ML19259D382

Revised Final Deficiency Rept Re Potential Excessive Water Hammer Forces in Main Feedwater Sys.Util Failed to Evaluate Water Hammer Forces at Design Stage.Main Check Valves Analyzed.Valves Adequate for All Accident Conditions

ML19259D382
Person / Time
Site:	Sequoyah
Issue date:	10/10/1979
From:	TENNESSEE VALLEY AUTHORITY
To:
Shared Package
ML19259D374	List: ML19259D373 ML19259D382
References
NCR-MEB-79-1, NUDOCS 7910180399
Download: ML19259D382 (2)

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P00R UstiEL ENCLOSURE SEQUOYAH NUCLEAR PLANT UNITS 1 AND 2 POTENTIAL EXCESSIVE WATER HAMMER FORCES IN THE MAIN FEEDWATER SYSTEM NCR MEB 79-1 REVISED FINAL REPORT Description of Deficiency Westinghouse i'nformed TVA of the potential for excessively high water hammer forces associated with the main feedwater line check valves.

This deficiency involves excessively high water hammer forces gener-ated by slamming of the three main feedwater line check valves as a result of a postulated pipe rupture in the other main feedwater line.

The water hammer force caused by slamming of the three check valves might be high enough to adversely affect the integrity of those three check valves or feedwater piping and hence lead to a multiple steam generator blowdown.

The failure of TVA to evaluate in detail at the design stage the capability of the maia feedwater check valves and feedwater piping to absorb the water hammer forces associated with a closure of those valves following a postulated break in one leg of the main feedwater tystem piping resulted in this deficiency.

Safety Implications If this deficiency had remained uncorrected, a main feedwater line break in the feedwater line to one steam generator would have had no adverse effect on the safety of the plant. A line break could lead either to a single steam generator blowdown or loss of the main feed-water system's ability to supply the four steam generators, dependent on break location. However, these accidents have been analyzed (see FSAR chapter 15) and the ability of the plant to reach a safe shutdown condition is not adversely affected.

Corrective Actions IVA has completed a detailed study of the main feedwater check valves installed at Sequoyah' Nuclear Plant. Evaluations of the check valves were made using two procedures. The first procedure was a comparison of the energy level that the Sequoyah feedwater check valves are capable of sustaining with the energy levels that similar design check ' calves used in the main steam system at several other TVA nuclear facilities and at other non-TVA nuclear facilities are

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capable of sustaining. The second procedure of the evaluation con-sisted of analysis of the main feedwater check valve discs by equating the disc strain energy to the disc kinetic energy at closing. A fluid transient analysis of the pipe break / water hammer event was performed to determine the disc Kinetic energy at closure. This evaluation shows the closing kinetic energy to be less than the potential strain energy capacity of the disc and a stress slightly above yield at the disc center. This study has shown that the main feedwater check valves at Sequoyah would maintain their integrity and would continue to func-tion as designed following the most severe main feedwater line break postulated. Based on these studies and analyses, TVA is satisfied with the adequacy of the Sequoyah main feedwater check valves to per-form their intended function under all expected accident conditions.

The allowable pipe peak pressure calculated in accordance with ASME Section II, paragraph NC-3611.2 is 3,300 psi, compared to worst case calculated transient pressure of 2,750 psi. Circumferential pressure stress produced by the calculated 2,750 psi pressure is 26,097 psi com-pared to a minimum material yield stress of 30,000 psi at system operating temperature.

Means Taken to Prevent Recurrence ,

The analyses of the main feedwater line check valves for all of TVA's future nuclear plants will be documented at the design stage. A veri-fication of the capability of the main feedwater check valves to sus-tain the water hammer forces resulting from a closure of those valves following a postulated break in one leg of the main feedwater system piping will be made and documented for each of TVA's nuclear plants now under construction.

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