Fracture and Creep in the All-Tungsten ARIES Divertor

Jake BlanchardUniversity of Wisconsin – Madison

(presented by Mark Tillack)

Japan/US Workshop on Power Plant Studies and Advanced Technologies

26-27 February 20131

2

Introduction

The ARIES Project is exploring the feasibility of using tungsten as a structural material for plasma-facing components

For this analysis, we assumed the material is pure tungsten, but alloys may be necessary

This talk addresses two key failure modes that must be addressed by these designs◦ Fracture◦ Thermal creep

3

We examined the ARIES plate divertor design

Temperature distributions were simulated using ARIES design loads with simplified convection cooling

q”=11 MW/m2

q’’’=17.5 MW/m3

P=10 MPa

Tcoolant=600 ᵒC

Max. Tarmor= 2000 ᵒC

Max. Tstructure=1310 ᵒC

Min. Tstructure=725 ᵒCoC

Stresses in uncracked structure identify regions of concern

MPa MPa

X- Stress Distribution when Hot (MPa)

x

• During operation: tensile stress along the cooling channel, compressive stress in the grooves.

• High tensile stress occurs at the base of the grooves after cool-down, resulting from plastic deformation.

x

X- Stress Distribution after Cool-down (MPa)

tension

6

Simulated cracks were introduced in high-stress regions

Crack-Free Stress State

2. Coolant channel surface

1. Base of grooves

7

Stress intensity should remain below the fracture toughness

B. Gludovatz, S. Wurster, A. Hoffmann, R. Pippan, “Fracture Toughness of Polycrystalline Tungsten Alloys.” 17th Plansee Seminar 2009, Vol. 1.

(unir

radia

ted)

8

0 0.1 0.2 0.3 0.4 0.5 0.60

2

4

6

8

10

12

14

16

c/a=2c/a=6c/a=10

Crack Depth (mm)

Stre

ss In

tens

ity

(MPa

-m1/

2)

Fracture Results

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.550

1

2

3

4

5

6

7

8

c/a=2

c/a=6

c/a=10

Crack Depth (mm)St

ress

Inte

nsit

y (M

Pa-m

1/2)

Crack on coolant surface (hot)

Crack in notch (at shutdown)

2c

a

Stress intensities for crack perpendicular to coolant flow direction

Crack Face

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.550.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

c/a=2

Crack Depth (mm)

Stre

ss In

tens

ity (M

Pa-m

1/2)

Z- Stress Distribution when Hot (MPa)

Lower stress intensities are obtained

Conclusions on fractureStress intensity is highest at full

power.

Critical crack is in the notch between “tiles” under shutdown conditions.

This may change if thermal creep is taken into account.

Fatigue (growth rate) has not yet been explicitly included.

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Thermal Creep

12

Thermal analysis predicts temperatures in the 1100 – 1300 oC range in the W structure

q”=11 MW/m2

q’’’=17.5 MW/m3

P=10 MPa

Tcoolant=600 ᵒC

Max. Tarmor= 2000 ᵒC

Max. Tstructure=1310 ᵒC

Min. Tstructure=725 ᵒC

• While structural temperatures are only ~0.4 T/Tm data indicate that pure tungsten can creep at these temperatures.

• Power law creep model was added to ANSYS analysis to evaluate creep behavior

oC

1300 C

1300 C

Large uncertainties exist in creep strain rate data• Most W creep data is for higher temperatures and

lower stress

• Limited available data for temperatures/stresses of interest

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A. Purohit N. A. Hanan S. K. Bhattacharyya E. E. Gruber “Development of a steady state creep behavior model of polycrystalline tungsten for bimodal space reactor application.” Argonne National Lab., IL 1995

14

Using the more conservative data, excessive creep deformation is obtained

Mid Channel Displacement

Time (hr)

Dis

pla

cem

ent

(mm

)

15

q”=6.7 MW/m2

q’’’=17.5 MW/m3

P=10 MPa

Tcoolant=600 ᵒC

Max. Tarmor=1433 ᵒC

Max. Tstructure=1310 ᵒC

Min. Tstructure=714 ᵒC

• Reducing the surface flux to a value of 6.7 MW/m2 reduces the maximum armor temperature to 1433 oC

• Structure temperatures where creep occurs are in the 900-1000 oC range

Additional studies were performed using reduced heat flux

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Creep behavior of tungsten armor over two year exposure

0 5000 10000 15000 200000.0000

0.0002

0.0004

0.0006

0.0008

0.0010

0.0012

0.0014Total CreepThermal CreepPressure Creep

Time (hr)

Cre

ep S

train

Creep Strain After Two Years

Pt. A

Total Creep Strain along with Pressure Only and Thermal Only Creep at Pt. A

•Creep from combined thermal and pressure loads is considerably greater than sum of the individual components

•Thermal creep rate is initially high, but slows as stress is relieved. Pressure creep rates are constant.

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0 5000 10000 15000 200000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

baselineq +20%q +10%q -10%q -20%

Time (hr)

Cre

ep S

train

(m

m/m

m)

Sensitivity of Creep Rates to Changes in the Thermal and Pressure Loads

0 5000 10000 15000 200000

0.0005

0.001

0.0015

0.002

0.0025

base-lineP +20%P +10%P -10%P -20%

Time (hr)

Cre

ep S

train

(m

m/m

m)

Creep Sensitivity to Surface Heat Flux

Creep Sensitivity to Coolant Pressure

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Results show creep rates are very sensitive to changes in temperature and stress

0.0

1.0

2.0

3.0

4.0

5.0

6.0

Heat Flux

Pressure

Normalized Parameter Value

Norm

alize

d C

reep S

train

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Reduce Tile Notch Depth by 1 mm

3 mm

•Reducing the notch depth increases minimum wall thickness to 3 mm (from 2 mm) reducing pressure stress in wall with some increase in thermal stress.

•Two year creep strain is reduced by 23% and rate is also significantly reduced.

0 5000 10000 15000 200000

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

Baseline

Reduced Notch

Time (hr)

Cre

ep S

train

20

Remove Notch Completely

0 5000 10000 15000 200000

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

BaselineSolid Wall 3.5mmSolid Wall 4mm

Time (hr)

Cre

ep S

train

•Removing alleviates stress concentrations and lowers surface temperature. Thermal stresses increase however.•Two year creep strain is reduced by 7% for a 3.5 mm wall and 15% for a 4 mm wall.•4 mm wall reduces maximum surface temperature by 162 oC vs baseline.

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Conclusions on creepCreep appears to be a significant

problem for design heat fluxes above 10 MW/m2.

Design changes can alleviate the problem to some extent, but

Materials with higher creep strength are needed (compared with pure W).

Reliable data are essential.

Data NeedsFracture toughness of tungsten

(alloy)◦As-manufactured, at temperature◦Irradiated

Crack growth rates (da/dN)Creep ratesCreep rupture dataCreep-fatigue interaction data