Four workers suffered internal radiation exposure due to inhalation of a large amount of plutonium during an inspection at a nuclear research facility in Ibaraki Prefecture on Tuesday, the operator of the facility said Wednesday.

In the wake of what appears to be an unprecedented internal radiation exposure accident, the state’s nuclear safety regulator and local labor authorities inspected the scene to see if there were any flaws in safety management.

The accident occurred at the fuel research building of the Japan Atomic Energy Agency’s Oarai Research & Development Center when a bag covering a container for nuclear fuel materials, including powder samples of plutonium and uranium, tore during inspection on Tuesday.

Up to 22,000 becquerels of plutonium 239 were detected in the lungs of a male worker in his 50s. Up to 14,000 becquerels of radioactive materials were found in the three other workers, officials of the Japan Atomic Energy Agency said.

The Nuclear Regulation Authority said the worker with the higher reading has been exposed to an extreme amount of radiation and the situation is considered grave.

While none of the workers have complained of health problems so far, an official with the facility’s operator said it “cannot rule out the possibility of future health effects.”

The agency assumes that the amount of radiation exposure of the male worker in his 50s translates to up to 12 sieverts over 50 years, well above the legal limit set for workers who deal with radiation.

For its part, the labor office said that it estimates the man with the highest exposure to radiation has exceeded the annual limit of radiation exposure, which is 50 millisieverts a year and 100 millisieverts in five years.

Plutonium is known to emit alpha rays over a long period, damaging surrounding organs and tissues. If it is deposited into the lungs, it could increase the risk of developing cancer

The five workers have been transported to the National Institute of Radiological Sciences and given medication to help discharge radioactive materials from their bodies.

“I saw such a (high) figure for the first time,” said Makoto Akashi, a senior official at the National Institutes for Quantum and Radiological Science and Technology, referring to the reading of 22,000 becquerels. The institute oversees the National Institute of Radiological Sciences.

“It is very clear from a scientific viewpoint that the internal exposure to radiation would increase the risk of cancer (for the workers),” Akashi said.

“I have never heard of such a large amount as a reading for internal exposure to radiation,” Shunichi Tanaka, who heads the NRA, told a separate news conference.

The workers wore masks but could have inhaled radioactive material from the small gaps between the masks and their faces.

Kunikazu Noguchi, an expert on radiological protection and associate professor at Nihon University, said it is hard to conclude the impact of the 22,000 becquerels, as the actual amount of radioactive substances he inhaled is still unknown.

“It is possible, however, that the worker could have been exposed to more radioactive materials than the legally allowable maximum limit,” Noguchi said. He said it is necessary to get to the bottom of the incident, especially whether workers followed guidelines, as it is hard to imagine a plastic bag containing nuclear substances could tear in such a facility as the Oarai center.