The safe disposal of nuclear waste is a critical concern. In many countries, high-level radioactive waste is immobilized in borosilicate glasses intended for deep geological disposal. A key factor in assessing the long-term safety of these repositories is understanding the long-term behavior of the glass in contact with groundwater. Water alters the glass through a series of processes, including ion exchange and hydrolysis of chemical bonds within the glass network. This alteration leads to the release of radionuclides into the environment, a process that needs careful investigation and modeling to ensure long-term safety. The alteration process typically involves three distinct stages: an initial interdiffusion stage dominated by ion exchange, an initial dissolution stage with a rate controlled by the hydrolysis of the glass network, and a residual dissolution stage where the alteration rate decreases due to several factors. This research focuses on understanding how external irradiation, simulating the effects of self-irradiation in nuclear waste glasses, affects these alteration processes. Previous research has shown conflicting results regarding the impact of irradiation on glass alteration, with some studies showing increased alteration rates while others have not found a significant impact. This inconsistency necessitates further investigation into the relationship between structural changes within the glass and the mechanisms of alteration.

Extensive research exists on the aqueous alteration of borosilicate glasses. Reviews summarize the current understanding of the initial dissolution and residual dissolution regimes, highlighting the importance of hydrolysis reactions and the formation of passivating layers. The interdiffusion stage, characterized by ion exchange and hydrolysis of B-O-Si linkages, is also well-studied, with its dependence on pH and glass composition being established. However, the effects of self-irradiation from radioactive decay on glass alteration are less well understood. While it's known that self-irradiation modifies the glass structure, leading to changes in density, boron coordination, hardness, and fracture toughness, the impact of these structural modifications on the alteration kinetics remains a topic of ongoing research. Studies using heavy-ion irradiation to simulate self-irradiation have yielded mixed results, with the effects of irradiation appearing more pronounced in simpler glasses. The lack of a comprehensive understanding of these effects motivates the present study, which aims to clarify the influence of irradiation-induced structural changes on the alteration kinetics of a simplified borosilicate glass.

This study employed a simplified 4-oxide borosilicate glass (CJ2 glass), lacking Ca and Zr found in the International Simple Glass (ISG) glass. CJ2 glass was chosen for its low non-bridging oxygen (NBO) content (approximately 0.037 per tetrahedron of Si + Al), indicating that sodium primarily serves as a charge compensator for BO4 and AlO4 units. Samples were irradiated using 7 MeV Au ions at a fluence of 2 × 10¹⁴ at/cm², a dose sufficient to induce maximum structural changes within the first two microns of the material. The aqueous alteration behavior of irradiated and non-irradiated CJ2 glass was compared through a series of experiments. Interdiffusion was studied at 30 °C and pH 1 using ToF-SIMS to analyze the altered layers. Initial dissolution rate was measured at 90 °C and pH 9 using a static mode experiment, measuring the release of Si, B, Na, and Al into the solution via ICP-OES. The residual dissolution rate was studied at 90 °C and pH 9 in a solution close to saturation with respect to amorphous silica, again using ICP-OES to measure the release of B. Raman spectroscopy was used to characterize the structural changes in the glass before and after irradiation. ToF-SIMS was also extensively used to analyze the alteration layers and gels formed during residual rate experiments, allowing detailed characterization of elemental and isotopic distributions within the altered layers. The analysis included the use of exogenous tracers like Potassium to identify the depth of alteration, and Oxygen isotopes to monitor the hydrolysis/condensation processes. This comprehensive experimental approach enabled a detailed examination of the impact of irradiation on the glass alteration process across different stages and conditions.

Raman spectroscopy showed that irradiation caused a shift in the Si-O-Si bond angle, depolymerization of the silicate network, and an increase in the fraction of tri-coordinated boron (BIII) and the number of NBOs. Interdiffusion experiments at 30 °C and pH 1 revealed a dramatic increase (more than an order of magnitude) in the interdiffusion rate for the irradiated glass compared to the control. The congruent dissolution of boron and sodium strongly suggests that the rate-limiting step involves the breaking of B-O-Si bonds. In the initial dissolution experiments at 90 °C and pH 9, the irradiated glass exhibited a 2.3 times higher dissolution rate than the non-irradiated glass. The residual dissolution rate experiments at 90 °C and pH 9 in silica-saturated solutions showed that more glass was altered in the irradiated sample than in the control, indicating that the passivation was less efficient in the irradiated glass. ToF-SIMS analysis revealed the thickness of the gel layer formed in the residual rate regime and the incorporation of ¹⁸O from water molecules into the gel structure, showing that the gel of the irradiated glass reorganized more rapidly. It was noted that the gel on the irradiated sample showed faster incorporation kinetics of ¹⁸O, and lower retention of B. The tracing experiment, performed after the main residual rate experiments using ¹⁰B and ¹⁸O, showed similar gel thicknesses and confirmed the slow diffusivity of B within the gel layers. In this case, the glass alterations were increased by more than an order of magnitude after 8 hours and 34 times after 3 days.

The results demonstrate a clear effect of pre-irradiation on the alteration of the CJ2 glass. The increased interdiffusion rate under acidic conditions can be attributed to a combination of factors, including possible glass swelling, an increased proportion of ¹¹B (which dissolves faster), and stored energy within the irradiated glass. The increase in the initial dissolution rate at basic pH is less pronounced and might be explained by the increased fraction of tri-coordinated boron and the presence of more small-sized rings in the silicate network, making the network less resistant to dissolution. The enhanced residual dissolution rate is attributed to the reduced passivation efficiency of the gel formed on the irradiated glass. This difference in passivation likely arises from the faster restructuring of the gel layer in the irradiated glass, as evidenced by the more rapid incorporation of ¹⁸O into the gel. These findings highlight that the impact of irradiation on glass alteration is strongly dependent on the alteration conditions, with the most significant effects observed in acidic conditions where interdiffusion is prominent. This could influence long term behavior of glass for nuclear waste disposal, where simultaneous processes of alteration and radiation damage can occur.

This study demonstrates that irradiation significantly affects the chemical durability of a 4-oxide borosilicate glass, particularly impacting interdiffusion and, to a lesser extent, initial and residual dissolution. The results highlight the importance of considering the combined effects of radiation damage and aqueous alteration in predicting the long-term performance of nuclear waste glasses. Future work should investigate the dynamics of disorder creation and gel reorganization in more detail to improve predictive models for long-term behavior of nuclear waste glasses, particularly those incorporating the effects of self-irradiation alongside alteration.

The study utilized a simplified 4-oxide borosilicate glass as a model for more complex nuclear waste glasses. While this simplification aids in mechanistic understanding, the results might not fully translate to the complex compositions of actual nuclear waste glasses. The irradiation was performed ex situ before the alteration experiments, which might not fully capture the complex interplay of simultaneous radiation damage and aqueous alteration in real nuclear waste environments. Further research is needed to investigate the effects of in situ irradiation on glass alteration.