ML20136J106

PNO-IIT-86-001A:on 851226,loss of Integrated Control Sys Occurred.Caused by Simultaneous Deenergizing of All Redundant Dc Power Supplies.Licensee Preparing Action Plans for Systematic Troubleshooting.Sequence of Events Encl

ML20136J106
Person / Time
Site:	Rancho Seco
Issue date:	01/05/1986
From:	Hebdon F NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION V)
To:
References
PNO-IIT-86-001A, PNO-IIT-86-1A, TAC-60462, NUDOCS 8601130198
Download: ML20136J106 (11)

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January.5, 1986' PRELIMINARN NOTIFICATION OF EVENT.OR UNUSUAL OCCURRENCE.PNO-IIT-86-1A

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Thic: preliminary notification constitiutes EARLY notice of events of POSSIBLE safety or s-; publicfinterest~ significance.

,' FACILITY: Sacramento Municipal. Utility District.

Licensee Emergency Classification:

Rancho ~Seco Notification of Unusual Event Docket No.L50-312-

-Alert?/

Site (Area; Emergency General Emergency

-X Not Applicable h

SUBJECT:

~STATUSREPORTFROMTHENRCINCIDENTINVESTIGATI'ON[ TEAM

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Th3 " Incident;Investig'ation Team (IIT) remains onsite gathsringland analyzing data,

~cr!. viewing-licensee action plansiTand analyzing [the. sequence"of. events.> A preliminary-sequence of events'has been. developed'by the IIT :nd is,att'ach'ed.

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t Alltinitialsinterviews with,the licensee acaff,on-shift during the_12/26/85 ov:rcooling event have been completed. zThe lisensee is' pre" paring detailed [ action.

.~ plans for.the' systematic troublesh'ooting of' failed ejuipment,on the q6arantine~ list.

Thn; team expects'toicomplete the review of these' plans no,later than January 8, 1986.'

t 1Thn~IIT' expects'to complete its initial on-site investigation'and data gathering E

' cctivities by ' aid-week.: The team will then return to Bethesda,' Maryland to continue

th;analysisofthedata,and'tobegin' interviews,with}NRC;staffmembers'whowere

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=inv:1ved with regulatory activities and issue's that may have had a: direct (impact on

?the course of this event..The team expects ~to return to the site when the. licensee

,7 completes the troubleshooting work. The actual troubleshooting work will be monitored

byfrepresentatives-from-Region V.

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Contact:

Frederick}J.Hebdon','(209).748-2791 f"

1 DISTRIBUTION:

?H2 Street!

MNBB'

.Phillips E/W Willste MAIL:

Chairman Palladino' EDO

-NRR-IE NMSS ADM:DMB Comm. Roberts.

PA OIA' RES DOT: Trans Only Comm.'Asselstine MPA' AEOD Applicable' State Comm. Bernthal ELD.

7 5.Comm.t Zech Air Rights INPO

.SECY;

.SP NSAC

ACRS'

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Licensee:

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B601130198 860105 l:

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INCIDENT. INVESTIGATION TEAM i

sCg PRELIMINARY SEQUENCE OF EVENTS y

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INITIAL PLANT CONDITIONS

- Unit operating at steady state power of 76% [710 MW(e)).

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-- Reactor. Coolant System,(RCS) average-temperature is 582 F.

- RCS pr'esssre'is' 1150 pdig'.

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, This ' plant does not have Main Steam Isolation Valves (MSIVs). -

--;The-plant had started up on December 24, 1985 following an outage of 2 days.

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- Integrated'Co'ntrol System (ICS) in full automatic.

- The Bailey compu'ter was out of ' service (one of the planti's two main computer

,' systems. in the Control Rcom). Consequently, the Bailey post-trip review Bailey: alarms printout, and Bailey inputs to the Interim Data Aquisition and' Display System (IDADS) are not available.. IDADS inputs

<from sources other than the Bailey computer are available.

