Workers at the crippled Fukushima Daiichi nuclear power plant have finished sending seawater into an underwater tunnel that has been built to release treated and diluted water from the facility into the ocean.

The plant's operator, Tokyo Electric Power Company, started the process on Monday afternoon. This came after work to dig the underwater structure was completed in April.

Once filled with seawater, the tunnel will guide treated water from the plant to a point about 1 kilometer offshore.

TEPCO says the tunnel has been filled with some 6,000 tons of seawater.

The water release system is almost complete, except for a reservoir that will store treated water before its release. The utility plans to complete all construction work by the end of this month.

The project still faces opposition from local fishery workers who are worried the plan could hurt the reputation of their seafood.

TEPCO has built the tunnel in an effort to address ever-increasing amounts of water on the premises of the plant.

Water used to cool molten fuel at the facility mixes with rain and groundwater. Accumulated water is treated to remove most radioactive materials and stored in tanks on the plant's premises.

The government plans to dilute the filtered water to bring the concentration of tritium well below the percentage permitted by national regulations.

The amount of tritium in the diluted water is also expected to be below World Health Organization guidance levels for drinking water quality.