ML20024E936

Provides Brief Summary of Util Submittals Re Thermal Shield Problem

ML20024E936
Person / Time
Site:	Maine Yankee
Issue date:	08/30/1983
From:	Heitner K Office of Nuclear Reactor Regulation
To:	Lainas G Office of Nuclear Reactor Regulation
References
NUDOCS 8309070435
Download: ML20024E936 (1)

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Docket File PDR ORB #3 Rdg Memo File KLHeitner MEMORANDUM FOR:

Gus C. Lainas, Assistant Director PMKreutzer for Operating Reactors JRMiller THRU:

James Miller, Chief Operating Reactors Branch #3 Division of Licensing 30) 0 FROM:

Kenneth L. Heitner, Project Manager Operating Reactors Branchif3 Division of Licensing

SUBJECT:

REVIEW OF MAINE YANKEE SUBMITTALS ON THERMAL SHIELD PROBLEM As ypu requested. I have briefly sumarized all the Maine Yankee s ub-mittals on the subject topic. Also, attached is a copy of each sub-mittal for your convenience.

1.

August 9,1983 - Letter - Comunits to removal of core barrel

& thermal shield for inspection, Comits to submit flow blockage analysis by September 9, 1983, 2.

May 25, 1983 - Our letter and JCO.

3.

April 14, 1983 - Letter - Update on Thermal Shield status indicating both actions taken and concurrence with CE safety analysis on shield failure.

4.

February 28, 1983 - Report - Complete report prepared by CE on the dislodged position pins.'at Maine Yankee.

Ngnalsiped by:

Kenneth L. Heitner, Project Manager Operating Reactors Branch #3 Division of Licensing Attachments:

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Docket File PDR ORB #3 Rdg Memo File KLHeitner MEMORANDUM FOR:

Gus C. Lainas, Assistant Director PMKreutzer for Operating Reactors JRMiller THRU:

James Miller, Chief Operating Reactors Branch #3 Division of Licensing G

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FROM:

Kerineth L. Heitner, Project Manager Operat,19 Reactors Branchif3 Division of Licensing

SUBJECT:

REVIEW OF MAINE YANKEE SUBMITTALS ON THERMAL SHIELD PROBLEM As ypu requested I have briefly sumarized all the Maine Yankee s ub-mittals on the subject topic. Also, attached is a copy of each sub-mittal for your convenience.

1.

August 9,1983 - Letter - Commits to removal of com barrel

& themal shield for inspection, Comits to submit flow blockage analysis by September 9, 1983.

2.

May 25, 1983 - Our letter and JCO.

3.

April 14, 1983 - Letter - Update on Themal Shield status indicating both actions taken and concurrence with CE safety analysis on shield failure.

4.

February 28, 1983 - Report - Complete report prepared by CE on the dislodged position pins at Maina Yankee.

EMpalsiped by:

Kenneth L. Heitner Project Manager Operating Reactors Branch #3 Division of Licensing Attachments:

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August 9,1983 E83-167 M.83-202 4,

Y Director of Nxlear Reactor Rqulation thited Stat e fbcieir Regulatory Comhsim

'tishingtcri, D. C. 20555 Attentim: Mr,. Robert A. cluk, Oiief Operating Pas: tors Branch No. 3 Divisim of 1.icairg Rafcrence: (a) Licente No. [FR-M (Docket % 50-309)

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Subjmt: Tne:mlShield i

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This letter is in resconse to a request frm your staff to sutmit additional infomtion rega.rding the Wtine Yenkee trenal shield, I

Darirg the last refoelirg outace a comlete inspection of the them!

snield ns cordrted by remving the core barrel. The mly irdications.cf I

vest noted Wre on three of the nine top positionirg pins which were dislodged. Wear irdicatims and radiation levels en the pins indicate tre j

dislodgenmt was not :ecent. During the next refuelirg outage, we plan to Imove the core barrel and conduct another thorough inspection of the theInal i

I shield.

l We have ccepleted an initial evaluatim of the theImel shield support

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systen integrity using the loose parts monitoring systeni and neutron noise analysis tecMiques. Our emsultants, who have extensive experlerce in

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, similar monitoring progrars, hBYe Conc u el d d that there is no ey

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irc: eased therral shield movement since our last inspection. Based on the results of this analysis, we have concluded that the Maine Yankee theral

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- shield swoort syitem is not experiencing any sigficant cegradatlon, For the rerainoer of this operating cycle, we vill closely monitor the

..a.u ex reacter vessel loose parts system and periodically check neutrm noise to assure that abnaImal thenal shield movement is not occurrirg. If we have 4-eviderce of any siWficant degradatim in the thermal shield support system, h~'

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. we plan to shut the plant doen for a thorough inspection.

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States Mlear Regulatory Comission August'9, ISO y

tention: Rr.RobertA, Clark, Chief Ri-0-167 We have completed a preliminary evaluation of the consequerces of a postulated failure of the themal shield support system.

The evaluation shows that the themal shield rould likely he stopped by the core support barrel snubbers and in any event wou.ld be stopped by tre core stops. The evalmtim

.also indicates that a dropped themal shield V0Jld not have h significant effectoncoreflow.

Weexpecttobeabletocomleteourevaluationby Septeter 9,1983.

We vill continue to keep you informed of our status..

Please feel hee to contret e if you crould ha$,*e any gosstion.in this

metter, Very truly yi. ursi MIfE Y#FfE ATDXIC POE' R C0f#fY N AY 4#ohnH.Carrhty,SeniorDirector J

l Relear Engineering srd 1.icensirg JHG/bjD l '

cc: Dr. Thuus E. Murley Mr.PaulD.Setland l

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UNITED STATES

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i WASHIN4'ON, D.k. 20555 MAY 2 5 1983 Docket No. 50-309 Mr. John H. Garrity, Senior Director Nuclear Engineering and Licensing Maine. Yankee Atomic Power Company 83 Edison Drive Augusta, Maine 04336

SUBJECT:

THERMAL SHIELD SUPPORT INTEGRITY I

3 We have completed our review of your letter dated April 14,1983 on! the status of Maine Yankee's thermal shield.

We requested this letter from you in light of the severe problem with the thermal shield support s.tructure at St. Lucie Unit 1.

