ML20214N812

Safety Evaluation Accepting Mods to Emergency Diesel Generator Load Sequencer Described in .Mods Assure That Loading of ECCS Pump Motors on Emergency Diesel Generators Will Be Sequenced Per Plant Design Basis

ML20214N812
Person / Time
Site:	Duane Arnold
Issue date:	09/11/1986
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20214N804	List: ML20214N800 ML20214N812
References
NUDOCS 8609170036
Download: ML20214N812 (2)

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\...../ SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION ECCS LOAD SEQUENCER MODIFICATION IOWA ELECTRIC LIGHT AND POWER COMPANY -

DUANE ARNOLD ENERGY CENTER DOCKET NO. 50-331

1.0 INTRODUCTION

On August 22, 1985, with the reactor in normal power operation, the licensee determined that the loss of one of the two transformers that supply vital (emergency) power buses from offsite power, concurrent with a LOCA,-could defeat-the load sequencing logic of the emergency diesel generators. The effect would have been that a core spray and two residual heat removal pumps per diesel would have been on demand simultaneously loaded onto the diesel rather than in intended sequence of five second intervals. The essential bus loads, in this potential situation, could have caused the diesel generator to trip during loading. Following this determination on August 22, 1985, both diesel generators were declared inoperable. In the interim, keylock test switches were placed in the

" test" position in the RHR and the core spray pump start logics to ensure that bus vital loads always sequence upon bus transfer. The licensee was asked by the staff to find a permanent solution to the problem of load sequencing onto the diesel generators during emergency conditions. By letter dated May 9, 1986, the licensee submitted the information for a permanent solution.

2.0 EVALUATION In the original design, a loss of offsite power (LOOP) signal is initiated only if the output voltage of both the standby and startup transformers drops to 65% or less of nominal voltage or if a degraded undervoltage condition exists on the emergency bus. Therefore, if the emergency loads were powered by the standby transformer and a loss of coolant accident (LOCA) occurred at the same time as the standby transformer failed, the output voltage of the startup transformer (which was still in operation) would have prevented from sending the loss of offsite power signal to the pump start circuitry. This would have caused all core spray and RHR pumps to load their respective emergency buses sirhultaneously and, thereby, stalling the diesel generators due to excessive load resulting from simultaneous motor starting currents. In the modified design, the relays used in the automatic start circuitry of core spray pump and RHR pumps on loss of auxiliary power have been disconnected to bypass the automatic instantaneous start feature on the pumps and to direct the pump start signal to the appropriate time delay pickup relay. As a result, the pumps will sequence onto their respective buses in their proper order.

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3.0 CONCLUSION

Based on the above evaluation, the staff concludes that in the modified design, the core spray and the two RHR pumps will sequence to the emergency buses in the proper order, following a LOCA signal regardless of the status of offsite power and thereby, ensure that the diesel generators are loaded in accordance with plant design basis. Therefore, the licensee's modifications are acceptable.

Principal Contributor:

N. Trehan Dated: September 11, 1986

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