Japan's Atomic Energy Agency said Wednesday that five workers at a nuclear facility that handles plutonium have been exposed to high levels of radiation after a bag containing highly radioactive material apparently broke during equipment inspection. The state-run agency said the incident occurred Tuesday at its Oarai Research and Development Center, a facility for nuclear fuel study that uses highly toxic plutonium. The cause of the accident is under investigation. The mishap poses a major nuclear security concern as well as a question as to whether the handlers and their health were adequately protected at the facility. The agency said its initial survey found contamination inside the nostrils of three of the five men - a sign they inhaled radioactive dust. All five were also found to be contaminated on their hands and feet, but the radioactive material was likely to have been removed by taking off their gloves, shoe covers and other protective gear, and by taking a shower. Agency spokesman Masataka Tanimoto said one of the men's survey indicated high levels of plutonium exposure in his lungs, with the dose showing nearly 1,000 times that of his earlier nostril survey. The figure, 22,000 Becquerels, could mean his exposure levels in the lungs are not immediately life-threatening, but are well above an average annual dose limit for nuclear workers. The workers did not have any visible signs of health problems, Tanimoto said. They were taken to a special radiation medial institute for further health checks. Japan's possession of large numbers of plutonium stockpiles, resulting from the country's struggling nuclear fuel reprocessing program, has already faced international criticism. Critics say Japan should abandon its spent fuel recycling ambitions because nuclear plant startups are still coming slowly amid persistent anti-nuclear sentiment since the 2011 Fukushima crisis.