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ScienceDirect Procedia Materials Science 7 (2014) 163 – 171

2nd International Summer School on Nuclear Glass Wasteform: Structure, Properties and Long-Term Behavior, SumGLASS 2013

Open scientific questions about nuclear glass corrosion S. Gin CEA Marcoule, DTCD SECM F-30207 Bagnols-sur-Cèze, France

Abstract Nuclear glass corrosion by groundwater involves coupled processes at various scales that need to be well understood to develop predictive kinetic models. Here we highlight and discuss key scientific questions arising from the current state of knowledge and presentations given at the SumGLASS 2013 summer school. These issues concern fundamental processes as well as how chemical, hydraulic, thermal, and mechanical constraints related to the design of the multi-barrier system in the geological disposal facility affect the rate-limiting mechanisms. Those that are deemed of high importance have to be addressed to improve the robustness and reliability of predictions of radionuclide release from the glass on a geological timescale. © 2014 2014The TheAuthors. Authors. Published Elsevier © Published by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selectionand andpeer-review peer-review under responsibility ofscientifi the scientific committee of SumGLASS Selection under responsibility of the c committee of SumGLASS 2013 2013. Keywords: nuclear glass; borosilicate glass; glass durability; corrosion; mechanisms; modeling

1. Introduction Borosilicate glasses of nuclear interest have been intensively studied in recent decades. These materials produced at industrial scale by robust processes in hot cells must immobilize a wide range of chemical elements for tens to thousands of years (Ojovan and Lee, 2007; Donald, 2010). The presence of water in the vicinity of glass is likely especially once the canister and overpack are breached, and makes this objective challenging (Gin et al., 2013a). Research in the field of glass corrosion is thus motivated by the need for reliable prediction of the fate of radionuclides in relation with the safety assessment of geological repositories (Poinssot and Gin, 2012). Recent papers have synthesized the current state of knowledge on glass corrosion (Grambow, 2006; Conradt, 2008; Frugier et al., 2008; Vienna et al., 2013). The current knowledge convergee on the general idea that in a smart multi-barrier

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2211-8128 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of the scientific committee of SumGLASS 2013 doi:10.1016/j.mspro.2014.10.022

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S. Gin / Procedia Materials Science 7 (2014) 163 – 171

system, glass packages could last from few hundred thousand to a few million years. It is acknowledged that more work is needed to reduce uncertainties and increase the reliability of predictions. Here we briefly review the state of the art and discuss the lack of knowledge, trying to focus on the main questions that need to be addressed in the future to improve current models. 2. Glass corrosion mechanisms Borosilicate glasses are thermodynamically metastable and tend to transform into more stable compounds, especially in presence of water (Paul, 1977; Jantzen et al., 2010). Molecules of water and its dissociated species OHand H+ are expected to diffuse within the glass structure, exchange with alkalis and to a lesser extent with alkaline earths, and hydrolyze iono-covalent bonds (Si-O-M, M = Al, Zr, Fe, etc.) (Doremus, 1975; Bunker, 1994; Rebiscoul et al., 2007). The term interdiffusion refers to ion exchange, whereas dissolution refers to hydrolysis of the silicate network. As shown by first principle calculations, these two reactions are strongly coupled (Geneste et al., 2006) and lead to a structurally modified, hydrated and dealkalinized material generally called hydrated glass. The backward reaction, i.e. condensation of more or less detached silica species, occurs in most of cases and leads to the formation at the glass surface of a porous, amorphous, hydrated and fully reorganized material called a gel (Iler, 1979; Cailleteau et al., 2008, 2011; Gin et al., 2011). It is also very common to observe the precipitation of secondary minerals at the gel surface (Ebert and Bates, 1991). These four mechanisms (ion exchange, hydrolysis, condensation, precipitation) generally take place in parallel, and one of them becomes rate-limiting depending on the reaction progress (Gin et al., 2013a; Vienna et al., 2013). The overall features and properties of the resulting materials (hydrated glass, gel and crystalline phases) are not completely documented. For example sharp chemical gradients have been observed in hydrated glass formed during a 25-year leaching test of SON68 glass (Gin et al., 2013c). These gradients have been related to the formation of a passivating layer. However we do not know the properties of this hydrated layer. Moreover, the diffusion coefficients derived from the chemical gradients remain apparent as it is not known why they are lower than water diffusion through the pristine glass. Diffusion but also water activity could be modified at this location (Briman et al., 2012). A better characterization of the materials formed during glass corrosion could help understand the rate-limiting mechanisms and reduce the empiricism of the rate laws. The role of the gel also remains unclear. As a highly porous material it could be seen as an inert and nonprotective barrier. It seems however that the gel is not a single phase and that it can be reactive (Gin et al., 2001a) and rate-limiting, especially when part of its internal porosity is clogged (Rebiscoul et al., 2005; Petit et al., 1990; Jollivet et al., 2008; Cailleteau et al., 2008). 3. Glass dissolution kinetic regimes Prior to glass corrosion by liquid water, a stage of alteration by water vapor is likely in some scenarios involving carbon steel overpack corrosion because of the large amount of hydrogen produced in the vicinity of glass (ANDRA, 2005). At high relative humidity, water can condense on the glass surface and initiate corrosion processes. Some data are available at high temperature but there is a lack of knowledge under realistic conditions (typically between 30 and 90°C) (Neeway et al., 2012). In presence of liquid water, it has been known for decades that the dissolution rate of borosilicate glass varies by several orders of magnitude depending on the experimental conditions (temperature, pH, solution composition and flow rate) and reaction progress (Nogues et al., 1985; Gin et al., 1994; Vernaz et al., 2001; Frugier et al., 2005). In practice, four kinetic regimes are distinguished according to the rate-limiting mechanism: initial dissolution rate, rate drop, residual rate, and alteration resumption. Figure 1 illustrates these stages and the proposed rate-limiting mechanism. There is a fair agreement on the initial dissolution rate (rate-limiting mechanism, parameter dependence, experimental procedures). However it remains unclear how interdiffusion and hydrolysis are coupled. This question highlights the needs of a better understanding of the physical and chemical processes at molecular level. At larger scales, the initial stage of alteration is expected to be negligible because the high level of confinement near the glass would allow the rate to drop quickly.


