bulletin cover story Ensuring waste glass longevity: How studying ancient glasses can help predict the durability of vitrified nuclear waste

By Jamie L. Weaver, John S. McCloy, Joseph V. Ryan, and Albert A. Kruger

Long-term mechanisms of glass to study glasses that contain similar alteration have remained elusive.1 mass percentages of the baseline These mechanisms are of particular oxides of silica, aluminum, sodium, interest to the international nuclear and, if possible, boron. The difficulty Ancient glass artifacts provide a surprisingly rich waste glass community, because of finding a one-to-one analogue is source of analogues to study long-term mechanisms they strive to ensure that vitrified in part due to the complex elemental of glass alteration for design of new glasses for products will be durable for thou- composition of nuclear waste. sands to tens of thousands of years. In general, the two most common nuclear waste disposal. For the past thirty years, this com- types of vitrified nuclear wastes are munity has been attempting to fill low activity waste (LAW), and high- ow does glass alter with this research gap by partnering with level waste (HLW). Classification of archeologists, museum curators, these wastes is most often based on time? For the last hun- H and geologists to identify hundred- present radionuclides and regulations dred years, this has been an important to million-year-old glass analogues regarding how these elements are that have altered in environments immobilized and isolated from the question to the fields of object con- representative of those expected environment. Additionally, research- servation and archeology to ensure at potential nuclear waste disposal ers further distinguish glasses by their sites. Thre process of identifying relative elemental concentrations. preservation of glass artifacts. This a waste glass relevant analogue is For U.S.-based LAW glasses, same question is part of the develop- challenging, because it requires baseline components are typically scientists to relate data collected 45 mass% SiO and 20 mass% ~ 2 ~ ment and assessment of durable glass from short-term laboratory experi- Na O, mixed with 6 mass% 2 ~ ments to observations made from Al O , 9 mass% B O , and 20 waste forms for the immobilization of 2 3 ~ 2 3 ~ long-term analogues and extensive mass% of other waste-derived nuclear wastes. Researchers have devel- geochemical modeling. oxides. Alternatively, U.S.-based oped experiments ranging from simple HLW glasses contain ~30–55 Choosing an appropriate ana- mass% SiO , 15–20 mass% Na O, 2 ~ 2 to highly sophisticated to answer this 4–22 mass% Al O , and 10–20 logue: Initial considerations ~ 2 3 ~ question and, as a result, have gained When initially approaching the mass% B2O3, with other oxides challenge of choosing a glass altera- comprising the balance, either significant insight into the mechanisms tion analogue, one could choose to added in the frit or from the waste that drive glass alteration. However, limit the types of glasses to those stream. HLW glasses also may, but that have compositions similar to do not have to, contain more iron gathered data have been predominately nuclear waste glasses. Although it is than LAW glasses. Depending on applicable to only short-term alteration very unlikely that an analogue will the origin of the waste stream—i.e., have exactly the same composition as industrial waste from used nuclear times—i.e., over the course of decades. a nuclear waste glass, it is important fuel reprocessing or legacy defense 18 www.ceramics.org | American Ceramic Society Bulletin, Vol. 95, No. 4 Capsule summary

THE CHALLENGE WHAT’S OLD IS NEW AGAIN FUTURE MODELS Studying how glass is altered over time, Ancient artifacts represent a rich source of ma- A better understanding of long-term glass altera- especially large spans of time, is a challenging terial to study long-term glass alterations under tion will allow a more accurate prediction of the research problem. However, thorough under- various environmental and exposure conditions. performance of vitrified nuclear waste and could standing of these mechanisms is critical for Although artifacts often have compositions help develop waste glasses that will be durable prediction and design of glasses that can safely different from new glasses, they offer many for years to come. store nuclear waste for long periods of time. potential analogues for study. nuclear waste—these other elements can vide insight into how a slight change in and granite, which will dictate chemistry vary widely in identity, concentration, composition affects the overall durability of the environment in contact with the and oxidation state. of a sample. This is important because, glass. An appropriate analogue will have Once