*Article* **Long-Term Oxidation of Zirconium Alloy in Simulated Nuclear Reactor Primary Coolant—Experiments and Modeling**

**Iva Betova <sup>1</sup> , Martin Bojinov 2,\* and Vasil Karastoyanov <sup>2</sup>**


**\*** Correspondence: martin@uctm.edu

**Abstract:** Oxidation of Zr-1%Nb fuel cladding alloy in simulated primary coolant of a pressurized water nuclear reactor is followed by in-situ electrochemical impedance spectroscopy. In-depth composition and thickness of the oxide are estimated by ex-situ analytical techniques. A kinetic model of the oxidation process featuring interfacial reactions of metal oxidation and water reduction, as well as electron and ion transport through the oxide governed by diffusion-migration, is parameterized by quantitative comparison to impedance data. The effects of compressive stress on diffusion and ionic space charge on migration of ionic point defects are introduced to rationalize the dependence of transport parameters on thickness (or oxidation time). The influence of ex-situ and in-situ hydrogen charging on kinetic and transport parameters is also studied.

**Keywords:** zirconium alloy; nuclear reactor primary coolant; electrochemical impedance spectroscopy; oxidation model; compressive stress; ionic space charge
