A day after the decision the South Korean foreign ministry called in Japan’s top envoy to Seoul on Tuesday.
Second Vice Foreign Minister Choi Jong-moon met with Japanese Ambassador Koichi Aiboshi, according to officials.
Choi was expected to express regrets and lodge a protest during the meeting.
The decision was taken in a meeting of Prime Minister Yoshihide Suga’s cabinet.
Suga said in the meeting that getting rid of the water was an “inevitable task” in the decades-long exercise of decommissioning the nuclear plant.
He added that the release will take place only “after ensuring the safety levels of the water” and alongside measures to “prevent reputational damage.”
The operator Tokyo Electric Power Company (TEPCO) - which has more than 1,000 tanks at the site - is tasked with discharging the water.
TEPCO had claimed that the space for tanks would run out by 2022, although some officials and experts disagree.
Locals, neigboring nations concerned
The decision comes a decade after the reactor at the Fukushima Daiichi Nuclear Power Station was wrecked in a meltdown following a tsunami in 2011.
According to government officials and TEPCO, tritium - which doesn’t cause harm in small amounts - cannot be removed from the water, but all other selected radionuclides can be brought down to a level suitable for release.
Some scientists have pointed out that the long-term effects on marine life from low-dose exposure to such large amounts of material is not yet known.
There has been strong local opposition to the move, and neighboring countries such as China have also expressed their concerns.
“China has expressed grave concern to the Japanese side through the diplomatic channel, urging Japan to handle the issue of wastewater disposal from the Fukushima nuclear power plant in a prudent and responsible manner,” Beijing’s Foreign Ministry spokesperson Zhao Lijian said on Monday.
South Korea’s Yonhap News agency quoted the country’s foreign ministry official Choi Young Sam, saying that Japan’s decision “can have direct and indirect impact on the safety of our people and the surrounding environment.”