the waste glass of interest is iden- occasionally during the vitrification pro- been altered in a similar environment. tified and its compositional range defined, cess, compositions of produced glasses are then it is possible to begin looking for an designed to be slightly different from one Analogues are samples with a past appropriate analogue or set of analogues. melt batch to the next. These differences Before they decide if a glass artifact An ideal analogue, as stated above, should in composition are designed to accommo- is a viable analogue, scientists usually contain most, if not all, of the main ele- date the special chemistry of a particular collect background data on the glass, ments in the waste glass in mass% that fall waste stream and to efficiently incorporate including information regarding its within the composition range defined for the waste elements into the glass. provenance.3 Three additional questions the waste glass. Many previously studied In addition to choosing an analogue should be asked: analogues (see below) have two or three of that has a similar composition as the • Is it ethical or feasible to analyze the main elements of their corresponding waste glass of interest, it also is impor- this artifact? waste glasses. In these cases, a second set tant to choose an analogue that has • What ex-situ factors may have of analogues that contains the relevant been altered under relevant conditions. affected how the glass was altered, and mass% of the other primary elements—and This is because glasses altered in various what effects did they have? do not deviate more than a few mass% of environments can form various types of • How does studying this glass help the other elements—could be identified alteration layers with time (Figure 1).2 validate current glass alteration models? and studied. Chosen analogue glasses should come Researchers should be aware of any Combining both data sets could allow a from alteration environments similar cultural importance of the glass artifact researcher to develop a well rounded view to those that have been suggested or and should carefully consider what phys- of how major elements affect the long-term chosen for the disposal of nuclear waste ical and philosophical effects the analy- durability of waste glass. Additionally, glasses. Disposal areas are often designed ses might have on an artifact’s long-term studying two or more analogues may pro- to have engineered barriers, such as clay preservation. Researchers should ask whether analyzing the glass will change how it may be interpreted by future gen- erations. It is important that research- ers make these considerations with the advice of conservators, archeologists, art historians, and museum curators who specialize in the time period and geo- graphical area associated with the arti- fact. Nondestructive analysis is preferred, if possible. However, if some destructive Figure 1. Locations analysis is warranted to obtain critical and timeline of various types of excavated information that otherwise could not vitreous materials used be obtained, researchers must consider as analogues to study additional precautions. For instance, durability of modern how might the glass alter when it is nuclear waste glasses. sampled for analysis? How might the glass behave under an ion beam? Will

American Ceramic Society Bulletin, Vol. 95, No. 4 | www.ceramics.org 19 Ensuring waste glass longevity: How studying ancient glasses can help . . . the artifact be chemically changed if it is ed the data collected from an analogue. and it can be challenging to separate exposed to X-rays? Glass alteration models are developed out the effects of each factor. However, Researchers also should consider how from data collected on controlled experi- simplified experiments performed in a collected data could help validate or inval- ments, but analouges are rarely altered laboratory setting can help disentangle idate glass alteration models. As stated under controlled conditions. Variability these factors. in question three, a researcher should in alteration conditions could include Data collected in the analysis of be aware of what part of the alteration annual changes of the pH of the ground- ancient glasses also can be utilized in model is being tested, which requires water that was in contact with teh ana- preservation and historical interpretation knowledge of glass corrosion science and logue, or fluctuations in microbial popu- of the artifact itself or similar pieces. theory. Additionaly, the researcher should lations in teh soil surrounding a buried For example, archeologists continue to be aware of how outside factors (such as analogue. It can be difficult to know all attempt to gain an understanding of the variable pH conditions, microbial