Laboratory of Reactor Physics and System Behavior

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Miniature and Minimalistic Neutron Detectors for Online High-Resolution Experiments in the Zero-Power Reactor CROCUS

Fanny Vitullo

The continuous strive for more efficient and reliable nuclear power plants is leading to increasing complexity in the design and operations of reactors. As a consequence, comprehensive analyses are required to assess the reactor's safety during normal and accidental conditions. Validated simulation tools with high-fidelity capabilities, i.e., at the pin or sub-pin scale, are necessary to predict local neutronics effects in the core. However, the scarcity of high-resolution in-core experimental data imposes limitations to the validation of such high-fidelity pin-resolved neutronics codes. The challenges in performing online high-resolution in-core experiments are numerous: from the accessibility of in-core locations to the availability of neutron detection technologies with adequate dimensions and online capabilities. In the present thesis, the development of a novel miniature and minimalistic (MiMi) neutron detection technology is presented, allowing unprecedented spatial resolution for the online study of the neutron flux in zero-power research reactors. The MiMi neutron detectors are tested in the EPFL zero-power reactor CROCUS and used to build a data set of high-resolution neutronics experiments in CROCUS, including measurements of local gradients and directionality of neutron flux, and the local impact of a fuel rod displacement. As the next level of development, a three-dimensional (3D) full-core mapping system named SAFFRON, consisting of 149 MiMi neutron detectors distributed in-core, is designed and installed in CROCUS. Static thermal neutron flux maps are measured in absolute terms and between different core configurations, e.g., water level vs. control rod operation at criticality. The obtained results open up the investigation of a variety of space-dependent neutronics phenomena in CROCUS.

EPFL2022

CFD Simulation of Particle-Laden Flow in a 3D Differentially Heated Cavity Using Coarse Large Eddy Simulation

Abdelouahab Dehbi, Konstantin Mikityuk

Particulate flow in closed space is involved in many engineering applications. In this paper, the prediction of particle removal is investigated in a thermally driven 3D cavity at turbulent Rayleigh number Ra = 1E9 using Coarse Large Eddy Simulation (CLES). The depletion dynamics of SiO2 aerosol with aerodynamic diameters between 1.4 and 14 µm is reported in an Euler/Lagrange framework. The main focus of this work is therefore to assess the effect of the subgrid-scale motions on the prediction of the particulate flow in a buoyancy driven 3D cavity flow when the mesh resolution is coarse and below optimal LES standards. The research is motivated by the feasibility of modeling more complex particulate flows with reduced CPU cost. The cubical cavity of 0.7 m side-length is set to have a temperature difference of 39 K between the two facing cold and hot vertical walls. As a first step, the carrier fluid flow was validated by comparing the first and second-moment statistics against both previous well-resolved LES and experimental databases [Kalilainen (J. Aero Sci. 100:73–87, 2016); Dehbi (J. Aero. Sci. 103:67–82, 2017)]. First moment Eulerian statistics show a very good match with the reference data both qualitatively and quantitatively, whereas higher moments show underprediction due to the lesser spatial resolution. In a second step, six particle swarms spanning a wide range of particle Stokes numbers were computed to predict particle depletion. In particular, predictions of 1.4 and 3.5 µm particles were compared to LES and available experimental data. Particles of low inertia i.e. dp

2022

Unmoderated molten salt reactors design optimisation for power stability

Andrea Bellè, Axel Guy Marie Laureau, Andreas Pautz

The critical region of unmoderated molten salt reactors consists in a cavity filled with a liquid fuel. The lack of internal structure implies a complex flow structure of the circulating fuel salt. A preliminary core shape optimization has been performed during the EVOL European project to limit recirculation and hotspots. This optimization was based on a Reynolds Averaged Navier Stokes (RANS) approach, but the latter only provides time-averaged values for velocity and temperature. However, the power stability is sensitive to thermal fluctuations induced by the flow turbulence itself, even at steady state without pump flow rate or heat extraction variation. This phenomenon is studied using a Detached Eddies Simulation approach to solve the turbulence in the reactor and get a time dependent temperature distribution and then the reactivity fluctuations. A new geometry is proposed to limit the total power fluctuations from 7.5% for the preconceptual EVOL geometry down to 1.2%. (C) 2022 Published by Elsevier Ltd.

PERGAMON-ELSEVIER SCIENCE LTD2022