Flow rate measurement in a pipeline using thermal noise detection
Flow measurement is a key factor in process management, particularly in the nuclear and industrial sectors. However, current measurement methods require complex installations, especially in environments with strict regulations, such as in the nuclear sector. To address these challenges, the CEA has developed an innovative method for measuring flow in non-isothermal fluids, based on the analysis of thermal fluctuations. This technique, which uses two temperature sensors installed upstream and downstream on the pipeline, is simple to implement and involves minimal constraints. The temperature variations are carried by the flow from one sensor to the other, and by comparing the signals recorded by these sensors, it is possible to calculate the thermal transit time between them, which allows the flow velocity, and consequently, the flow rate, to be determined. The goal of this thesis is to optimize this method by enhancing its reliability. To achieve this, the propagation of thermal noise within the flow will be studied, and both the type and placement of the sensors will be optimized. This work will be carried out within the Core and Circuit Thermohydraulics Laboratory in collaboration with the Instrumentation, System and Method Laboratory, which has state-of-the-art experimental equipment. Numerical simulations will complement the experimental work to validate the obtained results. In parallel, artificial intelligence approaches will be explored to improve the processing of thermal signals. By the end of the thesis, the doctoral candidate will have acquired extensive skills in experimental and numerical work and will be able to leverage these in future endeavors.
Experimental study of the two-phase natural convection and vaporization regimes in the cooling pool of a nuclear facility
Nuclear energy, with low CO2 emissions, is one of the major players in France's energy transition. In this context, the management of the cooling of irradiated fuel elements is a matter of utmost importance. This thesis focuses on two-phase natural convection flows and vaporization phenomena that can develop in the cooling pools of various nuclear facilities, particularly those having a significant vertical variation in the saturation temperature of the coolant due to their great depth. These pools are used to dissipate the residual heat from irradiated fuels in many types of nuclear reactors, both existing and planned. In an accident scenario with a significant heat release from the fuels, the water in these pools can vaporize, eventually limiting their cooling capability. Among the possible phase change mechanisms in deep pools is the gravity-driven flashing, a phenomenon found in various natural or industrial systems analogous to vertical channels heated from below. However, this phenomenon has been little studied in the specific configuration of a pool and was only recently observed in this context. Therefore, the objective of this thesis is to better understand the phenomenon, as well as the turbulence induced within the coolant by the bubbles it generates, in order to improve state-of-the-art thermal-hydraulic models for simulating such pools. The proposed research, of an experimental nature, will be conducted in collaboration with the Catholic University of Louvain (UCLouvain, Belgium) and the LEGI laboratory of CNRS Grenoble, with a significant portion of the research carried out at UCLouvain. The candidate will be affiliated to the Core and Circuit Thermal-hydraulics Laboratory (LTHC) of CEA IRESNE, specialized in the study of two-phase flows in nuclear facilities. During the thesis, finely resolved experimental data in both space and time will be acquired and interpreted, contributing to a better understanding of the phenomenon. To achieve this, advanced techniques such as stereo particle image velocimetry (3D PIV) in two-phase media, thermometry and shadowgraphy will be employed. During this thesis project, the PhD student will be able to develop skills in the field of experimental thermal-hydraulics through the definition, execution, and interpretation of tests, as well as the use of advanced two-phase flow measurement techniques.
Eco-designed materials for encapsulating new-generation flexible photovoltaic modules
The lifetime of thin-film devices such as Organic Photovoltaic (OPV) devices or new-generation lightweight and/or flexible Silicon (Si) photovoltaic modules is critical to their commercialization. In particular, it is crucial to encapsulate them with highly gas-barrier materials to avoid degradation through various water/oxygen insertion mechanisms that can be coupled to illumination. This objective is all the more complex when the device and its encapsulation need to be flexible. Moreover, the eco-design of this new generation of flexible modules raises the question of the nature of the encapsulation materials used, as well as that of the end-of-life of the materials making up the modules. For example, the current use of fluorinated polymers for encapsulation generates toxic products at end-of-life, and could be replaced by the use of eco-designed materials, potentially bio-sourced, if the performance is adapted to the photovoltaic technology employed and the use.
The aim of this thesis will be to study the physico-chemical properties (gas barriers, mechanical, thermal, etc.) of bio-sourced encapsulants developed as part of a national PEPR BioflexPV project. These studies will cover both sealing materials and flexible caps. In addition, these materials will be used to encapsulate real OPV and flexible Si devices, in order to study their degradation under different illumination, temperature and humidity conditions. These studies will help define the degradation mechanisms involved, depending on the photovoltaic technology used (OPV or Si), and thus define the desired properties for bio-sourced encapsulants.
