ML24128A035
| ML24128A035 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 05/07/2024 |
| From: | Marshall T Tennessee Valley Authority |
| To: | Louis Mckown Office of Nuclear Reactor Regulation, NRC/RGN-II, Document Control Desk |
| References | |
| IR 2024090, EA-24-008 | |
| Download: ML24128A035 (1) | |
Text
Tennessee Valley Authority, Sequoyah Nuclear Plant, P.O. Box 2000, Soddy Daisy, Tennessee 37384
May 7, 2024
U.S. Nuclear Regulatory Commission Region II Attn: Mr. LouisMcKown Branch Chief - Division of Reactor Projects Marquis One Tower 245 Peachtree Center Avenue, NE., Suite 1200 Atlanta, GA 30303-1257
Sequoyah Nuclear Plant, Units 1 and 2 Renewed Facility Operating License Nos. DPR-77 and DPR-79 NRC Docket Nos. 50-327 and 50-328
Subject:
Apparent Violation Supplemental Information (Tennessee Valley Authority -
Sequoyah Nuclear Plant)
Reference:
EA-24-008, Sequoyah, Units 1 and 2 - NRCInspection Report 05000327/2024090 and 05000328/2024090 and Preliminary Greater-Than-Green Finding And Apparent Violation, letter dated March19,2024
In the referenced letter, the Nuclear Regulatory Commission (NRC) documented a finding and associated apparent violation that hasbeenpreliminarilydetermined to be of greaterthan very low safety significance (greater-than-Green, or GTG)regarding a failure of the 1B Diesel Generator. The Tennessee Valley Authority(TVA) was provided with the opportunityto request a Regulatory Conference or to submit a position on the proposed GTGfinding in writing.
TVA and NRC participated in a Regulatory Conference on May 2, 2024, in which TVA presented information regarding the apparent violation.
The purpose of this letter is to provide additional Informationin support of TVAs position.
Documents providecorrespondence between TVA and the 1B Diesel Generator vendor, Engine Systems, Inc. (ESI), regarding further investigation of the September 2023failure. The documents were not available prior to the NRC Significance and Enforcement Review Panel (SERP) conducted in February 2024.
U.S. Nuclear RegulatoryCommission Page 2 May 7, 2024
There are no new regulatory commitments in this letter. If you have any questions, please contact Mr. Rick Medina, Site Licensing Manager, at (423) 843-8129 or rmedina4@tva.gov.
Sincerely,
Thomas B. Marshall
Site Vice President Sequoyah Nuclear Plant
Enclosures:
1.ESI Response - TVA Lash Adjuster Potential Part 21, dated April 19, 2024 2.TVA Response to ESI Response, dated April 24, 2024 3.ESI Response - Plausibility of Lash Adjuster Failure Scenario, dated April 29, 2024
cc: NRC Regional Administrator - Region II NRC Region II Branch Chief NRC Senior Resident Inspector - Sequoyah Nuclear Plant NRC Document Control Desk Enclosure 1
ESI Response -
ENGINE SYSTEMS, INC. _________________ __________
175 Freight Rd., Rocky Mount, NC 27804 Telephone: 252/977-2720 Fax: 252/446-1134
ESI Response - TVA lash adjuster potential Part 21 Page 1 of 2
To: Tennessee Valley Authority James Edmonds
Ref: Lash Adjuster P/N 40118613 (D/C 05D)
TVA PO# 00001690, release 00403 ESI SO# 3001209
Date: April 19, 2024
The root cause evaluation associated with TVA report SQN CR 1900849 indicates the lash adjuster assembly internal spring had a pre-existing crack formation and failed due to fatigue. This was screened as a potential Part 21 reportable issue and ESI was asked to review the root cause and provide a formal, written response with ESIs position, including any counter arguments. The purpose of this letter is to provide ESIs response to the root cause.
ESI acknowledges the comprehensive analysis performed within SQN CR 1900849 and in general agrees with many of the conclusions. However, there are several key items that, in ESIs opinion, disagree with the final conclusion and call into question the deduction of the lash adjuster as the cause.
These items are discussed below:
Thread damage to rear valve bridge adjustment screw The upper 13 threads of the adjustment screw were undamaged and damage occurred only to the lower 13 threads. This indicates the adjustment screw was in the fully retracted position before the event occurred since 13 exposed threads coincides with a fully retracted adjustment screw position. For a typical installation approximately 6 threads are exposed above the valve bridge.