E TIME DESCRIPTION OF EVENT DATA SOURCE

' Transient Ibitiation F

04:13:47 " Loss of ICS or: Fan Power" Annunciator Alarm.

IDADS Print out Loss of ICS is caused by the simultaneous de-

+ energizing of all. redundant ICS DC power supplies.

ICS demand signals.go'to midscale.

(The ICS works on +/- 10 volt scale, with zero volts being 50% demand). The startup and Main

'Feedwater (MFW) valves close to 50%ebecause of this decrease in demand signal. The loss of ICS power, however, causes the MFW pump speed to

' decrease to.the minimum speed of 2500 RPM.

L With the plant initially at 76% power, this reduction

.in MFW flow increases RCS pressure.-

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The loss of ICS DC power,also sends demand' signals ~

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.to one of two sets of Auxiliary Feedwatert('AFW)'fe

,f flow control valves,ihe! Atmospheric DumpiValves (ADVs) and the Turbine Bypass ValvesgTEVs),t'o l ope'nI to >50% demand.

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(Note: The plant h,as; pao Tarallel sets of AFW valves.S;lt',

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One set is controlled by the ICS and one set is controlled by the Safety Features Actuation System).

' Operator / System Response to the Less of ICS Power

-04:13:7 Control room operators notice MFW flow Operator decreasing rapidly. Also, they notice RCS StateLent pressure increasing. Operators open the pressurizer spray valve in an attempt to stop the RCS pressure increase.

Due to the rapid overheating of the RCS by the reduction in MFW flow (it appears that MFW flow actually decreased to zero), the actuation of pressurizer spray la not sufficient to reverse the RCS pressure increase.

04:14:01 The reduction in MFW pump speed causes a low IDADS MFW pump discharge pressure of less than Print 850 psig which automatically starts the out motor driven AFW pump.

04:14:03 Reactor trips on high RCS pressure. The t'urbine IDADS Print trip is also initiated by the reactor trip. A out Control Room operator closes the pressurizer spray valve.

04:14.04 Peak RCS pressure of 2298 psig.

Several Main Steam fIDADS Print Safety Valves are believed to have lifted'and o'u t.

reseated early in the event.,

04:14:06 AfW dual drive (i.e. steam I electric) pump:

IDADS autostarts on low MFW pump' discharge [pr'e~ssure' Print (850 psig).-

out ThisAFWpumpissteam-driventhrough[ut'this~

transient.

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04:14:06 Peak RCS hot leg temperature'of 606.5 F.

IDADS Print out Operator / Systems Response to the Plant Trip and Overcooling 04:14:7 Immediately upon reactor trip, many fire alarms, Operator The Technical Support Center (TSC) spray actuation Statement alarm, the seismic trouble alarm, and Spent Fuel Pool (SFP) temperature high alarms are received. The significance of this is still being assessed.

The operators perform the actions of the Emergency Procedure section E.01 (Reactor Trip Inmediate Actions). This includes reducing RCS letdown flow.

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Operators. then proceed _. wit:h. Emergency Procedures'section E.02,(Vital System. Status.

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R Verification).

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.. 04:14:11'l:' AW flow begins to b'oth Once Through' St:eam Generators IIDADS Print

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{(OTSGs) through the ICS-controlled AFW Flow Control out

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Valve'.

IO4':14:25 70perators. note pressurizer _ level decreasing, Operator

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.and ' f ully open' the A" injection valve ' for more Statement makeup flow to RCS.:

IDADS Print out

'04:14:30 The loss _of'ICS power also results in loss'of Operator l' ~

emanual-(i.e.._ hand) control of ICS controlled-Statement valves from the Control Room. Therefore, non-y

_. licensed operators;are sent to close the TBVs, TADVs, and AFW flow control valves.- (Note: The ADVs

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and-TBVs could have been shut from the Remote Shut-down Panel. However, the operators failed to remember this fact).:

s The operators recognize the beginning of an 1

overcooling transient due to.the open startup and main MFW valves, the half open TBVs and ADVs, the open AFW flow control. valves, along with M W speed' remaining at around-2500 RPM.

