Investigation of Proton Irradiation-Induced Creep of

Ultrafine Grain Graphite

Anne A. Campbell & Gary S. Was

University of MichiganResearch Supported by

US DOE under NERI Contract # FC07-06ID14732

And INL under Contract # DE-AC07-05ID14517

Outline

• Objective• Background• Experimental Setup & Procedure• Research Approach• Results• Comparison with Neutron Data• Summary

2

Objective

• Investigate the irradiation-induced creep of ultra-fine grain graphite– Study the effect of stress, dose rate, and temperature on

creep rate

• Compare results with neutron irradiation-induced creep work

3

Background

• Using POCO grade ZXF-5Q– Particle size < 1 µm– Density 1.78g/cm3

– Tensile Strength 79 MPa– Anisotropy < 1.03 BAF– Young’s Modulus 14.5 GPa

• Using 3 MeV protons for radiation damage and heating– 3 MeV used to keep damage rate ratio

from front to back of sample below 1.5 (from SRIM calculations)

– 70 µm maximum sample thickness– Final sample size 3mm x 0.07 mm x 33

mm (cut via EDM)

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POCO Grade ZXF-5Q Data sheet.

Experiment – Irradiation Chamber

5

Campbell, A.A. and G. S. Was, Journal of Nuclear Materials, 433 (2013) 86-94.

Experiment – Irradiation Stage

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Laser Speckle Extensometer (LSE)

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Camera

Laser

AnalysisProgram Sample

2D Thermal Imager

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Irradiation Creep Experimental Procedure

• Assemble stage, test heater, mount samples, apply and remove tensile load on bench-top, align and test beam apertures

• Assemble chamber on beam-line, insert load, insert stage, align LSE, apply load to sample, seal top port, pump out chamber, start LSE DVRT and Stage data collection, condition vacuum, align 2D thermal imager start temperature collection

• Obtain desired proton current in Faraday cup, increase beam scanner to wider than necessary, apply beam to samples, balance beam one apertures, narrow scanner width until desired temperature is achieved, set alarms for temperature and dose rate monitoring and control

• Analyze LSE and DVRT data to determine linear fit and 95% CI of slope, continue at same conditions until 95% CI < 1% of the slope or desired final dose is achieved, change to other conditions or end experiment

• Insert Faraday cup, cool system to room temperature, remove stress from sample, vent vacuum chamber, remove stage, remove samples from stage, put individual samples in boxes labeled with all relevant irradiation information

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Irradiation Creep Example Data

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Dose Rate & Temperature Control

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Irradiation Creep Experimental Conditions

• Applied tensile stress (1000ºC, 1.15x10-6dpa/s)– 5 MPa, 10 MPa, 20 MPa, 40 MPa

• Dose Rate (700ºC, 20MPa)– 2.95x10-7dpa/s to 5.51x10-7dpa/s

• Temperature (20MPa, variable dose rate)– 700ºC, 900ºC, 1000ºC, 1100ºC, 1200ºC

• Two samples used for each experiment, one with stress and one without stress– Residual stress from EDM machining resulted in curvature of the

unstressed sample

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Applied Stress Dependence

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Dose Rate Dependence

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Temperature Dependence

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Accumulated Dose Dependence

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Stress Dependence Comparison

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Gray, Carbon, 11, (1973) 183 Oku et al., JNM, 152, (1988) 225Oku et al., JNM, 172, (1990) 77

Dose Rate Comparison

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Veringa and Blackstone, Carbon, 14, (1976) 279.Oku et al., JNM, 152, (1988) 225 Oku et al., JNM, 172, (1990) 77

Accumulated Dose Comparison

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Neutron Data for H-451 from:Burchell, T.D., JNM, 381, (2008) 46.

