ML20039C270
| ML20039C270 | |
| Person / Time | |
|---|---|
| Site: | Byron, Braidwood  |
| Issue date: | 12/15/1981 |
| From: | Tramm T COMMONWEALTH EDISON CO. |
| To: | Harold Denton Office of Nuclear Reactor Regulation |
| References | |
| 3053N, NUDOCS 8112290153 | |
| Download: ML20039C270 (12) | |
Text
.
[ )4 Commonwealth Edison
) one First National Plaza. Chicago. Ilbnois 8kC Address Reply to: Post Office Box 767
\\
Chicago, Illinois 60690 December 15, 1981 s.
b.
S RECEgygg Mr. Harold R. Denton, Director Office of Nuclear Reactor Regulation jp OEC2 8 ;sgg, 29 U.S.
Nuclear Regulatory Commission tems Washington, DC 20555 gggas 77 g
Hz
/
/
Subject:
Byron Station Units 1 and 2 4
Braidwood Station Units 1 and 2 Auxiliary Feedwater System Reliability NRC Docket Nos. 50-454/455/456/457 References (a):
September 18, 1981, letter from T.R.
Tramm to H. R.
Denton.
(b):
October 27, 1981, letter from T.R.
Tramm to H. R.
Denton.
(c):
October 30, 1981, letter from D. G. Eisenhut to L. O. De1 George.
(d):
November 10, 1981, letter from T. R.
Tramm to H. R.
Denton.
Dear Mr. Denton:
This is to provide additional information regarding the reliability o f the Byron /Braidwood Auxiliary Feedwater System ( AFWS).
References (a), (b) and (d) provided the results of analyses of AFWS reliability for Byron and Braidwood.
During the review of these analyses with the NRC staf f, certain recalculations were requested.
The recalculations have been completed and are summarized in the enclosure to this letter.
The reanalysis conforms to SRP 10.4.9 by taking credit for a second auxiliary power supply to the feedwater pump.
An unavailability o f 9.4 x 10-5 per demand has been demonstrated.
Upon receipt of staf f concurrence with this approach, the response to FSAR Question 10.53 will be revised to include the results of this reanalysis.
Please address questions regarding this matter to this office.
8112290153 811215' k\\k PDR ADOCK 05000454 A
H. R. Denton Decembe r 15, 1981 One (1) signed original and fif ty-nine (59) copies o f this letter and the enclosure are provided for your review and approval.
Very truly yours,
~hhr W
T.R.
Tramm Nuclea r Licensing Adminis trai.or
Enclosure:
" Byron /Braidwood Stations, Auxiliary Feedwater System Reliability Analysis, Second Recalculation," December 14, 1981.
cc:
I. Papazoglou, Brookhaven Nt'l Lab.
TRT/Im 3053N 1
._,--,.,c.---..
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December 14, 1981 Byron /Braidwood Stations
Subject:
Auxiliary Feedwater System Reliability Analysis Second Recalculation af Summary of Results
~
Unavailability per Demand Initiating Report (l)
First Second Recalculation (2)
Recalculation.
Event 6.4 X 10-4 8.9X10-5 9,4X10-5 LOOP Assumptions 1.
LMFW coincident with LOOP (3)
LOOP considered on the unit under study only 2.
from Bus 241 Credit taken for manual breaker closure to BUS 141 3.
4.
Other assumptions per NUREG-0611 Discussion ESF Bus 141, the bus that supplies the 1A motor driven auxiliary is capable of being supplied from one of three sources:
f eedw ater pump, The system aux transformer (SAT 142-1); the diesel generator (DG 1A); or the Unit 2 ESF Bus 241.
(See Figure 1).
