Diagram showing the main exhaust stack of Fukushima Daiichi Unit 1/2, where ultra-high radiation fields in excess of 10 sieverts/hours were discovered by TEPCO on 1 August 2011. Yomiuri ShimbunAugust 3 (Yomiuri Shimbun) – Record-high radiation levels of more than 10 sieverts (10,000 millisieverts) per hour have been detected on a pipe at the crippled Fukushima No. 1 nuclear power plant, according to Tokyo Electric Power Co.

TEPCO said Monday no workers trying to bring the plant under control have been working near the pipe. Exposure to radiation of that level for just six minutes would be enough to cause nausea and other acute symptoms. According to TEPCO, the pipe connects the containment vessels of the Nos. 1 and 2 reactors to a main exhaust stack. No work was scheduled to be conducted in the area, and TEPCO has prohibited anyone from coming within three meters of the pipe. The flow of air through the pipe had been turned off, the utility said. The previously highest level detected at the plant was 4 sieverts per hour inside the No. 1 reactor building. […] However, Kenzo Miya, a professor emeritus at the University of Tokyo and an expert in nuclear engineering, suggested another possible explanation. “As well as radiation spilling out when the vents were opened, we can’t rule out that radioactive substances poured into the pipe during the hydrogen explosion” that damaged the reactor on March 12, Miya said. “Radiation levels also could be high in the exhaust stacks of the Nos. 3 and 4 reactors. This should be closely checked to ensure the safety of workers at the plant,” he said.

Record radiation at Fukushima N-plant / More than twice previous peak level

August 2 (Mainichi Japan) – Tokyo Electric Power Co. (TEPCO) said on Aug. 1 that it had detected radiation doses exceeding 10 sieverts per hour, the highest level of radiation measured since the outbreak of the crisis at the Fukushima No. 1 Nuclear Power Plant, outside the buildings for two reactors — a new discovery that could hamper efforts to bring the troubled reactors under control. The dosage, which exceeded the capacity of measuring equipment, was detected near the surface of an exhaust pipe between the No. 1 and 2 reactors at the Fukushima nuclear complex, said TEPCO, the operator of the crippled nuclear facility. People risk death if they are exposed to radiation doses of 10 sieverts per hour, or 10,000 millisieverts. The high levels of radiation mean that a person could be exposed to 250 millisieverts of radiation — the upper limit set by the government for workers engaged in restoration work at the Fukushima plant — within 90 seconds. […] The exhaust pipe where the radiation doses were detected is used in the event of an emergency to release gas from reactor containment vessels. Steam containing high levels of radioactive substances may have remained in the pipe after TEPCO vented steam from the No. 1 reactor on March 12 in an attempt to protect the reactor containment vessel. Radioactive particles may have also adhered to the outside of the pipe. The exhaust pipe is not in use now. Previously, radiation of 4,000 millisieverts — the then highest level of radiation since the outbreak of the nuclear crisis — had been detected on the first floor of the reactor building of the No. 1 nuclear reactor. […] In a related development, TEPCO said on Aug. 2 that there were two more pipes nearby that apparently showed radiation in excess of 10 sieverts. The utility said it had not actually measured the radiation there and it had no plans to do so because there would be no one working there.

Highest levels of radiation since March 11 detected at Fukushima nuclear reactors