Rainwater that overflowed Sunday from the concrete-ringed enclosures around the water storage tanks at the Fukushima No. 1 nuclear plant had excessively high radiation readings, Tokyo Electric Power Co. disclosed Monday.
Strontium-90 in the rainwater, which had accumulated from recent downpours, was above the limit of 10 becquerels per liter near six tank clusters, with the reading in one area reaching 710 becquerels, Tepco said.
It said some of the radioactive water has seeped into the ground, but noted that most of the affected surface water probably didn’t flow to the Pacific Ocean because barrier mounds have been built outside the flood enclosures to prevent water from entering the drainage channels that lead to the sea.
Yet Tepco couldn’t totally rule out that surface water had reached the sea.
The concrete flood fences, about 30 cm high, were built to keep water from spreading if a tank leaks. There are 23 enclosed tank clusters. The tanks store highly radioactive water that was used to cool the melted cores of the three crippled reactors.
When rainwater accumulates in an enclosure, Tepco transfers it to other containers and checks the radiation level before discharging it. But Sunday’s rainfall was so heavy that it overflowed.
The rainwater that day overflowed the fences of 11 of the tank enclosures, Tepco said.
Simultaneous overflows had never taken place at so many clusters before.
Tepco also found water leaks from a concrete joint in a barrier at another tank cluster where excessively radioactive substances were found in the past. As a result, tainted water in such barriers has leaked from over half of the plant’s 23 tank enclosures.
In the past, radioactive materials in excess of the provisional limits set by Tepco have been detected in water in some of the tank cluster enclosures.
The overflows and leaks are the latest in a series of the Japanese government’s water problems at the plant.
On Sunday, the utility started draining water from inside the barriers at six tank areas, including five of the 11 overflowed areas, after confirming that radiation levels had fallen below the provisional limits.
At two other tank areas, Tepco transferred the enclosure water to sunken reservoirs. Although the utility stopped using the reservoirs to store the highly radioactive coolant after discovering in April that they were leaking, it took the emergency step this time because of the looming rainfall threat when Typhoon Wipha approached last week.
The total amount of water that overflowed and leaked Sunday is not known, the company said.
In one location among the 11 tank areas, a maximum of 29,000 becquerels per liter of strontium and other radioactive materials emitting beta-ray particles were detected in the past.
Tepco’s provisional limit on radiation is 10 becquerels per liter for strontium-90, a substance linked to bone cancer that is also believed to account for about half of beta ray-emitting radioactive materials. Airborne radioactive materials have fallen into the enclosures, while tainted tank water is thought to have leaked into some of the areas.