Liquid film condensation modelling for passive: from experiment towards CFD and system codes
Passive systems are being considered for innovative reactors owing to their enhanced safety reliability. Particularly, the Safety Condenser (SACO) ensures the decay heat removal through a passive cooling of the secondary side: steam generated is condensed in a submerged vertical heat exchanger and the condensate returns back to the steam generator by natural circulation. It is therefore essential to accurately predict condensation in numerical codes.
CATHARE-3 is the reference thermalhydraulic code at system scale used in France for safety assessment of pressurised water reactors (PWRs). In particular, CATHARE standard film condensation models are validated against the COTURNE separate effect tests. Designed to validate reflux condensation mode in PWRs, the database involves gravity-driven flows with a certain extent of shear stress. However, the standard model is no longer valid for the SACO operating region, which is dominated by the sheer stress.
Recent works on SACO have shown a systematic overestimation of condensation by CATHARE. The main goal of this thesis is to improve CATHARE-3 condensation heat transfer models by means of experimental data (to be generated within the EASI-SMR European project) and by means of an upscaling methodology from CFD tools, namely Neptune_cfd.
Modeling condensation and solidification of air gases on a cold wall: application to the simulation of the Loss of Vacuum of a liquid hydrogen tank
The increasingly widespread use of liquid hydrogen (LH2), particularly for low-carbon mobility, raises safety issues given its highly flammable nature. One of the major accidents involving cryogenic systems is the air ingress following a rupture of the outer shell of a vacuum-insulated tank. In such an event, the gases in the air liquefy and solidify on the cold walls, resulting in a high heat deposit and sudden system overpressure. The discharge line and the safety devices must be sized to evacuate the cryogenic fluid safely and avoid any risk of explosion. The aim of this thesis is to develop a model to simulate this type of scenario using the CATHARE code. A particular effort will be made to model heat exchange by liquefaction and solidification through the tank wall. This work will benefit from the loss of vacuum experimental campaign to be carried out in LH2 by CEA as part of the ESKHYMO ANR project. In addition, the use of a CFD local-scale simulation tool such as neptune_cfd could help in the construction of models in CATHARE by up-scaling. Finally, the methodology developed will be applied to simulate a system representative of an industrial facility.
Biosourced alditol anhydrides, tunable molecular architectures for a sustainable approach to the uranium extraction
Although current applied processes for extracting uranium in sulfuric, phosphoric and nitric media, are efficient enough to justify their large-scale application, they require improvements to increase their efficiency and reduce their environmental impact. This doctoral project aims to improve these performances by focusing on the liquid-liquid extraction stage. This consists of selectively transferring uranium, extracted after crushing, grinding and leaching rocks, to an oil phase containing a lipophilic ligand compatible with the leachate medium. The ambition here is to develop new extractants analogous to trialkylamines (AMEX process), trialkylphosphines and phosphoric diesters (URPHOS process), and trialkylphosphates (refining). The PhD student will synthesize chiral amphiphilic extractants, derived from bicyclic anhydrides of biosourced alditols (isosorbide, isomannide and isoidide). He will evaluate their affinity towards uranium and their selectivity in the presence of competing ions. He will then characterize the molecular and supramolecular mechanisms of these new extractants (coordination, aggregation) using state-of-the-art methods such as UV, IR, multinucleus NMR, X-ray scattering and neutron scattering. The doctoral training will help the PhD student to integrate easily into academic or industrial environments, particularly in the fields of the nuclear fuel cycle, separative chemistry and formulation. Research will take place in the LTSM laboratory of the Institut de Chimie Séparative de Marcoule, renowned for its expertise in the chemistry and physical chemistry of extractants for hydrometallurgy. The PhD student will benefit from high-quality supervision and a collaborative working environment, surrounded by PhD students, post-docs and engineers, in a serene and stimulating setting.
Multiphysical modeling of a dual-frequency induction-heated metallothermic reactor
The recycling of uranium extracted from spent fuel (reprocessed uranium or URT) is of major strategic interest as regards both closure and economics of the cycle as well as for national sovereignty. France has initiated the development of a reprocessing route for this URT, involving an entire production chain relying on SILVA laser enrichment technology.
In this context, the CEA is in charge of developing all the processes in this chain, in particular the steps involved in the conversion of uranium oxide into uranium metal required for laser enrichment. For this purpose, the “Laboratoire d'étude des technologies Numériques et des Procédés Avancés” (LNPA) is studying the transposition of the historical metallothermy process to a cold crucible type reactor. This dual-frequency inductive furnace is designed to melt a two-phase charge consisting of a fluorinated slag and a metal produced in situ by the metallothermic reaction.
