Department of Energy (DOE) Advanced Reactor ......and Property Changes due to Fast Neutron Irradiation”, Juel-4183, 2005 10 Main Material Properties of Interest Irradiation Dimensional

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Nuclear Graphite Research Needs

Will WindesIdaho National Laboratory

DOE-NE NEET Cross-cut Coordination Meeting August 15-16, 2016

Department of Energy (DOE)Advanced Reactor Technologies (ART) R&D Program

Why Research Graphite? Lessons we’ve learned from other graphite core reactor programs

– After Reactor Start-up…• Interestingly - fuel is not considered life limiting component after start-up• Graphite is life limiting component of reactor

– Degradation issues of graphite – normal and accident operations– Changes resulting from irradiation – structural integrity, cracks (irradiation creep)

– NRC will require understanding of the primary structural core material• Material properties needed for license approval before reactor start-up, and • Predicting core behavior during normal and off-normal operation

Primary objectives of graphite components– Must keep the fuel safe

• Must keep temperature “low” – thermal conductivity• Must maintain structural integrity – strength and irradiation behavior

– Creep behavior → Reduces cracks resulting from irradiation stresses– Provides core structure

• Cooling channel integrity, control rod insertion, and stable fuel configuration

− After Reactor Start-up…• Interestingly - fuel is not considered life limiting component after start-up• Graphite is life limiting component of reactor

• Degradation issues of graphite – normal and accident operations• Changes resulting from irradiation – structural integrity, cracks (irradiation creep), fracture

− NRC will require understanding of the primary structural core material• Material properties needed for license approval before reactor start-up, and • Predicting core behavior during normal and off-normal operation

Primary function of Graphite R&D activities Defines the safe working envelope for nuclear graphite

− Current and future nuclear graphite components− Applicable for multiple DOE reactor designs (cross-cutting):

• HTR (both PB and Prismatic) and MSR designs

Data/analysis will be codified− Data will be used in new ASME Code− Code requires use of irradiation data and high

temperature behavior

Graphite Core Components

ComponentMax.

Normal Operation

Off-Normal DPA

Graphite fuel block 900-1200ºC ~1400ºC ~ 0.8/yr

Reflector blocks 600-900ºC ~1200ºC ~ 0.5/yr

Core support columns 1000-1200ºC ~1200ºC ~0.001/yr

Other components 250 - 350ºC ~300-600ºC Varies

Material Issues

Degradation

-4

-2

0

2

4

6

8

10

12

14

0 5 10 15 20 25

∆L/L

[%]

dose, dpa

1100 C900 C700 C500 C300 C

0.40.50.60.70.80.9

11.11.21.31.4

-5 5 15 25C

TE/C

TEo

dose, dpa

300 C

500 C

700 C

900 C

1100 C

0

0.5

1

1.5

2

2.5

0 10 20

Tens

ile s

tren

gth,

σ/σ

o

dose, dpa

400 C

500 C

700 C

900 C

1100 C

Property changes from irradiation and environment

O id ti i iti

Fission product transport/retention

Main Material Properties of Interest

6

Irradiation Dimensional

Change


7


Change

8



Change

9



Change

G. Haag,” Properties of ATR-2E Graphite and Property Changes due to Fast Neutron Irradiation”, Juel-4183, 2005

10



Change

Irradiation creep

(Residual stress remover)


Change


Dose, dpa

Structural failure

Low Thermal Conduct

Relieves stress Irradiation

creep(Internal stress

remover)

Longer component

lifetime

Structural failure

Turnaround

Higher core temperature


12

Internal stress build-

up

Cracking&

Fracture


Change

Larger block gaps (lowers

thermal transport)

Thermal properties

Thermal expansion

change

Lower Failure

Degradation

StrengthlossLower

ThermalTrans KIc

Extends turnaround

Strengthdecrease

USA’s 5 different research areas

Graphite R&D Program

Defines the safe working envelope for nuclear graphite and

protection of fuelBehavior models- Predicts irradiated

material properties and potential degradation issues

- Irradiation behavior for continued safe operation

Mechanisms and Analysis- Data analysis and interpretation- Understanding the damage mechanisms

is key to interpreting data

Irradiation- Determines irradiation

changes to material properties- Irradiation behavior for

continued safe operation

Virgin Properties- (Statistically) Establishes as-

received material properties- Baseline data used to

determine irradiation material properties

Licensing & Code- Establishes an ASME

approved code (for 1st time)- Develops property values

for initial components and irradiation induced changes

AGC Experiments

Irradiation Testing:• Creep Specimens• Button “piggyback”• Changes to properties• Irradiation of new materials• Fundamental irradiation damage

AGC Experiment : Irradiation changes

AGC - 3

AGC - 5

Database for previous nuclear graphite grades

800 ºC

1100 ºC

600 ºC

1400 ºC

AGC - 4

AGC - 6

Dose (dpa)1 3 4.5 6

HTV

AGC - 1AGC - 2

• By comparing between test series• Property change by dose• Property change by temperature • Property change by stress

• Three pairs of test capsules• 3 Temperatures • 3 Stress levels• Continuous dose (0.5 – 7 dpa)

