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Contribution to the Study of the Pseudobinary Zr1Nb‐O Phase Diagram and Its Application to Numerical Modeling of the High‐Temperature Steam Oxidation of Zr1Nb Fuel Cladding

M. Négyesi, J. Krejčí, S. Linhart, L. Novotný, A. Přibyl, J. Burda, V. Klouček, J.

Lorinčík, J. Sopoušek, J. Adámek, J. Siegl, V. Vrtílková

17th

International Symposium on Zirconium in the Nuclear Industry

February

3‐7, 2013, Taj Krishna, Hyderabad; Hyderabad, Andhra Pradesh, India

CHEMCOMEX, NRI, UNIPETROL, AS, JEPU, MU

Outline

• Introduction• Main goals• Material & Methods• Calculations• Results & Discussion:

• O distribution• Zr1Nb‐O• Experimental vs. Calculations

• Conclusions

26/02/2013 M. Negyesi, Zr1Nb‐O & Its Application to Modeling of the Cladding Oxidation, 17th

International Symposium on Zirconium in the Nuclear Industry 2

Introduction



• modeling of Zr1Nb HT oxidation:

• Zr1Nb‐O (Cα/α+β

andCβ/α+β

) is of high importance: exp. x calc.(CALPHAD)

• experimental: new experimentalprocedure for assessment of Zr‐alloy‐Obased on O concentration measurementinside cladding wall after HT oxidation

• assumption: equilibrium conditions arefulfilled at interfaces, solutesredistribution upon quenching isnegligible

Introduction

• first tests with Zry‐4 with variable H content=> validation of the procedure=>

influence of H can be treated

• tests with Zr1Nb –

troubles with α‐Zr(O) thickness

• comparison with CALPHAD calculations – satisfactory agreement => new tests follow

• modeling of the (α+β)‐Zr

region



Main Goals

• determination of Zr

rich Zr1Nb‐O phase diagram (Cα/α+β

and Cβ/α+β

) based

on O concentration measurements inside quenched Zr1Nb cladding wall

after HT

isothermal steam oxidation

• comparison

of

the

experimentally

determined

Zr1Nb‐O

with

CALPHAD

calculations

• modeling

of

double‐sided

HT

steam

oxidation

using

Zr1Nb‐O

including

(α+β)‐Zr

region

for T = 1,100‐1,300 °C until O saturation

• validation of the calculations using experimental data

526/02/2013 M. Negyesi, Zr1Nb‐O & Its Application to Modeling of the Cladding Oxidation, 17th International

Symposium on Zirconium in the Nuclear Industry

Material & Methods

• alloy: modified E110 (improved oxidation properties => lower H uptake!!!)

• 30 mm long tubes (outside diameter: 9.1 mm, wall thickness: 0.7 mm)

• as‐received x corroded (non‐irradiated) in steam 425 °C/10.7 MPa – 2 (<50 wppm

H), 10 (~150 wppm

H), or 20 µm (~600 wppm

H),

• HT

oxidation (double‐sided) in steam (0.1 MPa) T = 850‐1,300 °C

‐

T measured by a thermocouple inside the tube, samples quenched in ice water



Sample exposed at 1,200 °C/9 min

Material & Methods

• H content measured by vacuum extraction• microstructure observation – LM & SEM• microhardness

& nanohardness

measurements• O concentration measurement:

– WDS

(Wavelength‐Dispersive Spectrometry)– SIMS (Secondary Ion Mass Spectrometry)– TEA (Thermal Evolution Analysis)



O concentration Cα/α+β


Symposium on Zirconium in the Nuclear Industry 8

determination of α/α+β

is

not so obvious in some

cases – O, Nb

profiles,

metalography

O

saturation in

β

solutes redistribution

elevated nanohardness

in

β

‐

precipitated α‐Zr(O)

more measurements

–

substantial

experimental scatter

α/α+β

α/α+β

α+β/β

α+β/β

Nanohardness

vs. O content



• nanohardness

depends strongly on O concentration

• the relation is equal for both Zry‐4 & Zr1Nb

• nanohardness

and O concentration at

the metal/oxide phase boundary(in the

metal) are also involved

• nanohardness

measurement followedwith advantage of higher throughput

O ceiling in β‐Zr



• SIMS & TEA ‐

determination of Cβ/α+β

(WDS only for higher T)‐

larger analyzed volume

• assumption: O solubility limit in β

is achieved and exceeded aftercertain exposure time upon HT oxidation approx. in whole βat the same time

