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ACCESSION NBR:8810070001 DOC.DATE: 88/09/28 NOTARIZED: NOFACIL:50-275 Diablo Canyon Nuclear Power Plant, Unit 1, Pacific Ga

AUTH.NAME .AUTHOR AFFILIATIONSISK,D.P. Pacific Gas & Electric Co.SHIFFER,J.D. Pacific Gas & Electric Co.

RECIP.NAME RECIPIENT AFFILIATION

DOCKET05000275

SUBJECT: LER 88-025-00:on 880830,reator trip due to lo-lo level ingenerator 1-4 following the trip of feedwater pump 1-2.

W/8 ltdDISTRIBUTION CODE: IE22D COPIES RECEIVED:LTR L ENCL SIZE-TITLE: 50.73 Licensee Event Report (LER), Inci ent Rpt, etc.NOTES:

REC1PIENTID CODE/NAME

PD5 LAROOD,H

COPIESLTTR ENCL

1 11 1

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On August 30, 1988, at 2042 PDT, a low-low steam generator water level tripoccurred following the trip of main feedwater pump (HFHP) 1-2. The unit wasstabilized in.Hode 3 (Hot Standby) and the 4-hour nonemergency report required by10 CFR 50.72(b)(2)(ii) was made at 2142 PDT.

Initial evidence indicated that the event was caused by the failure of the one ofthe two redundant speed probes on HFHP 1-2 combined with the speed controllermalfunction detection circuit not detecting the failure and switching to theredundant speed probe. The suspect probe was repl.aced, with a spare and returned tothe vendor for analysis and determination of applicable failure sensing circuitdesign changes. Further investigation now indicates the root cause may be in thespeed control system pneumatic high select modules. Both modules for HFHP 1-2 havebeen replaced. The root cause is still under investigation and a supplementalreport .will be submitted after the root cause is determined and applicablecorrective actions determined.

2313S/0063K

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U.S. NUCLEAII IIEGULATOIIYCOMMISSION

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DIABLO CANYON UNIT 1

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Unit 1 was in Hode 1 (Power Operation) at approximately 99 percent power.

A. Event:

On August 30, 1988, at 2042 PDT, Unit 1 experienced a reactor trip due tolow-low water level in steam generator (SG) 1-4 (AB)(SG). The low-lowsteam generator water level condition occurred as the result of thetripping of main feedwater pump (HFHP) 1-2. HFHP 1-2 (S3)(P) tripped onoverspeed when the speed control system (Love)oy) malfunctioned andincreased the steam supply, which increased the pump speed and resultedin the pump overspeed trip.The unit was stabilized in Mode 3 (Hot Shutdown) in accordance withapproved plant emergency procedures.

The four hour nonemergency report required by 10 CFR 50.72 was made at2142

PDT.'.

Inoperable structures, components, or systems that contributed to theevent:

None.

C. Dates and approximate times for ma)or occurrences.

l. August 30, 1988, at 2041 PDT: HFHP 1-2 tripped.

2. August 30, 1988, at 2042 PDT: Event Date — Reactor tripped due tolow-low water level in SG 1-4.

D.

3. August 30, 1988, at 2142 PDT: 4 hour nonemergency notification toNRC as required by 10 CFR 50.72.

4. August 30, 1988, at 2200 PDT: Unit stable in Hode 3.

Other systems or secondary functions affected:

None

E. Hethod of discovery:

The event was immediately apparent due to alarms and other indications inthe control room.

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F. Operator actions:

When the MFWP tripped,. operators reduced the turbine load in anunsuccessful attempt to reduce the steam demand on the steam generators.Upon receipt of reactor trip alarms, the plant operating personnelentered the appropriate emergency operating procedures and stabilized theunit in Mode 3 (Hot Standby).

G. Safety system responses:

1. Reactor trip breakers (JC)(BKR) opened.

2. The control rod drive mechanisms (AA)(DRIV) allowed the control rodsto drop into the reactor (AC)(RCT).

3. The turbine (TA)(TRB) tripped.

4. Auxiliary feedwater pumps (BA)(MO)(P) started as per design.

5. Main feedwater (SJ) was isolated.

A. Immediate cause:

Reactor tripped due to low-low water level in steam generator 1-4. Thelow-low steam generator level was caused by the overspeed trip of MFWP

1-2.

B. Root cause:

1. Initial Investigation

An initial investigation was performed as part of the Unit 1 restartprocess. The initial investigation indicated 'the root cause of thisevent to be in the speed pickup sensors. This conclusion was basedon the following:

The Electronic Speed Control System was designed and constructedwith two speed pickup sensors and a circuit to detect a pickupsensor failure and automatically transfer control to the backupsensor.

The Electronic Speed Control System vendor, Love]oy, stated thatthey had not received any prior reports of problems with the speedprobe failing in the low state. They stated that the probemanufacturer had indicated the probe should fail in a "High" TTL(Transistor - Transistor Logic) state. Therefore, Love)oy designed

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the malfunction detection circu1try to only recognize this mode offailure. The pickup, 1n this case, failed low and was sensed asunderspeed which resulted in the control system demanding increasedsteam supply thus increasing MFHP speed. The suspect probe wasreturned to the vendor for analysis. of cause of the failure anddetermination of actions necessary to prevent, recurrence.

