Japan's Atomic Energy Agency says five workers at a nuclear facility that handles plutonium have been exposed to high levels of radiation after a bag containing highly radioactive material broke during equipment inspection.
Up to 22,000 becquerels of radioactive materials were detected in the lungs of one of the workers, JAEA said.
The accident occurred at the Oarai Research and Development Center outside Tokyo where the men were researching improving nuclear fuel. Three of the men's nostrils were also contaminated, indicating they had inhaled toxic dust.
Officials said the five staff have not yet complained of health problems with one assuring that "the amount is not enough to cause acute radiation damage", according to the Japanese newspaper The Asahi Shimbun. It raised nuclear security concerns as well as questions about whether the workers were adequately protected. The figure, 22,000 Becquerels, could mean exposure levels in the lungs may not be immediately life-threatening.
Yoshihide Suga, Japan's chief cabinet secretary, said in a news conference that he had been told the exposure would not pose an immediate risk to the workers' health.
Shunichi Tanaka, chairman of the Nuclear Regulation Authority (NRA), which has frequently criticized the JAEA for the conditions at its facilities, said "workplace complacency" was possibly to blame. They were taken to a special radiation medial institute for further health checks.
The NRA said the workers had never experienced a similar plastic rip before, and as a result, did not feel the need to complete their research in a tightly sealed environment.
A suspected case of plutonium inhalation occurs at Japan Nuclear Fuel's reprocessing plant in the village of Rokkasho, Aomori Prefecture, but a check for internal exposure turns out negative.
Japan has remained on edge over radiation since the 2011 Fukushima crisis.
To reduce the stockpile, Japan plans to burn plutonium in the form of MOX fuel - mixture of plutonium and uranium - in conventional reactors. But nuclear plant startups are still coming slowly amid persistent anti-nuclear sentiment since the 2011 Fukushima nuclear reactor meltdown in the wake of an natural disaster and tsunami.
Since then, just a handful of reactors have come back online due to public opposition and as legal cases work their way through the courts.