Diffusion of Lanthanides and Actinides from Graphite at High Temperatures

Measurements were made of the diffusion losses of lanthanide and actinide elements from graphite at temperatures from 1600 to 2600°C. In the lanthanide series, a close correlation was observed between the diffusion rates from graphite and the boiling points of the metals. Where boiling point measurements permit comparison, a similar correlation was also noted for the actinides. The most volatile of these elements are the ones for which lower oxidation states can be attained chemically [Sm(II), Eu(II), Yb(II), and possibly Am(II)]. Conversely, those elements which exhibit oxidation states higher than (III) show high-temperature stability in graphite. Although the diffusion loss of uranium is not negligible above 2100°C, no loss was detected from a small graphite pin heated for four hours at 2050°C; under the same conditions, about 50% of the plutonium was lost. The possible significance of these measurements to high-temperature reactor design is discussed briefly.