nuclear fuels and materials - 151-2017-00l - psi · pdf filetoughness is measured with impact...

WIR SCHAFFEN WISSEN – HEUTE FÜR MORGEN

Nuclear Fuels and Materials - 151-2017-00L

Manuel A. Pouchon :: Head of LNM :: Paul Scherrer Institut

Master of Nuclear Engineering ‐ Spring Semester 2016

Lecture 3: Toughness, Fatigue, Irradiation Effects, Corrosion, ….

Nuclear Fuels and Materials - 151-2017-00L ⌸ Lecture 3 - Page 2/88

• Toughnesso Impact test – Charpyo Fracture Toughness (KIC)o J-Integral (JIC)

• Fatigueo Wohler S-N diagram

• RPV & irradiation doseo Partso Constructiono Weldingo Low carbon steelo Radiation dose

• Irradiation Damage / Effecto Defect formation and annealingo Hardening / Embrittlemento Creep / Swelling

• Corrosion / Segregation / IASCC

TOC


When dropping a ceramic piece to ground it breaks – metals usually not. The quantity relating the

• resistance of a material to sudden (impact) loads

• is called toughness.

Construction materials must have high toughness. For many metallic materials (including RPV-steels) the toughness is temperature dependent with a sudden change at a so-called transition temperature. This transition temperature can change during service (ageing) which means an increase of failure risk.

Toughness is measured with impact testing machines or with fracture mechanics samples as shown in the next viewgraphs.

Toughness of materials


See also : Impact testing

http://www.twi.co.uk/technical-knowledge/job-knowledge/job-knowledge-71-mechanical-testing-notched-bar-or-impact-testing/

Impact Testing


The typical SEM images of different impact fracture surface of steel: (a) fracture 1 (brittle fracture, tempered at 200 °C); (b) fracture 2 (mixed ductile–brittle fracture, tempered at 300 °C); (c) fracture 3 (ductile fracture, tempered at 400 °C); (d) fracture 4 (ductile fracture, tempered at 500 °C).

Fracture surface

Trans‐granular Intra‐granular


Fracture surface

http://practicalmaintenance.net/?p=1135

(a) Necking(b) Cavity formation(c) Cavity coalescence to form a crack(d) Crack propagation(e) Fracture

(a) Highly ductile fracture in which the specimen necks down to a point

(b) Moderately ductile fracture after some necking(c) Brittle specimen without any plastic deformation



Pile-up of dislocations at a barrier which might be a grain boundary or an included particle may result in a micro-crack. Within crystal lattice micro-cracks are created when three unit dislocations combine into a single dislocation.

It is possible that micro-cracks may exist in the metal due to the previous history of solidification or working. However, even an initially sound material may develop cracks on an atomic scale.

A mechanism of crack formation is:

Fracture surface



Cold Working: Hardness-Strength vs. Ductility


[1] http://www.efunda.com/formulae/solid_mechanics/fracture_mechanics/fm_lefm_K.cfm[2] http://www.efunda.com/formulae/solid_mechanics/fracture_mechanics/fm_lefm_stress.cfm

Crack tips produce a 1/√r singularity. The stress fields near a crack tip of an isotropic linear elastic material can be expressed as a product of 1/√r and a function of θ with a scaling factor K:

where the superscripts and subscripts I, II, and III denote the three different modes that different loadings may be applied to a crack. The detailed breakdown of stresses and displacements for each mode are summarized in [2].

The factor K:Stress Intensity Factor

Stress Intensity Factor and Crack Tip Stresses


“Goofy duck” analog for three modes of crack loading. (a) Crack/beak closed. (b) Opening mode. (c) Sliding mode. (d) Tearing mode. (Courtesy of M. H. Meyers.)

Goofy Duck Analog for Modes of Crack Loading


Fracture Toughness: a quantitative impact measure


Fracture Toughness or different MaterialsGraphite/ Ceramics/ Semicond

Metals/ Alloys

Composites/ fibers

Polymers

5

KIc

(MP

a ·

m0.

