Zircaloy-4 cladding corrosion model covering a wide range of PWR experiences Byung-Ho Lee * , Yang-Hyun Koo, Jae-Yong Oh, Dong-Seong Sohn Korea Atomic Energy Research Institute, P.O. Box 105, Yuseong, Daejeon 305-600, Republic of Korea article info Article history: Received 8 August 2007 Accepted 3 April 2008 abstract A phenomenological corrosion model for Zircaloy-4 cladding was developed by focusing on the effect of the metallurgy of cladding and the water chemistry combined with the thermo-hydraulic conditions. The metallurgical effect was formulated by considering the Sn content in the cladding and the heat treatment of the cladding. Concerning the effect of the water chemistry, it is assumed that lithium and boron have an influence on the corrosion under the condition of subcooled void formation on the cladding surface. The developed corrosion model was implemented in a fuel performance code, COSMOS, and verified using the database obtained for the UO 2 and MOX fuel rods irradiated in various PWRs. It was elucidated that the corrosion by lithium was enhanced in the case where the fuel rods were irradiated with a high linear power so that a significant subcooled void could be formed on the cladding surface. On the other hand, there was no evidence of the lithium effect even though its concentration was high enough if the void in the coolant was negligible. This result shows that the acceleration of corrosion by an increased lithium concentration occurs only when subcooled voids are formed on the cladding surface. In addition, the comparison between the measurement and the prediction for the MOX fuel rods indicates that no dis- tinguishable difference is found in the corrosion behavior between the MOX and the UO 2 fuels as expected. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Corrosion of the Zircaloy cladding in PWRs has become more important due to  a higher fuel discharge burnup to reduce fuel cycle costs,  a higher coolant inlet temperature to increase plant thermal effi- ciency, and  an increase of the coolant pH and lithium concentration to reduce plant radiation levels. Even though the corrosion mechanism of Zircaloy is still not fully understood as yet, the main factors determining its corrosion rate are the metallurgical characteristics of the cladding, alloy composition, and the irradiation environments of the fast neutron flux, water chemistry, and the thermo-hydraulic condition of the coolant. Particularly, concerning the water chemistry, an increase in the corrosion rate of 10–30% has been observed when the maximum coolant lithium content was 2.2–3.5 weight ppm [1,2]. On the con- trary, results from some other reports showed no discernible oxi- dation enhancement in the presence of an elevated lithium concentration [3–5]. Keeping in mind these conflicting corrosion behaviors, a phe- nomenological corrosion model for Zircaloy-4 cladding was devel- oped to consider the lithium acceleration and boron retardation coupled with the thermo-hydraulic condition of the coolant as well as the metallurgy in the cladding alloy. The developed model after incorporation into the fuel performance code, COSMOS [6], was verified by four cases of various cladding oxidation behaviors for the UO 2 and mixed oxide (MOX) fuel rods irradiated in PWRs. 2. Development of the corrosion model Since an in-pile fuel performance can be properly predicted with the precise estimation of corrosion behavior of fuel claddings, a phenomenological corrosion model for the cladding in PWRs has been developed and implemented into the fuel performance code, COSMOS [6,7]. In the oxide layer thickness range up to 2.1 lm (pre-transition range), oxide layer formation has a cubic characteristic. At a layer thickness above 2.1 lm, there is a change to the linear corrosion kinetics. The oxidation process of Zircaloy-4 cladding can be estimated using semi-empirical correlations divided into pre-transition and post-transition kinetics. Since the Zircaloy corrosion process is essentially a diffusion-controlled reaction, the Zircaloy oxidation kinetics is characterized by the Arrhenius equation as a function of the temperature, activation energy and additional acceleration factors. 0022-3115/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2008.04.019 * Corresponding author. Tel.: +82 42 868 8984; fax: +82 42 864 1089. E-mail address: bholee@kaeri.re.kr (B.-H. Lee). Journal of Nuclear Materials 378 (2008) 127–133 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat