More than 1 million tonnes of contaminated water has accumulated at the plant since it was struck by a tsunami in March 2011, triggering a triple meltdown that forced the evacuation of tens of thousands of residents.

Tokyo Electric Power (Tepco) has struggled to deal with the buildup of groundwater, which becomes contaminated when it mixes with water used to prevent the three damaged reactor cores from melting.

Tepco has attempted to remove most radionuclides from the excess water, but the technology does not exist to rid the water of tritium, a radioactive isotope of hydrogen. Coastal nuclear plants commonly dump water that contains tritium into the ocean. It occurs in minute amounts in nature.