5)

1

Mg alloys

Al alloys

Ti alloys

Steels

Si crystalGlass -soda

Concrete

Si carbide

PC

Glass 6

0.5

0.7

2

4

3

10

20

30

<100>

<111>

Diamond

PVC

PP

Polyester

PS

PET

C-C(|| fibers) 1

0.6

67

40506070

100

Al oxideSi nitride

C/C( fibers) 1

Al/Al oxide(sf) 2

Al oxid/SiC(w) 3

Al oxid/ZrO 2(p)4Si nitr/SiC(w) 5

Glass/SiC(w) 6

Y2O3/ZrO 2(p)4

K1c – plane strain stress concentration factor – with edge crack; A Material Property we use for design, developed using ASTM Std: ASTM E399 - 09 Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness K1c of Metallic Materials

Composite reinforcement geometry is: f = fibers; sf = short fibers; w = whiskers; p = particles. Addition data as noted (vol. fraction of reinforcement):1. (55vol%) ASM Handbook, Vol. 21, ASM Int.,

Materials Park, OH (2001) p. 606.2. (55 vol%) Courtesy J. Cornie, MMC, Inc.,

Waltham, MA.3. (30 vol%) P.F. Becher et al., Fracture

Mechanics of Ceramics, Vol. 7, Plenum Press (1986). pp. 61-73.

4. Courtesy CoorsTek, Golden, CO.5. (30 vol%) S.T. Buljan et al., "Development of

Ceramic Matrix Composites for Application in Technology for Advanced Engines Program", ORNL/Sub/85-22011/2, ORNL, 1992.

6. (20vol%) F.D. Gace et al., Ceram. Eng. Sci. Proc., Vol. 7 (1986) pp. 978-82.


http://www.keytometals.com/page.aspx?ID=CheckArticle&site=ktn&NM=184

Development of a crack


http://www.chemeurope.com/en/encyclopedia/J_integral.html

Physically the J-integral is related to the area under the curve of a load versus load-point displacement

J. R. Rice, Appl. Mech. 35(2), 379-386 (1968) doi:10.1115/1.3601206

J-Integral and Fracture Toughness

The J-integral can be described as follows

where• F is the force applied at the crack tip • A is the area of the crack tip • is the change in energy per unit length • σ is the stress • is the change in the strain caused by the stress

Fracture toughness is then calculated from the following equation

where • K1c is the fracture toughness in mode one loading • v is the Poisson's ratio • E is the Young's Modulus of the material


Griffith's criterion

• E: Young's modulus of the material• γ: surface energy density

• a: flaw length• σf: and the stress at fracture

The growth of a crack requires the creation of two new surfaces and hence an increase in the surface energy. Griffith found an expression for the constant C in terms of the surface energy of the crack by solving the elasticity problem of a finite crack in an elastic plate. Briefly, the approach was:

• Compute the potential energy stored in a perfect specimen under a uniaxial tensile load.

• Fix the boundary so that the applied load does no work and then introduce a crack into the specimen. The crack relaxes the stress and hence reduces the elastic energy near the crack faces. On the other hand, the crack increases the total surface energy of the specimen.

• Compute the change in the free energy (surface energy − elastic energy) as a function of the crack length. Failure occurs when the free energy attains a peak value at a critical crack length, beyond which the free energy decreases by increasing the crack length, i.e. by causing fracture

The growth of a crack requires the creation of two new surfaces and hence an increase in the surface energy. Griffith found an expression for the constant C in terms of the surface energy of the crack by solving the elasticity problem of a finite crack in an elastic plate. Briefly, the approach was:

• Compute the potential energy stored in a perfect specimen under a uniaxial tensile load.

• Fix the boundary so that the applied load does no work and then introduce a crack into the specimen. The crack relaxes the stress and hence reduces the elastic energy near the crack faces. On the other hand, the crack increases the total surface energy of the specimen.

