ML20138B487

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PNO-IIT-85-002D:on 851121,unit Tripped.Incident Investigation Team Will Complete Remaining Interviews & Review Results of Licensee Investigaton.Prime Hypothesis Explaining Events Described in PNO-IIT-85-002B Encl
ML20138B487
Person / Time
Site: San Onofre Southern California Edison icon.png
Issue date: 12/10/1985
From: Lanning W, Martin T
NRC - INCIDENT INVESTIGATION TEAM
To:
References
PNO-IIT-85-002D, PNO-IIT-85-2D, NUDOCS 8512120310
Download: ML20138B487 (3)


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  • 1 Date: 12/10/85 PRELIMINARY NOTIFICAT10N OF EVENT OR UNUSUAL OCCURRENCE--PNO-IIT-85-2D This preliminary notification constitutes EARLY notice of events of POSSIBLE.

safety or public interest significance. The information presented is preliminary, requires further evaluation and is basically all that is known by the IIT on this date.

FACILITY: Southern California Edison Company Emergency Classification Unit 1 X Notification of Unusual Event Docket No. 50-206 Alert Site Area Emergency General Emergency Not Applicable

SUBJECT:

STATUS REPORT FROM NRC INCIDENT INVESTIGATION TEAM The Incident Investigaticn Team (IIT) returned to the San Onofre Unit 1 site today to continue its investigation of the loss of power and water hammer event that occurred on November 21, 1985. The team will complete remaining interviews and review the results of the licensee's investigation of the extent of damage and the root causes of the event.

The IIT developed and continues to test a prime hypothesis (attached) which, to date, seems to explain the significant events described in PNO-IIT-85-2B.

Recent results of the licensee's investigations have provided some confirmatory evidence supporting the hypothesis, and the IIT has decided to release it for information purposes. The attached hypothesis should be considered preliminary and could be modified based on further analysis.

CONTACT: T. Martin W. Lanning (714) 492-2641 (301) 492-4433 DISTRIBUTION H St. MNBB Phillips E/W Willste AR Mail:

Chairman Palladino EDO NRR IE NMSS ADM:DMB Comm. Roberts PA R. Dudley OIA RES DOT:Trans Only Comm. Asselstine MPA AEOD Comm. Bernthal ELD Comm. Zech Regions:

SECY INP0 NSAC ACRS Licensee:

CA (Reactor Licensees)

POR Resident Inspector  ;

I l es1212o31o e51210 l PDR I&E I PNO-II I-85-OO2 PDR EE -M

Attachment to PNO-IIT-85-2D Prime Hypothesis Being Investigated as of November 26, 1985 Ground fault on "C" auxiliary transformer.

Transformer trip deenergizes " East" main feedwater (MFW) pump.

" West" MFW pump remains energized, due to alternate power source alignment from the unit main electric generator.

" East" MFW pump discharge-check-valve fails to seat properly.

(This 14" check valve has since been found cocked open, supported by a loose attachment in the check valve body.)

" West" MFW pump over pressurizes " East" feedwater heater-condensate train.

" East" #4-5 feedwater heater ruptures, due to over pressurization.

Both main turbine " East" rupture discs fail.

Operator trips unit, deenergizing " West" MFW pump.

Feedwater (FW) flew to all steam generators stop.

"B" feedwater regulating valve discharge-check-valve fails to seat properly.

(This 10" check valve flapper has since been found in the bottom of the valve body with hinge pin and nut missing.)

Electric auxiliary feedwater (AFW) pump gets start signal, but lacks electric power to start.

Steam AFW pump gets start signal, but takes three .ninutes to start delivering flow, due to automatic warm-up cycle.

"A", "B", and "C" steam generator steam spaces are interconnected and begin blowing "B" feedwater line dry, throrgh the stuck open check valves ahead of the "B" feedwater regulating valse and the " East" MFW pump.

Steam AFW pump begins to deliver relatively cold auxiliary feedwater to all three main feedwater lines.

The auxiliary feedwater to the "B" steam generator feedwater line flows to both the malfunctioning check valves and to the long horizontal run of FW pipe in the primary containment.

Operators reduce AFW flow to minimize rate of cooldown of primary system.

Steam in "B" feedwater line finally encounters cool water laying in the horizontal pipe, resulting in rapid condensation.

Water slug forms in "B" feedwater line in containment, is accelerated by steam flow toward the point of condensation, encounters multiple pipe turns, and damages associated "B" feedwater line and supports.

4. ,

. o 2

1 Water hammer pressure pulse stretches "B" f.eedwater regulating valve bypass line check-valve top-hat-bolts, extrudes gasket and produces  ;

. substantial steam-water leak. I

! (This 4" check valve flapper has since been found to have suffered

impact damage.)

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"B" steam generator boils dry with all "B" AFW flow being cai cted by

. steam out check-valve. leak.

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