Simplified modelling of calcination in a rotating tube
As part of the reprocessing of uranium oxide spent fuel, the final high-level liquid waste is packaged in glass using a two-stage process, calcination followed by vitrification. Calcination gradually transforms the liquid waste into a dry residue, which is mixed with preformed glass in a melting furnace. The calciner consists of a rotating tube heated by a resistance furnace. The calcined solutions consist of nitric acid and compounds in their nitrate form or insolubles in the form of metal alloys. In order to improve control of the calciner, it is proposed to model it.
The modelling will consist of creating and then coupling three models:
- A thermodynamic model to represent the transformations undergone by the material. This part will almost certainly involve ATD and ATG measurements, coupled with a design of experiments type approach (1st year).
- A material flow model. The literature already contains very simplified principles for representing the flow in a rotating tube calciner, but we will have to be innovative, in particular by defining tests to characterise the flow of the material during the calcination process (2nd year).
A thermal model that will take into account exchanges between the furnace and the calciner tube as well as exchanges between the material and the tube. The exchange coefficients will have to be characterised (1st year).
Combining these three models (3rd year) will give rise to an initial simplified calcination model. This model will be used to help control the calcination stage and also to train operators to control this apparatus.
You will be working in the LDPV, a multidisciplinary team (process, chemistry, fluid mechanics, modelling, mechanics, induction) comprising 16 engineers and technicians. A team with 30 years' experience in vitrification processes, recognised both nationally and internationally.
Deterministic neutron calculation of soluble-boron-free PWR-SMR reactors based on Artificial Intelligence
In response to climate challenges, the quest for clean and reliable energy focuses on the development of small modular reactors using pressurized water (PW-SMR), with a power range of 50 to 1000 MWth. These reactors aimed at decarbonizing electricity and heat production in the coming decade. Compared to currently operating reactors, their smaller size can simplify design by eliminating the need for soluble boron in the primary circuit water. Consequently, control primarily relies on the level of insertion of control rods, which disturb the spatial power distribution when control rods are inserted, implying that power peaks and reactivity are more difficult to manage than in a standard PWR piloted with soluble boron. Accurately estimating these parameters poses significant challenges in neutron modeling, particularly regarding the effects of the history of control rod insertion on the isotopic evolution of the fuel. A thesis completed in 2022 explored these effects using an analytical neutron model, but limitations persist as neutron absorbers movements are not the only phenomena influencing the neutron spectrum. The proposed thesis seeks to develop an alternative method that enhances robustness and further reduces the calculation biases. A sensitivity analysis will be conducted to identify key parameters, enabling the creation of a meta-model using artificial intelligence to correct biases in existing models. This work, conducted in collaboration with IRSN and CEA, will provide expertise in reactor physics, numerical simulations, and machine learning.
Study of the transitions of flow regimes in post-burnout
Dispersed two-phase flows are part of many fluid systems such as the cooling of nuclear reactors. Depending on the heat flux in the reactor core, the flow rate, the subcooling or the pressure, different flows may occur: single phase, bubbly or annular flows (with a liquid film on the wall and a vapour core).
During a loss of primary coolant accident, the reactor core, containing the fuel rods, increases in temperature until the boiling crisis when the heat flux is high enough. The different regimes of two-phase flows that occur in this type of accident are illustrated in figure 1. A vapour film appears rapidly and thermally insulates the rods, while some liquid remains in the centre of the flow. The rods are dried up, thus their surface are cooled down by the single vapour, and the heat exchange at the wall is reduced [1], which corresponds to the « inverted annular film boiling » flow. When the liquid gradually vaporises, the vapour film thickens and the induced turbulence tends to form waves at the vapour-liquid interface, and to destabilise the interface until the formation of liquid slugs (inverted slug film boiling). Then, the evaporation and fragmentation of these slugs lead to the formation of a dispersed flow with droplets (dispersed film boiling).
The transitions of flow regimes in this configuration are not well-identified [1], [2] although their understanding is significant to study the cooling of a nuclear reactor core. One of the main obstacles in experimental studies is that the walls need to be strongly heated up in order to form and maintain a vapour film, which leads to opaque test sections. Thus, a direct visualisation is particularly complex to obtain, as much as measuring local parameters such as temperature and velocity fields. The experimental results available in the literature on this topic are insufficient to develop a physical model [1], [3], [4], [5].
