A warning alarm was ignored and 100 tons of highly radioactive water overflowed from a storage tank and seeped into the ground at the Fukushima No. 1 nuclear plant, the plant operator said Feb. 20.
A warning alarm was ignored and 100 tons of highly radioactive water overflowed from a storage tank and seeped into the ground at the Fukushima No. 1 nuclear plant, the plant operator said Feb. 20.
The water did not reach the ocean, Tokyo Electric Power Co. said.
According to TEPCO, a worker on patrol spotted the water leaking from a storage tank in the H6 area at 11:25 p.m. on Feb. 19.
The area is surrounded by a barrier, but the leaking water entered a gutter for rainwater at the tank’s top plate and flowed outside the barrier, creating pools covering 870 square meters, TEPCO officials said.
Workers dismissed an alarm that sounded shortly past 2 p.m. on Feb. 19 that indicated dangerously high water levels in the tank, TEPCO said.
They believed that water levels could not be rising because no radioactive water was being transferred into the tank, and attributed the alarm to faulty instruments, such as the water level gauges, the utility said.
They did not check on the situation.
But they later concluded that highly radioactive water was leaking from the tank after detecting beta-ray levels of 50 millisieverts per hour and gamma-ray levels of 0.15 millisievert per hour from the water that had escaped.
They also found 230 million becquerels of beta-ray sources, including radioactive strontium, per liter of water in the rain gutter, TEPCO said.
The leak was apparently caused by open valves along a radioactive water conduit leading into the storage tank, a flange type that uses bolts to fasten steel panels. TEPCO officials said radioactive water likely entered the tank via the valves, filled it beyond capacity and escaped through openings in its top plate.
Workers closed the valves and transferred radioactive water to other tanks to lower the water level in the tank. They confirmed the leak stopped at 5:40 a.m. on Feb. 20.
TEPCO officials said workers were collecting the radioactive water that had escaped, adding that they will investigate why the valves were open.