• Compute the change in the free energy (surface energy − elastic energy) as a function of the crack length. Failure occurs when the free energy attains a peak value at a critical crack length, beyond which the free energy decreases by increasing the crack length, i.e. by causing fracture ht

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Mode I loading of a elastic-perfectly plastic material:

02

Iyy

K

r

Plane stress assumed (1)

crackx

r1

Y

r2

(1)

(2)

(2)

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The Irwin approach I


Redistribution of stress due to plastic deformation:

Plastic zone length (plane stress):

Irwin’s model = simplified model for the extent of the plastic zone:

- Focus only on the extent of the plastic zone along the crack axis, not on its shape

2

11

2 I

Y

Kr

- Equilibrium condition along the y-axis not respected

yy

real crack x

Y

fictitious crack

Stress Intensity Factor corresponding to the effective crack of length aeff =a+r1

1 1,I effK a r K a r

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In plane strain, increasing of sY. : Irwin suggested in place of sY3 Y

(effective SIF)

The Irwin approach II







TOC


Fatigue


R = σmin/σ max

Fatigue Basics


High Cycle Fatigue (HCF)

Low Cycle Fatigue (LCF)

N~5000

HCF: Loading far below yield stress,Caused by vibrations, crack initiation is important. Usually, stress controlled tests

LCF: Plastic strains occur, caused by transient loads or on notches, crack propagation Is important. Usually, strain controlled tests

Cycles to Failure (N)

Stress (S) or strain

HCF-LCF


Different failure modes for a structure

Repeated loading for high cycle fatiguehttp://www.finiteelementanalysis.com.au/featured/what-is-fatigue/http://www.finiteelementanalysis.com.au/featured/what-is-fatigue/


S-N curve (Wohler S-N diagram):nominal stress amplitude S versus cycles to failure N

http://www.fea-optimization.com/ETBX/stresslife_help.html

Material (Curve ) A:endurance limit which represents a stress level below which the material does not fail and can be cycled infinitely. e.g. Steel & Ti

Material (Curve ) B:do not exhibit well-defined endurance limits. These materials instead display a continuously decreasing S-N response. Fatigue strength Sf for a given number of cycles must be specified.e.g. non-ferrous metals and alloys (Al, Cu, Mg, …)

http://www.energy.kth.se/compedu/webcompedu/WebHelp/S5_Aeroelasticity/B1_Introduction_to_Aeroelasticity/C7_Introduction_to_High_Cycle_Fatigue/ID122_files/S-N_diagram.htm

http://www.energy.kth.se/compedu/webcompedu/WebHelp/S5_Aeroelasticity/B1_Introduction_to_Aeroelasticity/C7_Introduction_to_High_Cycle_Fatigue/ID122_files/S-N_diagram.htm

Fatigue curves


Creep fatigue life: Average curves and experimental values at 550 oC

Creep fatigue life


Fatigue & Creep Interaction







TOC


http://rpmedia.ask.com/ts?u=/wikipedia/commons/7/7d/Reactorvessel.gif

Parts in a nuclear reactor


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RPV with its installations


RPV Weldments

Stainless steel cladding

Attachment welds: Lower and

upper RPV head penetrations and

reactor internal supports

Stainless steel cladding

Attachment welds: Lower and

upper RPV head penetrations and

reactor internal supports

Forged nozzles

A 508 Cl.2

Feed‐water, steam, recirculation and

emergency cooling piping systems

Forged nozzles

A 508 Cl.2

Feed‐water, steam, recirculation and

emergency cooling piping systems

1st Generation:

Bent plates with vertical

and horizontal welds

A 302 B, A 533 B Cl.1

1st Generation:

Bent plates with vertical

and horizontal welds

A 302 B, A 533 B Cl.1

3rd Generation:

Forged rings without weld

in beltline region

A 508 Cl.3

3rd Generation:

Forged rings without weld

in beltline region

A 508 Cl.3

2nd Generation:

Forged rings with

horizontal welds only

A 508 Cl.3

2nd Generation:

Forged rings with

horizontal welds only

A 508 Cl.3


Shielded metal arc welding (SMAW)

Submerged arc welding(SAW)

Welding techniques

Gas tungsten arc welding (GTAW) / tungsten inert gas (TIG) welding

Gas metal arc welding (GMAW)(1) Direction of travel(2) Contact tube(3) Electrode(4) Shielding gas(5) Molten weld metal(6) Solidified weld metal(7) Workpiece


Video on submerged arc welding (SAW)


Cracks developing in girth welds

• Type I damage is oriented either longitudinal or transverse, located in the weld metal and remains within the weld metal.

