Strontium-90 was detected in groundwater near the plant at levels 30 times above the government safety standard, officials said Wednesday.
Strontium-90 is a by-product of the fission of uranium and plutonium in nuclear reactors as well as nuclear weapons. People exposed to it are at a greater risk of developing cancer.
High levels of tritium, a less harmful substance, had also been found, Toshihiko Fukuda, a general manager at Tokyo Electric Power Co. (TEPCO), told a news conference.
They suspect the radioactive material was released amid the meltdowns that rocked the plant following the March 2011 earthquake and tsunami, and has been circulating in groundwater. All but two of Japan’s 50 nuclear reactors have been shut down since the disaster, pending a review of safety procedures.
TEPCO officials say it's unlikely the contaminants are leaking into the ocean because there have been no major changes in radiation levels in nearby seawater.
The company has constantly revised announcements about radiation levels and other problems at the plant since the disaster, however. Earlier this month it reversed a claim that the groundwater flowing into the damaged basements of reactor buildings was not contaminated.
"TEPCO needs to carry out more regular testing in specific areas and disclose everything they find," said Michiaki Furukawa, a nuclear chemist and professor emeritus at Nagoya University.
Hundreds of tonnes of contaminated water accumulates everyday at the Fukushima Daiichi nuclear complex, most of which contains radioactive cesium.
Japan's nuclear watchdog on Wednesday formally approved new safety requirements for atomic plants, paving the way for the reopening of facilities shut down since the Fukushima disaster.
The new requirements by the Nuclear Regulation Authority will take effect on July 8, when operators will be able to apply for inspections. If plants pass inspection, they can reopen.
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All but two of Japan's 50 reactors have been offline since the 2011 earthquake and tsunami led to multiple meltdowns and massive radiation leaks at the Fukushima plant.
The new requirements for the first time make it compulsory that plants take steps to guard against radiation leaks in the case of severe accidents, install emergency command centres and enact anti-terrorist measures.
Safety was previously left up to the operators.