ML20059M726
| ML20059M726 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 08/17/1993 |
| From: | Rueger G PACIFIC GAS & ELECTRIC CO. |
| To: | |
| References | |
| OLA-2-I-MFP-015, OLA-2-I-MFP-15, NUDOCS 9311190267 | |
| Download: ML20059M726 (15) | |
Text
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- acific Gas and Hectric Company 7 Bea:e 5::ee: F::rn 1451 egorytr @e';f.I N E 70 E:n 770000 Stnior vice Prestcent ana
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- n esv3 25n April 27, 1993 793 C*,i 23 P6 :15 PG&E Letter No. DCL-93-098 e .. ;. U.S. Nuclear Regulatory Commission 48/'I' ATTN: Document Control Desk 2
L Washington, D.C. '20555~
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Re: Docket No. 50-2/5, OL-0PR-80 Docket No. 50-323, OL-DPR-82 Diablo Canyon Units 1 and 2 Licensee Event Report 1-93-005-00 Medium Voltage Cable Failures Gentlemen:
Pursuant to Item 19 of Supplement I to NUREG-1022, PG&E is submitting the enclosed voluntary Licensee Event Report 1-93-005-00 regarding the failure of certain medium voltage 4kV and 12kV cables. These events.did:
not affect- the health and safety of the public.
Sincerely, Mr Gregory M. Rueger cc: - Ann P. Hodgdon John B. Martin Mary H. Miller Sheri R. Peterson CPUC Diablo Distribution 1NPO ocae ufc-2115-CLA_ cum on k 7AFA-M tn m e.aa:, NMWL.MSarst_ELtATAM, Ce DCl-92-EM-N054 w' '
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RECElVED APR 2 71993 '
CHRISTOPHER J. WARNE kIM"79g7 o
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LICENSEE EVENT REPORT (LER) '.f i
F AC1WTY h AAdi (1) DOCR E'T WMtm (?) PAOf f?t DIABLO CANYON UNIT 1 0l5l0l0l0l2l7l5 1 l ' l '*14 mum MEDIUM V0LTAGE CABLE FAILURES rvt=r cArc isi ,
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UCFM8ET CONT ACT POR T>es LEP fitt TELEDMCwE MMBER DAVID P. SISK, SENIOR REGULATORY COMPLIANCE ENGINEER ^"5^
- 05 545-4420 COMPLETE ONE UNE FOR EACH COMPONENT F Aall#LE D&aCRISG 4N T644 nEPORT (13)
CAust $75 FEM CCPWONEht MA FAC. ArPosta ( CAUSE $Y3tcM COMPONENT 844 8AC. a Poa A (
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EXPECTED-SUBMIS110N lX l YES (if yes, ccrolete EXPEcfED submiss 10N DATE) l l NO 08 01 93 A85tAACT (16)
This voluntary LER is being submitted for information only purposes as described in Item 19 of Supplement 1 to NUREG-1022.
Diablo Canyon has experienced three 4kV and two 12kV circuit underground cable failures. The cables were manufactured by Okonite and were installed _in 1973 and 1974. .. . . e .
Laboratory analyses have established that the 12kV cable failure mechanism is chemical attack. The cause of the 4kV cable failures is still under-investigation; however, laboratory analysis has also established that the contaminants found in the 12kV cable jacket and shield are not present in the 4kV cables. PG&E believes that the 4kV and 12kV cable failures were caused by different mechanisms.
Final corrective actions will be identified following completion of the~ root cause investigation.
60895/85X
j i LICENSEE EVENT REPORT (LER) TEXT CONTINUATION ' ,
LEn ansets (61 Paet (3)
FOC3Liff NAME (1) tioCEET enseCa (2) ggvampn VMA SOWm&L DIABLO CANYON UNIT 1 0l510l0l0]2l7l5 93 -
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- 1. plant Conditions 1
Units 1 and 2 have been in- various ' modes _and at various- power levels.-
II. Descriotion of Event ,
A. Summary: .
Diablo Canyon Power Plant (DCPP):has: experienced threet4kV ~and two '
12kV circuit cable (EA)(CBL)-failuresk These; failures have occurred!
only in underground duct bank conduits.(FA)(DUCT)(CND) between the-turbine building (NM)'and the intake structure (NN) (see Figure 1).:
Both 12kV cable failures occurred.on Unit 1 between the first pull box-outside tha turbine- building'_ and the. next pull ' box tow'ard the discharge structure elevation'.
