Investigation of high-temperature chemical interaction of calcium silicate insulation and cesium hydroxide

The interaction of cesium hydroxide and a calcium silicate insulation material was experimentally investigated at high-temperature conditions to evaluate the possibility of unprecedented cesium retention under severe accident of boiling water reactor. The temperature where the interactions occurred and chemical species of cesium after the interaction were examined in this study. A thermogravimetry equipped with differential thermal analysis was used to analyze thermal events in the samples of mixed calcium silicate and cesium hydroxide under oxidizing and reducing atmospheric conditions with a maximum temperature of 1100°C. Before being mixed with cesium hydroxide, a part of calcium silicate was pretreated at high temperature to evaluate the effect of possible structural changes of this material due to a preceding thermal history and also for the sake of thermodynamic evaluation. It was found that for the original material, as xonotlite (Ca6Si6O17(OH)2) crystal, the endothermic reaction with cesium hydroxide occurred over the temperature range 575–730°C; meanwhile, for heat-treated material, which varied the crystal phase of original material to wollastonite (CaSiO3), the interaction occurred over the temperature range 700–1100°C. The X-ray diffraction analyses have indicated that both types of calcium silicates regardless of the atmospheric conditions, cesium aluminum silicate, CsAlSiO4, was formed with aluminum in the samples as an impurity or adduct. The insolubility of this formed cesium suggested the potency of cesium localization in the primary containment vessels on other structural materials that possess similar elements to that of calcium silicate insulation; hence, an effective decommissioning process could be developed.本研究针对氢氧化铯与硅酸钙绝热材料在高温环境下的相互作用开展实验探究，以评估沸水堆（Boiling Water Reactor）严重事故下实现前所未有的铯滞留的可能性。本研究考察了相互作用发生的温度，以及相互作用后铯的化学形态。研究采用配备差热分析功能的热重分析仪，在氧化与还原大气环境下，对硅酸钙与氢氧化铯的混合样品进行热行为分析，最高测试温度达1100℃。在与氢氧化铯混合前，部分硅酸钙样品先经高温预处理，以评估该材料因前期热历史引发的结构变化所带来的影响，同时也用于热力学评估。研究发现，对于原始硅酸钙材料（其晶型为硬硅钙石（xonotlite，Ca₆Si₆O₁₇(OH)₂）），其与氢氧化铯的吸热反应发生在575–730℃的温度区间内；而对于经热处理后的材料，其原始晶型转变为硅灰石（wollastonite，CaSiO₃），二者的相互作用则发生在700–1100℃的温度区间。X射线衍射分析表明，无论大气环境如何，两种硅酸钙样品中均会生成铯铝硅酸盐（CsAlSiO₄），其中铝以杂质或加合物的形式存在于样品中。该生成产物的难溶性表明，在反应堆主安全壳容器内，铯可滞留于与硅酸钙绝热材料元素组成相似的其他结构材料表面；据此可开发出高效的退役处理工艺。