[en] ¹⁸O/¹⁶O and ¹³C/¹²C ratios measurements in foraminiferal calcite are applied to the paleoclimatology of the North Atlantic and to the reconstruction of deep water exchanges between the Atlantic and Pacific Oceans, from middle miocene time (15 m.y. ago) to the present, on samples from 2 DSDP wells. Chapters 1 to 4 describe the structural frame and hydrological setting of these sites, and the stratigraphy of the deposits. A .4 m.y. lag between the initiation of the first boreal ice-caps and their extension to northern Europe is explained by the persistency of the North-Atlantic Drift. In chapters 5 to 8, the ¹³C/¹²C ratio of dissolved mineral carbon is used as a tracer of the residence time of the deep waters, the indications of which are preserved in benthonic foraminiferal calcite. It is shown that present-day type thermo-haline circulation was initiated 13.2 m.y. ago in the northern Atlantic, when the volcanic Scotland-Iceland-Greenland ridge subsided; that the closure of the Mediterranean sea during the Messinian (6.2 to 5 m.y. ago) caused this circulation to stop, and that the present circulation started again when the Mediterranean re-opened, at the beginning of the Pliocene

[en] Suspensions of Neurospora crassa conidia were inactivated by blacklight (BL) radiation (300 to 425 nm) in the absence of exogenous photosensitizing compounds. Carotenoid-containing wild-type conidia were less sensitive to BL radiation than albino conidia, showing a dose enhancement factor (DEF) of 1.2 for dose levels resulting in less than 10 percent survival. The same strains were about equally sensitive to shortwave ultraviolet (uv) inactivation. The kinetics of BL inactivation are similar to those of photodynamic inactivation by visible light in the presence of a photosensitizing dye (methylene blue). Only limited inactivation by visible light in the absence of exogenous photosensitizers was observed. BL and UV inactivations are probably caused by different mechanisms since wild-type conidia are only slightly more resistant to BL radiation (DEF = 1.2 at 1.0 percent survival) than are conidia from a uv-sensitive strain (upr-1, uvs-3). The BL-induced lethal lesions are probably not cyclobutyl pyrimidine dimers since BL-inactivated Haemophilus influenzae transforming deoxyribonucleic acid is not photoreactivated by N. crassa wild-type enzyme extracts, whereas uv-inactivated transforming deoxyribonucleic acid is photoreactivable with this treatment

[en] Empty shells of spinose planktonic foraminifera on the seabed are significantly enriched in oxygen-18 as compared with the shells of their living counterparts in surface waters. This enrichment is due to gametogenic calcification, which extracts calcium carbonate from deeper and colder waters as the shell sinks below the euphotic zone

[en] The uranium ore body in Oklo is a unique subject in the world, as natural fission reactions occurred there two billion years ago. It provides opportunities for the study of natural analogy with deep radioactive waste disposal, specially radionuclide mass transfer processes to the surface. The ongoing program is co-funded by the CEC, and it involves several directorates in the CEA, that is to say the IPSN, plus DTA and DCC. Other, non-CEC agencies also take part in the studies, such as SKB (Sweden), AECL and ONTARIO-HYDRO (Canada). It can be subdivided into several different tasks: 1. In situ sampling, in close collaboration with the mining company (C.O.M.U.F., Compagnie des Mines d'Uranium de Franceville, Mounana, Gabon). 2. Study and characterization of the source term (mostly in CEA laboratories). 3. Studies on the geochemical systems ruling the migrations, implying collaboration between CEA laboratories and other institutions: CREGU (Centre de Recherches sur la Geologie des matieres premieres minerales et energetiques, formerly 'de l'Uranium', Nancy), Centre de Geochimie de la surface (CNRS, Strasbourg), and Ecole Nationale Superieure des Mines de Paris (ENSMP, Centre de Geologie Generale et Miniere and Centre d'Informatique Geologique, Fontainebleau). 4. Modelling: Part of the modelling will take place in each laboratory involved, but the final coupling of models will be the responsibility of IPSN and ENSMP. 1 fig

