ML19253B416

Final Deficiency Rept Re Steam Generator Water Level Ref Column Error.Caused by Design oversight.Low-low Water Level Setpoint Will Be Adjusted Before Unit 1 Fuel Loading

ML19253B416
Person / Time
Site:	Sequoyah
Issue date:	09/27/1979
From:	TENNESSEE VALLEY AUTHORITY
To:
Shared Package
ML19253B414	List: ML19253B413 ML19253B416
References
NCR-MEB-79-32, NUDOCS 7910150524
Download: ML19253B416 (1)

Text

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ENCLOSURE SEQUOYAH NUCLEAR PLANT UNITS 1 AND 2 STLiM GENERATOR WATER LEVEL REFERENCE COLUMN ERROR NCR MEB 79-32 10 CFR 50.55(e)

FINAL REPORT Description of Deficiency The steam generator (SG) secondary side water level instrumentation utilizes a reference water -alumn external to the SG but inside the SG enclosure. The column has a condensate reservoir at the top with the top portion of the reservoir connected to the SG steam space. A dif-ferential pressure transmitter is connected between the bottom of the column and the SG water space. The column thus provides a reference water column of constant height, to which the water depth in the SG is compared through the transmitter. In the event of a high energy line break in the containment, the reference column could become heated and the reduced water density could cause the SG water level to be indicated erroneously high.

In the event of a main feedwater line break, this would delay actuation of the,SG low-low level signal which initiates a reactor trip. A timely reactor trip is assumed in the plant safety analysis of a main feedwater ,

line break.

The deficiency results f rom a Westinghouse design oversight. Westinghouse reported tt, deficiency to TVA and other Westinghouse plant owners, and has also reported it to NRC under 10 CFR 21.

Safety Implications Although there are several redundant reactor trip signal sources, the steam generator low-low water level signal is the primary one assumed by Westinghouse in the plant safety analysis for a main feedwater line break. A delay in the initiation of that signal would therefore introduce a condition not analyzed in the safety analysis.

Corrective Action Westinghouse has completed the analysis of the main feedwater line break, considering the effect on the steam generator water level reference column.

They have determined that a low-low water level setpoint of 21 percen; of full scale will ensure reactor trip within the time assumed in the safety analysis. This setp; at will be implemented in the pisnt design hardware, and a corresponding technical specification change is ueing written.

These corrective actions will be completed before fuel loading of unit 1.

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