The subject cables were manufactured by.0konite in 1972,fand are - .
insulated with black ethylene-propylene-rubber (EPR) and jacketed with- '
neoprene. The cables were installed'at DCPPrin 1973 and 1974; The' cable circuits were then energized. intermittently until'1984,'when the '
circuits were then placed into. service.. J Laboratory analyses have established that-the 12kV~ cable failure '
mechanism was chemical attack. The cause of the 4kV cable _:failuresLis- ,
still under investigation; however, laboratory, analysis has also established that the contaminants four.d in.the 12kV~ cable jacket and :
shield are not present in the 4kV cables. , ,
B.
Background:
- =- .aw, n.
a Cable Construction The 4kV cables are rated for up to SkV.applic'ations and the 12kV cables are rated for up to 15kV applications'.-
The cable construction is similar.for both the 4kV and 12kV cables.- 1 The cables are a shielded ' single-conductor' cable design and are i constructed.in concentric layers:(see Figure 2). There are four cable layers of interest for this LER: (1) the insulation (EA)(CBL)(INS) 1ayer, which is approximately.115 mils ~ of ~ black EPR for the.4kV cables '
and approximately 220 mils-of black EPR for-the 12kV cables;':(2) the~
~
1 tinned copper shield tape, which acts to equalize the: electrical. :)
stress; (3) the wax coated synthetic. binder tape, which" holds the'- l copper shield against the cable, thereby. allowing the outer jacket ~to be extruded over the cable.during manufacture;' and (4).the outside 1ayer, which is a neoprene jacket. The function'of the outer neoprene i jacket is to protect the cable during installation'(impact and' ~
abrasion resistance) *and to act as = a physical -barrier. between the..
6089S/85K I
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION moon - m on -.. m u .g. . e ~m DIABLO CANYON UNIT 1 0l5l0l0l012l7l5 93 -
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0l0 3 l"l 1 f.II 417) outside environment and the shield once the cable is installed. The cable Jackets are not bonded to the cable shield or. insulation layers.
Duct Bank Construction The 4kV feeders consist of three separate single-conductor cables (one for each phase), routed in a single conduit (one circuit). The 12kV motor-driven circulating water pump (CWP) (KE)(MO)(P) feeders consist of six separate single-c:r.ductor cables (two for each phase), routed as two separate three phase circuits in two single side-by-side conduits (two circuits).
The subject cables are routed in two' separate sets of duct' bank conduits, one for each unit, between the turbine building and the intake structure. Concrete vaults are located at various intervals to serve as pull boxes for the circuits. These duct bank conduits a.e directly buried in sand and are covered for their entire length by a six-inch thick concrete cap. The duct bank conduits include 12kV, 4kV, and 480V (CBL4) power cables,120V ac control cables (EF)(CBL3),
125V de control cables (EJ)(CBL3), and instrument cables (EF)(CBL1).
The pull boxes immediately outside of the turbine building have drains (DRN), which are routed to common sump vaults (manholes) (FA)(PBX) for Units 1 and 2. These manholes are equipped with automatic submersible Class II sump pumps (PBX)(P).
The Unit I and Unit 2 trenches are similar, except that the Unit I duct bank rises to cross over the circulating water discharge tunnel and then slopes downhill towards the intake structure. This design makes the Unit I section of cable conduits near the turbine building
- sust-eptible to submergence if the pull box sump oumps are not functional and if the water within the pull boxeTrises above the conduit openings.
Cable Testina In accordance with applicable industry standards, DCPP performs direct-current, high-potential testing.(hi-pot testing) as a maintenance activity each refueling outage. Normal practice for hi-pot testing of DCPP 4kV motors (MO) is to hi-pot the motors from the switchgear (SWGR) end of the circuit, through associated cables and motor terminations. The normal hi-pot test voltage level used at DCPP for 4kV motors is 10.5 kV dc.
