Highly radioactive water has leaked from the crippled Fukushima nuclear plant in Japan after unexpectedly heavy rain on Sunday, its operator said.
Water with high levels of the toxic isotope Strontium-90 overflowed containment barriers around water tanks, operator Tepco said.
The tanks are being used to store contaminated cooling water from the plant's damaged reactors.
Fukushima has been hit by a series of toxic water leaks in recent months.
The plant was badly damaged by the earthquake and tsunami of March 2011. Cooling systems for reactors were knocked out, causing meltdowns at three of them.
- 21 Oct: Radioactive water overflows a containment barrier after heavy rain
- 7 Oct A plant worker accidentally switches off power to pumps used for cooling damaged reactors
- 3 Oct Tepco says there is a radioactive water leak after workers overfill a storage tank
- 21 Aug Japan's nuclear agency upgrades Fukushima alert level
- 20 Aug Tepco says 300 tonnes of radioactive water has leaked from a storage tank into the ground
- July Tepco for the first time admits radioactive water is going into the sea
- June Tepco says radioactive water leaking from a storage tank to the ground
- April Tepco suspects a fresh radioactive water leak at Fukushima
- March Tepco suspects a rodent may have been behind a power cut that shut down cooling systems
- Dec 2011 Contaminated water leaks from a treatment system, caused by a crack in the foundation
Water is being pumped in to cool the reactors. However, this creates large amounts of contaminated water that must be stored securely.
Some of the water has leaked from the tanks, pipes and damaged structures, leading to concerns contaminated water is mixing with groundwater that is flowing into the sea.
This time, the rainwater run-off - which becomes contaminated when it hits polluted surfaces areas - overflowed concrete barriers surrounding a group of tanks, reports citing the Tokyo Electric Power Company (Tepco) say.
Tepco says that in one area, readings of Strontium-90 were at 70 times the legal limit for safe disposal.
"Our pumps could not keep up with the rainwater. As a result, it flowed over some containment areas," Tepco spokesman, Yoshikazu Nagai, told Reuters news agency.
The company was quoted by Kyodo news agency as saying that some of the toxic water had seeped into the ground, but that it was unlikely to have flowed into the sea.
Strontium-90 is a radioactive by-product of the fission of uranium and plutonium inside a nuclear reactor. It is easily absorbed by the human body and is thought to be a cause of bone cancer.
More rains are expected in Japan, with meteorologists forecasting a typhoon to make landfall this week.
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