ML17252A960
| ML17252A960 | |
| Person / Time | |
|---|---|
| Site: | Dresden  |
| Issue date: | 06/22/1973 |
| From: | Worden W Commonwealth Edison Co |
| To: | Anthony Giambusso US Atomic Energy Commission (AEC) |
| References | |
| WPW Ltr.#474-73 | |
| Download: ML17252A960 (2) | |
Text
1**-,
Commonwe* Edison One First NationWi'aza, Chicago, Illinois v
Ada~ss Reply to: Post Office Box 767 Chicago, Illinois 60690 WPW Ltr.#474-73 Mr. A. Giambusso Deputy Director for Reactor Projects Directorate of Licensing U. s. Atomic En~rgy Commission Washington, D. C.
20545 lt~fatory*"** *-~ Fl~ c~
50-249 Dresden Nuclear Power Station R. R. Ill -
60450
SUBJECT:
LICENSE DPR-25, DRESDEN NUCLEAR POWER STATION, UNIT #3, SECTION 6.6.B.2 OF THE 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 thermal power was 1800 megawatts.
This condition was related to Mr. F. Maura of Region Ill 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 INVESTIGATION During surveillance inspection.of the low vacuum scram bypass switches, the setpoint of Meletron pressure switches PS-3-263-51A and B were found to have drifted-.
Pressure switch ?S-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 r.each~s 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 ~he opening of the pressure switches, a reactor scram will occur.
When reactor pressure is below 600 PSIG,. these" switches bypass _the low condense_r vacuum reactor scram.
With the pressure switches 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 5024
- June 22, 1973 of two logic; the-scram would not have occurred -un,tj.,1-the pressure setting of the PS-3-263-SlB was reached.
The 51B sw'itch would have opened first since its setting was 609 PSIG and the 51A switch setting was 633 PSIG.
Since these switches were open in tthe safety system circuits, they could not have prevented a scram.
Therefore, the continued safe operation of the unit was not ~eopardized.
An investigation into the problem was conducted but:;.,the cause for the drift could not be determined.
This type of pressure switch has been known 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 remains unknown.
CORRECTIVE ACTION The immediate 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 problem has been determined, your office will be notified.
WPW:d~
Sincerely,
~k_Jl ~- tM.~*
()_/\\_ W.
- P. Worden *
~ - Superintendent