ASSESSMENT OF DOSE-RATE PROFILES AND ACCESSIBILITY INSIDE THE BUILDING OF THE EXPERIMENTAL FUSION-REACTOR, ITER, DURING OPERATION AND AFTERSHUTDOWN

During the D-T operation of the International Thermonuclear Experiment al Reactor (ITER), the dose inside ITER building is attributed to neut rons and gamma-rays streaming from various openings in the machine, wh ereas after shutdown, it is solely due to the decay gammas from activa ted components inside the building. In the present study, two cases of different geometrical arrangement are considered (Case A and Case C) in which the impact of the size of the opening of the NBI horizontal p orts and presence of the NBI structure on the dose profiles everywhere in ITER building was studied and compared to the case (Case B) where all the horizontal ports are plugged. Contours and mapping of the dose rate profiles during operation, at shutdown, and at one-day, one-week , and one-month after shutdown, were generated for the three cases. Zo ne classification requirements for each hall/room and gallery of the b uilding in terms of the maximum allowed exposure dose during operation and after shutdown were examined to identify where these requirements are met during 'hands-on' maintenance activities. It is shown that th e requirements are satisfied at all times after shutdown in the halls above the ground, even during operation. For the conservative Case A, the dose rates are excessively high inside the equatorial NBI hall and inside the access halls above and below (divertor hall). In Case C, a ll the halls/vaults and galleries are accessible 1-week after shutdown , except inside the NBI hall whose dose rate (6.2 x 10(5) mu Sv h(-1)) exceeds the required zone classification (3-10 mu Sv h(-1)). All area s are accessible a week after shutdown in Case B which represents the building side away from the NBI ports. However, in all these cases, th e accessibility criterion inside the rooms beneath the cryostat is not met at all times after shutdown due to the rather thin bottom bioshie ld (50 cm) which may necessitate some design changes. (C) 1998 Elsevie r Science S.A. All rights reserved.