Fuel and Waste Management Company (SKB) supports this programme, with special interest in the Bangombe reactor, a shallow reaction zone possibly affected by surficial alteration processes. The Oklo study comprises hydrogeology, groundwater chemistry, isotopic analyses (environmental isotopes, U series, 235 U/ 238 U), and modelling. Two sites are being thoroughly investigated : the less perturbed OK84 reactor zone in Okelobondo (200 m south from Oklo) and the Bangombe reactor zone, 30 km south of Oklo. We focus our study on uranium migration from these reactor zones, using tracers such as the 235 U/ 238 U isotope ratio. After preliminary field campaigns, a conceptual model was constructed, both for Okelobondo and Bangombe. For this purpose, groundwaters have been characterised for three years in different areas around Oklo : Okelobondo groundwaters in mines and boreholes and surface waters, and Bangombe, both in boreholes and surface waters. Detailed investigations were then conducted in order to validate our conceptual models, and finally to enabling us to model U migration from the reaction zones, and to evaluate the performance assessment of deep geological disposal of radioactive wastes. After the presentation of regional and local geology and hydrogeology, we give a complete description and interpretation of groundwater chemical evolution both in Okelobondo and Bangombe. It concerns major-element behaviour, redox control and uranium behaviour. Preliminary conclusions for the conceptual model drawn from hydrogeological studies and from groundwater chemistry agree closely. In addition, we have successfully tested the use of 235 U/ 238 U isotope ratios in groundwaters to evaluate the migration of uranium from reactor zones.

On the Use of Spectroscopic Techniques for Interaction Studies, Part I: Complexation between Europium and Small Organic Ligands

In the framework of environmental studies, it is important to understand the interaction of humic substances with cations (heavy metals, radionuclides) and to determine their complexation constants in order to evaluate their potential impact on their fate. For this purpose, two techniques have been used: electrospray ionization mass spectrometry, a newly used technique in speciation studies, and time-resolved laser-induced fluorescence spectrometry, a well-known technique for such studies. As a first step, for simplification purposes and to compare both techniques, simple molecules having functional groups present in humic substances have been selected, such as acetic, glycolic, and 4-hydroxyphenylacetic acids. Both techniques have been used to obtain stoichiometries and complexation constants between these simple molecules and europium (III).

Trace-metal behaviour in natural granitic waters

Alkaline thermal waters were sampled in granitic areas of southern Europe. In this study attention is focused on the behaviour of trace elements likely to be present in a radioactive waste disposal. We demonstrate that, in this type of waters: 1. (1) most transition metals are limited to very low concentrations probably by sulfide minerals (CoS, MoS3, ZnS, Sb2S3); 2. (2) uranium is limited in solution by uraninite; 3. (3) for tri- and tetravalent metals such as Zr, Hf, etc., and rare-earth elements (REE), we observe that they are well correlated with Th, with a slope of 1 in log-log diagrams. These elements are associated with particles suspended in solution. The different Th to REE or Th to Zr, Hf, etc., ratios are constant in solution and in the particulate fraction.

Modified acid-base behaviour of resin-bound pH indicators

Abstract A fibre-optic device was used to characterize the acid-base properties of pH indicators bound to polymeric resins either by π-π or ionic interactions. The ability of an indicator to give up a proton was found to be significantly modified by the nature of its binding to the resin. When involved in hydrophobic interactions, the increase in its apparent p K i value accounts for the electron-donating role of the resin. In contrast, when ionically bound to the resin, it displays a higher acidity; in the instance, the resin plays the role of an electron-withdrawing entity. Mixed influences have also been observed.

Long term evolution of spent nuclear fuel in long term storage or geological disposal. New findings from the French PRECCI R&D program and implications for the definition of the RN source term in geological repository

This paper aims to give a brief overview of the wide research undertaken in France in order to elucidate the potential long term evolution of spent nuclear fuels in long term storage or geological disposal. Scientific key issues related to the potential long term evolution in closed system, in presence of and oxidative phase and in presence of water are presented as well as the anticipated trends. A particular emphasis is put on the major outcomes of this research which is a new definition of the radionuclides source term for the geological disposal: we estimate that we have now to allocate a higher fraction of the radionuclides inventory to the so-called instant release fraction which is instantaneously released in presence of water.