According to Tepco test results released on Thursday, that water contains about up to 63 becquerels of tritium per litre, below the World Health Organisation drinking water limit of 10,000 becquerels per litre. A becquerel is a unit of radioactivity.
Tepco expects the process of releasing the wastewater – currently totalling more than 1.3 million metric tons – to take decades.
Civic groups have launched protests in Japan and South Korea, although South Korea’s government has said its own assessment found no problems with the scientific and technical aspects of the release.
The Fukushima Daiichi plant was destroyed in March 2011 after a massive 9.0 magnitude earthquake off the Japanese coast generated powerful tsunami waves that caused the meltdowns of three of its reactors.
The water release is a key step in a dauntingly long and difficult process of decommissioning the plant, including the removal of molten fuel.