Decontamination tests of dust under load for the ITER Blanket Remote Handling System

Abstract

Radioactive dust will accumulate in the vacuum vessel (VV) of ITER after plasma operations. Consequently, the ITER Blanket Remote Handling System (BRHS) will be deployed into the VV to stably handle the blanket modules 1.5 m long, weighing up to 4.5 ton with high positioning accuracy. The BRHS itself must also undergo regular hands-on decontamination and maintenance in the Hot Cell Facility (HCF). However, the workers will be exposed to any activated dust that remains on BRHS component surfaces after remote decontamination. Past studies estimated the contaminated surface area of the BRHS, however, in this study, decontamination tests were performed and the dose rate to maintenance workers was calculated using the Monte Carlo N–Particle Transport Code (MCNP5). Decontamination tests were performed by using multiple test pieces of varying surface roughness made from SUS329J4L and two different types of brushes (nylon and SUS304) to simulate decontamination of the BRHS surface. Tungsten dust was pressed on the test pieces to simulate the loading by the rollers. After the test pieces were brushed the surface of each test piece was observed by using both optical and scanning electron microscopes. Dust was reduced by approximately 99% in all cases where the SUS304 brush was used regardless of surface roughness. The decontamination rate was used to estimate how much dust will be able to be cleaned from the BRHS surface. This paper describes the optimal decontamination tools with respect to the BRHS surface conditions to calculate and hopefully reduce the dose rate to maintenance workers so as to optimize the BRHS maintenance plan in the HCF.