Tainted rainwater was discharged into the ocean Monday to prevent the damaged Fukushima No. 1 power plant from being flooded by the passage of Typhoon Man-yi, Tokyo Electric Power Co. said.
The radioactivity of the rainwater, which had accumulated within circular barriers around makeshift storage tanks holding water tainted by emergency cooling operations, was low enough to release into the sea, the beleaguered utility said.
Tepco said it decided to discharge the rainwater, tainted by strontium-90, because it was threatening to spill over into the rest of the complex. So it opened the barriers and released the water from the storage area, mainly through rainwater ditches, into the sea.
The barriers are designed to contain leaks from the more than 1,000 hastily built storage tanks at the plant, which suffered three core meltdowns after the March 11, 2011, earthquake and tsunami.
The utility said the rainwater was discharged from barriers at seven locations and contained strontium 90, which can cause bone cancer if ingested. The beta radiation given off by the strontium did not exceed the government's limit of 30 becquerels per liter, it said.
Tepco said the radiation level of the water overall, including strontium 90, which accounted for about half of the beta ray emissions, maxed out at 24 becquerels per liter.
In areas where water samples were highly toxic, however, Tepco took a different approach and transferred it elsewhere through makeshift pumps. One of those areas contained rainwater that was emitting 170,000 becquerels per liter, far higher than allowed.