Assessment of dose rate profiles and accessibility inside the building of the experimental fusion reactor, ITER, during operation and after shutdown

Abstract

Abstract During the D–T operation of the International Thermonuclear Experimental Reactor (ITER), the dose inside ITER building is attributed to neutrons and gamma-rays streaming from various openings in the machine, whereas after shutdown, it is solely due to the decay gammas from activated 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 ports 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. Zone classification requirements for each hall/room and gallery of the building 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 the 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, all the halls/vaults and galleries are accessible 1-week after shutdown, except inside the NBI hall whose dose rate (6.2 × 105 μSv h−1) exceeds the required zone classification (3–2 μSv h−1). All areas are accessible a week after shutdown in Case B which represents the building side away from the NBI ports. However, in all these cases, the accessibility criterion inside the rooms beneath the cryostat is not met at all times after shutdown due to the rather thin bottom bioshield (50 cm) which may necessitate some design changes.