TOKYO - Some of the rainwater overflowed from concrete barriers around clusters of tanks on Sunday in Fukushima Daiichi nuclear power plant was found to have radiation levels above the regulatory limit, Tokyo Electric Power Co said Monday.  

According to Kyodo News, strontium-90 in water that had accumulated was above the limit of 10 becquerels per liter around six clusters of tanks due to heavy rainfall. The data in one area even reached 710 becquerels.

According to TEPCO, some of the toxic water is unlikely to have flown into the Pacific Ocean with the help of mounds created outside the barriers, though some of the water has seeped into the ground.

The barriers, about 30 centimeters high, have been created to block water from spreading outside when a leak occurs in the tanks, which hold highly radioactive water. Such barriers exist around each of the 23 clusters of tanks.

When rainwater accumulates inside, TEPCO transfers it to other containers and checks its radiation level before discharging it. But the rainfall on Sunday was so heavy that water overflowed from the barriers.

TEPCO said on Sunday that water near a total of 12 clusters had spilled from the barriers, but it corrected the number to 11 on Monday.

Japanese Nuclear Regulation Authority has told TEPCO it will authorize the Fukushima plant to discharge water from within the barriers only if radiation readings are below 10 becquerels per liter for strontium-90. While the legal limit for this kind of elements into the sea outside the plant is set at 30 becquerels per liter.    

Strontium tends to accumulate in bones and is feared to cause bone cancer and leukemia.