ML20058K927
| ML20058K927 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 06/02/1972 |
| From: | Mayer L NORTHERN STATES POWER CO. |
| To: | Anthony Giambusso US ATOMIC ENERGY COMMISSION (AEC) |
| References | |
| NUDOCS 9104260280 | |
| Download: ML20058K927 (4) | |
Text
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NSF NORTHERN STATES POWER COMPANY MlN N E A PO LI S. MIN N E S OTA 55401 P
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Deputy Director for Reactor Projects $
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Dear Mr. Giambasso:
MONTICELLO NUCLEAR GENERATING PLANT Docket No. 50-263 License No. DPR-22 Momentary Opening of "A" Reactor Saf ety Valve This letter is being submitted for your information on the subject momentary opening.
At 0530 hours0.00613 days <br />0.147 hours <br />8.763227e-4 weeks <br />2.01665e-4 months <br /> on February 26, 1972,while performing the turbine stop valve 10% closure surveillance test, all four stop valves tripped closed simultan-eously causing a reactor scram. Actuation of the safety / relief valves limited the maximum transient reactor pressure to a recorded value of 1118 psia.
In-cident to this event was a 0 9 psia increase in dryvell pressure (see attach-ment) and a 1000F increase in "A" saf ety valve disebarge nozzle temperature.
Mid-level drywell temperature was 930F prior to the scram and an approximate 6 F temperature rise was recorded at mid-level following the scram. Follow-ing this indication of a momentary actuation of the "A" safety valve, it rescated with no detectable subsequent leakage.
There was no evidence of instrument or equipment malfunction or damage due to the momentary actuation.
The stop valve closure resulted from a momentary loss of turbine emergency trip oil pressure, which was caused by a malfunction of the test-exercise controls for the No. 2 stop valve.
Because of the unusual manner in which the trip oil pressure was momentarily lost, the turbine control valves were not automatically tripped closed as they would be for a normal turbine trip.
The pressure control system responded to the steam preceure transient by fully opening the turbine control valves to a position equivalent to 110% of rated steam ficv. However, no steam flow could take place through the control valves since the stop valves, located immediately upstream, were already closed. The control system is designed to limit opening of turbine valves to prohibit steam flows greater than 110% of rated, therefore the bypass valves were prevented from opening and the pressure transient was more severe than for a normal tur-bine trip.
W 9104260200 720602^
CF ADOCK 05000263 CF:
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N RTHERN OTATES POW ER CMPANY Mr. A Gia::.busso June 2, 1972 On Saturday, May 13, 1972, during an outaEe which included scheduled mainten-ance work in the dryvell area, an inspection of the dryvell equipment was perfomed and this inspection confirmed the indicated momentary actuation of the "A" safety valve.
The aluminum cover on a blank flange downstream of "A" safety valve had dislocated.
The surrounding surfaces had insulation deposits resulting from steam impingement on the flange insulation. An inspection in the reciprocal direction of the tee discharge disclosed the dislocation of a cable tray cover.
The nominal setrng of "A" safety valve is 1210 psig.
'he respective loca-tions of the 6at'ety/ relief valve (EV2-71A) and the safety valve (RV2-70A) on the "A" main steu line are indicated in the second attachment.
Yours truly,
. O.
L 0 Mayer, P.E.
Director-Nuclear Support Services LOM/br cc: E H Grier Attachment
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