The 12kV motors, due to hi-pot test equipment limitations, are hi-pot tested locally at the motor with the cables disconnected. The cables are hi-pot tested separately from the motor. The maximum voltage level used for the in-service 12kV cables is 30 kV dc.
50895/85K
- LICENSEE EVENT REPORT (LER) TEXT CONTINUATION
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C. Event
Description:
4kV Cable Failures I On October 29, 1989, ground current alarms (EA)(GI)(IA) associated with the Unit 2 Auxiliary Saltwater (ASW) Pump (BA)(MO)(P) 2-2 annunciated twice and immediately cleared both times. The pump was removed from service. A ground was found on one cable that runs between the turbine building pull box and the first pull box at the discharge structure. Cables in this pull box were found submerged-in water. The faulted portion and a similar length of the other'two cables for this circuit were removed, the water was pumped out of the pull box, a mandrel was passed through the conduit several times to remove standing water, and new cables were installed. Visual examination found no obvious physical defects in the removed cables.
Cable samples were sent to Okonite and to the PG&E Technical and Ecological Services (TES) laboratory for examination and testing. The cable testing determined that the cable met or exceeded the original mechanical and electrical stress limits set forth in'the original purchase specification, except for some minor loss of mechanical strength of the jacket material. The testing laboratories determined the failure to be an isolated event.
On May 3,1992, intermittent ground current alarms were received associated with Unit I nonsafety-related 4kV Bus D (EA)(BU). The 4kV Bus D was removed from service. The ground was determined to be located on one cable between the pull box located immediately outside of the intake structure and the Bus 14D transformer (EA)(XFMR) in the intake structure. The pull box located outside of the intake structure was found to have water inside, which was pumped out to j 4cilitate cable repbcement. Approximately 40 feet of the single i faulted cable was replaced. Visual examination found no obvious physical defects in the removed cable. Cable samples were sent to Okonite and to the PG&E TES laboratory for examination and testing.
The examination and testing determined that the physical properties of the insulation were normal and that the jacket properties displayed some loss of elongation which was considered normal for neoprene installed for almost 18 years. The electrical properties were normal for 18-year old cable. The testing laboratories could identify no definitive reason for the failure.
On October 31, 1992, ASW Pump 1-2 on Unit I was removed from service for refueling outage maintenance. As part of the procedure to return the pump to service, a motor hi-pot test was conducted. During the hi-pot test, the cable insulation developed a ground fault at approximately 6kV. Investigation determined that the fault was located on one cable between the first pull box outside the' turbine building and the next pull box at the discharge structure elevation.
Cables in this pull box were also found submerged in water. The 60895/85K
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faulted portion and a similarL length of;the'.other two cables <for this. ",
circuit were removed : thel water- was pumped out of the pull box,?a L mandrel was passed through1the. conduitiseveral times' to remove' ~ ..
standing water, and new , cables 1were installed. lOther than:at"the? l fault point, visual examination { revealed'no obvious physicalfdefects; ,
in -the removed cables. Cableisamples'weressent to'0konite,ethe(PG&E-. '
TES 1aboratory, :and' Cable Technologies Laboratory,(CTL): for?
axamination.and testing.- In February 1993, an.additionalisample was. :
sent to Altran Materi'als for chemicalJanalysis. ; Interim' progress-reports < indicate that the electrical = properties;of the cable are' ;
acceptable, no firm evidence of manufacturing ~defectsihas beeni identified, .and no ' indication.offinstallation. problems kor, abnormal- '
operating conditions canlbeyfound. The root cause investigationifor.- 1 this event-is in progress.; .
y 12kV Cable Failures' On February 5, 1993,. Unit 1 was ramped:down;from 100"to'46 percent; R power due to a ground current alarm for CWPJ1-1~. (While-the ground !
alarm annunciated. smoke was' reported in the 12kV switchgear: room 1, (NM)(EA)(SWGR) duel to ground; resistor bank heating.of accumulated ..
