3.1 Ongoing Materials Degradation Work Within the …Ongoing Materials Degradation Work within the Primary Systems Corrosion Research Program and the other EPRI “Issue Programs”

Ongoing Materials Degradation Work within the Primary Systems Corrosion Research Program and the other EPRI “Issue Programs”

John HicklingTechnical Fellow – Materials IssuesTechnology Group – Nuclear Sector

NRC Research PMDM MeetingCharleston, SC; May 11, 2006

2© 2006 Electric Power Research Institute, Inc. All rights reserved.

EPRI Primary Systems Corrosion Research (PSCR) Program: Overview

• Goal: To improve the useful life of BWR and PWR primary system components through a better understanding of crack initiation and early crack propagation processes leading to SCC and IASCC degradation of materials

• Corrosion Research addresses Barrier 1.1 in the Material Degradation & Aging Action Plan:– Limited fundamental understanding of environment related

degradation phenomena

– Lack of mechanistically based predictive models

– Need for new, more corrosion resistant materials

• Planning of current and future projects draws heavily on the information gained during establishment of the MDM


CRCRProgramProgram

CHEMISTRY

International SCCInitiation Program

(PEACE)

Cooperative IASCCResearch (CIR)

UCB “in-situ”Autoclave Work

SGMPMRP

Corrosion Research Roadmap

MEOG/MTAG Materials Initiative

(DM/IMT)

MaterialsInformation Portal

Mechanistic “Gap” Studies

BOP Corrosion

BWRVIP


Key Corrosion Research Projects: Scope

1. Cooperative IASCC Research (CIR) Program

Objectives • Develop a mechanistic understanding of key parameters (material,

fluence, temperature, chemistry and stress) that control IASCC initiation and growth

• Formulate predictive models for IASCC based on a mechanistic understanding

• Identify suitable countermeasures for IASCC

International collaborative research program managed by EPRI and co-sponsored by organizations from U.S., Sweden, France, Belgium, Spain, Japan, Germany, Finland, Norway and Czech Republic

• Members include utilities, regulators, vendors and research organizations


Key Corrosion Research Projects: Scope (cont’d)

2. Participation in an International Program on Life Time Prediction of EAC (work at FRI, Tohoku University, Japan) with following goal:

– Develop predictive models based on better mechanistic understanding of EAC of stainless steels and Ni alloys in BWR and PWR environments

– This program is co-sponsored by Japanese government, utilities, vendors and several international organizations

– Allows significant leveraging of EPRI resources

3. EPRI-managed program at UC Berkeley on:

– In-situ characterization of surface films on SS and Alloy 600 in LWR environments and their role in SCC

– Development of thin film SCC initiation sensors


• Development of Understanding of the Interaction between Localized Deformation in Materials and EAC

– Recent findings for several alloys suggest that the tendency to undergo localized deformation is a key element in determining EAC susceptibility

– A coordinated effort across a range of material/environment systems may offer a more proactive approach to managing long-term degradation

• Development of Additional Understanding on Initiation of EAC and the Growth of “Short Cracks”

– Component behavior in the field is often governed as much or more by this phase of degradation than by the behavior of long (deep) cracks as usually studied in the laboratory

– Improved understanding in this area is key to developing successful EAC mitigation techniques

Corrosion Research: Additional Work funded by the U.S. Materials Management Initiative


Objectives of new PSCR Project on the Interaction between Localized Deformation in Materials and EAC

• EAC has been studied for 30 years → improving and optimizing of practices in nuclear power plants (repair, replacement of components or materials) → considerably reduced the susceptibility of LWR to EAC.

• 4 Objectives :

• To identify possible strain localization mechanisms in Nuclear Materials

• To review different types of EAC – strain localization interactions in LWR water environments

• To focus on key-gaps and to identify the main issues

• To propose recommendations


Localization of plastic flow in nuclear materials

1. Origin of strain localization• Strain localization results from a complex competition between

material hardening versus softening, geometry and loading conditions.

