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.
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).