enhanced hydrogen uptake und reaction kinetics during ......imaging at antares (frm-2) 50 % argon 10...

19th International Symposium on Zirconium in the Nuclear Industry

Manchester UK, May 19-23 20191/21

KIT / Institut für Angewandte Materialien

www.kit.edu

Enhanced Hydrogen Uptake und Reaction Kinetics During

Oxidation of Zircaloy-4 in Nitrogen Containing Steam

Atmospheres

M. Grosse, S. Pulvermacher, M. Steinbrück (KIT – IAM)

B. Schillinger (TU Munich)



Introduction

Why the reaction of Zircaloy in steam-nitrogen mixed atmospheres is an

issue?

➢ Air ingress during SFP accidents and after vessel failure in severe reactor

accidents.

➢ It is well known that the presence of nitrogen can accelerate the reaction of

zirconium alloys with oxygen or steam.

Basic reactions, strongly simplified :

with oxygen 𝑍𝑟 + 𝑂2 → 𝑍𝑟𝑂2with steam 𝑍𝑟 + 2𝐻2𝑂 → 𝑍𝑟𝑂2 + 2𝐻2with nitrogen 2𝑍𝑟 + 𝑁2 → 2𝑍𝑟𝑁

Re-oxidation of nitrides:

ZrN + O2 → ZrO2 + N

𝑍𝑟𝑂2

0 5000 10000 15000 200000

200

400

600

800 N2 conc., %

0

0.1

0.2

0.5

0.7

1

2

5

10

50

70

80

90

100

m

, g

/m²

time, s

H2O+N2

H2ON2

Mass gain of Zircaloy-4 oxidized at 800°C

in steam-nitrogen mixtures,

(Steinbrück et al., NUMAT 2014)



Introduction

If ZrN precipitate, cracks are formed and

the ZrO2 layer is not longer protective

against further oxidation.

The behavior of the reaction of zirconium

with oxygen, steam and nitrogen was

investigated in a large number of

experiments.

Modeling in severe accident codes is not

yet satisfying.

𝑍𝑟𝑂2

𝒁𝒓

𝒁𝒓𝑵

The results of tests using different experimental setups are contradictory.

One reason for the differences are different gas flow rates applied in the tests.

A model describing the influence of the gas flow rates on the reaction behavior

was developed at KIT.



Flow rate model[M. Grosse et al. ICAPP 2016]

𝐻2O g + (Zr𝑂 + 𝑉𝑂2+)(𝑜𝑥) = 𝑍𝑟𝑂2 ox + 2𝐻𝑎𝑑

+

Basic reactions with steam if an oxide

layer is already formed

𝑂𝑂 𝑜𝑥 + 𝑍𝑟 𝑚 = (𝑍𝑟𝑂 + 𝑉𝑂2+ + 2𝑒−)

Wagner, C., Die Löslichkeit von Wasserdampf in ZrO2-Y2O3-Mischkristallen,

Ber. Bunsen-Ges. Phy. Chem., Vol. 72, 1968, pp. 778-781




Basic reactions with nitrogen if no

oxygen or steam is available

𝑁2(𝑔) + 4 𝑍𝑟𝑂 + 𝑉𝑂2+ + 2𝑒− (𝑜𝑥) = 2𝑍𝑟𝑂2(𝑜𝑥) + 2ZrN(ox)

𝑂𝑂 𝑜𝑥 + 𝑍𝑟 𝑚 = (𝑍𝑟𝑂 + 𝑉𝑂2+ + 2𝑒−)



1 2 3 4 5 6 70.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

(, g

m-2

s-1)1

/4

Oxygen concentration, wt%

900°C

1000°C

1100°C

1200°C

1300°C

𝑁2(𝑔) + 4 𝑍𝑟𝑂 + 𝑉𝑂2+ + 2𝑒− (𝑜𝑥) = 2𝑍𝑟𝑂2(𝑜𝑥) + 2ZrN(ox)


Experimental evidence: reaction rate K ~ xO4 for the reaction of N2 with Zr(O)




ሶ𝑛𝑉𝑂2+ = ሶ𝑛𝑜𝑥𝑦𝑔𝑒𝑛,𝑟𝑒𝑎𝑐𝑡 =

𝐾𝑛𝑜𝑥2 𝑡∗

Do we know the oxygen vacancy flux to the surface?

