Uranium facilitated transport by water-dispersible colloids in field and soil columns

International audienceThe transport of uranium through a sandy podzolic soil has been investigated in the field and in column experiments. Field monitoring, numerous years after surface contamination by depleted uranium deposits, revealed a 20 cm deep uranium migration in soil. Uranium retention in soil is controlled by the < 50 µm mixed humic and clayey coatings in the first 40 cm i.e. in the E horizon. Column experiments of uranium transport under various conditions were run using isotopic spiking. After 100 pore volumes elution, 60% of the total input uranium is retained in the first 2 cm of the column. Retardation factor of uranium on E horizon material ranges from 1300 (column) to 3000 (batch). In parallel to this slow uranium migration, we experimentally observed a fast elution related to humic colloids of about 1-5% of the total-uranium input, transferred at the mean porewater velocity through the soil column. In order to understand the effect of rain events, ionic strength of the input solution was sharply changed. Humic colloids are retarded when ionic strength increases, while a major mobilization of humic colloids and colloid-borne uranium occurs as ionic strength decreases. Isotopic spiking shows that both 238U initially present in the soil column and 233U brought by input solution are desorbed. The mobilization process observed experimentally after a drop of ionic strength may account for a rapid uranium migration in the field after a rainfall event, and for the significant uranium concentrations found in deep soil horizons and in groundwater, 1 km downstream from the pollution source

Mesure, par une méthode d'émanation, des rendements des gaz rares dans la fission de 238U et 232Th par neutrons de 14 MeV

A direct method, using emanation of rare gases by uranyle stearate and thorium stearate, has been applied to the measurement of cumulative fractional yields of certain isotopes of krypton and xenon, in the fissions of 238U and 232Th by 14 MeV-neutrons. The independent yields of the same isotopes were measured previously by means of isotopic on-line separation. Combining these results we have calculated the widths of the mass and charge distributions, the relative chain yields, the fractional cumulative yields of certain bromine and iodine isotopes, the values of Z p, the most probable charge, in the isobaric chains 87-93 and 137-142, and the elemental yields of krypton and xenon.On a mesuré, par une méthode directe utilisant l'émanation des gaz rares à partir des stéarates d'uranyle et de thorium, les rendements cumulatifs fractionnels de divers isotopes du krypton et du xénon, dans les fissions de 238U et 232Th par neutrons de 14 MeV. En couplant ces résultats à des mesures antérieures, par séparation isotopique en ligne, des rendements indépendants des mêmes isotopes, on a calculé les largeurs des distributions en masse et en charge, les valeurs relatives des rendements de chaîne, les rendements cumulatifs fractionnels de divers isotopes du brome et de l'iode, les valeurs de la charge la plus probable dans les chaînes isobariques 87 à 93 et 137 à 142, et les rendements élémentaires du krypton et du xénon

Experimental studies of the formation and decay of hot nuclei

What is the contribution of Computer Supported Collaborative Learning (CSCL) technologies to the tutorial activity ? Do they lead to a significant change in the relationships between tutor and students ? between tutor and professor ? Do they contribute to renew the pedagogy ? This paper is based on an analysis of an action learning program on project management at the national school of information and library sciences (enssib). The first results of this on-going research give answers to some of these questions through the study of a corpus of interactions on 3 forums, and the representations given by the tutors of their own use of the variety of way they would communicate with the students or with their peer groups