Influence of Pre-Oxidation on Mechanical Properties of Zr1Nb Alloy

Nové technologie - Výzkumné centrum v západočeském regionu Západočeská univerzita v Plzni

Influence of Pre-Oxidation on Mechanical Properties of Zr1Nb Alloy z Olga Bláhová New Technologies - Research Centre in Westbohemian Region

University of West Bohemia, Plzen

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Zirconium Alloys z The use of Zr alloys expanded in 1950’s z Application in nuclear power generation: nuclear fuel cladding z Favourable mechanical properties and corrosion and radiation resistance z Further development: alloys for fuel with a long burnout period z The present alloy: 1.0-1.1 % Nb, 3 ppm H, 20 ppm N, 100 ppm C, 840 ppm O 2


Focus of Research z Nuclear plants security enhancement in case of LOCA type accident – a project of the Ministry of Industry and Trade, CZ

z UJP Praha a.s.: corrosion testing, metallography, hydrogen content measurement, pressure tests, safety criteria

University of West Bohemia in Pilsen: nanoindentation measurement, X-ray diffraction, SEM, EDAX

z Aim of the study: - Clarification of mechanism of oxidation processes (hydriding, long fuel cycle, oxide dissolution) - Formulation of oxide dissolution model - Construction of pseudo-binary phase diagrams - Proposed modification of oxidation and safety criteria 3


LOCA Accident z Loss Of Coolant Accident z Failure of primary circuit piping ⇒ Loss of coolant and pressure ⇒ Temperature increase in the core (1,000°C – 1,200°C) ⇒ Rapid material oxidation in water - steam mixture ⇒ Emergency water cooling ⇒ quenching of material z Negative impact on mechanical properties – requirement for safe handling of material z Oxidation criteria: ECR 17% equivalent cladding reacted - Zircaloy-4 K-criterion (by UJP) - Zr1Nb 4


LOCA Simulation z z z z

z z z z

Pre-oxidation – simulation of long-term operation HTO – high temperature oxidation Quenching – water + ice Parameters: temperature, time, heating and cooling rates Migration of oxygen and hydrogen atoms in material: Oxidation Hydriding of material - embrittlement, degradation of mechanical properties Occurrence of stresses 5


Experimental Specimens and Methods z Specimens of cladding tubes, length 30 mm, ∅ 9 mm, wall thickness 0.6 mm z Corrosion testing various environments and z conditions z Corrosion kinetics z Mechanical testing (compression, microhardness) z Metallography, X-ray diffraction, z H2 and O2 levels, EDAX, WDX (UJV Řež) 6


Microstructure of Material upon HTO z External oxide ZrO2 z α-Zr(O) phase: α-Zr phase stabilized by oxygen – greater brittleness & hardness z α-Zr (prior phase β-Zr) – higher elongation and toughness, lower hardness {provides required mechanical properties (toughness)

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Nanoindentation Measurement z Parameters NI XP:

NanoIndenter XP (MTS)

{ Load: 10 μN to 10 N { Loading unit resolution: 0.050 μN (≅ 5.1 μg). { Max. indentation depth: 500 μm { Depth measurement resolution: 0.01 nm { Indenters: Berkovich, Vickers, ball, cube corner tip { Methods: indentation test cyclic indentation CSM method scratch test profilometry 8


Nanoindentation Measurement z h = f(F) function: indentation curve Fmax z Indentation hardness:

H IT =

Ap

z Indentation modulus of elasticity: 2 ν 1 − ( ) S. π s E = Er = IT 2 1 − ( ν ) 1 i 2. A p

Er

−

Ei

(ISO/DIS 14577 – Metallic materials – Instrumented indentation test for hardness 9 and materials parameters. ISO Central Secretariat, Geneva 2002)


Nanoindentation Measurement • Berkovich indenter • 8 mN load • Indent spacing 5 µm • Determination of • α-Zr hardness

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Experimental Specimens Pre-oxidation - simulation of long-term operation - simulation of the reactor environment - steam at 425 °C, the pressure of 10.7 MPa, till 365 days → oxid thickness: 0, 2, 10, 30 µm HTO – high temperature oxidation - different temperature: 950 - 1200°C - different time: 0 - 300 min Different cooling rates - quenching – water + ice - furnace

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Exposure Time vs. Hardness

1200°C 1200°C-p 1150°C 1150°C-p 1100°C 1100°C-p 1050°C 1000°C 950°C

6

H IT [GPa]

5

4

3

7 6

2

HIT [GPa]

0

5

5

10

15

t [min]

4

950 1000 1050 1100 1200

3 2 0

100

200 t [min]

300

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Exposure Time vs. Modulus 1200°C 1200°C-p 1150°C 1150°C-p 1100°C 1100°C-p 1050°C 1000°C 950°C

1200°C 1200°C-p 1150°C 1150°C-p 1100°C 1100°C-p 1050°C 1000°C 950°C

40

110

EIT /H IT

EIT [GPa]

120

30

100 0

5

10

15

t [min]

20 0

5

10

t [min]

1315


Hardness vs. Ductility Brittle samples: HIT > 4 GPa

ductility [%]

60

1200°C 1200°C-p 1150°C 1150°C-p 1100°C 1100°C-p 1050°C 1000°C 950°C

40

20

0 2

3

4

H IT [GPa]

5

6

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Conclusions z Use of nanoindentation measurement: - Evaluation of changes upon high-temperature oxidation, exposure time and temperature - Additional input in Corrosion and Oxidation databases - Evaluation of influence pre-oxidation on properties z Correlations with results of other analytical methods: - Formulation of models of oxidation, hydriding and oxygen dissolution - Specification of safety criteria with greater accuracy

- Construction of pseudo-binary phase diagrams The study was conducted under project no. 2A - 1TP1/037 of the Ministry of Industry and Trade of the Czech Republic

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Influence of Pre-Oxidation on Mechanical Properties of Zr1Nb Alloy

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