Saturday, 15 July 2006

Effects of a Climate Cooling on the Behavior of Radionuclides in Soils.

Sophie Maillant1, Guillaume Echevarria1, Elisabeth Leclerc-Cessac2, and Jean-Louis Morel3. (1) Laboratoire Sols et Environnement UMR 1120 INPL-INRA, 2 av de la Foret de Haye, Vandoeuvre-lès-Nancy,, 54500, France, (2) Andra, Direction Scientifique, Service Transferts, 1-7 rue Jean Monnet, Châtenay-Malabry, 92298, France, (3) INPL(ENSAIA)/INRA, Laboratoire Sols et Environnement, BP 172, Vandoeuvre-les-Nancy cedex, 54505, France

For the safety assessment of an underground radioactive waste disposal conducted by Andra, it is required to estimate the behavior of radionuclides in soils for the next five hundred thousand years (Andra, 2005). Within this time scale, soils would probably evolve from their actual status, as the Earth climate would turn from interglacial into glacial. It is then necessary to assess the effects of these changes on the behaviour of the radiochemicals that would enter the soils. Here, we present a method that allows the prevision of the pedological changes and their consequences on the behaviour of radiochemicals in soils. This method was applied to a limestone plateau (later refered to as the “reference region” in the text) which would be under periglacial conditions during a glaciation, according to climatic modelling (Texier, 2003). We have focused the study on the retention of mobile radionuclides: iodine (I) and technetium (Tc). The method relies on the study of analogous soils, sampled in regions similar to the reference region regarding the geochemical and topographical contexts but undergoing a cold climate, which mimics the future periglacial conditions. The characteristics of these analogous soils are interpreted to predict the future soil cover of the study area. For the predicted soils we can estimate the retention and the mobility of the radionuclides through the study of their sorption onto the sampled analogous soils. The analogous soils sampled have developed under boreal forest and cold steppe conditions from limestone or calcareous parent materials. The observed soils were eutric Leptosols under boreal forest, and Chernozems under cold steppe. In both biomes, under hydromorphic conditions, OM accumulated in the soil. Evolution scenarios constructed from these results indicated that the periglacial conditions would probably favor the persistence of soils fairly similar to the present ones in the reference region, especially during the boreal stage. However these soils would display higher OM content than the actual soils. In the steppe-tundra biome, a pergelisol would favor hydromorphy in most soils (stagnic Cryosols). The consequences of these pedological changes on the behavior of mobile radionuclides were studied through the retention of Tc and I in the analogous soils at field capacity in batch experiments. The experiment showed that the dehydration of soils could favor the retention of both elements under aerobic conditions, although the latter are not usually favourable to Tc and I retention (Kd > 1 in analogous soils of boreal forest after one year of contact time). These results were used to establish maps of the retention of each element for both periglacial biomes. It is suggested that Tc would be retained in the soils in the cold steppe soils whereas I would be more retained in soils of the boreal biome. In both biomes, soils with the highest retention towards I and Tc would be peaty soils located in the bottom of marly valleys. The proposed method underlain by analogy and laboratory retention studies should help to understand and forecast the behavior of the long-lived radionuclides. It should help in the determination of the site specific parameters for the dose computation of safety assessment in a cold biosphere scenario.

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