ML20235E102

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Ro:On 710918,HPCI Sys Declared Inoperable & Remained So Until 710924.Caused by Recurrent Increase in Flow Elbow Differential Pressure Leading HPCI Steam Line to Isolate. Flow Rate Testing Procedure Revised to Isolate B Steam Line
ML20235E102
Person / Time
Site: Monticello, 05000000
Issue date: 09/28/1971
From: Duncanson R
NORTHERN STATES POWER CO.
To: Morris P
US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20235B311 List: ... further results
References
FOIA-87-111 NUDOCS 8709250481
Download: ML20235E102 (2)
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i NORTHERN 5TATES POWER COMPANY l Minneapolis, Minnesota 55401 )

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September 28, 1971 th SO N9p'W q

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D: vision of Reactor La censing e co SEP3 01971 C l, I  %

l United States Atomic Energy Commission Washington, D.C. 20545 y

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Dear Dr. Morris:

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IGTICELLO MJCLEAR GENERATI NG PLANT Docket No. 50-263 License No, DPR-22 Inoperability of the High Pressure Coolant injection System l

The High Pressure Coolant Injection (HPCI) system at the Monticello Nuclear Generating Plant was declared inoperable on September 18, 1971, and remained inoperable until September 24, 1971. This occurrence is being reported in accordance with the reporting requirements of section 6.6.B.2 of Ap A, Technical Specifications of the Provisional Operating License DPR 22. pendix The Region ill Compliance O,fice f has been notified of the occurrence.

Summarv Descrintinn nr Occorrence in our letter of September 15,1971, (copy attached) concerning an in-operability of the HPCI system which had occurred during the previous week, it was explained that the differential pressure measured at the elbow taps on the HPCI steam line had been found to have increased substantially, thereby causing an automatic isolation of the HPCI steam line during HPCl testing. Ii was noted that following backflushing of the instrument lines, the system had been retested and the differential pressure at the elbow taps was found to have returned to previously measured values. Ii was also reported that weekly testing of the HPCI system would be conducted until such time that the Operations Committee was satisfied that the change in the flow elbow differential pressure was not a recurring problem. During the first of these weekly tests, conducted on September 18, 1971 while operating at 90% power, it was found that the flow elbow differential pressure had again increased, causing the HPCI steam line to isolate. The HPCI system was declared inoperable and the i

RCIC, LPCI, and core spray systems were immediately demonstrated to be operable as required by section 4.5.D.2 of the Technical Specifications.

A review of all previous HPCI tests showed that all successful testing had

- been completed under low main steam flow conditions and that the recent 8709250481 870921 7 ,,

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. unsuccessful tests were all. conducted with approxima ely 90% of rated.

main steam flow. I t was theorized that the dif ferent;al pressure at the one foot of the flow elbow, point which where the HPCIis located within steam line exitsapproximately" from the rain steam line, might be affected by the main steam flow.

Tests were performed during which the "B" main steam ;ine was isolated, unisolated, and reisolated with the HPCI system in operation. These tests conclusively proved that the increase in the indicated differential pressure -

at the flow elbow was caused by flow in the "B" main steam line. With the "B"' steam line closed, the system operated properly with expected differential pressure indications. With main steam flows up to approx-imately 50% of rated flow, the differential pressure :ndications increase, however, the HPCI system remains in operation. For rain steam flows greater than approximately 50% of rated, the HPCI system automatically isolates after 45 seconds because the differential ressure indications remain higher than the reset. values of the 150,000 #p/hr. flow sensors.

Testing was also conducted at 90% of rated steam flow to verify that the flow in the main steam line did not result in a trip er the 300,000'#/hr HPCI flow sensors which would cause an instantaneous -PCI system isolation.

Also, on September 5th, the HPCI system operated properly following a  ;

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Iow-low reactor water isolation from 90% power, even -hough the HPCI system had been declared inoperable the day before because of the differ-ential pressure indication problem discovered during testing.

On September Operations Commi 24,ttee concluded that the HPCl was operable on the basis 1971 that conditions which initiate HPCI (low-low reactor water level and high drywell pressure) also result in a reactor scran and a main steam line isolation. Thus, for an automatic HPCI initiation, the main steam flow will drop to zero within 5 seconds and allow the 150,000 #/hr flow sensors to reset well before the end of the 45 second time delay.

In the future,"B" isolating the steam line. flow rate testing of the HPCI system will be conducted by A detailed report of this occurrence is being written and will be available for review by the Region ill Compliances inspector during his next  !

site visit.  !

l Yours very truly f h . .-

h)fC F.0. Duncanson, Jr. , P.E.

Gen. Supt. of Power P1 ants Mechanical {

Chai rman Monticello Safety Audit Committee FOD/caf l l

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