The transport or diffusion of uranium (as a stand-in for plutonium) was investigated under conditions approximating those of the primary coolant loop in a high-temperature gas-cooled reactor. Profiles were obtained for uranium penetration in H-451 graphite at temperatures ranging from 900 to 1400°C. Profile data for given temperatures were considered in terms of the following expression: where C is the concentration of uranium at time t, for distance x, into the pellet; C0 is a constant representing the uranium concentration at x = 0 for all t, and D is the diffusion coefficient. Diffusion coefficients for uranium initially present as dicarbide at 1000 and 1400°C were found to be defined byFor uranium initially present as dioxide at 900, 1000, and 1400°C, diffusion coefficients are defined bywhere R is the gas constant and T is the temperature in degrees Kelvin.