Surface Alteration of Borosilicate and Phosphate Nuclear Waste Glasses by Hydration and Irradiation

Abstract

We examined the degradation of nuclear waste borosilicate and phosphate glasses containing strong alpha-emitter 238Pu at a specific activity of 6.33 × 105 MBq/g in comparison with similar non-radioactive, non-radioactive irradiated and radioactive samples containing beta- and gamma-emitters, namely radionuclides 134Cs and 137Cs. For irradiation and leaching experiments, we used borosilicate and phosphate glasses, which are well-known and currently used to immobilize high-level radioactive waste. The main focus was the observation of the surface of altered glasses. Comparative analysis of hydrolytic surface alteration of borosilicate and phosphate nuclear waste glasses reveals that the behavior of radioactive samples differs significantly from that of non-radioactive glasses.

Long-Term Chemical Alteration of 238Pu-Doped Borosilicate Glass in a Simulated Geological Environment with Bentonite Buffer

“…During the first examination using optical microscope, formation of a secondary phase on the glass surface was clearly observed (Figure 7). However, the alteration of the glass observed in wet bentonite medium is much less pronounced than for borosilicate glass in contact with distilled water at temperature 90 • C [9,10]. The reasons for the discrepancy are not yet fully understood, but variations in pH and eventual partial radiation-induced destruction of Teflon TM container with release of fluorine compounds in experiments described in [9] may be responsible.…”

“…Thus, using 238 Pu, more pronounced aging of the glass can be achieved in a shorter period of time; more extensive radiation damage of the surrounding bentonite can be expected as well. The Pu-doped borosilicate glass has been synthesized in 2016 [9,10] by melting the oxide mixture with a suitable frit at temperature of 1400 • C for 2 h in air atmosphere (Figure 1). The glass was doped with 0.42-0.45 wt.…”

“…These effects include chemical degradation of the glass matrix, thermal and radiation damages in bentonite, and local change of reducing and oxidizing conditions due to water radiolysis. Since experiments with real vitrified HLW are expensive and complicated, information about the long-term behavior of highly radioactive glass under self-irradiation and contact with water is very limited [5][6][7][8][9][10]. However, it was reported earlier that the chemical alteration of a real highly radioactive glass doped with 238 Pu in water is much more intense in comparison with a simulated non-radioactive glass of similar chemical composition; pronounced glass degradation is observed [9,10].…”

“…Radiation effects in bentonite and other clay materials had been studied mainly in terms of the stability of clay materials under external gamma-irradiation [9][10][11]. It was shown that bentonite has high radiation stability and preserves crystallinity with no significant changes in its structure at accumulated doses as high as 3 × 10 10 rad at room temperature and at 3.5 × 10 9 rad at 300 • C [12,13].…”

Forty years of durability assessment of nuclear waste glass by standard methods

“…Where higher-level wastes are concerned, radiation and radiation damage may influence glass durability and therefore studies on active glasses are extremely valuable. Such studies are few with some studies indicating greatly increased degradation in active glasses [184][185][186][187][188] whist others imply little or no change 189,190 .…”