Rapid loosening of the locknut ESI believes the likelihood for the adjustment screw locknut to loosen due to a failed lash adjuster is low (ESI is not aware of an instance). The probability is even lower for the locknut to rapidly loosen in the timeframe required of the TVA technical report. The report indicates locknut loosening occurred after the failed lash adjuster caused the spring seat to fracture which led to the dropped exhaust valve. The dropped exhaust valve would have caused near immediate catastrophic engine damage that resulted in an engine shutdown. In this scenario, the loosening would have taken place over the relatively brief time from dropping the valve until engine shutdown. For a typical installation, the adjustment screw must rotate approximately 7 turns to reach fully retracted position. ESI finds it unlikely for the locknut to loosen and for the adjustment screw to turn 7 revolutions in this timeframe.
With regard to the lash adjuster spring, ESI has not reached the conclusion of a reportable defect with the lash adjuster supplied on TVA PO# 00001690, release 00403. While the TVA report indicates there may have been a preexisting fatigue crack in the lash adjuster spring, it is not conclusive to ESI this condition existed. There are no known lash adjuster failures due to an internal spring fracture nor is ESI aware of any other failures of lash adjusters from this specific batch, of which qty 527 were supplied.
Quality Performance With Pride ENGINE SYSTEMS, INC. _________________ __________
175 Freight Rd., Rocky Mount, NC 27804 Telephone: 252/977-2720 Fax: 252/446-1134
ESI Response - TVA lash adjuster potential Part 21 Page 2 of 2
Based on a review of the information obtained throughout this evaluation process, ESI has been unable to conclude the issues identified are reportable per 10CFR part 21. ESI will continue to assist in any further analysis pursuant to the power assembly failure.
- Regards,
John Kriesel
Engine Systems Inc.
Engineering Manager
Quality Performance With Pride Enclosure
To:EngineSystemsINC JohnKriesel
Ref:ESIResponseTVAlashadjusterpoten alPart21(April19,2024)
MPRle er(0047 0059 LTR001,Rev.0)
MPRcalcula on(0047 0059 CALC 001,Rev0)
Date:April24,2024
TheESIreviewoftheTVAlashadjusterpoten alPart21associatedwithrootcauseevalua onreport SQNCR1900849determinedthatitcannotbeconcludedtheissuesiden edarereportableper10CFR Part21.ThiswasbasedonndingsthattheLashAdjusterfailureduetoacrackedinternalspringisthe onlyreportedfailureforthebatchof527providedunderPO00001690andinconclusiveevidenceof whenthecrackwaswas Inaddion,theESIreportprovidedcounterargumentstoconsider regardingTVAsconclusionthatthelashadjusterfailurewastherootcausefortheEDG1B2Engine Failure.ThepurposeofthisleeristoprovideESIaddi onalinformaonandsolicitESIsinputon plausibilityofaLashAdjusterfailurebeingapossibleiniatoroftheEDG1B2Enginefailure.
Part21Disposion:
TVAagreesthattheLashAdjusterfailuredoesnotmeettherequirementstoreportper10CFRPart21.
First,withESIsndingsthattheLashAdjusterfailureduetoacrackedinternalspringistheonly reportedfailureforthebatchof527providedunderPO00001690,itisevidentthisisanisolatedevent.
Second,thereisnoconclusiveevidencethatamanufacturingrelateddefectexistedinthespringwhen ESIprovidedthea ectedlashadjustertoTVA.Instead,afaguecrackini atedandpropagatedinthe springduetospecicEDGservicecondi onsover me,mostlikelyasaresultofstressesexperienced duringnumerousEDGstarts,includingfaststarts.Thetermpreexisngusedtodescribethefa gue crackintheTVArootcausereportwasintendedtoconveytheconclusionthatthecrackwaspresent,but withthespringnotyetcompletelyfractured,whentheEDGwasstartedthedayofthefailurein September,2023.Thecrackcon nuedtopropagateandthespringul matelyfracturedatsomepoint duringthefailurerunandiniatedallofthevalvetraincomponentdamageexperiencedduringthefailed run.TheLashAdjusterwasinstalledin2005.Ifamanufacturingdefectwaspresentatthe meof installaon,theEDGwouldhavefailedmuchsoonerthantheeventin2023.TheEDG1B Bhad approximately2,000hoursofoperaonwithmul pleidleandfaststartsinthatmeframe.Itis acknowledgedthatthepublishedLashAdjusterservicelifeis16,000hoursbasedonEMDtes ngof locomo veengines,whichareoperateddi erentlythanNuclearapplicaons.TheTVADieselProgram Managerwillpresenttheinforma onfromtheevalua onndingstotheEMD/ESIDieselUsersGroup forconsideraontoevaluate .
