05000250/LER-1980-024-03, /03L-0:on 801119,flow/feedwater Flow Mismatch W/ Low Steam Generator Water Level in Steam Generator a Caused Reactor Trip.Caused by Broken Valve Stem FCV-3-478 Due to Improper Load Distribution.Stems Replaced

/03L-0:on 801119,flow/feedwater Flow Mismatch W/ Low Steam Generator Water Level in Steam Generator a Caused Reactor Trip.Caused by Broken Valve Stem FCV-3-478 Due to Improper Load Distribution.Stems Replaced

ML17340A758
Person / Time
Site:	Turkey Point
Issue date:	12/19/1980
From:	Pace P FLORIDA POWER & LIGHT CO.
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II)
Shared Package
ML17340A756	List: 05000250/LER-1980-024, Forwards LER 80-024/03L-0 05000250/LER-1980-024-03, /03L-0:on 801119,flow/feedwater Flow Mismatch W/ Low Steam Generator Water Level in Steam Generator a Caused Reactor Trip.Caused by Broken Valve Stem FCV-3-478 Due to Improper Load Distribution.Stems Replaced
References
LER-80-024-03L-01, LER-80-24-3L-1, NUDOCS 8102250678
Download: ML17340A758 (6)

LER-1980-024, /03L-0:on 801119,flow/feedwater Flow Mismatch W/ Low Steam Generator Water Level in Steam Generator a Caused Reactor Trip.Caused by Broken Valve Stem FCV-3-478 Due to Improper Load Distribution.Stems Replaced

Event date:
Report date:
2501980024R03 - NRC Website

text

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REPORTABLE OCCURENCE 250-80-24 LICENSEE 'EVENT REPORT P QE TWO Additional Event Descri tion and Probable Conse uences:

During power operation, a reactor trip resulted from steam flow/feedwater flow mismatch coincident with steam generator low level on the 'A'team generator.

The apparent cause of the reactor trip was valve FCV-3-478,steam generator

'A'eedwater Flow Control valve, failing closed.

The USNRC Operations Center was notified of. the reactor trip via Emergency Notification System (ENS) in accordance with the recgxirements of 10 CFR 50.72.

Following discovery and tightening of a loose connection on the signal converter-associated with the feedwater control valve, the nuclear unit, was returned to power.

The generator was off-line for approximately two and one-half hours.

During the power ascension

stage, feedwater control problems were experienced on all three steam generators.

To stabilize the condition a second feedwater pump was started'o increase the feedwater pressure.

and to help stabilize the level oscillations in the steam generators.

Subsequent to initiation of the second feedwater

pump, secondary system vibration increased".

Based on this occurence,.

load was being reduced in order to remove the unit fxcm the line.

During this load. reduction, a. two inch line connecting, an alternate feedwater source-.from.

fossil Unit No.

2 to the 3B., feedwater,,regulator bypass; line ruptured

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resu1ting,in a.reactor trip,.due.. to steam, flow feedwater flow mismatch coincident.,with low steam generator level on 'C'team generator.

The pipe rupture (through which feedwater was leaking and flashing to steam) was manually isolated within thirty minutes.

The USNRC was again notified of the reactor trip via the ENS..

Investigation revealed that the plug had separated from the stem on valve FCV-3-489, steam generator

'B'eedwater Flow Control Bypass valve.

Repairs to the bypass flow control valve and the ruptured piping were effected and-the nuclear unit was returned to power.

The generator was off-line for approximately 20 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br />.,

I During. power ascension, feedwater control problems were again exhibited by inability to pass flow through FCV-3-478, steam generator

'A'eedwater Flow Control valve.

While evaluation of the situation. was in progress, the load increase was terminated at approximately 90 MWe.

A control problem was. also associated with FCV-3-479, steam.

generator

'A'eedwater Flow Control Bypass valve, in that the controller would only respond to permit flow between 80 and 100 percent.

The unit was removed from the line..

Investigation revealed that the stem had broken away from the plug in the 3A Feedwater Flow Control valve-and that the 3A Feedwater Flow Control Bypass valve was out of calibration.

Repairs were effected and the unit was returned to power.,

The generator was off-line for approximately 17 hours1.967593e-4 days <br />0.00472 hours <br />2.810847e-5 weeks <br />6.4685e-6 months <br />.

K Public health and safety werenot affected.

This occurence is bounded by the analysis presented in FSAR Section 14.1.1.

During these occurences the auxiliary feedwater system was operable and available.

This report is pursant to TS 6.9.2.b.

Aditional Cause Descri tion and Corrective Actions:

The root cause of. the series of events was a broken valve stem in FCV-3-478, steam generator

'A'eedwater Flow Control valve..

The apparent

MPVAXMMUl VLI~LMlD cUV QV LICENSEE EVENT REPORT PAGE THREE r

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Additional Cause Descri tion and Corrective Actions (cont'd):

cause of the stem failure was improper load distribution between the stem and plug due to the taper on the valve stem.

The original.cage, plug and stem in the Feedwater Flow Control valves were replaced with a modified

cage, plug and stem in accordance with a Plant Change/Modification (PC/M) originated in 1974.

This PC/M provided for strengthening the cage, reducing the size of the orifices, and changing the shape of the plug to go along with the change".in:-the size and shape of the orifice in the cage.

The taper on the field fabricated stem was found following this occurrence"not to have the exacting tolerence on dimensions to distribute load between the stem and plug.

The stem failure due to improper'oad distribution was the most probable cause of the initial trip.

The trip caused the stem to drive back into the plug on the feedwater isolation signal.

This is supported by the fact that three rows of threads above the break in the stem are damaged.

The: feedwater control problems that resulted in the second trip were apparently induced when the. plug dislodged from the stem.

The upward forces on the plug closely approximating the weight of the plug resulted in the oscillation of the. plug in the cage until the level oscillations in the steam generators caused a reactor trip on steam flow/feedwater flow mismatch coincident with low steam generator level on 'B'team generator.

However, this was not discovered because of the visible damage to FCV-3-489 and to the alternate feedwater pipe rupture.

Additionally, the main feedwater valves successfully passed stroke testing following the trips.

The valve stems on all, three valves of both Unit No.. 3 and Unit No.

4 have been replaced.

Additionally, the broken stem in the 3B Feedwater Flow Control Bypass Valve has been repaired.

All connections to the alternate feedwater system from fossil Unit No.

2 have. been removed and caps have been welded in place.

Feedwater Flow Control Bypass valve FCV-3-479 (steam generator A)

FCV-3'-489 -(steam generator B)

Copes-Vulcan Division of Blaw-Knox Co.

4 inch 900 psi USA Standard Model D-100-160-24 Feedwater Flow Control valve FCV-3-478 -(steam generator A)

Copes-Vulcan Division of Blaw-Knox Co.

12 inch 900 psi USA Standard 14 inch inlet and outlet Model, D-100-160-24

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