The operator of Japan’s stricken Fukushima nuclear power plant said Saturday it had found new radiation hotspots near tanks storing toxic water, with one reading peaking at 1,800 millisieverts per hour — a potentially lethal dose.

Tokyo Electric Power Co (TEPCO) detected high radiation readings at four sites around the tanks although none of the containers showed any visible fall in their water levels, according to a statement reported by two Japanese news agencies.

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Last week the plant operator admitted 300 tonnes of toxic water had seeped out of one of the vast containers — one of around 1,000 on the site — before anyone had noticed.

The spill, which sparked fears the toxic water may have seeped into the nearby ocean, was categorised as a Level 3 event, making it the single most serious incident since three reactors went into meltdown after being swamped by a 2011 earthquake-sparked tsunami.

The latest radiation hotspots were discovered during daily inspections on Saturday at three tanks and one pipe connecting the containers to the crippled plant, the Kyodo news agency reported.

Although it was unclear if the hotspots indicated a fresh spill of toxic water, traces of water leak measuring 230 millisieverts per hour were found below the pipe, the agency added.

At the time of last week’s discovered leak the plant operator said the radioactivity of the puddles was around 100 millisieverts per hour.

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Jiji news agency said the highest reading of 1,800 millisieverts per hour was found at one of the tanks, adding that exposure to that level for four hours would be fatal to humans. The other readings measured between 70 and 230 millisieverts, the agency added.

A TEPCO official said the operator could not rule out the possibility of new leaks of radioactive water at the four sites, the agency reported, adding that the operator had not noticed a decline in water levels inside the tanks.

Both agencies said that the new radiation hotspots were coming from different tanks to the one that was discovered leaking water earlier this month.

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TEPCO has long struggled to deal with growing volume of now contaminated water it has used to cool the broken reactors.

It said last week that some of the 300 tonnes that leaked from the tank could have made its way through drainage systems into the Pacific Ocean.

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The pools of water near the holed tank were so toxic, the operator said, that anyone exposed to them would receive the same amount of radiation in an hour that a nuclear plant worker in Japan is allowed to receive in five years.

That came on top of the admission that groundwater contaminated by water from the plant was flowing into the sea at a rate of 300 tonnes a day, taking its low-level radioactive load with it.

In response to growing domestic and international criticism over TEPCO’s handling of the crisis, Japanese Prime Minister Shinzo Abe on Thursday promised the world his government would play a greater role in stopping leaks of highly radioactive water.

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“The accident in Fukushima cannot be left entirely to Tokyo Electric Power. There is a need for the government to play a role with a sense of urgency, including taking measures to deal with the waste water,” he said at the time.

Abe’s pledge came as the world’s nuclear watchdog urged Japan to explain more clearly what is happening at Fukushima and avoid sending “confusing messages” about the disaster.

The International Atomic Energy Agency questioned why the leak last week of 300 tonnes of highly radioactive water merited a rating on its International Nuclear Event Scale (INES), when no other incident since the March 2011 meltdowns had.

[Image via Agence France-Presse]