ML18096A982

LER 91-004-01:on 910206,turbine Runback from 100% to 90% Power Occurred Due to Overtemp/Delta Temp Signal.Caused by Equipment Failure.Recorders Attached to RPS Comparator Circuits & Failed Capacitors replaced.W/920910 Ltr

ML18096A982
Person / Time
Site:	Salem
Issue date:	09/10/1992
From:	Pollack M, Vondra C Public Service Enterprise Group
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-91-004-01, LER-91-4-1, NUDOCS 9209210054
Download: ML18096A982 (6)

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• Public Service Electric and Gas Company P.O. Box 236 Hancocks Bridge, New Jersey 08038 Salem Generating Station September 10, 1992 U. S. Nuclear Regulatory Commission Document Control Desk Washington, DC 20555

Dear Sir:

SALEM GENERATING STATION LICENSE NO. DPR-70 DOCKET NO. 50-272 UNIT NO. 1 SUPPLEMENTAL LICENSEE EVENT REPORT 91-004-01 This Supplemental Licensee Event Report is being submitted pursuant to Code of Federal Regulations 10CFR50.73. The report has been modified based upon completed event causal factor investigation .

. A. Vondra General Manager -

Salem Operations MJP:pc Distribution 180055

//)/1 The Energy People /yl'i' I 9209210054 920910 95-2189 (10M) 12-89

~DR ADOCK 0500Q??2

NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION 16-89) APPROVED OMB NO. 3150-0104 EXPIRES: 4/30/92 ESTIMATED BURDEN PER RESPONSE TO COMPLY WTH THIS INFORMATION COLLECTION REQUEST: 50.0 HRS. FORWARD LICENSEE EVENT REPORT (LER) COMMENTS REGARDING BURDEN ESTIMATE TO THE RECORDS AND REPORTS MANAGEMENT BRANCH (P-530), U.S. NUCLEAR REGULATORY COMMISSION, WASHINGTON, DC 20555, AND TO THE PAPERWORK REDUCTION PROJECT (3150-0104), OFFICE OF MANAGEMENT AND BUDGET, WASHINGTON, DC 20503.

FACILITY NAME (1)

Salem Generating Station - Unit 1 TITLE l*l Reactor Protection System actuation: Turbine runback on overtemperature delta temp. signal.

EVENT DATE (5) LER NUMBER (6) REPORT DATE (7) OTHER FACILITIES INVOLVED (8)

MONTH DAY VEAR VEAR  :{:{ SE~~~~~~AL t< ~~~~~~ MONTH DAV VEAR FACILITY NAMES DOCKET NUMBER(S) oI 2 ol 6 9 i 9 I1 - al o14 - oI i o19 ii o -~ 2 THIS REPORT IS SUBMITTED PURSUANT TO THE R~QUIREMENTS OF 10 CFR §: (Check one or morB of th* following) (11)

--

OPERATING MODE (9) 1 20.406(c) 50.73(1)(2)(iv) 73.71(b)

I-60.38(c)(1)

- 50.73(1)(2)(v) 73.71(c)

-- x 50.38(c)(2) 60.73(1)(2)(;)

-

-

50.73(1)(2)(viil 60.73(1)(2)(viii)(A)

OTHER ISP11cify in Absrr1ct b*low and in Tt1xr. NRC Form 366A)

- li0.73(1)(2)(ii) 60.73(1)(2)(ilil

......_

LICENSEE CONTACT FOR THIS LER (12) 50.73(1)(2)(vili)(B) 50.73(1)(2)(x)

NAME TELEPHONE NUMBER AREA CODE M.rr. Pollack - LER Coordinator 61 0 19 3 13 19 I - 12~ 'I 0 12 12 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT 113)

MANUFAC- MANUFAC-CAUSE SYSTEM COMPONENT SYSTEM COMPONENT TURER TUR ER I I I I I I I I I I I I I I I I I I I I I I I I I I I I SUPPLEMENTAL REPORT EXPECTED (14) MONTH DAY YEAR EXPECTED

~NO SUBMISSION l YES (If yos, complete EXPECTED SUBMISSION OA TE)

DATE (151 I I I ABSTRACT (Limit to 1400 SpactJs, i.tJ., approximately fiftetJn single-space typewrittt1n lines} (16)