Non homogeneous loading(crack, notch…)

Non homogeneous response(heterogeneous material,

plastic instabilities…)

Strain localization• Intensity• Length scale• Stationary / propagative• Transient regime / persistent


2. Four main types of plastic instability in nuclear environment (Estrin-Kubin classification)

Spatial material heterogeneity (type O)Strain softening (type h) : “the more you strain the easier you strain”Strain rate softening (type S) : “the quicker you strain the easier you strain”Fatigue localization (type F)

Localization of plastic flow in nuclear materials Homogeneous loading →Heterogeneous response


Spatial material heterogeneity (type O) : GBS plastic instability

304L Stainless steel

CERT (5.10-8 s-1)

PWR environment (360°C) 600 Ni base alloy

CERT (5.10-8 s-1)

PWR environment (360°C)


Strain softening (type h) : Lüders (plastic instability)

Mild steel

Tensile test in air (25°C)

S. Graff, Material Science and Engineering A, Vol. 387-389, 2004

El.

Load

Lüdersband

Unyieldedmetal


Strain rate softening (type S) : PLC in 718 alloy tested in air

718L Ni base alloy

Tensile test (5.10-5 s-1) in air

L. Fournier, Materials Science & Engineering A269 (1999) 111-119

Tensile test in air at 500°C Tensile test in air at 470°C


Fatigue localization = type F plastic instability

PSB in pure copper

Fatigue straining (∆ε)

Localization in PSB

Crack nucleation + damage percolation

Number of cycle to fracture


EAC & GBS (type O) alloy 600 tested in PWR environment

Time to failure vs. Intergranular viscosity ηAlloys 600 and 690 RUBs specimens tested in primary environment at 360 °CFrom J.D. Mithieux ph-D (EDF)

CGR vs. Intergranular viscosity ηAlloys 600 and 690 CERTs (5.10-8 s-1) in primary environment at 360 °CFrom J.D. Mithieux ph-D (EDF)

Time to initiation of SCC Crack growth rate of SCC

GBS GBS


0

20

40

60

80

100

120

140

160

0 0,1 0,2 0,3 0,4

γ

Cra

ck d

epth

(µm

)

β = −1

β = 0

β = +1

IGSCC

0

200

400

600

800

1000

1200

0 0,1 0,2 0,3 0,4γ

Cra

ck d

epth

(µm

)

β=−1β=0

β=+1

TGSCC

EAC & substructure instabilities (type h) in 304L tested in PWR environment

45°

135°

90°

β = +1

β = −1

β = 0

γ = tan αα

τ

τ


Fe-δ

Localized strain around δ-ferrite

σ

IGSCC for a complex strain path

TEM observation

SEM observation

500 nm


IGSCC for a complex strain path

→ Strain localization/incompatibilities promote IGSCC

Initiation of cavities at the grain boundary

Grain boundary

σ


200 nm

Area under residualstresses

Twin boundary

DFZ

Pile up of dislocations / micro crack

Transgranular crack-tip observation

100 nm


EAC & Strain localization (type S) in 304L/316L tested in PWR environment

1E-121E-111E-101E-091E-081E-071E-061E-051E-041E-031E-021E-01

0 200 400 600 800 1000

Temperarure (°C)

Cre

ep ra

te (1

/s)

50 MPa - 316LN (Usami)



250-400 MPa - 304L (EDF-MMC)

170 MPa - 304 (Frost)

340 MPa - 304 (Frost)

PLC

Observed by Ehrnsten in 316LN (dε/dt=5.10-5 s−1) with an increasing

effect of pre-strain hardening

Time

Tensile load at 360°C

Initiation & propagation

of SCC

No initationof SCC


EAC & fatigue instability (type F)Fatigue straining (∆ε)

Localization in PSB

Crack nucleation + damage percolation

Decrease of number of cycle to fracture


Type of

instabilities Physical cause

Governing parameters

microstructure,loading Example in nuclear materials

Type O : macrosopic

heterogeneity

Heterogeneity at the mesoscopic or

macroscopic level Castings, weldings

Scale and connectivity of the soft and hard zones, contrast

between the zones Direction of loading with

respect to the heterogeneities

- PWR primary coolant pipes : Thermally aged cast stainless steels → Hardened ferrite in austenite → mechanical embrittlement +,sensitivity to IASCC - welds - “ghost lines”

Type O : intragranular heterogeneity

Interaction of the GB with chemical species, vacancies

Width of the PFZ, state of precipitation at the GB,

direction of loading with respect to the grain

morphological texture Width of the precipitated free

zone and depleted zone.

