C. Duriez, 1 S. Guilbert, 1 A. Stern, 2 C. Grandjean, 1 L. Bělovský, 3 and J. Desquines 1 Characterization of Oxygen Distribution in LOCA Situations ABSTRACT: Oxygen distribution in the rod cladding tubes during a Loss of Coolant Accident LOCA situation is known to play a key role regarding the post-quench mechanical properties of fuel rods. Analyti- cal solutions of the diffusion equations in the complex multilayer and multiphase systems describing the cladding material tube during high temperature oxidation are limited to stationary isothermal conditions. To address the problem of non-stationary transient LOCA situations, a numerical tool named DIFFOX has been developed. In the first part, the main assumptions and main features of the code are described. In parallel to the code development, an experimental program has been launched to provide a validation database as well as to input data for improvement of the code models. Zircaloy-4 cladding samples have been pre-oxidized at low temperature then vacuum annealed or steam oxidized at high temperature. Oxy- gen distribution profiles are investigated by Electron Probe Microanalysis. The ability of the code to de- scribe the chemical reduction of the pre-oxide layer and oxygen diffusion into the metal during high tem- perature annealing is demonstrated. Regarding high temperature steam oxidation, the pre-oxide layers are observed to be either protective or not on a same given sample. While a good code to experimental data agreement is obtained in the first case, the growing of a high temperature oxide underneath the pre-oxide is not well reproduced by the code. The effect of hydrogen has also been investigated by performing small scale oxidation tests on 600 wppm hydrogen charged Zy-4 cladding. Samples have been long-term an- nealed for homogenization after oxidation. The main purpose of these experiments is to provide quantita- tive data on the modification of the /+ and +/ transus of the Zy4-O pseudo binary phase diagram induced by the presence of hydrogen. A variation of the transus locations deduced from these experiments is proposed. KEYWORDS: LOCA, DIFFOX, oxygen diffusion, modeling, hydrogen, pre-oxide Introduction The French “Institut de Radioprotection et de Sûreté Nucléaire” IRSN launched in recent years a re- search program named Characterization and Investigation for Improved Calculation of LOCA Addressing the Determination and Evaluation of Safety CYCLADES, aiming at improving the knowledge on fuel behavior during Loss of Coolant Accidents LOCA. The IRSN’s strategy is detailed in Ref 1. The CYCLADES program covers three main topics of interest for LOCA issues: Cladding mechanical behavior and failure conditions, axial fuel relocation, and the coolability of fuel rods after flow blockage. The present study addresses part of the research program on oxygen-induced cladding embrittlement, which mainly depends on oxygen distribution in the Zirconium -phase 2–5. The influence of oxygen content on cladding embrittlement is usually expressed using a single parameter: The Equivalent Cladding Reacted ECR, which is directly related to the weight gain during the LOCA transient 3. However the presence of the corrosion scale grown during normal operation was not taken into account in the original ECR criteria. This pre-existing oxide layer is potentially a source of oxygen for oxygen diffusion in the under- lying metal. Actually, at elevated temperatures, partial or full dissolution of this pre-existing oxide layer is sometimes observed and its oxygen content moves inwards by diffusion. As a consequence of this disso- lution process, significant oxygen embrittlement can be observed without any noticeable weight gain 6. Furthermore, the hydrogen content as well as the cooling rate were recently demonstrated to play a key role in the oxygen distribution and consequently in cladding embrittlement 4,5,7,8. It was thus decided Manuscript received May 7, 2010; accepted for publication December 17, 2010; published online January 2011. 1 IRSN/DPAM, BP3, Saint Paul-Lez-Durance 13115, France. 2 Post-Doctorate, IRSN, BP3, Saint Paul-Lez-Durance 13115, France; currently at CEA/DEN-DANS/DMN/SEMI/LM2E, CEA/ Saclay, Gif-Sur-Yvette 91191, France. 3 ALIAS CZ, s.r.o., Bachova 1586/16, Praha 149 00, Czech Republic. Journal of ASTM International, Vol. 8, No. 2 Paper ID JAI103156 Available online at www.astm.org Copyright © 2011 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959.