tutorial irradiation embrittlement and life management of rpvs

Znojmo (CZ) 18 October 2010

TutorialIrradiation Embrittlement and

Life Management of RPVs

Structural Integrity IssuesF. Gillemot

http://capture.jrc.ec.europa.eu


What is structural integrity?

No answer in the WEB or in other documents!

Safe operation of passive components in normal andnon-normal conditions. The structural integrity meansthat the sructure, or component not only safe, butsurvives the service and environmetal effects withoutany serious damage.

Structural Integrity Issues



Passive components

Passive components: all pressurized or loadedstructures

Examples:Reactor Pressure Vessel Steam generator PressurizerHouse of main coolant pump PipesCrane structures etc.




b


Safety – whats that?

Safety means protection of the environment and populations from radioactivecontamination, or from other harness

The deteministic safety assessment methodology uses a technique in with adefence in depth assessment assure success in each level of the defence.Design/safety limits are specified for each level of defence.

The probabilistic safety assessment uses a methodology to calculate the risk offailure, and determines the acceptable risk level. No 100% safety, too high safetyrequirements are damaging the society. Very high responsibility for the engineers.




Structures, systems components(SSC) integrity




IAEA Safety Standards and Guidelines on PLiM and AM

ProgrammaticGuidelines

ComponentSpecificGuidelines(13)

AMPReviewguideline

SGonAMP

SafetyGuideon PSR

Safety of NPPDesign NS R-1

RPV and PLiM

SafetyGuideon MSI

Safety Guideon PersonalQualification

Human AgeingGuideline

NENPNSNI

SafetyRequirement

Safety Guide

Tech.Guidelines



Steam generators (TECDOC-981)Concrete containment buildings (TECDOC-1025) PWR pressure vessels (TECDOC-1120) PWR vessel internals (TECDOC-1119)Metal components of BWR containment (TECDOC-

1181) In-containment I&C Cables (TECDOC-1188) Volume

I In-containment I&C Cables (TECDOC-1188) Volume

II PWR primary piping (TECDOC-1361) BWR Reactor Pressure Vessel (TECDOC-1470) BWR Rector Pressure Vessel Internals (TECDOC-

1471)





Operating time

Ageing effects

50% failureprobability

Design safetylevel

Operatingstrategy I

Operating strategy II Operating strategy III

Safe operatinglife I

Safe operatinglife II




Ageing mechanism

- Radiation embrittlement- Thermal embrittlement- General corrosion- Stress corrosion cracking- Pitting corrosion- Irradiation assissted corrosion- Hidrogen embrittlement- Liquid metal embrittlement- Wear- Fatigue and low-cycle fatigue- Creep- High temperature rupture- Errosion- Etc...



Event selection (PSA Probabilistic Safety analyses)


High pressure, safety valveopen

Safety valve mailfunction,coolant pressure drop

Reactor stop, emergency corecoolant pumps operating

Rapid cooling

PTS

Safety valve closed,normal shut down

Failureprobability isbelow of the

acceptance level

Structural integrityassessment

Probability isbelow of the

acceptance level

No integrity

Event tree




Failure Analysis

Failure of a component indicates it has become completely orpartially unusable or has deteriorated to the point that it is

undependable or unsafe for normal sustained service.

Typical Root Cause Failure Mechanisms

1. Fatigue failures

2. Corrosion failures

3. Stress corrosion cracking

4. Ductile and brittle fractures

5. Hydrogen embrittlement

6. Liquid metal embrittlement

7. Creep and stress rupture




Fatigue Failures

Metal fatigue is caused by repeatedcycling of of the load. It is aprogressive localized damage due tofluctuating stresses and strains onthe material. Metal fatigue cracksinitiate and propagate in regionswhere the strain is most severe.

The process of fatigue consists ofthree stages:

Initial crack initiation Progressive crack growth

across the part Final sudden fracture of the

remaining cross section

Schematic of S-N Curve, showingincrease in fatigue life withdecreasing stresses.




Stress Ratio

The most commonly used stress ratio is R, the ratio of the minimum stress to themaximum stress (Smin/Smax).

1. If the stresses are fully reversed, then R = -1.2. If the stresses are partially reversed, R = a negative number less than 1.3. If the stress is cycled between a maximum stress and no load, R = zero.4. If the stress is cycled between two tensile stresses, R = a positive number

less than 1.

Variations in the stress ratios can significantly affect fatigue life. The presence of amean stress component has a substantial effect on fatigue failure. When a tensilemean stress is added to the alternating stresses, a component will fail at loweralternating stress than it does under a fully reversed stress.




