Mrs. Emmanuelle Lacaze, Director of Research, CNRS - Sorbonne University, President
Mr. Alfred Weber, Professor, University of Clausthal, Reporter
Mr. Khaled Hassouni, Professor, University of Paris 13, Rapporteur
Ms. Rosine Coq Germanicus, Senior Lecturer, University of Caen, Examiner
Mr. Olivier Pluchery, Professor, Sorbonne University, Director
Mr. Christian Grisolia, Director of Research, CEA, Co-Director
Mr. François Gensdarmes, Research Engineer, IRSN, invited member

Abstract

 

The mobility and containment of radioactive dust produced by plasma/wall interactions taking place in the heart of a tokamak have become, over the years, major topics for the safety assessment of the ITER installation. Under normal operating conditions, this reactor will accumulate several hundred kilograms of metal dust (from materials facing the plasma) which may be radioactive, exhibit acute chemical toxicity or even form, with air and water vapor, a potentially explosive mixture. To answer these safety issues, I have adopted complementary approaches based on experimental and numerical work. First, a dust sampling device was designed and manufactured in order to collect dust deposited on the walls of a fusion reactor. The in situ sampling campaign carried out with this device in the WEST tokamak (CEA/IRFM) made it possible to identify tungsten particles of spherical shape and micrometric size. With these samples, I was able to perform an in-depth study using an atomic force microscope (AFM). Distributions of adhesion forces for different particle/surface systems were thus obtained. The results of this study are in very good agreement with an analytical model which makes it possible to describe the adhesion forces according to the size of the particles and the roughness of the surfaces. I continued the study by performing measurements of the electric charge of particles when labeled with tritium using Kelvin Probe Microscopy (KPFM). The sensitivity of this technique allowed me to demonstrate a difference in surface potential between "neutral" steel particles and steel particles marked with tritium. The electric charge of these particles could be explained using a self-charging model and Monte Carlo simulations. Finally, resuspension experiments with tungsten particles loaded with tritium were carried out. The results of these experiments are in agreement with a resuspension model in the case where the adhesive force distributions obtained previously by AFM are taken into account. The results of these experiments, combined with the validation of a particle resuspension model, provide robust data for dust management, safety analysis and definition of radiation protection plans for future nuclear fusion facilities.