Two years ago this summer, crews working at the sprawling Department of Energy campus in Kentucky noticed high levels of radioactive "green salt" in one of the buildings.
Initially thought to have been an accident, the incident became the focus of a nearly yearlong investigation suggesting that someone at the Energy Department's Portsmouth Paducah Project Office had intentionally placed the toxic substance in an air monitor.
But who was responsible remains unclear.
An investigator with the Energy Department's Office of Inspector General concluded that because the building where the green salt was stored "is not monitored and has multiple access points, we were unable to determine who was in the building and when they obtained the green salt," according to documents recently obtained by The Washington Times through the Freedom of Information Act.
A form of uranium, green salt is toxic and can cause cancer.
There is no indication in the documents reviewed by The Times that anyone came into physical contact or inhaled the substance, and officials said workers leaving the facility's controlled area are monitored for radioactive contamination.
"This is how the presence of green salt was identified," Tim Echland, a spokesman for the project office, wrote in an email to The Times responding to questions about the incident.
Mr. Echland said the facility, known as the C-410 complex, is within a larger area that restricts access to authorized site personnel, who are "trained and approved to be on the site."
"They are trained to follow all safety requirements, including the wearing of protective equipment and the appropriate use of monitoring devices," he wrote, adding that the incident, which occurred on June 29, 2010, was investigated initially by Energy Department oversight staff under the assumption that the discovery of green salt in a breathing zone monitor happened because of "unintentional human error or equipment failure."
But the Energy Department's probe took an unexpected turn when "it was determined that the cause may have involved intentional violation of requirements," Mr. Echland said.
The Energy Department's inspector general took over the case, interviewing multiple personnel, according to documents. The names of those interviewed were redacted in records provided to The Times for privacy and other reasons, according to the inspector general's office.
Investigators talked with one person who said that because the green salt had been found behind an air monitor's clean filter, it would have had to have been placed there intentionally.
"Holy crap, this did not get sucked through the filter," one person was quoted as saying, according to the report.
According to the Energy Department's website, the Portsmouth Paducah Project Office manages environmental projects at the department's gaseous diffusion plants in Paducah, Ky., and Portsmouth, Ohio. Over the years, according to the Energy Department, the plant has been used to enrich uranium for use in military reactors, nuclear weapons and commercial nuclear reactors. The Energy Department, through employees and contractors, also has been conducting remediation work to clean up the site.
In recent years, tens of millions of dollars from the Obama administration's economic stimulus program have funded cleanup efforts. In one fact sheet on the stimulus-funded cleanup efforts, the C-410 facility is referred to as part of the "feed plant" and one of several inactive facilities slated for decontamination and decommissioning.
The inspector general's report on the green-salt release raises questions about the physical controls over the building where the incident happened.
In one portion of the highly redacted summary of the investigation, an interviewee noted that "because building C-410 is not monitored and radiological control technicians were only in the building when a crew is scheduled to work in it, anyone could have entered the facility in one of its many doors and placed the green salt in [redacted] air monitor."
Mr. Echland said the lack of monitoring referred to the fact that employees were not tracked electronically or through surveillance inside the facility.
Still, he added, entry into the facility was "controlled through authorization by supervision and via limited access/entry points."
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