The P and R reactors, first brought online nearly 60 years ago, were part of the site's role in the manufacture of nuclear weapons materials.
Recovery Act funding was used to deactivate and perform the decommissioning of the obsolete reactors. About 260,000 cubic yards of concrete grout were used to fill the underground areas and reactor vessels, stabilizing them for at least the next 1,400 years.
As part of a ceremony Wednesday before the doors were welded shut, officials placed a time capsule, about the size of a 5-gallon paint bucket, containing items that depict both the history of the site and current events in the region and nation.
In addition to Department of Energy documents and other agency records, the materials included a copy of People magazine on Prince William's wedding to Kate Middleton and other news items.