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Neutronic, thermohydraulic and thermomechanical coupling for the modeling of criticality accidents in nuclear systems

Abstract : This thesis was developed within in the framework of the multi-scale and multi-physics models for the simulation of criticality accidents, carried jointly between the CNRS and the IRSN. A multi-physics and multi-scale approach aims to produce a numerical model taking into account all the relevant physical phenomena existing in nuclear systems as well as their coupling. This approach makes possible to improve the predictive capacities of the single physics models and to numerically study the behavior of a nuclear system under conditions that would be difficult to achieve or reproduce by experiments. The multi-scale / multi-physics approach is, therefore, particularly useful for the study of nuclear reactor criticality accidents, or more generally, for all nuclear systems where a tight coupling exists between neutronics, mechanics (of solids and fluids) and heat transfers.The objectives of the thesis were, firstly, to develop a new numerical scheme for the coupling between the neutronic code Serpent 2 (Monte Carlo code) and the Computational Fluid Dynamics (CFD) code OpenFOAM. Secondly, to develop the physical models that allow greater flexibility for criticality accidents studies in terms of type of transients, systems and phenomena considered. Among the various physical models developed during the work, it can be mentioned the transient neutronic models based on a quasi-static Monte Carlo approach and on the deterministic SP1 and SP3 methods. A porous medium model was also developed during the work to allow performing studies on nuclear systems containing a solid nuclear fuel cooled by a fluid. The numerical implementation of the multi-physics coupling was performed in C/C++ in the OpenFOAM code. This code is very well suited to numerically solve continuous mechanics problems using a finite volume method. It also provides very large library of CFD algorithms (RANS, LES et DNS). The thesis work specially focused on the study of the strategy to be followed to implement the quasi-static method numerically with a Monte Carlo type code in the same platform through internal coupling.The performances of the coupling and the developed models were studied for different scenarios and nuclear systems: the transient Godiva experiments, an international benchmark for multi-physics codes for Molten Salts Reactors and the case of a hypothetical criticality accident in a Boiling Water Reactor (BWR) spent fuel pool. These diverse scenarios and systems were selected because they are characterized by presenting a multitude of highly coupled physical phenomena which required a very careful modeling. One can mention: the Doppler and fuel density effects, the thermal expansion and thermomechanical stresses, the presence of laminar or turbulent flows in the coolant or liquid fuel, the delayed neutrons precursors convection, and the energy and mass transfers and the phase change in porous media. The different comparisons between the multi-physics tool and the available data show a very good agreement and confirm that the selected approach is pertinent for the study of criticality accidents and allows obtaining very good precision and flexibility while maintaining satisfactory computational costs.
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Submitted on : Thursday, March 18, 2021 - 10:28:08 AM
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  • HAL Id : tel-03172978, version 1

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Juan Antonio Blanco. Neutronic, thermohydraulic and thermomechanical coupling for the modeling of criticality accidents in nuclear systems. Génie civil nucléaire. Université Grenoble Alpes [2020-..], 2020. English. ⟨NNT : 2020GRALI078⟩. ⟨tel-03172978⟩

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