Thirty-one Hanford nuclear reservation workers inhaled very small amounts of radioactive material after a take-cover order for a contamination spread June 8.
An alarm sounded that morning at the Plutonium Finishing Plant when airborne radioactive contamination was detected as open-air demolition using heavy equipment was being done. Work was underway at the most contaminated part of the plant.
Initially, workers were not believed to be at risk because no contamination was found on the skin or protective clothing of workers, but employees were offered testing for internal contamination as a precaution.
But concerns increased when some of the initial tests showed contamination. Then Washington Department of Health air sampling results came back in August showing radioactive plutonium and americium particles in very low levels at the Rattlesnake security barricade three miles from the plant.
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Some 305 workers had bioassay tests — checks of bodily waste — to see if they had radioactive particles lodged within their body from inhaling or ingesting airborne contamination.
Of the 31 workers who tested positive for measurable levels of internal radiation, 18 had levels that would give them a radiation dose of less than 0.5 millirem total over 50 years.
The remaining 13 had higher doses, but the highest measurement would mean 10 millirems total over 50 years.
All were well below the Department of Energy administrative limit of 2,000 millirems in a single year. The average person receives about 300 millirems a year annually from natural background radiation.
DOE and its contractor at the Plutonium Finishing Plant, CH2M Hill Plateau Remediation Co., plan to bring in independent, outside experts to answer any questions employees may have regarding their results, DOE said in a statement Wednesday.
“Our objective is to keep employee radiological doses as low as reasonably achievable,” DOE said.
It expects Hanford Site workers to be protected from harmful levels of contaminated materials during demolition activities, and the controls that were in place significantly limited the June exposure to “very small amounts,” it said.
After the contamination spread, DOE and CH2M implemented additional controls. They included additional dust suppression during demolition and expanding boundaries where radiological materials could be present.
The revised safety boundary around the demolition area included several trailers used to support workers. Employees and equipment were moved to other trailers outside the expanded boundary.
The spread of contamination came as workers were demolishing an addition, the Plutonium Reclamation Facility, at the end of the Plutonium Finishing Plant’s main processing area.
They were using a shear at the end of an excavator arm to peel off the outer face of a large glove box running the length of the facility, after extensive preparations to clean out the facility and fix remaining contamination in place.
Work has been done since the June 8 incident to tear down other parts of the plant. An explosive demolition brought down the 200-ventilation stack for the plant in July.
In July workers started tearing down the main part of the plant, nicknamed “Z Plant” because it was the last stop for plutonium production at Hanford.
Demolition at the highly contaminated Plutonium Reclamation Facility where the contamination spread is expected to resume in a few weeks.
Workers have hooked up large exhausters in preparation of resuming work there. The exhausters provide for negative ventilation, according to DOE.
“We believe we have taken positive steps to further reduce the possibility of contamination during the remaining demolition work,” DOE said in the statement.
The Plutonium Finishing Plant was used in the plutonium production process during the Cold War. About two-thirds of the plutonium for the nation’s nuclear weapons program was produced at Hanford.
The plant’s Plutonium Reclamation Facility was used to recover valuable plutonium from waste material during the height of the Cold War.
Work has been underway since the ’90s to prepare the plant for demolition, including stabilizing plutonium left in a liquid solution at the plant when it shut down.
DOE missed its legal deadline to have the plant demolished by the end of September.
However, DOE said Wednesday that work was on track to complete demolition by the end of this year or early in 2018.