Volumen II - SAM

Congreso SAM/CONAMET 2009 Buenos Aires, 19 al 23 de Octubre de 2009
CALCULATION OF OXYGEN DIFFUSION PROFILES IN ZIRCONIUM BASE ALLOYS
CLADDING TUBES AT HIGH TEMPERATURE: coupling of the diffusion code “EKINOX”
and the thermodynamic database “ZIRCOBASE”
C. Corvalán-Moya (1) , C. Desgranges (1) , C. Toffolon-Masclet (1) , C. Servant (2) , J.C. Brachet (1)
(1)CEA, Nuclear Materials Department, 91191 Gif-sur-Yvette Cedex, France
(2) Université de Paris-Sud, Laboratoire de Physico-Chimie de l'Etat Solide, ICMMO, 91405 Orsay, France
E-mail : carolina.corvalan-moya@cea.fr
ABSTRACT
Thermodynamic and kinetic data are generally essential for quantitative modelling of materials processing,
structure, and property. In this frame a new tool has been developed by coupling the diffusion code
"EKINOX" and the thermodynamic database "ZIRCOBASE" using TQ (ThermoCalc® program
interface).Particularly, in order to predict the behaviour of cladding tubes under conditions of "Loss Of
Coolant Accident (LOCA)", a tool able to simulate the oxygen diffusion at different times and high
temperatures is relevant. Simulations of oxygen diffusion profiles at 1100°C, 1200°C, and 1250°C are
presented.
Palabras clave: Zy-4, cladding tubes, diffusion, modelling, LOCA.
1. INTRODUCTION
During some hypothetical Pressurized Water Reactor (PWR) accidental scenario such as Loss Of Coolant
Accident (LOCA), the nuclear fuel cladding tubes made of Zr base alloys are subjected to high temperature
oxidation (~1200°C) due to the steam environment. This leads to the growth of a zirconia layer (ZrO2), but
also to the growth of αZr(O) from the parent (ductile) βZr phase due to the oxygen diffusion within the suboxide
metallic layer (see Fig. 1). The zirconia and the αZr(O) phase layers are brittle at low temperatures, then, the
residual ductility of the high temperature oxidised cladding tubes depends mainly on the concentration of the
dissolved oxygen in the prior or ex-βZr phase inner layer. Thus, the oxygen concentration has a key-role for
predicting the amount of ductile phase remaining in the cladding tube after a LOCA scenario. To anticipate
the oxygen concentration in the prior-βZr phase with a better quantification of the diffusion phenomena, a
new tool using thermodynamic and kinetic data is then necessary. The EKINOX numerical model has been
developed in order to treat this issue. The first simulation results are presented and compared to an analytical
model and to previous experimental results.
ZrO
Oxygen concentration (wt.%)
α(O
0,7
0,6
0,5
0,4
0,3
0,2
0,1
~Brittle failure mode Ductile failure mode
100 200 300 400 500 600
Distance from the outer clad surface (µm)
Prior-β layer
a) b)
Fig. 1 a) Typical prior-β fractograph of an impact tested Zircaloy-4 sample at 20°C and associated oxygen
diffusion profile after one size steam oxidation for 120s at 1250°C and quenching (from [1]). b) Zr-O phase
diagram illustrating the phase transformations occurring with an increase of oxygen content at 1250°C.
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