ML18038B996

LER 97-007-00:on 971028,Unit 2 Automatically Scrammed.Caused by Momentary Pressure Drop in electro-hydraulic Control Sys at Turbine Control Valves.Scram Contactor Replaced

ML18038B996
Person / Time
Site:	Browns Ferry
Issue date:	11/25/1997
From:	Deroche M TENNESSEE VALLEY AUTHORITY
To:
Shared Package
ML18038B995	List: ML18038B994 ML18038B996
References
LER-97-007-02, LER-97-7-2, NUDOCS 9712020132
Download: ML18038B996 (10)

Text

NRC FORM 366 U.S. NUCLEAR REGULATORYCOMMISSION APPROVED BY OMB NO. 3150<104 (4-95) EXPIRES 04130rss ESTIMATED BURDEN PER RESPONSE To COMPLY WITH THIS MANDATORY INFORMATION COLLECTION REQUEST: 50.0 HRS. REPORTED LESSONS LICENSEE EVENT REPORT'(LER) I.EARNED ARE INCORPORATED INTO THE LICENSING PROCESS AND FED BACK To INDUSTRY. FORWARD COMMENTS REGARDING BURDEN ESTIMATE To THE INFORMATIONAND RECORDS MANAGEMENTBRANCH (T4 F33). U.S.

(See reverse for requirednumber of NUCLEAR REGULATORY COMMISSION. WASHINGTON,.OC 205554001. AND To THE PAPERWORK REDUCTION PROJECT (3(504(04), OFFICE OF digits/charactersfor each block) MANAGEMENI'ANOBUDGET. WASHINGTON. DC 20603.

FACILITYNAME (1) DOCKET NUMBER (2) PAGE (3)

Browns Ferry Unit 2 050000260 1OF5 Reactor Scram Resulting from Pressure Perturbationin the Electro-HydraulicControl System Caused by Testing ¹1 Turbine Control Valve EVENT DATE (5) LER NUMBER (6) REPORT DATE (7) OTHER FACILITIESINVOLVED(6)

SEQUENTIAL REVISION MONTH OAY YEAR FACILITYNAME DOCKET NUMBER NUMBER NUMBER 25 97 NIA FACILITYNAME DOCKET NUMBER 10 28 97 97 007 00 N/A OPERATING THIS REPORT IS SUBMITTED PURSU ANT.TO THE REQUIREMENTS OF 10 CFR g: (Check one or more MODE (9) 20.2201(b) 20.2203(a)(2)(v) 50.73(a)(2)(i) 50.73(a)(2)(viii)

POWER 20.2203(a)(1) 20.2203(a)(3)(i) 50.73(a)(2)(ii) 50.73(a)(2)(x)

LEVEL (10) 070 20,2203(a)(2)(i) 20.2203(a)(3)(ii) 50.73(a)(2)(iii) 73.71 20.2203(a)(2)(ii) 20.2203(a)(4) 50.73(a)(2)(iv) OTHER rf'>

20.2203(a)(2)(iii) 50.36(c)(1) S0.73(a)(2)(v) Specify in Abstract below 20.2203(a)(2)(iv) 50.36(c)(2) 50.73(a)(2)(vii) or in NRC Form SBBA P!Ihaee(

LICENSEE CONTACT FOR THIS LER (12)

TELEPHONE NUMBER (IIICNde Area Code)

Mark DeRoche, Industry Affairs Specialist (205) 729-7559 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT (13)

CAUSE SYSTEM COMPONENT MANUFACTURER REPORTABLE CAUSE SYSTEM COMPONENT MANUFACTURER REPORTABLE To NPRDS To NPRDS SUPPLEMENTAL REPORT EXPECTED (14). EXPECTED MONTH DAY YES NO SUBMISSION X DATE (15)

(If yes, complete EXPECTED SUBMISSION DATE)

ABSTRACT (Umit to 1400 spaces, i.e. approximately 15 single-spaced typewritten lines) (16)

~

On October 28, 1997, at,1 509 Central Standard Time (CST)I Unit 2 automatically scrammed from approximatelp70 percent power while cycling ¹1 Turbine Control Valve after repair. The scram resulted from an actuation of scram channel B1 caused by a pressure perturbation in the electro-hydraulic control (EHC) system at the turbine control valves. The pressure perturbation was induced by, opening ¹1 turbine control valve. A haif scram was previously present in RPS A due to cycling

¹1 control valve. Subsequent testing showed that, the seal-in contact for one of the contactors in scram channel B1 (5A-K14B) exhibited chattering when subjected to reduced voltage. The scram contactor was replaced. Other corrective actions include procedure revisions to mitigate the consequences of pressure perturbation experienced while testing control valves, the installation. of orif(ces in the EHC lines on the turbine control valves, and conducting a failure analysis of the RPS contactor that was replaced.

TVA is reporting this event in accordance with 10 CFR 50.73(a)(2)(iv), as an event that resulted in automatic actuation of an engineered safety feature including the reactor protection system.

