Some 31 of 305 bioassays collected from workers after the airborne spread of radioactive contamination at the Hanford Site’s Plutonium Finishing Plant (PFP) in June have tested positive for internal radioactive contamination.
Doses were very low, the Department of Energy said in a statement on Wednesday. Of the 31 workers with positive results, 18 had doses calculated at less than 0.5 millirem over 50 years. The remaining 13 workers had higher doses, with the highest calculated at 10 millirems total over 50 years.
“All doses were well below administrative limits,” DOE said. The average exposure to a person in the United States from natural sources of radiation is about 300 millirems per year.
The Energy Department and PFP demolition contractor CH2M Hill Plateau Remediation Co. will bring in independent, outside experts to answer any questions employees may have regarding their results. “The department expects that Hanford Site workers will be protected from harmful levels of contaminated materials during demolition activities,” DOE said.
About 350 workers were ordered to take cover the morning of June 8 during open-air demolition at the PFP Plutonium Reclamation Facility, an addition at one end of the main processing area of the plant. A shear at the end of an excavator arm was peeling off the front face of one of four glove boxes running the length of the facility’s central canyon. Workers took cover for more than three hours after an air monitor detected radioactive particles and activated an alarm.
No contamination was found on the skin or protective clothing of site personnel in initial scans, but workers were offered bioassay tests as a precaution initially. Concerns increased as some of the earliest bioassay tests came back positive. Then, in August, Washington state Department of Health air samples collected 3 miles from the plant on June 8 showed very low levels of airborne plutonium and americium. The contamination did not exceed air quality standards.
Precautions taken to control radioactive contamination during demolition significantly limited exposures to workers in the June 8 incident, DOE said. “However, the release of contamination caused the department and its contractor CH2M to review these controls and implement additional controls,” it said. Steps included additional dust suppression, expanding the area in which precautions must be taken for possible radiological contaminants, and moving people and equipment near the demolition area to support trailers farther away.
Demolition stopped at the Plutonium Reclamation Facility following the incident, but is expected to resume in the next few weeks. Workers recently hooked up large exhausters at the facility to provide negative ventilation as an additional contamination control measure. Teardown has continued in other areas of the Plutonium Finishing Plant. Significant progress has been made to take down the main processing area of the plant, nicknamed Z Plant by workers, and the ventilation stack was brought down in an explosive demolition.
The Energy Department missed its Tri-Party Agreement deadline to bring the plant to slab on grade by the end of fiscal 2017 on Sept. 30. However, DOE said Wednesday that demolition should be completed by the end of the calendar year or early in 2018.
During the Cold War, about two-thirds of the nation’s plutonium for its nuclear weapons program came off of the plant’s two main production lines. Plutonium in a liquid solution was turned into buttons the size of hockey pucks and oxide powder for shipment to a nuclear weapons manufacturing plant from 1949 to 1989. The plant’s Plutonium Reclamation Facility was used to recover plutonium from scrap material to increase Hanford’s overall plutonium production during the height of the Cold War.
