VTT TECHNICAL RESEARCH CENTRE OF FINLAND LTD

IGSCC in a BWR steam lineafter 30 years of operation

U. Ehrnstén1, J-M. Autio1, P. Holmström2

1 VTT Technical Research Centre of Finland Ltd2 TVO Teollisuuden Voima17th International Conference on EnvironmentalDegradation of Materials in Nuclear Power Plants –Water Reactors, Ottawa, 9-12.8.2015

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AcknowledgementsThe failure analysis was performed as a customer assignment forTVOMore detailed investigations where made as part of the nationalreactor safety program SAFIR2014 in the ENVIS project(Environmental influence on cracking susceptibility and ageing ofnuclear materials), which is funded by the

Nuclear Waste Management FundVTT LtdSSM (Strålsäkerhetsmyndigheten, Sweden)OECD-Halden projectFortum Ltd

Their funding and interest to the topic is highly appreciated.

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ContentIntroduction, and reasons for reporting this caseBackgroundPerformed investigations and resultsDiscussion, lessons learned

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IntroductionIntergranular stress corrosion cracking plagued the BWR fleet in the70 - 80´s, with a remarkable effect of the utilisation rateMost of the failures occurred in Type 304 stainless steel with highcarbon content, resulting in welding induced sensitization and lowtemperature sensitisation during long term operationLarge research programs were launched on all three parameters, i.e.,material, stress and environment

Nuclear grade 316NG was developedNarrow gap welding and improved welding procedure specifications(WPS) appliedImproved water chemistry guidelines and innovative water chemistrieswere developed…and much more

Consequently the amount of BWR IGSCC failures has decreasedIGSCC has also occurred in steam dryers, but is IGSCC in steamlines a new issue?

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Background

Finland has two BWR units of ABB Atom type, i.e., Ol1 and Ol2;commercial use in 1979 and 1982; nominally 710 MW, upgradedto 880 MWAlso these suffered from IGSCC in the 80´s, and primary pipingreplacements to Type 316NG/L stainless steel was performedOnly a few IGSCC cases has been detected during the lastdecades, and the plant utilisation rate is on a world top levelIn December 2012, a leak in a steam pipe line was observedduring a walk down inspection in one of the two BWR unitsThe leak was temporarily repaired using a patch, and replacedwith an insert during the next outageThe steam line in question was originally made from carbon steel,but was replaced in 1983 due to flow assisted corrosion usingSS2333 (Type 304) stainless steel

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Background, cont.

The dimensions of the leaking pipe is 406,4x8 mm (i.e. thinnercompared to primary piping)The operating temperature is 170 C and the pressure is 8 barsThe environment inside the pipe is a steam-water mixtureThe leak is close to an assembly TIG-weld, which was re-weldedduring the assembly twice due to observed flaws in the weld

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Performed investigationsMacroscopyChemical compositionMicroscopyFractographyHardness measurementDetermination of the degree of sensitization using oxalic acidetching and DL-EPRDiscussion and conclusions

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The BWR steam line

sea

www.nrc.gov/reading-rm/basic-ref/teachers/03.pdf

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The leak

A leak in a 406.8x8mm steam line pipe was observed during a walk-down inspectionThe insulation was removed, the leak located and a temporary repairby welding a patch on the leak was doneA permanent replacement was performed at the next outage

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The weldThe weld was an assembly weld, which had to be re-welded twicedue to defects in the weld

Result after first welding sequence:

The weld was partly opened, re-welded, inspected, rejected, re-welded and approved

Incomplete penetrationUndercut, root protrusion, porosityIncompleted weld, porosity, root protrusion

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Macroscopy of section sent for failure analysis

The pipe with the leak was made from welded SS2333 (Type 304)pipe, while the pipe on other side was a seamless pipeThe crack in Material 1, with the leak, was 15 mm long and locatedat a distance of about 8-11 mm from the fusion lineThe width of the weld root is larger at the leak location compared toelsewhere indicating this is the location of the re-welding

8-11 mm!

crack Material 1

Material 2

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Chemical composition

Material C Si Mn S P Cr Ni Mo

Material 1 0.038 0.44 1.54 0.007 0.030 18.2 8.67 0.07

Material 2 0.040 0.46 1.54 0.006 0.031 18.1 8.71 0.07

Both materials fulfils the requirements of SS2333 (Type304)The carbon content in both materials is high, higher thanwhat would be specified today

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Fracture surface, macroscopy

The macroscopic crackconsists of numerousindividual cracks thathas initiated at a rathersimilar distance fromthe fusion line andcoalesced

