파이로 공정에서 발생하는 희토류 폐기물 담지체 개발 및 침출 거동

Abstract

Pyro-processing technology is used for recovering radioactive uranium and trans-uranium elements from the spent fuels of pressurized water reactors. Several hazardous wastes produced during pyro-processing should be transformed into stable materials for safe storage. In particular, wastes resulting from the electro-winning stage of the pyro-processing process consist of eight rare earth (RE) oxides, primarily CeO2 and Nd2O3. Therefore, specific waste forms need to be developed to sequester these wastes containing high concentration of RE oxides. In this study, we fabricated glass-ceramics containing vitusite [Na3Ce(PO4)2], Ca-silicate [Ca2Nd8-xCex(SiO4)6O2] crystals and borate glasses for immobilization of lanthanide ion wastes produced from pyro-processing. We investigated the distributions of rare-earth ions in the glass-ceramics and evaluated basic characteristics as wasteforms such as waste loading, chemical durability and process-ability. In addition, we investigated the local structure of RE ions in the vitusite glass-ceramics to understand the mechanism of RE dissolution inside the crystals. First, glass-ceramics wasteforms containing vitusite [Na3Ce(PO4)2] and Ce-rich crystals were synthesized to immobilize lanthanide oxide wastes from pyro-processing of spent fuels of pressurized water reactors. Approximately 80 wt.% of total lanthanide elements were concentrated inside crystalline phases. Ce4+ and Nd3+ released during the product consistency test were below the detection limit (0.1 ppm). Local environments of Nd ions in vitusite glass and glass-ceramic waste forms were analyzed using EXAFS spectroscopy. A second shell observed in the spectrum of glass-ceramics was attributed to the combination of Nd-Na and Nd-P pairs. Preferred incorporation of Nd3+, P5+, and Na+ inside the crystalline phases surrounded by the glass matrix, resulted in the increase of chemical durability of glass-ceramics. These results can be used in the development of new rare-earth waste forms exhibiting a high chemical durability. Second, we have synthesized glass-ceramics wasteforms containing calcium neodymium (cerium) oxide silicate [Ca2Nd8-xCex(SiO4)6O2] crystals to immobilize lanthanide oxide wastes generated from pyro-processing of spent fuels of pressurized water reactors. Controlled crystallization of alkali borosilicate glasses by heating at T ≥ 750°C for 3 h formed hexagonal Ca-silicate crystals. Maximum lanthanide oxide waste loading was > 26.8 wt%. Approximately 44 wt% of Ce and Nd ions were partitioned inside the crystalline phases and this value is ~ 2.6 times larger than that in the amorphous matrix. The normalized amounts of Ce and Nd ions released from specimens were ~106 g·m-2 for PCT test and below the detection limit of ICP-MS (< 0.1 ppb) for MCC-1 test. Glass-ceramics wasteforms containing Ca-silicate crystals are good candidates for the immobilization of the lanthanide wastes generated by pyro-processing technology. Finally, borate glasses with composition (mol %) of 20 CaO - 15 Al2O3 - 45 B2O3 -20 Nd2O3 were fabricated by melting at 1300 ºC for 30 min. Maximum solubility of the rare-earth oxides in the glasses was evaluated as 22 mol% (56.8 wt%). Processing temperature of these glasees were ~150 ºC lower than the melting points of competitive lanthanide aluminosilicate glasses with a slightly higher RE-loading. In addition, the borate glasses were satisfied with the requirements to adjust to cold crucible induction melter (CCIM) such as viscosity (10-100 poise) and electrical conductivity (0.1-1 S/cm). The normalized released amounts of Ca, Al and B were 10-3-10-4 g/m2day and the released concentration of Nd were below the limit of detection of ICP-MS (< 0.1 ppb). As addition of RE dramatically decreased released amounts, we proposed how to formations of RE-precipitates at the glass surface during dissolution and this can retard a dissolution of the glasses.