ML19317H018
| ML19317H018 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 03/31/1978 |
| From: | Mattimoe J SACRAMENTO MUNICIPAL UTILITY DISTRICT |
| To: | |
| References | |
| NUDOCS 8004020695 | |
| Download: ML19317H018 (6) | |
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Decke: No. 50-312 A
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-a Reper:a.le Occurrence ic-1 o
Dear d'r. Ingelken :
In accordance tri:h -'echnical Specifica:icas f:: ?ancho Seco Suclear Generating Sta: ion, Se: tion 6.9.4.1.b, the Sacranen:o l'unicipal U:ility D4s.ri
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discuss the analysis perforced on the transien: and i:s effec:s on the KCS, and se: forth the corrc::ive enian that has and rill be taken to prevent recurrence and to insure the integrity of plant sys: ens.
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3, 7 THIS DOCUMENT CONTAINS b004020 N 6 i
POOR QUALITY PAGES
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4 Dire:::: :f Re gula:ery Operations March 21, 197S b ~.
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- o :ero in response to f aulty signals.
The reduction in feedwater flew caused ?.0S pressure :o 'ncrease, with a reactor trip :: curring on high pressure ~
a: 0425.
In the period follewing the rea::cr trip,,the eperators were still harpered by the lack of ins:rucenta: ion available and by equipment responding :o indccu-rate si:na*.s.
For apo. roxinstelv, nine =inutes following :he : rip, pressure sicwly dec ye in :he RCS, remainin a: aoo.rexi=a:ely 2C00 psig.
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p e. d.. d..a.e.'.. e cooling pr:vided by makeup flew into the RCS and to the lif:ing of a ?ressuri er Code Safe:y '.'alve below its se: point of 2500 psig.
An auxiliary feedvater punp had s:arted on the loss of feedwa:er flew, h vever, :he auxiliary feedwa er valves rensined ci: sed in response to erroneous Once Through Stea: Generator (OTSG) s:ar up level signals.
These two signals were rendered ineperable by the NNI-Y D: power failure, the "A" steam generater level signal drif ting to
- cro indi:atien over a nine-:'nute period while :he "3" staa: generator level drif ed full scale.
The actual plant conditicas showed that both O!SG's ven:
dry during :his peried.
When the startup level for t'he "A" OTSG drif ted belew the low level setpoint, the ICS opened the auxiliary feedwater' valve admit:ing wa:er to :he shell side of the ' A" O!SG.
This inficw of water crea:ed a heat sink for the RCS resulting in a rapid pressure drop.
The operators also ray have. increased the main feed purp flew at this time, providing another source o f feed fi:w to the
'A" stes: genera:or.
The rapid dr p in pressure tock the RCS through the Safe,- Fea:ures A::ua: ion Syste: (SFAS) se:peint (1600 psig).
On SFAS signal, bo-5 auxiliar/ feedwa:er bypass valves opened which bege" j
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filling cf bo:h steam senera: ors with water.
i L'ntil power was res:ored :o SMI-Y approximate'y one hour and ten minu:es after :ne reactor : rip, the opera: ors con:inued the inje:: ion of auxiliary feedwa t e: that was s:ar:cd on SFAS signal.
I: did no: appear :ha: any RCS tarperature indica: ion was reliable, so the opera: Ors main:ained KCS pressura as well as possible utilizing the pressur':er level indi a:icn and :he RCS pressure indica: ion :ha: was available.
Cen:rol was ob:ained by adjusting high pressure inje:: ion flew.
The pressurizer hea:crs were not available due to the a*1 power loss, preventing their use for pressure centrol.
The continuous injection of auxiliary feedwater resulted in cc:plete filling of bo:h s:ca generators, af ter which wa:er beg:n :o enter the stea lines.
This large hea; sink continued :o coo.' off :he RCS, al: hough the operators were not aware of sys:e t e:pe ra:ure.
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Director of ?.c gula:Ory Opera:icns-March 31, 1973 operating region, incidding spraying :he pressurizer o reduce pressure, keeping
- hree RCP's opera:ing (pump combinations were changed) :o increase te:perature, shu::ing ef f auxiliary feedwater flow, and draining :he CTSC's.