The vendor supplied technical manual states that the probe should betested by operation of the applicable equipment. The manual did notprovide a m1nimum speed for the test. Since previous observationsof the A and B probes indicated a readout prior to reaching 300 rpm,300 rpm was selected as'the test speed for the root causeinvestigation. Hi th the probes installed in the same positions aswhen the failure occurred, the pump was brought up to 300 rpm. TheA probe did not provide an indicat1on, but the B probe did provide aspeed indicat1on signal. The pump was stopped. The A and B probeswere interchanged and the test repeated. The B probe installed inthe A channel provided indication before the pump exceeded 100 rpmand the A probe installed in the B channel failed to prov1de anind1cation. The probes were interchanged back to their originalpositions and the test repeated. Again the A probe failed toprovide a speed indicat1on. Next the failed. A probe was replacedwith a new probe and the test repeated. The new A probe startedproviding a speed indication before the pump reached 100 rpm. Basedon th1s testing, combined with observations of the pump trip, it wasinitially determined that the A channel circuitry was operable andthat the cause of the lack of speed indication was the failure ordegraded condition of the A probe.

Further 1nvestigation

Based on the results of the. initial 1nvestigation, the vendoranalysis of the probe, and continuing malfunction indications fromthe pump's speed control system, a further investigation ensuresthat the root cause of the pump speed controller problems isident1fied and appropriate corrective actions are taken. Thisfurther 1nvest1gat1on now indicates the root cause may be in thespeed control system pneumatic h1gh select modules. Both modulesfor MFHP 1-2 have been replaced. The root cause is still under1nvest1gation and a supplemental report will be submitted after theroot cause 1s determined and applicable corrective actionsdetermined.

C. Contributory cause:

A contributory cause of th1s event was the low-low steam generator levelreactor trip set point set at the Technical Specification specif1ed15 percent.

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IV.

License Amendment Request (LAR) 88-03 was submitted to the NuclearRegulatory Commission on April 18, 1988, requesting that the steamgenerator low-low water level and low water level coincident withsteam/feedwater flow mismatch reactor trip setpoints be reduced from15 percent to 7.2 percent. Had the set point been set to the licenseeamendment request proposed 7.2 percent, operator intervention may haveprecluded the reactor trip.

A.

V. rr

Safety Analysis

The loss of normal feedwater is a previously analyzed Condition IIevent. The reactor trip on low-low steam generator level and theautomatic start of the two motor driven auxiliary feedwater pumps ensuredan adequate supply of water in the steam generators to provide for thecooldown of the reactor per design. Thus the health and safety 'of thepublic were not affected by this event.

v A

VI.

A.

B. ~

Immediate Corrective Actions:

The speed probe and both pneumatic high select modules were replaced forHFWP 1-2. The failed probe was returned to the vendor for analysis.

Corrective Actions to Prevent Recurrence:

A supplemental report will be submitted after the root cause isdetermined and applicable corrective actions have been determined.

A.

B.

Failed Components: Failed components will be identified in a supplementto this report when the root cause and applicable corrective actions havebeen determined.

Previous LERs:

LER 2-86-012, "Reactor Trip Due to Hain Feedwater Pump 2-2 Trip AfterFailure of the Thrust Bearing Wear Detector Probe", was similar in that areactor trip occurred after the failure of a main feedwater pumpcomponent caused the trip of a main feedwater pump. The correctiveactions for the wear detector probe failure event, since it was the firstfailure of the probe, consisted of replacing the failed probe. Sincethis failure occurred two and a half years later and involves entirelydifferent components that also do not have a history of previousfailures, the corrective action taken as a result of the wear detectorfailure probe would not have been expected to have prevented thisfailure.

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'e A C i0 IIM 444 A/9 43 I

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Pacific Gas and Electric Company 77 Scale Street

San Francisco, CA 94106

415I972 7000TWX 910 372 6587

)

James 0. Shiffer

Vice PresidentNuclear Power Generation

September 28, 1988

PGhE Letter No. DCL-88-228

U.S. Nuclear Regulatory CommissionAttn: Document Control DeskHashington, D.C. 20558

Re: Docket No. 50-275, OL-DPR-80Diablo Canyon Unit 1

Licensee Event Report 1-88-025-00Reactor Trip Due To Low-Low Level In Steam Generator 1-4Following the Trip of Hain Feedwater Pump 1-2

Gentlemen:

Pursuant to 10 CFR 50.73(a)(2)(iv), PGiEE is submitting the enclosedLicensee Event Report concerning a reactor trip due to low-low waterlevel in steam generator 1-4 following the trip of main feedwaterpump 1-2.

This event has in no way affected the public's health and safety.

Kindly acknowledge receipt of this material on the enclosed copy ofthis letter and return it in the enclosed addressed envelope.

S cerely,

t./. D. S fer

cc: J. B. MartinH. H. HendoncaP. P. NarbutB. NortonH. RoodB. H. VoglerCPUCDiablo DistributionINPO

Enclosure

DC1-88-TI-N091

231 3S/0063K/DY/2159

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