growth, factors that might have influenced altera- methods used to make glass recovered at and human intervention) may have affect- tion of an ancient glass during its history, the Broborg site in Sweden (Figure 2). Broborg hillfort case study Archeologists define vitrified forts as stony fortifications in which a dry-wall structure has been bound by molten or calcined materials.3 Over the past 60 years, scien- tists have produced three theories of how hillforts were vitrified: incidental, or resulting from cooking-hearth fires, forges, or even lightning; constructive, meaning materials were purposefully selected and melted to fortify dry-wall structures; and destructive, meaning walls surrounded by organic matter, such as timber, that was set on fire either by (left) Jamie Weaver at the Broborg hillfort near Uppsala, Sweden, where remains accident or during an attack. of a vitrified wall are protruding from the snow. (right) Close-up of a molten section from a hillfort, showing impressions of the charcoal used to vitrify the materials, In the case of the Broborg hillfort, located despite ~1,000 years of weathering. near Uppsala, Sweden, scientists believe that vitrification was intentional, and its construc- and has lead researchers at Washington of bellows. Other studies have found that tion was completed around 500 CE during State University and Pacific Northwest amphibolite collected from the site could be the Migration Period (prior to the Viking age). National Laboratory to dig into the geology melted only with the help of a forced draft Several excavations of the site have resulted of this region of Sweden. The amphibolite and a furnace hearth covered with turf.4 in evidence of house foundations, suggest- used at the site is metamorphic doleritic rock These results suggest that to create vitrified ing that at one point it held a permanent formed at high water pressure and contains portions of the wall, ancient people most settlement. Walking along the edge of the mainly hornblende and feldspars. Studies of likely had to control redox condition and partially vitrified wall at the site, a casual the amphibolite left at the site have uncov- water content of the melt—a technology they observer will find it initially difficult to dis- ered evidence of a wide range of localized most likely developed based on their experi- cern rock from vitrified material. However, melting and moisture evolution, from slight ences with iron smelting. Understanding how Peter Kresten—a geologist who has spent heating to complete liquefaction. WSU scien- these melts were made could inform their most of his professional career investigat- tists have found that temperatures >1,400˚C chemistry, which is invaluable in determina- ing vitrified forts—can provide guidance are needed to create a molten material if the tion of the long-term durability of Broborg that makes differences between the two melting is completed in air without the aid glasses. materials apparent. Atop and between about one-third of the well-weathered black-and- white-speckled granite and gneiss boulders lie. With further survey of the site, one can see that the areas where the boulders were not covered and fused with vitrified matter are more heavily eroded then their fortified counterparts. When the dust is lightly swept away from vitrified sections, one discovers a slightly cloudy material that ranges in colors from dark brown to almost clear and, in Credit: Jose Marcial and Joseph Osborn some areas, still bears marks of the charcoal (left) Melt series based on chemistry of melted amphibolite composition from the used in its firing. Broborg site. Melting in atmosphere and without the use of bellows required heating Amphibolite appears to have played an the material to temperatures >1,400˚C, which is not likely to be reached in ancient important role in the construction of Broborg times. (right) Final glass produced from the melting study.

20 www.ceramics.org | American Ceramic Society Bulletin, Vol. 95, No. 4 Roman glasses An excellent example of using anthro- pogenic glass analogues for modeling glass alterations can be found in the recent works of Verney-Carron and the Commissariat à l' Énergie Atomique (CEA) on fractured, ~1,800-year-old glass blocks excavated in 2003 from a Roman shipwreck near the French island of Embiez.9 Many glasses—includ- ing nuclear waste glasses—fracture upon cooling, and these fractures increase the reactive surface area of the glass. Because Figure 3. Fragment of a Roman glass cup Iulia Felix of the slow flow of water in and out handle found in the shipwreck, showing iridescent alteration layers. of some cracks, increased surface area results in alteration rates in the cracks On this side of the Atlantic, glass different from those on the exterior glass scientists at Pacific Northwest National Figure 2. Examples of hillfort glasses, surfaces. The effects of these cracks on Laboratory (PNNL) have been working which are manufactured glasses with com- long-term alteration rates have been diffi- with Italian archeologists to investigate the positions similar to some nuclear waste cult to replicate in short-term laboratory formation of alteration layers on second- to glasses. An amphibolite rock source for tests because of the slow rate at which third-century CE Roman glass recovered vitreous material, found at Broborg in borosilicate glass alters. 