- Chemical heterogeneities due to grain boundary precipitation or segregation (chromium carbides in 600 alloys and stainless steels in BWR → sensitised stainless steels) - Chemical heterogeneity due to in service (irradiation or thermal ageing) induced segregation at grain bondary (thermal ageing : pressurizer : temper reversible embrittlement ; irradiation : stainless steels internals)

Type O : Grain

boundary slidings

Stress gradients incompatibilities

Grain size, state of precipitation at the GB

-Nickel base components in PWR primary circuit components : Influence of grain boundaries sliding on Alloy 600 SCC

Strain localization in nuclear materials : type O instabilities

Example of spatial material heterogeneities…


Type of




Type h : Dislocation

burst

Unpinning from obstacles or brutal multiplication of

mobile dislocations

Solid solution composition, Prior Static ageing, state of

recristallisation, Temperature, strain rate

- Corrosion -fatigue of LAS components (static strain ageing),

Type h : Obstacle

destruction

Destruction of chemical order or localised obstacles by the motion of

dislocations

Strengthening effect of the chemical order, density and size of precipitates, density and size of irradiation loops

- Channelling in irradiated stainless steels (internals) - Possible Short and Long range ordering in 690 located in “hot” parts of Inconel components, irradiation (supports M) - Precipitation in alloys 718 and X-750 - Demixtion of ferrite in duplex stainless steels

Type h: substructure instability

Destabilisation of a well developed

substructure by a change path

State of organisation of the substructure prior to the strain

path change Severity of the change

Cold worked stainless steels under complex mechanical loadings

Strain localization in nuclear materials : type h instabilities

Example of strain softening…


Type of




Type S : PLC

instability

Dynamical strain ageing due to the

interaction of dislocations with

mobile solute atoms

Solid solution composition, Strain rate and temperature,

strain threshold

- PWSCC of alloys 718 and X-750 - LAS - Austenitic stainless steels

Strain localization in nuclear materials : type S instabilities

Example of strain rate softening…


Type of




Type F : fatigue

Local softening partial plastic reversibility

Strain amplitude, temperature, possible precipitation.

Stainless steels fatigue Fatigue corrosion of LAS

Strain localization in nuclear materials : type F instabilities

Example of fatigue instabilities …


Key gaps : Ni base alloys in PWRs

1. Effect of strain localization (due to strain hardening mode) on the time to initiation and on the CGR ?Time to initiation obtained on alloy 600 exposed to hydrogenatedprimary PWR environment at 325°C → deleterious effect of the complex strain path in RUBs

2. Effect of strain localization on the diffusion of oxygen at grain boundaries (IO model) ?

3. Possible strain localization in HAZ ?

964No initiation after 3840 h586Cylindrical

2142

Predicted tinitiation (h)

2143480RUB

Measured tinitiation (h)σ325°C (MPa)Type of specimen


Key gaps : austenitic stainless steels in PWRs

1. Effect of strain localization due to complex strain hardening on the time to initiation and on the CGR ?

→ Complex strain hardening promotes IGSCC

2. Effect of strain localization due to cyclic loadings (Persistent Slip Bands, Shear Bands) on the time to initiation ?

→ No initiation under pure static loading

3. Effect of the interaction between (C + N) and plasticity on the initiation of IGSCC ? on the CGR ?

→ low strain rates promote IGSCC while high strain rates promote TGSCC

4. Physical mechanism ?

→ strain localization seems to be a necessary condition for SCC


Recommendations : strategy

1. Two main directions of advance :Comprehension of the local coupling between strain localization with :

Oxidation at the surface or at the crack tipTransport of species (H, vacancies, O)Fracture

Modeling of initiation and propagationQuantitative predictionBased on physical mechanisms (oxidation, transport, fracture, mechanical

behavior)

2. Three main tasks :Choice/development of tools of investigation

SEM, TEM, specific tests, numerical simulationQuantification of strain localization effect on EAC

Empirical modeling, then physical modeling of oxidation, transport and fractureEvaluation of possible EAC-strain localization synergy in components

Selection of critical components (fabrication, in service loading), criteria for survey (ti, CGR), mitigation


Recommendations : quantification of interactions between EAC and plastic flow instabilities

Modeling of SCC

Range of T-dε/dt promoting the GBS. Correlation with initiation of SCC.

Range of T-dε/dt promoting interactions between solute atoms and mobile dislocations (reducing intragranular creep). Correlation with initiation of SCC.

Evolution of the surface reactivity during the incubation period as a function of pre-strain hardening.

Rate of appearance of PSBs and shear bands at the surface. Correlation with initiation of SCC.