A We believe that it is important for you to carefully follow and monitor the course of the investigatiort into the failure mechanism at St. Lucie

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You should 'be prepared to prrovide any add'ifionil in~alyses that may be

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identified as a-result of that investigation to support your justification for continued operation.

In addition you should follow-up your recent submittal with a plant specific analysis to support the assumption that the snubbers and/or the core stops will arrest the fall of the thermal shield so that flow to the core would not be impaired.

We have concluded that continued operation at this time of Maine Yankee is l

justified, based on your submittal and ability to perform loose parts j

monitoring. We have also considered the potential for performing neutron l

noise analysis to detect motion of the thermal shield.

Further justifica-tion is provided by your commitment to perform a limited visual inspec' ion at the next refueling outage.

No response is needed to this 1etter. Our safety evaluation is enclosad.

Sincer y,

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Enclosure:

l Safety Evaluation l

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See next page MwOSM3Q$

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Maine Yankee Atomic Powsr Company cc:

E. W. Thurlow, President Mr. Robert H. Groce Maine Yankee Atomic Power Company Senior Engineer - Licensing Edison Drive Maine Yankee Atomic Power Company Augusta, Maine 04336 1671 Worcester Road Framingham, Massachusetts 01701 Mr. Donald E. Vandenburgh Vice President - Engineering U. S. Environmental Protection Agency Yankee Atomic Electric Company Region I Office 1671 Worcester Road ATTN:

Reg. Radiation Representative Framingham, Massachusetts 01701 JFK Federal Building Bos. ton, Massachusetts 02203 John A. Ritsher, Esq.

{d Ropes & Gray 225 Franklin Street Boston, Massachusetts 02110 State Planning Officer Executive Department 189 State Street Augusta, Maine 04230 Mr. E. C. Wood, Plant Manager Maine Yankee Atomic Power Company P. O. Box 3270 Wiscasset, Maine 04578 Regional Administrator U. S. Nuclear Regula' tory Commisd9an Region I 631 Park Avenue King of Prussia, Pennsylvania 19406 First Selectman of Wiscasset Municipal Building U. S. Route 1 Wiscasset, Maine 04578

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Mr. Paul Swetland Resident-Inspector c/o U. S. Nuclear Regulatory Commission P. O. Box E Wiscisset, Maine 04578 Mr. Charles B. Brinkman l

Manager - Washington Nuclear Operations Combustion Engineering, Inc.

7910 Woodmont Avenue l

Bethesda,. Maryland 20814 0

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a Safety Evaluation Maine Yankee Atomic Power Company Maine Yankee Docket No. 50-309 aina' Yankee 1as request.ed.c submitia letter relatirve to their intentions on'the issue of thermal shield integrity in the light of the damage that had occurred at St. Lucie which was discussed in a meeting on April 12,

-1983.

The-licensee's response dated April 14, 1983 stated that they would:

Follow closely the progress of the investigation of the thermal o

shield situation at St. Lucie, and reflect the knowledge gained in assessments of the imolications of the St. Lucie problem for the Maine Yankee plant.

. Review the conclusions concerning Maine Yankee's thermal shield o

presented in the report of February 18, 1983 in light of the information-'obtained from St. Lucie.

Maintain the loose parts conitoring system (LPMS) operable.

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Review L?MS indications from past operations.

Reflect knowledge gained from the above as appropristMn ;&ans-for

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o internals inspection at the next refueling outage.

Consider the feasibility of additional monitoring of reactor internals o

. performance, especially focused on characterizing internals motions i

through neutron noise analysis and characterization of hydraulic

forcing functions acting on internals.

x Additionally, Maine Yankee concurs with the Combustion Engineering position that reactor shutdown is not warranted, advanced at the April 12 meeting.

Their concurrence is based on:

the detailed knowledge of the conditions of the thermal shield support system and-internals gained.in the recent i

10 year insec.vice inspection, conducted with the internals removed; on i

the knowledge that core flow cessation would not occur even if the thermal shield were to drop because motion would be arrested by either.the bacrel snubbers or core stops; and on the evidence that thermal shield support system distress might start early but progresses gradually over a period of years rather than suddenly or as a' result of a single plant event, thus allowing periodic assessment based on refueling interval, inspections.

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Maine Yankee had previously committed to a limited visual inspection of l'

the thermal shield pins at the next refueling outage.

A representative sample of thermal shield positioning pins and support lugs can be viewed without removal of the core support barrel from the reactor vessel.

This is accomplished by lowering a televisiori camera down through the sur-veillance capsule access holes in'the core support barrel upper flange.

The circumference, of the thermal shield visible through the surveillance r.ur,i

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capsule eccess holes is inci: ate: b;. : e cross hatchec =~ y t-9 -

a attacnec figure.

The representative inspecticn cf

?e thermal scie!c support systet, tnrougn the core support barrel u;cer flange would not

-guarantee that there are no 1:ose or missing positioning pins as four of the remaining upper positioning pins cannot be seen with tnis pro-cedure, but it would show any wear on the thermal shield _ supports

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resulting from excessive motion of the thermal shield relative to the core support barrel.

The reactor vessel bottom head-can also be inspected, partially, through the four television access ho,les-in the lower support structure plates.

The television access holes will pro-vide a view of uo to fifty percent of the reactor vessel bottom head.

The areas of the-reacter vessel bottom head exterior to the flow skirt accessible for inspection are shown in the attached figure.

Eased on a review of the above, the staff concludes that continued operation of Maine Yankee is justified based on their acility to perform loose parts monitoring and the potential for performing neutron noise analysis.