ε, σ

, ρ, α

, E, C

TE

Dose (dpa)

Tirr = 600, 800, 1100Cσ = 14, 17, 21 MPa

15

Areas of pertinent graphite research DOE-ART focusing on large program activities

– Irradiation (AGC), Unirradiated data, Oxidation, ASME Code

Areas of maximum interest– Fundamental (i.e., small) irradiation studies

• Irradiated defect structures - creep mechanisms– Material property changes

• Affect of irradiation and molten salt– Fracture and strength

• Fracture toughness and multi-axial fracture behavior– Degradation (oxidation – salt erosion)

• Development of oxidation/degradation models• Development of new oxidation resistant grades/components

(dopants and coatings)– NDE flaw detection

• Flaw prediction and evolution during irradiation or degradation

Fundamental irradiation studiesA real need to determine irradiated defect structures and how they affect behavior/performance.

• Grains shrink parallel to planes and grow in perpendicular direction

• Overall volume shrinkage

Cracks formed during fabrication accommodate swelling – for a while

Creep - Dislocation climb/glide

• Intriguing TEM images under electron beam

– ║to Basal planes

– Interaction of basal planes under irradiation

– Does this show dislocation glide possibilities?

• Not supposed to happen in current model

C. Karthik et al. J. Nuc Mat 412 (2011) 321–326

2 IL bonds

3 IL bonds

Dislocation climb/glide

2 IL bonds3 IL bonds

T. Trevethan et al, U of Sussex & U of Leeds, INGSM-13, Sept 2012

Evidence of irradiation damage in graphite

A. Asthana et al, J. Appl. Cryst., (2005) 38, 361-367

• Tirr = 333 K (60°C)

• φlow 0.004 dpa

• φhigh 0.24dpa

Unirr

γlow

γhigh

No indication of sub-plane formation Different from current model

Questions• What /Which defect structure affects material properties

– What are real creep mechanisms? Defect structures– What defect structures affect strength/modulus?

– How does this affect fracture behavior?– How is thermal diffusivity/conductivity affected?– Thermal expansion doesn’t match up with current explanation of

microstructure/defect changes. Why?• How does defect structure behave/interact

– Microstructure evolution• How does the defect microstructure change over increased dose

and temperature?• Defect accumulation

– How is porosity affected?• Pore generation in tertiary creep – but how/why?

• Irradiated specimens - at National Labs (INL/ORNL)– Collaborations and sharing of irradiated specimens is a priority– Independent (small) studies with university reactors is needed, too

Material Property Changes

-4

-2

0

2

4

6

8

10

12

14

0 5 10 15 20 25

∆L/L

[%]

dose, dpa

1100 C

900 C

700 C

500 C

300 C

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

0 5 10 15 20 25

CTE

/CTE

o

dose, dpa

300 C

500 C

700 C

900 C

1100 C

0

0.5

1

1.5

2

2.5

0 5 10 15 20 25

Tens

ile s

tren

gth,

σ/σ

o

dose, dpa

400 C

500 C

700 C

900 C

1100 C

Irradiation shown to affect graphite properties− Dimensional changes, internal stress− Increased stiffness and strength− Dramatic decrease in thermal diff.

What about Molten Salt?− If salt infiltrates graphite pores− Are basic properties the same?− Combination effects from irradiation and salt?

Fracture and Strength Irradiation shown to affect graphite

strength− Initial strength due to dislocation pinning

from point defects− But what about pore structure?

− Microstructure evolution over dose?

− Increased flaw population must change fracture behavior

How does molten salt penetration into pores affect fracture behavior?− Solidification of salt inside pores will create high

internal tensile stresses− Pore/crack growth will result.− Exacerbates fracture?

G. Haag,” Properties of ATR-2E Graphite and Property Changes due to Fast Neutron Irradiation”, Juel-4183, 2005

Degradation - oxidation

0123456789

1011

0 0.5 1 1.5 2 2.5 3

PCEA

IG-110

NBG-18

Pure PCEA NBG-17

Wei

ght l

oss,

%

PCEA

NBG-18

• Lots of data and studies on graphite oxidation

• Need to develop improved models based on new data

Radioactivity – tritium and fp build-up• Build-up of tritium

− Especially important for dissolved fuel

• Does graphite retain it? Diffusion rate?− Any build-up?

• Important for HTR design as well− Source term for accident calculations.

− Waste disposal issues− How does temperature affect release rates (accident)?− Diffusion rate at accident temperatures?

Nondestructive Evaluation of Graphite• Desperately need development of NDE Technologies for large components

and In-Service Inspection (ISI)• Conventional NDE Inspection technologies applicable to Graphite

– Eddy Currents, X-Ray Radiography, and Ultrasonics

Ultrasonics

Laser Based UT

Contact UT

X-Ray Source

X-Ray Radiography

Induction CoilAC Magnetic Field

Eddy Currents

Test Piece

Eddy Currents

27

Will Windes Idaho National [email protected] (208) 526-6985

Documents

Department of Energy (DOE) Advanced Reactor ......and Property Changes due to Fast Neutron Irradiation”, Juel-4183, 2005 10 Main Material Properties of Interest Irradiation Dimensional