Austenitic stainless steels

Modeling of IGSCC

Strain softening effect (due to complex strain paths) on the CGR. 2006-2007

Strain localization in HAZ and correlation with the crack propagation. 2006-2007

Strain localization in HAZ and correlation with initiation sites. 2007

Strain softening effect (due to complex strain paths) on time to initiation. 2006-2007

Wrought Ni base alloy

Modeling of IGSCC

Rate of appearance of PSBs and shear bands at the crack-tip. Correlation with CGR of IGSCC.

Strain localization in welds and correlation with the crack growth path.

Rate of appearance of PSBs and shear bands at the surface. Correlation with time to initiation of IGSCC.

Strain localization in welds and correlation with initiation sites.

Weld Ni base metal

5

4

3

2

1

Task order


CONCLUSION : “Products” for Utilities

Examples of « end products » for Utilities :

1. Strain localization maps vs. EAC

2. Local fracture criteria for EAC

Temperature

Stra

inra

te

DSAPLC

SRO

GBS

IGSCC

Strain localization maps vs. EAC for a couple material/environment

Local fracture criteriafor IGSCC

Strain map (from Zinkle)


• Materials aging due to SCC is the greatest single challenge facing the LWR nuclear industry

• Research into EAC (SCC and corrosion fatigue) mechanisms, mitigation and key factors affectinginitiation and propagation of cracks are important for long-term asset management

• Initiation and short crack growth can dominatecomponent life

• The EPRI Primary Systems Corrosion Research program has used MEOG/MTAG funding in 2005 to develop a review of knowledge “gaps” in this area

New PSCR Project on Initiation and Short Crack Growth: Introduction


PSCR “Gap” Project on Initiation and Short Crack Growth: Objectives of Review

• Summarize the current knowledge of crack initiation and short crack growth in nickel base alloys, stainless steelsand carbon/low alloy steels in typical PWR and BWR environments

• Identify key gaps in the knowledge base • Recommend experimental work to fill these gaps that

will likely contribute to improved management or mitigation of EAC in PWRs and BWRs

• EPRI report # 1011788 (prepared by Framatome-ANP and GE-GRC) was issued in December 2005

• Question and answer session (telecon) was held on 12/05/05 with the principal investigators (Scott/Andresen)


PSCR “Gap” Project on Initiation and Short Crack Growth: Stages in the Development of SCC


PSCR “Gap” Project on Initiation and Short Crack Growth: Report Structure

• Introduction, objectives, definition of terms - P. Scott and P. Andresen

• Surface oxidation, and crack initiation in deaerated hightemperature water – P. Combrade

• Passive layer formation, breakdown and crack initiation in oxygenated conditions – P. Andresen and Y. Kim

• Field experience of crack initiation and growth in austenitic SSand C&LAS, together with associated laboratory testing – R. Kilian and A. Roth

• Quantifying crack initiation and short crack growth – P. Andresen• Multiple crack initiation and coalescence – P. Scott• Conclusions and recommendations – P. Scott and P. Andresen


PSCR “Gap” Project on Initiation and Short Crack Growth: Definitions

Proposed Working Terminology for Crack Initiation

Metallurgically small cracks, which are cracks smaller than one grain diameter, or some other metallurgical feature.

Phenomenology – Metallurgy

Mechanically small cracks, which are shallow surface cracks growing under plane stress conditions, or where linear elastic fracture mechanics cannot be applied with confidence and certainty.

Phenomenology – Mechanics

Chemically short cracks, in which a mature, long crack chemistrytypical of deeper cracks has not formed.

Phenomenology – Environment

Detectability that is likely to be achievable in in-situ autoclave investigations in the laboratory, e.g., ≈ 50 µm crack depth.

Practical Definition

Formation of a physically distinct geometry that will tend to grow in preference to its surroundings as a sharp crack.

