On mass transport in nuclear carbides by evaporation-condensation

Self-diffusion studies in the U-C system show a mass transfer between diffusion specimens. This phenomenon is shown to be due to rate-controlling metal-atom transport through the gas phase by evaporation-condensation, and fast carbon transport through solid-state diffusion. The activation energy of the process being about 5.0 eV is thus in agreement with literature data on the heat of vaporization of U from UC. The mass transfer can be measured very accurately and vapor pressures can be deduced. The same mechanism can also explain the observed pore mobilities in nuclear carbide fuels. (Auth.)