The clean-up after the Fukushima nuclear disaster could cost five times more than estimated, figures have revealed, as Tokyo Electric Power said on Wednesday that steam had been seen again in a reactor building.

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 ($58 billion), far more than the 1 trillion yen the government has so far allocated.

The institute, in a report released Tuesday, 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.

"We hope the study will be helpful in drafting plans for decontamination of forests and farmland, as well as plans for residents to return to their homes," the institute said.

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.

"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 on Tuesday for a delay in admitting that radiation-polluted groundwater was flowing into the sea.

Earlier this month, the utility had reported spiking levels of possibly cancer-causing materials in soil from underneath the plant, but maintained that toxic groundwater was likely contained.

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 Tuesday 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".

On Wednesday, TEPCO said workers had noticed steam around the fifth floor of the building housing Reactor No. 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.

The firm has said there has been no increase in the amount of radioactive material being released, although it does not know where the steam is coming from.

TEPCO said it was looking at the possibility that accumulated rainwater had been the source.

The roof of the building was blown off in a hydrogen explosion after meltdowns in the days after the tsunami swept ashore.

Although the natural catastrophe is known to have killed more than 18,000 people, no one is officially recorded as having died as a direct result of the radiation released at Fukushima.

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TEPCO