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Optimization of Metakaolin-Based Geopolymer for Solidification of Cesium and Molybdenum
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The sustainability of the nuclear power industry hinges increasingly on the safe, long-term disposal of radioactive waste. Despite significant innovations and advancements in nuclear fuel and reactor design, the quest for a permanent solution to handle accumulating radioactive waste has received comparatively less attention. Conventionally, two widely recognized solidification methods, namely cementation for low and intermediate-level waste and vitrification for high-level waste, have been favored due to their simplicity, affordability, and availability. Recently, geopolymers have emerged as an appealing alternative, gaining attention for their minimal carbon footprint, robust chemical and mechanical properties, cost-effectiveness, and capacity to immobilize a broad spectrum of radionuclides, including radioactive organic compounds. This study delves into the synthesis of metakaolin-based geopolymers tailored for the immobilization of fission products like cesium (Cs) and molybdenum (Mo). The investigation unfolded in two key steps. In the initial step, we optimized the alkali content to prevent the occurrence of efflorescence, a potential issue. Remarkably, as the Na2O/Al2O3 ratio increased from 0.82 to 1.54, we observed significant enhancements in both compressive strength (11.45 to 27.07 MPa) and density (up to 2.23 g/cm3). This suggests the importance of careful adjustment in achieving the desired geopolymer characteristics. The second phase involved the incorporation of 2wt% of Cs and Mo, both individually and as a mixture, into the geopolymer matrix. We prepared the GP paste, which was poured into cylindrical molds and cured at 60°C for one week. To scrutinize the crystallinity, phase purity, and bonding type of the developed matrix, we employed XRD and FTIR techniques. Additionally, we conducted standard compressive strength tests (following ASTM C39/C39M-17b) to assess the stacking durability and robustness of the developed waste form, vital for storage, handling, transportation, and disposal in a deep geological repository. Furthermore, to evaluate the chemical durability, diffusivity and leaching of the GP waste matrix, we employed the ASTM standard Product Consistency Test (PCT: C 1285-02) and American nuclear society’s devised leaching test (ANS 16.1). It is noteworthy that the introduction of Cs and Cs/Mo in the GP matrix led to a reduction of more than 50% and 60% in compressive strength, respectively. This outcome may be attributed to the interference of Cs and Mo with the geopolymerization process, potentially causing the formation of new phases. However, it is crucial to emphasize that both developed matrices exhibited an acceptable normalized leaching rate of less than 10-5 g·m-2·d-1. This finding underscores the promising potential of the GP matrix for the immobilization of cationic and anionic radioactive species, paving the way for more sustainable nuclear waste management practices.