In the event of a LOOP event, Bus 141 automatically transfers to DG 1A.
the operator is able to close two breakers f rom If DG 1A fails to start, the control room to f eed Bus 141 f rom Bus 241, w hich still has power f rote the system aux transformer associated with that bus or f rom diesel The operator is capable of closing these breakers within generator 2A.
in this analysis.
the 20 minute steam generator boil dry time assumed The capability of supplying the Unit 1 ESF busses from the Unit 2 ESF a new feature and has existed in the B/B design since its busses is notIn f act these crossties are the alternate source of off site beginning.
The power to the ESF busses reauired by our Technical Specifications.
NRC Staf f has reviewed this system and has f ound it to be acceptable.
. The crosstie between Bus 141 and safety loads of Bus 141.
Bus 241 is capable of feeding all the to handle the safety loads of both Bus 141 and Bus 241, hence the the crosstie does not compromise the safety of Unit 2.
The assumption of loss of off site power to only one unit the results presented is supported by in Table 1.
is 0.605 events / year.The total frequency of loss of offsite
-power 1to a single unit
"~
The major contribution to the frequency is f ailure of the SAT's, which is 0.3 per transformer with I
a total of 0.6 for the two transformers supplying a single unit.
second' contributor to the LOOP per unit The line supplying the ring bus section that feeds the SAT's of a unit.is a f ault assoc contribution is.0.005 events / year.
This 1
As can be seen from. Table 1, 4 -
to both, units is 3.6 X 10-4, the events that cause loss of off site power the major contributor being tornado damage
.(A'ttachment A).
This is a small number compared to the overall estimated f requeicy,ef LOOP. to the unit, 1s applicable to only one unit.and hence, we have concluded that the LO in1t:iating event how e,v e r,
e fad;f rom Bus 241 wheh _ Bus 241 is being supplied from diesel
~
<2 A.,
There are no breaker interlocks to prevent this operation.
~rt The crosstie operation will be conducted under a specific set of circ 6msta'nces and controlled by a well defined emergency operating p96c ed u re.
The procedure will cover the following conditions:
f l),
LOOPhasoccurredondnit being supplied to the Unit I and there is no au<iliary feedwater I steam generators because the 1A diesel / generator f ails to, start and the 1B Auxiliary Feedwater pump cr fails to start.
/
(2) a)
LOOP nas not occurred on Unit 2.
The crosstie breakers may be closed to Bus 141 to pick up AFW pump 1A.
or i
t b)
LOOP has occurred on Unit 2.
Unit and only/ normal shutdorn loads are present.2 AFWS is operating normally Both of the Unit 2 emergency olesels have started and are carrying their i
respective ESF busses.
pick up Bus 141 and the 1A AFW pump.The crosstie breakers may be closed to Nonessential loads on Bus 2el may be stripped in_ order to accomodate the lA AFW pump load.
Diesel generator 2A operation must be monitored closely
' to ensure that the< generator is not overloaded.
e t
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a 1
e 1 A
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. The above operations can be completed within 20 minutes of the initiating event, ahich is the steam generator boil dry time assumed in this analysis.
He wish to emphasize that LOOP to both units is not an event that we consider to be credible.
Nevertheless the capability exists to ensure sufficient auxiliary f eedw ater f low to the faulted unit.
The number associated with remote manual closure of the two breakers reauired to supply Bus 141 from Bus 241 is 1.2 X 10-2 per demand.
There are two components to this unavailability number:
1X10-2 per manual action (this is considered to be one manual action) and IX 10-3 unavailability per demand f or each of the two breakers.
The unavailayflity per demand of diesel generator l A was chosen to be 3X10-2, (J)
Conclusion (4)
Credit for use of two auxiliary power supplips to Bus 141 leaves us with an AFWS unavailability per demand of 9.4X10-3, which meets the requirements of SRP 10.4.9.
LAB:mnh 0984b*
Notes (1)
Table 4.3, under " Automatic Startup w ith Manual Backup" p 4-14, Byron Units 1 and 2, Braidwood Units 1 and 2, Auxiliat y Feedwater System Reliability Analysis, Final Report, dated August, 1981.
(2)
October 27, 1981 letter from T.R. Tramm to H.R. Denton.