1 dust, as is expected;during ground fault conditions. An Unusual Event- 9 was declared at 2156 PST due toia precautionary assistance / request to an offsite_ agency. An immediate emergency report was made'to: report ;
the declaration of an' Unusual' Event'in accordance withT10 CFRL 50.72(a)(1)(i). ,
~ ;
Investigation determined that the ground was: located on one cable- ;
between the first pull box outside the turbine. building-and the next1 pull box at the discharge: structure; elevation. . When the cables were
- _ removed, cables ofthe the neoprene circuit' for jacket a distance
~
was o found,f approximately 200 teet.'to beJ
~
The faulted portion and, a'similar lengthLof. the otherL two cables-for; this circuit were removed,'the water was . pumped out off the pull, box, a" mandrel was. passed through theLeonduit several times to remove 1 standing water, and new cables'were installed; WhenLthe newEcable sections were spliced in, no visual' degradation was foundlat the splice locations. Cable samples.were-sent to 0konite,; the;PGLE TES- ,
laboratory, and Altran . Materials for comprehensive examination!and 1 testing. Interim progress reports; indicate that-the outer neoprene _ 1 Jacket had been chemicallyadegradedand the copper' shield:shows' -
evidence of corrosion. -However, el.ectrical properties,of the cable-are acceptable,'no evidence of; manufacturing-defects has!been identified, and no indication of anlinstallation' problem or abnormal- :1 operating conditions can;be found. The; root:cas e, investigation for -i this event is in progress.
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DOCKEY MuMBLR (2)
F1CILITT NAME () """"
. C DIABLO CANYON UNIT 1 0l5l0l0l0l2l7l5 93 -
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While removing the failed CWP l-1 cable from the conduit, water was introduced into the conduit from the discharge structure elevation pull box in order to lubricate the cables for removal. No water came out the pull box at the turbine building end of the conduit run.
l Investigation determined that the six-inch acrylonitrile-butadiene
! styrene (ABS) conduit was broken, and that this damage had resulted from the initial attempts to remove the cable.
On March 12, 1993, intermittent ground current alarms annunciated for Unit 1 CWP 1-2. The Unit was ramped to less than 50 percent power and the pump was secured. Investigations determined that one cable was-shorted to ground between the first pull box outside the turbine building and the next pull box at the discharge structure elevation.
When the cables were removed, the neoprene jacket was found to be separating from all three cables of the circuit for a distance of l approximat ly 200 feet. There was evidence (pull box water marks) that the cables had previously been submerged in water.
i The faulted portion and a similar length of the other two cables for this circuit were removed, the water was pumped out of the pull box, a mandrel was passed through the conduit several times to remcVe standing water, and new cables were installed. When the new cable sections were spliced in, no' visual . degradation was found at +,he splice locations. Cable samples were sent to Okonite, the PG&E TES laboratory, and Altran Materials for examination and testing. Interim progress reports indicate that the outer neoprene jacket.has been chemically degraded and the copper shield shows evidence of corrosion.
However, electrical properties of the cable are acceptable, no evidence of manufacturing defects has been identified, and no indication of installation or abnormal operating conditions can be found. The root cause inyestigation for this, event is in progress.
Following the March 12 event, PG&E conservatively replaced all Unit 1 l non-failed medium voltage circuits between the pull boxes outside the I turbine building and the next pull box at .the discharge structure elevation (ASW Pump 1-14kV cables and the second 3-phase 12kV circuits for CWPs 1-1 and 1-2). Also, one complete circuit, between l the turbine building switchgear and the motor terminations at the l intake structure, of the Unit 2 12kV CWP 2-1 motor feeder circuits was replaced. The neoprene jacket on the Unit 1 12kV CWP cables showed i evidence of chemical degradation, similar to that on the previously .
l replaced Unit 1 12kV CWP circuits. Visual examination of the ASW 1-1 l 4kV cables and the CWP 2-1 12kV cables revealed no defects.
D. Inoperable Structures, Components, or Systems that Contributed to the Event:
SM-1 and SH-2 are the pull box drain systems and associated sump pumps for Units 1 and 2, respectively, for the pull boxes immediately 60895/85K 4
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outside~of the turbine building..' Investigation' determined that: water had accumulated in the pull 1 boxes as a result 'of'the pull' box: drain -
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systems and associated = sump ~ pumpst not being . functional .for; a; period of- ,
time preceding the cable failure ~ events.. t E. Dates-and Approximate Times;for Majorf0ccurrences:- .
