ML20059C800

Intervenor Exhibit I-MFP-92,consisting of LER 1-92-016-00, Re Dockets 50-275 & 50-323,dtd 921130

ML20059C800
Person / Time
Site:	Diablo Canyon
Issue date:	08/19/1993
From:	Rueger G PACIFIC GAS & ELECTRIC CO.
To:
References
OLA-2-I-MFP-092, OLA-2-I-MFP-92, NUDOCS 9401060091
Download: ML20059C800 (6)

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92. bgara hg'/fc2. , gggg Pacific Cas and Electric Company 77Se eS" > G'epy f/ Rep

.i Sr Hn::s:o CA 94106 $e J Vice Pres' dent and Q NPA-T2- w m.wa Gunmy .

tu.a Poe Gre:3Mn November 30, 1992 53 0' E 'c 11 PG&E Letter No, DCL-92-264 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C. 20555 2d Re: Docket No. 50-275, OL-DPR-80 Docket No. 50-323, OL-DPR-82 Diablo Canyon Units 1 and 2 Licensee Event Report 1-92-016-00 Feedwater Flow Control Bypass Line Snubber Failure Due to Stress Corrosion Cracking Gentlemen:

PG&E is submitting the enciosed voluntary Licensee Event Report (LER) concerning a feedwater flow control bypass line snubber failure due to stress corrosion cracking (SCC). Following a structural inspection walkdown that identified the failed snubber, inspection into the root cause was initiated. Material analysis subsequently has shown that the snubber verge wheel failed due to SCC and that the SCC developed as a result of a combination of material, environment, and stress. The verge wheel failure apparently resulted in the snubber locking in place and then buckling under subsequent compressive thermal loading.

While this event was determined to be not reportable, PG&E believes that the information in this LER may be of benefit to the industry and other utilities that may have the possibility for a similar event. Therefore, this report is submitted for information purposes only as described in Item 19 of Supplement 1 to NUREG-1022.

This event has in no way at'fected the health and safety of the public.

Sincerely,

) 9401060091 930839 DR ADOCK 0500 Np- 5 Gregory 'i. Rugr cc: Ann P. Hodgdon rr om mum to'm:a John B. Martin , , , 7 y,g ,g g

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UCENSEE EVENT REPORT (LER) '99993.9 FActWTY NAMI(11 Ew)C A TT NWMM (U P A< f 41 3 DIABLO CANYON UNIT 1 0l5l0l0l0l2l7l5 1l'l 5 mta m FEEDWATER FLOW CONTROL BYPASS LINE SNUBBER FAILURE DUE TO STRESS CORROSION CRACKING EVENT DATE [8) Ltyt tim m REPOMT DATE (78 OTHER FACILITIES INVOLVED (8)

MON DAY YR YM S(QUENTIAL REM 6 SON MON DAY VM DOCF.ET NLSA8ER (8)

Ntsassa Nwarn DIABLO CANYON UNIT 2 04 RA 25 G

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0l0 11 30 92 TH13 R(PORT 15 5UBMIT TLC PUR$UANT TO Ynt REQUt aEME NT5 DF 30 CFR; (11) 0 5 0 0 0 1 -

LNt 10 CFR

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1l0l0 X OTHER - VOWNMR Y (Specify in Abstract below and in text, WRC f orm 366A)

UCfwSEF. CONT ACT FOM TMS (fP (1U Yt(( PHON [ M N@(9 DAVID P. SISK, SENIOR REGULATORY COMPLIANCE ENGINEER ^"'t**

805 545-4420 COMPtTTE ONE UNE FOR EACH COMPONfMT F AILLRE DESCMBED 194 TMS R& PORT 031 CAuSE SYSTEN CCMPONE N T MA FAC- O POR E CAJ5E $,5 FEM CtNPohENT MANUFAC. E PO E A Sl J Sl Nl 8l Al 3l 9l 1 N l lll lll I lll lll l lll Ill svPPtauENT At Rtroni EmCTED oel EXPECTED "I" DA' 'E^"

SUBMISSION l l YES (if yes, conplete EXPECTED SUBMISSION DATE) {X l WO DATE W l 4

A95 TRACT (16)

This voluntary LER is submitted for information purposes only as described in 1 Item 19 of Supplement I to NUREG-1022.

l 1

On May 14, 1992, with Unit 1 in Mode 1 (Power Operation) at 100 percent power, during a PG&E structural inspection walkdown of components on the Unit 1 pipe )

rack. a mbber at pipe support 1-171SL on a main ':edwater flow contr ' bypass  !

line was found damaged. The snubber had apparently locked in place during a plant trip on April 25, 1992. Subsequent compressive thermal loading caused the snubber to buckle, The snubber is an Anchor Darling 50ll with a 10-inch (nominal) stroke.

