ACCEISRATED DISTRIBUTION DEMONSTRATION SYSTEM

REGULATORY INFORMATION DISTRIBUTION SYSTEM (RIDS)

ACCESSION NBR:8811080073 DOC.DATE: 88/10/28 NOTARIZED: NO DOCKETFACIL:50-315 Donald C. Cook Nuclear Power Plant, Unit 1, Indiana 6 05000315

50-316 Donald C. Cook Nuclear Power Plant, Unit 2, Indiana & 05000316 ~;~AUTH.NAME AUTHOR AFFILIATION

ALEXXCH,M.P. Indiana Michigan Power Co. (formerly Indiana & Michigan Elef(RECIP. NAME RECIPIENT AFFILIATION

Document Control Branch (Document Control Desk)

SUBJECT: Provides info re degradation of A-193 Grade B6 Type 410stainless steel retaining block studs in check valves.

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Indiana MichiganPower Company

~ P.O. Box 16631Columbus, OH 43216

AEP:NRC:1054

Donald C. Cook Nuclear Plant Unit Nos. 1 and 2Docket Nos. 50-315 and 50-316License Nos. DPR-58 and DPR-74VOLUNTARY REPORT: DEGRADATION OF RETAINING BLOCKSTUDS IN DARLING VALVE AND MANUFACTURING COMPANYCLEAR WATERWAY CHECK VALVES

U.S. Nuclear Regulatory CommissionAttn: Document Control DeskWashington, D.C'. 20555

Attn: A. B. Davis

October 28, 1988

Dear Mr. Davis:

The purpose of this letter is to provide you with informationconcerning recently observed degradation of A-193 Grade B6Type 410 stainless, steel retaining block,.studs in Darling Valveand Manufacturing Company Clear Waterway check valves, installedat the Cook Nuclear Plant. The'bserved condition did,not. resultin 'any check valve failures,,'and we have determined. that, thecondition"was not reportable under Title'0 CFR or„ our technicalspecifications (T/Ss). However, because degradation of the typeobserved at the Cook Nuclear Plant has been of general industryinterest in the past (e.g., INPO Significant Operating ExperienceReport [SOER] 86-03), we have elected to submit this voluntaryreport. A summary of the observed condition and actions we havetaken is provided below.

In conjunction with the performance of other maintenance on 8"Darling Clear Waterway swing check valve (2-SI-151W) installed inthe low pressure emergency core cooling syst: em (ECCS), aninspection of the valve internals was performed in accordancewith the maintenance program that we established in response toINPO SOER 86-03. During this inspection, one of the tworetaining block studs was,found broken and the other cracked. Adiagram of the valve type in question is provided in Figure 1.The"retaining block studs (Part No. 11542-61-5) retain the blocks(Part Nos. 1'1542-60/60-1),that hold the valve disc assembly inplace. As a result of this finding, the" corresponding check *valve (2-SI-151E) in the redundant low pressure-ECCS train was

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Mr. A. B. Davis -2- AEP:NRC:1054

inspected. Again, one of the two retaining block studs was foundbroken and the other cracked. Discovery of this second instanceprompted the expansion of the inspection to all Unit 2 Darlingcheck valves of the same design as those in which the degradedstuds were found. There are 12 valves of this design installedin the ECCS and RHR systems in each unit at Cook Nuclear Plant.All of these valves are classified as pressure isolation

valves'PIVs)

and leak tested in accordance with our IST valve program.They are:

o (4) 10" check valves at the accumulator outlet(SI-166L1, L2, L3, L4)

o (4) 10" check valves ECCS injection to cold legs(SI-170L1, L2, L3, L4)

o (2) 8" check valves low pressure ECCS(SI-151E & W)

o (2) 8" check valves normal RHR (RH-133, -134)

Figure 2 provides a simplified flow diagram which identifies thelocations of these check valves in either unit at Cook NuclearPlant.

Soon after the decision was made to initiate the inspection ofUnit 2 check valves however, Unit 1 went from power operation tohot shutdown (Mode 4) due to an unrelated event. As a result, adecision was made to immediately inspect all Unit 1 check valvesof this design accessible in Mode 4. In Unit 1, the only checkvalves accessible for inspection during the Mode 4 forced outagewere 1-SI-151E, 1-SI-151W, 1-SI-166L1, and 1-SI-166L4. TheUnit 1 inspections found one broken stud in each of the checkvalves installed in the low pressure ECCS (1-SI-151E & W) andstud material with an appearance not typical of Type 410stainless steel in each of the two accessible accumulator outletvalves (1-SI-166L1, & L4).