104:14:48 0 Makeup tank (MUT) level decreasing rapidly.- Operators operator

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open ' the.Borat'ed' Water Storage. Tank (BWST)' suction Statement-valve.'on-the "A" sidesto provide an additional-sodrce of makeup water.

04:15304 Operators start the_"B" HPI pump to increase-IDADS Print

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=e makeup flow to the RCS from the BWST.'+ a.

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{IDADSPrint

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f MFW flow indication on thM Control _ Room strip' operator

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charts indicates about'3.5:million' pounds l ;- ' /

' Statement per hour., However,I this MFW flowmindication<.

3 passesthroughmodules. powered,bytheICSand,,\\

A therefore, the loss of ICS power causes'this.

' indicator-fail to midscale.- The actual (MFW flow?

rate indicated by.the IDADSfprfnt out decreased to zero upon= reactor'tripa -.

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before the reactor operators. trip the

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s MFW pumps.. The. actual-MFWjflowrAte remained at:zero due to the increased pressure'in.both OTSGs and'the low speed demand to both MFW pumps.

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AW flow-is greater than 1000 gpm. to' each,0TSG.

,IDADS Print

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je 04:16:7; An operator isolates' pegging steam, which is'u' sed Operator to heat MW during low power operation..The Statement operator isolates it because he believes that a M W heater relief valve may have lifted.

04:16:57 RCS pressure has' decreased to11600 psig. Pressur-IDADS izer-level is 15 inches. The Safety Features Print Actuation System (SFAS) automatically initiates.

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A', ~ B, C 'and D HPI inj ection valves open t'o.

pre-determined positions. Selected SFAS equipment,. including the motor-driven A W pump, automatically sheds off the vital busses Land sequence loading of SFAS equipment begins.

- AW (SFAS-controlled) valves travel. full open.

"A" & '?B" Low Pressure _ Injection / Decay Heat-Removal;(LPI/DHR) pumps autostart in the recirculation mode. -Diesel generators autostart but do not close onto the vital busses as there has been no loss of

. power to the vital busses.

SFAS also actuates containment (i.e. reactor) building isolation. This

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actuation isolates suction to the containment building radiation monitors, which subsequently causes one of the associated sample pumps to overheat and damage its' seals.

04:16:59 "A"-HPI pump autostarts on SFAS signal.

IDADS Print out Operator / Systems Response to SFAS Initiation

- 04:17:10(?) Operator overrides SFAS signal to the A W Operator (SFAS-controlled) flow control valves and closes Statement /

them.

IDADS Print out 04:17:15 A" & "B" Control Room / Technical Support Center IDADS Print

-(CR/TSC) Essential Heating, Ventilation and out/

Air Conditioning (HVAC) units start on the Operator SFAS signal. This significantly increases the Statement noise level in the Control Room.

04:17:27 Motor-driven AW pump automatically IDADS Print sequences back on its vital-bus and restarts, out The dual-drive AFU pump has been running continuously and powered by steam since it-

' initially started.

04:18:58. RCS temperature decreases below 500 F.

IDADS Print out l

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5' 04:19:15l;0perators: secure "A" CR/TSC Essential HVAC to

.IDADS Print reduce-the noise level in the control room.

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Operator Statement

,104:20:00 - Pressurizer. level-offscale low. Subcooling IDADS Print margin is 85 F and increasing.

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04:20:+: _ Shift Supervisor sends a computer technician-

' Operators Statement-

' to. check'the ICS-power supply, e

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The technician reports that all. four,ICS 24 VDC

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-power. supplies are de-energized.

The Automatic y

Bus' Transfer (ABT)'has not transferred,and is-still on.the "C" bus (vital-bus)!which'is-still energized.

It is later determined that the ICS "J" bus-loads-(non-vital bus)-hadiearlier been connected ' to ' the "F'J. power. bus. 'The "F" _ power-bus

-(non-vital) also supplies power tolthe TSC-

' Fire System (which alarmed upon' re' actor; trip). ;Also,

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portions of the security;1ighting which~is pawered -by ^

the 2C bus _which' powers the'F, bus was momentarily out'about:thetimeofthe:reactortfip.

