Study of coupled neutronic and thermal-hydraulic behavior in fast reactors

Numerical simulations are essential for the design of nuclear reactors and the analysis of operational and accidental scenarios. Two key physical phenomena must be accurately modeled: neutronics and thermal-hydraulics. Although these aspects are often studied separately, they are strongly coupled. Neutronic calculations determine the spatial distribution of fission power, which acts as a volumetric heat source and gives rise to temperature distributions in the fuel and coolant. In turn, the fuel and coolant temperatures affect the reactor reactivity through phenomena such as Doppler broadening and density variations. Accurate reactor predictions therefore require a coupled treatment of neutronics and thermal-hydraulics.

The goal of this thesis is to perform integrated neutronic – thermal-hydraulic calculations to obtain accurate predictions of the temperature distributions and the reactivity (changes) of the reactor. The idea is to consider a Monte Carlo (MC) approach for the neutron transport coupled to a Computational Fluid Dynamic (CFD) simulation for the coolant and the solution of the heat equation for the fuel. The thesis will deal with (quasi-)steady-state operation of the reactor. High-fidelity coupled MC – CFD simulations are essential to validate the accuracy of more reduced approaches with a limited resolution in physical and energy space. Owing to the statistical noise associated with MC simulations, the coupled calculations will be restricted to a limited number of neutronic – thermal-hydraulic iterations. The focus of the thesis is therefore on quantifying the trends induced by multiphysics feedback rather than on developing a fully accurate and computationally efficient coupling strategy.

The thesis will be carried out by two students. One student will focus on the neutronic calculations using the Serpent MC code. This includes the development of the reactor geometry model, the calculation of neutron flux and power distributions, and the assessment of the model accuracy. The second student will focus on the CFD simulations using OpenFOAM. This work includes the implementation of fission heat sources and the calculation of coolant flow and temperature fields. Both students will collaborate on the development of a flexible neutronic – thermal-hydraulic coupling interface. Particular attention will be given to the mapping of data between non-conforming computational meshes and to the modularity of the interface, enabling the future integration of alternative neutronic solvers.

The work will initially focus on simplified benchmark geometries, after which the geometric complexity will be gradually increased towards more realistic reactor configurations. Due to the available experimental and numerical data, a sodium-cooled fast reactor (SFR) will be the primary focus of the thesis. The final objective is to assess the accuracy of simplified neutronic and thermal-hydraulic modeling approaches by comparison with the high-fidelity coupled MC-CFD simulations developed in this thesis.

Het vereiste minimumdiploma van de kandidaat

• Master in de ingenieurswetenschappen

De vereiste achtergrondkennis van de kandidaat

• It is recommended that at least one of the students has obtained a master with sufficient focus on fluid mechanics and heat transfer. CFD experience is highly recommended, preferentially with OpenFOAM.

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