1'
- 1. October _29, 1989: A Unit _2. ASW Pump 2-2,4kV_ cable ground -
alarm was recei.ved. , ,
2._May 3,- 1992: 'A' Unit:1Lnonsafety-related'Busi14D 4kV 1
. cable ground: alarm was received-3; October 31, 1992: . Unit 31IASW: Pump:1-2;4kV cable l '
~degradationLwas. detected during; mairstenanceihi-pot testing.
- 4. February 5, 1993: 'A: Unit linonsafety-related CWP 1-l'12kV-cable ground' alarm was received.. , ,
- 5. March 12, 1993:' 1A.UnitInonsafety-relatedCWPil-2.12kV cable ' ground _' alarm was received..
F. Other Systems or Secondary Functions'Affected: L None. .
s G. Method of. Discovery: ,
.c
- 1. The four "in-service" cable' failures ~ were:immediately apparent-
~
- to plant operatorsidue-to groundifault alarms.;and. indications- '
received in the control room:(NA)..
- 2. The ASW Pump 1 4kV cable degradation'was found during .
maintenance when Electrical Maintenance _ personnel l observed that the cable failed'to pass itsi hi-potitest.,
H. Operator Actions:
For the four."in-service" events,-the associated equipment was removed from service and troubleshooting activities;were.. initiated.
I. Safety System Responses:
None.
- 50895/85K
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. LICENSEE' EVENT REPORT (LER) TEXT CONTINUATION t :s uwege ' en - C*et (3)
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III. Cause of the Event A. Immediate Cause:
- 1. For the four "in-service" cable failures, tthe'immediate causeL was a conduction path lto' ground, which resulted.in sufficient.
current to. flow through the ground. detection circuitry.-
- 2. For the ASW' Pump 'l-2: 4kV. cable degradation,l the immediate- causa !
was failure of the cable' to withstand the. hi-pot : test 1 voltage - ,
during post-maintenance > activities.
B.. Root Cause:
Extensive' root cause investigation has.been conducted since~the.first ,
cable failure.in'1989. Threeipotentialtroot'cause,hypothesesLand/or' ?
contributory causes are being investigated::
~
l '. Manufacturing Extensive cable dissection,' wafer, slicing and staining, and examination have not found any evidence of' insulation breakdown. 1 i
due to moisture in any of the failed 4kV.or.'12kV cables.. ,
Based on evidence evaluated to date .PG&E believes that-the failures in the 4kV and 12kV cables are noticaused by. l i
manufacturing defects.
- 2. Installation ,
o Cable pulling tensions' for. the 4kV and 12kVLcables r' outed between:the turbine building and the~ intake structure.were.
evaluated.- TheJevaluation concluded that the vendor specified pulling tension and sidewallf pressure-limits were'not exceeded. ,
~
Based on video probe inspections and Lthe< ease of cable . ,
replacement, the duct' bank:is' intact,1except for the conduit section damaged during CWP.1-1 cable removal,,which isino longer used.
Operating Environment !
3.
l' Moisture Intrusion:: The cable designJ basis:is for wet?and/or-dry conditions. The cable can bel submerged for prolonged;' <
periods. ' Inquiries to'other nuclear'. power plants did'not . :
. identify any trends for medium voltage:EPR. insulated cable: . l failures in similar applications.. The Okonite black EPR cable 1 is widely used in electric utility distributiontsystems,: '
including PG&Es(five million feet has been installed in the PG&E S.
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0l0 '9 l'"l 1 Yiti (37) distribution system for a longer period of time than the DCPP cables), and has been highly reliable in similar applications.
Chemical Attack: Laboratory chemical analysis results have established that the 12kV. neoprene jacket was. attacked by a chloride and/or fatty acid.- High ambient' temperatures accelerate the' chemical related jacket degradation.- PG&E'has evaluated the cable operating temperature and estimated that,.in the worst case, the 12kV CWP cables are. operating with insulation temperatures in the range of_80_to 85ac. The 4kV' cables operate at a significantly lower temperature (60 to 70*C, based on self-heating) due to loading requirements. The hypothesized method of chemical ~ migration and-degradation is by way of water that intruded.into the conduits through the cable pull boxes that are located irumediately outside of the. turbine.
building; the water intrusion resulted from the inoperable sump pumps and associated pull box drainage system.