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This'isLthe first of four-[ individuals who.indpedndently check the ICS power supply, cabinets and ch) ~not note ~

'that the power supply switches (S1 add S2) are open.

'04:20:20 OTSG pressures have decreased to;500 psig.

9 IDADS Print-out At this, pressure the running condensate pumps begin'to supply MFW to the OTSGs-thrcugh the idle MFW pumps.- This adds'approximately 1000 gpm of flow to each OTSG.

04:21:25 Minimum RCS pressure of 1064 psig is reached.

IDADS Print

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out 04:22 The plant exceeds the B&W recommended pressure /

IDADS Print temperature _ limits for pressurized thermal ~ shock

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- (PTS) of the reactor vessel (i.e., the PTS region).

,The nil ductility temperature limits in the-Technical Specifications are not violated during o

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- this event.

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/ 4:22

' Operator initially throttles HPI injection 0

flow'(Note: RCS-pressure is beginning'to increase, but pressurizer level is still off-scale low).

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-04:22:50' 0TSG pressures _have decreased to 435 psig.

IDADS Print out d.

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3 Main Steam Line Failure Logic actuates

.which closes the startup and main MFW valves.

MFW flow from the condensate pumps is stopped.

04:23 ADV and TBV isolation valves shut locally Operator (i.e., by handwheels) by operators.

Statement 4

04:23:10. Non-licensed operator attempts to close "B" AFW IDADS Print (ICS-controlled) control valve using the valve out/

handwheel.. The valve is partially closed by the Operator operator, however, much of the flow appears to Statement have been merely redirected to the "A" AFW valve.

The operator thought he had completely IDADS Print closed the valve at this point. AFW out flow to the "B" OTSG, however, has decreased by only about 60%.

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04:25:30 Operator opens the HPI pump SFAS-controlled recircu-IDADS Print.

lation valves, opening the recirculation path to the out makeup tank, to prevent overheating the pumps when flow is subsequently further throttled.

However, the discharge valve from the Makeup Tank is still closed at this time.

04:26:15 CR/TSC Essential HVAC train "B" is secured to Operator further reduce noise levels in the Control Room.

Statement Operator notes that the signal to start it in the high temperature /high radiation level mode was present. The significance of this fact I

is still being assessed.

04:26:?

Operator attempts to close "A" AFW (ICS-IDADS Print controlled) control valve using the valve out handwheel.

04:26:22 "A" AFW valve closed.

However, much of the flow Operator appears to have been merely redirected to the Statement partially.open "B" AFW valve.. Operator believes the "A" AFW valve is only 80% closed and leaves to locate a valve wrench.

04:26:47 Pressurizerlevelisbackon~scaleandincre$ sing.

IDADS Print Subcooling margin is 170 F.

Operators throttle out HPI injection valves to decrease the rapid increase of reactor pressure.

04:28:00 Makeup tank level offscale high. IPressure Relief IDADS Print Valve opens and discharges to the, Flash. Tank.

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04:28:00 Operators manually stop "C",-RCP p.er core, lift, IDADS Print restrictions. The procedures require.that the out u

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• i RCP be tripped when RCS temperature decreased to y,

500 F at 04:18. RCS temperature is now 490 F.

04:28:59 Operators stop "A" HPI pump.

IDADS Print out 04:29:40 Non-licensed operator uses valve wrench on "A" Operator AFW (ICS-controlled) flow control valve. ' Manual Statement

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portion of valve operator is apparently damaged.

The valve suddenly fails to the open position.

Operator calls Control Room and is told to close the "A" AFW manual isolation valve.

04:29:40- RCS pressure peaks a second time at 1616 psig. RCS IDADS Print temperature is 422 F.

out 04:29:45 Operator closes "C"

& "D" HPI injection valves IDADS Print to reduce the repressurization while temperature out is still decreasing.

