Pyrocarbon-coated microspheres of UC2, ThC2, and (Th, U)C2 utilized in fuel for high-temperature gas-cooled reactors will migrate up an imposed thermal gradient during service life. The degree of kernel migration is limited by appropriate core design to retain coating integrity. The kernel migration (amoeba effect) results from carbon transport in the fuel phase and is characterized by a rejected graphite layer on the cool side of the kernel. The thermal gradient provides the dominant driving force for the rate-controlling process, which is the self-diffusion of carbon in the fuel phase. All dicarbide kernel materials show similar kernel migration behavior; however, ThC2 has the most rapid migration rate. The migration rates may be empirically described over the temperature range of 1250 to 1900°C by the expressionwhere

T= absolute temperature, K
ΔT/Δx= linear temperature gradient across the coated particle, K/cm
β= pre-exponential constant, (cm2 K)/sec
R= gas constant
ΔH= activation energy, in kcal/mole, for self-diffusion of carbon in the di-carbide.
The pre-exponential constant and activation energy determined for the three fuel types were as follows.