• Type II damage is similar to Type I, but grows out of the weld metal into the adjacent HAZ and base material.

• Type III damage is located in the coarse-grained region of the HAZ.

• Type IV damage is located in the fine-grained/partially transformed region of the HAZ

Chan W, McQueen R, Prince J, Sidey D (1991) Metallurgical experiences with high temperature piping in ontario hydro ASME PVP 22. Service experience in operating plants, New York

Welding


Welding

Joining to large components welding

Welding introduces:

• Thermal influence (heating / cooling)

• new phases / microstructures

• Transition zones between two alloys (dissimilar welds)

doi: 10.1115/1.4031129For DMW joints preparation, two root passes have been employed using TIG process and Inconel 82/52 TIG rods for the respective weld joints. The close chamber purging was provided during root passes of the both joints. The subsequent fill passes applied with shielded metal arc welding process using Inconel 182 and Inconel 152 electrodes at reverse polarity.


A weld consists of:• Base metal• Heat affected zone• Fusion line• Weld metal

Different kinds of steel can develop. Particularly for dissimilar welds. After welding proper post weld heat treatment (PWHT) is important

A weld consists of:• Base metal• Heat affected zone• Fusion line• Weld metal

Different kinds of steel can develop. Particularly for dissimilar welds. After welding proper post weld heat treatment (PWHT) is important

Dissimilar weld (microstructure)


http://engineers.ihs.com/document/abstract/VYJOIBAAAAAAAAAA

Low Alloy Steels

Fine-grained structural steels with bainitic (BCC) microstructure and high toughness

Quenched + tempered (Q+T) + post-weld heat treatment (PWHT)

Mn-Mo-Ni-type (SA-508 Cl. 3 forgings, SA-533 Gr. B Cl. 1 plates, …)

Ni-Mo-Cr-type (SA 508 Cl. 2 forgings, …)

S ≤ 0.01% (in very old plants up to 0.04% S)

S ↑ EAC susceptibility ↑, fracture toughness ↓

Cu ≤ 0.05% (in old plants, weldments contained up to 0.35% Cu)

Cu ↑ irradiation embrittlement ↑, toughness ↓


http://www.jsw.co.jp/en/product/material/vessel/fabsequence.html

Production


Video of steel making process


A ladle containing 150 tons of liquid steel is lowered into the tank degasser at Pennsylvania Steel Technologies to remove hydrogen from steel for harder railheads.

http://www.memagazine.org/backissues/membersonly/april98/features/vacuum/vacuum.html

Ladle degassing


Ring Forging Penetration welding

http://www.kntc.re.kr/openlec/nuc/NPRT/module2/module2_6/2_6.htm

Reactor pressure vessel production steps


http://radona.de/index.htm

http://www.kntc.re.kr/openlec/nuc/NPRT/module2/module2_6/2_6.htm

Austenitic Cladding Application


End of (Design) Life n-Fluence (E > 1 MeV)







TOC


http://hss.energy.gov/nuclearsafety/techstds/docs/handbook/h1019v1.pdf

Neutron Spectra of Thermal and Fast Reactors


Seeger A (1962), Radiation damage in solids 1. IAEA Vienna: 101

Defect formation


• (a–c) MD simulation snapshots of initial intermediate and final dynamic stageof a displacement cascade

• (d–e) vacancy and self interstitial defects

• (f) vacancy-solute cluster complexformed after long-term cascade aging

An illustration of cascade primary-damage production(iron atoms not shown in a–c and f):

Cascade illustration


http://dx.doi.org/10.1016/j.jnucmat.2012.05.024

Point defect reactions


Typical isochronous annealing curve for pure Cu after irradiation at 4.2 K with fast neutrons to typical doses of 10-5 dpa. The annealing temperature T is normalized to the melting temperature of Cu. 10 minutes at successively higher temperatures

FD: Frenkel Defects

Tipping PG (ed) Understanding and mitigating ageing in nuclear power plants, WoodhaedPublishing Ltd, Cambridge