11 Sweden, shows a molten top portion. from the Iulia Felix shipwreck. The Iulia In addition to showing that alteration Felix wreck is of particular interest, because Scientists hope that further analysis of mechanisms of archeological glass are its cargo included blue-green Roman ancient glasses may uncover redox condi- similar to those described by a modern glass fragments—referred to in archeology tions used by these ancient peoples and glass alteration model, the authors also literature as “naturally colored” glasses— demonstrate a connection between iron discovered that alteration kinetics slightly and colorless and red shards, “artificially working from that time and creation of varied depending on fracture location. colored” by additions of reduced copper the glasses. They calculated a lower alteration rate for (Figure 3). Because ancient glassmakers internal cracks that were not continually usually added coloring agents in measured Previous studies exposed to water as compared with exte- amounts to colorless glasses, artificially col- Natural glasses rior surfaces that were most likely in con- ored glass usually has a base composition Most early studies of ancient glasses stant contact with water. Without renewal similar to naturally colored Roman glasses. for the purpose of model validation have of fluid into cracks, the altering solution This slight modification makes it possible focused on alteration of natural glasses, became saturated with dissolved glass ele- to investigate the effect of chemical com- such as basalts, tektites, and rhyolites.5–7 ments, making it unfavorable for the glass position on glass alteration within a set Rhyolitic glass, or high-silica natural matrix to undergo further dissolution. environment. Further research is currently volcanic glass, also is known as obsidian. In a recent follow-up to this study, underway on these artifacts. Natural glasses range from hundreds of Verney-Carron et al.10 used chemi- PNNL researchers have also focused millions of years old (such as terrestrial, cal modeling to simulate alteration of on parts of these glasses that were encased meteoritic, and lunar glasses) to only the glass in a variety of solutions, and in cemented sediments (Figure 4).11 The thousands of years old (some basaltic they compared the results with reactive cementing material around the glass is and rhyolitic glasses). transport modeling of alterations in the magnesium-rich calcite, likely formed Tektites and rhyolitic glasses generally cracks. They calculated that cracks inside through corrosion of the glass in seawater have high SiO2 concentrations (70–75 the archaeological blocks had one to two saturated with naturally occurring dolo- mass%), whereas basaltic glasses have lower orders of magnitude less alteration than mite and magnesium silicate precipita-

SiO2 content (45–50 mass%). Unlike most the external surface because of a strong tions. These magnesium silicate species nuclear waste glasses, natural glasses tend coupling between rate of glass alteration form during alteration of some HLW to contain low mass percentages of alkali and slow renewal of the internal crack glasses, and the researchers identified elements. This low alkali content has made altering solution. Additionally, narrow similar species in the altering solution of application of the alteration data collected internal cracks precipitated crystals nuclear waste glasses that were subjected on these natural glasses to most alkali-rich because of supersaturation of the solu- to corrosion with a 1 mmol/kg MgCl2 waste glasses difficult. However, even with tion in the crack with silicon, calcium, solution.12,13 Both cited experiments these limitations, basaltic glasses have been and aluminum. Formation of these crys- were short term, but, when coupled with used as fruitful analogues to some HLW tals plugged the cracks, causing flow of results from the Iulia Felix glasses, the glass formulations.8 water and alteration processes to halt. researchers determined that, in undersat-

American Ceramic Society Bulletin, Vol. 95, No. 4 | www.ceramics.org 21 Ensuring waste glass longevity: How studying ancient glasses can help . . .