Investigations have concluded that failure of an internal part (verge wheel) caused the snubber to lock in place. Failure of the verge wheel was due to stress l corrosion cracking. l All ex.su.ng Anchor Darling snutaers in the sa...e u.ivironment (io:atac vatdoors) will be replaced or overhauled and functionally tested. During the procurement of  ;

new snubbers, if a snubber is to be reused, its interior surfaces will be cleaned, ,

degreased, sealed and environmental protective covers (boots) . zialled.  !

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LICENSEE EVENT REPORT (LER) TEXT CONTINUATION 1 9 9 9 3,9 4 FACILITY NANC (1) DOCkli NUMBER (2) LES M 8f8 16) PAGE (3)

YEAA 96M&W R$WEEKM DIABLO CANYON UNIT 1 0l5l0l0l0l2l7l5 92 -

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1. Plant Conditions  :

Unit I was in Mode 1 (Power Operation) at 100 percent power, t II. Description of Event A Summary:

On May 14, 1992, with Unit 1 in Mode 1 at 100 percent power, during a PG&E structural inspection walkdown of components on the Unit 1 pipe rack (*PRK), a snubber (SJ)(SNB) at pipe support (SJ)(PSX) 1-171SL on  ;

a main feedwater (MFW) flow control bypass line (SJ) for Steam Generator (SG) 1-1 (AB)(HX) was found damaged.

B.

Background:

i Technical Specification (TS) 3.7.7.1 requires that all snubbers shall l be operable. The only snubbers excluded from this requirement are those installed on nonsafety-related systems, and then only if their f ailure, or f ailure of the system en which they are installed, would have no adverse effect on any safety-related system.

' This TS requirement is applicable in Modes 1, 2 (Startup), 3 (Hot Standby), 4 (Hot Shutdown), 5 (Cold Shutdown) and 6 (Refueling) for snubbers located on systems required operable in those Modes. l TS 3.7.7.1 requires that with one or mc c snubbers inoperable on any system, within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> replace or restore the inoperable snubber (s) to operable status and perform an engineering c'taluation on the attached component or declare the attached system inoperable --d t folloo the appropriate act;on statement for tLt system.

l C. Event

Description:

On May 14, 1992, with Unit 1 in Mode 1 at 100 percent power, during a PG&E structural inspection walkdown of components on the Unit 1 pipe

rack, a snubber at pipe support 1-171SL on the 6-inch MFW flow control bypase 'in' ^ Sr ' 1 as fo H W ged ar.d w s renlaced that una day. '

Based on a review of the as-found condition and the Unit 1 operating history, it was determined that the snubber had apparently locked in place during the plant trip of April 25, 1992 (reported in LER l '

004-00, DCL-92-124, dated May 20, 1992). Subsequent compressive thermal loading caused the snubber to buckle. The rack (SJ)(RK) and i the shroud (5J)(*SD) were bent approximately 30 degrees and the annular stroke position indicator (SJ)(PIC) was cracked. The snubber i is an Anchor Darling 50ll with a 10-inch (nominal) stroke.

105BS/85K

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DIABLO CANYON UNIT 1 0l5l0l0l0l2l7l5 92 -

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0l0 3 l'l5 D. Inoperable Structures, Components, or Systems that Contributed to the Event:

k ne.

E. Dates and Approximate Times for Major Occurrences:

1. April 25, 1992: Event date. Snubber at pipe support 1-171SL apparently locked in place. Compressive thermal loading caused the snubber to buckle during the plant trip.

2. May 14, 1992: Discovery date. Snubber at pipe support 1-171SL was found damaged and was replaced the same day.

F. Other Systems or Secondary Functions Affected:

None.

G. Method of Discovery:

The damaged snubber at pipe support 1-171SL was found during a PG&E structural inspection of components on the L' nit 1 pipe rack.

H. Operator Actions:

None.

