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Dust mobilization by flashing of liquid jet: Application to the problematic of loss of vacuum due to water ingress in the ITER facility

Benjamin Blaisot has defended his thesis on ​on 3rd February 2020 in Saclay, France

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Research Unit > IRSN/PSN-RES/SCA/LPMA

Authors > BLAISOT Benjamin

Publication Date > 03/02/2020

Summary

As the ITER fusion reactor is being built, IRSN is conducting research to assess the consequences of various accident scenarios that could lead to damage to the vacuum chamber of the tokamak and potentially lead to a risk of radioactive materials being released into the environment. The scenario considered in this study is the loss of vacuum by water ingress or ICE (Ingress of Coolant Event), the consequences of which are likely to lead to the formation of hydrogen, thus generating a risk of explosion.

In order to evaluate the quantity of dust that could be mobilized in the vacuum chamber of the tokamak (to which depends the quantity of hydrogen produced) it is necessary to identify and quantify the mechanisms contributing to the resuspension of particles at low pressure during a vacuum loss by ingress of coolant or ICE scenario. During an ICE, the water from the cooling circuit enters the vacuum chamber at a temperature above its boiling point due to the ITER like conditions (low pressure and high temperature) and undergoes atomization by thermodynamic effect known as flash boiling.
The phenomenon of flash atomization in a vacuum and its impact on dust mobilisation is a particularly complex problem. Flash atomization falls under two-phase flow in the presence of thermodynamically metastable phases and extremely rapid phase changes.
Moreover, the atomisation into fine droplets and the intense vaporization that takes place at the breach can give rise to strong compressibility effects that can result in the appearance of supersonic two-phase flows, shocks and this at the limit of rarefied environments.

To conduct this experimental study, the development of a new experimental bench (FAAMUS, Flash Atomization and Aerosols Mobilization Under vacuum System) associated with metrology adapted to the characterization of two-phase flows based on high speed PIV and shadowgraphy techniques was the subject of the first part of the thesis work. Postprocessing
algorithms were also developed to extract qualitative and quantitative data on the morphology of the two-phase flow and on the velocities of the drops leaving the breach. Experiments have been carried out to study flash atomization phenomena for experimental conditions representative of those of an ICE in ITER as well as for extended thermodynamic conditions.

Under ITER like conditions, we have been able to clearly demonstrate and describe the supersonic expansion of the two-phase jet as it exits the breach. This is a complex flow forwhich we were also able to quantify its sensitivity to the nature of the two-phase flow regime developed in the circuit upstream of the breach.

The experiments carried out allowed us to obtain the evolution of the spray morphology as well as the velocity of the drops produced over a large range of overheat. Thus, the spray angle reaches an angle of 150° while the velocity of the drops exceeds 60 m/s at high overheat. The evolution of these characteristics is not linear with superheat and depends on many geometrical and thermodynamical parameters.

Preliminary experiments on aeraulics resuspension of particles at low pressure by liquid-jet flashing have been carried out and provide many insights into the problem of dust behaviour under an ICE scenario in ITER.
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