97i2020i32 'T)7i125 PDR ADOCK 05000260 S PDR NRC FORM 366 (4-95)

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NRC FORM 366A U.S. NUCLEAR REGULATORYCOMMISSION Cq (445)

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITYNAME 1 DOCKET KERNSHBER 6 PAGE 3 YEAR SEQUENTIAL REVISION NUMBER 2 OF 5 Browns Ferry Unit 2 05000260 97 007 00 TEXT (Ifmore spaceis mquired, use additional copies of NRC Form 366A) (17)

I. PLANT CONDITIONS At the time of the scram, Unit 2 was at 70 percent power. ¹1 turbine control valve had been closed for post-maintenance testing, resulting in an expected half scram. Unit 3 was operating at rated power and Unit 1 was shutdown and defueled. Units 1 and 3 were not affected by this event.

,II. DESCRIPTION OF EVENT A. 'Event:

On October 28, 1997, at 1509, Unit 2 automatically scrammed while cycling ¹1 turbine control valve after repair.

When ¹1 turbine control valve was cycled for post-maintenance testing, one of the reactor. protection system (RPS) [JC] channel 81 scram contactors dropped out. Opening the control valve resulted in a short duration pressure transient in the electro-hydraulic control (EHC) system [TG] at the control valves. The pressure switch for the ¹2 turbine control valve, which provides an input'to RPS logic when pressure decreases below setpoint, indicating control valve fast. closure, was subjected to the pressure transient. Although the.transient was of such a short duration that the pressure switch did not completely operate, it is believed that the transient resulted in a decreasing voltage to the scram contactors in RPS channel 81. One of the pair of contactors dropped out, although its companion did not.

Since a half scram was already present in RPS A due to ¹1 control valve maintenance, when the 81 channel scram'contactor dropped out, 93 control rods (in groups 1 and 4) scrammed and one of the two backup scram valves operated, venting the scram pilot air header. The consequent drop in pressure in the scram pilot air header resulted in a full scram signal.

The scram resulted in the actuation or isolation of the following Primary Containment Isolation [JE] (PCIS) systems/components.

~ PCIS group 2, shutdown cooling mode of Residual Heat Removal [BO] system; Drywell floor drain isolation valve; Drywell equipment drain sump isolation valve [WP].

~ PCIS group 3, Reactor Water, Cleanup [CE].

~ PCIS group 6, Primary Containment Purge and Ventilation [JM]; Unit 2 Reactor Zone Ventilation [VB];

Refuel Zone Ventilation [VA]; Standby Gas Treatment (SGT) [BH] system; Control Room Emergency Ventilation (CREV) [Vl].

~ PCIS group 8, Tranverse Incore Probe [IG].

B, lno erable Structures Com onents or S stems that Contributed to the Event:

None.

NRC FORM 366 (4-95)

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NRCFO M366A U.S. NUCLEAR REGULATORYCOMMISSION g (4-95)

LtCENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITYNAME 1 DOCKET 'LER NUMBER 6 PAGE 3 YEAR SEQUENTIAL REVISION NUMBER 3OF5 Browns Ferry Unit 2 05000260 97 007 00 TEXT (Ifmote space is iequiled, use additional copies of NRC Form 366A) (1 7)

C. Dates and A roximate Times of Ma'or Occurrences:

October 28, 1997 at 1140 (CST) During plant rounds, a non-licensed operator identified a leak on the EHC system.

October 28, 1997 at 1223 (CST) A senior reactor operator located the EHC leak on the servo valve for ¹1 turbine control valve.

October 28, 1997 at 1305 (CST) The operating crew began power reduction in preparation for closing ¹1 turbine control valve.

October 28, 1997 at 1322 (CST) Reactor power approximately 70 percent. The operating crew closed ¹1 turbine control valve, isolating the EHC leak. The expected half scram in RPS A was received.

October 28, 1997 at 1450 (CST) Repairs to ¹1 turbine control valve completed, satisfactorily.

The valve was re-opened and the associated half scram was reset.

October 28, 1997 at 1507 (CST) The operating crew closed ¹1 turbine control valve for post-maintenance testing. The expected half scram in RPS A was received.

October 28, 1997 at 1509 (CST) Shortly after opening ¹1 turbine control valve, the Unit 2 reactor automatically scrammed.

October 28, 1997 at 1838 (CDT) A four-hour non-emergency report is made to the NRC.

pursuant to 10 CFR 50.72(b)(2)(ii).

D. Other S stems or Seconda Functions Affected:

None.

E. Method of Discove The operating crew received alarms associated with the foll reactor scram.

F. 0 erator Actions:

Operators responded to the scram in accordance with the emergency operating instructions and stabilized the unit in hot shutdown.

G. Safet S stem Res onse The safety systems listed in section IIA of this report responded to the reactor scram as designed. However, immediately prior to the scram, RPS contactor 5A-K14B dropped out prematurely in response to a momentary voltage drop which did not affect the companion contactor, 5A-K14F, or initiate any upstream RPS relays.