it with a synthetic solution at a flow rate chosen to test the glass under diffusion- controlled conditions. The researchers also used this alteration routine on synthetic archeological slags that had the same composition as excavated samples. They determined Figure 4. Glass collected in-situ with con- significant similarities in tacted soil at the Aquileia site in Italy. the increase of glass altera- Aquileia was a major glass-producing tion in the presence of iron center in Roman times. from comparison of the two altered samples. This Figure 5. Scientific collaboration at work in Uppsala, urated conditions, dissolution of the glass result supports previous Sweden, showing investigation of vitrified iron furnace matrix is the main cause of formation of studies that have shown a walls. Shown (left to right) are John McCloy, glass cementing phases. deleterious effect of reduced scientist of Washington State University; Eva Hjärthner- iron, which results in an Holdar, archaeologist in Arkeologerna, Geoarkeologiskt Iron slag increased rate of glass disso- Laboratorium; and Rolf Sjöblom, geochemist with Tekedo and Luleå University of Technology. Many scientists have gained insight lution, an important result from other ancient vitreous materials that researchers can use to aid modeling for Hanford LAW glasses allow up to a in addition to glasses. A few studies in of long-term durability of glasses stored in few vol% of spinel crystals. The build- recent years have focused on alteration of iron-based containers.15 ers either buried these hillfort glasses 14 metallurgical slags —glassy materials pro- under near-surface conditions or exposed duced when metals are smelted from ore. Swedish hillfort glasses them to surface weathering cycles.4 Slag is comprised of a mixture of metal Researchers recently have identified Although research on these ancient vitre- oxides, typically calcium oxide, and silica. glasses that were produced almost 1,500 ous materials remains in its early stages, Iron blast furnace slags, for instance, are years ago on a hillfort in Sweden that WSU researchers have made promising formed from silica in the iron ore and have been altered by surface atmospheric headway in an attempt to synthetically added limestone or a similar calcium conditions, which have analogies to the reproduce these glasses. In addition, source. Aged slags produced during iron- near-surface disposal planned for some PNNL researchers have been working working represent an alteration condition low-level nuclear waste glasses, including with Swedish archeologists, scientists, and in which silica-rich materials are altered those at Hanford.16 A hillfort is a build- state officials to acquire fresh near-surface in the presence of metallic iron (Figure ing structure used as a fortified refuge, altered samples from the Broborg site. 5). This situation is similar to what may trading post, or defended settlement, happen to nuclear waste glass as it ages in where an elevated location relative to the Medieval glasses stainless-steel containers. surrounding area provides a defensive It is sometimes helpful to analyze a Researchers identified such altered advantage.17 There are many hillforts in glass that has been altered under two th slags from the 16 century CE and Europe, and these fortifications usually or more conditions simultaneously. In recovered from an excavated blast fur- follow the contours of a hill, consist of these cases, the focus of understanding nace site located in Normandy, France. one or more lines of earthworks (includ- is placed on whether a glass alteration Geological conditions of the burial (satu- ing stockades or defensive walls), and are model can be extrapolated to include rated clay) are similar to those described surrounded by external ditches. More multiple altering conditions. Alteration for disposal of some waste glasses in than 100 of the European hillforts, most data from several environments can be France. French scientists at the CEA from the Iron Age, are vitrified—much extremely helpful when attempting to have studied these samples using short- controversy continues to surround how decide what disposal setting and geology term wet laboratory experiments as well this occurred.17 is best for a specific type of waste glass. as analysis of long-term alteration layers A selection of glasses from the Broborg Researchers have discovered medieval 14 on the artifact surfaces. hillfort site in Sweden contain iron at stained and nonstained glass windows Researchers designed wet laboratory concentrations between those detected to be ideal analogues for these studies, experiments to closely reproduce the in basaltic and Roman glasses. Similar because the windows often have under- proposed French nuclear waste disposal to basaltic glasses, some of the Broborg gone alteration under two or more set- settings. They covered SON68, a nonra- glass samples contain embedded crystal- tings18—glass window panels facing out- dioactive version of a French HLW glass, line phases, such as magnetite and other ward are exposed to a alteration environ- wih an iron–steel composite and injected spinels. The current acceptance criteria ment different from those facing inward.