04:30 Shift _ supervisor declares Unusual Event. The Operator Senior Control Room Operator notifies State Statement and County agencies.

'04:30:30.0perators start depressurizing RCS, in an attempt to Operator return to condition outside PTS region, using Statement &

normal-pressurizer spray.

RC Pressure

_ Plot 04:33:20- Another non-licensed operator; arrives at thet"B" AFW Operator control valve and closes it all'the way.' AFW.

Statement &

to the "B" OTSG is now stopped;Lhowever, some' IDADS Print of this flow may have been merely' diverted to

.out-the "A" OTSG.

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04:33:40 The "A" OTSG is full to the top of the steam

~ IDADS Print shroud-and begins to spill water into the steam out annulus and into the main steam lines. At this '

time the AFW flow to the "A"'OTSG i's in" excess'of 1300 GPM.

04:35' Operatorsclosethe"A"boopBWSTsuctionvalve Operators in an attempt to mitigate the high level in Statements the mskeup tank (MUT). However, the discharge valve from the MUT is still shut.

(Note: -on the simulator,. shutting the BWST suction valve automat-ically opens the discharge valve from the MUT.

This ia not the case in the plant). This action

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isolates the suction to the makeup pump, the "A" HPI, and the "A" LPI/DHR pump.

04:36 Non-licensed operator attempts to close the Operator "A" AFW manual isolation valve but it will Statement not move, even with valve wrench.

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. +i fRCS-subcooling margin' reaches peak of 201JF IDADS Print'

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._ and begins'to' decline.'L(RCS_ temperature = 390 F,.

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EiRCS pressure = 1430 psig) ~This'is approximately

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f800 psi.into-thePTSregion..

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4 04:40%

An SRO~ discovers that:the switches-(S1 and!S2)

Operators

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<to.thelICS.DC power supplies'are tripped to.'the.

Statement /

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off position. ;ICS power is restored by closing ~

IDADS Print.

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Jswitches S1'and:S2 in ICS cabinet:3.. (Note: ~

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the:ICSjDC Power; Supplies.are located in.ICS

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l Operator / Systems Response totRestoration of'ICS Power ~

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,c-A Initially -the' ADVs,: TBVs and AW valves

' receive sidemand'. signal to go fully open i

when power is restored. However, the 1

iisolation valves for all:but.the "A" A W valve e

Lhad.been closed. !The Control Room Operators s

.ismeediately switch to manual (i.e., hand) control N

_. cand shut the. valves.'

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A11 ~AW flow to both 'OTSGs has ceased.' RCS l

d' ibegins'to heat.up.: Lowest RCS temperature of

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-386LF was reached and at this. time'RC pressure

,-(1413 psig) is being' reduced.to achieve-

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. conditions outside the. PTS region.- The' plant 1has= cooled'down.180 F'in 24 minutes.

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04:41':00 50perat'oricallsl Control Room and reports.

Operator

' that the,"A"-A W manual-isolation valve is stuck:

-Statement

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open. The operators

are direc.ted to disengage theinanual handwheel on the "B"

-A W flow contro1~ valve. Other; operators.are

" directed.to unisolate the ADVs!and TBVs.

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IDADS Print

-04:41:101'"A" OTSG 1evel decreases belowl steam shroud.:

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.-04:42:42'_ Operator stops the.l"B" HPI pump.DThe makeuptpump-y IDADS Print-y Q, ~' [;,3." -

out lis continuing to run.

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404:42:56.- Operators close the "A'.' & "B"*HPIIinjection.