Scientific Definition


PSCR “Gap” Project on Initiation and Short Crack Growth: Implications


PSCR “Gap” Project on Initiation and Short Crack Growth: Dynamic Effects during Static Loading

Crack tip

Stra

in

Crack tip

Stra

in

Area where redistribution of strain induces slip offsets at the crack tip

Crack Tip Plastic Strain304 Stainless at 288oC

0.00

0.05

0.10

0.15

0.20

0 10 20 30 40 50

Distance from crack tip (µm)

Cra

ck T

ip P

last

ic S

trai

n

K = 25 MPa√mn = 3.43

E = 172 GPacgr = 1.4x10-7 mm/sec

σYS = 161 MPa

σYS = 550 MPa

Crack tip

Crack Tip Plastic Strain304 Stainless at 288oC

0.00

0.05

0.10

0.15

0.20

0 10 20 30 40 50

Distance from crack tip (µm)

Cra

ck T

ip P

last

ic S

trai

n

K = 25 MPa√mn = 3.43

E = 172 GPacgr = 1.4x10-7 mm/sec

σYS = 161 MPa

σYS = 550 MPa

Crack tip

At constant load, crack tip strain rate occurs due to strain redistribution as the crack grows.↑ yield strength ↑ dε/da

Crack Tip Strain Rate from Redistribution


PSCR “Gap” Project on Initiation and Short Crack Growth: General Conclusions

• Oxide film structure– multi-layered structures on all materials in both PWR

and BWR environments– inner layer provides protection against EAC – formation of outer layers depends on cation concentration in water

(may prevent access of Zn, e.g.)• Crack initiation processes

– IG oxide penetrations observed in Alloy 600 in PWR primary water but not• in C&LAS or SS in this medium• in any of these materials in BWR environments (as far as known)

– pitting and dissolution of sulfide inclusions important for C&LAS


PSCR “Gap” Project on Initiation and Short Crack Growth: : Oxide Films on Ni-based Alloys in PWRs


PSCR “Gap” Project on Initiation and Short Crack Growth: : Oxygen Penetration in Grain Boundaries

SIMS image of ions 16O- at different depths under the oxide layera - 4 µm, b - 1.9 µm, c - 1.1 µm, d - 0.7 µm, e - 0.5 µm, f - 0.3 µm

After Delabrouille

20 % Cr model alloy oxidized 1000 hrs at 360° C in simulated primary coolant.


PSCR “Gap” Project on Initiation and Short Crack Growth: : Oxide Films on SS in BWRs

Steel Barrier layer Inner layer Outer layer Deposited layer

(Fe1-xNix)(Cr1-yFey)2O4 (NixFe1-x)(Fe1-yCry)2O4 NiFe2O4

VM´´´Vm VM

´´´

VO••

VO••

Mi•• Mi

••

Maqy+

Maqx+

Maqx+

Vm



VM´´´Vm VM

´´´

VO••

VO••

Mi•• Mi

••

Maqy+

Maqx+

Maqx+

Vm



VM´´´Vm VM

´´´

VO••

VO••

Mi•• Mi

••

Maqy+

Maqx+

Maqx+

Vm


PSCR “Gap” Project on Initiation and Short Crack Growth: : Role of Pitting for LAS

Pre-pitting and fatigue in air (top) vs. active pitting during fatigue (bottom) Disproportionate effect of early cycles

under “bad” water chemistry


PSCR “Gap” Project on Initiation and Short Crack Growth: General Conclusions (con.)

• Fabrication factors affecting probability of crack initiation:– surface chemical inhomogeneities from oxidation, chemical etching,

pre-segregation…– surface mechanical inhomogeneities, including cold work, handling

damage, casting defects/porosity, welding folds and trapped slag…– material inhomogeneities from microstructure (e.g., grain size,

compositional banding…)– inhomogeneities from local stresses and strains, including those

from grain boundary (GB) orientation (e.g., GB mis-orientation energy…)

– regions of material that undergo accelerated aging during plant operation (e.g., more prone to sensitize, exhibit IG attack/oxidation…)

– complex interactions of the above


PSCR “Gap” Project on Initiation and Short Crack Growth: Field Experience with Pre-existing Defects

Defects formed in service

Pits as crack initiationside

Manufacturing flaws: cold work > SCC Manufacturing flaws:

root notch > SCC


PSCR “Gap” Project on Initiation and Short Crack Growth: General Conclusions (con.)

