Four workers have suffered internal radiation exposure, with one found with up to 22,000 becquerels of plutonium in his lungs, following an inspection accident at a nuclear research facility in Ibaraki Prefecture on Tuesday, the operator of the facility said Wednesday.

In one of the worst accidents involving internal radiation exposure in Japan, up to 5,600 to 14,000 becquerels of plutonium 239 have been detected from the other three workers, the Japan Atomic Energy Agency said.

Another worker who was engulfed in the accident Tuesday with the other four is also highly likely to have been internally exposed, agency officials said.

The accident occurred at the fuel research building of the 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.

A labor standards inspection office in Ibaraki conducted an inspection Tuesday and Wednesday at the building, while the Nuclear Regulation Authority, the nuclear safety watchdog, also dispatched an inspector to the scene to check whether there were any violations of safety regulations.

The agency estimates that the amount of radiation exposure of the man with the highest level translates to up to 12 sieverts over 50 years.

The labor office believes that the man in his 50s has exceeded the annual limit of radiation exposure of 0.1 sievert in five years set for those who handle radioactive materials.

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

The Japan Atomic Energy Agency has said the operation by the workers was carried out as usual.

Nuclear Regulation Authority Chairman Shunichi Tanaka said of the incident at a press conference, "Perhaps (the workers) have become too accustomed to plutonium. I urge careful handling."

"As (a level for) internal radiation exposure it's an amount unheard of," he said.

"We shouldn't downplay the situation," said NRA Commissioner Nobuhiko Ban, a specialist in radiological protection.

While none of the workers has complained of health problems so far, an official of the facility operator said it "cannot rule out the possibility of future health effects."

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

Since radioactive materials were found on hands and faces of four of the five workers, they have been decontaminated, said an official of the National Institutes for Quantum and Radiological Science and Technology, an umbrella organization of the National Institute of Radiological Sciences.

The workers wore masks to cover their mouths and noses but could have inhaled the radioactive materials from the small gaps between the masks and their faces.

The Japan Atomic Energy Agency has previously come under criticism for lacking safety awareness, following revelations of a massive number of equipment inspection failures at its Monju prototype fast-breeder nuclear reactor in Fukui Prefecture.

The Japanese government decided to decommission Monju last year after it has barely operated over the past two decades despite its envisioned key role in the country's nuclear fuel recycling policy.