Development of pyrometallurgical partitioning technology of long-lived nuclides. Measurement of thermodynamic properties of americium in molten chloride/liquid metal systems

Americium is known to be the most difficult transuranic element to separate from rare earth elements using pyrometallurgical partitioning processes. But the thermodynamic properties necessary to estimate its separation behavior is not as sufficient as those of uranium, neptunium, and plutonium. Therefore, the thermodynamic properties of americium in molten chloride/liquid metal systems were measured at 400degC and 450degC. Measurement of its standard potential in molten LiCl-KCl eutectic salt gave a value of -2.981 V at 400degC and -2.850 V at 450degC (vs. Cl2/Cl-), and that of its activity coefficient in liquid cadmium gave values of 2.9 x 10-5 at 400degC and 1.3 x 10-4 at 450degC. These values at 450degC are in good agreement with the values which had been already reported, and by using these values, those at 500degC were estimated as -2.808 V (vs. Cl2/Cl-) and 4.5 x 10-4. Additionally, its activity coefficient in liquid bismuth at 450degC was also newly obtained as 3.9 x 10-10. Next, Am(2)/(Am(2) + Am(3)) existence ratio in the slat phase facing liquid cadmium was calculated. This ratio was found to be nearly zero when the cadmium potential was positive over about -1.10 V (vs. Ag/AgCl) and almost unity when cadmium potential was more negative than about -1.70 V (vs. Ag/AgCl). However, almost all of Am existed as Am(3) in the salt phase in contact with liquid bismuth due to high potential of the liquid bismuth. (author)