Yes, we know!

ሶ𝑛𝑁2 =𝐾𝑛𝑜𝑥2 𝑡∗

−ሶ𝑛𝑂22− ሶ𝑛𝐻2𝑂 /4How many nitrogen reacts?

𝑛𝑁 = න

𝑡

𝑘𝑛𝑂2 𝑡∗

−ሶ𝑛𝑂22 + ሶ𝑛𝐻2𝑂

2ሶ𝑛𝑂22 + ሶ𝑛𝐻2𝑂

𝑑𝑡 + 𝑛0𝑁Molar concentration of

nitrogen taken up:




❑ Number of cracks can be assumed to be proportional to the number of

ZrN precipitates.

❑ No hints that the size of the ZrN precipitates depend on time or

temperature.

❑ Therefore, the fraction of cracks should be proportional to the nitrogen

concentratiion in the oxide.

𝑓𝑐𝑟𝑎𝑐𝑘𝑠 = 𝐴න

𝑡

𝐾𝑛𝑂2 𝑡∗

−ሶ𝑛𝑂22

+ ሶ𝑛𝐻2𝑂

2ሶ𝑛𝑂22

+ ሶ𝑛𝐻2𝑂

𝑑𝑡 + 𝑛𝑁0

❑ The reaction rate is weighted sum of the reaction rate of positions with

cracked and undisturbed oxide.

ሶ𝑛𝑍𝑟 = 𝑓𝑖𝑛𝑡𝑒𝑟𝑓𝑎𝑐𝑒𝑐𝑟𝑎𝑐𝑘𝑒𝑑𝐾𝑛𝑍𝑟 + 1 − 𝑓𝑖𝑛𝑡𝑒𝑟𝑓𝑎𝑐𝑒𝑐𝑟𝑎𝑐𝑘𝑒𝑑𝐾𝑛𝑂

2 𝑡∗




• total starvation (starvation of all gases reacting): The lower the

oxygen, steam and nitrogen flow rate, the lower is the reaction rate

• partial starvation (starvation only of oxygen and stream): The lower

the oxygen and steam flux, the higher is the reaction rate.

• global starvation: starvation at the whole material

• local starvation: starvation only at locations with increased reaction

rate (e.g. cracks) or if oxygen and steam was already consumed by

the material located before in flow direction.

2 x 2 kinds of starvation

ሶ𝑛𝑁2 =𝐾𝑛𝑜𝑥2 𝑡∗

−ሶ𝑛𝑂22− ሶ𝑛𝐻2𝑂 /4

Determining parameter: quantity

of steam and oxygen starvation



Experimental validation of the model

0 500 1000 1500 2000 2500 30000

50

100

150

Time, s

m

, g

/m2

SF1

SF2

SF4

SF6

SF8

SF10

INFLUENCE OF THE STEAM AND OXYGEN

FLOW RATE ON THE REACTION OF

ZIRCONIUM IN STEAM/NITROGEN AND

OXYGEN/NITROGEN ATMOSPHERES, (M.

Grosse et al., ICAPP 2016)

The lower the oxygen and

steam flow rate the earlier is the

transition in the reaction kinetics

and the higher is the reaction

rate after the transition.



Neutron radiography

Which are the consequences for the hydrogen uptake and why can

we study these processes by in-situ neutron radiography?

Crack formation in the zirconium oxide layer is connected with enhanced

hydrogen uptake by the metallic zirconium. Hydrogen concentration is a

marker for oxide cracks.

