This thesis is part of the framework of safety studies associated with Severe Accidents in Sodium-cooled Fast Neutron Reactors, and more particularly when the melted core relocates toward the core catcher at the bottom of the tank by gravity. A corium jet (mixture of combustible and structural elements of the melted core) then interacts violently with the coolant fluid of the reactor. This phenomenon is called FCI for Fuel Coolant Interaction. The interaction involves among others a fragmentation of the corium jet (dispersed particles) coupled to film boiling of the coolant. Characteristics of steam films are determining to study and model the fragmentation step of fuel leading potentially to sodium steam explosion.
DNS (Direct Numerical Simulation) of film boiling in these conditions is very costly especially, because of the small thickness of films, mass transfer to take into account at the smallest scales and constraints of the compressibility.
The goal of the thesis is then to simulate film boiling with an out of equilibrium compressible model able to relax these constraints with a great generality. Indeed, the knowledge of phases disequilibrium allow to evaluate locally the exchanges, for instance of heat thanks to semi-empirical correlations, while conserving a proper solving of the main flow scales. The proposed model will have to take into account the evaluation of the interfacial area in order to evaluate accurately mass and heat transfers at the liquid-vapour interface.
On this basis, the thesis work will be broken down into 3 parts, in addition to the initial bibliographical study. The first part will concern the choice or proposal of envelope macroscopic models, on the one hand, and high-precision models on the other, enabling the simulation of film boiling. The second part of the project will then involve the implementation, in the Computational Fluid Dynamics (CFD) code SCONE based on TRUST (an Open Source code developed at CEA), of models and numerical methods enabling reliable resolution of the problems considered. Finally, the last part of the work will be dedicated to prospective work and sensitivity studies of the numerical models, particularly in terms of geometry and thermal conditions (undercooling of the coolant, temperature field in the solid), in order to determine the validity range of the proposed work and potential avenues for improvement.
In a general manner, the thesis will allow characterizing numerically steam film boiling. Improvement of the modelling of film boiling goes far away the context of FCI phenomena and will be therefore applicable to a large variety of industrial and academic problematics. This work thus opens up career prospects particularly in research centers and R&D departments in industry.
A master internship is proposed by the team in addition to the thesis.