As a first step towards an accurate identification of the regime transitions, this thesis focuses on the single effect of the hydrodynamics, by coupling experimental and analytical approaches. In order to clarify the physics of the different phenomena, the configuration of a liquid flow inside a gas flow is proposed. Indeed, the interface deformation and the gas and liquid velocities may influence the transition from one regime to another [6], [7]: the smooth interface is therefore perturbed by waves (Kelvin-Helmholtz instabilities) and droplets could be entrained from the interface. A parametric analysis is considered by varying the gas and liquid flow rates and the thickness of the gas film, in order to observe these different phenomena and to understand the influence of each parameter on the regime transitions. An experimental facility has recently been conceived at DM2S/STMF/LE2H to study these transitions by a visualisation of the interface deformations, and may be adapted with new measurements or new methodology if necessary.
Dimensionless numbers will be identified or defined from the experimental results to describe the phenomena. Then, the regime transitions will be characterized, based on these dimensionless numbers, in order to establish a diagram of the transitions of flow regimes.
The combination of the results obtained in this thesis will enable to reinforce the physical models used in the system code CATHARE, developed at CEA for thermal-hydraulic studies about nuclear safety. This thesis presents a strong academic interest thanks to an innovative experimental facility and production of original results. Besides, it also presents an interest on the industrial level since it contributes to enhance the expertise of significant phenomena in the demonstration of nuclear reactor safety.
References:
[1] M. Ishii et G. De Jarlais, « Flow visualization study of inverted annular flow of post-dryout heat transfer region », Nuclear Engineering and Design, 1987.
[2] G. De jarlais, M. Ishii, et J. Linehan, « Hydrodynamic stability of inverted annular flow in an adiabatic simulation », Argonne National Laboratory, CONF-830702-9, 1983.
[3] T. G. Theofanous, « The boiling crisis in nuclear reactor safety and performance », International Journal of Multiphase Flow, vol. 6, no 1, p. 69-95, févr. 1980, doi: 10.1016/0301-9322(80)90040-3.
[4] N. Takenaka, T. Fujii, et others, « Flow pattern transition and heat transfer of inverted annular flow », Int. J. Multiphase Flow, 1989.
[5] M. A. El Nakla, D. C. Groeneveld, et S. C. Cheng, « Experimental study of inverted annular film boiling in a vertical tube cooled by R-134a », International Journal of Multiphase Flow, vol. 37, p. 37-75, 2011.
[6] Q. Liu, J. Kelly, et X. Sun, « Study on interfacial friction in the inverted annular film boiling regime », Nuclear Engineering and Design, vol. 375, 2021.
[7] K. K. Fung, « Subcooled and low quality film boiling of water in vertical flow at atmospheric pressure », PhD Thesis, Argonne National Laboratory, 1981.
Modeling of the fall of a drop in a volume, in support of the system code CATHARE
This thesis focuses on the study of droplet fall in free volumes, as part of the continuous improvement of the physical models in the CATHARE code, used for safety studies of Pressurized Water Reactors. The current models are based on the work of Ishii and Zuber, who model the fall velocity of droplets in a two-phase fluid. The objective of the thesis is to refine the precision of this model by incorporating additional parameters and validating it through experiments such as those of Dampierre and CARAYDAS. The PhD candidate will be required to develop a more representative mechanistic model, based on experimental data or CFD simulations if necessary. The innovation lies in developing a more accurate model of droplet fall processes, paving the way for specific applications such as spray modeling, and thus contributing to the validation of the CATHARE code in additional fields.
Study of the dynamics of molten salt fast reactors under natural convection conditions
Molten Salt Reactors (MSRs) are presented as inherently stable systems with respect to reactivity perturbations, due to the strong coupling between salt temperature and nuclear power, leading to a homeostatic behavior of the reactor. However, although MSRs offer interesting safety characteristics, the limited operational experience available restricts our knowledge of their dynamic behavior.
This research work aims to contribute to the development of a methodology for analyzing the dynamics of MSRs, with the goal of characterizing complex neutron-thermohydraulic coupling phenomena in an MSR operating in natural convection, identifying potentially unstable transient sequences, prioritizing the physical phenomena that cause these instabilities, and proposing simple physical models of these phenomena.
This work will contribute to the development of a safety-oriented methodology that will help MSR designers better understand and model the reactor dynamic behavior during transients, through dimensional analysis and the study of the flow stability. This methodology aims to define simple and robust criteria to ensure the intrinsic safety of a fast-spectrum MSR, depending on its design and operational parameters allowing compliance with the operating domain limits.
This PhD lies at the crossroad of theoretical analysis of the physical phenomena governing the MSR’s behavior, particularly the study of unstable regimes (oscillatory or divergent in nature) due to neutron-thermohydraulic coupling under natural convection conditions, and the development of analytical and numerical tools for conducting calculations to characterize these phenomena.