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-valves.-

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'!O4:43i50' Operator notes loss o'f RCP' seal injection,

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Statement ib4:43:54' Operator.restartsthe"B"'HPIpum[ tor-IDADS. Print l establish RCP ' seal l injection flow.f out~

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04:49:-

Leakage. (steam). from the damaged makeup Operator

~.e pump is released via the auxiliary building Statement

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- ' ventilation-system. The-release' is"within

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. Technical' Specification limits /R The "

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, auxiliary building stack radiation monitor f; m.,, ;

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alarms an'd: isolates the auxiliary building '

g, ventilation system, stopping-the release. L. " f y L

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j, 04:50:19;.OperatorLat' ops the "B"..HPI pump.7- (Note:

,;IDADS Print 4

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~ Operator.had re-aligned valvesiin the seal e

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injection flow path.)-

'04:50:30 _ Operator again notices loss of seal flow,: and operator

. restarts. the '?B" HPI pump.

(Note: loss of RCP Statement d

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-seal flow is due to'the' failure of the makeup pump..The operators are not yet

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aware of the. failure.)~

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d4:52'

.One.of the four SR0s onishift (not the shift Operator

' supervisor) collapses in front;of the control Statement:

panel,iThis SRO had:previously_ assisted in closing the ADV and TBV manual isolation

. valves.,

'05'00, Operator in' Control' Room-hears a' loud noise.

Operator

He.looks down-at makeup pump ammeter and Statement-

no'tes itiisireading about 1/3 of normal' 1 running current. He realizes the makeup pump

-r ha's.been; damaged (due to' lack of suction).

b e s f05:00:10 10perator, trips makeup pump. Operator opens:

Operator-makeup: tank discharge valve, which allowed Statement

'wa.ter>to spill out of'the damaged' makeup

' pump.onto the pump room floor._ The operator

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e subsequently. shuts the' discharge valve.

Approximately 450 gallons is spilled.

05:05:

RCS pressure' decreases-out.of. PTS' region.'A 'hree Operator t

Statement /-

- hour soak lis, initiated. >(RCS Pressure = 870 psig,

-RCS temperature = 428 F).~

IDADS Print out 05:05 lAn Ambulance is called for SRO'who collapsed.

Operator

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Statement

.Both-AFW pumps are manually stopped while OTSG IDADS Print 05:09' g

1evel is reduced via the OTSG drain lines out to' allow re-establishment of normal MFW flow with the c.'.aensate pumps.

,05:27l

= Operators isolate the makeup pump. Note: Operators Operator isolate'the pump by entering the pump room Statement which contains airborne radioactivity and 3 to 4

~ inches.of contaminated water on the floor.

'3 e operators do not wear respirators because k.

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10 they were not available in the area and thus become as contaminated due to water on the floor.)

~05:29:04f 0perators stop'the second ("A") RCP.

IDADS Print out 05:33 A smoke detector locks-out Rad Waste Area Operator

-Exhaust ~ Fans.

It is believed that this is Statement caused by damage to the Waste Gas Compressor

-which was damaged due-to water from'the flash' tank which was. filled when the makeup tank' overflowed.

05:40 Main Steam Line Failure Logic is " inhibited."

Operator t

This permits MFW flow to the OTSGs.

Statement 05:46:?.

Reactor building radiation monitor fails.

due to shutting the suction valve when SFAS initiated and isolated the reactor building.

Security 05:54

.A non-licensed ~ operator' loses his security badge.

He is escorted to the Control > Room by a Security

' Report /

Guard. The Control Room calls security and re' quests Operator that a spare security badge be b'rought to the Statements Control Room.

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06:06:00 Operators " Bypass" SFAS.

' Operator Statement

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06:11 Momentary " Loss of ICS or Fan Power" alarm.

. Operator The S1 and S2 switches remain closed and the Statement alarm clears without operator action. No

' equipment response is noted.

06:14-

" Loss of ICS or: Fan Power" alarm.

ICS-controlled Operator valves again receive 50% demand signals.

Statement

.0perators immediately reset switches S1 and S2

,to restore ICS power.

ICS-controlled valved again reveive 100% demand signals. -Operators manually shut the valves from the control room.

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06:15 Security brings a spare security badge to the Control Security Room.

Report 07:00 SRO released from hospital.

Operator Statement 08:41:

Terminated Unusual Event.

Emergency Coordinator Log a