Characterization of Oxygen Distribution in LOCA Situations
Résumé
The French « Institut de Radioprotection et de Sûreté Nucléaire » (IRSN) launched in
the recent years a research program named CYCLADES aiming at improving the knowledge on fuel
behaviour during Loss Of Coolant Accidents (LOCA). The CYLCADES program covers three main
topics of interest for LOCA issues: cladding mechanical behaviour and failure conditions, axial fuel
relocation, and 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 β-phase. The influence of oxygen content on cladding embrittlement is
usually expressed using a single parameter: the Equivalent Cladding Reacted (ECR) directly related
to the mass gained during the LOCA transient. 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 underlying metal. Actually,
at elevated temperature, 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 dissolution
process, significant oxygen embrittlement can be observed without any noticeable mass gain.
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. It was thus decided
to reach better physical determination of the cladding embrittlement under LOCA conditions relying
on the oxygen distribution through the cladding wall rather than relying on the ECR parameter alone.
IRSN develops the DIFFOX diffusion code in order to achieve this objective.
The present paper focuses on the IRSN action plan, established to obtain an accurate simulation of
the oxygen profile within the cladding thickness using the DIFFOX code. This diffusion code will
later be applied to the many configurations of interest for PWR LOCA safety issues. Three main
subjects of interest are under investigation: the study of oxidation and oxide reduction kinetics at
high temperature, the study of the hydrogen influence on oxygen distribution after a LOCA transient,
and the study of the oxygen concentration at phase boundaries under non-stationary cooling
conditions. Only the two first topics are handled in this paper, since the last one started being
investigated too recently.
First, the paper describes the main assumptions for diffusion modeling in the DIFFOX code and
associated numerical methods. Next, the test devices and experimental protocols supporting this
study are presented. Then, experimental results relevant for oxidation and reduction kinetics are
discussed and compared to open literature data. Particular attention is paid to oxygen concentration
profiles, obtained by Electron Probe Micro-Analysis (EPMA), that are compared to DIFFOX
predicted distributions. Regarding investigation on the influence of hydrogen on oxygen distribution,
results obtained with 600 wppm hydrogen charged cladding samples have been obtained. The
influence of hydrogen is mainly focused on measurements at thermodynamic equilibrium, that lead
to new data on the Zr-O phase diagram for a hydrogen containing alloy.