Stages - I: initiation self-interstitial atom migration (correlated & uncorrelated)II: long-range migration of SIA clusters and SIA-impurity complexesIII: associated with vacancy migration IV: mitigation of vacancy clusters and vacancy-solute complexesV: thermal dissociation of (displacement cascade produced) vacancy clusters

Frenkel defect retention


fcc bcc

http://link.aip.org/link/doi/10.1063/1.1880013

Cascade Development is depending on Lattice Type


Early period of the development of the center of a cascade in copper as result of a molecular dynamics simulation

http://msg.igcar.gov.in/mpd/ibcss/index.php/research/77-general/122

MD cascade simulation


Duration (ps)

Event Result

10‐6 Transfer of recoil energy

from irradiation particle Primary knock‐on atom

10‐6 to

0.2

Slowing down of PKA,

generation of coll ision cascade

Vacancies and low energetic

recoils, subcascades

0.2‐0.3 Spike formation Low density hot molten droplet, shock front

0.3 to 3 Spike relaxation, interstitial ejection, transition from heated to undercooled l iquid core

Stable self interstitials atomic mixing

3 to 10 Spike core solidification and cooling to ambient temperature

Depleted zone, disordered zone, amorphous zone, vacancy collapse

More than 10

Thermal intercascade recombination, thermal migration of point defects from the cascade,

reaction of migrating point defects

Surviving defects, migrating interstitials and vacancies, stationary fluxes of vacancies and interstitials to sinks,

growth/shrinkage of point defect clusters, solute segregations

Irradiation stages


Comparison of MD simulations of displacement cascades in Fe for different PKA energies http://dx.doi.org/10.1063/1.1880013

Cascade dependency on energy


• LASREF: Los Alamos Spallation Radiation Effects Facility(Be(d,n) neutrons)

• RTNS-II: Rotating Target Neutron Source-II (14 MeV neutrons)

• OWR: Omega West Reactor (fission neutrons)

http://dx.doi.org/10.1016/0022-3115(94)90004-3

Irradiation hardening of solution annealed 316 steel

Irradiation Damage

neutrons per cm²

displacements per atom


ν(T) = T/2.Eth

Displacement per atom «dpa»: R/N

ISBN 978-1-4471-2915-8

flux

displacement number from pka

x-section

required energy

Energy transfer

Recombination

vacancy rate equation

interstitial rate equation

Irradiation Damage


Transmutation for fast neutrons

Transmutation for Ni and thermal neutrons

http://dx.doi.org/10.1016/0022-3115(88)90235-8

Transmutation: Nickel and Iron cross sections


Effect Consequence in material

Kind of degradation in component

Displacement damageFormation of point defect clusters and dislocation loops

Hardening, embrittlement

Irradiation-induced segregation

Diffusion of detrimental elements to grain boundaries

Embrittlement, grain boundary cracking

Irradiation-induced phase transitions

Formation of phases not expected according to phase diagram, phase dissolution

Embrittlement, softening

SwellingVolume increase due to defect clusters and voids

Local deformation, eventually residual stresses

Irradiation creep Irreversible deformationDeformation, reduction of creep life

Helium formation and diffusion

Void formation (inter- and intra-crystalline)

Embrittlement, loss of stress rupture life and creep ductility

Radiation Damage


Which damage in which temperature region

Temperature (Homologous Temperature)

Ranges over which Radiation Damage Occurs


Effect of addition of 5 % Ni on the dislocation loop density in irradiated copper

http://dx.doi.org/10.1016/0022-3115(94)90043-4

(1) The loop density in the alloy is considerably higher at all irradiation rates.

(2) The difference diminishes with increasing damage rates. It is largest for typical reactor damage rates.

Alloying element effect on dislocation density


http://dx.doi.org/10.1016/0022‐3115(93)90128‐L

Influence of irradiation temperature on the formation of different obstacles taking an austenitic steel as an example

At lower temperatures the irradiation induced defects are predominant, whereas with increasing temperature the point defect concentration of thermal equilibrium governs microstructural development and irradiation damage starts to disappear. This can also be seen from an analysis of the diffusion coefficients shown in the next slide.

Saturation densities in austenitic steels


Wiedersich H (1986) In: Physics of radiation effects in crystals Elsevier 237

Dth: Thermal diffusion

Drad: radiation induced diffusion

Dm: ion mixing

• At high temperatures thermal diffusion is predominant.

