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Influence of Small Specimens on FractureToughness of Five Irradiated RPV Materials

Randy K. Nanstad,

Mikhail A. Sokolov, and Michael K. MillerOak Ridge National Laboratory, Oak Ridge, TN, USA

Enrico Lucon(National Institute for Standards and Technology, Boulder, CO (formerly of SCK-

CEN, Mol, Belgium)

17th International Conference onEnvironmental Degradation of Materials

in Nuclear Power Systems – Water ReactorsOttawa, Ontario, Canada

9-13 August 2015

Presented by Roger E. Stoller, ORNL

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Five Reactor Pressure Vessel (RPV)Materials Were Irradiated at High Flux forComparison with Results at Lower Flux

• The Heavy-Section Steel Irradiation (HSSI) Program had previously irradiatedvarious RPV steels at fast fluxes of about 4 to 8x1011 n/cm2/s (>1 MeV) tofluences from 0.5 to 3.4×1019 n/cm2 and at 288C. Specimens of 12.5 and 25.4mm thickness (0.5T and 1TCT) were used for unirradiated and irradiatedtesting.

• The HSSI Program provided tensile and 5x10-mm three-point bendspecimens (0.2TSEB) to SCK-CEN for irradiation in the in-pile section #3 (IPS-3) of the Belgian Reactor BR2 at fluxes >1013 n/cm2/s to the same fluences asprevious low flux irradiations to enable direct evaluations of flux effects.

• BR2 irradiations were conducted at about 2 and 4x1013 n/cm2/s withirradiation temperature from 295 to 300C (water temp). Unfortunately,fluences ranged from about 6-10x1019 n/cm2 (0.10-0.16 dpa) more than doublethe desired fluences.

• Thus, direct comparisons of irradiation-induced T0 and evaluations of fluxeffects are confounded by significantly different fluences.

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Mechanical Property and MicrostructuralSpecimens of Two Base Metals andThree Weld Metals Were Irradiated

• For every material• 6 subsize tensile, 2.4 mm diam.• 12 three-point bend, 5x10mm (0.2TSEB)• 1 atom probe blank, 0.5 mm thick• 1 SANS blank, 0.25 mm thick

• Only for HSST Plate 02• 12 miniature C(T), 4.2 mm thick (0.16TCT)

Material C Mn Si S P Cr V Cu Mo Ni W Al

PW0.11 1.25 0.18 0.017 0.014 0.04 0.003 0.20 0.55 1.2 - -

MBW0.09 1.607 0.622 0.009 0.017 0.12 0.005 0.256 0.41 0.574 <0.01 0.015

HSST-020.23 1.55 0.20 0.014 0.009 0.04 0.003 0.14 0.53 0.67 <0.01 0.019

73W0.10 1.56 0.45 0.005 0.005 0.25 0.003 0.31 0.58 0.60 <0.01 0.005

JRQ0.18 1.42 0.24 0.004 0.017 0.12 0.002 0.14 0.51 0.84 <0.01 0.014

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Irradiation-Induced Changes in YS at RoomTemperature Show Somewhat Disparate Results

• Palisades andPlate 02 show rapidhardening with noincrease at highfluence.

• JRQ showsindication ofsaturation, followedby increases at highfluence.

• Midland weld andWeld 73W showrapid hardeningwith slight increaseat high fluence.

Low High

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Tensile Results for Palisades Weld at High and LowFlux/Fluence Show Greater Overall Hardening atHigher Fluence as Expected – Flux Not a Factor

1.0E20 @ 4.45E13

6.2E19 @ 1.85E13

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Midland Beltline Weld at High Fluence (6.5E19)Exhibits Relatively Low Fracture Toughness ShiftWhen Unirradiated ReferenceT0 From 0.5T/1TCTSpecimens

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Determination of T0 for Midland Beltline WeldShows Use of Different Size/Type Specimens inUnirradiated and Irradiated Conditions May Lead toSignificantly Non-Conservative Results

• Hi fluence result initially used T0

from unirradiated 1TCT tocalculate T0.

• Hi fluence result used T0 fromunirradiated 0.2TSEB to calculate T0

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Embrittlement Predictions* with Five Models ShowHigh Scatter

*Predictive calculations by Lucon.