The PhD student will be based within a research unit dedicated to innovative nuclear systems. He/she will develop skills in MSR modelling and safety analysis, and will have the opportunity to present his/her work to the international MSR research community.
Experimental and numerical analysis of fluid-structure interactions in the propagation of rarefaction waves through complex structures in pressurized water reactors
Loss of coolant accident (LOCA) in pressurized water reactors (PWR) leads to fast transient phenomena, such as the propagation of rarefaction waves within the reactor's internal structures. These waves generate transient pressure loads between different areas, such as the reactor core and the bypass zone, which places stress on the baffle. The deformation of this critical structure can compromise the structural integrity of the reactor and complicate the handling of fuel assemblies, particularly their removal after the accident.
The main scientific objective is to develop, implement, and validate new numerical models that allow for a more accurate simulation of rarefaction wave propagation through complex obstacles. The current state of the art relies on simplified models, validated only for simple configurations such as single-orifice plates. However, there is a need to extend these models to more complex geometries, such as plates with multiple holes, using different numerical methods.
The development of a porosity model to represent fuel assemblies is also crucial. The expected results will be validated experimentally and have direct applications for industrial partners EDF and Framatome, enhancing the industrial relevance of this research.
The thesis will adopt a combined approach, both experimental and numerical. The use of the MADMAX platform will allow for the testing of various complex obstacles and the collection of detailed experimental data using specialized sensors. This data will be used to validate the numerical models developed in the EUROPLEXUS software. Additionally, the simulations will include innovative approaches such as a new porosity model for the internal structures of the reactors. Participation in international conferences and publication of results are planned to ensure the scientific dissemination of the findings.
The thesis will be conducted at the DYN laboratory of CEA Paris-Saclay, equipped with unique experimental facilities, such as the MADMAX platform, and has strong expertise in numerical modeling. Several industrial (EDF, Framatome) and academic collaborations will provide a rich environment for the doctoral candidate, with regular exchanges within international networks.
The ideal candidate should possess solid skills in fluid mechanics, structural dynamics, numerical modeling (finite element, finite volume), and programming. Previous experience with tools like EUROPLEXUS will be a plus. An M2 internship may be offered to familiarize the candidate with the methods and tools used in this thesis.
This thesis will enable the doctoral candidate to acquire highly specialized skills in fluid-structure interactions, numerical modeling, and experimentation in an industrial context. These skills are in high demand in the energy, aerospace, and advanced simulation technology sectors, paving the way for careers in applied research or engineering within the industry.
Mass transfers and hydrodynamic coupling: experimental investigation and models validation and calibration
In the context of the energy transition and the crucial role of nuclear power in a low-carbon energy mix, understanding and then mitigating the consequences of any accident leading to a reactor core meltdown, even a partial meltdown, is an imperative research direction.
During a core meltdown accident, a pool of molten material, known as corium, can form at the bottom of the reactor vessel. The composition of the pool can change over time. The corium bath is not homogeneous and can stratify into several immiscible phases. As the overall composition of the corium changes, so do the properties of the different phases. The vertical stratification order of the phases may change, leading to a vertical rearrangement of the phases. During this rearrangement, one phase passes through the other in the form of drops. The order of the phases and their movements are of prime importance, as they have a major influence on the heat flows transmitted to the tank. A better understanding of these phenomena will enable us to improve the safety and design of both current and future reactors.
Initial models have already been produced, but they lack validation and calibration. Prototype experiments are difficult to set up and none are planned in the short term. This thesis proposes to fill this gap by carrying out an experimental study of the phenomenon using a water-based simulating system that allows local instrumentation and large-scale test campaigns. The aim is to validate and calibrate the existing models, and even develop new ones, with a view to capitalising on these results in the PROCOR software platform, which is used to estimate the probability of a reactor vessel breach. The experimental set-up would be built and operated at the LEMTA laboratory at the University of Lorraine, where the PhD student would be seconded. In terms of experiments, two cases will be studied, the single drop case, and the stratified case with drop formation via Rayleigh-Taylor instabilities.
The work will be mainly experimental, with a component involving the use of code for calibration and validation, and may include a modelling component. It will be carried out entirely at the LEMTA laboratory in Nancy. The PhD student will benefit from LEMTA's expertise in the development of simulating experimental devices, fluid transfers and metrology. They will be part of a dynamic environment made up of researchers and other PhD students. The candidate should have knowledge of transfer phenomena (mass transfer in particular), as well as a definite interest in experimental science.