Atmospheres M. Grosse, S. Pulvermacher, M. Steinbrck (KIT IAM) B. - - PowerPoint PPT Presentation

19th International Symposium on Zirconium in the Nuclear Industry Manchester UK, May 19-23 2019

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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)

19th International Symposium on Zirconium in the Nuclear Industry Manchester UK, May 19-23 2019

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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 𝑎𝑠 + 𝑃2 → 𝑎𝑠𝑃2 with steam 𝑎𝑠 + 2𝐼2𝑃 → 𝑎𝑠𝑃2 + 2𝐼2 with nitrogen 2𝑎𝑠 + 𝑂2 → 2𝑎𝑠𝑂 Re-oxidation of nitrides: ZrN + O2 → ZrO2 + N 𝑎𝑠𝑃2

5000 10000 15000 20000 200 400 600 800

N2 conc., % 0.1 0.2 0.5 0.7 1 2 5 10 50 70 80 90 100

m, g/m² time, s

H2O + N2 H2O N2

Mass gain of Zircaloy-4 oxidized at 800°C in steam-nitrogen mixtures, (Steinbrück et al., NUMAT 2014)

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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

• f

experiments. Modeling in severe accident codes is not yet satisfying. 𝑎𝑠𝑃2 𝒂𝒔 𝒂𝒔𝑶 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.

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Flow rate model

[M. Grosse et al. ICAPP 2016]

𝐼2O g + (Zr𝑃 + 𝑊

𝑃 2+)(𝑝𝑦) = 𝑎𝑠𝑃2 ox + 2𝐼𝑏𝑒 +

Basic reactions with steam if an oxide layer is already formed 𝑃𝑃 𝑝𝑦 + 𝑎𝑠 𝑛 = (𝑎𝑠𝑃 + 𝑊

𝑃 2+ + 2𝑓−)

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

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

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Flow rate model

[M. Grosse et al. ICAPP 2016]

Basic reactions with nitrogen if no

• xygen or steam is available

𝑂2(𝑕) + 4 𝑎𝑠𝑃 + 𝑊

𝑃 2+ + 2𝑓− (𝑝𝑦) = 2𝑎𝑠𝑃2(𝑝𝑦) + 2ZrN(ox)

𝑃𝑃 𝑝𝑦 + 𝑎𝑠 𝑛 = (𝑎𝑠𝑃 + 𝑊

𝑃 2+ + 2𝑓−)

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1 2 3 4 5 6 7 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

(, gm

• 2s
• 1)

1/4

Oxygen concentration, wt%

900°C 1000°C 1100°C 1200°C 1300°C

𝑂2(𝑕) + 4 𝑎𝑠𝑃 + 𝑊

𝑃 2+ + 2𝑓− (𝑝𝑦) = 2𝑎𝑠𝑃2(𝑝𝑦) + 2ZrN(ox)

Flow rate model

[M. Grosse et al. ICAPP 2016]

Experimental evidence: reaction rate K ~ xO

4 for the reaction of N2 with Zr(O)

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Flow rate model

[M. Grosse et al. ICAPP 2016]

ሶ 𝑜𝑊

𝑃 2+ = ሶ

𝑜𝑝𝑦𝑧𝑕𝑓𝑜,𝑠𝑓𝑏𝑑𝑢 =

𝐿𝑜𝑝𝑦 2 𝑢∗

Do we know the oxygen vacancy flux to the surface?

Yes, we know!

ሶ 𝑜𝑂2 =

𝐿𝑜𝑝𝑦 2 𝑢∗ − ሶ 𝑜𝑃2 2 − ሶ

𝑜𝐼2𝑃 /4 How many nitrogen reacts?

𝑜𝑂 = න

𝑢

𝑙𝑜𝑃 2 𝑢∗ − ሶ 𝑜𝑃2 2 + ሶ 𝑜𝐼2𝑃 2 ሶ 𝑜𝑃2 2 + ሶ 𝑜𝐼2𝑃 𝑒𝑢 + 𝑜0𝑂

Molar concentration of nitrogen taken up:

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Flow rate model

[M. Grosse et al. ICAPP 2016]

❑ 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. 𝑔

𝑑𝑠𝑏𝑑𝑙𝑡 = 𝐵 න 𝑢

𝐿𝑜𝑃 2 𝑢∗ − ሶ 𝑜𝑃2 2 + ሶ 𝑜𝐼2𝑃 2 ሶ 𝑜𝑃2 2 + ሶ 𝑜𝐼2𝑃 𝑒𝑢 + 𝑜𝑂0 ❑ The reaction rate is weighted sum of the reaction rate of positions with cracked and undisturbed oxide. ሶ 𝑜𝑎𝑠 = 𝑔

𝑗𝑜𝑢𝑓𝑠𝑔𝑏𝑑𝑓𝑑𝑠𝑏𝑑𝑙𝑓𝑒𝐿𝑜𝑎𝑠 + 1 − 𝑔 𝑗𝑜𝑢𝑓𝑠𝑔𝑏𝑑𝑓𝑑𝑠𝑏𝑑𝑙𝑓𝑒

𝐿𝑜𝑃 2 𝑢∗

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Flow rate model

[M. Grosse et al. ICAPP 2016]

• total starvation (starvation of all gases reacting): The lower the
• xygen, 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 ሶ 𝑜𝑂2 =

𝐿𝑜𝑝𝑦 2 𝑢∗ − ሶ 𝑜𝑃2 2 − ሶ

𝑜𝐼2𝑃 /4

Determining parameter: quantity

• f steam and oxygen starvation

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Experimental validation of the model

500 1000 1500 2000 2500 3000 50 100 150

Time, s m, g/m

2

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.

19th International Symposium on Zirconium in the Nuclear Industry Manchester UK, May 19-23 2019

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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.

3600 7200 10800 14400 18000 200 400 600 800 1000 1200 1400 1600

cH, wppm

• xidation time, s
• M. Grosse et al.,
• Nucl. Instr. & Meth. A 651 (2011). 253

Zircaloy-4, 1000°C

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Basis Neutron radiography

Intensity measured at the detector pixel x,y: Total macroscopic neutron cross section:

( )

s y x I y x I y x T y x I

total 

 −  =  = exp ) , ( ) , ( ) , ( ) , ( ) , , ( ) , , ( ) , , ( ) , ( ) , , ( ) , , (

4

t y x N t y x N t y x N y x t y x N t y x

O O N N H H Zry total i i i total total

total total total

 +  +  +  =  = 

−



   

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

total, cm

• 1

H/Zr atomic ratio

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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

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Test matrix of experiments with different flow rates

4 temperatures

800°C 900°C 1000°C 1100°C

5 flow rates

24 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

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In-situ Neutron imaging – influence of flow rate

36 l/h 72 l/h 24 l/h 48 l/h steam-nitrogen 900°C

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Results 900°C

absorbed hydrogen released hydrogen

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In-situ Neutron imaging – different atmospheres

800°C 1000°C steam-nitrogen steam steam-air

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In-situ Neutron imaging – different atmospheres

Breakaway at the very beginning

• f 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

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In-situ Neutron imaging – different atmospheres

absorbed hydrogen released hydrogen Released and absorbed hydrogen during the tests at 1000°C

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Summary and conclusion

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

hydrogen absorption during the reaction

• f

Zircaloy-4 in nitrogen-steam atmosphere were observed.

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

𝐿𝑜𝑝𝑦 2 𝑢∗ − ሶ 𝑜𝑃2 2 − ሶ

𝑜𝐼2𝑃. It was confirmed qualitatively.

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

during severe accidents!

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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

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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)