In late October, an environmental sampling crew with Consolidated Nuclear Security — the government’s managing contractor at the Y-12 nuclear weapons plant — noticed some abnormal brown stains on the exterior of ventilation stacks at the plant’s 9212 uranium-processing complex.

The stains were reportedly from stacks tied to a ventilation system for operations that dissolve uranium metal for processing.

According to a Nov. 7 report by staff of the Defense Nuclear Facilities Safety Board, the system is set up to help protect workers by capturing contaminated vapors released when the “tray dissolver units” are opened during operations, but in this case the emissions may have gone through a stack without scrubbers or high-efficiency air filters.

On Oct. 20, the work crew “was changing out filter paper in the stack monitors for exhaust stacks 47 and 114 when they noted abnormal stains on the exterior of stack 114,” the DNFSB report said.

“It appeared that a brownish substance had been ejected from stack 47 onto stack 114 and the surrounding roof,” the report said.

A followup investigation found that a significant roof area — about 2,800 square feet — had low levels of radioactive contamination, and the same type of contamination was also found on “ground-level areas adjacent to the facility.” The contractor set up radiological “boundaries,” apparently to warn workers of the contamination, and samples were sent to a lab for analysis and to pinpoint the source.

Steven Wyatt, a spokesman with the National Nuclear Security Administration’s Production Office, said the Y-12 contractor has completed its investigation of the situation and sent a “corrective action plan.” But that plan was not immediately available for public release.

“Operations of the tray dissolver system remain suspended until actions are completed to return the system to normal operation,” Wyatt said Thursday. “There is no indication of any employee exposures as a result of this incident.”

The federal spokesman said decontamination efforts by CNS had “significantly” reduced the size of the contaminated area of roof. “Other areas containing low levels of contamination are being controlled in accordance with all requirements for employee and environmental safety. ”

Wyatt confirmed that the brownish substance on the roof was uranium, but further details were not released.

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