• Radiation induced diffusion depends on the density of sinks. Sinks reduce the amount of excess point defects which decreases the respective diffusion coefficient as shown here with „no sinks“, p=10-4 and p=10-3.

• Ion mixing which happens at low temperatures independent of the temperature will not further be discussed here.

Temperature & radiation induced diffuison


Gilbert ER, Kaulitz DC, Holmes JJ, Claudsen TT (1972) In: Proceedings conference on

irradiation embrittlement and creep in fuel cladding and core components. British Nuclear

Energy Society London 1972, pp 239–251

Comparison of thermal and irradiation creep strains in 20 % cold worked 316 SS

Irradiation Creep


http://dx.doi.org/10.1016/0022-3115(84)90626-3

General form of swelling versus dose with the various stages involved.

Void Swelling


Schilling W, Ullmaier H (1994) Physics of radiation damage in metals. Mater. Sci. Technol. VCH 10B:187

Development of helium bubbles and voids with time (schematically):cHe: helium concentrationcB: bubble concentrationrB: void/bubble radius

Helium bubble formation


Irradiation hardening in a ferritic-martensitic steel. At temperatures above 400 C hardening starts to disappear as a result of annealing

http://aries.ucsd.edu/LIB/PROPS/FS/FS.htmlDOE/ER-0313/23 (1997) 179 http://dx.doi.org/10.1016/S0022-3115(98)00395-X

Shift in the fracture appearance temperature as a result of irradiation embrittlement. FFTF: Fast Flux Test Facility

Load vs. normalized crosshead displacement tensile curves for F82H irradiated to 3-34 dpa at 200-600°C

Klueh RL, Alexander DJ (1992) ASTM STP 1125 American society for testing and materials Philadelphia, p 1256

Irradiation Hardening and Embrittlement


Atom Probe Analysis


Atom Probe Analysis of an irradiated steel


Atom Probe Scheme







TOC


Types of Corrosion

Type of Corrosion Comments Attack

Uniform Corrosion Uniform corrosion, as the name suggests, occurs over the majority of the surface of a metal at a steady and often predictable rate

homogeneous

Galvanic Corrosion Can occur when tow different metals are placed in contact with each other

localized

Pitting Corrosion Pitting corrosion occurs in materials that have a protective film such as a corrosion product or when a coating breaks down

localized

Crevice Corrosion If two areas of a component in close proximity differ in the amount of reactive constituent available the reaction in one of the areas is speeded up

Intergranular Corrosion

Preferential attack of the grain boundaries of the crystals that form the metal

localized

Corrosion Fatigue Corrosion + Fatigue interaction

Fretting Corrosion Corrosion + Friction interaction

Stress Corrosion Cracking

Corrosion + Stress interaction


• Corrosion can cause local attack such as pitting or grain boundary degradation which can be considered as crack-like defects from which fatigue cracks can propagate.

• The formation of intrusions and extrusions can destroy a protective layer and expose fresh material to the corrosive atmosphere leading eventually to severe degradation of the newly created fresh surface and consequently to local damage.

• Protective layers (e.g. oxides) can spall off removing material and creating a new surface which can be attacked again.

• Reaction between the atmosphere and the material (nitriding, oxydation etc.) can lead to microstructural changes like carbide-or nitride formation or the dissolution of aluminum-containing phases when aluminum oxide is formed which can result in a change of the mechanical properties. ISBN 978-1-4471-2915-8

Corrosion effects


Different types of oxide layers.

• Dense, protective layer at the left side and a

• porous layer at the right side. The porous layer can easily spall off and it also allows penetration of corrosive species. https://192.107.58.30/D19/Heikinheimo.pdf

Development of Surface Layers


A Pourbaix diagram for iron. Similar to phase diagrams which are based on thermodynamic equilibrium a potential/phdiagram mapping out equilibrium phases of an aqueous electrochemical systemcan be derived form the Nernst equation.

Reduction:Fe2+ + 2H2O Fe(OH)2 + 2H+ (cathodic) Oxidation:Fe Fe2+ + 2e- (anodic)

Electrochemical reactions during corrosion in liquid media.