Preventing Fatigue Failure

The most effective method of improving fatigue performance is improvementsin design:

1. Eliminate or reduce stress raisers by streamlining the part

2. Avoid sharp surface tears resulting from punching, stamping, shearing,or other processes

3. Prevent the development of surface discontinuities during processing.

4. Reduce or eliminate tensile residual stresses caused by manufacturing.

5. Improve the details of fabrication and fastening procedures

Fatigue Failure Analysis

Metal fatigue is a significant problem because it can occur due to repeatedloads below the static yield strength. This can result in an unexpected andcatastrophic failure in use.

Because most engineering materials contain discontinuities most metal fatiguecracks initiate from discontinuities in highly stressed regions of thecomponent. The failure may be due the discontinuity, design, impropermaintenance or other causes. A failure analysis can determine the cause of thefailure.




High Temperature Failure Analysis

Creep occurs under load at high temperature. Boilers, gas turbine engines, andovens are some of the systems that have components that experience creep. Anunderstanding of high temperature materials behavior is beneficial in evaluatingfailures in these types of systems.

Failures involving creep are usually easy to identify due to the deformation thatoccurs. Failures may appear ductile or brittle. Cracking may be either transgranularor intergranular. While creep testing is done at constant temperature and constantload actual components may experience damage at various temperatures andloading conditions.

Creep of Metals

High temperature progressive deformation of a material at constant stress is calledcreep. High temperature is a relative term that is dependent on the materials beingevaluated. A typical creep curve is shown below:




In a creep test a constant load is applied to a tensile specimen maintained at aconstant temperature. Strain is then measured over a period of time. The slope ofthe curve, identified in the above figure, is the strain rate of the test during stage II orthe creep rate of the material.

Primary creep, Stage I, is a period of decreasing creep rate. Primary creep is aperiod of primarily transient creep. During this period deformation takes place andthe resistance to creep increases until stage II. Secondary creep, Stage II, is aperiod of roughly constant creep rate. Stage II is referred to as steady state creep.Tertiary creep, Stage III, occurs when there is a reduction in cross sectional area dueto necking or effective reduction in area due to internal void formation.




Stress Rupture

Stress rupture testing is similar to creep testing except that thestresses used are higher than in a creep test. Stress rupture testingis always done until failure of the material. In creep testing the maingoal is to determine the minimum creep rate in stage II. Once adesigner knows the materials will creep and has accounted for thisdeformation a primary goal is to avoid failure of the component.




Corrosion Failures

Corrosion is chemically induced damage to amaterial that results in deterioration of thematerial and its properties. This may result infailure of the component. Several factorsshould be considered during a failure analysisto determine the affect corrosion played in afailure. Examples are listed below:

Type of corrosion Corrosion rate The extent of the corrosion Interaction between corrosion and other

failure mechanisms

Corrosion is is a normal, natural process.Corrosion can seldom be totally prevented, butit can be minimized or controlled by properchoice of material, design, coatings, andoccasionally by changing the environment.Various types of metallic and nonmetalliccoatings are regularly used to protect metalparts from corrosion.




Stress Corrosion Cracking

Stress corrosion cracking is a failure mechanism that is caused by environment,susceptible material, and tensile stress. Temperature is a significantenvironmental factor affecting cracking.

For stress corrosion cracking to occur all threeconditions must be met simultaneously. Thecomponent needs to be in a particular crackpromoting environment, the component must bemade of a susceptible material, and there must betensile stresses above some minimum thresholdvalue. An externally applied load is not required asthe tensile stresses may be due to residual stressesin the material. The threshold stresses arecommonly below the yield stress of the material.

Stress Corrosion Cracking Failures

Stress corrosion cracking is an insidious type offailure as it can occur without an externally appliedload or at loads significantly below yield stress.Thus, catastrophic failure can occur withoutsignificant deformation or obvious deterioration ofthe component. Pitting is commonly associatedwith stress corrosion cracking phenomena.




Pitting CorrosionPitting is a localized form of corrosive attack. Pitting corrosion is typified by theformation of holes or pits on the metal surface. Pitting can cause failure due toperforation while the total corrosion, as measured by weight loss, might be ratherminimal. The rate of penetration may be 10 to 100 times that by general corrosion.Pits may be rather small and difficult to detect. In some cases pits may be maskeddue to general corrosion. Pitting may take some time to initiate and develop to aneasily viewable size.

Pitting occurs more readily in a stagnantenvironment. The aggressiveness of the corrodentwill affect the rate of pitting. Some methods forreducing the effects of pitting corrosion are listedbelow:

Reduce the aggressiveness of theenvironment

Use more pitting resistant materials Improve the design of the system




Mihamaaccident

Orifice



Davies-Besse NPP





Fracture Analysis

1. sizes of flaws which must be detected during nondestructiveexaminations of components

2. needs to replace or repair structures and components which arefound to have flaws present.

