ML20056B704
| ML20056B704 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 01/28/1972 |
| From: | Mayer L NORTHERN STATES POWER CO. |
| To: | Morris P US ATOMIC ENERGY COMMISSION (AEC) |
| References | |
| NUDOCS 9102080390 | |
| Download: ML20056B704 (3) | |
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NORTHERN STATES POWER COMPANY j
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O United States Atomie Energy Commissio Washington D. C.
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Dear Dr. flor r is:
LOTR ICELLO TUCLEAR GETERATIIG PLAIR Docket !b. 50-263 Li cence Ib. DPR-22 Reset Failure of 'D' Reactor Safety / Relief Valve This letier is bei ng submi t ted fo r you r information on the subject failure.
At 0125 hours0.00145 days <br />0.0347 hours <br />2.066799e-4 weeks <br />4.75625e-5 months <br /> on September 28, 1971 following a reactor scram and isolation, the 'D' safety / relief valve (one of four Target Rock safety / relief valves in the I
f!onticello system) was manually actuated to maintain reactor pressure between 950 psig and 1050 psig as outlined in the plant operating procedures. On the second manual actuation the 'D' safety / relief valve failed to seat af ter the conirol swi tch was returned to the 'close' position.
Reactor pressure continued to drop. The 'D' safety / relief valve was given another open and close signal, at whi ch tire the valve reseated.
The duration of the blowdown was estimated to be 37 seconds. The reactor pressure dropped f rom 1040 psig to 670 psic corresponding to a drop of approxi-mately 50 F in saturatien tercerature.
Based on the following facts it was determined that reactor operation could '
safely continue.
The 'D' safety / relief valve is not part of the automatic pressure relief 1.
system which is requi red to be operable during power operation by the Techni cal Speci fi cations.
2.
Even if the 'D' relief valve was not operable, the three remaining safety /
relief valves would provide adeq; ate overpressure protection.
(Reference Section 3.6.E of the Technical Specifications.)
Any of the other safety / relief valves could be used to manually control 3.
reactor pressure in the event of a reactor isolation.
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The vessel temperature transient was well within the technical specification requi rement of 1000 f averaged over a one hour period.
The handswitch for the 'D' safety /r el 4f valve was tagged, instructine the control room operators to use one of the r Ler safety / relief valves to manually econtrol reactor pressure in the event of 'uiure reactor isolations. Plans were made to inspect the 'D' safety / relief vr ve at the earliest convenient opportunity.
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iThejreactor was made critical again at 238 hours0.00275 days <br />0.0661 hours <br />3.935185e-4 weeks <br />9.0559e-5 months <br /> on September 28, 1971.
i On November 12, 1971, the reactor was placed in the cold shutdown condition for plant maintenance activities.
All of the work described in the remainder of i
this report was completed during the extended maintenance outage.
Du ri ng the i
week of November 14, 1971, plant maintenance personnel inspected the 'D' safety /
relief valve to determine the cause of the earlier failure. The air actuator was removed and stroked using an air supply, the air solenoid was disassembled and inspected, and the second stage piston was checked for freedom of movement.
No cause for the faulty valve operation could be determined.
New set pressure adjusting springs were installed in the four safety / relief i
valves. The new springs are made of type A286 alloy which is designed for hi h E
tempe ratu res.
This eliminates the possibility of set point drift due to spring i
relaxation ai prolonged high temperature.
Du r i ng t he week of December 13, 1971 a Target Rock representative was at the si te to supervise the installation of the new set pressure adjusting springs. T he as found and as lef t set point data for each of the valves was as follows:
l Previous setii nn S/R Valve A 1060 psig
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As Found As left 1072 psie 1067 psig S/R Valve B 1068 psig 1070 psig 1068 psig
- n te S/R Valve C 1063 psig 1030 psig 1067 psig S/R Valve D 1068 psig 1080 psig 1068 psig I
fDTE:
Because of uncertainties found in the calibration of the pressure gauge during the test on the
'C' and 'D' safety / relief valves the exact as found pressure settings are not known. The as found settings were approx-imately 1080 psig.
i The Tarpet Rock representati ve was asked to give special attention to the
'D' safety / relief valve to identi fy a cause for the earlier reset f ailure.
His inspection of the pilot valve assembly, the second stage piston, and the ai r actuator revealed no cause for the reset failure.
On January 12, 1972, the manual control ci rcuit for the 'D' safety / relief A ain, no abnormali ties were discovered.
valve was tested.
g On January 17, 1972, the 'D' safety / relief valve was disassembled and the main valve piston assembly was inspected.
The stem on the main disc was found to be severely galled and had to be forced out of its bushing.
It is believed that the galled stem caused the valve to stick open on September 28th. On January 19, 1972, the 'C' safety / relief valve was disassembled and inspected. The stem on the. main disc was also found to be lightly galled, however, it was found to be free in its bushing.
Inspections of the remaining two safety / relief valves revealed A
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that the ' A' valve stem was scratched and the 'B' valve stem was lightly called.
The main disc stems in both the ' A' and 'B' safety / relief valves were found to be free in their bushings. A review of past operating history indicated the 'D' safety / relief valve had been actuated approximately 20 times whereas the remaininc valves had been actuated approximately 4 times each.
Further inspections revealeil
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'that the inside edges at the ends of the stellite stem bushings were not chamferred, which probably led to the galling action.
On January 23, new main discs were installed in four safety / relief valves.
The new discs have stellite coated stems to make them more resistant to galling.
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Following a discussion with a Target Rock representative the inside edges of the stem bushings were also chamferred to reduce the possibility of shaving or galling the main disc stems. The above action should prevent similar problems in the fu tu re.
During the forthcoming startup each of the safety / relief valves will be manually actuated at 150 psig reactor pressure to verify their operability.
l Yours very t ruly, g d.
L 0 flayer, P.
Di rector-fluclear Support Servi ces l
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