Experts probing the cause of the plutonium-inhalation accident involving five employees at a fuel research facility in Ibaraki Prefecture are trying to determine whether failures in safety equipment or procedures allowed the deadly powder to escape its container.

The accident might have been caused by the long-term buildup of helium emitted by the plutonium, one expert says.

The accident took place at around 11:15 a.m. Tuesday when five men from the Plutonium Fuel Research Facility at Oarai Research & Development Center were taking stock of a radioactive substance in an old storage container. This process usually involves placing the container into a special machine that adjusts the air pressure to prevent the material inside from being blown into the air.

Masked, gloved and donning other protective gear, a worker in his 50s along with a coworker standing by, removed the sealing bolts of a stainless steel container and opened the lid only to see a black powder burst forth.

“The plastic bags were thick and we did not expect them to burst,” said an official at Japan Atomic Energy Agency, the facility’s operator. “I have no idea why (the plutonium powder) flew out of the container,” another said.

The powdery substance had originally been encased in a plastic container double-wrapped in plastic. It was then placed inside a stainless steel container sealed with six bolts. The container had not been opened since 1991, and held about 300 grams of uranium oxide and plutonium oxide that had been used in past experiments, JAEA officials said.

“The container may have been filled with helium (which can be emitted by plutonium) from extended storage, and that may have increased the pressure inside it,” according to Kazuya Idemitsu, an expert on nuclear fuel engineering and a professor at the Graduate School of Engineering at Kyushu University.

Although masks were covering the workers’ noses and mouths, radioactive material was detected inside the noses of three of the exposed employees.

The agency said Wednesday that internal radiation exposure was detected in four of the five workers and that a fifth is suspected as well.

Up to 22,000 becquerels of plutonium were detected in the lungs of the worker in his 50s who opened the lid. Based on that figure, the agency estimates his body has likely has 360,000 becquerels of material inside it overall, they said Thursday.

Under current labor standards, that translates into 1.2 sieverts over a year, and perhaps a 12 sieverts over 50 years, the officials said.

The government allows designated nuclear workers to be exposed to a maximum of 0.05 sievert per year, or 0.1 sievert over five years.

“This is an unusually high amount of radiation. We must carefully look into whether the workers took proper steps,” Nobuhiko Ban, an expert on radiological protection and a member of the Nuclear Regulation Authority, said at an NRA meeting Wednesday.

Plutonium decay can continuously damage cells in the body so it is imperative to make sure workers don’t inhale it, Ban acknowledged.

The main threat from internal plutonium exposure this is bone cancer.

“There are very limited cases of treatment for internal exposure to plutonium in Japan,” Kazuhiko Maekawa, an expert on the subject, said.Gen Suzuki, an expert on radiation epidemiology and professor at the International University of Health and Welfare, said the amount of radiation in their bodies can vary based on the size and character of each particle of plutonium.

March 1997: Radioactive material leaks after a fire and explosion at the Ibaraki branch of now-defunct Power Reactor and Nuclear Fuel Development Corp., later absorbed by Japan Atomic Energy Agency. Thirty-seven employees were exposed.

September 1999: A self-sustaining chain reaction is triggered by the use of mixing buckets at uranium processing firm JCO Co. in the village of Tokai, Ibaraki Prefecture. The accident eventually kills two of three employees, after tainting more than 600 residents.

June 2006: A suspected case of plutonium inhalation occurs at Japan Nuclear Fuel’s reprocessing plant in the village of Rokkasho, Aomori Prefecture, but a check for internal exposure turns out negative.

July 2008: A worker at Global Nuclear Fuel Japan Co. is exposed to uranium in Yokosuka, Kanagawa Prefecture, followed by the exposure of four workers to a uranium-tainted liquid a month later.

March 2011: Three workers stepped in to a puddle during the meltdown crisis at the Fukushima No. 1 power plant, exposing two to high radiation.

May 2013: Thirty-four researchers at JAEA’s Japan Proton Accelerator Research Complex (J-PARC) in Tokai are exposed to an exotic soup of isotopes during an experiment.