Bubble formation in nuclear glasses: A review
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Bubble formation in nuclear glasses: A review Laura Leay1,a)
Mike T. Harrison2
1
Dalton Cumbrian Facility, The University of Manchester, Moor Row, Cumbria CA24 3HA, U.K. Waste and Residues Processing Services, National Nuclear Laboratory, Sellafield, Seascale CA20 1PG, U.K. a) Address all correspondence to this author. e-mail: [email protected] 2
Received: 26 July 2018; accepted: 16 January 2019
Highly radioactive waste is incorporated into a glass matrix to convert it into a safe, passive form suitable for long-term storage and disposal. It is currently known that alpha decay can generate gaseous species, which can nucleate into bubbles, either through the production of helium or from ballistic collisions with the glass network that liberate oxygen. An effective method to probe this phenomenon utilizes ion beams to either directly implant helium or investigate the damage due to ballistic collisions. This paper provides an overview of the methodology, summarizes the results of current studies, and draws comparisons between them. We find that the irradiation scheme as well as the temperature and composition of the glass are important in determining whether bubble formation will occur. We also explore how analytical techniques can promote bubble formation and suggest avenues for further work.
Introduction Vitrification is used by various countries around the world to immobilize high-level radioactive waste (HLW) in a glass matrix. This wasteform generally has an amorphous structure, although it can also incorporate crystalline regions. The composition of these glasses is complex: HLW contains around 30 fission products, and the exact composition can vary depending on the origin. These fission products are radioactive, and the decay of these species involves a variety of pathways. Alpha, beta, and gamma radiation will produce electronic effects, which involve excitation of electrons. Alpha decay also causes recoil of the parent atom. Both the recoil nucleus and the alpha particle itself generate ballistic collisions where the incident ion interacts with the nucleus of the target atom. There are many outstanding questions regarding the effects of radiation on HLW glass [1]. These mechanisms can create defects in the glass structure, which can cause the glass to swell or compact depending on the composition of the glass. Swelling, and the associated decrease in density, can lead to more free space (interstitial sites) in the glass [2]. The radiolytic breakdown of the glass can also generate gas molecules. These can diffuse through the glass and become trapped in these interstitial sites. The number of interstitial sites is related to the solubility of the gas and, once all interstitial sites are filled, gas molecules can cluster and so
ª Materials Research Society 2019
could form bubbles [3]. The complexity of the glass composition and structure and the variety of mechanisms of radioactive decay provide a challenge for studying bubble formation. It is important to understand this phenomenon, so that the long
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