ML20235E968
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NSF NORTHERN STATES POWER COMPANY MIN N E A POLIS MIN N E SOTA $5404 cu June 2, 1972
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1 Mr. A Giambusso 75 i I @.n Deputy Director for'Beactor Projects Ip gg ggy 7.,
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Directorate of Licensing
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m United States Atomic Energy Commissio @
DOCKET 9L!AK N
Washington, D c 20545 t*
Dear Mr. Giambusso:
MONTICELLO NUCIJAR GENERATING PIANT Docket No. 50-263 License No. DPR-22 Momentary Opening of "A" Reactor Safety 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. Actuationofthesafety/reliefvalveslimited 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 attaen-ment) and a 1000F increase in "A" safety valve discharge nozzle temperature.
Mid-level dryvell temperature was 93 F prior to the scram and an approximate 60F temperature rise was recorded at mid-level following the scram.
i Follow-ing this indication of a momentary actuation of the "A" safety valve, it reseated 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 nomal turbine trip.
The pressure control system responded to the steam pressure transient by fully opening the turbine control valves to a position equivalent to 110% of rated steam flow. 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 j
from opening and the pressure transient was more severe than for a normal tur-3 I
bine trip.
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Mr. A Giambusso June.2,' 1972 On Saturday, May 13, 1972, during an outage which included scheduled mainten-ance_vork in the drywell area, an inspection of the drywell equipment was performed and this inspection confinned the' indicated momentary, actuation of the "A" safety valve., The aluminum cover on a blank flange-downstream of."A" safety valve'had dislocated... 'lhe 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.
of "A" safety valve is 1210 psig. 'lhe nepective loca-The nominal setting / relief valve (RV2-71A) and the safety valve (RV2-70A) on tions of the safety the "A". main steam line are indicated in the second attachment.
Yours truly, h.
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L 0 Mayer,-P.E.
Director-Nuclear Support Services LOM/br -
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