Photo/IllutrationRuptured plastic bags come out of a metallic container in the Japan Atomic Energy Agency's Oarai Research and Development Center in Ibaraki Prefecture. (Provided by Japan Atomic Energy Agency)

More than 20 years ago, the Japan Atomic Energy Agency (JAEA) confirmed the kinds of abnormalities that resulted in five workers being exposed to radioactive substances in June.

Although workers at the agency's Oarai Research and Development Center in Ibaraki Prefecture changed the swollen plastic bags and other items at the time, they continued to store the radioactive substances in the same manner for the next two decades, agency officials said July 18.

That apparently resulted in the leakage of plutonium from a plastic container through ruptured plastic bags on June 6 when a worker opened a metallic container that contained the double plastic bags and the plastic container.

After the accident, the JAEA said that it has not inspected the inside of the metallic container for 26 years since 1991, when the plastic container that contained the radioactive substances was placed in it.

When the agency looked into the container's storage records to determine the cause of the accident, however, it found the records of inspections conducted in 1996, which had descriptions of not only the swelling of plastic bags but also damage to the plastic container.

The 1996 inspections checked 63 metallic containers, including the one that was the source of the leak in June. Abnormalities, such as the swelling or tarnishing of plastic bags and changes in color of plastic containers, were confirmed in 23 of the 63 metallic containers.

“We have not been able to confirm whether staff in those days took measures to find causes (of the abnormalities) or prevent a recurrence,” said a JAEA official.