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than the geometric area. Various empirical methods and thermomechanical modeling applied to R7T7 glass confirmed an increase in the reactive surface (Barth et al., 2012, 2013). Until now, all performance assessment models have assumed a constant reactive surface area. However, the question of whether the reactive surface could increase or decrease over time remains open. In theory several phenomena could affect the reactive surface area over time: i) slow release of residual stresses, ii) stress corrosion, iii) self-irradiation, iv) external stress loading in a geological repository, and v) precipitation of secondary minerals. The first two phenomena are not expected to have a significant effect on the glass block, but further examination of stress corrosion under realistic conditions is necessary to confirm this conclusion. Self-irradiation should not have a detrimental effect as it has been established that it improves the mechanical properties of the glass (increased toughness and decreased Young’s modulus and microhardness). Glass canisters in a geological repository could be subjected to additional stress loading by lithostatic pressure. According to the French scenario studied by ANDRA, preliminary simulations showed that some cracks could close (unpublished data). Finally, it has been shown in fractured Roman glass blocks after 1800 years on the Mediterranean seafloor that the contribution of the crack network to glass alteration diminishes dramatically over time, as the precipitation of secondary minerals within cracks slows down the transport of reactive species (Verney-Carron et al., 2010). Moreover, no evidence of stress corrosion was observed. Despite conditions much less favorable for the Roman glass blocks than for nuclear waste glass because of the very high seawater renewal rate, no glass fragments were detached from the Roman glass blocks due to corrosion. Sealing the voids within glass blocks by alteration products could also play a major role in the case of nuclear glasses, but this remains to be demonstrated under realistic conditions. 5. Influence of glass composition The glass composition is a first-order parameter. Glass composition effects are investigated using complementary methodologies based either on statistical approaches for complex glasses or a more mechanistic approach on simple materials. The former approach gives insight into major component effects and potentially on synergetic effects on glass durability, whereas the latter can help understand the relations between glass composition, glass and gel structure, gel properties, and dissolution rates (Frugier at al., 2005; Ledieu et al., 2005; Trotignon et al., 1994; Bergeron et al., 2010; Pierce et al., 2010; Gin et al., 2012; Molières et al., 2013). It is important to note that each kinetic regime has a specific composition dependency. In other words, the effect of the glass composition cannot be anticipated in different kinetic regimes simply by examining a specific regime. For example, a higher Zn concentration in borosilicate glass significantly decreases the initial dissolution rate, but also increases the residual rate (Gin et al., 2013b). Moreover, the glass composition effect is generally nonlinear and strong synergies have been observed (for example between Ca and Al or Ca and Zr in the alteration layer) (Gin et al., 2013b). This is why there is only a partial understanding of glass composition effects on glass durability. Another consequence of the complexity described above is that kinetic models are generally not parameterized for a large compositional domain. Note that a recent attempt has been made to extend the GRAAL model to the entire R7T7 domain (glasses produced by Areva at La Hague facility) by adapting the value of the diffusion coefficient of reactive species through the passivating layer to the glass composition variations (Fleury, 2013). Composition effects on glass durability are an active field of research with important spin-off applications in glass formulation as well as in performance assessment. New glasses could be formulated in the future for various kinds of wastes. 6. Influence of nearfield materials Various nearfield materials (iron, stainless or carbon steel, concrete, bentonite, etc.) have been considered for placement between the glass canisters and the host rock (Godon et al., 1992; Curti et al., 1993; Gin, et al., 2001b; Bennett and Gens., 2008; Corkhill et al., 2013). Additives such as amorphous silica, glass frit, or graphite have also been considered to improve the properties of the engineered barrier system (EBS) (Godon et al., 1992; Van Iseghem et al., 2001). Nearfield materials are expected to react with the glass, more or less rapidly depending both on