• Other factors affecting crack initiation/ short crack growth:– when KI<KISCC it cannot be assumed that SCC will not initiate (or that

short cracks will not propagate)– multiple SCC initiation is a common observation in engineering

components– short cracks may grow rapidly initially, slow down exponentially, and

exhibit long periods of dormancy until coalescence with newlyinitiated cracks nearby reactivates growth• ratio of growth of short cracks in depth and at the surface is contrary to

expectations from LEFM analyses– study of databases of generic SCC operating experience can yield

useful practical information for improving fabrication and operating conditions so as to reduce the risk of crack initiation


PSCR “Gap” Project on Initiation and Short Crack Growth: Crack Dormancy/Coalesence


PSCR “Gap” Project on Initiation and Short Crack Growth: Crack Dormancy/Coalesence

Dormancy and/or coalescence of multiple cracks (left: PWSCC “craze” cracking in a RPV head penetration; right: in a SG tube roll transition) is a key issue that can best

be described using Weibull statistics and modeled using Monte Carlo methods


PSCR “Gap” Project on Initiation and Short Crack Growth: Fabrication Effects


PSCR “Gap” Project on Initiation and Short Crack Growth: Field and Lab Experience for SS in BWRs

• The primary objective is to avoid crack initiation and subsequent crack growth by the application of proper, qualified fabrication processes

• Need to rule out detrimental effects such as– sensitization due to grain boundary chromium depletion– weld imperfections (e.g., root notches, shrinkage defects, mismatch,

misalignment, excessive penetration lack of fusion, slag inclusions)– surface cold work exceeding the critical hardness– excessive residual stresses in the component

• dK/da shown in laboratory testing to have a major influence on growth rates (and thought to represent component behavior)


PSCR “Gap” Project on Initiation and Short Crack Growth: Role of Stress Intensity Control


PSCR “Gap” Project on Initiation and Short Crack Growth: General Recommendations

• Ensure that experiments undertaken to study EAC initiation are carried out under optimal test conditions (& yield statistically valid data)

• Measure the properties of oxide films that impact the rate determining processes that lead to a failure to protect the underlying metal from crack initiation (also relevant to the issue of activation of corrosion products in LWR coolant circuits)

• Identify and, if possible, quantify and model the kinetics of the damage processes that occur in LWR structural materials prior to detectable crack initiation


PSCR “Gap” Project on Initiation and Short Crack Growth: General Recommendations (con.)

• Document and further evaluate field experience of SCC in LWRs to identify critical fabrication, environmental and operational factors influencing crack initiation in service

• Devise suitable in-situ test and service crack initiation monitoring equipment

• Improve or develop robust predictive models for generic SCC phenomena, including situations where multiple crack initiation and coalescence occurs (e.g. by applying techniques evolved in the high-pressure, gas transmission pipeline industry)


PSCR “Gap” Project on Initiation and Short Crack Growth: Specific Recommendations (1)

• Experimental requirements– Use adequate numbers of samples for statistical significance– Tapered specimens are suitable for studying multiple initiation,

coalescence and growth (in combination with conventionalmicroscopy and layer grinding)

– Test designs should avoid complications of stress relaxation associated with fixed-deformation testing

– Good control of specimen surface finish and simulated reactorquality water is essential

– On-line DCPD is well adapted to measure short crack growth rates in appropriately designed specimens - electrochemical noise alsouseful

– ATEM could be usefully deployed to help identify the factors involvedin transitions from incipient crack damage to short crack growth and finally to LEFM controlled growth



• Surface oxide properties:

– Determine the composition, crystallography, microstructure and semi-conducting properties of material/environment combinationssubject to generic SCC (as a necessary precursor for establishingrate determining processes and predictive models)

– Establish the influence of temperature on oxidation rate, cation release rate and damage to the underlying metal



• Damage processes prior to initiation

– For Ni-base alloys in PWR primary conditions:• determine the mechanism, kinetics and temperature dependence of GB

oxidation embrittlement• evaluate local stresses due to oxide growth and selective

oxidation/dissolution• determine the stress/strain conditions necessary for developing the first

microcrack from an oxidized grain boundary

– For stainless steels in PWR primary conditions :• examine cold worked and sensitized stainless steels for the existence of

similar damage processes




– For austenitic alloys in BWR conditions:• establish the cause and statistics behind crack initiation in uncreviced,

unsensitized SS and Ni alloys in relevant (good) water chemistries• study the films on stressed and unstressed specimens to identify

whether precursor phenomena can be identified in the oxide film or underlying metal

• quantify the benefit of improved alloys and weld metals, such as Alloy 690 and its weld metals, on crack initiation as a function of stress and water purity

• determine how memory effects from periods of "bad" water chemistry can accelerate crack initiation and short crack development in cold worked and sensitized austenitic materials during long periods of acceptable water chemistry