Furtner justification is previded by the ::r.mitment to perform tne limicec visual inspection at the next refueling outage.

Date:

Principal Contributor:

C. 3. Sellers, MTIS S

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Febbuary 28, 1983 MN-83-33 JHG-83-38 t

Director of Nuclear Reactor Regulation United States Nuclear Regulatory Commission Washington, D.C.

20555 Attention: Mr. Robert A. Clark, Chief Operating Reactors Branch No. 3 Division of Licensing

References:

(a) License No. OPR-36 (Docket No. 50-309)

(b) USNRC Letter to MYAPCo dated November 2, 1982

Subject:

Thermal Shield Positioning Pin Report

Dear Sir:

The enclosed report provides a summai,f of the~information regarding---

oislodged thermal shield positioning pins presented at the October 27, 1982 meeting with the staff.

Additionally, radiation measurements have been taken of the three These readings are found at Attachment I to this letter.

recoveIed pins.

These readings tend to support the theory that these positioning pins lost their preload and became disloeged early in-plant life.

We trust that you find this information satisfactory. However, should you have further questions, please feel free to contact us.

Yours very truly, MAINE YANKEE ATOMIC POWER C0bf'ANY l

John H. Garrity, Senihr Director Nuclear Engineering and Licensing

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i JHG:pjp Attachment (1 page)

Enclosure (30 pages) cc: Mr. Ronald C. Haynes h

Mr. Paul A. Swetland bbbE2 0 0 9

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TABLE QF CONTENTS SECTION TITLE

1.0 INTRODUCTION

2.0 CONCLUSI0tl 3.0 C0!iPONENT DESCRIPTION CORE SUPPORT ASSCtBLY 3.1

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3.2 THER!ML SHIELD 3.2.1 COMPONENT A'ID SUPPORT SYSTEM DESCRIPTION 3.2.2 THER:AL SHIELD FIELD INSTALLATION 4.0 INSPECTIO!!

4.1 REACTOR IIITERNALS IllSPECTION SU;'JMRY 4.2 THEP!!AL SHIELD AtlD THER!!AL SHIELD SUPPORT SYSTB1

-4.2.1 POSITI0tlIllG PI!! IllSPECTION RESULTS 4.3 LOCK-BAR INSPECTION 4.4 DISLODGED POSITI0t!ING PIN HOLE I!!SPECTIO!!

4.5 THER!ML SHIELD SUPPORT LUG AllD SUPPORT PINS 5.0 POTENTIAL FAILURE t1ECHA!! ISMS 5.1 LOSS OF PRELOAD EVALUATIO!!

5.2 FLOU INDUCED FORCE EVALUATIOil 5.3 POSITIONING PIN AND SLPPORT LUG LOADS AND STRESSES 5.4 RADIATION INDUCED EFFECTS EVALUATION 5.5 THERI%L EXPANSION EVALUATIO!!

5.6 IMTERIAL DEFECTS EVALUATION

5. 7.

CORR 05 ION EVALUATION 5.8 UEAR EVALUATION 5.9 INSTALLATION EVALUATION 6.0 tiOST PROBABLE FAILURE SCENARIO 7.0 SUlitARY 7.1 I

DATA EVALUATIOil AND ANALYSES 7.2 k.

EVALUATION OF FLOW BLOCXAGE FRQt LOOSE PARTS 7.2.1 EVALUATION OF A POSITIONI!!G PIN AS A LOOSE PART 7.2.2 EVALUATION OF A LOCK-BAR AS A LOOSE PART 8.0 RECOMMBIDATIONS

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1.0 INTRODUCTION

During the September - October 1,982 ostage, the Maine Yankee reaccor internals were removed from the reactor vessel and inspected as part of the required ten ~ year Inservice Inspection. The inspection, conducted by C-E, disclosed that the condition of the reactor internals was nomal with the following exceptions. Two of the nine upper themal shield positioning pins were dis-lodged along with two lock-bars while a third upper themal shield positioning pin was loose in the hole in which it was located. Of the two positioning pins dislodged, c'ne was found outside the flow skirt in the bottom of the reactor vessel, see Figure 5, and the second was found wedged between the core support barrel and the themal shield. Retrieval of the three worn positioning pins was completed on October 27, 1982. The two missing lock-bars were not found and it is suspected that they wore away to metal particles.

Following the discovery of the missing themal shield positioning pins, the scope of the inspection was

.. expanded to include the thermal shield support system in more detail, and other

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associated areas of the reactor vessel internals. All inspections were perfom-3 ed remotely with an undemater television camera and the findings were documented on video tape. The reactor internal structures were previously inspected in 1974 at which time no unusual conditions were noted.

2.0 CONCLUSION

Failure of the th'ree positioning pins was most probably thrresult of a loss of preload followed by buffeting of the positioning pins from hydr.aulic. fptces.

This produced relative motion between the positioning pin and the thermal shield. The loose positioning pins wore as a ressit of flow induced motion.

The inspection revealed no evidence of wear, looseness'or excessive motion at any of the thermal shield support luas or any other positionino pin interface, j

The inspection of the reactor vessel scubbers and core support barrel snubber lugs has shown no wear which would be the result of unanticipated motion of the* core _ support barrel. A review of the inspection results and l

CG-232(II) i

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- evaluations conducted from start-up testing in 1072 throug5 the current

' Inservice Inspection has shown th,at the reactor vessel internals have respond-ed as expected.

Analysis for steady state and transient conditions has shown that loss of the three positioning pins has not detectably changed the response of the themal shield-core support barrel assembly or the themal shield support system. The analytica1' evaluation has confimed that the themal shield, support lugs, and core barrel assembly design is sufficient to prevent relative lateral motion of the thermal shield without any of the remaininc upper positioning pins.

The path a dislodged positioning pin would travel in the flow stream was examined.

