Tokyo Electric Power Co. said it dumped about 1,130 tons of tainted rainwater Monday into the Pacific Ocean after it accumulated at the Fukushima No. 1 nuclear plant due to a typhoon.
The radioactivity of the rainwater, which had built up within circular barriers installed around makeshift storage tanks holding water tainted during emergency cooling operations, was considered low enough to be able to be released into the sea, the utility said Tuesday.
Tepco said an estimated 8.8 million becquerels of radioactive substances emitting beta rays, such as strontium-90, were released.
The barriers were built to prevent water, possibly containing radioactive substances, that is leaking from the tanks from spreading further outside. Then rainfall from Typhoon Man-yi, which hit Honshu on Monday, generated fears that the water inside the barriers could overflow.
To deal with the situation, Tepco released the accumulated rainwater by opening some of the valves attached to the barriers.
The water that was discharged contained a maximum 24 becquerels per liter of radioactive substances emitting beta rays, lower than the allowable level of 30 becquerels.
As for rainwater with a radiation level confirmed to be close to the allowable limit or even higher, Tepco said it decided to store it in tanks instead of discharging it.
A huge number of tanks are being used at the plant to store toxic water, which is increasing daily as a result of continuing water injections into the three reactors where meltdowns occurred.
Prime Minister Shinzo Abe is scheduled to visit the plant on Thursday to check the progress of measures to tackle the contaminated water.