ML20127L117

Ro:On 720717,rupture Disc on HPCI Turbine Exhaust Line to Suppression Chamber Ruptured on Overpressurization. Investigation Showed No Measurable Activity Release from Reactor Bldg Ventilation Exhaust

ML20127L117
Person / Time
Site:	Monticello
Issue date:	07/27/1972
From:	Mayer L NORTHERN STATES POWER CO.
To:	Giambusso A US ATOMIC ENERGY COMMISSION (AEC)
References
NUDOCS 9211230429
Download: ML20127L117 (2)

Text

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RCgulatory File Cy, NORTHERN STATES POWER COMPANY WNNEAPOW .

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55401 July 27, 1972  %

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- cg fb w:7 Mr A Giambusso Deputy Director for Reactor Projects p - !p.sf p/f Directorate of Licensing 4 ,

United States Atomic Energy Commission -* /

Washington, D.C. 20545

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Dear Mr Giambusso:

MONTICELLO NUCLEAR GENERATING PLMIT Docket No. 50-263 License No. DPR-22 Reporting of High Pressure Coolant Injection System Failure A condition occurred at the Monticello Nuclear Generating Plant recently which we are reporting to your office in accordance with provisions of Section 6.6.B.2 of Appendix A, Technical Specifications, of the Provisional Operating License DPR-22.

The Region III Compliance Office has been notified of the occurrence.

During quarterly flow rate surveillance testing of the HPCI system on July 17, 1972 the rupture discs on the HPCI turbine exhaust line to the suppression chamber ruptured on overpressurization. The HPCI syste'n was declared inoperable and the other ECCS systems and the RCIC systems were tested as required by Technical Specification  :

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4. 5. D. 2.

Investigation shcwed no measurable activity release from the Reactor Building Ventila-tion Exhaust.

Direct cause of the high exhaust pressure was determined to be failure of stop check l valve HPCI-lO, which is located in the HPCI turbine exhaust line immediately upstream )

of the suppression chamber. This valve was open for inspection during a subsequent outage which commenced July 21, 1972. 'Jhe valve disc pin had failed allowing the  ;

disc to separate from the valve hinge. The disc was lodged in the outlet portion I of the valve body restricting the flow path. i A modified disc pin has been installed in accordance with the valve manufacturers recommendation. The new disc pin ! es a one inch diameter section for threaded attachment to the hinge compared to 3/4 inch diameter threads on the original dise pin. Disc and ring seating surfaces were repaired and the valve reassembled. A valve leakage test will be conducted prior to plant startup.

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9211230429 720727 hDR ADOCK 05000263 PDR bM

,' NOF MERN OTATES POWER Ct JPANY Mr A Giambusso July 27,1972 Pressure switches on the HPCI turbine exhaust line are designed to initiate an alarm and turbine trip at 150 psig turbine back pressure in the exhaust line. Design burst pressure for the rupture disc pressure relief is 175 psig. High steam flow isolation of the HPCI steam supply did occur. A HPCI turbine trip occurred, but the high exhaust pressure alarm was not recorded.

Pressure switches and associated alarm and HPCI turbine trip circuitry were examined.

The circuitry wa6 found to be operable and pressure switches properly calibrated.

The pressure switch sensing lines are provided with snubbers. In the rapid exhaust pressure rise wnich is believed to have occurred in this event, it is conceivable that the rupture disc coitld have relieved before the pressure switches could be actuated and HPCI turbine trip may have been in$ tiated by overspeed trip. The sensing line snubbers will be examined and measurements made to determine whether less restrictive snubbers can be used which will s till accommodate the surges related to normal HPCI system operation.

Steam issuing fror ~.he rupture disc impinged directly onto four of the temperature switches for the B1CI steam leakage detection system. The steam caused sufficient mechanical damage to the system wiring, to render three temperature swit"hes in-operable. The box that houses the temperature switch wiring has been modified using heavier gauge material.

During the startup following the July 21st outage, surveillance testing on the HPCI system will be performed. The Operations Committee will review the results of the tests to determine HPCI operability.

An Abnormal Occurrence Report will be available at the site for the Regulatory Operations inspectors.

Yours very truly, L 0 Mayer, P.E.

Director of Nuclear Support Services LOM/ma cc: B H Grier

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