Non-Nucleir Instruments::en ? ver loss The sher: caused by :he light bulb drew excessive curren: thrcugh the 24-vol: 2c pcwer supplies which service componen:s in NN: cabine:s 5, 6, an.d 7.
The power fer these cabinets is designited NNI-Y, with :he pcVer for cabinets 1, 2, 3 'and 4 being designated NNI-X.
The four power supplies for NNI-Y are operated current-lini:ed with a se: point of 7.5 aspe.
The subsequent reductionn in vol: age :eused an undervel: age monitor to epera:e, opening the :wo shun:
breakers through which AC power fro: inver:er D and inverter J is supplied to the CC power supplies.
Loss of these power supplies mean
- hat eve ry co:penent in cabine:s 5, 6, or 7 operating en DC power was no: functioning properly.
An NNI signal could have been affec:ed :wo ways between its source and :he receiving :::penent.
The signal could be interrupted cc ple:ely due to a centact opening en Scing deenargized.
3ecause =est of :he signals are
-10 veits :o +10 veits, this would have resulted in a =id-scale reading or in some cases a reading anywhere between -10 volts and +10 volts being trans itted to the indicator er sent to the ICS as an actual plant parare:er.
If a signal condi:i:ning component (buffer a:plifier, square root extractor) was affec:cd, this would have =eant that the desired conditi:cing veuld no:
have been perforced on the signal or that :he component night not pass the true signal, resulting in erveneous values being sen: to the indicater or :o the ICS.
Since signal paths in the NNI are no: res:ricted to either the N or the Y cabine:s, abou: two-thirds of the signals passed through at least one co:ponent in cabine: 5, 6 or 7 and were thus rendered invalid.
These spurious readings had several effects.
I: was difficult for the opera:crs to ascertain which of their indica: ors were valid, given :he changing plant conditicas and the wide variety of possible errors : hat were int:0duced.
Only a select few parace:ers were known :o be valid readings, and :he cpera:crs had to cen:rol the plan: based on :ha: infor:::ica.
The second effec: was : hat spurious signals were fed into the ICS, so equip:en: was Operated au:ena:ically, #
withou: regard to ac:ual conditicas.
The first evidence of this was :he runback of :he rain feed purps to cere, which caused the rea::Or trip.
la:er, the ac:oza:ic actions. involved with adding feedva:er to the dry stes: genera: Ors hindered operator actions and precipi:ated the rapid depressurizatien leading to SFAS ini:iation.
Power was finally res:cred :: the NN -Y when opera: Ors realized :ha: the shunt breakers were open.
Restering ;;wer re:urned the ncn-nu: lear ins:ru-
=en:a:icn :: opera:ica, perni::ing prcper opera::: response to the plan:
condi:icn, which at :his :ize was cu: side :he pernissible opera:ing region.
V"
6 Oire::cr of Regula: ry Operations
-4 Ma rch' 31, 19 73_~
Transier-2nsivsis There was very li::le per:anen: recosd of ' the - plant parameters during the ::ansient.
A na. tor source of infor:a: ion was the ?os:LTrip Transien:
Reviev, which prints out selected computer points periodically following a rea:::: : rip.
I:.vas no: possible to ex:ensively analy:e this and the c:her da:a available (recorder cutputs, hourly logger typer, e:c.) during the transient.
Over a perici of several days following :he incident, engineers were able ts trace which signals vere valid, deter =ine wha equip =ent operated at which ti=es,
and then interpela:e :o arrive at a te perature trace.fer the RCS.
I=cedia:ely following :he trip, :e:peratures slowly increased while pressure decayed away, :ain:aining approximately 2000 psit.
At approx 1:ately nine =ine:es af ter the trip, feedwa er began to. enter the "A" stea: generator, resulting in an RCS depressuri:ation leading to SFAS initiation.
With the full auxiliary feedwa:er flew initiated by STAS, RCS :e:perature fell fro: the high of about 595*F to 235*F in the span of slightly more than one hour.
This cooldown ra:e of approxizately 300*F per hour is well above :he permitted rate of 100*F per hour stated on Figure 3.1.2-2.
When NNI power was restored with :he RCS at 235*F, measures were taken to bring the plant back within limits by increasing terperature and reducing pressure.
