ML20085K237

Ro:On 730614,during Surveillance Insp of Low Vacuum Scram Bypass Switches,Meletron Pressure Switches PS-3-263-51A & B Found Drifted Above Tech Spec Limits.Cause Not Determined. Switches Recalibr

ML20085K237
Person / Time
Site:	Dresden
Issue date:	06/22/1973
From:	Worden W COMMONWEALTH EDISON CO.
To:	Giambusso A US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20085K240	List: ML20085K237 ML20085K242 ML20085K246
References
474-73, NUDOCS 8310200360
Download: ML20085K237 (2)

Text

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" . ['Lj$4 'Oy Fug Address t inal Plaza Chgo.]nois ly to: Post Othee Box 767 Chicago, llhnois 60690 50-249 UPU Ltr.d474-73 Dresden Nuclear Power Station R. R. #1 Norris, Illinois 60450 p ._

June 22, 1973 Od1EQ N Y6)Nk Q

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Mr. A. Giambusso g- -

Deputy Director for Reactor Projects 3.$ 'ef 'h k.- Q Directorate of Licensing 'h ,

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Wa shington, D. C. 20545

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SUBJECT:

LICENSE DPR-25 DRESDEN UUCLEAR PO'JR STATION, UNIT #3, SECTION 6.6.B.2 0F TliE TECHNICAL SPECIFICATIONS. .

Dear Mr. Giambusso:

This is to report a condition relating to the operation of the unit in which on June 14, 1973 at 2100 hours0.0243 days <br />0.583 hours <br />0.00347 weeks <br />7.9905e-4 months <br />, two low vacuum scram bypass switches were found to have drifted. Pressure switch 3-263-51A was found to trip at 633 PSIG, and switch 3-263-51B was found to trip at 609 PSIG.

These trip settings are above Technical Specification limits of less than 600 PSIG as described in section 3.1. The reactor was in the run mode and therr:al power was 1800 =cgawatts. This condition was related to Mr. F. Maura of Region III Regulatory operations at 1500 hours0.0174 days <br />0.417 hours <br />0.00248 weeks <br />5.7075e-4 months <br /> on June 15, 1973.

PROBLEM AND INVESTICATION During surveillance inspection of the low vacuum scram bypass switches, the cetpoint of Maletron pressure switches PS-3-263-51A and B were found to have drifted. Pressure switch PS-3-263-51A had drifted to a setting of 633 PSIG, and switch PS-3-263-51B had drifted to a setting of 609 PSIG. The switches were previously checked on December 19, 1972 and were found to be within Technical Specification limits at that time.

The purpose of the pressure switches is to detect when reactor pressure reaches 600 PSIG. When reactor pressure reaches 600 PSIG, the PS-3-263-51A and B switches are designed to open. If condenser vacuum has not reached 23 inches of mercury prior to the opening of the pressure switches, a reactor scram will occur. When reactor pressure is below 600 PSIG, these switches bypass the low condenser vacuum reactor scram.

With the pressure switchies set above 600 PSIG, the reactor operator is allowed to raise pressure above.600 PSIG without sufficient condenser vacuum. Under these conditions a scram should occur, however, with both switches set above 600 PSIG a scram would not take place until one of the switches trips. Since the switches are arranged in a one out M 05000 PDR 9 e

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Mr'. A. Giambusd (3 -

June 22, 1973 i

i of two logic, the scram would not have occurred until the pressure setting of the PS-3-263-51B was reached. The 51B switch would have opened first since its setting was 609 PSIG and the SLA switch setting was 633 PSIG.

Since these switches were open in the safety system circuits, they could not have prevented a scram. Therefore, the continued safe operation of the unit was not jeopardized.

An investigation into the problem was conducted but the cause for the drift could not be determined. This type of pressure switch has been kno a to experience drifting setpoints (Reference Incident letter dated May 31, 1973). The switch drift problem has been under investiga-tion by the manufacturer as well as the station instrument department.

At this time, the cause still re::nins unknown.

CORRECTIVE ACTIO!:

The 1::=ediate corrective ' action was to recalibrate the switches.

Future corrective action will be based on the outcome of the investigations.

When the cause of the drift problea has been determined, ycur office will be notified.

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h>At . blm (g) W. P. Worden v Superintendent WPW:do 1

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