Photo/IllutrationThe fuel research building of the Japan Atomic Energy Agency's Oarai Research and Development Center in Oarai, Ibaraki Prefecture, where the accident occurred June 6 (Asahi Shimbun file photo)

The Japan Atomic Energy Agency (JAEA) now considers the most likely cause of last month’s nuclear contamination accident at its Oarai research center to be gas produced by decomposing resin containing plutonium and other radioactive substances.

The June 6 incident at the facility in Ibaraki Prefecture, northeast of Tokyo, exposed five workers to plutonium when they handled 26-year-old radioactive waste stored there.

Resin was used to stick the radioactive waste on an aluminum sheet and stored in a tightly sealed polyethylene container, which was wrapped in plastic bags and placed in a stainless steel container.

The accident occurred when the workers opened the steel container and were exposed to radioactive particles that seeped out of the polyethylene container in gas that ruptured the plastic coverings and escaped into the room they were in.

“Decomposition of resin by a radioactive substance is considered as the most likely cause of the gas’s formation,” said Toshio Kodama, JAEA president, at a July 3 meeting with the science and technology ministry’s special investigation team that was set up to determine the cause of the accident.

JAEA found that the polyethylene container inside the plastic bags contained powdered plutonium set in pieces of epoxy resin.

The agency is looking at other possibilities, but now considers decomposition of the resin as the most likely cause.

On the same day, the National Institute of Radiological Sciences in Chiba announced that three of the five workers have been admitted to its facility for the third time to receive medication via an intravenous drip that speeds the excretion of radioactive substances from their bodies as urine.

The health of the five workers has not changed, according to NIRS.