High-temperature Thermal Decomposition of Cs-adsorbed CHA-Cs and CHA-PCFC-Cs Zeolite System, and Sr-adsorbed 4A-Sr and BaA-Sr Zeolite System

Abstract

For the immobilization of high-radioactive nuclides such as Cs and Sr by high-temperature thermal decomposition, this study was carried out to investigate the phase transformation with calcined temperature by using TGA (thermogravimetric analysis) and XRD (X-ray diffraction) in the Cs-adsorbed CHA (chabazite zeolite of K type)-Cs and CHA-PCFC (potassium cobalt ferrocyanide)-Cs zeolite system, and Sr-adsorbed 4A-Sr and BaA-Sr zeolite system, respectively. In the case of CHA-Cs zeolite system, the structure of CHA-Cs remained at up to 900°C and recrystallized to pollucite (CsAlSi2O6) at 1,100°C after undergoing amorphous phase at 1,000°C. However, the CHA-CFC-Cs zeolite system retained the CHA-PCFC-Cs structure up to 700°C, but its structure collapsed in 900~1,000°C, and then transformed to amorphous phase, and recrystallized to pollucite at 1,100°C. In the case of 4A-Sr zeolite system, on the other hand, the structure of 4A-Sr maintained up to 700°C and its phase transformed to amorphous at 800°C, and recrystallized to Sr-feldspar (SrAl2Si2O8, hexagonal) at 900°C and to SrAl2Si2O8 (triclinic) at 1,100°C. However, the BaA-Sr zeolite system structure began to break down at below 500°C, and then transformed to amorphous phase in 500~900°C and recrystallized to Ba/Sr-feldspar (coexistence of Ba0.9Sr0.1Al2Si2O8 and Ba0.5Sr0.5Al2Si2O8) at 1,100°C. All of the above zeolite systems recrystallized to mineral phase through the dehydration/(decomposition)→ amorphous → recrystallization with increasing temperature. Although further study of the volatility and leachability of Cs and Sr in the high-temperature thermal decomposition process is required, Cs and Sr adsorbed in each zeolite system are mineralized as pollucite, Sr-feldspar and Ba/Sr-feldspar. Therefore, Cs and Sr seen to be able to completely immobilize in the calcining wasteform/(solidified wasteform).