Maintenance Testing: The routine maintenance hi-pot testing of the 4kV and 12kV cables is within vendor recommended values and does not electrically overstress' the' cable. -
Summary PG&E has concluded that the most probable failure mechanism for'the 12kV cable insulation failures is long-term chemical degradation of The neoprene jacket, probably due to chloride and/or a fatty acid, followed by corrosion of the copper shield. When the shield deteriorates, uneven electrical stresses occur,. ultimately. resulting in a cable ground.
- m. ,u ,
PG&E believes that the 12kV cable failures occurred over an extended period of time (greater than a year). .This conclusion is based on the relatively mild pH (approximately 8.5)Lof the liquid found beneath the neoprene jacket and the copper shield binding tape showing no. evidence of high cable operating temperatures (the binding tape melts at approximately 85"C).
PG&E concludes that the failure mechanism of the 4kV cables was not chemical degradation. This conclusion is based on chemical analysis' indicating: (1) lack of excess chlorine or fatty acids for the 4kV cables; and (2) much lower levels of "self-heating." Laboratory.
testing has determined that the 4kV cables meet or exceed the electrical stress' requirements' set' forth in the original purchase specification. However, the root cause investigation is' continuing.
Cables rated 600 volts or less experience lower electrical . stress and, therefore, do not require a shield to equalize the electrical equipotential between the conductor and ground. Visual examination of--
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i em im cables rated 600 volts and lower show no obviousTsigns"of physical l
i degradation.
L
! IV. Analysis of the Event The 4kV power and associated control- circuits' potentially ~ affected by these.
cable failures are associated with the ASW pumps. ' The 12kV power and' associated control. circuits potentially =affected by these cable failures are'-
l associated with the CWPs.
~
The'ASW system has_a safety-related functionLto' remove he'at*from'other .
safety-related system components during normal operation and plant? shutdown-via the component cooling water'(CCW)(CC) systemi .The ASWLsystenfalsoc provides . vital cooling necessary for. the Lengineered. safeguards-feature (ESF) systems to perforr their functions.- Each DCPP unit has:two redundant ASW' trains, each consisting of.a full capacity ASW: pump and' associated' piping to-supply.the CCW heat exchanger. The"ASW~ pump motors are' supplied by vitali 4kV power. The ASW trains within' a unit: can' be cross-tied to allow"either* '
of the ASW pumps to supply either CCW heat. exchanger.L : InLaddition,!both units' ASW trains can'be cross-tied to add further. redundancy toithe: system.
The motor-driven CWPs are part of the' saltwater system which removes energy-from the turbine exhaust steam (SG) entering the main condenser-(SG)(COND) by providing cooling' water.to the condenser. Each' unit is provi.ded with two:
nonsafety-related 12kV motor-driven pumps 11ocated 'in the. intakeistructure.-
At 100 Jercent unit power level, both pumps mist ~runf to support full load operation.
In all cases, DCPP has ground detection alarms that provide indicatinn that.
A potential cable problem exists +- Upon receipt of;tMs alarm, . .
troubleshooting activities will determine the location.of. the problem, and the situation will be remedied.
A portion of the ASW pumps control circuitry is fed from the 125V de system,.
which is an ungrounded system. ~The: occurrence'of a ground in any 125V dc _
circuit is alarmed, but does not ~ affect the operation of1the circuit. itself. -
The 12kV, 4kV, and 480V systems;have high-resistance grounding, which allows <
continued operation for a limited time in the: event 'of a: single-line-to-ground fault. ~0perators have received simulator training on ground fault' incidents. The normal plant procedure,ias demonstrated.in the.past failure incidents, is to declare the associated component: inoperable, tand then troubleshoot and repair the' faulted circuit. The ground' detection system,.
as well as additional control room indication -(i~.e., red /greenilights; associated with the' motors), provide sufficient time tofidentify;and correct a problem prior to another failure causing a portion of a mutually redundant:
system from becoming -inoperable.
