Tokyo Electric Power Co. has determined an unidentified worker at its crippled Fukushima No. 1 nuclear power plant left valves open along a conduit, which resulted in 100 tons of highly radioactive water overflowing from a storage tank, sources said Feb. 21.
While the utility at first said valve malfunctions could not be ruled out, it concluded the cause was human error after examinations of a photo and water level gauge records.
Three valves line a conduit that sends radioactive water into the storage tank that overflowed. Two of them were open and the remaining one was closed when workers spotted the leak at around 11:30 p.m. on Feb. 19.
Radioactive water normally cannot flow into the tank unless all valves along the conduit are open. But water did flow in, leading TEPCO to initially suspect a malfunctioning valve.
However, a photo shot around 11 a.m. earlier on the day as part of a work procedure showed the third valve in the open position. In addition, readings of a water level gauge in the overflowing tank rose around noon on the same day.
These circumstances led TEPCO officials to believe that somebody opened the valve at around 11 a.m.
TEPCO has interviewed people who were working that day but has yet to identify the worker who operated the valves, the sources said.
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