ML20043G651

LER 90-008-00:on 900514,determined That Fault Current Could Develop Across Engineered Safeguards 480-volt Bus Output Breakers.Caused by Failure to Perform Adequate Short Circuit Analysis.Deficient Trip Devices replaced.W/900614 Ltr

ML20043G651
Person / Time
Site:	Crystal River
Issue date:	06/14/1990
From:	Boldt G, Stephenson W FLORIDA POWER CORP.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
3F0690-11, 3F690-11, LER-90-008, LER-90-8, NUDOCS 9006200528
Download: ML20043G651 (5)

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1-m coR POR ATiON June 14, 1990 3f0690-11 U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D. C. 20555

Subject:

. Crystal River Unit 3 Docket No. 50-302 Operating License No. DPR-72 Licensee Event Report No. 90 08

Dear Sir:

Enclosed is Licensee Event Report (LER) 90 08 which is submitted in accordance with 10 CFR 50.73.

Should there be any questions, please contact this office.

Sincerely,

/

G. L. Boldt Vice President, Nuclear Production WLR: mag Enclosure xc: Regional Administrator, Region II ,

Senior Resident Inspector 9006200528 900614 N' ~

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On May 14, 1990, Crystal River Unit 3 (CR 3) was in MODE 6 (REFUELING) with the reactor core fully unloaded and the refuel canal flooded. During verification of the electrical design for CR-3, Florida Power Corporation engineers determined that a fault current could develop across the Engineered Safeguards (ES) 480V Bus output breakers which would exceed the design capacity of the installed current interrupting devices. In this situation, l

the ITE 600 Ampere breakers with 00-4 trip device could be outside of their l

design.

l The root cause of this event is the failure to perform an adequate Short Circuit Analysis. This event has no significant im)act on nuclear safety. The Final Safety Analysis Report (FSAR) evaluation of tie potential loss of one ES train remains valid and this event does not significantly increase the l

probability of losing both ES trains simultaneously.

l The deficient trip devices have been replaced and the short circuit analysis i satisfied for these devices. The Enhanced Electrical Calculation Program (EECP) portion of the Configuration Management Program will assure an adequately performed and documented Short Circuit Analysis is available for other systems and components at Crystal River Unit 3.

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,u, a - . m. - o .mu m, EVENT DESCRIPTION At 1600 on May 14, 1990, Crystal River Unit 3 (CR-3) was in MODE 6 (REFUELING) with the reactor (RCT] core fully unloaded and the refuel canal flooded. Florida Power Corporation (FPC) engineers were performing a new Short Circuit Analysis of the plant as part of the Electrical Calculation Enhancement Program.

During verification of the electrical design for CR-3, the utility design engineers determined that a three phase fault current could develop which would exceed the design capacity of the installed current interrupting devices

[ED,57+]. The calculated current at Engineered Safeguard (ES) 480 volt (V) Bus 3A (ED,BV) is 29,812 Am)eres (A) and at ES 480V Bus 3B [ED BV) it is 29,279A.

CR 3 uses ITE 600A brea(ers with 00-4 Trip devices (BKR,57+] in eight service locations associated with the ES 480V Buses 3A and 3B. Testing performed by ITE on its 800A frame breakers (BKR), which employ OD 4 electromechanical trip devices (57+], showed that at currents of 26,000A and greater, the contacts on the breaker welded shut and the breaker failed to open. The contacts of the 800A and the 600A frame breakers are alike and the failure mechanism would be the same for the 600A frame breakers as it was for the 800A frame breakers.

The eight breakers are specified in the Enhanced Design Basis Document (DBD),

Section 4, Electrical, Rev. O page 50 of 78, as having the capability to interrupt current flow up to 22,000A. With the potential fault current in excess of that value, the breakers are used in a situation which could be in excess of  ;

their design capability.

CAUSE

. The root cause of this event is the failure to perform an adequate short circuit ,

fault analysis. The initial short circuit fault analysis from the 1960's time frame did not use the maximum possible 230 KV substation voltage and did not adequately consider the 480V motors as contributing to the short circuit current.

Each of these errors increases the short circuit current available. Using this incomplete information produced the analysis results which indicated the fault current would not exceed the capability of ITE breakers using the OD-4 current interrupting devices.

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[YINT ANALY3IS If one of the loads on either of the 480V ES Buses faulted, the breaker to that load may be incapable of opening and removing the fault from the Bus. The feeder breaker [ED,BKR] to the Bus would then sense the fault current and would open to isolate the affected 480V ES Bus. If the feeder breaker failed to isolate the fault, the ES 4160V to 480V supply breaker (EB,BKR] would open thus de-energizing the Bus and protecting the rest of the electrical system. The ES 4160V buses (EB,BU) remain within the design basis of the Final Safety Analysis Report (FSAR) and DBD. This fault would result in the loss of all loads on the faulted 480V bus. One train of Engineered Safeguard components will be rendered '

inoperable, a condition analyzed in the FSAR.

Separate short circuit failures on both ES 480V Buses is not considered to be credible. The loads affected on ES 480V Buses A and B are provided in Enclosure

1. The single failure criteria was not compromised by this design oversight even though the oversight affects both trains of ES. The probability of a simultaneous failure of both ES trains due to a short circuit on the "A" 480V ES Bus and a short circuit on the "B" ES 480V Bus remains within the FSAR analysis.

CORRECTIVE ACTION .

1) The OD-4 electromechanical trip devices have been replaced with 00-3 or 0D-5 trip devices (57+] which will detect the fault more rapidly and therefore the breakers will interrupt the fault sooner. The breakers have the capacity to handle the short circuit current calculated by the EECP.

The modification was completed prior to CR-3 entering MODE 4 (HOT j SHUTDOWN).

2) The EECP portion of the Configuration Management Program recently implemented at CR-3 will assure an adequately performed and documented short circuit analysis is available.

PREVIOUS SIMILAR EVENTS There have been three occurrences of calculational error or faulty assumption in electrical design at CR 3.

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