Effects due to an obstacle presence in stardust mobilization experiments

Dust mobilisation is a significant concern for the safety of fusion facilities in case of accidents due to loss of vacuum or loss of coolant, because the dust exiting from the vacuum vessel represents a radiological source term influencing both doses to the workers and doses to the population. The STARDUST device (ENEA laboratories, Frascati - Italy) was employed in the past years (2003-2004) as a simple experimental apparatus to demonstrate the effects of loss of vacuum accidents on dust mobilisation; and to obtain indications for the basic assumptions necessary to fix the boundary conditions for the analysis of such accidents. The results obtained from the first experimental campaign demonstrated that the initial mobilization factor of 100% for all the dusts deposited in the ITER vacuum vessel, and used in the Generic Site Safety Report, was too conservative, for all the types of dust tested in STARDUST. This first experimental result was obtained without the presence of any obstacle inside the STARDUST tank. The purpose was to test the most conservative configuration. In reality, inside the ITER vacuum chamber, the air entering through a hypothetical break, in the equatorial port or in the divertor port, faces several obstacles To study the influence of these obstacles on dust mobilization, a new experimental campaign was planned for the same apparatus. Because in real fusion devices (i.e., JET) the dust accumulates mainly in the divertor lower region, an obstacle was placed in the bottom of the STARDUST tank to simulate analogous conditions. A preliminary series of tests with the dust placed in different locations, in and around the obstacle, have been performed. The divertor port level configuration has been chosen as the most interesting to study the mobilization effects. A pressurization rate of 300 Pa/s was selected as representative for failure of the port seals. The same type of dust (carbon, stainless steel and tungsten) used in the past experimental campaigns have been employed. The results confirmed that it is not possible to resuspend all the dust present in the vacuum vessel volume and that a significant fraction of dust is not affected at all by the air inflow. The amount of dust remaining in its initial position is high for the tests in which it was placed inside the obstacle representing the ITER divertor. For the other configurations the mobilization fraction is higher, but significantly lower than the 100%, stated in the GSSR. In conclusion, the fraction of mobilized dust from worst-case accidents is far less than what was assumed in the ITER accident analysis. Consequently, public doses from dust release should be far less than what has been reported in the GSSR. (orig.)