– For C&LAS in both PWR and BWR media:

• determine critical flaw sizes and shapes for SICC or CF as a function of potential and Cl- and SO4

– concentration

• determine the role of thick, porous oxide deposits on local water chemistry and decoupling from bulk environment



• Analysis of operating experience

– Invest in building databases of generic SCC problems and theiranalysis (e.g., by neural network methods)

– Characterize crevice conditions with respect to stress/strainconcentrations and critical hardness

– Characterize typical industrial surface finishes with high resolutiontechniques prior to HT water exposure and then the characteristicsof oxide layers that form in HT water (especially those associatedwith crack initiation)



• Multiple crack initiation and coalescence– Test tapered specimens of materials subject to generic SCC in

LWRs in order to determine crack initiation density as a function of stress and time (and analyze whether the crack density is sufficientto give rise to coalescence)

– Determine pit birth, growth and death statistics in those systemswhere this process occurs and risk of crack initiation from pits

– Examine LEFM solutions for multiple cracks (developed for fatigue crack growth evaluations) for their applicability to SCC growthmodeling

– Assess importance of dK/da on propagation rates– Establish a theoretical basis for crack dormancy– Develop a Monte Carlo model of multiple crack initiation,

coalescence and growth


PSCR “Gap” Project on Initiation and Short Crack Growth: Suggested Priority Areas

1. Determine frequency of multiple initiation as a function of stress and time and determine whether the crack density in typicalmaterial/environment combinations with known SCC susceptibility issufficient to lead to coalescence

2. Determine the effect of dK/da load control on the growth rate of short cracks

3. Create/expand systematic databases of SCC field failures and analyze them so as to determine critical fabrication parametersassociated with SCC initiation

4. Determine kinetics of IG oxidation in Alloy 600 and similar alloys and depth/strain combinations required to initiate a crack – thendemonstrate that Alloys 690/152/52 and SS are not susceptible to the same mechanism


BWR Vessel Internals Program (BWRVIP)Example #1 of Proactive Materials Research

• International workshop held in 2003 identified a key issue• Evaluate and quantify the effects of high radiation levels on

behavior of stainless steel components with regard to– Crack growth through IASCC– Fracture toughness

• BWRVIP Inspection and Evaluation Guidelines for internals require supporting data on irradiated stainless steels

• Data in hand show significant degradation due to irradiation– Irradiation at intermediate fluence accelerates SCC growth rate in

stainless steels by a factor of 5 or more– Some data indicate HWC is less effective at fluences approaching

1x1022 n/cm2

• There are data gaps, especially at end of license fluences


BWRVIP ProgramExample #1 (con.)

• BWRVIP issued a Request for Proposals (RFP) in early 2004• GE and Studsvik selected as primary contractors to conduct testing

– GE Team• GE Vallecitos – crack growth testing• Battelle – material characterization• University of Michigan – post test SEM

– Studsvik Team• Studsvik – fracture toughness and crack growth testing• Nippon Fuels – fracture toughness testing• Both organizations to conduct post test characterization for materials

tested at their respective facilities• Scope includes 22 fracture toughness and 13 CGR tests (NWC / HWC)• Final report will include a critical assessment of existing BWRVIP

correlations and flaw evaluation methodologies


BWRVIP ProgramExample #2 of Proactive Materials Research

• Assess potential advanced mitigation technologies– Demonstration of On-line Noble Metal Chemical Application (NMCA)– Development of Zirconia coatings

• The conventional NMCA process is applied during an outage and requires two days of critical path time– New cracks formed during off-hydrogen periods may continue to

grow

• On-line NMCA process was developed by GE to reduce critical path time required for the conventional NMCA process and deposit noble metal on new crack surfaces which may be created during off-hydrogen periods

• On-line NMCA technology will be demonstrated at an international BWR-4 in mid-2005 (including fuel surveillance)


BWRVIP ProgramExample #2 (con.)