It was concluded the positioning pin would nost likely come to rest at the bottom of the reactor vessel, trapped between the flow skirt and reactor vessel.

Transit-in the flow stream beyond the flow skirt is unlikely since the positioning pin must have its axis aligned with the axis of the flow holes in the flow skirt.

Positioning pin travel beyond the flow skirt is improbable due to the size and location of the flow holes and the geome.try and mass of the positioning pin.

Based on these inspection data, analyses, and evaluation of the design and his-torical record's, it was recommended that the utility reinstall the reactor vessel internals and continue plant o'peration with the three positioning pins retrieved

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but not replaced. Further recommendations included; an inspection, at the next i

refueling outage, of the themal shield supports and positioning pins -tha-t are accessible for television camera or boroscope viewing through the surveillance capsule access holes and periodic review and evaluation of data from the loose parts monitoring and neutron noise systems. The plant has been returned to service without replacement of the three positioning pins.

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3.0 C0ilp0t!EIC DESC M PTIO:t The rajor conconents of the reactor ipternals, see Figures 1 and 2, is the core support assembly. This assembled structure consists of the core support barrel, the lower' support structure, the core shroud and the thernal shield.

The major material for the construction is Type 304 stainless steel.

3.l Core Suocort Assenbly The core support assembly is supported at its upper end by the upper flange of the core support barrel which rests on a ledge in the reactor vessel flange.

The lower flange of the core support barrel supports and positions the lower support structure.

The lower support structure provides support for the l'

y core by means of a core support plate supported by columns resting on beam

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assemblies.

The lower support structure also provides guidance for the bottom-l h'

mounted instrumentation to the fuel assemblies.

The core suoport plate provides support and orientation for the fuel assemblies.

The core shroud which envelopes the core and directs the coolant flow for the fuel assemblies is supported by the core support plate.

The lower end of the core support barrel is restrained i

radially by six core barrel to pressure vessel snubbers.

3.2 TilEF11AL SHIELD 3.2.1 Component and Support System Description The three inch thich, Type 304 stainless steel thermal shield is'a one piece cylindrical structure supported by the core support barrel. At the upper end, see Figure 2, the shield is supported by nine equally spaced lugs on the outer periphery of the core support barrel and is positioned radially by nine ecually l

spaced positioning pins which pass through the shield and butt against the core support barrel, see Figure 3.

The lower :nd of the therral shield is positioned i

radially utilizing seventeen equally spaced positioninn pins.

CEH-232(M) f w.

4-The thermal snield positioning pins are 304 stainless steel pins which are installed in the themal shield and preloaded against the core support barrel (CSB). The 2-1/4-16UN-2A threads on the positioning pin which engage in the themal shield are chrome plated and Neolubed prior to installation. The end of the positioning pin which bears against the CSB is hardfaced with Stellite Six.

The bearing area on the CSB is a pad of Stellite Twenty-fivh machined tangential to the CSB.

3.2.2 Themal Shield Fi&id Installation The installation of the themal shield onto the CSB was a field assembly.

The themal shield was jacked vertically to the elevation where the nine support lugs from the CSB bottomed out in the mating slots of the themal shield. The positioning pins were then used to equalize the gap between the CSB anii the thermal shield. With the themal shield concentric to the core support barrel, the five inch diameter themal shield support pins were ' installed and welded to the themal shield. The support pins had been trial fit in the shop and lapped to the mating tapered hole in the thermal shield.

The support pins were machined to provide a.0005 to.002 inch lateral clearance (.see Figure 4) between the support lug of the Core Support Barrel and the mating slot in the support pin. A.000 to.005 inch' veri:ical clearance between the bottom of the themal shield slot and the support lug.on the core support barrel was o.btained by the themal shield support shim (see Figure 4). The positioning pins were then torqued 'to 250 I 15 ft. lbs. and locked into place with 3/8 inch square lock-bars of 304 stainless steel. The lock-bars were rotat2d in the direction of increasing torque on the positioning pin prior to the welding of the lock-bar to the themal shield. The jacks supporting the thermal shield were then removed.

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4.0 INSPECTION 4.1 Reactor Internals Insoection' Summary The reactor internals were inspected as part of the Inservice Inspection of the reactor vessel and the results are detailed in a separate report, reference;

" Reactor Pressure Vessel Inservice Examination for 'faine Yankee Power Company".

Re' port Number IR-ISI-014, (to be issued).

In summary, the rea' tor internals c

did not exhibit any signs of abnonnal motion. Areas of the reactor internals inspected include the snubbers on the core support barrel, the snubber lugs on the reactor vessel the alignment keys in the core support barrel upper flange, and the alignment key slots in the mating components. No conditions were noted which would indicate excessive motion of the reactor internals resulting from the dislodged thermal shield positioning pins.

4.2

'Themal'ShieTd'an?Themal 5hiek Succort System Th'e inspection of the thermal shield support lugs, themal shield support pins, themal shield support pin welds, positioning pins, lock-bars, and three damaged positioning pins and their respective holes iere perfomed using an undenvater television camera and documented on videc tape. The camera system had been shown to meet Section XI ASM.E Code criteria for clear resolution of a one thirty-second of an inch wide black line on an eighteen percent neutral gray background. The video tapes showed the resolution to be much better than the ASME Code requirement. One of the damaged positioning pins had-threads which were worn from approximately the pitch diameter in, one area to below the minor diameter in another area as observed by going around the circumference of the positioning pin. Machined features on this positioning

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pin of less.than ten thousandths of an inch were readil'y discernible.

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. 4.2.1 Positionine Pin Inscection Results The ccmponents in the thermal shield support system were identified by an alpha-numeric designation which was stamped into each component during fabric-ation. The support lugs were identified by a number "One" through "Nine".

The upper positioning pins were designated "R" through "Z", see Figure 5.

The lower positioning pins were identified by letters "A" through "Q".