Alongside a multi-year technology development program on reduced-scale inactive pilot plants, numerical modeling studies of the reactor are undertaken in order to consolidate the change in working scale and enable system parameters to be optimized before deployment of the technology in active operation on depleted uranium for validation tests. The aim of the proposed thesis work is to develop the magneto-thermo-hydraulic (MTH) multiphysical model of the cold crucible metallothermic furnace.
fragmentation of a liquid jet against an obstacle : application to sodium spray fires
The sodium spray fires, generated by a leak in a sodium pipe, are of major concern for the safety of the Sodium Fast Reactors. The main consequences of these fires are the temperature and pressure increase within the involved building or containment as well as the production of a chemical source term that could be released into the environment.
The key phenomenon to consider for spray fire studies is the fragmentation of the liquid sodium jet against an obstacle (room wall or ceiling). This mode of fragmentation has to be carefully assessed with a special focus on the droplet population generated at the impact.
To do so, the LESC lab in Cadarache is currently preparing an experimental loop involving a water jet impacting a horizontal or vertical surface.
The Ph-D work will then consist in:
- Performing the impact experiments with varying jet initial diameter, velocity and orientation;
- Analyzing the images from the high-speed camera with an in-house software and extract the droplet size and velocity distributions;
- Suggesting a model that could represent this mode of fragmentation and the resulting droplet population;
- Implementing this model into the CEA canoP CFD software;
- Validating the developed model against the results from the above image analysis.
Effects of structural heterogeneities on flows through reinforced concrete structures
The containment building is the third safety barrier in nuclear power plants. Its role is to protect the environment in the event of a hypothetical accident by limiting releases to the environment. Its function is therefore closely linked to its tightness, which it must maintain throughout its operating life. Traditionally, the estimation of the leakage rate is based on a good knowledge of the hydric state and potential mechanical disorders, associated with transfer laws (such as permeability) in a chained (thermo-)hygro-mechanical simulation approach. While the mechanical behaviour of the structure is now generally well known, using advanced simulation tools, progress is still needed to improve the understanding and quantification of flows. This is particularly the case in the presence of heterogeneities (cracks, honeycombs, reinforcement, cables, etc.), all of which can locally disrupt permeability. This is the context of the proposed thesis topic. The aim is to improve the understanding and representation of flows through a reinforced concrete structure using an approach that combines experimental tests and modelling. An initial analysis will be used to define an optimised experimental design based on several configurations (leak paths, type of flow, temperature, saturation, etc.), which will then be implemented during the thesis. The results will be analysed in order to characterise empirically the influence of the leakage path on the macroscopic laws classically used (Darcy's law). A more refined simulation approach will then be developed, based on the finite element method. The aim will be to reproduce the experimental results and extend them to the behaviour of containment vessels, thereby improving the modelling tools currently available.
Modelling and simulation of concrete melting and fracture in corium-concrete interaction
In certain accidental situations, it is important to assess the consequences of severe thermal loading on the mechanical behaviour of concrete structures, particularly with regard to potential cracking. This is particularly the case in the study of corium-concrete interaction. As part of the assessment of the consequences of a hypothetical severe accident, a core meltdown may be considered. The molten mixture, known as corium, then spreads into the reactor and comes into contact with the concrete. Various phenomena can occur, leading to partial ablation of the material. Given the stakes involved in terms of environmental protection, it is essential to have modelling tools that can represent the mechanisms involved. The aim of this thesis is to develop a comprehensive simulation methodology to represent the mechanical consequences of corium-concrete interaction, including local-scale modelling to represent the ablation of the cementitious material. Particular attention will be paid to the concrete cracking model (development of a model adapted to severe thermal loading, concrete ablation criteria) and to the thermal-mechanical-flow chaining of tools for representing the penetration of corium into cracks. This work will be carried out in collaboration between CEA SACLAY (which has the first tools for simulating thermomechanical behaviour) and CEA Cadarache (which has numerical and experimental expertise in corium-concrete interaction).
Modeling two-phase flow transitions in the hybrid formalism continuous/dispersed
In the nuclear industry, simulating two-phase flows may require modeling gas pockets and/or plumes of bubbles with varying shapes. These flows transition between dispersed bubbly flows and separated regimes, characterized by large continuous interfaces, and vice versa. The challenge lies in accurately modeling the transitions between these regimes to better understand the complex phenomena that arise. Currently, two different approaches are used: a statistical method for bubbles and an interface reconstruction method for large, highly deformed bubbles or gas pockets. However, combining these methods within a unified framework remains a key scientific challenge.
The proposed PhD work aims to develop a method capable of modeling both the transitions between continuous and dispersed phases as well as their coexistence. This will involve analyzing experimental data, developing numerical tools within the NEPTUNE_CFD code, and validating the approach through academic and industrial case studies. Applications include the modeling of Taylor bubbles, the study of transitions in the METERO H experiment, and the analysis of flows in tube bundles. The expected results will enhance the simulation of these complex flows in industrial contexts.