The Government said on Tuesday that 17 workers had been exposed, but that figure was downwardly revised to 16 late early Wednesday.

All 16 workers underwent full body scans on Wednesday. Officials at the INL said radioactive contamination was detected on the skin or clothing of at least six of the workers after Tuesday’s mishap, and two were confirmed to have inhaled radioactive particles, Reuters reported

The results of the scans conducted on the exposed workers prompted doctors on Wednesday to order additional lung examinations of three workers, and urine and stool samples to be taken from all 16, lab officials said. All are technicians employed by laboratory contractor Battelle Energy Alliance.

INL spokesman Earl Johnson told Reuters that none of the workers reported feeling ill or showed symptoms of radiation sickness. All were allowed to return home after initial laboratory tests.

But a statement from the INL said it might be weeks until the extent of the workers’ exposure is known.

“We’re still collecting and analyzing samples from all 16; there is a lot of work to be done to get the correct information,” another INL spokeswoman, Sara Prentice, told reporters.

There was no evidence that radiation was released outside the aging facility, and there was no risk to the public or the environment, the lab said the statement.

Bellona nuclear physicist and executive director, Nils Bøhmer, who has visited the Idaho National Laboratory with other Bellona staff said that aging equipment and facilities at the 62-year-old facility could have played a roll in Tuesday’s accident.

A special team of Energy Department investigators was expected in Idaho next week to launch its own inquiry. In the meantime, new curbs have been placed on the handling of some radioactive materials at the INL complex, US press reports said.

Health implications for the exposed workers

Investigators in Idaho were at the site trying to determine what went wrong.

The accident, which lab officials said occurred during routine procedures, was believed to be the worst in at least four years at the sprawling site, which occupies 1432 square kilometres in the high desert of eastern Idaho.

The lab’s statement said there was no release of radiation beyond a single room in the reactor building, which lies about 60 kilometres from the city of Idaho Falls. An initial equipment inspection pointed to possible damage inside a small plutonium fuel container that was the source of the exposure.

The health impacts of plutonium vary depending on the type of plutonium and whether the contaminant becomes trapped in the body. If plutonium gets trapped in the lungs, for example, it could lead to damage to the body’s cells, lab officials said.

The 16 employees exposed to the radiation Tuesday were offered intravenous treatments with calcium or zinc, which binds to the plutonium and expedites its elimination from the body to limit potential harm, the lab’s statement said.

But according to spokeswoman Prentice, just four of the exposed technicians agreed to the precautionary treatments, which are designed to flush radioactive particles from their bodies.

Further lung examinations were ordered for two workers whose body scans tested positive for radiation in their lungs and for a third worker whose lungs showed an “anomaly.”

Bellona’s Bøhmer said that “the number of exposed workers is quite high, and I hope that there will be full explanation into what caused the accident and how much radiation the [16] workers were exposed to.”

Questions about the accident

The technicians were exposed when a foot-long rectangular container of plutonium fuel inside the facility’s decommissioned Zero Power Physics Reactor (ZPPR) was opened during regular work to prepare it for shipment to another facility, World Nuclear News reported.

“This was a procedure that has been done many times before,” Prentice told Reuters.

An inspection of the container, which Prentice said is shaped like a covered cake pan, did not reveal that the exterior had been damaged. But she said that “there may be damage” to the stainless steel that covers the fuel inside.

“One of the issues with US nuclear facilities is that a large proportion of them are of substantial age,” said Bellona’s Bøhmer. “This could be part of the reason for this accident at [the INL].”

Spokesman Johnson said that technicians were dressed in lab coats and wearing gloves, but the work they were performing requires no respirators or other special protective gear. He said all 16 workers were present in the room but that only two had handled the fuel container in question.

Tons of radioactive waste at INL

Before the ZPPR was decommissioned in 1992, researchers used it to build and test nuclear reactors more cheaply than the cost of constructing an entire power plant.

Last year, cleanup workers had finished removing millions of kilograms of steel and other materials that made up the reactor core, but its shell remains — along with plutonium fuel that once powered the reactor.

Some 6,000 employees and contractors work at the Idaho lab, which opened in 1949 as a national reactor testing station.

INL’s history of accidents – first fatality in US nuclear history

According to lab records, the last serious accident to take place at the site was in 2007, when a worker was treated for minor burns and smoke inhalation from a small laboratory fire, though no radiation release was reported in connection with that incident.

In January 1961, the Idaho lab was the site of the only fatal nuclear accident in US history so far, killing 3, when an explosion occurred at the INL’s Stationary Low-Power Reactor Number One (SL-1). A control rod for absorbing extra neutrons during nuclear reactions, was extracted too far, leading to a core meltdown and an explosion.

The reactor vessels jumped some three metres and concussion and blast killed all three military enlisted personnel working on the reactor. Because of severe radioactive contamination, all three men had to be buried in lead coffins.