Temperature Comparison

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Veringa and Blackstone, Carbon, 14, (1976) 279. Burchell, T.D., JNM, 381, (2008) 46.Gray et al., Carbon, 5, (1967) 173 Kelly and Burchell, Carbon, 32, (1994) 119.Mitchell et al., Nuclear Energy, 41, (2002) 63 Oku et al., JNM, 152, (1988) 225Oku et al., JNM, 172, (1990) 77 Perks and Simmons, Carbon, 1, (1964) 441.Perks and Simmons, Carbon, 4, (1966) 85.

Possible Source of Discrepancy

• Protons provide both displacement damage and heating from electronic excitation– Damage comes from PKA interaction– Fundamentally similar PKA interaction within material

• Neutron irradiations have both neutron and γ-rays interacting with material

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γ – Ray Effects

• Work in Russia– Examined effect of

Radiation Composition Factor (RCF=ϕn/ϕγ) on Turn-around dose (Fcrit)

• Use equation to determine effective temperature (i.e. temperature without γ-rays)

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MR (0.13)

SM-2 (0.1)

BOR-60 (4)

2

5 2

0.18 19.85

2.440.0721* 7.49 10

ncrit cm

irr irr

F E MeV

T C x T CRCF

Nikolaenko et al., Atomic Energy, 87, (1999) 480.

Burchell, T.D., "Neutron Irradiation Damage in Graphite and Its Effects on Properties", Presented at International Carbon Conference CARBON 2002, Beijing, China, September 15-20, 2002.

Neutron Results – H-451

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Temp (ºC)

Fluence (n/m2 E>0.1MeV)

Fluence (n/cm2 E>0.18MeV)

RCF ϕn/ϕγ

Equivalent Temp (ºC)

(ϕγ=0)

kE0 (dpa)-1

600 2.5x1026 20.5x1021 0.156 950 2.65900 1x1026 8.2x1021 0.154 1100 4.56

2

5 2 2.440.18 19.85 0.0721* 7.49 10n

crit irr irrcmF E MeV T C x T C

RCF

Proton Results

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Temperature (ºC)

Dose Rate (dpa/s) Drad (cm2/s) kE0 (dpa)-1

from DVRT900 8.1x10-7 2.30x10-18 27.55 (6.04)1100 15.6x10-7 2.81x10-17 51.51950* 9.9x10-7 4.72x10-18 32.88

0

0

1100 51.511.29

4.56900

p

n

kE C

kE C

0

0

950 32.8812.41

2.65600

p

n

kE C

kE C

17

18

1100 2.81 1012.2

2.30 10900rad

rad

D C x

xD C

Discussion of Analysis

• Russian researchers proposed that γ-rays interact and produce fast electrons that stimulate diffusion processes

• Recent work in China investigated the effects of γ-rays, on graphite, at room temperature– Shows that γ-rays can increase graphitization of graphite at room

temperature

• Possible that γ-rays are annealing damage as it is being caused by neutrons in-reactor, effectively reducing the # of defects available to assist with driving creep

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Li, B. et al., Carbon, 60, (2013) 186. Xu, Z. et al., Materials Letters, 63, (2009) 1814.

γ dose (MGy) D002 (nm) Lc (002) (nm) D002 (nm) % Graphitized

0 0.3372 [Li] 26.4 [Li] 0.3376 [Xu] 75 [Xu]

0.200 0.3361 [Li] 31.7 [Li]

2.00 0.3366 [Li] 31.0 [Li] 0.3371 [Xu] 81 [Xu]

Summary

• Proton Irradiation-Induced creep experiments preformed on POCO ZXF-5Q graphite– Investigated stress, dose rate, temperature, and accumulated

dose effects on creep behavior

• Linear dependence on stress, dose rate, and temperature trends agree with neutron creep measurements

• Compliance values for protons factor of 6-10 higher than neutron values– Experimental results suggest creep is driven by defect

population and γ-rays reduces the defect population, which reduces creep rate

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Investigation of Proton Irradiation-Induced Creep of

Ultrafine Grain Graphite

Anne A. Campbell & Gary S. Was

University of MichiganResearch Supported by

US DOE under NERI Contract # FC07-06ID14732

And INL under Contract # DE-AC07-05ID14517