Workers at the Fukushima Daiichi nuclear power plant used a camera attached to a long pole with a dosimeter to explore highly radioactive areas near the combined exhaust stack for Unit 1 and Unit 2 reactors. Part of the decommissioning process requires the reinforcement and dismantling of portions of the exhaust stack, but the contaminated areas make it impossible for workers to access some of the required areas.
The investigation found radiation doses of 95 millisieverts per hour 5 feet from the SGTS piping at the base of the exhaust stack.
Cracks were previously identified in the exhaust stack some 200 feet above the ground during seismic safety evaluations. Radiation levels inside the pipe were measured at more than 10 Sieverts per hour.
TEPCO assumes that the high dose areas identified by investigations, is likely due to two or more highly radioactive sources in the piping. It is estimated that the sources may around 25 Sieverts per hour near-surface, with a dose rate of some 15 Sieverts per hour.
The investigation did not determine whether Unit 1 or Unit 2 was the source of the contamination.