On 2/6/91 at 1511 hours0.0175 days <br />0.42 hours <br />0.0025 weeks <br />5.749355e-4 months <br />, during routine power operations, a turbine runback from 100% to 90% power occurred due to an Overtemperature Delta Temperature (OTDT} signal. The plant was stabilized at 90%

power and remained there pending further investigation. At the time, the power range Nuclear Instrumentation System (NIS) channels were being recalibrated with revised currents. Two of the channels recalibration was complete and a third channel was being recalibrated when the runback occurred. To support NIS recalibration, the NIS channel bistable is placed in the tripped position deenergizing the OTDT turbine runback relay. This gives a 1/4 signal to the protection circuitry for OTDT turbine runback which uses 2/4 relay logic. One of the other channel signal comparators tripped to the "off" condition deenergizing a second turbine runback relay to satisfy the turbine runback relay logic. The root cause of this event is equipment failure. The OTDT runback signal was caused by Resistance Temperature Detector (RTD} faulty connectors in combination with the OTDT setpoint summator higher than normal output signal which was due to failed electrolytic capacitors. Recorders were attached to the RPS comparator circuits; however, the failure mechanism did not recur.

The Unit was subsequently returned to full power operation on 2/8/91.

The failed connectors were identified following a second event (which did not involve a runback since the necessary logic was not made up).

New connectors with gold plated pins have been installed.

NRC Form 366 (6-89)

a LICENSEE EVENT REPORT (LER) TEXT CONTINUATION Salem Generating station DOCKET NUMBER LER NUMBER PAGE Unit 1 5000272 91-004-01 2 of 5 PLANT AND SYSTEM IDENTIFICATION:

Westinghouse - Pressurized Water Reactor Energy Industry Identification System (EIIS) codes are identified in the text as {xx}

IDENTIFICATION OF OCCURRENCE:

Reactor Protection System actuation: Turbine runback on overtemperature delta temperature signal Discovery Date: 2/06/91 Report Date: 9/10/92 This report was initiated by Incident Report Nos.91-084 and 91-612.

CONDITIONS PRIOR TO OCCURRENCE:

Mode 1: Rx. Power 100% Unit Load 1160 MWe DESCRIPTION OF OCCURRENCE:

On February 6, 1991, at 1511 hours0.0175 days <br />0.42 hours <br />0.0025 weeks <br />5.749355e-4 months <br />, during routine power operations, a turbine runback from 100% to 90% power occurred due to an Overtemperature Delta Temperature (OTDT) signal. The plant was stabilized at 90% power and remained there pending further investigation.

At the time, the power range Nuclear Instrumentation System (NIS)

{IG} channels were being recalibrated with revised currents (done every 92 effective full power days). The 1N41 and 1N44 channel recalibrations had been successfully completed and the 1N43 channel was being recalibrated when the runback occurred.

During recalibration, the NIS channel bistable is placed in the tripped position deenergizing the OTDT turbine runback relay. This gives a 1 out of 4 signal to the protection circuitry for OTDT turbine runback which uses 2 out of 4 relay logic for Reactor Protection System {RPS) actuation. One of the other channel signal comparators tripped to the "off" condition deenergizing a second turbine runback relay thereby satisfying the turbine runback relay logic.

The OTDT turbine runback is preset at a 200%/min rate for 1.5 seconds with intervals of 25 seconds between each power reduction. The OTDT runback signal lasted approximately 30 seconds resulting in two (2) runback pulses (approximately 5% power/pulse). After the two (2) runback pulses, the signal ceased and the comparator returned to its non tripped condition.

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION Salem Generating Station DOCKET NUMBER LER NUMBER PAGE Unit 1 5000272 91-004-01 3 of 5 DESCRIPTION OF OCCURRENCE: (cont'd)

The console bezel alarm for OTDT Rod Block and Turbine Runback was lit, due to the 1N43 bistables being tripped, at the time of this event. This alarm therefore went to "reflash" indicating that a second power range channel bistable had actuated. When the operator acknowledged the alarm it returned to a solid light. Also, when the runback occurred the overhead alarm for "EH Runback Oper" alarmed.

The Nuclear Regulatory Commission (NRC) was notified of the automatic actuation of the Reactor Protection System (RPS) {JC}, for turbine runback on OTDT, on February 6, 1991, at 1900 hours0.022 days <br />0.528 hours <br />0.00314 weeks <br />7.2295e-4 months <br /> in accordance with Code of Federal Regulations lOCFR 50.72(b) (2) (ii).

