Oxide glass structure evolution under swift heavy ion irradiation
Introduction
Glass is the primary containment barrier in the case of high-activity nuclear waste disposal. The minor actinides, fission products, and other elements in the vitrified waste solution form an integral part of the glass structure. The main sources of radiation are beta decay of fission products and alpha decay of the actinide elements. Spontaneous fission of some of the actinide isotopes and alpha–neutron reactions are also sources of fission fragments and neutrons, but these radiation sources can generally be ignored in disposal conditions because of their low production rates [1]. After a few centuries, most of the radioactivity will be due to alpha decay of minor actinides. It is therefore important to assess the impact of accumulated alpha decay damage on the glass structure.
Recent studies [2], [3] have shown that the alpha decay dose received by curium-doped glass has a direct impact on its fictive temperature and that the energy dissipated by the recoil nucleus during alpha disintegration results in the formation of a new structure similar to that of very rapidly quenched glass. This phenomenological description evokes the formation of ion tracks described by the inelastic thermal spike model [4], [5] during irradiation by swift heavy ions (SHI). In this model, the passage of a swift heavy ion produces a localized high energy deposit by electronic interaction, resulting in local melting along the ion track followed by very rapid thermal quenching. It is of interest to compare the consequences of this type of irradiation on oxide glasses with the consequences resulting from alpha decay of minor actinides.
The effects of ion tracks have been widely investigated in amorphous silica [6], [7], [8], [9], [10], [11], [12], [13], [14], [15] and metallic glass [10], [16], [17], [18] but no studies have been conducted to date on their impact on the structure of borosilicate glasses. The objective here was to assess the impact of ion tracks on simple borosilicate glass specimens (containing 3 and 6 oxides), which are in fact simplified versions of the French R7T7-type nuclear glass [19]. The glass specimens were irradiated by krypton ions (74 MeV) and xenon ions (92 MeV). A silica glass specimen was also irradiated to study the irradiation response of a simple single-oxide glass with anomalous high-temperature behavior compared to that of borosilicate glass [20]. The effect of irradiation on the glass structure was then examined by Raman spectroscopy and nuclear magnetic resonance (NMR) spectroscopy. Changes in the glass properties were observed by optical interferometry and micro-indentation.
Section snippets
Samples
A 1 mm thick Suprasil 300 glassy SiO2 plate was used as a reference material. The plate was cut into 1.5 × 1.5 cm samples.
The BS3 and BS6 glasses (see Table 1 for compositions) were synthesized by melting oxides, carbonates and nitrates. The mixture was melted for 3 h in a platinum–rhodium–yttrium crucible without stirring at 1523 K for the BS3 glass and 1673 K for the BS6 glass, and then poured into a graphite crucible. The glasses were then annealed above the glass transition point (∼873 K) from
Density variations
When subjected to ion irradiation several processes are known to lead to dimensional changes of glasses, density changes induced by the glass structural modifications, irradiation creep associated to stress relaxation and ion hammering induced by irreversible macroscopic deformations perpendicular to the ion path [33].
The density variation can be estimated from the step height and transverse step measurements performed by optical interferometry and knowing the damaged depth. The damaged depth
SiO2
After irradiation by 74 MeV krypton ions, silica exhibits about 3.3% densification and a 24% hardness decrease. This suggests that the observed decrease in hardness is not due to a decrease in the atomic density of the glass but more likely to an increase after irradiation in the concentration of weak points such as dangling bonds, as suggested by Bibent based on the infrared spectrum of silica after ion irradiation [54].
Irradiation also increases the intensity of the Raman D2 band at 607 cm−1.
Conclusion
The behavior of two borosilicate glass compositions containing 3 and 6 oxides was evaluated under irradiation by 74 MeV krypton ions and 92 MeV xenon ions.
A combination of Raman spectroscopy analysis with a multinuclear NMR approach (11B, 23Na, 27Al and 29Si) has shown that the glass structure after irradiation resembles a vitreous state frozen from a very high-temperature liquid (depolymerization of the silicate network, lower boron coordination number, increased disorder) but also exhibits
Acknowledgments
This project was carried out under a research program jointly funded by CEA and AREVA.
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