ML20085L609

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LER 76/010B:on 760917,during Power Ascension Tests,High Heat Load Revealed for Region 29.Caused by Hot Helium Entering Control Rod Drive to Liner Annulus.Control Rod Drive Changed Out
ML20085L609
Person / Time
Site: Fort Saint Vrain Xcel Energy icon.png
Issue date: 01/22/1977
From: Hillyard H, Swart F
PUBLIC SERVICE CO. OF COLORADO
To: Howard E
NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV)
Shared Package
ML20085L596 List:
References
LER-76-010B, LER-76-10B, P-77030, NUDOCS 8311020116
Download: ML20085L609 (6)
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.. UCENSEE EVENT REPO CONTFiOL DLOCK: l l l l l l (PLEASE PRINT ALL CEOUIREJ INFOTICTIDN) 1 tl NAME LCINSE NUMBtR y E I ( )

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Janua ry 22, 1977 UNUSUAL EVENT 75/10A REPORT DATE:

Page 1 of 4 OCCURRENCE DATE: _ Sep t embe r 17. 1976 FORT ST. VRAIN NUCLEAR CENERATING STATION PUBLIC SERVICE COMPANY OF COLORADO '

P. O. BOX 361 PLATTEVILLE, COLORADO 80651 REPORT No. 50-267/75/10A Supplement IDENTIFICATION OF OCCURRENCE:

During routine monitoring of PCRV liner cooling tube outlet temperatures, as required by SUT B-3, cooling tubes in refueling region 29, of the PCRV were observed to be indicating a higher than expected temperature rise. This is being reported as a supplement to Unusual Event Report No. 50-267/75/10A.

EVENT DESCRIPTION:

Routine scanning of cooling water outlet temperatues, taken to evaluate top head cooling effectiveness, showed that the tube associated with the lower end of refueling penetration #29 was experiencing a greater temperature rise than the tubes of other refueling penetrations. This was noted during the B series (rise to power) tests at between 20% and 30% reactor power.

Hot gas from the upper plenum was apparently being drawn into the annulus between the control rod drive housing and the refueling penetration. In nor-mal operation, such a flow of gas could result from the lower static pressure which exists inside the control rod drive housing relative to the static pres-sure associated with the core inlet plenum (see Figure 1).

CAUSE DESCRIPTION:

The static pressure inside a control rod drive housing is slightly greater than that at the core outlet, but less than that of the core inlet plenum.

These pressure dif ferences are intended to cause normal purge flow into the drive housing to flow downward, exit the control rod guide tubes, flow over the control rods for cooling, and discharge into the core outlet plenum.

Due to the equipment maintenance access holes in the control rod drive housing, plus the articulated lower guide tube and orifice valve assembly, no positive seal exists between the annulus surrounding the drive unit and its encased l inte rnals . Therefore, a pressure difference sufficient to draw hot gas from the upper plenum into the . annulus is present whenever the primary coolent is being circulated. The temperatures of the hot gas are equal to the core inlet primary coolant temperature, which varies from 210*F at zero power, up to 760*F at full power.

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UNUSUAL EVENT 75/10A Page 2 of 4 CAUSE DESCRIPTION (continued): , .,,...

The cooling tubes are physically welded to the refueling penetration nozzles at the upper headliner, and therefore they responded promptly to the presence of hot helium flowing into the penetration between the control rod drive assembly and the penetration liper.

. The control rod drive had been modified as described in Unusual Event Report No. 50-267/75/10, but some leakage of hot helium gas into the penetration between the liner and the control rod drive was still occurring.

CCRRECTIVE ACTION:

The control rod drive in Region 29 was removed and another modified (as des-cribed in Unusual Event Report No. 50-267/75/10) control rod drive was in-stalled. The reactor power has been increased to above 28% and there has been no recurrence of high cooling water temperature in Region 29.

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O O UNUSUAL EVENT 75/10A l Page 4 of 4 l Prepared by: (fff/ 2. d#M41r '

William J. Francy Reactor Engineer Reviewed by: t[ -

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.Y. W'. H1'11yj(rd , Jr. / -

Technical services Supervisor Reviewed by:

Frank M. Mathie Superintendent, Maintenance

( f I Approved by: 1.;;_ , a r$_c C. s _ ,,m Fre3eric E. Swart '

Nuclear Production Manager i

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January 22, 1977 Fort St. Vrain p' Unit No. 1

/ qg 'g P-77030 Mr. E. Morris Howard, Director Nuclear Regulatory Commission Region IV Office of Inspection and Enforcement Suite 1000 Arlington, Texas 76012 REF: Facility Operating License No. DPR-34 Docket No. 50-267

Dear Mr. Howard:

Enclosed please find a copy of Unusual Event Report No. 50-267/75/10A, Supplement, submitted per the requirements of the Technical Specifica-tions.

Also, please find enclosed one copy of the Licensee Event Report for Unusual Event Report No. 50-267/75/10A.

Very truly yours, '

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Frederic E. Swart Nuclear Production Manager FES/alk cc: Mr. Roger S. Boyd 945

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