Electrokinetic Phenomena in Pressurised Water Reactor Corrosion and Deposition

Abstract

The deposition of corrosion products within light water reactors is known to adversely effect their safety and efficiency. Where flow is accelerated through orifices, corrosion deposit formation is enhanced. It is speculated that electrokinetic effects, arising from the advective shearing of the double layer, may influence both the location and morphology of corrosion deposits. This work develops a computational model of the Electric Double Layer (EDL) both at inert and active metal surfaces. The EDL models use novel applications of modified Nernst-Planck-Poisson theory to account for dielectric saturation and surface adsorption. Generalised Frumkin-Butler-Volmer (gFBV) kinetics models have been applied to elucidate the structure of the EDL with Faradaic processes. These models are then coupled to a model of the fluid flow through an orifice to examine the resulting streaming current and charge distributions under changing chemistries (pH, [H2 ], [FeII]) and flow rates. This new approach can recreate the expected trends in ionic concentration profiles, potential distributions, differential capacitance and polarisation curves. Surface charge arising from adsorption on surfaces with Faradaic reactions taking place is negligible. The model predicts that the magnitude of the electrokinetically stimulated Faradaic currents are not sufficient to explain the observed rates of corrosion product deposition. Overall, orifices are expected to be slightly cathodically polarised by the flow with small anodic regions observed on the front face of the orifice. Soluble FeII had a negligible effect on the model. A decrease in the ionic strength of the solution has the most profound effect on both the interfacial current densities and the electric field around the orifice. A new model explaining corrosion product deposition as a electrically influenced but mass controlled, particulate fouling process is proposed.

Date of Award	4 May 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Nicholas Stevens (Supervisor) & Fabio Scenini (Supervisor)