Diffusion of uranium through graphite

Abstract

AbstractDiffusion rates of uranium through graphite were determined in the temperature range of 3,000° to 4,350°F. The diffusion couples consisted of sintered UC2 disks in contact with graphite rods. The observations indicated two distinct types of uranium transport which could be associated with volume diffusion and with migration along pores respectively.Volume diffusion was characterized by steep concentration gradients and shallow penetration. The diffusion coefficient Dv, in sq. cm./sec. between 3,300° and 4,250°F., is given by an equation. Above 4,250°F. incipient melting of the UC2 was evident and the diffusion coefficients were much higher than those given by the equation.As an example of the penetration resulting from volume diffusion, calculations show that, after 1,000 hr. at 4,200°F. the uranium concentration at 0.1 cm. from the interface will be 1,000 mg./cc., compared with 10,000 mg./cc. for pure UC2.Pore migration resulted in uranium penetration far beyond that arising from volume diffusion at equivalent temperatures and diffusion times. However, uranium concentrations were very small compared with those corresponding to volume diffusion. Pore migration is strongly temperature dependent.To estimate the practical importance of pore migration, the uranium flow through a graphite wall at 3,000°F. was measured. With a wall thickness of 0.32 cm., the average flow per unit area was 0.015 mg./ (sq. cm.)/ (hr.) for a 40‐hr. test.