NRC FORM 366 (4-95)

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NRCFO M366A U.S. NUCLEAR REGULATORYCOMMISSION

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LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION "FACILITYNAME1 DOCKET 'LER NUMBER 6 PAGE 3 SEQUENTIAL

'EAR REVISION'UMBER 4OF5 Browns Feny Unit 2 05000260 97 007 00 TEXT (Ifmoite space is mquired, use addttional copies of NRC Form 366A) (1 7)

III. CAUSE OF THE EVENT A. Immediate Cause:

The immediate cause of the scram was the dropping out of RPS contactor 5A-K14B with an RPS A half scram previously present.

B. Root Cause The root cause of the scram was the momentary pressure drop in the EHC system at ¹2 turbine control valve as a result of opening ¹1 turbine control valve.

C. Contributin Factors:

None.

IV. ANALYSIS OF THE EVENT The transient was initiated from an unexpected partial scram (i.e., one of the contactors in automatic scram channel B1 dropped out) as a result. of EHC pressure perturbations during turbine control valve testing. The dropping out of this contactor, with RPS A previously tripped; resulted in the actuation of one of.two backup scram valves. The actuation of the backup scram valve caused the scram pilot air header to be vented. The resulting low scram pilot air header pressure signal completed the scram.

On-line corrective maintenance to repair an EHC oil leak was in progress when the scram occurred. While performing post-maintenance testing for repair of an oil leak on the ¹1 turbine control valve, momentary pressure decreases were experienced on the other control valves causing the scram. Such pressure decreases were confirmed by testing to occur any time a control valve is stroked in the test mode. Pressure drops on other control valves during single control valve testing increases the probability of a scram and is undesirable system performance. A recently issued vendor document recommends that restricting oriTices be installed in the EHC oil lines at the control valves to minimize these pressure drops.

RPS contactor 5A-K14B dropped out prematurely in response to a momentary voltage drop whigh did not affect the companion contactor, 5A-K14F, or initiate any upstream RPS relays. Subsequent testing showed that the seal-in contact on relay 5A-K14B exhibited chattering when subjected to reduced voltage. This problem was not found on the other scram contactors. Relay SA-K14B was replaced.

V. ASSESSMENT OF, THE SAFETY CONSEQUENCES All safety systems operated as expected in response to this event. Operator actions were appropriate and consistent with plant procedures. There were no equipment failures during or following the scram that complicated recovery. As a result, there were no threats to public health and safety.

Reactor water level decreased to approximately -35 inches and was recovered by the Feedwater Level Control System [JB), as designed. This is well above the top of the active fuel.

This report is submitted in accordance with 10 CFR 50.73(a)(2)(iv) as any event or condition that resulted in manual or automatic actuation of any engineered safety feature including the reactor protection system.

N RC FORM 366 (4-95)

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, NRCFORM366A U.S. NUCLEARREGULATORYCOMMISSION (4-95)

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITYNAME 1 DOCKET LER NUMBER 6 PAGE 3 YEAR SEQUENTIAL REVISION NUMBER 5 OF 5 Browns Ferry Unit 2 05000260 97 007 00 TEXT (Ifmote space is required, use additional copies of NRC FomI 3664) (17)

Vi. CORRECTIVE ACTIONS A. Immediate Corrective Actions:

Operators responded to'the scram in accordance with the emergency operating instructions and stabilized the plant in hot shutdown.

B. Corrective Actions to Prevent Recurrence:

The scram contactor (5A-K14B) was replaced prior to unit restart. The remaining Unit 2 scram contactors were tested satisfactorily. TVA will conduct a failure analysis of this scram contactor. TVA will revise procedures to mitigate the consequences of the EHC pressure drops experienced while testing control valves.'VA will install orifices in the EHC lines to the turbine control valves. These orifices will control the rate at which the operaling mechanism of a previously closed valve is refilled and re-pressurized when the valve is opened, resulting in smaller pressure decreases at the other control valve pressure switches.

Vll. ADDITIONALINFORMATION A. Failed'Com onents:

None.

B. Previous Similar Events:

On September 16, 1983, during testing of the ¹2 turbine control valve following replacement of the EHC servo, Unit 2 scrammed from approximately 90 percent power. The cause of the reactor scram could not be definitely determined, but the best estimate was 'indicated'urbine control valve fast closure. The investigation concluded that as ¹2 turbine control valve was re-opened and the fast-acting solenoid reset, air in the EHC lines caused a pressure. disturbance which actuated the pressure switches which sense turbine control valve fast closure.

Discussion with a vendor revealed a recommendation for the installation of orifices in the EHC lines. However, .

at that time, the recommendation was limited to intercept valves, so orifices were only installed on the intercept valves. Therefore, implementing that recommendation would not have prevented the October 28, 1997 scram.

Vill.COMMITMENTS.

TVA will install orifices in the EHC lines at the turbine control valves, as recommended by the vendor. This action will be completed for Unit 2 by April 30, 1999 and completed for Unit 3 by November 30, 1998.

1. These actions are being tracked by TVA's Corrective Action Program and are not considered regulatory commitments.

Energy Industry Identification System (EIIS) system and component codes are identified in the text with brackets (i.e.,

[Nq).

NRC FORM 366 (4-95)

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