Diffusion of Lanthanides and Actinides from Graphite at High Temperatures

Abstract

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.