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COSI6 : a Tool for Nuclear Transition Scenarios Studies

Abstract : Nuclear systems, composed of reactors with varied fuel and cycle facilities (enrichment plant, fabrication plant, reprocessing plant,) are complex and in constant evolution. Decision makers need to have all the technical elements, based on dynamic nuclear fleet transition scenarios studies. These scenarios are tools to compare different options of nuclear systems evolution, and identify strengths and drawbacks. To have a complete overview of nuclear systems, it is required to follow precisely material flows at each step of the fuel cycle front-end and back-end, and at each date of the operation period. The evolution in time and under flux of materials isotopic composition has also to be taken into account, which can give access to other interesting values (activity, decay heat, toxicity,).Since 1985, CEA has been developing the COSI software, simulating in detail the evolution in time of a nuclear reactors fleet and its associated fuel cycle facilities. It is designed to study different options for the introduction of various nuclear reactor types and the use of the associated nuclear materials.The general principle and the physical models of the current version in 2014 (COSI6 7.0) is described in the paper. The main physical models are the equivalence models in the fabrication plant, which determine the initial composition of mixed fuels in order to provide an equivalent efficiency whatever the isotopic composition of constitutive materials, and the depletion models for irradiated fuels in the core and cooling materials in storage the reference model is CESAR of which several versions are available. CESAR is the reference code at the AREVA NC La Hague reprocessing plant and is used to calculate the isotopic composition of spent fuel by solving Bateman equation, using one-group cross sections libraries coming from neutronic codes (APOLLO2 in thermal spectrum and ERANOS in fast spectrum). An exercise of validation of COSI6 previously carried out on the French PWR historic nuclear fleet until 2010 (and not presented in this paper) allows us to validate the essential phases of the fuel cycle computation and highlights the credibility of the results provided by the code. Otherwise, a methodology of propagation of inputs uncertainties on results has been developed and could be implemented to quantify the uncertainties associated to scenarios results.Finally, a methodology of optimization of scenarios is currently developed. It will consist in a module associated to COSI6, to find the appropriate input parameters defining the best scenarios for a given problem. Indeed COSI6 is a deterministic code in the sense that the scenario is simulated chronologically as function of the input parameters defined by the user, without any decision taken by the code. Nevertheless the user needs more and more often to identify solution scenarios to different problems (e.g. how to minimize natural resources consumption, waste production and toxicity) while respecting industrial constraints (maintaining the energy production, limiting the interim storages, stabilizing the plants capacities,...).
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C. Coquelet-Pascal, M. Tiphine, G. Krivtchik, D. Freynet, R. Girieud, et al.. COSI6 : a Tool for Nuclear Transition Scenarios Studies. GLOBAL 2015, Sep 2015, Paris, France. ⟨cea-02509673⟩

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