50895/85K
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION <
F ACILITV sumE (1) DOCIET NLMSER (2) F ES 98UMS(8 14) # AGE (3) viam ,
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DIABLO CANYON UNIT 1 Ol5l0l0l0l2l7l5 93 -
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loss of 120V ac control or low power instrument circuits does not affect the operability of any safety-related components.
The ability to operate for a. limited time with a ground on the CWPs ensures.
that time is available to bring the plant to 'a stable condition where the-affected pump can be removed'from service and the circuit repaired without challenging any safety systems.
In summary, since both ASW trains are electrically separate and independent, I a cable fault in one ASW train would not have prevented the other train from l performing its safety-related function. Furthermore, since the failures have been separated in time, and diagnostic examinations of the failed cables and additional "non-failed" cables'show no evidence that additional failures are imminent, the probability of a design basis accident followed by a random 4kV cable failure is considered to be very low. In addition, as discussed 1bove, the ASW system can be cross-connected to the ASW sy.cte- of the other unit. This helps maintain the' ability to perform the ASW system safety function of the unit with a design basis accident. Thus, the health and safety of the public were not affected by this event.
V. Corrective Actions A. Immediate Corrective Actions:
- 1. All Unit 1 ASW pump and CWP cablec becween the turbine building and the discharge structure elevation pull boxes have been-replaced.
- 2. PG&E TES, Okonite, and two independent laboratories were sent 4kV and 12kV cable samples for chemical, electrical, and
_ _ mechanical examination and 6dsting.
- 3. After the event on March 12, 1993, one circuit of the 12kV CWP 2-1 motor feeder circuit was replaced from the switchgear to the intake structure to verify the condition of the Unit 212kV cables. Visual examination revealed no defects in the removed cabl e. Unit 2 has not experienced any 12kV CWP cable failures.
- 4. The pull boxes were pumped dry and the sump pumps for the first outside pull boxes have been overhauled and returned to service.
B. Corrective Actions to Prevent Recurrence: 1
- 1. Hi-pot testing will continue for the 4kV and 12kV affected circuits during each refueling outage.
- 2. A formal preventive maintenance program has been established for the sump pumps and drains immediately outside the turbine building.
$0B95/85K
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. LICENSEE EVENT REPORT (LER) TEXT CONTINUATISN ~
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- 3. Final laboratory reports.on the' chemical' analyses and' electrical testing are scheduled for completion in May 1993.and the'results of these evaluations, including any further corrective. actions, will be reported in a supplemental LER.
VI. Additional-Information T A. Failed Components:
- Component: Medium Voltage Cable Manufacturer: Okonite Model Number: SkV.& 15kV Rated, w/ 133 percent' insulation Type: EPR Black Insulation w/ neoprene' jacket. ;
B. Previous LERs on Similar Problems:
None.
C. NRC Information Notice (IN) 92-81, " Potential Deficiency of Electrical -
Cables ~ with. Bonded Hypalon Jackets," dated. December II,1992 As part of the root cause investigation, IN.92-81.was reviewed fori applicability to the cable failures..at DCPP. 'IN 92-81.was: determined to not be applicable to the DCPP cable failures-since neither the 4kV nor the 12kV cable jackets are bonded to the' cable shield / insulation. .
9 50895/85K n .f d . ., . , , . .w.. .w .- . = w,.-.. =
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION on -.. m u. ., .
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,u, un DIABLO CANYON UNIT 1 0l5l0l0l0l2l7l5 93 0l0l5 0l0 13llId ;
1 Figure 1: Site Overview of Conduit Trench for Units 1 & 2 ;
l 3rd Event, ASW 1-2 PuI 10/31/92 h 4th Event, CWP 1-1 l l
02/05/93 5th Event, CWP 1-2 l 03/12/93
/ Unit 2 l&C Maint. y Pull Box o 4. - _ 1st Event, ASW 2 2, Puli 10/30/89 ;
Box Unit 1 gN Pull Box Discharge N Unit 2 Structure l
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Box Pull Box Unit 1 Unit 2 2nd Event, 4kV Bus D and Bus 14D, 05/03/92 Intake Structure l .
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