Radiation control report on intermediate heat exchanger replacement and related works

Kano, Yutaka; not registered; not registered; not registered; Hoshiba, Hideaki; not registered; Shindo, Katsutoshi

The 13th periodical inspection of the experimental fast reactor JOYO is being made from Jun. 2000 to Jan. 2003. While this inspection, from the end of oct. 2000 to Nov. 2001, the MK-III modification work on heat transport system was made in lower region of the reactor containment vessel in the reactor facility (under floor area). In the MK-III modification work, the works important to radiation control were the replacement of intermediate heat exchangers (IHxs) and fixtures, and the picking out of the surveillance material from primary heat transport piping carried out in the maintenance building. Because the working areas of these works were executed in small space around the complicated primary heat transport piping, workability was bad and dose rate from the corrosion products (CP) in piping or fixtures was high. In such condition, radiation control was performed mainly concerned about external exposure. The planned total external exposure of the IHX replacement and related works was 7135 manmSv (target of total dose control: less than 5708 manmSv, 80% of the plan), derived from special radiation work plans for segmental works, concerned about work procedure, number of workers, period of work, dose rate of working area and surface dose rate of equipments. The special radiation control organization was established for such long and large-scale work. The special organization held detailed discussion about radiation control of this work with the execution section and contractors appropriately, performed carefal external/internal exposure control and surface contamination control and made effects to reduce the external exposure thoroughly. As a result of these action, the total external exposure was 2386 manmSv ( 33% of the plan, 42% of the target) and the maximum individual exposure were 24.7 mSv for staffs and 21.7 mSv for contractors. The dose rate, surface contamination and air contamination while the works were ...