A partial power outage at the damaged Fukushima Daiichi nuclear power plant in northeastern Japan stopped the release of treated and diluted water into the ocean for about six and a half hours.

Plant operator Tokyo Electric Power Company says one of the systems supplying electricity to the plant stopped at 10:43 a.m. on Wednesday.

The utility says the safety of critical areas of the plant has been ensured by sourcing power from other lines. This includes cooling of spent nuclear fuel and nuclear fuel debris in damaged reactors.

The company also says there have been no irregularities in data from radiation monitoring posts.

Power was restored by shortly after 4:00 p.m.

But the loss of power led to an automatic stoppage of the discharge of treated and diluted water from the plant into the sea until 5:16 p.m.

In addition, a building where officials monitor decommissioning work experienced a blackout shortly after the trouble, and again for about 20 minutes a few hours later.

The Fukushima Daiichi plant suffered a triple meltdown in the 2011 earthquake and tsunami.

Water used to cool molten fuel has been mixing with rain and groundwater. The accumulated water is treated to remove most radioactive substances, but still contains tritium. The treated water is being stored in tanks on the plant's premises.

Before releasing the treated water into the sea, Tokyo Electric dilutes it to reduce the tritium levels to about one-seventh of the World Health Organization's guidance level for drinking water.

The utility began the fifth discharge round on Friday.

Tokyo Electric says that at around the time of the power outage, a worker who was drilling near a power cable was taken to hospital with burns to the face and right arm. The worker's life is not in danger.

The company says the power cable may have been accidentally damaged during the drilling work.