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Alkanes And PE Radiolysis

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Aging, radiolysis and radio-oxidation of polymers is of tremendous importance all along the nuclear fuel cycle. Polymer irradiation is a difficult topic because of the multiscale nature of the material. It is a well establish topic at CIMAP, where the MADIR team has an important experimental research activity. An attempt has been made to organize the research activity at the national level with the COPPOLA CPR. Unfortunately this research program did not really succeed, mainly because the teams joining the project limited themselves to the use of phenomenological models based on homogeneous kinetics rather than understanding the basic underlying mechanisms, which are likely to be more transferable.
The experience of SIMUL team in heterogeneous simulation combined with its competence in quantum chemistry creates a favorable framework to develop a new simulation research theme in this field. We propose to investigate the reaction pathways for several specific reactions involving alkyl groups, and then to simulate the heterogeneous kinetics following radiation interaction with a medium made of long alkane chains in the spirit of our previous work in water radiolysis. This work will be performed in collaboration with our colleagues from MADIR and some researchers of CEA-DEN.
At the microscopic level, we will investigate the significance of migration along alkane chains and from one chain to another one. This will be done by a multiscale approach combining accurate quantum chemistry calculation of the active part of the chain surrounding the radical, with a much faster, though more rudimentary, approach of the global motion of the chains by means of tight-binding force field method.
At the mesoscopic level, we will simulate the kinetics of radical reactions for several chain lengths in solid or liquid phases. The chemical kinetics is partially controlled by the radical diffusion which can be quite complex in a medium made of long chains. The necessary inputs for a simulation are the effective diffusion constant of the reactants, either radical or molecular species like O2, and the bimolecular reaction rates. Some of them can be obtain from the literature, but some others are missing. The aforementioned microscopic studies will help us to supply reasonable values for these constants. We shall investigate in particular the significance of the heterogeneous character of the kinetics, which has been so far ignored in previous simulations.