Yuanzineng kexue jishu (Jul 2023)
Thermodynamic Analysis for Effect of Waste Resin Irradiation Gas on Mineral Composition of Ordinary Portland Cement
Abstract
Recently, cement matrixes become a mainstream solution to address the stabilization of radioactive spent resins. Generally, the cumulative dose of spent resin cemented waste form will reach approximately 106 Gy over a disposal lifetime of 300 years. Due to the organic properties of spent resin, namely producing gas if being irradiated, it inevitably affects the mineral phase of cement waste forms. Therefore, the content and composition of the mineral phase could be key factors determining the performance of cement waste forms. With that in mind, in this paper, the influence of the above factors on ordinary Portland cement (OPC) was explored especially. Specifically, based on the Gibbs free energy minimization principle, the influence of three gases generated by irradiation of waste resin, namely H2, CH4 and CO2, on the mineral phase composition and total volume of ordinary Portland cement was discussed. Thereafter, the evolution of the mineral phase composition of ordinary Portland cement over time was investigated based on the rate of irradiated gas production. In this work, all the experiments were produced by GEMS software based on the thermodynamic Gibbs free energy minimization method. Meanwhile, the formulation for cement solidification of spent resin meets the standard of GB 14569.1-2011. From the experimental results, several important findings were obtained. Firstly, only a small amount of H2 and CH4 reacts with the cement hydration products, which causes the dissolution of the mineral phase with high Ca/Si, while the total volume of the mineral phase is almost invariant. Secondly, in the prior phase, CO2 first reacts with the portlandite in the cement hydration products. After the portlandite dissolute completely, the silicate minerals start to dissolve, forming a system composed of abundant carbonate and a few clay minerals, which decreases the total volume of the mineral phase. Thirdly, during the predicted 160 years, the effects of spent resin irradiation gases on the cemented waste form are consistently dominated by CH4 and H2, while the performance of the cemented waste form is stable. However, the risk of rupture and explosion caused by the accumulation of H2 and CH4 in the waste packaging containers and the risk of radioactive release due to carbonation of the cemented waste form by CO2 should not be overlooked. In summary, the above results reveal the evolution law of degradation of cement solidified body caused by waste resin irradiation gas. The results of this paper provide effective data support for the safety evaluation of waste resin treatment by cement solidified form.