0 3600 7200 10800 14400 18000

0

200

400

600

800

1000

1200

1400

1600

cH, w

pp

m

oxidation time, s

M. Grosse et al.,

Nucl. Instr. & Meth. A 651 (2011). 253

Zircaloy-4, 1000°C



Basis Neutron radiography

Intensity measured at the detector pixel x,y:

Total macroscopic neutron cross section:

( )syxIyxIyxTyxI total −== exp),(),(),(),( 00

),,(),,(),,(),(

),,(),,(

4 tyxNtyxNtyxNyx

tyxNtyx

OONNHHZry

total

i

i

i

totaltotal

totaltotaltotal +++=

=

−

0,0 0,1 0,2 0,3 0,4 0,5 0,6

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

900°C

1000°C

1100°C

1200°C

1300°C

to

tal,

cm

-1

H/Zr atomic ratio



Experiments - Antares beam line (FRM-2)

Experiments were performed at the

ANTARES neutron imaging beamline

at the FRM-2 research reactor (TU Munich,

Garching, Germany)

using the KIT-INRRO furnace

Lateral resolution: ~ 0.25 mm (l ~ 225 mm,

L/D ≈ 971)

exposure time: 29 s

readout time: 1.2 s



Test matrix of experiments with different flow

rates

4 temperatures800°C

900°C

1000°C

1100°C

5 flow rates24 l/h

36 l/h

48 l/h

60 l/h

72 l/h

The oxidation times applied

depended on temperature.

Analysis of the reaction gases

by mass spectrometry in the

pre-tests,

Analysis of the hydrogen

absorption by in-situ neutron

imaging at Antares (FRM-2)

50 % argon

10 % steam

40 % nitrogen

Steam-air-tests

Several tests with 40% air and

10% steam with 24 l/h and 60 l/h,

respectively, at 800°C

2 in-situ with 24 l/h tests,

respectively, at 800°C and 1000°C



In-situ Neutron imaging – influence of flow rate

36 l/h 72 l/h24 l/h 48 l/h

ste

am

-nitro

gen

900°C



Results 900°C

absorbed hydrogenreleased hydrogen



In-situ Neutron imaging – different atmospheres800°C

1000°C

steam-nitrogen steam steam-air



In-situ Neutron imaging – different atmospheres

Breakaway at the very beginning

of test

(the first image after starting the

injection of the reactive gases

show breakaway)

Starting time for breakaway < 30 s

1000°C 24 l/h

steam-nitrogen



In-situ Neutron imaging – different atmospheres

absorbed hydrogenreleased hydrogen

Released and absorbed hydrogen during the tests at 1000°C



Summary and conclusion

• Hydrogen concentration is an indicator for cracks in the oxide layer. An enhanced

hydrogen absorption during the reaction of Zircaloy-4 in nitrogen-steam

atmosphere were observed.

• At 800, 900 and 1000°C a change from parabolic-linear to pure linear oxidation was

observed. Partial starvation occurred.

• The lower the flow rates, the higher are the oxidation rates and the later are

the transitions in the kinetics

• Breakaway at 1000°C starts very early (during the first 30s)

• Reactions at 1100°C show the largest mass increase. The kinetics are linear from

beginning, because of total starvation.

• The KIT model of the influence of the gas flow rate shows that the reaction of

zirconium alloys in nitrogen containing atmospheres depends on the quantity of

steam and oxygen starvation:𝐾𝑛𝑜𝑥2 𝑡∗

−ሶ𝑛𝑂22− ሶ𝑛𝐻2𝑂. It was confirmed qualitatively.

• Main conclusion for nuclear reactor safety: Beware having nitrogen in the reactor

during severe accidents!



Acknowledgment

Thanks to

FRM-2 for providing beamtime at the ANTARES neutron imaging beamline,

the ANTARES team in particular D. Bausenwein for their support in the neutron

imaging experiments and

U. Stegmaier and P. von Appledorn from KIT for their support in the re-

commissioning of the INRRO furnace.

Thank you for your attention



KIT / Institut für Angewandte Materialien

www.kit.edu

Enhanced Hydrogen Uptake und Reaction Kinetics During

Oxidation of Zircaloy-4 in Nitrogen Containing Steam

Atmospheres

M. Grosse, S. Pulvermacher, M. Steinbrück (KIT – IAM)

B. Schillinger (TU Munich)

enhanced hydrogen uptake und reaction kinetics during ......imaging at antares (frm-2) 50 % argon 10...

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