The positioning pins which were dislodged were at locations "V", "X",

and "Y".

A description of the condition of these positioning pins follows:

Positioning pin letter "V" was installed in the upper end of the thermal shield at the 150 degree location (see Figure 5) with the angular position being defined relative to zero degrees on the reactor vessel. When the onsitioning pin became dislodged it fell from the worn threaded hole in the thermal shield, falling outside the thermal shield to the bottom of the reactor vessel where it became lodged between the flow skirt outer surface and the reactor vessel bottom head, the location from which it was subsequendy retrieved. As shown~irr ~~

r Figure 6, a groove is circumferential1y worn in the positioning pin 'V" originating just below the bottom of the lock-bar slot. With the exception _of an area of remaining threads which are worn from the pitch diameter to the minor diameter, all of the threads are worn from the pin. The area of daxinun wear on the positioning pin is shown at the top of the fioure where tSe threaded section was worn nearl'y to the ibvel of two inch dianeter section. The harifaced end, opposite the slotted end of the positioning pin, is worn at a shallor angle, extend-j l

ing two thirds of the way across the two inch diameter with no apparent-decrease in overall length. The worn areas on the dislodced positioninn pin as well as

'xy the other two were smooth with norsigns of marks which would be associated with excessive motion of the positioning pin. The smoothness of the unrn areas indicate that the wear' took place over an extender' period of time.

CEl-232(".)

-7 Positiening pin letter "X" was installed at the 230 degree locatien of the reactor internals. The positioning pin, although loose, remained captured in its threaded hole in the thermal shield from which it was recovered.

It can be seen from Figure 6 that the threads have been worn off with the except-ion of a small area at the lock-bar end. The top of the figure shows a hollow worn to approximately the two inch disneter while the' lower lock-bar end of the'. figure shows a rounding of the corner and some extrusion of naterial beyond the lock-bar end of the positioning pin.

There is a groove in the area of the extruded material, apparently worn there by the remainder of the failed lock-bar which shadowed the threaded hole. As seen on positioning pin letter "V", the hardfaced end of positioning pin letter "X" was worn at a shallow angle with no apparent decrease in the overall lenoth. The major portion of t1e lock-bar for location letter "X" had broken off,' however a small piece remained, preventing the positioning pin from becoming a loose w.

part in the flow stream. The snall piece. ofdock-bar_ shadowing._the_ hole __

was' removed to remove the positioning pin. The failed section of lock-bar i

was not recovered after a thorough search and it was postulated to have been reduced to small particles by motion which took place after it fell to the botten head of the reactor vessel.

Positioning pin letter "Y" was installed at the 270 degree location of the reactor internals.

Inspection ' revealed that the lock-bar recained in place l

while the positioning pin became loose, was buffeted in its installation Sole, worn to the point where it became dislodged from its hole and was trapped hori: ental.ly in the thermal shield to core support barrel annulus, belou its installation hole. The positioning pin was subsequently retrieved fron this location. A large amount of wear was visible on the hardfaced end of the

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positioning pin as shown on Figure 6.

The worn end apoears to be snooth and 2*

at an angle of approximately 30' with a semicircular step. The contour of the CE';-232(: )

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step matches the approximate height and shape of the hardfaced surface on the core support barrel which the positioning pin bears against, and likely occurred as the positioning pin worked its way into the annulus inside the thermal shield. The local hollow worn in the positioning pin shown at the top.of Ehe figure and the rounded edge on the lock-bar end are_ consistent with the wear which would be anticipated based on the 30* wear angle as the positioning pin proceded into the annulus between the thermal shield and the core support barrel.

The remainder of the upper positioning pins and the seventeen lower positiening pins were inspected visually. The positioning pins showed no signs of loosening or wear. The six upper oositioning pins were also visually inspected for in-dications of looseness at the core support barrel interface.

Mo gap was jr-visible between the positioning pin hardface and the hardfaced ein on the core support barrel.

4.3 Lock-Bar Insoection

~

Detailed inspection of the lock-bar which remained in position "Y" indicated that the. lock-bar and attachment welds remained intact while some wear was evidentIon'the inner surfaces of the lock-bar. The lock-bars, which were desioned to be 3/8 square by 2-15/16 long, (see Figure 3) were welded in place at -

both ends'with 1/4 inch fillet welds, both sides of each end, for a total N

of fou'r welds. The inspection of the other lock-bars on the six upper positioning pins and the seventeen positioning pins at the botton indicated no. detectable deterior'ation of the lock-bars or welds.

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The wear on the lock-bar at location "Y" occurred on the surfaces which act to prevent rotation of the positiching p'in.

The wear was the appar'ent result of oscillatory rotation as well as lateral motion of the positionina pin. The wear appeared uniform or similar on each end of the lock-bar with the maximum wear estimated to be approximately 1/16 inch.

The lock-bar did not fail as the positioning pin went inward, away from the lock-b'ar, to become lodged in the ' annulus formed by the core support barrel and thermal shield.

4.4 Dislodoed Position Pin Hole Inspection The positioning pin holes in the thermal shield appeared to have been worn below the major thread diameter. The exterior of positioning pin holes letters "V" and "Y" appear circular while the hole at location letter "X" is worn elliptical to approximately the three inch ccunter bore diaceter along the major axis of the ellipse.

Inside the thermal shield, the position-7~.--

ing pin holes.were worn more noticeably. The holes were elliptical and the edges of the holes were rounded, particularly in locations designated by letters "X" and "Y".

4.5 Themal Shteld'Succort tuo and Succort Pins The thermal shield support lugs were inspected for any detec' table changes in the close gap at the bottom of the support lug to the botton of the support l'ugs slot in the thermal shield and the thernal shield support pin to support

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. lugs gaps, see Figure 4.

.The gaps appeared as fabricated indicati_ng.that_the thermal shield had not experienced excessive motion which would lead to near and an increase in the gap sizes. The support pin welds attachinn the support pins to the thermal shield were visually inspected. The welds appeared sound with no indications of deterioration.

