Digging of an undersea tunnel was also due to begin later on Thursday.
The construction of the Fukushima Daiichi plant follows formal approval by the Nuclear Regulatory Authority last month of a detailed wastewater discharge plan that TEPCO submitted in December.
The government last year announced a decision to discharge the sewage as a necessary step for the ongoing dismantling of the plant.
A massive earthquake and tsunami in 2011 destroyed the cooling systems at the Fukushima Daiichi power plant, causing a triple meltdown and releasing large amounts of radiation. The water that was used to cool the three damaged and highly radioactive reactor cores has since drained into the basements of the reactor buildings, but has been collected and stored in tanks.
TEPCO and government officials say the water will still be treated to levels well below clearance standards and the environmental and health impacts will be negligible. Of more than 60 isotopes selected for processing, all but one – tritium – will be reduced to meet safety standards, they say.
Local fishing communities and neighboring countries have raised concerns about potential health risks from radioactive sewage and reputational damage to local products, and oppose the release.
The scientists say the impact of long-term, low-dose exposure to not only tritium but also other isotopes on the environment and humans is still unknown and a release is premature.
The contaminated water is stored in approximately 1,000 tanks which require a lot of space in the plant complex. Officials say they must be removed so that facilities can be built for its dismantling. The reservoirs are expected to reach their capacity of 1.37 million tonnes in the fall of 2023.
TEPCO said it plans to transport the treated, releasable water through a pipeline from the reservoirs to an inshore pool, where it will be diluted with seawater and then sent through an underwater tunnel with a exit approximately 1 kilometer (0.6 miles) to minimize impact. on local fishing and the environment.
TEPCO and the government have won approval from the heads of the plant’s host towns, Futaba and Okuma, for the construction, but local residents and the fishing community remain opposed and could further delay the process. The current plan calls for a gradual release of treated water to begin next spring in a process that will take decades.
TEPCO said Wednesday that weather and sea conditions could delay completion of the installation until summer 2023.
Japan has requested assistance from the International Atomic Energy Agency to ensure that the water release meets international safety standards and reassures local fisheries and other neighboring communities and countries, including the China and South Korea, which opposed the plan.
IAEA experts who visited the plant earlier this year said Japan was taking appropriate action for the expected release.
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