Dust produced by plasma off normal event in the vacuum vessel: Experimental analyses of the deposition inside the pressure suppression tank during a LOCA

This paper deals with experimental tests simulating the dust deposition, produced in the Vacuum Vessel of the nuclear fusion reactor ITER, and carried by steam and gases inside the Pressure Suppression Tanks during a Loss of Coolant Accident (LOCA). The dust, mainly tungsten and beryllium, is produced by the erosion of the divertor or of the first wall due to plasma off-normal events, like plasma disruptions or vertical displacement events. The activity is performed in the frame of a research program performed at the University of Pisa, funded by the ITER Organization, for studying the behaviour of the Vacuum Vessel Pressure Suppression System (VVPSS). A reduced scale of VVPSS was built at the University of Pisa (1/32 geometrical scale). The experimental rig is composed by an electric steam generator and steam and air mass flow rate control lines. Steam with (or without) air is sent in a condensation container, partially filled with water. These fluids entrain a mass of dust measured by a dust mini-dosing system. The air and the non-condensed steam flow in the atmosphere through a demister and HEPA filters. A tests campaign was carried out injecting a simulant of the actual dust (aluminium oxide) in a flux of steam and air into the condensation tank. The tests were performed at different values of steam, air and dust mass flow rate. During the test, the dust concentration in the water was measured versus time, in order to determine the settling time. The dust deposition on the surfaces of the condensation tank was measured as well as the decontamination efficiency of the tank. The results demonstrated that about 99 % of dust is deposited on the tank surfaces and only a quantity less than 1 % is carried by the water during the tank discharge.