PNNL

How might getting old—really old—affect the glass waste form chosen to immobilize radioactive waste at the Hanford Site in southeastern Washington state? Researchers are seeking answers from walls containing ancient glass in Europe. These walls, known as hillforts, are defense structures that date as far back as the Bronze Age. The researchers progress is the cover story in the May 2016 American Ceramic Society Bulletin.

Vitrification is a term used to describe the process whereby a material is transformed into glass. In terms of waste treatment, the vitrification process melts a special mix of glass-forming silica with radioactive waste, which then hardens into a solid glass log. Millions of gallons of radioactive waste at the Hanford Site will be immobilized in this way and stored safely for thousands of years. This process is a key part of the Department of Energy's cleanup plans at Hanford and the primary mission of the Hanford Tank Waste Treatment and Immobilization Plant, otherwise known as the Vit Plant.

An important question is how the glass may deteriorate over thousands of years. While researchers can run accelerated aging tests to simulate the effects of age on samples of vitrified waste, they want to know more about how accurately the tests reflect the actual aging process in the appropriate environmental setting. But without actual old vitrified waste, they lack a baseline for validating the results. That's where the ancient glass comes in.

Out of the almost 30,000 hillforts in Europe, there are about 200 known hillforts (17 in Sweden) which have been found vitrified. The process converted materials to glass that adhered the stones of the wall together. Early studies suggested that the hillfort stones may share similar properties with some of the glasses the DOE plans to use for vitrification. This makes them ideal for helping with simulated aging studies.The researchers will characterize the alteration layers that may have formed on the glass surfaces using advanced techniques and instrumentation housed at PNNL and at the Smithsonian Institution’s Museum Conservation Institute. Additional plans may include the synthesis of "new" hillfort glass based on compositional analysis of the authentic, pre-existing hillfort samples received from Sweden. After fabrication of a “new” hillfort glass, samples of both the "new" and "authentic" glass can be subjected to a simulated alteration experiment that is already used to study the durability of nuclear waste glasses.

Depending on the results of these studies, researchers may seek approval to excavate a larger sample of hillfort rock for more testing to further validate the advanced aging process.

Funded by the DOE Office of Environmental Management's International Program, this project is a partnership among the DOE Office of River Protection, PNNL, Washington State University (WSU), the Luleå University of Technology in Sweden, and the Smithsonian Institution’s Museum Conservation Institute.

A part of the glass testing will take place at the Applied Process Engineering Laboratory and DOE's Environmental Molecular Sciences Laboratory under the guidance of David Peeler (PNNL) and Jamie Weaver (WSU, PNNL), researchers in PNNL’s Radiological Materials and Detection group, and by John McCloy (WSU, PNNL).

Bronze Age Vitrification?

A century-long debate among historians and archaeologists questions whether the melting of the glass in the hillforts was accidental or intentional, and in the latter case, whether it was for constructive or destructive purposes. One line of thinking is that the builders set fire to the rocks to cause a melting effect that would strengthen the mounds in the absence of mortar. Others think that the fires may have been set by enemies attacking the mounds. However it happened, the hillforts were definitely built to last. At some sites the forts are built on granite, and the glass rocks have actually held up better than the granite.