This handout illustration taken on February 7, 2024 and released by Tokyo Electric Power Company (TEPCO) on February 8 shows two photos - one the location of a contaminated water leak (L) - and a second image (R) showing a close-up of the vent (R) where the water was released, at part of the Fukushima Daiichi nuclear power plant that processes contaminated water in Okuma, Fukushima prefecture. (Photo by Handout/Tokyo Electric Power Company (TEPCO)/AFP)

TOKYO, Japan -- An estimated 5,500 liters of radioactive water leaked from Japan's stricken Fukushima nuclear plant but no sign of contamination has been detected outside the facility, its operator said Thursday.

A spokeswoman from Tokyo Electric Power Co (TEPCO) told AFP that the leak was detected at part of the plant that processes contaminated water.

"We estimate that roughly 5.5 tons (5,500 liters) of water leaked" on Wednesday morning, but "there have been no significant changes" at posts monitoring radioactivity around the power station, she said.

Even so, TEPCO plans to remove soil from around the area that may have been contaminated, the spokeswoman said, without providing specific details on the location of the leaked water.

The Fukushima plant was wrecked by a huge earthquake and tsunami in 2011 that killed 18,000 people. It was one of the worst nuclear disasters in history.

The clean-up operation is expected to take decades, with the most dangerous part -- removing radioactive fuel and rubble from three stricken reactors -- yet to begin.

In August, Japan began gradually releasing into the Pacific Ocean 1.34 million tons of treated wastewater that has collected since the catastrophe, saying it is harmless and heavily diluted with seawater.

This view is backed by the United Nations atomic watchdog, but China and Russia have criticized the release and banned Japanese seafood imports.

Wednesday's leak took place at a facility which processes the water before most radioactive elements are filtered out at a different, advanced facility known as ALPS.

TEPCO said the leak from a vent was spotted by a worker who was cleaning the vent before operating the facility.

"Vents should be closed during cleaning, but this time they were open," the spokeswoman said.

In a separate incident, smoke and sparks were detected on Thursday at a fan of Tsuruga nuclear power plant, which is in the process of decommissioning.

"The situation was resolved immediately with no injury or leak of radioactive materials," a spokesman of the operator Japan Atomic Power told AFP.