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5.0 POTENTIAL FAILURE MECHANISMS A number of potential fai. lure mechanisms were evaluated to establish the cause of the failure.

These failure mechanisms include:

- Loss of Preload on the Positioning Pins

- Flow Induced Vibration

- Positioning Pin and Support Lug Loads and Stresses

- Radiation Induced Effects

- Thermal Expansion: 1.e., Thermal Shield - Positioning Pin - Core Support Barrel

- ilaterial Defects

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- Corrosion 3

- Hear A.\\

- Installation 5.1 Loss of Preload Evaluation i

Loss of positioning pin preload was evaluated and determined to be the most probable failure mechanism. The loss of preload acainst the core suoport barrel would leave the positinnina pin threads unloaded. Turbulence in the ficu around the thermal shield positioning pin would produce lateral loads on the positi'oning pin causing it to move in the threads of the thermal shield, resulting in wear of the threads in,the installation hole and on the positioning pins. Given sufficient time, the wear would progress to the point whe're the positioning pin could move axially, latarally, rotationally, causina failure

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of the lock-bar. The positioning pin would then no longer be captured in the thernal shield.

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5. 7.

Flow Induced Force Evaluation Evaluation of flow induced forces on the themal shield support systen has indicated that it was not the failure mecnanism.

Flow induced forces on the themal shield and CSB could cause relative deflection of the themal shield which would heatendencytoloadsomeoftheoositioningpinswhileun-loading others. A review of the scratch gage data taken as part of the pre-critical vibration monitoring program to measure the relative motion between the themal shield and the CSB, indicated.007 inches of relative motion midway circumferential1y between the themal shield support lugs and

.002 inches of relative motion at a point a few inches from the support lug.

The calculated deflection of the CSB relative to the shield from the position

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pin prelcad is.030 inches which is well above the deflections seen in the

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pre-critical vibration monitoring prografi.~Tiieiichia~1 cperating hydraulic loads on the CSB and themal shield are equal to or less than the flow loads experienced during pre-critical vibration testing.

5.3 Positionino Pin and Succort Luc Loads and Stresses Evaluation A review of the loads and stresses in the themal shield support system did not result in detemining an over stressed condition which could explain the position-ing pirt failure. The stress calculations for both the support lugs and positioning pins indicate that the positioning pin and support lug stresses are snall in comparison to the allowable values as established by Section III of the ASME Boiler and Pressure Vessel Code. The vertical loads on the thermi shield support lugs consist of the weicht of the themal shield, the axial hydraulic loads on the themal shield and a friction load as the CSB expan'is axially relative to the themal shield.. T'1ese vertical 1qads, when distributed anong the nine support lugs,- produce low stresses.

CEN-232(li)

5.4 Radiation Induced Effects Evaluation Radiation induced;. effects were evaluated and found to potentially contribute to a small redu ' ion in preload. The positioning pins are located in an area of relatively fluence. A high fluence level combined with the relatively low positioning pin stress levels results in a low percentage relaxation in stress. The additional effects of irradiation, the slight loss of ductility and increase in tensile strength, did not contribute to the positioning pin _

failure.

5.5

' Thermal Exoansion Evaluation Evaluation of thermal expansion at the positoning pin, core supoort barrel, '

and thermal shield interface has shown that it increases the load in the positioning pin by a fraction of the preload based on a maximum design temperature difference. The' load from thermal expansion increases the preload stresses in the positioninc pin, but stress intensitie,s remain well below the allowables established in Section III of the ASi!E Boiler and pressure Vessel Code for 304 stainless steel. The maximum effect of axial thermal expansien would be seen at the lower positioning pins which are the farthest frem the support lug. No signs of positioning pin failure were seen at the lower elevation. The calculated stress intensity with the effects of axial expansion of the thermal shield show.the positioning pin to be well within the ASIEE Code allowables.

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5.6 Material Defects Evaluation

~______ __

The reports for the positioning pin material and heat treat were reviewed for any condition which could lead to a naterial defect. The material was accept-able to the ASTil specification requires.ents and no condition was found in'the heat treat history which would tend to produce an unacceptable condition.

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' 5.7 Corrosion Evaluation The corrosion rate for 304 stainles.s steel is so low that it would not lead to a measurable loss of preload or failure of the threads.

?!o sions of unusual corrosion were observed on the failed ~ positioning pins or in other areas of the reactor internals during the remote visual inspection.

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P 5.8 ' Wear Evaluation

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If significant wear took place at the.. positioning 4 pin to CSB interface, there would be a resultant loss of positioning pin preload. \\ecr wear to occur.tnerei has t.o be relative motion between the parts.

The only me. tion which takes'placa at the positioning. pin to CSB interface is the result of the axial differentia 17 themal expansion between the thermal shield and the CSF.

Themdximentenperature

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s difference produces an axial differential growth of a fraction of an inch, at the positioning pin elevation of the thermal ' shield, with the CSB growing.down

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relative to the. thermal shiel... Since this_ temperature differential _

d occurs' during heat up and cool down, the numtier of cycles is low and the assagiated wear is n.egligible.

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5.9 Installation Evaluation

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s A review o'f the themal shield installation procedure indicates no instructions f

which would produce an overstressed condition.

The field installation documentation was reviewed and. no procedural steps were perfomed that unuld result in an overstressed ccndition.

It was determined that the sequence of operations in the.

Y, installation! racedure could have preferentially loaded certain support lugs g _

resulting irt a loss in positioning pin preload at the'affected support ino, 6.0 1105T' PROBABLE FAILURE SCEilARIO Of the potential failure mechanisms discussed, the one whic'n'has the hiohest probability is the loss of positioning pin preload. Therbisnooneeffect CC:-232(';)

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_14 wnich will lead to the loss of preload'by itself, but.the combination of effects

~4 c:uld produce the necessary deflection to eliminate preload in some of the s

upper pcsitioning pins. The combination of effects are as follows:

The core supp' ort barrel operates'.with 'an external static and dynamic pressure difference across the cylinder which tends to reduce the core. support barrel.

diameter. This ef'ect acts to produce a small loss in preload deflection.

