A mechanism for the effect of zinc in dose-rate reduction

摘要

It is clear that the effect of zinc in replacing cobalt is not one of simple thermodynamic substitution based on energetic considerations. That does not explain the reduction in thickness nor agree with the diminishing response at increased zinc concentrations. Also, oxides grown from fresh surfaces in the presence of zinc should have a consistent zinc concentration. That is not observed, indicating a different mechanism for zinc incorporation. The non-linear dependencies of oxide thickness, and reactor cobalt chemistry on solution zinc concentration also argue against a thermodynamic equilibrium model. A mechanism of zinc substitution only at grain boundaries and surfaces is consistent with the observed non-stoichiometry. Diffusion of zinc to grain boundaries within the inner oxide would be relatively rapid, as observed, and the diffusion would be facilitated by a concentration gradient derived from loosely - adsorbed species (e.g. Zn(OH)_2) at the outer surface of the oxide.