ML20235F038

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Ro:On 720721,main Transformer Fire Deluge Sys Initiated by Short in Manual Initiation Circuit.Caused by Malfunction of Steamline safety-relief Valve D.Valve Replaced
ML20235F038
Person / Time
Site: Monticello, 05000000
Issue date: 07/28/1972
From: Mayer L
NORTHERN STATES POWER CO.
To: Giambusso A
US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20235B311 List: ... further results
References
FOIA-87-111 NUDOCS 8709280387
Download: ML20235F038 (4)
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, ,}{gulatory Fit 'ty; MSP -

NORTHERN 8TATES POWER COMPANY Minneapolis, Minnesota 55401 July 28, 1972i.,g 7. ~.f. , ,

NV f N Mr. A. Gismbusso d UNc Deputy Director for Reactor Projects t , ,.

' Directorate of Licensing AUG3 1972 > '5 United States Atomic Energy Commission Washington, D. C. 20545 DOCKET CLERK g \,v

Dear Mr. Giambusso:

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MONTICELLO NUCLEAR GENERA ' BHT Docket No. 50-263 License No. DPR-22 Malfunction of the "D" Steamline Safety / Relief Valve and Inoperability of Bellows Leakage Monitoring Systems Conditions occurred at the Monticello Nuclear Generating Plant which we are reporting to your office in accordance with the provisions of Section 6.6.8.2 of Appendix A, Technical Specifications, of the Provisional Operating License DPR-22. The Region ill Compliance Office has been notified.

On July 21, 1972, while the reactor was ooerating at 100% power, the main trans-former fire deluge system was initiated by a short in the manual initiation circuit. '

The generator lockout relay was consequently tripped by the primary protection system for the main transformer's secondary circuit. This trip was initiated by A I phase arcing to ground and causing the A phase bushing to fail. A reactor c. cram was received at 1529 hours0.0177 days <br />0.425 hours <br />0.00253 weeks <br />5.817845e-4 months <br /> by control valve fast closure, as determined from the sequence of events log. A peak reactor pressure of 1112 psig was reached. Lack of response from the thermocouple and the discharge pressure switch on the D 1 safety / relief valve indicated that it did not open. Safety / relief valves A, B and C opened and closed automatically. As on two previous occasions, control rod drive 22-31 stopped at notch "02" and was manually inserted to the full in position, after the scram. The scram insertion time was well within Technical Specification require-ments. Plant conditions were stabilized following scram recovery procedures.

Analvsis and Corrective Action After plant conditions had stabilized, the plant was placed in a cold shutdown condition and the drywell was de-inerted to permit an investigation into the malfunction of the "D" safety / relief velve.

The recently installed safety / relief valve discharge pressure switch on the D valve was tested and verified to operate properly. This confirmed that the valve had not operated. The D valve was removed and subjected to a nitrogen test. The setpoint of the valve was found to be 1086 psig. The entire valve was disassembled and inspected in an effort to determine the cause(s) of the high setpoint and the failure of the valve to actuate. All valve internals were found in satisfactory condition with no dimensional abnormality and no sign of galling or wear. Since 9709290387 870921 PDR FOIA 5 N pge357 4259 MENZO7-111 PDR  !

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L no cause could be identified to explain the valve malfunction a replacement valve was procured and. installed.

An investigation of the pilot bellows leak detection system was' initiated to determine if a back pressure could be created which would affect the valve setpoint This system consists of a pressure switch which initiates an alarm on solenoid high pressure in the valves'and chamber control outside switches of the which pilottesting permit bellows; andpressure of the ~ air piping,tch swi and alarm from outside the.drywell. . (Refer to the attached figure for an explan-ation of the system).

On July 25, 1972, it was found that a small bellows leak could not be detected due to a system design error. It was the original design intent that solenoid valves SV2-32: and SVU-33 should provide redundant isolation to prevent pressur-izing of the bellows chamber in the event of, a failure of SV2-34 and to assure that pressurization would occur in the event of a bellows leak. It was found that the valves which were used for SV2-32 and SV2-33 are designed for tight shutoff only when pressure is applied to the " inlet" side (side away from the bellows chsmber). Tests were conducted to determine the backpressure which could be applied at the bellows chambers before leakage occurs. Tests were also conducted to determine the setpoint of the non-adjustable type bellows alarm pressure switches.- The results of these tests are shown below:

Backpressure (osio) vs. l eakap e Press Switch Bg}iefValve ,

Setocint. osio. Threshold 4.5 SCFH A 74 26-45 65 B 61 72-82 86 0 64 70-77 ,

83 D 78 68-70 74 These results indicate that the A & D leak detection systems would not have detected a small leak.

The bellows assemblies of all safety relief valves were leak tested with high pressure nitrogen. The installed valves in the A, B and C steam lines were

  • found to be leak tight. After being reassembled the valve which had been installed on the D line was found to have a small leak through an 0-ring seal located at the joint bstween the top of the bellows and the preload spacer. The I replacement valve was also found to have a small leak in the same location.

li is probable that the malfunctions of the D relief valve on July 10 and July 25 were due to a small leak in the bellows assembly. Such a leak could have increased

'the backpressure on the bellows, thereby increasing the effective trip setting.

The non-adjustible leak detection pressure switches have been replaced with switches

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set to trip at 5 psig. This setpoint was chosen to be well within the backpressure capability of the solenoid valves and to provide reliable trip and reset action.

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I The bellows assenbly leakage of the replacement valve installed on the D steam line was repaired. This valve will be tested during plant startup io verify proper manual operation.

Yours very truly,

. O.

L. O. Mayer, P...

Director Nuclear Support Services LOM/nunm At4.achment I

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4 l' ' Bellows Leakane- Alarm Test '

. A three posi tion " TEST /tORMAL/ vet #'.' switch and a test pushbutton are provided -

l in the control room.' The test sequence is as follows:

1. Test switch'in.the " TEST" position energizes SV 2-34, SV 2-32, and SV 2-33, which admits air to the bellows assembly and trips the bellows' leaking pressure switch alarm circuit.
2. Test Switch in " VENT" position de-energizes SV 2-34, and energizes SV 2-32 and SV 2-33 which ' vents air from the bellows assembly and resets the pressure switch ci rcuit.
3. -Test switch in "f0RMAL" position de-energizes all three solenoids.
4. The TEST pushbutton is-used to energize SV 2-34 (with the Test switch in FORMAL) to verify positive seating of SV 2-32 and SV 2-33.

Bellows leaking A alarm Bellows leaking L__ A pressure SV 2-34 SV 2-33 SV 2-32 U switch

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C A l 7 C 2 7

C 2 7 Air

. Y F' F' '

Relief Valve Supply Accumulator f Exhaust 7X U

Pelief Valve Bellows Leakace Test Svstem

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