The final results of worker tests after a December spread of contamination found that 11 Hanford workers had inhaled or ingested radioactive particles from demolition of the nuclear reservation’s Plutonium Finishing Plant.

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A total of 42 Hanford workers inhaled or ingested radioactive contamination from demolition of the nuclear reservation’s Plutonium Finishing Plant.

The final results of worker tests after a December spread of contamination at the plant found 11 Hanford workers had inhaled or ingested radioactive particles, according to information released Thursday.

That’s on top of the 31 positive test results after a similar spread of contamination in June at the plant in the center of the nuclear reservation.

Demolition at the plant has been halted since December. It will not restart until the Department of Energy approves a new demolition plan, and a plan is approved and implemented to prevent the airborne spread of small radioactive particles.

The Washington Department of Ecology, a regulator on the project, also has said it will not allow demolition to continue if it is not convinced it can be done safely.

Open-air demolition on the plant began in late 2016 using heavy equipment to tear down its walls. Extensive work already had been done to remove as much contaminated equipment as possible from the plant.

According to a draft report issued earlier in the month by CH2M/Jacobs Engineering with input from the U.S. Department of Energy, an air-monitoring system last fall failed to pick up the spread of radioactive contamination, giving management false assurance that controls were effective.

State monitoring has found that plutonium and americium particles traveled as far as 10 miles from the demolition site, near Richland. Vehicles, office buildings and workers have been tested for traces of radioactive contamination.

A plan for safer demolition has yet to be released.

The project has been troubled with radioactive contamination found outside worker offices at the plant and on worker cars and government vehicles.

Seven worker homes were checked for radioactive contamination, with none found.

The state health department found very small amounts of airborne radioactive contamination in the last year that could have come from the plant demolition miles away, near Highway 240 and the Columbia River.

The loss of control of the radioactive particles was a concern, although the amounts found were considered too small by state experts to pose a health risk.

In the round of worker testing prompted by the December incident, 281 workers were tested. Some 270 had no detectable internal contamination.

“We understand the concerns employees have related to internal exposure,” said a memo sent to employees of CH2M Hill Plateau Remediation, the contractor doing demolition, on Thursday.

Experts in determining radiation dose met with employees, the memo said. In addition, a brochure has been distributed explaining how doses were calculated.

And the government is making follow-up phone calls to answer any remaining questions that tested workers may have.

The tests, which were done on samples of bodily waste, can detect exposures well below any dose deemed harmful, according to the memo.

Of the most recent tests, the highest dose of internal radiation was calculated at 10 to 20 millirem during the next 50 years from particles within their bodies.

Eight workers are expected to receive radiation doses of 1 to 10 millirem, and two could receive doses of less than 1 millirem.

For comparison, the average U.S. resident is exposed to about 300 millirem a year from background and naturally occurring radiation.

About a dozen workers who initially requested the tests later changed their minds and did not submit samples for testing, dropping the number of tests done after the December spread to 281.

After the June contamination spread, the highest dose calculated for a worker was 10 millirem over 50 years. The majority of workers with positive results — 18 of them — then had radiation doses of less than 0.5 millirem.