The water, equivalent to about 500 Olympic-sized swimming pools, has been treated but needs to be filtered again to remove harmful isotopes. It will also be diluted to meet international standards before any release into the ocean.

In a statement, China’s foreign ministry called the move “extremely irresponsible” and said it reserved the right to take further action.

The water currently contains significant amounts of harmful isotopes despite years of treatment, according to TEPCO. The company plans further filtration to leave only tritium, a radioactive isotope of hydrogen that is hard to separate from water.

Five special rapporteurs from the United Nations said in March that the contaminated water remained a risk and that the ocean discharge plan could not be an “acceptable solution”.