• HVM ‐

determination of O saturation time

• SIMS & TEA results ‐

satisfactory agreement ‐

similar tendency withmicrohardness

• Cβ/α+β

can be then estimated

• LM confirm the

conclusions

Zr1Nb‐O



• CALPHAD ‐

database „Zr_BASE“

(involving Zr, Nb, O, and H) ‐

thermodynamic parameters ofphases from publicly accessible sources (no fitting, no optimization due to currently carriedout experiments)

• good agreement between the experimental results and

calculated phase diagram• comparison with

other authors

‐

satisfactory agreement[8] A. Stern et al., “Investigations of the microstructure and mechanical properties of prior‐Beta structure as function of the

oxygen content (0.9 wt %) in two zirconium alloys,”

J. ASTM Int., Vol. 5, No. 4, 2008.[15] C.E.L. Hunt, P. Niessen, “The Continuous Cooling Transformation Behavior of Zirconium‐Niobium‐Oxygen Alloys,”

J. Nucl.

Mater., Vol. 38, 1971, pp. 17‐25.

Numerical calculations

‐

JKOX



2nd

Fick

Law & mass balance equation,

satisfied on each interface:

rJrrrt

trC

1),(

rtrCTDJ r

),()(

ji

ji

CCtrJtrJ

dtds

,),(

• 1D finite difference method (implicit solution) ‐

cylindrical coordinates• code predicts the double‐sided oxidation ≥

1,100 °C• GB diffusion and diffusion of Nb

are neglected• α‐Zr(O) incursions are treated as an additional layer ‐

(α+β)‐Zr• O concentrations at phase boundaries in (α+β)‐Zr

& O diffusivity are chosenbased on experimental results

• Zr‐O & Zr1Nb‐O

phase diagram are used• diffusivities depend only on T:

k = 1.987 cal/mol/K

kTOX eD /35140127.0

kTeD /51000923.3

kTeD /282000263.0

Reaction layers development



• good agreement for allreaction layers ‐

until Osaturation

• measured oxide layersare slightly under‐predicted, mainly forhigher thicknesses

• higher

experimental scatter in case of α‐Zr(O)& (α+β)‐Zr

O pick‐up in β‐Zr



• satisfactory agreement• code under‐predicts experimental results• possible explanations:

• experimental results – O concentrations

measured in prior β

might be elevated by the

innermost α‐Zr(O) incursions

• no α‐Zr(O) grains precipitation in β‐phase is

modeled (O saturation) ‐

misfit for higher

exposure times

Conclusions• Zr1Nb‐O was experimentally assessed employing O concentration

measurements inside quenched cladding wall after HT steam oxidation

• experimentally determined Zr1Nb‐O was compared to CALPHAD

calculations with a satisfactory agreement

• proposed experimental procedure provides good estimation of the

Zr1Nb‐O phase diagram and can be also used for higher H content

• new diffusion model JKOX has been created using Zr1Nb‐O describing

Zr1Nb high‐temperature oxidation (≥1,100 °C), including (α+β)‐Zr, until

O saturation



Conclusions• numerical calculations were compared to experimental results with a

satisfactory agreement

• estimated

Zr1Nb‐O phase diagram may be applicable for O diffusion

models predicting the oxidation behavior of the Zr1Nb fuel claddings

upon HT oxidation



Acknowledgement



Authors

wish

to

thank

all

co‐authors,

J.

Sustr

for

metallographic

sections, and O. Blahova

for nanohardness measurements.

THANK YOU FOR YOUR ATTENTION!!!



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