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The themal, shield operates with an internal. pressure differential which also s

.. acts ti unibad the preload. The internal pressure is very small, producing an almost' negligible reduction in the preload.

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t The metaod of field installation of the thermal shield also acts to preferentially decretse $he. installation preload..on th'e pcsitioning pins.

The installation of the therral shield support system was completed with the torcuino of the position-ing pins to the design requirements while the weight of the themal shield was supported en jacks. The jacks were:then removed transferring the weight of the t.'.er-nal shield to the support lugs. This weight produces a moment on the succort lug at the core. support barrel which acts to deflect the core support barrel as 7

l shewn in. Figure 7 thus tanding to re. duce preload. The axial hydraulic loads i

on the themal shield and.the loads due to friction as the core support-s i

barrel' expands axially with resp'ect to the themal shield add to the nom,ent.

l 1

If.all of.the support lugs share the axial load equally, the reduction in l-positioning pin prelcad is noty. hat significant, however, during the assembly l

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'of the thermal shield to the core support barrel, it is likely that the vertical

' gap between the thermal shield. support pin and support lun varied'between the drawing tolerance of.000 and. 005. inches with the therial shield supported l

' by the jacks. As the jacks.were removed the support lugs with zero pap at the 1

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top of the support lug would carry the lead until they deflected to the point of gaining support from adjacent support lugs.

This would result in a more significant deflection of the core support barcel at the positioning pin elevation, tending to relieve preload.

9 T,he siiecification tolerance on the applied torque and the coefficient of friction would contribute to a variation of preload.

The contact surfaces were cleaned and the threads double coated with fleolube prior to hand installation of the positioning pins.

These precautions reduce the af#ects of variations in the coefficient of friction. Friction tests conducted at the time of the plant design for stainless steel bolts and mating threads, double coated with tieolube, indicated a tolerance of t 15 percent on the coef#icient of friction. The tolerance on the applied torque was smal., however the contribution from these effects, when taken in the least favorable combination, have the tendency to reduce'preload.

The positioning pins are located in an area of high radiation which makes then susceptible to stress relaxation.

The level of stress in the positionino pins and the radiation exposure were evaluated and detemined not to significantly reduce the preload.

The net effect of the combined contributions was a loss of prelpad to.the point where buffeting by_ lateral hydraulic forces produced. motion of the positioning pins at the threads in the thermal shield and lead to wear.

Given sufficient time, the wear progressed to the point where motion of the positioning pin prbduced wear of the lock-bar leading to its failure and the positioning pin was no longer captured. Some of the factors contributing

~

to a loss in preload such as the installa. tion process, would effect certain CD-232(M)

. positioning pins more tnan others. This ttnis to explain wny there is no symetry t 'the core support barrel centerl.ines of the failure locations and why positioning pin "W" remained as ' installed while the positioning pins "V" and "X" either side of "W" failed, see Figure 5.

7.0 Subt%RY 7.1 Data Evaluation and Analyses The positioning pins failed through wear at the threads.

The type of wear -

i present on the worn positioning pins indicates that the failure process occurred over a substantial period of time. The plant therefore, pas operated for some period of time without the support of these three positioning pins, with no

~ detectab'e degradation to the thermal shield support system or reactor internals.

The visual inspection of the themal shield support system including the seventeen (17) positioning pins at the bottom of the therral shield, showed no evidence of looseness or wear of the remaining positioning pins or the lock-bars.

The inspection of the themal shielti support lug showed no evidence of wear which would occur if the thermal shield were moving with respect to the suoport lug.

Inspection of the reactor vessel snubbers and core support barrel snub 5er vessel internals indicated no unusual wear.

Analysis has been completed which indicates that there is no measurable chance in the frequency of the themal shield or the reactor vessel internals with upper positioning pins missing, whether-the analysis is performed with positioning pins "V", "X", and "Y" or all nine positioning pins missing.

The resiilt's also indicate that the vertical down load on.the therral shield support lugs from the thermal shield weight and flow loads produces a frictional load which is sufficient to prevent lateral motion c.f.ne themal shield. Lateral notion of the thermal shield with respect to the core support barrel, is linited to.002 inch by the cl.earance between the thema1 shield support lugs and the themal shield support pin, see Figures 4 and 7.

The frictional load is conputed to be CEi!-232(*ri) t

sufficient to prevent lateral notion of' the thermal shield therefore the upper positioning pins could be removed to reduce the possibility of their becoming loose parts.

7.2 Evaluation of Flow Blockage From Loose Parts The potential of a loose positioning pin or lock-bar becoming lodged in a location where it would block flow to or affect the heat transfer characteristic I of a fuel assedly was evaluated. An analysis of the potential paths a loose positioning pin could take indicates it is highly unlikely for the positioning Din to reach the bottom of the fuel assembly where it would affect flow to the fuel assembly.

If the positioning pin or lock-har were to reach the botton of the fuel assembly, the flow blockage created would not produce a significant change in heat transfer.

~~

7.2El 1 EvaluawinnJof a.. Positioning pin as a Loose part To potentially cause a flow blockage problen, the positioning pin must reach tne lower end fitting on the fuel assembly. This condition requires that the positioning pin pass through three flow plates and that the positioning pin's t

wet weight be more tnan offset by the hydraulic forces acting on the positioning pin. The positioning pins advance would be prevented by ~the lower end fitting unless the positioning pin was worn to less' than 25 percent of its original diameter, approximately the lock-bar size, and oriented vertically.

