Abstract
Activity transport in the water circuit of nuclear reactors is an important problem in the nuclear power industry. The active species (particularly cobalt-60) are transported via particulates or in solution and are readily incorporated into the oxide films on stainless steel and Inconel. This thesis describes research using a novel high pressure water system and sophisticated surface analysis together with ion beam etching to reveal compositional and chemical state information about the incorporation of cobalt into the thin films that form on stainless steel immersed in water at temperatures up to 473 K. The mechanism by which this occurs and the influence of temperature and water chemistry have been examined. It has been shown that the change in film growth mechanism previously observed to occur between 433 K and 473 K, occurs at or close to 473 K. Dosing small quantities of cobalt into the water resulted in the incorporation of divalent cobalt in the oxide film both at 413 K and at 473 K. The proportion of incorporated cobalt was directly related to solution concentration and was considerably greater at 473 K than at 413 K, with a further increase in cobalt uptake when the water chemistry was modified to simulate reactor chemistry. The incorporated divalent cobalt is present at the expense of trivalent chromium. This is not explained by current models for film growth, and it has been proposed that it results from cobalt substituted magnetite having a greater stability than the mixed iron/chromium oxide otherwise forming on the surface. This would result in iron, normally lost into solution, being co-precipitated with cohalt in the film. This incorporated cobalt has no apparent effect on film thickness, and suggests that the cobalt containing oxide is as effective as the native oxide in controlling film growth kinetics. Advanced computer techniques have been developed in order to obtain a greater understanding of the chemical environment of the incorporated cobalt.