• Current mitigation technologies (HWC and NMCA) are not effective in mitigating cracking in components above the reactor core

• Prior work shows that zirconia coatings have the potential to mitigate cracking even in the absence of hydrogen

• Lab studies have demonstrated that adherent zirconiacoatings can be deposited from solution on pre-oxidized surfaces of stainless steel and Alloy 600

• Objective of present studies is to develop an in-situ process for the deposition of zirconia coatings on internals

• Future work will assess effectiveness in mitigating crack growth through IGSCC


Materials Reliability Program (MRP)Example #1 of Proactive Materials Research

• PWSCC of Ni-base alloys is an area of great current concern. In addition to responding immediate issues, the MRP Alloy 600 program is conducting longer-term studies on:– Mechanical and chemical mitigation approaches to preventing

crack initiation and slowing or stopping crack growth– Microstructural features affecting material susceptibility (including

detailed characterization of field cracks, HAZ testing, etc.)– Additional effects (fabrication defects, surface condition, grain

orientation, etc.) involved in cracking of the weld metals– Possible involvement of low-temperature crack propagation– Resistance to cracking of replacement materials (A 690/152/52)

• Goal is to improve both understanding and predictability


MRP ProgramExample #2 of Proactive Materials Research

• The Reactor Internals Group has program elements proactively addressing aging in several areas:

Screening, Categorization and Ranking of PWR Internals Components (includes consideration of individual and combined degradation mechanisms)Functionality Evaluations of PWR Internals Components (includes development of irradiated materials behavior models)Controlled Irradiation Experiment and Hot Cell Test addressing void swelling and IASCCInspection / Repair / Replacement / Mitigation Strategies

• Overall objective is to demonstrate that components can perform their required functions under aging/degraded conditions and are flaw and degradation tolerant.


MRP ProgramReactor Internals Aging Management Flow Chart

Options for Aging Management of PWRReactor Vessel Internals, and the

Associated Gaps and Needs

Use Inspections and Flaw Evaluationsfor Aging Management of PWR

Internals

ObtainNew Data

Document TechnicalBasis

Issue PWR InternalsInspection Guidelines

BOR DataSONGS DataBolts Samples

Perform FunctionalityAnalyses

ScreenComponents

Evaluate LeadComponents

WOG AnalysesBWOG Analyses

Gather EnvironmentalInformation and Materials Data


Irradiation Assisted Stress Corrosion

Cracking

Stress Corrosion Cracking

Irradiation Enhanced Stress

Relaxation

CASS Thermal Aging + Irradiation

Embrittlement

Irradiation Embrittlement

Irradiation Induced Void Swelling

Baffle-BoltsWelds

Thermal Shield & Core Barrel

Bolts

Bolted Joints

CASS Components

(Synergistic Effect)

Internals Components

(Increased Strength & Loss of Ductility)

Baffle-Former Plates and Bolts

(High Fluence and Temp. Regions)

Dimensional Changes and Functionality

Stress and Deflection

Due to LOCA and

SSE

UT Inspection EVT-1 or EVT-3 Inspection

1. Scope2. Preventive Actions3. Parameters Monitored4. Detection of Aging

Effects5. Monitoring and

Trending

Meet Current License Basis

IASCC SCC SR CASS TA/IE IE VS

6. Acceptance Criteria7. Corrective Action8. Confirmation Process9. Administrative

Controls10.Operation Experience

??

Meet Current License BasisMeet Current License Basis

Use Data, Generic Analyses and Models to Screen and Select Most-Affected Internals Components

Use Plant-Specific Analyses to Determine Critical Locations, Critical Crack Sizes and Flaw Tolerance for Actual Stresses, Fluences and Temperatures

MRP Program: Reactor Internals Issues for License Renewal

•Scope * Acceptance Criteria•Preventive Actions * Corrective Action •Parameters Monitored * Confirmation Process•Detection of Aging * Administrative

Effects Controls•Monitoring and * Operation ExperienceTrending


Steam Generator Management Program (SGMP)Example of Proactive Materials Research

• Original SG tubes were made with alloy 600 MA– Problematic with little hope for long service life– Challenge is to manage the problem until the scheduled replacement

• Some newer and replacement SG used alloy 600TT– Initially believed to be corrosion resistant, but growing evidence is

proving otherwise – In addition to chemistry control, the challenge is to detect and repair

flaws early to avoid potential unscheduled shut downs• New replacement SGs are made with alloy 690TT tubes– Proven to have corrosion-resistant superior to that of alloy 600TT and

so far has shown no service-induced degradation– Challenge is to be proactive and prevent the onset of potential problems

such as the effect of secondary side lead on 690 TT

3.1 Ongoing Materials Degradation Work Within the …Ongoing Materials Degradation Work within the Primary Systems Corrosion Research Program and the other EPRI “Issue Programs”

Documents