Assimilation of heterogeneous data in simulations of atmospheric dispersion of radionuclides at regional scale
Modeling and simulation provide essential knowledge on the aerial dispersion of gases and particles and the resulting environmental marking. This applies in particular to the releases that were generated by atmospheric nuclear tests carried out in the past by France in Polynesia. While regional-scale meteorological and dispersion calculations are reasonably reliable, their results have a degree of uncertainty and present discrepancies with heterogeneous measurements of activities or dose rates in the air, on the ground and in biological compartments. The thesis will aim to develop inversion methods, based on data assimilation, in order to reduce errors and uncertainties in simulations of regional dispersion of radionuclides. The application will concern certain nuclear tests in the atmosphere. However, the methods developed during the thesis, such as Monte Carlo sampling by Markov chains, will have a more general field of implementation. After a literature review on nuclear testing and data assimilation methods, original inverse modeling algorithms will be programmed, tested, and applied to the simulation of the dispersion of aerial releases from tests. This will allow us to estimate the anticipated important role of measurement assimilation in improving simulations.
Alteration mechanisms study of MOX spent fuel in the presence of cimentious bentonitic material (MREA). Experimental and modeling approaches
In France, the reference way remains the reprocessing of spent fuel and the recovery of certain materials such as uranium and plutonium through the elaboration of MOX fuels and its recycling. However, the direct storage of fuels (UOX and MOX) in deep geological repository is also being studied in order to ensure that French storage concepts (Cigéo) are suitable for spent fuels as requested and included in the National Plan for the Management of Radioactive Materials and Waste (PNGMDR). Therefore, it is essential to study the alteration mechanisms of the spent fuel matrices in the presence of environmental materials that are similar, on a laboratory scale, to the current storage concept of radioactive waste in deep geological disposal: HA cells dug in the Callovo-Oxfordian (COx) clay whose low-alloy steel liner is isolated from the clay by a cimentious bentonitic grout called MREA. There is various objectives : on the one hand, to determine the impact of the environment on the alteration mechanisms of the fuel matrix as well as on the radionuclides release, and on the other hand, to develop a geochemical model to account for the main physicochemical processes involved. These studies are carried out at the ATALANTE facility (DHA) of the CEA Marcoule, where leaching experiments and characterizations of MOX fuels are achievable. This work is performed as part of the COSTO project and is supported by Andra and EDF.