APPARENT CAUSE OF OCCURRENCE:

The root cause of this event is equipment failure. The OTDT runback signal was caused by faulty Resistance Temperature Detector (RTD) connectors in combination with the OTDT setpoint summator higher than normal output signal due to failed electrolytic capacitors.

Investigation, which included monitoring of the channels, after the February 6, 1991 event did not identify the cause of the OTDT actuation. A similar RCS Loop 11 OTDT failure occurred on September 2, 1991; however, an OTDT runback did not occur since the necessary logic (a second channel's bistable tripping) was not made up. After this event, monitoring equipment was again installed; however, additional failures did not occur and the channel was returned to service.

On September 13, 1991, the channel failed to midscale and remained failed. The RTD was replaced after this event. Investigation identified faulty RCS Loop 11 TH t and Tcold RTD quick connect connectors which were subsequent£y replacea. They were found to have increased resistance readings which were creating artificially high loop Tavg readings. Also, the OTDT setpoint summator was found to have fauity electrolytic capacitors which could have caused intermittent AC noise (spiking) that would result in an increased analog output signal. The combination caused a trip signal to a bistable for one of the four (4) channels in the OTDT runback logic.

Therefore, with the 1N43 channel bistable in the tripped condition, on February 6, 1991, the two (2) out of four (4) relay logic was met.

ANALYSIS OF OCCURRENCE:

Turbine runback, accomplished by the turbine electrohydraulic control (EHC) system {EG}, requires an auxiliary contact closure for initiation. The contact closure is part of the RPS OTDT circuitry.

Several variables are monitored (and applied) to ensure a departure from nucleate boiling (DNB) does not occur. These include thermal

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION Salem Generating Station DOCKET NUMBER LER NUMBER PAGE Unit 1 5000272 91-004-01 4 of 5 ANALYSIS OF OCCURRENCE: (cont'd) power, coolant flow, coolant temperature, coolant pressure, and core power distribution. The OTDT circuitry is part of the RPS used to mitigate the possibility of DNB. The Reactor Coolant System (RCS)

{AB} coolant temperature, upper and lower core power flux, and RCS coolant pressure develop the OTDT setpoint for turbine runback or reactor trip. In this event, the turbine runback output (of the dual signal comparator for OTDT) failed for the event duration. No other control room indications occurred which would indicate a problem with the actual RCS delta temperature channels or the OTDT setpoint channels which input to the signal comparators. The NIS work was coincident with the event and contributed to it by satisfying the 1 out of 4 logic to the RPS.

The Control Room indications which input to the OTDT circuitry were reviewed by System Engineering. This review concluded that plant parameters had remained normal prior to the February 6, 1991 event and did not contribute to the cause of the event.

The turbine runback signal was the result of equipment failure not a plant abnormal condition. The turbine runback occurred in accordance with design. Therefore, this event did not affect the health or safety of the public. However, since an automatic actuation of the RPS occurred, this event is reportable to the NRC in accordance with Code of Federal Regulations lOCFR 50.73(a)2) (iv).

CORRECTIVE ACTION:

A review of the procedure used for generating the current values, used in the recalibration of the power range channels, was performed. No discrepancies in procedure use, the resultant current values, or the procedure itself were identified.

Recorders were attached to the RPS comparator circuits (inputs and outputs) to determine which channel had momentarily failed; however, the failure mechanism did not recur. The Unit was subsequently returned to full power operation on February a, 1991 at 0130 hours0.0015 days <br />0.0361 hours <br />2.149471e-4 weeks <br />4.9465e-5 months <br />.

As discussed previously, a similar RCS Loop 11 OTDT failure occurred on September 2, 1991. After this event, monitoring equipment was again installed; however, additional failures did not occur and the channel was returned to service. On September 13, 1991, the channel failed to midscale and remained failed. After this, the faulty connectors were identified and replaced.

After the September 2, 1991 event, the OTDT setpoint summator was internally inspected and the failed capacitors were identified and replaced.

The procedure for RTD connector assembly was revised to address the

(,

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION Salem Generating Station DOCKET NUMBER LER NUMBER PAGE Unit 1 5000272 91-004-01 5 of 5 CORRECTIVE ACTION: (cont'd) recently implemented design change which installed replacement RTD's and new connectors. These new connectors are upgraded components with gold plated pins which decrease the possibility of the type connector failure experienced in this even~~

eneral Manager -

Salem Operations MJP:pc SORC Mtg.92-101