ML20148C610

SER Re Potentially Serious Degradation of Offsite Power Sys. Licensee Mod Would Change Loads from Being block-started to Being sequentially-started.Mod Acceptable

ML20148C610
Person / Time
Site:	Arkansas Nuclear
Issue date:	10/27/1978
From:	Lainas G Office of Nuclear Reactor Regulation
To:
Shared Package
ML20148C596	List: ML20148C591 ML20148C610
References
NUDOCS 7811020096
Download: ML20148C610 (2)

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.. O SAFETY EVALUATION REPORT DEGRADED VOLTAGE FROM 0FFSITE POWER SYSTEM APKANSAS NUCLEAR ONE *1 BACKGROUND In telephone conversation: with Arkansas Power & Light Company, the staff became aware of a potentially serious degradation of the offsite power system at Arkansas Nuclear One Unit 1. In the event of a LOCA and the simultaneous starting of all ESF loads onto the offsite power system, the voltaae at the onsite Clast IE 480-volt buses could dip to as low as 72L. According to an analysis performed by the licensee, such a low voltage could jeopardize the control circuit fuses in the 480V motor control centers which are required for the ECCS valves to function, i.e. could result in blown fuses.

DISCUSSION In its letter of October 25. 1978, the licensee proposed to modify the starting circuits of six (6) large Engineered Safety Features (ESF) loads on each 4160-volt Class IE system bus. The reason for this modification as stated in the licensee's letter would be to assure adequate power withouc the need to transfer to the diesel generators.

However, in a teiecon on October 26, 1978, he stated that there could be unacceptable under voltaae conditions that could result ii a loss of safety feature functicns, i.e. some safety loads could not be relief on to function.

The modificat:on would change these loaas from being block started to being sequential started on offsite power. This prm ised change is iderdical to the existir.c design for the ESF star loads when offsite power is not available and power is derivL the onsite diesel generator system. Without the change the lit "5 analysis indicates that an unacceptable under voltage cor.Jiti% would exist which :ould cause 480-volt control circuit fuses to fail. This in turn could result in a loss of the safety system functions.

Enclosed with the licensee's letter were six schematic diagrams whic.1 depict the intended modification. The modification consists of changing some wiring so that the automatic start circuits for both offsite power and onsite power will include the sequencer contacts for that load.

The statf has reviewed these diagrams and has discussed them with the licensee, and we conclude that the modification is acceptable.

In addition, the licensee has proposed tests which will confirm the functional intact of the modifi ation and which we find acceptable.

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2 The licensee has stated that the sequential loading of these selected l

loads is sufficient to maintain the bus voltage at a level above that which could jeopardize safety-related fuses. The staff intends to confirm the licensee's detailed analysis as part of the on-going generic review of " degraded grid voltage."

CONCLUSION Based on the inforrcation supplied by the licensee and our review thereof, we conclude that the proposed modification will reduce the potential jeopardization of the performance of the ECCS required and is acceptable.