OARAI, Ibaraki Prefecture--Five workers were exposed to radiation on June 6 during an inspection at a nuclear energy research center here, including one with 22,000 becquerels of plutonium in his lungs, the facility’s operator said.

Radioactive substances were found in the lungs of four of the workers.

So far, none of the workers has complained of health problems, according to the Japan Atomic Energy Agency, the often-criticized operator of the Oarai Research and Development Center.

A rough calculation of the 22,000-becquerel figure translates to internal radiation exposure of 1.2 sieverts over one year and 12 sieverts over 50 years. There have been no past examples of such a high level of internal radiation exposure.

However, an agency official said about the worker in his 50s, “The amount is not enough to cause acute radiation damage.”

The incident was reported at a regularly scheduled meeting of the Nuclear Regulation Authority held on June 7.

Nobuhiko Ban, an NRA commissioner who is a specialist in radiological protection, said at the meeting: “Detection of 22,000 becquerels is a situation that cannot be easily brushed aside. It is no small amount, although it may not be life-threatening. There will be a need to confirm if the work procedures were appropriate.”

The agency said no radioactive materials leaked outside of the center.

According to agency officials, the accident occurred at 11:15 a.m. in an analysis room of the fuel research building where studies are conducted on new forms of nuclear fuel for use in fast reactors.

After experiments are completed, the nuclear fuel, consisting of uranium and plutonium, is placed in a container that is then wrapped twice in plastic. The wrapped container is then placed in a metallic cylinder with a diameter of about 10 centimeters.

The accident occurred when the workers tried to open the metallic container to inspect the storage conditions. The plastic wrapping was ripped, and radioactive materials were spewed into the air. Officials believe a change in pressure may have caused the accident.

According to agency officials, the work was not carried out in a tightly sealed environment because the workers never assumed the plastic might rip. Instead, the work was conducted in a box with a partial opening.

The protective clothing and gloves worn by the five employees were contaminated. Although the workers all wore face masks that covered their mouths and noses, exposure of up to 24 becqerels apparently from plutonium was confirmed in the nasal cavities of three of them.

The five workers were injected with a substance that promotes the discharge of radioactive materials from the body and were transported to the National Institutes for Quantum and Radiological Science and Technology in Chiba Prefecture on June 7.

The NRA has instructed local safety inspectors to look over the accident site.

The NRA has severely criticized the agency in the past for shoddy management.

The agency was described as being unfit to safely operate the Monju prototype fast breeder-reactor in Fukui Prefecture.

It was also found to have handled radioactive waste in a slipshod manner at its Tokai reprocessing facility.