ESICounterArgumentsforLashAdjusterRootCause:
TVAisprovidingthebelowresponsesforconsideraontoelicitESIsfeedbackonplausibilityofthelash AdjusterfailurebeingthecauseoftheEDG1B2Enginefailure.Sincethecomple onoftheRootCause, MPRAssociateshasprovidedacalcula onandsummaryle erwhichprovidefurtherinsightsonhow theadjusngscrewlocknutmayloosenasaconsequenceofthelashadjusterfailureandallowthe adjustmentscrewtobacko .TheMPRle erandcalculaonareprovidedasreferencea achments.To aideinthediscussion,therapidlooseningofthelocknutwillbeaddressedrst,asitwillbeerexplain thethreaddamagetotherearvalvebridgeadjustmentscrew.
(1)Rapidlooseningofthelocknut:
Basedonfurtherinforma onprovidedbytheMPRcalcula onandESIresponse,itisacknowledgedby TVAthattheRootCausescenariosequenceprocessmaprequiresrevision.Theprocessmapshowedthe locknutlooseninga ertheexhaustvalvedroppedintothecylinder,butevaluaonsdemonstratethat looseningofthelocknutlikelyiniatedsoona erthelashAdjusterfailureoccurred,andtheloosening progressedoverminutes,ifnothours,duringtheEDGruninSeptember(TheEDG1B Bwas approximately4hoursintoa24 hourSurveillancerunwhenshutdownwasini atedbecauseofthe damagecausedbythedroppedexhaustvalve).
Looseningofthelocknutmostlikelyiniatedwellbeforetheexhaustvalvedropping.Whenthelash adjusterfailsandasignicantlocallashisintroduced,theloadingoftheadjustmentscrewandlocknut changedrama callyandimmediately.Theexcessivelashresultsinimpactforcesthataremuchlarger thantheforcesonthescrewduringnormalopera onwithoutafailedlashadjuster(ashighas1,900lbf forafullycollapsedlashadjuster).Thefailureofasinglelashadjusteralsoresultsinunbalancedloading ofthevalvebridge,whichcreatesabendingmomentonthebridge.Thisbendingmomentiscyclical witheachrockerarmactua on.Thebendingmomentresultsinaddi onallateralloadingofthe adjustmentscrewasopposedtopurelyaxialloadingofthescrewduringnormalopera on.The magnitudeofthelateralloadissignicant,ontheorderof600lbfforafullycollapsedlashadjuster basedontheMPRcalcula on.Theaddi onalimpactloadsduetothefailedlashadjusteroccuratthe enginespeedof900rpm,whichisafrequencyof15Hz.Overall,thefailedlashadjusterresultsin signicantvibraonalloadingoftheadjustmentscrewandlocknutinbothaxialandlateralorientaons.
Boltedconnec onsaresuscep bletoselflooseningduetovibra onalloading,inparculardueto lateralloads.Thisisespeciallytrueforsituaonswherefastenerpreloadismarginal.Thelocknutis usedtoestablishpreloadintheadjustmentscrew;thispreloadcreatesanelascclampingforcethat keepsthelocknutfromlooseningduringopera on.Thecalculatedpreloadfortheadjustmentscrew withthelocknuttorquedto80 lbisapproximately6000lbf,whichisrelavelylowfora5/8nominal diameterbolt.Fastenersaretypicallypreloadedto2/3to90%oftheirmaterialyieldstrength,with highervaluesusedinapplicaonsthataresuscep bletovibra on.Basedonthis,typicalpreloadvalues forasimilarlysizedboltofthesamealloyandhardness(AISI8620lowalloysteel,95HRBWcore hardness)wouldbeapproximately10,000 14,000lbf(assumingayieldstrengthof60ksi).Thedesignof therockerarm,adjustmentscrew,andlocknutalsoresultsinthescrewengagementlength(thee ec ve distanceoverwhichpreloadisdeveloped)beingrelavelyshort.Theoverallimpactisthatthereisonly 0.7mil(0.0007)ofelascelonga onintheadjustmentscrewbecauseofthe6,000lbfpreload.The smallelongaonmakestheadjustmentscrewsensi vetolossofpreloadduetolocknutcounter rotaon.Acounter rotaonofthelocknutofonly4.6degreeswillresultincompletelossofpreloadin theadjustmentscrew.Thevalvetraindesignhassu cientmargintomi gatelocknutlooseningduring normalopera onbecauseloadsarepurelyaxialandrela velysmall(approximately300700lbf) comparedtothescrewpreload.However,alashadjusterfailureresultsinaddionalimpactloadsof signicantmagnitudethataresubjectedtotheadjustmentscrewandlocknut(andothervalvetrain components)atrela velyhighfrequency.Ingeneral,thelashadjusterfailurecreatesvibraonalloading condionsthatarewell documentedintechnicalliteraturetoiniateanddrivetheselflooseningof boltedconnecons.
