Fractional packing densities and fast diffusion in uranium and other light actinides

The analysis of the α-, β- and γ-phases of uranium by a hard sphere model based on a simple bond description leads to fractional packing densities q which agree with the missing primary solute solubilities of C, N and O in α- and β-U. Yet this approach disagrees (i) with the concept of an 'open' structure of α-U and (ii) with the Wigner-Seitz concept of the electron theory of metals. Fast diffusion in γ-U, other bcc and fcc host phases like Nb, β-Zr, ε- and δ-Pu and α-Zr, can be explained by a modification of Haegg's rule based on the hard sphere model. This is, however, not possible for fast diffusion data of Fe and Co in the anisotropic host phases, α- and β-U. A likely solution of this problem is complex formation but this proposal has to be confirmed and quantified by diffusion experiments in α- and β-U single crystals. (orig.)