Improvement of the nuclear safety code CATHARE based on thermal-hydraulic experiments for the Jules Horowitz Reactor

The Jules Horowitz Reactor (JHR) is a material testing research reactor under construction at CEA-Cadarache (France). One of the computer codes employed in the safety analysis of this reactor is the thermal-hydraulic system code CATHARE. The physical models implemented in CATHARE have been developed and optimized for commercial Light Water Reactors which significantly differ from JHR in terms of both core geometry and operational conditions. In view of this, it is crucial to carefully assess the capabilities of CATHARE with respect to the JHR characteristics. The current thesis aims at improving the physical correlations used in CATHARE for JHR modeling. The work is based on the SULTAN-JHR experiments in narrow rectangular channels, that were carried out at CEA-Grenoble in order to investigate the thermal-hydraulics of the JHR core channels. The first part of the study is related to the assessment of the correlations for single-phase friction coefficients and for single-phase forced convection heat transfer. A more comprehensive modeling of single-phase flow in CATHARE is proposed by including a laminar-turbulent transition region. In addition, it is found that the turbulent heat transfer coefficient may be significantly under-estimated by standard correlations (e.g. Dittus-Boelter correlation) at high Reynolds numbers. Thus, new ad-hoc correlations were developed from the SULTAN-JHR data by making use of a best-fitting procedure. In the second part, the CATHARE two-phase heat transfer modeling is revised. Several correlations have been tested against the SULTAN-JHR experiments. The results show that the simplified Forster-Greif correlation may accurately predict the heat transfer in JHR-type channels under fully developed boiling conditions. Such a relationship is then added in CATHARE

Assessment and improvements of thermal-hydraulic correlations and methods for the analysis of the Jules Horowitz Reactor

Nuclear research reactors are used to test materials for current and future nuclear technologies, and to produce radioisotopes for medical purposes. Most of the existing Material Testing Reactors in Europe have operated for more than 50 years and new ones are needed. Therefore the Jules Horowitz Reactor (JHR) is under construction at the French Alternative Energies and Atomic Energy Commission (CEA), on the Cadarache site.<br /><br />The JHR will allow irradiation experiments with high neutron fluxes, at fast and thermal energies. In order to cope with the considerable heat fluxes generated during operations, the core configuration consists of fuel assemblies with parallel narrow channels, where coolant flows at high velocity. Such a design is unique and specific simulation capabilities have to be developed for the analysis.<br /><br />This doctoral research investigates possible improvements of the thermal-hydraulic modeling of the JHR, and is arranged in three parts.<br />In the first part, correlations for the single-phase turbulent friction and heat transfer, for the fully developed boiling heat transfer, and for the critical heat flux, respectively, are assessed and their accuracy is quantified, against the SULTAN-JHR experiments. These experiments were carried out in heated narrow channels comparable to the JHR ones. It is shown that the single-phase turbulent correlations valid for standard nuclear systems, can perform poorly when applied to the typical conditions of the JHR. Thus, new best-fitting relationships are derived. For fully developed boiling in narrow channels, the Forster-Greif correlation can be considered a reliable option. As regards the modeling of the critical heat flux, the Sudo correlation can provide satisfactory predictions. These results are then used to modify the thermal-hydraulic system code CATHARE for the purpose of a more realistic analysis of the JHR.<br /><br />The second part is focused on the onset of flow instability, which is a primary concern in systems with parallel channels as the JHR, since it can lead to undesirable boiling crisis. In view of this, several criteria are evaluated with experiments in narrow channels from both the SULTAN-JHR program and the literature. Conservative predictions can be obtained with Saha-Zuber KIT correlation. Furthermore, some criteria are optimized with respect to the available experimental data for narrow channels.<br /><br />In the third part, the analysis of a postulated accident in the JHR, namely a station black-out, is performed with a best-estimate plus uncertainty approach, combined with the CATHARE code as modified in the first part of the thesis. As a result, the impact of different input and modeling uncertainties on the simulation is estimated, and the most influential uncertain parameters are identified

A generalization of the CIRCE method for quantifying input model uncertainty in presence of several groups of experiments

