In-situ SEM Observation and FEM Analysis of Delayed ... · There was a qualitative agreement in...

NIPPON NUCLEAR FUEL DEVELOPMENT CO., LTD. 1

In-situ SEM Observation and FEM Analysisof Delayed Hydride Cracking Propagation in

Zircaloy-2 Fuel Cladding Tubes

*Toshio Kubo (NFD)Hiroaki Muta, Shinsuke Yamanaka (Osaka University)Masayoshi Uno (Fukui University)Keizo Ogata (JNES)


Outside-in crackingin ramp tests of BWR fuel rods

Cross-sectional view of a failed segmented rodBurnup, 61 GWd/t, Maximum power, 43 kW/m

100 µm

Outside-in crack propagationby DHC

(Delayed Hydride Cracking)

Radialhydrides

Overview images of an axial crack

Axial direction

1mm

Outer surface

1mm

Outersurface

Innersurface

Crack

Fracture surface

Axial crack


(1) DHC tests were carried out in the chamber of an SEMfor Zircaloy-2 BWR fuel cladding tubes,●to directly observe the propagation process of DHC, and●to measure the crack propagation rate in the radial

direction of the cladding tubes

(2) FEM analyses of stress distribution and hydrogen diffusionaround a crack tip were made to estimate the crack velocity.The DHC mechanism was considered by comparison betweenexperimental and analytical results.

Objectives


Apparatus for the DHC tests in an SEM

Objective

Tensile load

Micro-heater

Infrared thermometer

Ring specimen

Objective

Tensile load

Micro-heater


Ring specimen

Objectivelens

Micro-heater

Tensile load


Video

Objectivelens

Pre-crack

40°

GripTensileload

Ring specimen with a pre-crack


Test specimens

Test materials

Hydrogencharge

Thermal cyclingto obtain radial hydrides

DHC tests

●Cold-worked Zry-2 BWR fuel cladding tubes●Some were stress relieved at 723K and 783K

●Heated at 588K for 24h in LiOH aqueous solution●Hydrogen concentration ; 90-130 ppm

in Zry-2 matrix

Hoop stress of160MPa

Tem

pera

ture

/Str

ess

Time

663K

423K

Pre-cracking on outer surfaces

DissolutionPrecipitation


Zry-2matrix

Zr liner

100 µm

(a)Circumferentially-hydrided(After hydrogen charge)

(b)Radially-hydrided(After the thermal cycling)

Hydride distribution


Pre-cracking on specimen outer surfaces

This transverse cross section was polishedfor the observation under an SEM

Cyclic loading Methanol solution of iodine

Pre-crack

Cut at the middle of pre-crack

NIPPON NUCLEAR FUEL DEVELOPMENT CO., LTD.

a

W Uniform crack

Pre-crack

A

Axial direction

Outer surfaceLc

Inner surface

Evaluation of stress intensity factor, KⅠ

8

KⅠA for a long shallow crack≒ KⅠ for a uniform crack,when a/Lc < 0.2

M’ = ( 1.13 – 0.09α ) + ( - 0.54 + ) β2

α = a/Lc β = a/W

+ (0.5 – + 14.0(1-α)24)β2

ξ2 = 1 + 1.464α1.65

KⅠA =σ√πa M’ξ

0.89(0.2 +α)

1(0.65 +α )


●Test method : Ring tensile tests in an SEMat a constant crosshead speed

●Crosshead speed : 0.002 mm/min ( 2 µm/min )●Temperature : 523K and 548 K●Temperature cycling: Pre-heated at 598 K for 0.5 h

Test conditions

Time

598K/0.5h

Tem

pera

ture

/Str

ess

548K

Radially-hydrided material

Circumferentially-hydrided material


50

100

150

200

0 100 200 300 400Time (min)

Cra

ck d

epth

(µm

)

500750

250

①

⑥

Cra

ck d

epth

(µm

)

Time (min)

Stre

ss (M

Pa)

Crack depth

Stress

Time dependency of crack depth and stress

10

Temperature; 523 K, Circumferentially hydrided material


Observation of crack propagation

11

Temperature; 523 K,Circumferentially hydrided

0

25

50

200 250 300 350

Time (min)

Ave

rage

crac

k ve

loci

ty (

x 1

09 m/s)

①

②

③ ④

⑤

⑥

Prop

agat

ion

dist

ance

(µm

)

①

②

③

②

③④

Sharp crack

Crack blunting

③ ④

③④

⑤

Sharp crack

⑤

③④

⑤

⑥

Crack blunting

⑥

10µm

②


Effects of yield stress on DHC

12

Temperature; 548 K, Circumferentially hydrided materials

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

0 10 20 30

KⅠ (MPam1/2)

Cra

ck v

eloc

ity (

m/s

)○ Cold-worked△ 723KSR◇ 783KSR

Yield stress(MPa)

○ Cold worked 600△ Stress relieved at 723K 520◇ Stress relieved at 783K 310


Effects of hydride orientation on DHC

13

Temperature; 523 K

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

0 10 20 30

KI (MPam1/2)

Cra

ck v

eloc

ity (

m/s)

Hydride orientation○ Circumferential▲ Radial


DHC propagationin the radially hydrided material

BSE image taken after a DHC test. Test temp.; 523 K

10µm

Pre-existing radial hydride

Crack

Radial direction

Pre-crackCrack propagation

Pre-existing radial hydrides

Hydrogen diffusion

Hydrideprecipitation


Fracture of radial hydrides at room temperature

15

Crack

Fracture of radial hydrides

10μm

BSE image taken at room temperature

Radial direction

From The International Topical Meeting on Light Water ReactorFuel Performance, ANS, Seoul, Korea, October 19-23, 2008