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Fracture surface, microscopy

The crack is mainly intergranular with a small amount oftransgranular cracking

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Microstructure, macroscopy

Material 1 Material 2

In addition to themacroscopic crack inMaterial 1, a small crackin Material 2 wasobservedThe weld is wide and re-welded

Outside the crackedregion, a mis-fit of theweld is seen indicatingchallenging weldingconditions

cracks

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Microstructure

The crack is oxidised, mostlystraight, intergranular and withoutbranches, typical for IGSCC instainless steelAt a certain depth, the crack growsalmost parallel to the inner surface

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Microstructure

The part growing almost parallel to the inner surface seems tofollow a ferrite stringerThe crack path indicates also abending stress in the component

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Microstructure, sensitisation

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Microstructure, oxalic acid etching

Fusion line 4.5 mm

6.0 mm

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Oxalic acid etching

A cross-section close to, but outside the cracked region wasinvestigated for grain boundary attackGrain boundary attack was only seen between about 6 and 12 mmfrom the fusion lineNo clear indications of grain growth was observed in the HAZ

~6- 11 mm

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DL-EPR measurements

Twelve slices of with 1 mm intervals were cut parallel to the fusionline in Material 1 (with the macroscopic crack and subjected to DL-EPR measurements

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DL-EPR

The material outside the cracked region is sensitised at a distanceclose to the location of the crack

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Conclusions and discussionThe steam line pipe has suffered from IGSCCThe material is high carbon and sensitised which has clearlyaffected crack initiation and growthThe steam-water environment is oxidisingThe assembly weld was challenging to weld. Re-welding hascreated residual stresses

SO, WHY IS THIS WORTH REPORTING?The crack is further away from the fusion line compared to thattypically observed in primary pipingThe environment is steam-water mixture at relatively lowtemperature (170°C)IGSCC in SS steam lines have not been frequently reportedPossibility for further similar events

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Sensitisation issuesThe location of IGSCC is acombination of the sensitization andresidual stress/strain profilesIn thick-walled, sensitized Type 304primary piping is typically observed at adistance of about 3-5 mm from thefusionWhen the pipe wall thickness issmaller, the welding heat will spread tolarger distances from the fusion line,i.e., the region with maximum degreeof sensitization will be further awaycompared to a pipe with thicker wallthickness

IMPORTANT TO PERFORM NDEUP TO APPROPRIATE DISTANCES!

P. Andresen, Haldensummer school 2011

Xavier et al. MaterialsResearch. 2011

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Residual stress issuesThe residual stresses arehigher in smaller diameterpipes (with smaller wallthickness, my assumption)and stretches further awayfrom the fusion lineSteam line ~400x8mm,IGSCC at 8-11mm from FLPrimary piping ~400x20mm,IGSCC at 3-5mm from FL

IMPORTANT TOPERFORM NDE UP TOAPPROPRIATE DISTANCES!

Danko et al. 1989

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Environment issuesThe environment is a water-steam mixture at 170 CThe crack growth rate is higher at intermediate T, i.e., onceinitiated, leak will occur sooner compared to at 288°CMost impurities are non-volatile, and will not be present in thesteam, but in the waterIf water condensates on the surface, this can increase the localimpurity level, which may further concentrate due to repeatedcondensation – vaporization eventsFlushing is minimal in the steam lines

IMPURITIES MAY CONCENTRATE WITH TIME AND MAYINCREASE THE RISK FOR IGSCC IN STEAM LINES WITHOPERATION TIME

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Operation time issuesLTS is not an issue in this case,diffusion is too slow at 170 CDynamic loading is known topromote, or even be aprerequisite, for initiationAlthough the plant in questionhas a good history of steadyoperation, transients occur atleast during shut-down, typicallyonce a year

YEARS OF OPERATIONINCLUDING TRANSIENTS HASRESULTED IN CRACKINITIATION AND CONSEQUENTGROWTH, RESULTING IN THELEAK

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Final conclusionsIGSCC can occur in steam pipe lines

The CGR is higher at lower T compared to 288°CImpurities may concentrate from the water in the water-steam mixtureThe cracks in thinner piping may locate outside the region typical forprimary piping due to weld heat transfer issuesOperation time will increase the potential for crack initiation, so this isa LTO issueThe plant has reviewed manufacturing documents and madereplacement plans, starting with high risk welds, i.e., with high carbon& repairs

DO NOT FORGET YOUR PIPELINES AND COMPONENTSWHICH ARE NOT REGULARLY INSPECTED- A LONG PERIOD WITHOUT FAILURES DOES NOT MEANIMMUNITY

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