Tokyo Electric Power Co. (TEPCO), which operates the nuclear plant northeast of the capital, began disposing of 10,000 tonnes of water containing low-level radioactive substances in the Pacific Ocean from the nuclear power plant.
The dumping of the water that was about 100 times more radioactive than legal limits will help make room to store more highly polluted water filling the No. 2 reactor turbine building as it was hampering the plant's restoration work, TEPCO said.
"There is a need to release already stored water in order to accept the additional waste water," officials said.
Japan's Chief Cabinet Secretary Yukio Edano said it was the only available option. "So as to prioritise to stop the leakage of this water into the sea... we will release the water stored in the exterior building of the unit, which also unfortunately contains radioactivity but far lower than the highly contaminated water," he said.
The government said it poses no risk to human health.
Separately, the company also said it plans to release 1,500 tonnes of groundwater, also containing radioactive materials, near the No. 5 and No. 6 reactors. The plant operator poured 13 kilograms of milky white dye into an underground trench to find the source from where radioactive water is leaking into the Pacific Ocean, according to Kyodo news agency.
The move came after workers' effort to block the leakage from a cracked seaside pit connected to the No. 2 reactor turbine building showed no effect, it said.
Edano said the leak must be stopped "as soon as possible". "We must prevent radioactive water from spreading in the sea as soon as possible," he said, adding the longer the leakage continued, the larger the impact on the sea would be, even if radioactive materials were diluted.
The workers have been struggling for more than three weeks to regain control of the nuclear plant after the 9.0- magnitude earthquake and tsunami on 11 March killed 12,157 people and nearly 15,500 still missing.
The TEPCO is also considering installing "silt fence" barriers in areas where radioactive water is suspected to be flowing into the sea, Hidehiko Nishiyama, a spokesman for the government''s nuclear safety agency told reporters.
"We would like to set up these fences as soon as possible," he was quoted as saying by the Kyodo. He said it would likely take "several days" to complete the work.