This study aimed to evaluate the long-term stability of glass wool used as insulation material in domestic nuclear power plants and to quantify its degradation mechanisms and dissolution kinetics under highly alkaline conditions (pH ≥ 12) expected in cementitious environments associated with vault-type disposal systems for low-level radioactive waste. Experiments were conducted at 20 °C and 80 °C using cement-saturated groundwater (CGW) as the primary solution, while comparative tests were performed in NaOH and Ca(OH)₂ solutions at equivalent pH levels. ICP-OES, SEM-EDS, and XRD analyses revealed that dissolved Ca²⁺ significantly suppressed glass dissolution. The presence of abundant Ca²⁺ ions promoted densification of the surface alteration layer, retarding degradation, whereas depletion of Ca²⁺ resulted in a rapid increase in the dissolution rate. Although calcium silicate hydrate (CSH) precipitates are generally known to inhibit glass corrosion, the CSH phases formed in this study exhibited limited protective capability due to their low Ca/Si ratios and high porosity. Based on the dissolution rate constant at 20 °C, the complete dissolution of glass wool was estimated to require approximately 213 years; however, under conditions of limited Ca²⁺ availability, the dissolution rate could increase by up to 70-fold, approaching that observed in NaOH solution.

The long-term durability of metakaolin (MK)-based geopolymer waste forms was evaluated under simulated disposal environments. Specimens incorporating Cs, Sr, and Co were fabricated and subjected to ANSI/ANS 16.1 leaching tests in deionized water (DI), concrete-saturated groundwater (CGW), and CGW combined with gamma irradiation (CGW + γ). Compressive strength, leachability index (LI), microstructure, and surface chemistry were analyzed to evaluate mechanical and chemical stability. All specimens initially satisfied the disposal acceptance criterion (≥ 3.45 MPa). The compressive strength remained stable in DI but declined under CGW due to alkali-induced degradation. Under the CGW + γ condition, severe strength loss occurred, and several specimens fractured completely. Leaching tests revealed substantial LI reduction for Cs, Sr, and Co. Computed tomography imaging showed increased porosity under CGW + γ, while X-ray photoelectron spectroscopy analysis confirmed Cs and Sr surface enrichment with the reduction of Co3+ to Co2+. These results indicate that while MK-based geopolymers demonstrated strong resistance to individual degradation factors, they experienced significant structural deterioration under combined alkaline and radiation conditions, providing critical insights for the safe disposal of radioactive wastes.