CONSIDERING COBALT INCORPORATION IN OXIDE FILM UNDER BOILING WATER REACTOR CONDITIONS

Abstract

The phenomenon of radiation field build up-within Boiling Water Reactors (BWRs) is mainly dependent on the uptake of the highly radioactive 60Co into the surface oxide of the steel components. The incorporation of 60Co in the oxide film is strongly dependent on the type, composition and structure of the spinel-based oxide, which in turn is affected by reactor water chemistry, which further includes the oxidising potential and dissolved metal ions. The present research identifies and mechanistically investigates the oxides formed under a range of oxidising potentials, dissolved metal ion concentrations and surface finishes, using high-resolution characterisation techniques. As predicted within literature the oxides formed under hydrogen water chemistry conditions (HWC) conditions was shown to have a fine-grained Cr enriched spinel type inner oxide which contained voids while the outer layer was ferrite based. Zn injection prevented 59Co from incorporating into the oxide, which mechanistically occurs through the displacement of the Co2+ cation from the tetrahedral lattice sites within the spinel based oxide. Under Normal Water Chemistry (NWC) conditions, the inner layer was polycrystalline; however, the transpassivity of Cr formed a ferrite based inner layer which is considered less protective and contained extensive cavities/voids. The outer layer was composed of α-Me2O3 (Me: Fe, Cr and [Cr] << [Fe]). The beneficial effect of Zn injection under the oxidising conditions of NWC was less significant at reducing the Co uptake in the inner layer oxide. However, this was suggested through higher oxide dissolution rates. The Zn enriched to the inner layer oxide and not the outer layer, which disagreed with literature sources which utilised non site specific techniques. The literature sources characterisation techniques’ were indirect and had a low spatial resolution, thus, highlighting the effectiveness of Analytical Electron Microscopy (AEM) approaches to oxide analysis. The bulk of the experiments were conducted on type 316 stainless steel with a 600 grit finish, which is comparable to a machined surface alike those within nuclear power plants. However, fundamental insight into the oxidation was obtained from samples that were carefully polished to an Oxide Polishing Suspension (OPS) finish, which removed the surface damage. Specifically, it was found that the absence of the deformed, ultrafine outer layer, led to a thicker inner layer oxide which suggests Cr mobility into the inner oxide was reduced and that oxides formed on polished surfaces do not necessarily have increased protective properties. It was found that the effects of Zn injection on an OPS surface, formed a more protective, significantly thinner and less porous oxide, which was proposed to be attributed to a reduction in cation diffusion rates through the oxide. The effects of Zn injection on a 600 grit surface was less substantial, as the oxide retained its porous nature, thus maintaining the access of oxidant through the oxide film. The injection of both Ni and Zn was investigated under HWC conditions; however, the “synergistic� effects found under NWC conditions were not validated under HWC conditions. Overall, the findings support Zn injection technologies for both Co uptake mitigation and the corrosion inhibitive effects under both HWC and NWC conditions. However, surface preparation combined with Zn injection has been shown to yield an oxide with increased protective properties.

Date of Award	31 Dec 2018
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Fabio Scenini (Supervisor) & Grace Burke (Supervisor)