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thermodynamics and on the transport of reactive species. Many complex coupled systems have been studied to date, from laboratory mockups to in situ tests, but only simple coupled systems have been deeply investigated and understood (experimentally and by simulation). Compared to glass/water systems, the presence of reactive solids near the glass significantly increases the number of potential processes: sorption reactions, dissolution/precipitation of new phases, local changes in pH or in solution composition, local porosity clogging changing the transport properties of aqueous species, redox reactions, etc. (Inagaki et al., 1996; Gin et al., 2001b; Yamaguichi et al., 2006; Debure et al., 2012; Jollivet et al., 2012a; Jollivet et al., 2012b; Godon et al., 2013 ;Michelin et al., 2013a; MercadoDepierre et al., 2013). Moreover it has been shown that transport of reactive species at micrometer to millimeter scale is rate controlling in most of the glass-nearfield-materials systems already investigated (McGrail et al., 2001; Gin et al., 2004; Verney-Carron et al., 2008; Debure et al., 2013; Michelin et al., 2013b; Burger et al., 2013; Aradottir et al., 2013) Improving reactive transport models to account for and predict the behavior of such complex systems could help improve modeling of the reference case with limited uncertainties and later, optimize the EBS with potentially gain orders of magnitude in glass lifetime. 7. Influence of radioactivity Borosilicate glasses are known to be self-healing materials with respect to α, β and γ radiation. However, the structure and most of the physical and mechanical properties of the glass evolve slightly, mainly due to ballistic effects associated with α decay. For R7T7-type glasses, the properties typically evolve until the dose reaches about 2 × 1018 α decays/g, above which a steady state corresponding to a new fictive temperature is reached (Peuget et al., 2006; Maugeri et al., 2012). Do these structural changes and radiolysis effects impact the glass durability? Despite a large number of studies carried out on the leaching behavior of radioactive or irradiated glasses, showing no effect of radioactivity on the initial dissolution rate and little effect on the rate drop (Advocat et al., 2001), few data are available on the residual rate. A recent study suggested no significant impact of dose rate on the residual rate of a 239 Pu-doped glass altered under a dose rate of 150 Gy.hr-1 and on a 99Tc-doped glass providing a dose rate of 0.06 Gy.hr-1 (Rolland et al., 2013). Moreover the potential effects of radioactivity when glass is altered in the presence of nearfield materials require further work. 8. Kinetic modeling Several generations of kinetic models have been developed since the pioneering work by Aagaard and Helgeson in the 1980s (Aagaard and Helgeson, 1982). Initially based on a simple first-order law involving the rate of hydrolysis of the silicate network and thermodynamic parameters accounting for the rate drop, rate laws have subsequently coupled thermodynamic and transport-limiting phenomena within the alteration layer to better fit experimental data. GM2001 and GRAAL are the two advanced models used as tools for better understanding experimental results and for making predictions (at lab scale for designing new experiments or at larger scales) (Grambow and Muller, 2001; Frugier et al., 2008; Minet et al., 2010). Although such models are powerful tools, they only capture part of the basic glass corrosion mechanisms. More fundamental work is needed to validated or, if necessary, refine current models: this is the conclusion of several international workshops. Indeed it is well accepted that fundamental research on glass corrosion mechanisms will help reduce the remaining empiricism of the current models. 9. Validation by natural analogs Since the bright idea by Ewing at the end of the 1970’s, natural and archaeological glasses have been extensively used to consolidate predictions of glass performance in geological disposal (see the recent overview by Libourel et al., 2011). However, the key steps of the demonstration clearly need more work especially those focused on the jump between the human and geological timescales: are natural glasses altered at a residual rate in natural confined environments? If so, what is the corresponding rate, and are the limiting mechanisms the same as those of nuclear glasses? Several studies have shown large discrepancies between rate measurements of silicate mineral dissolution

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in the lab and in the field (e.g. Hochella and Banfield, 1995). Is this also the case with glassy natural analogs? Are the kinetic models used for predicting nuclear glass behavior able to account for the behavior of natural glasses on a geological timescale? Answering these questions could definitely bolster confidence in predictions. Because they are generally more recent, archeological glasses present the advantage of having been altered in better known environment (Verney-Carron et al., 2008, 2010). Recent investigations have been conducted on glass/iron interactions from slags recovered in a water-saturated clay medium in Normandie (Michelin et al., 2013). Other materials should be considered in the future. 10. Concluding remarks Thanks to parametric experiments, integrated mockups, in situ experiments, multi-scale characterization and modeling, tremendous progress has been made in recent decades in the field of nuclear glass corrosion. However, predicting the fate of radionuclides on a geological timescale is a major scientific challenge and gaps such as those discussed in this paper must be bridged and key questions addressed. Beyond these remaining questions, other scientific issues could arise in the future, for instance in relation with the formulation of new glasses, the optimization of the multi-barrier system, the definition of acceptance criteria of waste canisters in geological disposal, the need to maintain a high level of expertise until the final disposition of HLW glass, etc. Last but not least, another challenge for the international community would be to share pertinent criteria and standardized testing methods for laboratory tests and their transposition to geological repository conditions. This need would encourage the reinforcement of a broad international collaboration in the field. 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