Overall,thelooseningofthelocknutandadjustmentscrewlikelybeganearlyinthefailuresequence(as soonasthelashadjusterfailed)andtooksome metoreachtheas foundcondi onwiththelocknut fullyoandtheadjustmentscrewbackedout.Thelooseningcon nuedastheothervalvetrain componentsbecamedamagedandfailed,andthelooseningpoten allyacceleratedwitheach componentfailureduetohigherloads.
(2)Threaddamagetorearvalvebridgeadjustmentscrew:
Determinaonoftheadjus ngscrewlocknutlooseningearlierinthefailurescenario(justaerLash Adjusterfailureversusaertheexhaustvalvedropping)aidesinexplainingtheobserveddierencesin threaddamagetotherearvalvebridgeadjustmentscrew.TheESIconclusionsonthelowerthread damagewouldbecorrectassumingthatthelocknutloosenedandtheadjustmentscrewbackedoutat theveryendofthefailuresequencea ereverythinghadalreadyfailed.Thisishowevernotthecase withdeterminaonthatthelocknutwouldhaveloosenedmuchsoonerinthefailurescenario.The operangloadsbecomemoreabnormal,bothinmagnitudeandorienta on,aslashincreasesandthe resulngimpactloadsalsoincreaseandcauseaddi onalcomponentstofail.Theadjustmentscrewwas closetoorfullybackedoutattheendofthefailurewhenallcomponentshadalreadyfailed.Theloads onthethreadsshouldhavebeengreatestatthispointcomparedtoearlierinthefailuresequencewhen lashandtheresulngimpactloadsweremuchsmaller.Theforcesexertedontheadjus ngscrewwould havebeenlesswhenthelocknutrstlooseneda erlashadjusterfailureversusthemoresignicant forcesasthelashbecamegreater,componentsbegantofail,andtheexhaustvalvedropped(Note:The EDGcon nuedtorunfor2minutesa erthecrankcaseover pressurealarmcamein).Thisexplainswhy thedamagetothelowerthreadsontheadjustmentscrewweremoresevereun ltheadjus ngscrew fullybackedoutagainsttherockerarm.
ResponseRequested:
TVArequestESIprovidefeedbackonthetechnicalmeritsoftheaboveposi onsandtheplausibilityof theLashAdjusterfailurescenario.
- Regards,
JamesEdmonds TVANuclear SeniorProgram anagerEmergencyDieselGenerators Enclosure 3
ESI Response - Plausibility of Lash Adjuster Failure Scenario ENGINE SYSTEMS, INC. _________________ __________
175 Freight Rd., Rocky Mount, NC 27804 Telephone: 252/977-2720 Fax: 252/446-1134
ESI Response - Plausibility of lash adjuster failure scenario Page 1 of 1
To: Tennessee Valley Authority James Edmonds
Ref: ESI Response - TVA lash adjuster potential Part 21 (April 19, 2024)
TVA Response to ESI on SQN EDG 1B2 Failure (rev 00)
MPR letter (0047-0059-LTR-001, Rev.0)
MPR calculation (0047-0059-CALC-001, Rev.0)
Date: April 29, 2024
A conference call took place with teams from TVA, MPR, and ESI on April 25, 2024 to discuss the letters and calculations referenced above. As an outcome to the call, ESI agreed to review the information and provide feedback to the failure scenario presented. ESI has completed our review and agrees with the technical merit of the reports prepared by TVA and MPR and concedes the lash adjuster failure scenario presented is plausible.
- Regards,
John Kriesel
Engine Systems Inc.
Engineering Manager
Quality Performance With Pride