Contractors have completed the extraction of sludge and other non-liquid waste from the 11th single-shell tank at the Hanford site.

It's one of 177 that were buried to store chemical and radiological waste during Cold War plutonium production. URS Corp.-led consortium Washington River Protection Solutions announced Thursday that the 530,000-gallon tank known as C-110 now holds a volume of waste below the 360-cubic-foot regulatory threshold. Last month, the Department of Energy extended WRPS' initial five-year contract into 2016. It last completed waste extraction from a tank roughly a year ago.

The work is time consuming and complex. The pumpable, more easily retrievable waste has already been moved to double-shell tanks at the Washington site. Material remaining in the tanks is extremely hazardous, varies widely in consistency and must be accessed through narrow pipes. According to a DOE release, tank C-110 required removal of 178,000 gallons of sludge using sluicing equipment and another 17,200 gallons of so-called hard-heel waste on the tank floor. It necessitated a remotely operated, track-mounted device called a Foldtrack equipped with a plow blade and water jets. The operation also made use of a hot-water skid that heated 100 gallons per minute to 120 degrees Fahrenheit and helped dissolve elements of the sludge.