Unirradiated T0 Based on0.5T/1TCT

Unirradiated T0 Based on0.2TSEB

• The average “predicted-measured” values for the five materials:• -33 C in the case of the unirradiated T0 values from the larger specimens• -6C in the case of the unirradiated T0 values from the small bend specimens

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Determination of T0 for Weld 73W from DifferentSize/Type Specimens in Unirradiated and IrradiatedConditions Also Resulted in Significantly Non-Conservative Results

• Hi fluence result initially used T0

from unirradiated 1TCT tocalculate T0.

• Hi fluence result used T0 fromunirradiated 0.2TSEB to calculate T0

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T0 vs Fluence for Palisades Weld, Irradiated atLow and High Flux, Exhibits Generally IncreasingT0 with Fluence – Flux May Be a Factor

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T0 vs Fluence for HSST Plate 02 Irradiatedat Low and High Flux Exhibits IncreasingT0 with Fluence

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T0 vs Fluence for Plate JRQ ShowsIncreasing T0 with Fluence

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Although Scatter is High, Irradiation-InducedHardening (y) and Fracture Toughness Shifts (T0)are Typical of Other Reported Results

• For a largerdatabase of basemetals and welds,Sokolov andNanstad reported(ASTM STP 325):

T0 = 0.70y

r2 = 0.66

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Reference Temperature Bias Values for 0.2TSEBSpecimens of Five RPV Materials Range from 8 to-53C

Material

T0, C

0.5T&

1TCT

T0, °C

0.2TSEB

Bias

CPalisades

Weld -90 -115 -25Weld 73W -63 -116 -53Midland

Weld -54 -98 -44Plate 02 -26 -18 8JRQ Plate -54 -79 -25

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Small Fracture Toughness Specimens Often Exhibit aLow Reference Temperature, T0, Relative to LargerSpecimens – Giving a Specimen Size/Type Bias

• Precracked Charpy V-notch (PCVN) and smaller specimens are identifiedas a critical issue due to the important link to RPV surveillance programs,as CVN surveillance specimens may be reconstituted or machined,precracked, and then tested as PCVN or smaller specimens.

-200 -150 -100 -50 0 50 100

TEST TEMPERATURE(°C)

0

50

100

150

200

250

300

350

400

FR

AC

TU

RE

TO

UG

HN

ES

S,M

Pa

m0.5

1TC(T), 1st Batch HSSI Weld 72W

1st and 2nd Batches1TC(T), 2nd Batch

Master Curves

1TC(T) (To =-54.0°C)

MPCPCVN(To =-75°C)

Different materials responddifferently, particularly as an effectof irradiation. Examples of PCVNbias:

HSST Plate 02 (A533B-1) =5C.

HSST Plate 13B (A533B-1) = 36C.

JRQ Plate (A533B1) = 12 to -22C.

Other RPV steels = -12 to -45C

Annealed 9Cr-1MoV = 0C.

>200 PCVN for Weld 72W exhibited Bias = 21C

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Size/Type of Fracture Toughness Test Specimens MaySignificantly Influence Embrittlement Measurements• Five RPV materials were irradiated at high fluxes [>1013 n/cm2 (>1 MeV)] in

BR2 reactor to compare with results from previous irradiations at lowerfluxes (~ 6x1011).

– Comparisons are confounded by significant differences in fluences.Thus, evaluation of potential flux effects for this experiment isconstrained by above uncertainties.

• Use of 0.2TSEB specimens for irradiation and relatively large specimens(e.g., 0.5T & 1TCT) for unirradiated reference T0 exhibited significantly non-conservative determinations of T0 (0.2TSEB) for most of the materials.

– In the unirradiated condition, bias values for the 0.2TSEB specimensrelative to larger specimens ranged from -53 to 8C.

• For the five RPV materials tested, when measured shifts are based on theunirradiated reference T0 from 0.2TSEB specimens:

– Relationships between irradiation-induced hardening (y) and fracture toughnessshifts (T0) showed high scatter but a typical relationship shown in other studies.

– Embrittlement predictions with five different models show high scatter, but exhibitgood correspondence with measurements.

• Atom probe tomography characterizations of the higher flux, higher fluenceirradiations have not revealed any significant differences in the size andnumber density of the copper-enriched precipitates.