As many as 5,618 shipments of radioactive waste were made to the central Hanford landfill without the correct radioactive waste signs required on shipping containers.
However, no additional risk to workers resulted and the waste met requirements for disposal at the Environmental Restoration Disposal Facility, according to Washington Closure Hanford.
The landfill waste containers used at Hanford, which typically hold 18 to 23 tons of waste, all are routinely marked as carrying radioactive loads.
But the containers did not also have the required state Department of Transportation magnetic placards indicating a load of radioactive "low specific activity."
Never miss a local story.
The waste was collected at the 618-10 Burial Ground, which is about six miles north of Richland, and was transported on Route 4 South, which is open to the public until it reached the secure area beyond the Wye Barricade.
Much of the waste was soil, including soil dug up to get to radioactive debris in trenches at the burial ground, said Mill Lewis, waste services manager for Washington Closure. It also included some mixed-in debris.
Washington Closure discovered the mistake, which dated back to a single incorrect calculation in 2011, and reported it.
The calculation applied only to meeting Department of Transportation requirements, not the way the waste was loaded, and the waste was handled as radioactive material, according to Washington Closure.
The mistake was made when a checklist was revised with the intent of making it easier to use. But boxes indicating picocuries per gram and picocuries per kilogram were confused, causing the radioactivity to be under-reported by a factor of 1,000 for compliance with state regulations, according to Washington Closure.
Because the paperwork was done once for all shipments, the error carried through until discovered last week.
Most of the shipments were made between January and November 2012, with 155 made this year, Mill said.
Washington Closure immediately reviewed similar calculations used for other shipments to confirm the mistake had not been repeated at other Hanford projects, Mill said.
The 618-10 Burial Ground likely is one of the two highest hazard sites in a wide area along the Columbia River where Washington Closure is responsible for cleanup.
But work there has started with retrieval of waste from 12 trenches rather than 94 vertical pipe units. The vertical pipe units, made of five bottomless 55-gallon drums welded together and buried upright, were used as a dump for highly radioactive wastes.
The trenches are expected to hold up to 2,000 drums containing everything from mildly contaminated clothing and debris to highly radioactive laboratory equipment and liquids.
-- Annette Cary: 582-1533; firstname.lastname@example.org; Twitter: @HanfordNews