The semi-empirical nature of best-estimate models closing the balance equations of thermal-hydraulic (TH) system codes is well-known as a significant source of uncertainty for accuracy of output predictions. This uncertainty, called model uncertainty, is usually represented by multiplicative (log-)Gaussian variables whose estimation requires solving an inverse problem based on a set of adequately chosen real experiments. One method from the TH field, called CIRCE, addresses it. We present in the paper a generalization of this method to several groups of experiments each having their own properties, including different ranges for input conditions and different geometries. An individual (log-)Gaussian distribution is therefore estimated for each group in order to investigate whether the model uncertainty is homogeneous between the groups, or should depend on the group. To this end, a multi-group CIRCE is proposed where a variance parameter is estimated for each group jointly to a mean parameter common to all the groups to preserve the uniqueness of the best-estimate model. The ECME algorithm for Maximum Likelihood Estimation is adapted to the latter context, then applied to relevant demonstration cases. Finally, it is tested on a practical case to assess the uncertainty of critical mass flow assuming two groups due to the difference of geometry between the experimental setups.Comment: 26 pages, 7 figure

Efficient modeling of plasmonic organic hybrid electro/optic modulators

The work focuses on the modeling and simulation of plasmonic organic hybrid electro/optic modulators. Preliminary multiphysics-augmented simulations of the slot plasmonic waveguide phase modulator are presented. Instead of applying them to system-level models, they are combined with the results of 3D finite-difference time-domain (FDTD) simulations to achieve realistic physics-based simulations at moderate computational costs. The model is demonstrated on a Mach-Zehnder plasmonic modulator inspired to literature results and validated through a comparison with 3D-FDTD simulations of the entire device

Plasmonic-organic hybrid electro/optic Mach-Zehnder modulators: from waveguide to multiphysics modal-FDTD modeling

Plasmonic organic hybrid electro/optic modulators are among the most innovative light modulators fully compatible with the silicon photonics platform. In this context, modeling is instrumental to both computer-aided optimization and interpretation of experimental data. Due to the large computational resources required, modeling is usually limited to waveguide simulations. The first aim of this work to investigate an improved, physics-based description of the voltage-dependent electro/optic effect, leading to a multiphysics-augmented model of the modulator cross-section. Targeting the accuracy of full-wave, 3D modeling with moderate computational resources, the paper presents a novel mixed modal-FDTD simulation strategy that allows us to drastically reduce the number and complexity of 3D-FDTD simulations needed to accurately evaluate the modulator response. This framework is demonstrated on a device inspired by the literature

Towards an efficient simulation framework for plasmonic organic hybrid E/O modulators

Due to the large computational resources required, with CPU times of the order of several days, full-wave optical simulators can be hardly exploited for the modeling and optimization of plasmonic organic hybrid electro/optic modulators. With the aim to drastically reduce such complexity, in this work we present a divide-et-impera strategy reducing the number of FDTD simulations required to perform a full-wave simulation of the modulation response. This framework is demonstrated on 2D simulations of a device inspired by the literature, tracing a viable roadmap towards a computationally sustainable, yet accurate, comprehensive 3D simulation framework

Simulation and design of plasmonic directional couplers: application to interference-based all-optical gates

The paper is focused on the design of optical components based on plasmonic multi-slot directional couplers. In particular, the design of an all-optical gate is proposed, whose operation is based on the coupling between three plasmonic slots. The device input wavelength is 1550 nm, typical of long-haul telecommunication systems. The device footprint is as small as 11×6 µm 2 and the contrast ratio as an AND gate is about 5.8 dB. The two well-known Finite-Difference Time-Domain (FDTD) and Finite-Difference Eigenmode (FDE) methods are used for the device simulation and optimization

Simulation of electro optic modulators based on plasmonic directional couplers

In this paper, a new concept and geometry are proposed for plasmonic modulators, whose operation is based on the coupling between two plasmonic slots. An electro-optic polymer is exploited as an active material, and the device can be implemented within a Silicon Photonics platform. The device operates at 1550 nm wavelength, typical of data center or long-haul telecommunication systems. For a device length of around 16 μm, the simulated extinction ratio and optical insertion loss are 20.98 dB and 4.26 dB, respectively. Both performances compare favourably with those of Mach-Zehnder plasmonic modulators from the literature. The simulation is based on the Time Domain Finite Differences (FDTD) and Finite-Difference Eigenmode (FDE) methods