22 www.ceramics.org | American Ceramic Society Bulletin, Vol. 95, No. 4 A few rare glass samples that have been new insights into the effects of compo- found in architectural structures have sitional and environmental differences also been excavated from groundwater- on glass alteration. These include high- saturated soils,18 thus providing a third alumina glasses from India and some environment to compare alteration data. regions in Turkey, a special subset of Because builders, craftsmen, and Byzantine glasses that contain a moder- church officials kept excellent records, ate level of alumina plus some boron, medieval glass-manufacturing methods and certain Chinese glasses with high of most stained glass windows are well- barium oxide content.21 Researchers known. Therefore, glass scientists can currently are considering some of these straightforward identify conditions under glasses as possible analogues, although which the glasses were made (Figure 6). their rarity makes them challenging to In most cases, craftsmen made medieval acquire for study. glasses in either soda (sodium-rich) or Researchers also need to identify high- potash (potassium-rich) compositions concentration boron ( 10 mass% B O ) ~ 2 3 through melts of a blend of washed aluminosilicate glasses that have been siliceous sand and a flux.19 The flux altered in waste-disposal-relevant environ- material was either plant-based (beech or ments. Boron is a primary component fern ash) or mineral-based (natron) and, in all waste glasses and has increased, when added to other glass-forming mate- in certain cases, the durability of silicate Figure 6. Medieval stained glass windows rials, lowered the melting temperature. glasses.22 Boron made its mark on the at St. Vitus Cathedral in Prague, Czech Glassmakers achieved color either by glass industry in the early 1900s, when Republic. adding metals or oxides of cobalt, chro- Corning patented a series of boron- lyzing ancient glasses for this purpose. mium, manganese, copper, or iron and containing silicate glasses that could with- However, the payoff of having a com- controlling redox conditions of the melt, stand temperature shock—Pyrex.23 position- and environment-dependent or by applying a thin layer of a previously At this time, there were many model of glass alteration validated by colored glass to the surface of a clear recorded accidents related to breakage many data points, spanning days to mil- glass. Minor components in these medi- of hot glass railway lanterns during cold lions of years, will be momentous. A eval glasses are thus transition metals that rainstorms. Glass breakage caused the better understanding of long-term glass are relevant to some nuclear waste glasses. railway signals to fail, catching innocent alteration will allow for a more accurate Chemical durability different from the people unaware of oncoming trains. prediction of the long-term performance sodium-rich versus potassium-rich glasses Replacing soda in the glass with boron of vitrified nuclear waste and could aid as well as presence of multiple composi- oxide solved the glasses’ thermal shock continued formulation of waste glasses tions with various transition metals pro- problem. Today, we find borosilicate that will be durable for “thousands of vide a large dataset for studying composi- glasses in almost every home, office years”.1 In the process, knowledge gained tion-dependent alterations as well. space, and laboratory. In addition, com- on glass alteration can help create indus- Scanning electron microscopy stud- mercial uses have utilized the poor dura- trial glasses with engineered durability ies on medieval stained glasses from bility of some boron-containing glasses. as well as help museums and historical England, Italy, and France have revealed One example is Vycor, in which a phase- societies in the preservation of cultural that the type of weathering has an effect separated sodium borosilicate glass is heritage objects. on thickness and structure of the altera- selectively leached of its sodium borate tion layer(s). Potash glasses exposed to phase, leaving a skeletal silicate network About the authors an exterior environment produce thick, that is usually further consolidated to Jamie Weaver is a Ph.D. student uneven alteration layers with some produce a low-cost nearly pure silica. in the Department of Chemistry at pitting, along with pervasive micro- Clearly, chemical durability concerns Washington State University (Pullman, cracking.20 In contrast, potash glasses are relevant to ancient materials and to Wash.). Joseph Ryan is with the Energy that have been buried and exposed to today’s and likely tomorrow’s commer- and Environment Directorate at groundwater have nearly even alteration cial glass processing. Pacific Northwest National Laboratory layers with little to no cracking. (Richland, Wash.). John McCloy is Challenges remain associate professor in the School of Potential future glasses for study The studies reviewed above are only Mechanical and Materials Engineering at There are many less-well-studied a portion of those conducted to date on Washington State University and is with ancient glasses from around the world ancient glasses. However, their relevance Pacific Northwest National Laboratory. that deviate significantly in composition to the validation of a universal glass Albert Kruger is with the Office of River from the glasses described above. These alteration model is significant. There Protection at the Department of Energy glasses could provide researchers with are many challenges associated with ana- (Richland, Wash.).

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