3. remaining years of operating life of degraded components.




Ductile and Brittle Metal CharacteristicsDuctile metals experience observable plastic deformation prior to fracture. Brittlemetals experience little or no plastic deformation prior to fracture. At times metalsbehave in a transitional manner - partially ductile/brittle.Ductile fracture has dimpled, cup and cone fracture appearance. The dimples canbecome elongated by a lateral shearing force, or if the crack is in the opening(tearing) mode.Brittle fracture displays either cleavage (transgranular) or intergranular fracture.This depends upon whether the grain boundaries are stronger or weaker than thegrains.The fracture modes (dimples, cleavage, or intergranular fracture) may be seen onthe fracture surface and it is possible all three modes will be present of a givenfracture face.

Schematics of typical tensile test fractures are displayed above .




Brittle FracturesBrittle fracture is characterized by rapid crack propagation with low energy releaseand without significant plastic deformation. The fracture may have a brightgranular appearance. The fractures are generally of the flat type and chevronpatterns may be present.

Ductile FracturesDuctile fracture is characterized by tearing of metal and significant plastic

deformation. The ductile fracture may have a gray, fibrous appearance. Ductilefractures are associated with overload of the structure or large discontinuities



No defectless material, or at least we cannot prove that it existWe have to live with it.


Material behaviour

Depends on the three state facors:

-Temperature

- Stress state

- Strain rate

Brittle Ductile

Temperature

Toughness

Static load Dynamic load




Fracturetoughness

Thickness

Plain strainPlain stress



Stress intensity: ratio of the stress at crack tip/normal stressDenoted with K (K1 tensile K2 shear stress)


Crack tip

Stress atcrack tip

Normal stress




K1c = fracture toughness that is the K1 value where crack start topropagate in an absolute brittle material

No or negligable plasticdeformation =valid K1c

Limited plasticzone= J integral

General yielding, nofracture mechanics,leak before break

Jc=Je+Jp

Kjc=(E*Jc)/(1-ν)2)1/2

MPa√m

E=Young modulus

V=Poissons ratio




Stress Intensity Factor and Crack Tip Stresses

Crack tips produce a singularity.The stress fields near a crack tip of anisotropic linear elastic material can be

expressed as a product of and afunction of with a scaling factor K

where the superscripts and subscripts I, II, and III denote the three different modes thatdifferent loadings may be applied to a crack. The factor K is called the Stress IntensityFactor.




Infinite Plate with a CenterThrough Crack under Tension

Infinite Plate with a Hole and SymmetricDouble Through Cracks under Tension



b


Semi-infinite Plate with anEdge Through Crack under Tension

or

Infinite Stripe with a CenterThrough Crack under Tension




Charpy testing




LARGE SPECIMENS

SMALL SPECIMENS

95 %

5 %

95 %

5 %KJC

[MP

am

]

T [0C]

TYPICAL RAW DATA

STATISTICAL THICKNESS

ADJUSTMENT

LARGE SPECIMENS

SMALL SPECIMENS

95 %

5 %

95 %

5 %

KJC

[MP

am

]T [

0C]

MASTER CURVE ANALYSIS

“THE MASTER CURVE APPROACH”

THEORETICAL SCATTER DESCRIPTIONSTATISTICAL SIZE ADJUSTMENT

UNIFIED TEMPERATURE DEPENDENCE




PROBABILITY OF INITIATION(WEAKEST LINK)

CUMULATIVE FAILURE PROBABILITYOF A VOLUME ELEMENT

Pf

1 1 Pr I / ON

Pr{I/O} = Pr{I} (1- Pr{V/O})= Cleavage initiation without

prior void initiation

Pr{I/O} = (d, D, T, …. etc.). .




Master Curve

Kjc= 30+70*exp[0.019(T-T0)] Median

Kjc(5%) =25,4+37.8*exp[0.019*(T-T0)] Lower bound

Measurement: three point bend (precracked Charpy) or CTspecimens (8-10 pc minimum) at low temperature.

ASTM 1921-05 Evaluation T0 Margin=10-16 °C

Size adjustment included, small specimens can be used

Guidelines for Application of the Master Curve Approach to Reactor PressureVessel Integrity in Nuclear Power Plants Details Technical Reports Series No. 4292005,English, Full Text, (File Size: 1377 KB). 39.00 Euro. Date of Issue: 22 April 2005.

http://www-pub.iaea.org/MTCD/publications/ResultsPage.asp?p=2

Free download, 39Euro in printed form

tutorial irradiation embrittlement and life management of rpvs

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