Tepco on Monday discharged into the ocean filtered groundwater taken from wells around the damaged reactor buildings at the Fukushima No. 1 nuclear plant in an effort to curb the buildup of toxic water.
The project has been touted as one of Tokyo Electric Power Co.'s key measures in tackling the contaminated water problem.
Some 300 tons of untainted groundwater seeps into the buildings each day, where it mixes with water made radioactive by keeping the damaged reactors cool.
By pumping up groundwater through 41 wells and discharging it into the sea after treatment, the government and Tokyo Electric Power Co. hope to halve the amount flowing into the reactor buildings.
On Monday, Tepco released some 850 tons of filtered groundwater — part of some 4,000 tons pumped up last year on a trial basis and stored in tanks — after confirming that radiation levels were below measurable limits.
Tritium, which cannot be removed with existing technology, measured 330 to 600 becquerels per liter, well below the legally allowable limit of 1,500 becquerels, the utility said, citing analyses conducted by the company and an outside organization.
Fishermen in Fukushima Prefecture had long opposed releasing the water over concerns it would pollute the ocean and contaminate marine life, but signed off on the plan in August.
In exchange, the fishermen demanded among other things that Tepco and the government continue paying compensation for as long as the nuclear plant damages their business.
Tepco is running behind schedule on a project to build a huge underground ice wall at the site, another key measure to prevent groundwater from reaching the reactor building basements.