Local understanding of the corium-concrete interface through experimentation
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 even partial core meltdown is an imperative research direction.
In the event of a severe core meltdown accident, the amalgam of materials produced by core meltdown, or corium, may interact with the concrete of the plant's floor. The lack of understanding of local and interfacial physical phenomena during corium-concrete interaction (ICB) has led to the development of various international simulation tools. None of them has been able to explain the recent observations at the Fukushima Daiichi accident site. It is therefore crucial to improve the ICB simulation tools.
The aim of this thesis is to carry out a detailed, local experimental study of the corium/concrete interface with prototypical corium (depleted uranium). To this end, the candidate will design a test device to be introduced into the VITI inductive furnace of the PLINIUS platform dedicated to the study of severe accidents at the Cadarache center. After qualification of the experimental set-up, local corium/concrete interaction tests in VITI will be carried out on different types of concrete (including a sample from Fukushima) and with different coriums, enabling an incremental approach using separate effects. Ablation will be characterized via mass loss and hydrogen release. The interface will also be characterized after rapid corium removal. Samples will also be X-rayed (e.g. tomography). As the work progresses and the phenomenology of the Molten Corium Concrete Interaction is understood, a model may be developed and integrated into a simulation tool.
The thesis work will be carried out jointly in the experimental and severe accident modeling laboratories of the IRESNE institute at Cadarache, in a research environment of the highest international standard for the study of multiphysical phenomena at very high temperatures. This work will also be enriched by research carried out within the framework of the ANR IMMOC, in partnership with academics (CNRS Laboratoire Navier, AMU-CNRS Madirel...).
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.
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.
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.