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

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℃ and recrystallized to pollucite (CsAlSi₂O₆) at 1,100℃ after undergoing amorphous phase at 1,000℃. However, the CHA-CFC-Cs zeolite system retained the CHA-PCFC-Cs structure up to 700℃, but its structure collapsed in 900⁓1,000℃, and then transformed to amorphous phase, and recrystallized to pollucite at 1,100℃. In the case of 4A-Sr zeolite system, on the other hand, the structure of 4A-Sr maintained up to 700℃ and its phase transformed to amorphous at 800℃, and recrystallized to Sr-feldspar (SrAl₂Si₂O₈, hexagonal) at 900℃ and to SrAl₂Si₂O₈ (triclinic) at 1,100℃. However, the BaA-Sr zeolite system structure began to break down at below 500℃, and then transformed to amorphous phase in 500⁓900℃ and recrystallized to Ba/Sr-feldspar (coexistence of Ba_0.9Sr_0.1Al₂Si₂O₈ and Ba_0.5Sr_0.5Al₂Si₂O₈) at 1,100℃. 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/(solidifed wasteform)