The Hanford Site’s Office of River Protection has obtained samples of glass up to 2,000 years old to help it learn more about how the substance ages as the Department of Energy facility prepares to vitrify, or glassify, tank waste.
The ancient glass is being studied to predict how glassified waste might age over up to 10,000 years and also to help determine how much waste can be loaded into glass. The research is a joint project of the DOE office and the Lulea University of Technology in Sweden, with assistance from the Pacific Northwest National Laboratory in Richland, Wash., and the Smithsonian Institution’s Museum Conservation Institute. The national lab is assisting with analytical methods and the Smithsonian is providing additional analysis and the handling of historically significant samples.
The glass comes from hilltop forts in Europe, called hill-forts, whose walls were built 600 to 2,000 years ago using vitrified glass as mortar. Much of the glass under study comes from the Broborg hill-fort north of Stockholm, Sweden, which is estimated to have been built between 375 and 550 A.D., or possibly earlier, said Rolf Sjoblom, an adjunct professor specializing in waste science and technology at Uppsala University, who helped the Office of River Protection obtain glass samples.
The walls of the Broborg hill-fort were built during Sweden’s iron age by piling up rocks ranging in size from softballs to soccer balls into walls. Smaller rocks that would melt into glass at a lower temperature were packed around them and then heated with fire to fuse the larger rocks into a solid wall. As the walls have aged in a natural environment over more than 1,000 years, the glass has remained intact, Sjoblom said.
The Hanford Site today stores about 56 million gallons of radioactive and chemical waste produced during decades of plutonium production for the U.S. nuclear arsenal. That waste will largely be treated at the Waste Treatment Plant, or vit plant, being built by Bechtel.
Hanford officials are interested in developing an accelerated aging test that is more realistic than current methods, which grind up glass into small pieces and expose the material to heat. The Broborg hill-fort glass has some of the same metal oxides as the glass waste form that will be produced at the Waste Treatment Plant. By replicating the similar glass and subjecting it to new aging tests, scientists hope to validate testing methods by determining whether glass subjected to accelerated aging matches the condition of the ancient substance. If less conservative testing proves adequate, more waste could be loaded into glass, shortening the mission of the Waste Treatment Plant, said Albert Kruger, Office of River Protection glass scientist. Low-activity waste vitrified at the plant will be disposed of at a Hanford landfill, where the glass holding the waste must withstand the elements over thousands of years to prevent groundwater contamination.
