The plant operator discharged 7800 tons of treated water in each of the first two batches and plans to release the same amount in the current batch through to November 20.
Tokyo Electric Power Company Holdings said its workers activated the first of the two pumps on Thursday to dilute the treated water with large amounts of seawater, gradually sending the mixture into the Pacific Ocean through an undersea tunnel for an offshore release.
The plant began the first wastewater release in August and will continue to do so for decades. About 1.33 million tons of radioactive wastewater is stored in about 1000 tanks at the plant. It has accumulated since the plant was crippled by the massive earthquake and tsunami that struck northeastern Japan in 2011.
The wastewater discharges have been strongly opposed by fishing groups and neighbouring countries including South Korea, where hundreds of people staged protests. China immediately banned all imports of Japanese seafood, badly hurting Japanese seafood producers and exporters.
Prime Minister Fumio Kishida told reporters on Thursday that Japan has consistently provided transparent and scientific explanations about the discharge and gained understanding from many members of the international community, but "some countries are restricting Japanese seafood without scientific bases".
"We must continue to patiently explain to those countries bilaterally to request lifting of the restrictions," Kishida said.
"And it is also important to firmly show Japan's position at international meetings" and bodies such as the World Trade Organisation.
The International Atomic Energy Agency, which has sent several missions that include Chinese scientists to Japan over the past two years, concluded in July that if the release is carried out as planned, it would have a negligible impact on the environment, marine life and human health. IAEA mission officials said last month they were reassured by the smooth operation so far.
So far, results of marine samplings by TEPCO and the government have detected tritium, which they say is inseparable by existing technology, at levels far smaller than the World Health Organisation's standard for drinking water.
The IAEA said on Thursday its own analysis of water samples by agency experts stationed at its Fukushima office found the tritium concentration in the water "far below the operational limit".
In a recent setback, two plant workers were splashed with radioactive waste while cleaning piping at the water treatment facility and were hospitalised for exposure. The workers have since been released and were getting monitored, TEPCO said. It said none of the workers ingested any of the waste.
