Experimental and numerical analysis of dust resuspension for supporting chemical and radiological risk assessment in a nuclear fusion device

摘要

A recognized safety issue for future fusion reactors fueled with deuterium and tritium is the generation of sizeable quantities of dust. Several mechanisms like plasma disruptions, Edge Localized Modes (ELM) and Vertical Displacement Events (VDE) resulting from material response to plasma bombardment in normal and off-normal conditions are responsible for generating dust of micron and sub-micron length scales inside the VV (Vacuum Vessel) of experimental fusion reactors like the International Thermonuclear Experimental Reactor (ITER). The loss of coolant accidents (LOCA), loss of coolant flow accidents (LOFA) and loss of vacuum accidents (LOVA) are types of accidents expected in experimental fusion reactors like ITER, which may jeopardize the components and the plasma vessel integrity and cause dust mobilization risk for workers and public. In particular the mobilization of the dust and its release outside the VV threatening public safety because it may contain tritium, may be radioactive from activation products, and may be chemically reactive and/or toxic. Understanding of the behavior of dust in an accident scenario will be essential in order to support the prevention and protection activities. A small facility, Small Tank for Aerosol Removal and DUST (STARDUST), was set up to perform experiments concerning the dust mobilization in a volume with the initial condition similar to those existing in ITER VV. The aim of this work was to reproduce a low pressurization rate (300 Pa/s) LOVA event in ITER due to a small air leakage, for two different positions of the leak (equatorial port level and at the divertor port level) in order to evaluate a first re-suspension numerical model by the analysis of the: 1. Influence of obstacles and temperature on dust resuspension during both maintenance and operative conditions; 2. The velocity field inside STARDUST. For the first experimental purpose the dusts used were tungsten (W), stainless steel (SS) and carbon (C), similar to those produced inside the vacuum chamber in a fusion reactor when the plasma facing materials vaporizes due to the high energy deposition. The experimental campaign has been carried out by introducing inside STARDUST facility an obstacle that simulates the presence of objects, like divertor and the limiter-divertor gap inside ITER VV. The walls of the tank are heated at two different temperatures, 25℃ 110℃ in order to reproduce the maintenance conditions and operative conditions. For the second experimental purpose, local measurements of velocity have been acquired by a capacitive transducer. A two-dimensional (2D) modelling of STARDUST has been set up by using a commercial CFD code (FLUENT), in order to get a preliminary overview of the fluid dynamics behavior during a LOVA event. The results of these simulations were compared against the experimental data for the CFD code validation. For validation purposes, the CFD simulation data were extracted at the same locations as the experimental data were collected. In this work, the computer-simulation data and the comparison with data collected during the laboratory studies will be presented and discussed.