Tuesday's serious radiation exposure accident at a nuclear research facility in eastern Japan occurred when workers were handling a metal plutonium container that had been kept sealed for 26 years and never once opened for periodic checks, officials of the Nuclear Regulation Authority said Thursday.

The container was opened Tuesday morning for an inspection based on an instruction from regulators after they found the long-term storage of radioactive materials at locations other than their designated areas in Japan Atomic Energy Agency facilities, including its Oarai Research & Development Center in Ibaraki Prefecture where the accident occurred, could be problematic.

Five workers were at the scene when one of them, a male in his 50s, opened the container and a plastic bag inside it with plutonium and uranium powder samples suddenly ruptured, according to the agency. The man and three other workers suffered internal radiation exposure after inhaling the samples.

The man was later found with up to 22,000 becquerels of plutonium in his lungs. Although none of the workers has complained of ill health as of Thursday, radiation experts say long-term exposure to radiation from within the body raises cancer risks.

"The bag may have ruptured after long-term storage caused helium gas to accumulate and raise pressure within the container," said Kazuya Idemitsu, a professor of nuclear fuel engineering at Kyushu University's graduate school. "Storing (containers) for over 20 years is too long, and occasional checkups and bag replacements were likely necessary."

A Japan Atomic Energy Agency official said the agency has set no rules regarding how often such containers should be checked. The absence of such rules raises questions about the agency's management of radioactive materials at its facilities.

The accident occurred when the workers were checking the first of 21 containers kept at the Oarai Research & Development Center. The metal casing in question contained about 300 grams of radioactive samples that were leftovers from the manufacturing of nuclear fuel for use in experiments.

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 cancer.

The 22,000 becquerels of plutonium-239, found in the lungs of the worker in his 50s, mean that his body as a whole has taken in an estimated 360,000 becquerels, according to the agency. The radiation dose for the man translates into 1.2 sieverts for the first year and up to 12 sieverts over 50 years.

The maximum radiation dose for ordinary members of the public is set at 1 millisievert, or 0.001 sievert, per year. The limit for workers at nuclear facilities is set by the government at 0.05 sievert per year, or 0.1 sievert over five years, though the limit can be raised in emergencies.

In the 2011 Fukushima nuclear accident, the radiation doses for some workers exceeded 0.6 sievert when their internal and external exposure was combined.

The workers involved in Tuesday's accident have been given medication to facilitate the discharge of radioactive materials from their bodies.

The agency will monitor the health of the worker who suffered the most serious exposure to see whether it will have long-term effects on his health.

The workers were wearing masks to cover their mouths and noses but may 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 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.