I.  : fety System Responses:

l None. .

III. Cause of the Event l A. Immediate Cause:

Failure of the verge wheel caused the snub %r tc lock in place *n an

'ucrfed position.

Comore>sive saermal (oning 01 ne MFW iine during the April 25, 1992, reactor trip apparently caused the snubber to buckle.

B. Root Cause:

The root cause of this event is stress corrosion cracking (SCC) of an internal .nart (verge wheel) due to a combination of the following three factors:

1. Material. The verge wheel is made of 440C stainless steel, heat treated to a high strength level. This material is susceptible 1058S/85K to SCC when expose' to inclement environment. ,

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2. Environment. The snubber was located outdoors in coastal atmosphere that is humid and contains chloride salts. Water i intruded into the snubber, as evidenced by the rust found on some internal components including the vr.rle wheel. Chh ride in i solution with water can cause SCC in high-strength steels.

3. Stress. Out-of-tolerance conditions of the pinion gear and the verge wheel hole caused high hoop stresses in the interference fit connection. Stresses in the failed verge wheel due to this out-of-tolerance condition were calr.ulated to be 250 percent higher than the maximum stresses cxpected based on the manufacturer specifications.

The plant trip of April 1992, which caused a full thermal cycle on the 6-inch MFW line, induced the buckling of the snubber. There was no damage to any other components in the area.

Conclusion:

The out-of-tolerance pinion hole, which created high hoop stresses in the verge wheel, combined with the environment (ie. water, salt air) and material susceptible to SCC, caused the verge wheel to fail.

IV. Analysis of the Event A PG&E evaluation of the MFW bypass piping determined that the MFW system was operable with the damaged snubber in place. Subsequent to the identification of the snubber failure, several walkdowns were performed by Engineering and Mechanical Maintenance personnel to inspect the MFW bypass piping and adjacent feedwater lines for evidence of hydraulic phenomena that may h-"n overloaded the sn"S5ar. No indications of damage were noted.

An analysis was performed to determine if the MFW flow control bypass piping had been damaged due to this snubber failure. The governing load case was the thermal stress induced in the system assuming the snubber to be frozen at its hot setting and the pipe to be inhibited from reaching the cold position. The stress, strains, and loads calculated were not high enough to prevent tha sys'-m from cortin"ing to perform its des'gr functicq. t ,1 The locations of the calculated high stress points were nondestructively examined and no indications were found. The MFW flow control bypass piping was judgaJ operable. The snubber was r:placed r the engir.cering evaluation satisfied the requirements of TS 3.7.7.1. Therefore, this event bas in no way affected the health and safety of the public.

10585/85K

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION 1N' .

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0l0 5 l"l5 V. Corrective Actions A. Immediate Corrective Actions:

1. The damaged snubber at pipe support 1-171SL was replaced.

2. The entire MFW bypass line was visually inspected.

3. NECS Engineering pc-rformed a stress analysis of the MFW bypass line and determined that it was operable. '-

4. Inservice Inspection personnel performed an nondestructive examination on some sections of the MFW bypass line and found no indications.

J B. Corrective Actions to Prevent Recurrence:

1. During the Unit I fifth refueling outage, all existing Unit 1 Anchor Darling snubbers located outdoors were removed and functionally tested. All these snubbers passed their functional test requirements. A design change was issued to replace these snubbers with PSA snubbers. This design change was implemented on some snubbers using available PSA snubbers. The remaining Anchor Darling snubbers were either replaced with new Anchor Darling snubbers or internally cleaned, degreased, sealed, and

, tested. Environmental protection covers (boots) were installed on all the snubbers located outdoors.

2. During the Unit 2 fifth refueling outage, all existing Unit 2

' Anchor Darling snubbers located outdoors will be replaced or overhauled and functionally tested. A design change "ill be issued to replace these snubbers with available PSA c 'hers.

i The remaining snubbers (if any) will be either replaccc _ith new Anchor Darling snubbers or internally cleaned, degreased, sealed, and tested. Environmental protection covers (boots) will be installed on all the snubbers located outdoors.

. VI Add:+ ion;l Informstian A. Failed Components:

Componen'- Snubber Manufacturer: Anchor Darling Model: 50ll with 10-inch (nominal) stroke a '

B. Previous LFDs on Similar Problems:

I None.

1058S/85K

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