[en] The uranium ore body in Oklo is a unique subject in the world, as natural fission reactions occurred there two billion years ago. It provides opportunities for the study of natural analogy with deep radioactive waste disposal, specially radionuclide mass transfer processes to the surface. The ongoing program is co-funded by the CEC, and it involves several directorates in the CEA, that is to say the IPSN, plus DTA and DCC. Other, non-CEC agencies also take part in the studies, such as SKB (Sweden), AECL and ONTARIO-HYDRO (Canada). It can be subdivided into several different tasks: (1) In situ sampling, in close collaboration with the mining company (C.O.M.U.F., Compagnie des Mines d'Uranium de Franceville, Mounana, Gabon). (2) Study and characterization of the source term (mostly in CEA laboratories). (3) Studies on the geochemical systems ruling the migrations, implying collaboration between CEA laboratories and other institutions: CREGU (Centre de Recherches sur la Geologie des matieres premieres minerales et energetiques, formerly ''de l'Uranium'', Nancy), Centre de Geochimie de la Surface (CNRS, Strasbourg), and Ecole Nationale Superieure des Mines de Paris (ENSMP, Centre de Geologie Generale et Miniere and Centre d'Informatique Geologique, Fontainebleau). (4) Modelling: Part of the modelling will take place in each laboratory involved, but the final coupling of models will be the responsibility of IPSN and ENSMP. (author). 1 fig

[en] The safety analysis of long term radioactive waste disposal projects must take into account the evolution of the sites natural environment. The present paper aims at reassessing some questions relating to the methods and to some lack of knowledge which may appear when we try to forecast such evolutions and their results, and to some solutions that can be considered. We will particularly discuss the advantages and drawbacks of the deterministic approaches and the construction and working out of scenarios. The presentation is illustrated by reference to recent examples. 5 refs., 6 figs

[en] Natural nuclear fission reactors are only known in two uranium deposits in the world, the Oklo and Bangombe deposits of the Franceville basin: Gabon. Since 1982, five new reactor zones have been discovered in these deposits and studied since 1989 in a cooperative European program. New geological, mineralogical, and geochemical studies have been carried out in order to understand the behavior of the actinides and fission products which have been stored in a geological environment for more than 2.0 Ga years. The Franceville basin and the uranium deposits remained geologically stable over a long period of time. Therefore, the sites of Oklo and Bangombe are well preserved. For the reactors, two main periods of actinide and radionuclides migration have been observed: during the criticality, under P-T conditions of 300 bars and 400-500 degrees C, respectively, and during a distention event which affected the Franceville basin 800 to 900 Ma ago and which was responsible for the intrusion of dolerite dikes close to the reactors. New isotopic analyses on uranium dioxides, clays, and phosphates allow us to determine their respective importance for the retention of fission products. The UO₂ matrix appears to be efficient at retaining most actinides and fission products such as REEs, Y, and Zr but not the volatile fission products (Cd, Cs, Xe, and Kr) nor Rb, Sr, and Ba. Some fissiogenic elements such as Mo, Tc, Ru, Rh, Pd, and Te could have formed metallic and oxide inclusion in the UO₂ matrix which are similar to those observed in artificial spent fuel. Clays and phosphate minerals also appear to have played a role in the retention of fissiogenic REEs and also of Pu. 82 refs., 21 figs., 12 tabs

No abstract available

[en] This paper discusses the relationships between the French regulatory safety rule for radioactive waste deep disposal (RFS N. III.2.f), the geological (external geodynamics) data available when the rule was discussed, and the present geoprospective studies and scenarios to be taken into account in the safety assessment of an underground disposal site. Three periods are considered in the study: the initial period (0-500 years AP) during which only water circulations and heat transfers are considered in the waste-rock interaction problem; the permanent regime period (500-10000 years AP) and the subsequent period (10000-50000 years AP) during which changes in landscape morphology due to human activity and climate changes have to be considered. In particular, global climatic oscillations can lead to glaciations with an amplitude equivalent to those known for the Quaternary period and with possible influence on waste disposals integrity. The simulation of an ice-cap development over Europe has been attempted but no convincing model could be obtained so-far. (J.S.). 11 refs., 3 figs

[en] The purpose of this study is to identify and then to model the present transfer of groundwater which might set in motion elements produced in the reaction zone. Consequently, the first stage of this study consisted of a hydrochemical overview of the Oklo-Okelobondo site into which all previous and current information, gathered during missions in September 1990 and June 1991 was integrated. In the second stage of the study, a preliminary hydrodynamic model was built. In a plausible context and based on the collected data, the study area, the drainage of the domain and the aquifer layers were defined and a conceptual hydrogeological flow pattern proposed. The data made it possible to begin the hydrodynamic modelling of current migration basing it on a pattern of groundwater circulation and bearing in mind the morphology of the soil surface and of the geological strata and taking into account the first results of the hydrochemical analyses. (author). 7 refs., 1 tab