Vapour has begun rising again from a reactor at the Fukushima nuclear power plant, more than two-and-a-half years after its core melted down, the site's Japanese operator said Friday.
Tokyo Electric Power said it believed the steam was coming from a puddle sitting atop the reactor, but has not been able to clarify why vapour started appearing occasionally from July this year.
TEPCO said the Fukushima Daiichi nuclear plant remained stable, with sensors in and around it showing no increase in levels of radioactive release.
The steam appeared as TEPCO found more evidence that radioactive waste water at the plant was contaminating groundwater that is on its way to the sea.
Thousands of tonnes of water were poured on the reactors to tame the meltdowns sparked by the March 2011 earthquake and tsunami. TEPCO says their temperature is now stable but they need to be kept cool to prevent them running out of control again.
The continuing nuclear catastrophe at Fukushima has come under the international spotlight in recent weeks as Tokyo fought off challenges from Madrid and Istanbul for the right to host the 2020 Games.
Speaking to Olympic chiefs in Buenos Aires ahead of their decision to award the Games to Tokyo, Prime Minister Shinzo Abe said the situation at Fukushima was "under control".
Thousands of tonnes of radioactive water are being stored in temporary tanks at Fukushima. Much of it has been used to cool molten reactors at the plant.
TEPCO and Japanese officials are considering releasing some of it into the Pacific Ocean after filtering out radioactive materials, but face opposition from fisherman and neighbouring countries.
The Japanese government has decided to spend $470 million on a scheme to freeze the soil around the reactors to form an impenetrable wall of ice they hope will direct groundwater away from them.
Explore further: Fossil fuels at what price? EU analysis looks at energy costs