It is the third time steam has been observed in the battered structure over the last week.
The government-backed National Institute of Advanced Industrial Science and Technology said decontamination work in Fukushima prefecture will cost up to 5.81 trillion yen, far more than the 1 trillion yen the government has so far allocated.
The institute, in a report released yesterday, said the costs -- including for transportation and storage of radiation-contaminated soil over a large area -- would be in a range between 3.13 trillion yen and 5.81 trillion yen.
The study calculated costs for several decontamination models, including one under which surface soil on farmland is removed and stored elsewhere, and another that would only see that soil turned over.
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"It's important to examine the effects of several decontamination scenarios" as the ratio of evacuees who plan to return depends on the level of radiation after decontamination work, it said.
As the report was released, government officials scolded TEPCO yesterday for a delay in admitting that radiation-polluted groundwater was flowing into the sea.
On Monday it admitted its own study, completed days earlier, revealed the groundwater was leaking into the ocean, prompting criticism over the delay.
Trade minister Toshimitsu Motegi told reporters yesterday the slow release of data by TEPCO was "extremely deplorable", while Chief Cabinet Secretary Yoshihide Suga said: "This kind of data should be disclosed quickly".
Today, TEPCO said workers had noticed steam around the fifth floor of the building housing Reactor Number 3, which was wrecked by the tsunami of March 2011. It was the second time in two days and the third time in a week that steam had been observed.