The continuing Unit 2 inspections identified one additional checkvalve with one cracked stud (2-SI-166L4), and one valve (2-SI-166Ll) in which, although the studs were intact, the stud materialdid not have the appearance typical of Type 410 stainless steel,the material specified on the valve drawing for the retainingblock studs. Both valves are located on the accumulator outlet.A summary of inspection results for the valves inspected in bothUnit 1 and Unit 2 is provided in Table l.Actions Resultin from Check Valve Ins ections

In each of the cases discussed above, the cracked or brokenstuds, or studs of a material having an appearance not typical of

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Mr. A. B. Davis -3- AEP:NRC:1054

Type 410 stainless steel were replaced with A-193 Grade B8 studmaterial. This new stud material is recommended by the valvemanufacturer for this application. In addition, maintenance joborders were initiated to replace all A-193 Grade B6 Type 410stainless steel studs with the new A-193 Grade B8 materialregardless of whether any degradation is currently evident. Thisaction is also in accoidance with the valve manufacturer'srecommendation. To date, retaining block studs in 10 of the 12Unit 2 check valves and 4 of the 12 Unit 1 check valves have beenreplaced with the new A-193 Grade B8 stud material. The studs inthe remaining Unit 2 check valves will be replaced during thecurrent steam generator repair project outage. The studs in theremaining Unit 1 check valves are to be replaced during the nextscheduled outage, with the possible exception of those installedin the low pressure injection lines to the cold legs (1-SI-170Ll,L2, L3 and L4). Service conditions for these valves may not beconducive to the type of stud degradation observed in the othersystems inspected. Westinghouse, who supplied the check valvesunder the original NSSS contract, and Darling, the valvemanufacturer, were advised of the inspection findings discussedabove. Westinghouse is conducting metallurgical evaluations todetermine the root cause of the stud degradation.

Evaluation of Safet Si nificance

With regard to the evaluation of the safety significance of ourvalve inspection findings, the following key factors wereconsidered:

1) The check valves in their as-found condition had notfailed, nor was valve operability impaired.

2) Inadvertent pressurization of a low pressure ECCSsystem is precluded since in each case where a checkvalve with potentially degraded studs was found, at.least two valves were available to prevent back leakagefrom the reactor coolant syst: em.

3) 'The check valves would have performed their intendedfunction (i.e., opened) in the event of a LOCAregardless of whether the retaining block studs hadcompletely failed.

4) Of the 10 valves inspected on Unit 2, three showeddegradation of the retaining block studs and one wasfound to have questionable stud material. Thecorresponding Unit 1 valves which see the same serviceconditions as Unit 2 were inspected and studs replacedwith the new stud material. No anomalies were observedin the remaining Unit 2 check valves inspected.

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Mr; A. B. Davis -4- AEP:NRC:1054

5) The Unit 2 valves in the degraded condition hadfunctioned successfully in passing the required flowduring Mode 5 or 6 operation at the beginning of thesteam generator repair outage. Successful operation ofthese valves during this evolution is equivalent topassing the full flow test normally performed toconfirm valve operability.

Unit 1 was returned to service on September 15, 1988, and theactions discussed above to replace retaining block studs on bothunits have commenced.

This document has been prepared following Corporate procedureswhich incorporate a reasonable set of controls to ensure itsaccuracy and completeness prior to signature by the undersigned.

Sincerely,

M. P. Al xichVice President

ldp

cc: D. H. Williams, Jr.W. G. Smith, Jr. - BridgmanR. C. CallenG. CharnoffA. B. DavisNRC Resident Inspector - BridgmanG. Bruchmann

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TABLE 1

SUMMARY OF ANCHOR/DARLING CHECK VALVE INSPECTIONS

Valve Size Service LocationRetaining BlockStud Condition

Unit 1

1-SI-151E 8 II Safety Injection (SI)To Hot Legs

One Broken S tud 1

1-SI-151W 81I

1-SI-166L1 10"

SI To Hot Legs

Accumulator Outlet

One Broken Stud

Questionyble StudMaterial

Unit 2

1-SI-166L4 10" Accumulator Outlet Questionable StudMaterial

2-RH-133

2-RH-134

2-SI-151E

8 II

8 II

8 II

Residual Heat Removal(RHR) To Cold Leg

RHR To Cold Leg

SI To Hot Legs

OK

OK

One Broken Stud3One Cracked Stud

2-SI-151W 8 II SI To Hot Legs One Broken StudOne Cracked Stud

2-SI-166L1 10"

2-SI-166L2 10"

2-SI-166L3 10"

2-SI-166L4 10"

2-SI-170L1 10"

2-SI-170L4 10"

Accumulator Outlet

Accumulator Outlet

Accumulator Outlet

Accumulator Outlet

Low Pressure InjectionTo Cold Leg

Low Pressure InjectionTo Cold Leg

Questionable StudMaterial

OK

OK

One Cracked Stud

OK

OK

Table 1 Notes:

1) A broken stud is a stud that has completely shearedinto two parts. In each case where a broken stud isreported, the break occurred at or near the plane ofthe interface between the valve body and the retainingblock.

2) Questionable stud material refers to studs that lookedshiny instead of having the black appearance typical ofType 410 stainless steel, the material listed on thevalve drawings for the retaining block studs. Itappears that the "as-found" material is either Type 304or 316 stainless steel.

3) A cracked stud is a stud that has partially sheared buthas not parted into two pieces.

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