The diameter of the positioning pin.is comparable to, but smaller than the flow hole dimensions in the flow skirt, the core support structure bottom

' plate, and the core support plate. The length of the pin is substantially

~

larger than any of the flow hole diameters. To propel the positioning pin through a' series of relatively tight holes reruires that the positioning CE!!-232(il)

18-pin be aligned with its major axis nearly parallel to the axis of the flow holes.

In addition,'were the positioning pin to move throuqh the flow skirt, a minimum upward flow velocity on the order of 17.5 ft/sec is required to lift the pin towards the core support structure when the pin is in a vertical orientation. A smaller upward velocity is required if the pin is in a

~

horizontal orientation but this orientation will prevent the positioning pins advance. Upward flow velocities necessary to support the positioning pin in a' vertical orientation are not available in the general region of the lower plenum, except in the immediate vicinity of the core support structure bottom plate.

!iost probably, as was the case for positioning pin "V", a loose positioning pin would fall to the bottom of the downcomer because of the dounward directert flow and be trapped in the space between the lower end of the flow skirt and the reactor vessel wall.

If the ' positioning pin were to pass through the flou skirt, it would then most likely sink to the bottom of the vessel, because the available upward directed velocities are below the minimum required values to succort its weight.

In the,very unlikely event that a positioning pin was carried by the flow beyond the flow skirt, bottom plate, and the core support' plate (CSP), it would be captured by the lower end fitting (LEF) of the fuel assembly. There, the positioning pin would partially block the csp flow hole and several of the smaller flow holes in the lower end fitting plate. Such a flow blockaoe is expected to have little, if' any, adverse impact on heat transfer. This conclusion is based on the results of a critical heat flux (CHF) test in which the entrance to several' fuel assembly flow channels were blocked, and where no

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effect on the CHF could be detected.

CE?l-232(H) e

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7.2.2 Evaluation of a Lock-Bar as a Loose Part The lock-bar, due to its small size and weight;, could pass through any of the flow holes that it would encounter in the path from its present installed position to the lower end fitting of the fuel assembly. Flow velocities necessary to propel the lock-bar through. flow holes are generally available. However, because the diagonal distance between two corners of the lock-bar cross-section is larger than the diameter of the flow holes in the lower end fitting of'a fuel assembly, the lock-bar cannot enter these flow holes.

If the lock-bar reached the LEF, it would restrict the nost flow

'if it were captured in a horizontal position by the LEF plate where it would block up to five flow holes in the lower end fitting. The situation as described was evaluated and is not expected to have an inpact on heat transfer perfomance of the fuel assembly.

If. the corners of the lock-bar were word down sithat it could pass through the

~~

flow holes in the fuel assembly lower end fitting, the lock-bar would be captured and contained by the leading edge of the lowest spacer grid, which is located just downstream of the lower end fitting, _ As before, no quantifiable impact on heat transfer is expected.

i Given the further assumption that the lock-bar was worn down suff.iciently to pass farther up into the fuel assembly and come into contact with the hot fuel rod at the limiting location, no consequences on criti. cal heat flux is h' P.

expected. This conclusion is based on results from critical heat flux tests \\ \\\\.

in which an unheated rod was bowed to contact with two neighboring heated

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rods. Results showed that there was no impact on critical heat flux due to

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the centact between the heated and unheated rods.

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8.0 RECCtf4EI'DATI0ft Based on the considerations presented in this report, Maine Yankee returned the plant to power without replacement of the missing positioning pins. The following recomendations are made to verify the findings of this report.

During operation over the next fuel cycle, the loose parts monitoring system

- should be used to detect any unanticipated conditions such as, excessive notion of the thermal shield and/or reactor internals which produces an impact. The neutron noise monitoring system should be reviewed since it would provide an indication of motion of the reactor internals even if no impacting is involved.

Based on available data there is no reason to suspect any unusual conditions but use of these monitoring systems will aid in detecting any significant chance.

It is also recon 1 ended that the themal shield support system be visually inspected at the next refueling outage. A representative sample of thernal sh.ield positioning pins and support luqs can be viewed without removal of the core support barrel from the reactor vessel. This is accomplished by lowering a television camera down through the surveillance capsule access holes in the core support barrel upper flange. The circumference of the thermal shield visible through the surveillance capsule access holes is indicated by the cross hatched areas on Figure 9.

The representative inspection of the thermal shield support system through the core support barrel up'per flange would not guarantee that there are no loose or missing positioning pins as four of the remaining upper positioning pins cannot be seen with this procedure, but it would shown any wear on the thema1. shield supports resulting from excessive motion of the thennal shield relative to the core support barrel. The reactor vessel bottom head can also be inspected, partially, through the four television access holes in the lower support structure plates. The television access holes, will provide a view of up to fifty percent of the reactor vessel bottom head.

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The areas of the reactor vessel bottom head exterior to the flow skirt accessible for inspection are shown on Figure 9.

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I!!SPECTED CONDITIO!! 0F DISPLACED POSITIONIt!G PINS

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t-!!EARLY WORN TO 2' DIAMETER ORIGIt!AL LENGTH HARDFACED END Jl W.

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?! ON A ANGLE WITH A GR00VE AROUND END 50!1E ROLLED lETAL PIN y LOCATED AT 150*, R:.tKIEVED FROM BOTTOM OF REACTOR VESSEL.

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ROLLED OVER EDGE PIN X LOCATED AT 230',' RETRIEVED FR0ft ITS INSTALLATION HOLE IN THE THER!4AL SHIELD.

PIN WAS CAPTURED BY A PIECE OF LOCKBAR WHICH PROTRUDED INTO THE' HOLE.

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?IN Y LOCATED AT 270', WAS FOUND HORIZONTALLY LODGED IN THE THERMAL SHIELD / CORE S'UPPORT BARREL ANNULUS BELOU ITS I!!STALLATION HOLE.

STE? AT END APPEAP.S HOPS, NOT FRACTURED.

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