Stability of Glass-ceramic Solidified Body of High Level Chloride Salts from Pyro-reprocessing of Spent Nuclear Fuels

In comparison with spent fuels from pressurized water reactors, spent fuels from fast breeder reactors contain much more transuranic elements and fission products, and hence are of much higher radioactivity. This makes the traditional wet reprocessing method by solvent extraction difficult and as a result the pyro-reprocessing method by molten salts was developed. In this case, high level waste salts will be produced instead of high level liquid waste. In order to protect human beings and the environment from the danger caused by the highly water-soluble salts, proper solidification measures must be taken prior to the final geological disposal. When mixed alkali chloride salts are chosen in the pyro-reprocessing process, chlorine-containing waste salts will generated with high contents of alkalis, strontium and rare earths. One promising way to solidify this type of salt waste is the glass-ceramic composite method, in which the chlorides are converted to stable ceramic phases and bound with glass binders. This technology combines the advantages of higher chlorine-containing ability of ceramic and lower sintering temperature with glass. Based on the previous studies in ceramic formulations, sintering arts and development of glass binders, MCC-1 leaching test was carried out to investigate the chemical and thermal stability of prepared glass-ceramic composite. MCC-1 results show that the normalized leaching rates of Si, B, Na, Cs, Nd, and Al of the samples after 28 days of leaching test are all at a low level (0.382 g/(m2·d) for Si, 0.537 g/(m2·d) for B, 0.305 g/(m2·d) for Na, 0.587 g/(m2·d) for Cs, 0.004 g/(m2·d) for Nd, 0.282 g/(m2·d) for Al). Meanwhile, SEM images show that, after 28 days of leaching, the surface of samples is eroded, with slight changes in the regularity of the surficial shape. The prepared glass-ceramic samples were also heat treated at different temperatures (850, 900, 950 and 1 000 ℃) for 24 h. After the heat treatment, although the microscopic morphology and structure changes are not obvious, the production of Cl-sodalite is changed, with the highest production rate occurring at 850 ℃. The pores within samples reduce and the sample structure becomes denser at 950 ℃/1 000 ℃. It is founded that 850 ℃/900 ℃ are more helpful to improve the homogeneity of samples and the even distribution of chlorosiderite and glass phases. In summary, the prepared glass-bound Cl-sodalite samples are of good chemical and thermal stability.