模拟锕系核素和硫在钡硼硅酸盐玻璃陶瓷固化体中的赋存状态研究

The borosilicate glass waste forms for the immobilization of high level liquid waste (HLLW) have some drawbacks such as the low solubility of actinides and poor thermal stability and chemical durability. In addition, a separate yellow phase containing sulfur is usually formed during the vitrification process. In this project, barium borosilicate glass-ceramic waste forms containing zirconolite, titanite, and barite crystalline phases will be prepared by melt-heat treatment technique. It is expected that the simulated actinides (Nd and Ce) are immobilized in the zirconolite and titanite phases, while the sulfur is immobilized in the barite phase. The relationships among the composition, heat treatment processing, phase structure, microstructure, and physical properties of the glass-ceramic waste forms will be studied. The crystallization mechanism of the zirconolite and titanite phases in the barium borosilicate glass will also be discussed. The processing techniques of the barium borosilicate glass-ceramic waste forms will be optimized. Furthermore, the occurrence of simulated actinides (Nd and Ce) and sulfur in the barium borosilicate glass-ceramic waste forms will be investigated. Moreover, the chemical durability of the barium borosilicate glass-ceramic waste forms will be evaluated. It is expected that the research results can provide theoretical basis and technical support for the optimization and preparation of high-performance actinides and sulfur bearing glass-ceramic waste forms, and lay a good foundation for the applied basic research and safe disposal of the immobilization of HLLW using glass-ceramics.

针对目前高放废液硼硅酸盐玻璃固化体存在锕系核素溶解度较低、稳定性较差等不足，以及在玻璃熔制过程中产生分离的含硫黄色第二相（黄相）问题，本项目拟采用熔融-热处理工艺制备含钙钛锆石、榍石和重晶石晶相的钡硼硅酸盐玻璃陶瓷固化体，其中钙钛锆石和榍石相用来固定模拟锕系核素（Nd、Ce），重晶石相用来固定硫元素。系统研究钡硼硅酸盐玻璃陶瓷固化体的组分和热处理工艺与晶相组成、显微结构和物理性能之间关系，探讨钙钛锆石和榍石在钡硼硅酸盐玻璃中的析晶机制，优化集成固化体的制备工艺技术；揭示模拟锕系核素（Nd、Ce）及硫元素在钡硼硅酸盐玻璃陶瓷固化体中的赋存状态，评价固化体的化学稳定性。相关研究成果可为优化和制备高性能含硫锕系高放废液玻璃陶瓷固化体提供理论依据和技术支持，为推进高放废液玻璃陶瓷固化的应用基础研究及其安全处置奠定基础。

为提高锕系核素和硫元素在玻璃固化体中的包容量，本项目在钡硼硅酸盐玻璃中添加适量的CaO、TiO2和ZrSiO4，采用熔融-热处理工艺获得了含有钙钛锆石、榍石和重晶石等稳定晶相的致密的钡硼硅酸盐玻璃陶瓷固化体，并优化集成了固化体的制备工艺技术。利用Kissinger方法计算得到钙钛锆石和榍石的晶体生长活化能分别为124.38和166.13 kJ/mol，晶体生长因子分别为3.4和2.2，分别对应三维和二维晶体生长机制。系统研究了模拟锕系元素及硫元素含量对钡硼硅酸盐玻璃陶瓷固化体晶相组成、显微结构、抗浸出性能的影响，探讨了模拟锕系元素和硫元素在璃陶瓷固化体中的赋存状态。当Nd2O3含量为2~6 wt%时，玻璃陶瓷主要含钙钛锆石和榍石晶相；当Nd2O3的含量为8 wt%时，开始出现方形钛酸钙晶体，而Nd元素易在钛酸钙晶体中富集。对于Nd2O3掺量为6 wt%的玻璃陶瓷，Nd元素均匀分布在钙钛锆石、榍石和残余玻璃相中，约3.2 wt%的Nd2O3分布在残余玻璃基体中，42天后B、Ca、Nd元素的归一化浸出率分别为6.8×10-3、1.6×10-3和4.4×10-6 g•m-2•d-1。当SO3掺量大于或等于4 wt%时，玻璃陶瓷体内除了含钙钛锆石晶相外，还出现了颗粒状的重晶石晶体，S元素主要分布在重晶石晶体中。重晶石晶体在玻璃熔制阶段生成，当SO3掺量为6 wt%时，玻璃陶瓷中保留的SO3含量仅为1.3 wt%。通过本项目研究，验证了模拟锕系核素和硫元素能够被固定在钡硼硅酸盐玻璃陶瓷的稳定晶相中。本项目的顺利完成，为制备高性能含硫锕系高放废液玻璃陶瓷固化体提供了实验数据和理论依据，为推进高放废液的玻璃陶瓷固化及其最终安全处置提供了技术储备。