To assure that all co:ponen:s of the plant vere not da: aged by.the.:ransient,:
available infor:ation on the RCS pressure and :e:perature, CTSG pressure, te:perature and level, feedwater flow ( ;ain and auxiliary), pressurizer level and other relevant paraceters were transeit:ed to the reac:or vwdor, Sabcock and "ilcox, fer analysis.
On March 23, the Dis:rict received a response ~ fro:
35W i.!teating :ha: : heir analysis was co:ple:e.
After evaluating the effec:s 1
of :he transient on :he reac:or vessel, the reacter coolant piping, the ptessuricer, :he OTSG's :he fuel asse:blies, the RCP's and seals, and the con:rol rod driv'e techanists, it was recc : ended that Rancho Seco be permit:ed
- o return :o power, under certain specified conditions.
The analysis was sub=1::ed :o :he Of fice of Suclear Reac:or Regula: ion, and on March 2', : hey agreed :ha: ?.ancho Seco could return to operation, prov' ding the :endi: ions centioned in the 31W le::er were fulfilled.
When the Distri:: receives :he cerple:e analysis fro: 35W, a copy vill be forwarded to the SEC Regional Office.
Corre::ive A:: ion The incedia:e corre::ive a::ica taken foll:ving :he rapid cocidevn was
- :o re: urn the p'an: to :he per:Issible opera:ing regica of :he Technical Specifica:ica fisure.
The unit was then kept shu:d:wn while da:a was ga:hered and sent :o 3;W for analysis, and fur:her inves: iga:icas in:o :he inciden:
vere made
- :he plant.
The Yanage en: S af e ty Review
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- ae held a. ree:ing on the subje:: on Mar:h 21, and issued sever:1 directives.
A co- '- aa of three engineers frc: :he elec:rical, ze:hani:a1 and nuclear dis,ciplines was l
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Dire:::: of Regulatory. Opera: ices !! arch 31, 19 73
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r, if the emer;cncy shu:devn systens are adequa:e :: whether sete redesign is
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de: ermine if any conronen: dana.re had cecurred,~ and :0 rece :end corre::ive actiens.
The findings of this cc=rittec will be relayed to the NRC Regi:nal-
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Of fi:e when : hey a re available.
The PRC was directed :o review :he circunstances
-connec:ed wi:h the incident, and to review the 35% re::::endations which were
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the recontendations were a: ep::ble.
If any disagreements developed, no re:ura
- o power was permitted until the MSRC reviewed the nat:er and de:cr:ined' ac. o. r:pria:e action.
Upon receipt of the S&W rece==endations, the ?RC reviewed : hen and reques:ed : hat a special test procedure and a casurl:y procedure be wri::en
- assure cc pliance.
PRC concerns about the folletting 1:ers also had to be satisfied prior to star:up:
1.
A questien was raised concerning possible datage to stea lines frem the inje : ion of water, so the lines were checked for any defor ations.
,2.
A 2255 psig leak test was perforced on the P.CS to insure integrity.
3.
The overvoltage trip se: points on the NN! DC power supplies were increased from 27 volts to 29 volts to prevent spurious trips.
The special test procedure addressed the condicions imposed by 35%, such ~
as reac:or maneuvering licits for :he firs:-startup, increased survaillance of 7
the 1cese parts monitors for a week, an operability check of on-line and redundant "NI instrumentation, and daily surveillance of :he primary and secendary radiochemistry for a week to che:k for leaking cc ponen:s.
The casual:y procedure was written :c provide required :pera::: a:* ions for restora:1:n of NNI power following a : rip similar :o tha: e:g e rien ced.
The PRC also ::::itted to having a procedure wri: ten by April 7 giving opera:or ins: ue:icns if NNI power canno: be res:ored.
Curren: 5:atus Following.NRC's ' review of the 35W analysis of :he ::ansient, : hey agreed that a return to power was accep:able if :he S&W re:orrenda:icns ere icileved.
On Friday, " arch 21, the reactor was :aken cri:ical, and the initial pceer ascensi n was begun.
The one-week surveillan:e progra s required by 35% are
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Fellosing the reae:or trip mI caused when feedvarer flow as redu:ed free invalid si:n21s. lack i, i 31 i of ?.CS tencerature indica:ie resui:ed in racid 2001do -- be:ause
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