Contour lines ofstress

Fracture of radial hydrides

Crack

NIPPON NUCLEAR FUEL DEVELOPMENT CO., LTD.BSE image taken after a DHC test. Test temp.; 523 K 16

Circumferential hydride

Crack

Radial direction

10µm

Radial hydride

DHC propagationin the circumferentially hydrided material


SOFTWARE : ABAQUS Ver.6.7-1

STRESS DISTRIBUTION ANALYSES :●Zircaloy-2 ; Isotropic elastic- plastic body●Stress strain curve ;

●Yield stress ; Present experimental values●Others ; MATPRO HANDBOOK

HYDROGEN DIFFUSION ANALYSES

J ; Hydrogen fluxΦ; Normalized hydrogen concentrationp ; Hydrostatic pressures ; Hydrogen solubilityD ; Diffusivitykp ; Pressure stress factor

mnK ÷

øö

çèæ= -310

'ees

÷øö

çèæ

¶¶

+¶¶

-=xp

xsDJ pk

f

FEM ANALYSESon stress distribution and hydrogen diffusion

17

Hydrogen concentration is fixed at 100 ppm

Stress

Crack

W

L

W : 0.7 mmL : 0.05, 0.1 and 0.2 mm


Distribution of hydrostatic pressureand hydrogen concentration

18

98 ppm 155 ppm+20 MPa -970 MPa

Hydrostatic pressure Hydrogen concentration

Temperature : 523K Stress : 100 MPa

10 mm


Hydrogen flux rate versus KⅠ

19

KⅠ (MPam1/2)

Temperature : 523 K

0.15μm

Hydride

Thickness of the hydride at the crack tip:0.1 – 0.2μm ( from the observation)

1E-17

1E-16

1E-15

1E-14

0 5 10 15 20

KⅠ (MPam1/2)

Hyd

roge

n flu

x ra

te (g

/μm

2 /s)


V-KⅠ curvesCalculations and measurements

20

Temperature : 523 K

1.E-10

1.E-09

1.E-08

1.E-07

0 10 20 30

KⅠ (MPam1/2)

Cra

ck v

eloc

ity (m

/s)■ Calculation○ Experiment


Crack velocity versus yield stress

21

Temperature : 548 K

0

2

4

6

200 400 600 800

0.2% offset yield stress (MPa)

Cra

ck v

eloc

ity (

x108 m

/s)

Yield stress (Mpa)600 520 310● ▲ ■ Experiment● ▲ ■ Calculation


Hydrostatic pressure distributionin front of a crack tip

22

-1000

-750

-500

-250

0

0 2 4 6

Distance from crack tip (µm)

Hyd

rosta

tic p

ress

ure

(MPa

)

Yield stress (Mpa)◇ 310△ 520○ 600

Temperature : 548 K


Crack velocity versus yield stress

23

Temperature : 548 K

0

2

4

6

200 400 600 800

0.2% offset yield stress (MPa)

Cra

ck v

eloc

ity (

x108 m

/s)

Yield stress (Mpa)600 520 310● ▲ ■ Experiment● ▲ ■ Calculation


Summary

●The crack velocity was obtained as a function of KI, temperature,hydride distribution and material strength.

●The steady state crack velocity, Vs, tended to increase with anincrease in the 0.2 % offset yield stress.

●Effects of hydride orientation on DHC velocity was small●There was a qualitative agreement in crack velocity between FEM

analyses and experiment●FEM analyses showed that the increase in the 0.2 % offset yield

stress would accelerate the crack propagation by increasing thehydrostatic pressure at the crack tip.


Fracture surface

25

Crack propagation

20µmTest temp. ; 523 K

DHCPre-crack

100µm


-1500

-1000

-500

0

0 1 2 3 4 5 60.0

0.5

1.0

1.5

200 MPa300 MPa400 MPa

mm)

Hyd

roge

n flu

x ra

te (w

tppm

/mm

/s)

Distance from crack tip (µm)

Hyd

roge

n flu

x ra

te (p

pm/m

m/s

)H

ydro

stat

ic p

ress

ure

(MPa

)

□ 20○ 50△ 100▽ 200◇ 300

Applied stress (MPa)

Distributions of hydrostatic pressureand hydrogen flux rate

Temperature : 523K, Crack length:100µm

Contour lines ofHydrostatic pressure

Hydrogen diffusion

26

Hydrostatic pressure, p, is defined as

P = −Σσii /3

Hydrogen diffusion to the crack tip


Hydrogen flux rate versus KⅠ

27

0.01

0.1

1

10

0 5 10 15 20

KⅠ (MPam1/2)

Hyd

roge

n flu

x ra

te (p

pm/m

m/s)

Ave

rage

hyd

roge

n flu

x ra

te (p

pm/m

m/s

)

Hydrogen flux to crack tip Carck velocity

1.E-10

1.E-09

1.E-08

1.E-07

0 5 10 15 20

KⅠ (MPam1/2)C

rack

vel

ocity

(m/s)


V-KⅠ curvesCalculations and measurements

28

Temperature : 523 K

0.16μm

Hydride

Thickness of the hydride at the crack tip:0.1 – 0.2μm ( from the observation)

1.E-10

1.E-09

1.E-08

1.E-07

0 10 20 30

KⅠ (MPam1/2)

Cra

ck v

eloc

ity (m

/s)

■ Calculation

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In-situ SEM Observation and FEM Analysis of Delayed ... · There was a qualitative agreement in...

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