Further development of the Mixed-Conduction Model of oxide films in LWRs emphasising surface complexationand reprecipitation

The ultimate goal of this work is to develop a deterministic model for activity build-up and to increase the theoretical understanding of the oxide film build-up and breakdown, controlling the localised corrosion of structural materials in Light Water Reactors. The first step of the overall reaction of activity build-up in oxidised structural material surfaces in primary LWR circuits is the interaction of activated ionic species from the coolant with the oxide layers, involving surface complexation and redeposition stages. An experimental investigation of the interaction of the LWR oxides with coolant-originating species was performed to assess the thermodynamics of these stages. A series of high-temperature titrations at 280oC was performed, in which the competitive adsorptivity of Zn, Co and Ni cations onto iron oxide surfaces has been studied. The experiments provided the necessary information concerning the reactions at outer oxide / coolant interface. The obtained results were quantitatively interpreted using the diffuse layer and non-electrical surface complexation models and the extracted stability constants of the surface complexes were found to be in agreement with extrapolations of room-temperature data. The determination of the kinetic and transport parameters for the subsequent incorporation of Zn in the inner oxide layer on stainless steels and nickel alloys is based on the fitting of experimental depth profiles of these elements to the diffusion-migration equations for the non-steady state transport. The obtained parameter values are discussed in terms of the dynamics of the oxides on the structural materials.