Affiliation:Washington State University School of Mechanical and Materials Engineering

Title:Contributions of Geosciences to Nuclear Waste Management:  Understanding the History of 1500-Year-Old Glass at Pre-Viking Swedish Vitrified Hillfort Site

Abstract:

Geosciences have from the beginning been an integral part in robust planning and execution of nuclear waste management – from mineralogy of stable host phases for radioactive elements, to geochemistry of transport in the subsurface, to seismic and long term geophysical modeling of candidate repository sties for high level waste. This talk addresses yet another application, understanding and predicting the long-term degradation and alteration of emplaced glass using tools from various geoscience disciplines.

Nuclear waste must be immobilized and stored such that it does not cause significant impact on the environment or human health. In some cases, the integrity of the repositories will need to sustain for tens to hundreds of thousands of years. In order to ensure such containment, nuclear waste is frequently converted into a very durable glass. It is fundamentally difficult, however, to assure the validity of such containment based on short-term tests alone. To date, some anthropogenic and natural volcanic glasses, with ages of decades to thousands of years old, have been investigated for this purpose.

However, glasses produced by ancient cultures for the purpose of joining rocks in stonewalls have not yet been investigated as analogues, in spite of the fact that they might offer significant insight into the long-term durability of glasses in natural environments. Therefore, a project has been initiated with the scope of obtaining samples and characterizing their environment, as well as to investigate them using a suite of advanced materials characterization techniques. The objectives include determination of how the hillfort glasses may have been prepared, and to what extent they have altered under in-situ conditions. The ultimate goal is to obtain a better understanding of the alteration behavior of nuclear waste glasses and its compositional dependence, and thus to improve and validate predictive models for nuclear waste glass corrosion.

Broborg is an Iron Age site in Sweden dating ~450 AD. Evidence from field and experimental archaeology suggests that peoples were utilizing pyrotechnology to consolidate battlement walls. Sections of this “vitrified hillfort” still stand, indicating the durability resulting from this process. Excavations at Broborg and comparison with vitrified forts in other parts of Europe have suggested that at Broborg there was a practice of deliberate construction, based on melting amphibolite rock to consolidate gneissic-granite local boulders. Laboratory experiments have lent insight into requirements of local rock chemistry for melting using Iron Age pyrotechnology. Additionally, new excavations are planned for 2017 where paleomagnetic samples will also be collected, to confirm dating of the construction events based on recording of the earth’s magnetic field at the time of the crystallization of minerals like magnetite from the melted rock.

Given the presence of vitrified material of 1500+ years old, Broborg and similar sites provide a unique opportunity for studying long-term chemical durability of glass on a timescale relevant for application to models of nuclear waste glass alteration in geologic repositories.