Dose Rate in the Reactor Room and Environment During Maintenance in Fusion Reactors

According to the International Thermonuclear Experimental Reactor (ITER) conceptual design activity, after reactor shutdown, damaged segments are pulled up from the reactor and hung from the reactor room ceiling by a remote handling device. The dose rate in the reactor room and the environment is estimated for this situation, and the following results are obtained: First, the dose rate in the room is >10⁸ µSv/h. Since this dose rate is 10⁷ times greater than the biological radiation shielding design limit of 25 µSv/h, workers cannot enter the room. Second, lenses and optical fiber composed of glass that is radiation resistant up to 10⁶ Gy would be damaged after <100 h near the segment, and devices using semiconductors could not work after several hours or so in the aforementioned dose-rate conditions. Third, during suspension of one blanket segment from the ceiling, the dose rate in the site boundary can be reduced by one order by a 23-cm-thicker reactor building roof. To reduce dose rate in public exposure to a value that is less than one-tenth of the public exposure radiation shielding design limit of 100 µSv/yr, the distance of the site boundary from the reactor must be greater than 200 m for a reactor building with a 160-cm-thick concrete roof.