(Ox + z e− Red)

http://de.wikipedia.org/wiki/Nernst-Gleichung

Liquid Corrosion


Aqueous corrosion can be studied by measuring the potential of the corrosion system against a standard electrode.

In the polarization curve the potential is plotted as a function of the current density.

• The actual corrosion potential i.e. the potential difference between standard electrode and corrosion system becomes zero and no current flows.

• Increasing the potential leads at first to an increase in the current (active part) and

• turns back to a passive part where the current remains constant. In this area passivation (self-protection) of the surface happens.

• Further increase of the potential leads to increase of current which means that the passivation is no longer fully maintained. Radiolysis taking place mainly in the reactor core region changes the composition of the water which has an influence on the electrochemical behavior. ISBN 978-1-4471-2915-8

Polarization diagram


http://events.nace.org/library/corrosion/AnodProtect/passivecurve.asp

Hypothetical polarization diagram for a passivable system with active, passive and transpassive regions.

Only a few systems exhibit this behavior in an appreciable and usable way. The corrosion rate of an active-passive metal can be significantly reduced by shifting the potential of the metal so that it is at a value in the passive range.

The current required to shift the potentialin the anodic direction from the corrosion potential Ecorr can be several orders of magnitude greater than the current necessary to maintain the potential at a passive value.

The current will peak at the passivation potential value shown as Epp.

Polarization diagram


• AB represents cathodic behavior, • BG is the active zone. The metal is not passivated at its free

corrosion potential, B. • AC and DC are Tafel-type straight lines drawn for the reduction and

oxidation reactions of the normal metal dissolution (M=M++e). • At potentials more positive than B, corrosion rate increases, and

reaches a maximum at the passivation potential, G, which is often given the symbol, Epp.

• The transition from active dissolution occurs as a solid species becomes more thermodynamically stable than the metal ion. A protective film begins to form and causes a sudden drop in corrosion current density in the region G to J.

• From J to P, the passive zone, the current density is maintained at a steady, low level, until,

• at P, breakdown of the protective film begins. It is here that the likelihood of pitting is greatest, and consequently

• the potential Ec, often called the critical pitting or breakdownpotential. It is a useful parameter in assessing pitting properties of materials. It should be noted that it is not an absolute parameter, and varies according to both metallurgical and environmental conditions.

• At potentials more positive than P, the current density begins to rise as more and more pits propagatehttp://www.corrosionclinic.com/

corrosion_online_lectures/ME303L11.HTMhttp://www.corrosionclinic.com/corrosion_online_lectures/ME303L11.HTM

Potentiodynamic polarization behavior of passivating metals


Dependence of susceptibility to intergranular stress corrosion cracking (IGSCC) for two materials under different environmental conditions.

Was G, Busby J, Andresen PL (2006) ASM handbook:

corrosion: environments and industries, vol 13C.

Typical corrosion test:Samples are loaded in corrosive environment.

Above the threshold stress

(depending on material and conditions)

fracture occurs after some time. Below the threshold stress no corrosion fracture is expected

Intergranular Stress Corrosion Cracking


http://www.azom.com/article.aspx?ArticleID=102

Stage 1: below a threshold value of K1, called K1SCC, growth of a crack by SCC is not expected, but above this value the initial SCC growth rate increases with increasing K1, called stage 1 cracking

Stage 2: the crack growth rate is independent of K1 and depends instead on the corrosive environment and temperature. During stage 2 growth, K1 continues to increase and this leads to the rapid acceleration of the crack in …

Stage 3: …and final fast fracture when K1 reaches K1C which is the Fracture Toughness of the material.

Crack Growth under Corrosion


Revie I, Winston R (2008) Corrosion and corrosion control, 4th edn. Wiley, ISBN: 978-0-471-73279-2

Influence of corrosion on the fatiguecurves (schematically).

• At low number of cycles a weaker influence occurs than in the fatigue limit regime because of shorter time for corrosive attack.

• The effect is therefore frequency dependent.

