05000445/LER-1990-028, :on 900908,automatic Reactor Trip Occurred Due to Lightning Strike

:on 900908,automatic Reactor Trip Occurred Due to Lightning Strike

ML20065G452
Person / Time
Site:	Comanche Peak
Issue date:	10/09/1990
From:	William Cahill, Mcgee G TEXAS UTILITIES ELECTRIC CO. (TU ELECTRIC)
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-90-028, LER-90-28, TXX-90338, NUDOCS 9010220016
Download: ML20065G452 (7)

LER-1990-028, on 900908,automatic Reactor Trip Occurred Due to Lightning Strike

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv), System Actuation
4451990028R00 - NRC Website

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=M Log # TXX-90338

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g Ref. # 50.73(a)(2)(iv) 2 filELECTRIC October 9, 1990 s

% litism J. Cahill, Jr.

farcutove i ur henient U. S. Nuclear Regulatory Commission i

Attn: Document Control Desk Washington, D. C.

20555 SUBJECT: COMANCHE PEAK STEAM ELECTRIC STATION DOCKET NO. 50-445 MANUAL OR AUTOMATIC ACTUATION OF ANY ENGINEERED SAFETY FEATURE t

LICENSEE EVENT REPORT 90-028 00 f

Gentlemen:

Enclosed is Licensee Event Report 90-028 00 for Comanche Peak Steam Electric Station Unit 1, " Automatic Reactor Trip Caused by Lightning Strike."

Sincerel,

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fot, William J. Cahill, Jr.

DEN /daj Enclosure c - Mr. R. D. Martin, Region IV ResidentInspectors,CPSES(3)

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At 1428 on September 8,1990, Comanche Peak Steam Electric Station Unit 1 was automatically tripped from 38 percent power. A lightning strike is believed to have caused a surge in the input power resulting in the de energization of power supplies in the rod drive system, causing the rods controlled by one rod control cabinet to drop into the core. This resulted in the reactor trip from high negative flux rate.

No specific component or system failures were identified as the cause of this event. The inst @ation of surgo suppressors in the input supply to rod drive power supplies will provide additione! assurance that power supplies remain available during lightning strikes.

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DESCRIPTION OF THE REPORTABLE EVENT

A.

PLANT OPERATING CONDITIONS PRIOR TO THE EVENT On September 8,1990 at 1428, Comanche Peak Steam Electric Station (CPSES)

Unit 1 was in Mode 1, Power Operation, operating at 38 percent power.

B.

REPORTABLE EVENT CLASSIFICATION

An event or condition that resulted in an automatic actuation of any Engineered Safety Feature (ESF), including the Reactor Protection System (RPS)(Ells:(JC)).

C.

STATUS OF STRUCTURES. SYSTEMS. OR COMPONENTS THAT WERE INOPERABLE AT THE START OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT Not applicable - no structures, systems or components were inoperable at the start of the event that contributed to the event.

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NARRATIVE

SUMMARY OF THE EVENT. INCLUDING DATES AND APPROXIMATE TIMES On September 8,1990, during a lightning storm, two out of ten 25 Voltage Direct Current (VDC) power supply overvoltage protection circuits actuated (located in the rod drive (Ells:(AA)) power cabinets). This actuation resulted in loss of control power to the rod drive system and allowed the rods controlled by the two 25 VDC power supplies to fallinto the core. The dropped control rods actuated the Nuclear --

Instrumentation System (Ells:(JC)) power range high negative neutron flux rate reactor trip. All safety systems responded as expected.

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. _,... v orm w., O n An event or condition that results in an automatic actuation of any ESF, including the RPS is reportable within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> under 10CFR50.72(b)(2)(ll). At approximately 1618 on September 8,1990, the Nuclear Regulatory Commission Operations Center was notified of the event via the Emergency Notification System. The plant was stable in Mode 3, Hot Standby.

E.

THE METHOD OF DISCOVERY OF EACH COMPONENT OR SYSTEM FAILURE OR PROCEDURAL ERROR The overvoltage protection for the 25 VDC rod drive power supplies was found tripped upon investigation of a non urgent alarm on rod drive control cabinet 2AC received at the time of the trip.

II.

COMPONENT OR SYSTEM FAILURES A.

FAILURE MODE. MECHANISM. AND EFFECT OF EACH FAILED COMPONENT Not applicable - no failed components have been identified.

B.

Qd)fE OF EACH COMPONENT OR SYSTEM FAILURE Not applicable - no component or system failures have been identified.

C.

SYSTEMS OR SECONDARY FUNCTIONS THAT WERE AFFECTED BY FAILURE OF COMPONENTS WITH MULTIPLE FUNCTIONS Not applicable - there were no failed components with multiple functions that affected this event.

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FAILED COMPONENT INFORMATION

Not applicable - no failed components have been identified.

1 Ill.

ANALYSIS OF THE EVENT

A.

SAFETY SYSTEM RESPONSES THAT OCCURRED The Reactor Protection System (Ells:(JC)) and Auxiliary Feedwater System (Ells:(BA)) actuated during the event; all associated components within these systems functioned as designed.

B.

DURATION OF SAFETY SYSTEM TRAIN INOPERABILITY Not applicable - there were no safety systems which were rendered inoperable due to a failure.

C.

SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT The event consisted of a multiple control rod drop resulting in a reactor trip. This is bounded by FSAR, Section 15.4.3 and concludes that the successful automatic actua ion of the reactor trip will ensure there is no reduction in the margin to core thermallimits and that the Departure from Nucleate Boiling design basis is met.

Consequently, there were no adverse impacts to plant safety or the health and safety of the public.

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IV. CAUSE OF THE EVENT

ROOT CAUSE The initiating cause of this event is a lightning strike on the containment causing a surge in the 120 Voltage Alternating Current (VAC) input power creating a voltage surge in the 25 VDC output of the rod control power supplies. This surge actuated the overvoltage protection circuit on two 25 VDC power supplies which isolated the output of the supply.

The loss of this output voltage to the rod control system caused the rods controlled by the affected power supplies to fall.

V.

CORRECTIVE ACTIONS

A.

IMMEDIATE The Operator responded appropriately to the RPS actuation. Plant Emergency Operating Procedures were implemented and the plant was stabilized in Mode 3.

B.

CORRECTIVE ACTIONS TO PREVENT RECURRENCE The rod control power supplies tripped from the surge on the 120 VAC input. To prevent this, a design modification to install surge suppressors on the 120 VAC lines feeding for the system has been submitted. The installation of this modification will minimize the potential for an overvoltage trip of the 25 VDC power supplies from a lightning induced surge.

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VI.

PREVIOUS SIMllAR EVENTS Although there have been previous events that resulted in RPS actuation, the root cause of those events were unrelated to the root cause of this event. The corrective actions taken to resolve the root causes of the previous events would not have prevented this event. Therefore, no previous similar events have been reported pursuant to 10CFR50.73.

Vll. ADDITIONAL INFORMATION The times listed in the report are approximate and Central Daylight Savings Time (CDT).

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