Corrosion Fatigue


Reactor Internals mainly made of austenitic stainless steels (304, 316)


Predicted radiation temperature and flux effects on radiation induced segregation behaviour of austenitic steels. The left bar refers to typical reactor conditions

Was GS, Busby J, Andresen PL (2006) Effect of irradiation on stress-corrosion cracking and corrosion in light water reactors. ASM Handbook 13C corrosion environments and industries ASM international, pp 386–414

Temperature & Dose-Rate Effects on Segregation


http://dx.doi.org/10.1016/S0022-3115(99)00075-6

Neutron Fluence effects on irradiation-assisted stress corrosion cracking(IASCC)susceptibility of Type 304SS inLWR environments.

Damage developing with Time


http://www.nuclearplantcorrosion.com/pgs550/bbolt.html

Irradiation assisted stress corrosion cracking (IASCC) of a 20% cold worked Type 316 stainless steel baffle bolt

Irradiation assisted stress corrosion cracking


Radiation induced segregation (RIS):

Segregation: thermally driven movement of atoms to grain boundaries.

Temper embrittlement of steels is a very well known example for segregation related deterioration of toughness. Elements like phosphorus, sulphur or manganese diffuse to grain boundaries. The cohesion along the grain boundaries is weakened which leads to a reduction of toughness (reduction of fracture toughness or increase of ductile-to-brittle fracture transition temperature). Such grain boundaries can also act as preferential corrosion sites leading to stress corrosion cracking as discussed later.

Radiation induced segregation is the radiation-induced redistribution of alloy constituents at point defect sinks such as grain boundaries. Radiation induced segregation can be described in terms of the so called « Inverse Kirkendall Effect ». This inverse Kirkendall effect refers to cases where an existing flux of point defects affects the interdiffusion of A and B. In case of irradiation segregation in a homogeneous AB alloy occurs because the irradiation has produced excess point defects and a flux of point defects.

Radiation induced segregation


The above left figure explains the mechanism for a binary alloy more in detail. The ordinates represent the concentrations of vacancies and interstitials, respectively, in arbitrary units. The x-axis gives the distance from the grain boundary. The differences in the diffusion coefficients of A and B lead to a dilution of the concentration of atoms A and to an increase of the concentration of atoms of type B towards the grain boundaries.

The figure on the right shows the proton irradiated austenitic steel (304 SS) as an example. The chromium concentration diminishes at the grain boundary, whereas nickel increases

distance from the grain boundary

vaca

ncie

s

inte

rstit

ials

Radiation induced segregation


Main mechanisms of irradiation assisted stress corrosion cracking

http://dx.doi.org/10.1016/S0022-3115(99)00075-6

Mechanisms of IASCC (Irradiation Assisted Stress Corrosion Cracking)


• Irradiation damage leads to hardening of the matrix making basically the grain boundaries more attractive as path for growing cracks. This is what often happens as a result of hardening also without irradiation.

• Irradiation is also responsible for changes in grain boundary compositions by radiation induced segregation (primarily chromium depletion) which can further weaken the coherence of them.

• The surface of the cracks (particularly at the crack tip) is exposed to the radiolysis products which lead to chemical corrosion attack.

• Additionally, the crack can act as crevice supporting crevice corrosion.

Mechanisms of IASCC


Corrosion on pressure boundaries


Damage evolution over time


In March 2002, plant staff discovered that the borated water that serves as the reactor coolant had leaked from cracked control rod drive mechanisms directly above the reactor and eaten through more than six inches (150 mm) of the carbon steel reactor pressure vessel head over an area roughly the size of a football.

A breach most likely would have resulted in a loss-of-coolant accident.http://en.wikipedia.org/wiki/Davis-Besse_Nuclear_Power_Station

Davis-Besse


An incomplete mixing area of high and low temperature fluids in nuclear components, fluid temperature fluctuates with random frequencies. It induces random variations of local temperatures gradients in structural walls, which lead to cyclic thermal stress. When thermal stress and cycle number are large, there are possibilities of crack initiations and propagations. This couple thermal hydraulic and thermal mechanical phenomenon is called thermal striping, which should be prevented.

Thermal Striping

nuclear fuels and materials - 151-2017-00l - psi · pdf filetoughness is measured with impact...

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