(3)
NUREG-0611 assumed a diesel generator unreliability number of 1X10-2 We have not recalculated our system unavailability using the lower diesel generator unreliability, however it stands to t eason that the system unavailability would yet be lower than the 9.4X10-0 presented in the results section.
(4)
Other approaches to solving our dilemma of adequate AFWS reliability without a second motor driven pump supplied f rom the other ESF bus were considered.
They are discussed in Attachment 2.
LAB:mnh 0984b*
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psTtsAt w. u :s CEPAnn.ST November 23, 1981 Byron Station Loss of System Aux. Power Supply i
Approx.
Frequency Dur a t ion (Events / Year)
(Hour s)
Grid Collapse or Islanding
< 10' M 5 kV Switchyard d estroyed
- 3. 6 x 10 i
by tornado i
4-% 5 kV lines outaged due 2.5 x 10-(1) to independent and/or cocron cod e outages Subtotal - events isolating 4
the entire station from the
- 3. 6 x 10 interconnected system l
Cherry Valley line outage causes 0.005 1
( O wee,1)
LBB isolating SAT Subtotal - % 5 kV supply to SAT 0.0054 SAT outage (2) 0.6 110 Subtotal - events outaging "
one SAT 0,6 05 l
Total - all evcn ts outaging 0.605 SAT (1) Frequency increases dur ing line maintenance outages which average less than 1 day per line per year.
(2) Duration of out age on the faulted transformer.
The other transformer can generally be retained to service by switching within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />. Freqirency is twice that for sing 1 e full-sized transformer.
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CHERRY VALLEY L,062i BULK
[i]
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POWER l$
BLUE.
d ' I'u"stol SYSTEM g )-msE
[1 BE--4 mv
'4*-2 um wetsow t.issos o--[]
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WEMPLETOWN L.oG24 LJ [H i
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Bb5Es CHERRY VALLEY L.0622 14 2 v [ _ _ _ c.9 x.v 3
auscs 2W
- p_ 4.i kv aoses BYRON STATION 34 SKY BUS CONFtGURADOt 6YSTEM PLANMWG DEPG 'ratNr POVD4Bt.R 20,t%
(
l N~ k b b )3 kI Anc e r_/
N ovember 18, 1981 m pegg CEPART!ENT Complete Outage of Byron Station Off-Site Power Due'to Tornadoes
[
The fr equency of tornadoes outaging all sources of offsite power C
to Byron Station has beeen estimated.
Such outage could occur either from a j
tornado damaging each of the rights-of-way leading to the station, or from a t
tornado hitting the switchyard at the station.
j The present Byron transmission plan utilizes 3 tights-of-way leaving the station site in different directions:
to Cherry Valley (21 miles northeast; lines leave site in an easterly direction); Nelson (33 miles southwest; line leaves site in 'a southerly direction); and Wempletown (30 miles north); line i
leaves site in a northwesterly direction). A rear?nably straight tornado path cannot i g rsect all three rights-of-way except by hitting the relatively small area of the station switchyard.
The fr equency of this event is estimated as once in 2800 years.
{
If the Wempletown right-of-way were eliminated, tornadoes heading in the prevailing northeasterly direction and passing within five or six miles of the station could outage both of the remaining rights-of-way.
We estimate the frequency of this occurrence as once in 350 years, or eight times as frequent.
In this case the lines were modelled to the points where they intersect the present Nelson-Cherry Valley right-of-way, as there is a negligible risk of a tornado intersecting-two rights-of-way on the same line but separated by several miles.
The results are based on 102 tornado path legths and compass headings observed in the Chicago area (th? "Centennicl" data) whose summary statistics are as follows:
Standard Mean Deviation Range Path Length (miles) 6.3'e 10.45 0.52 - 68.4 Compass Heading (degr ees) 64.5 19.1 19 - 135 (00 = North, 90 = East)
The station switchyard was assumed to present a 900 foot wide " target" to any tornado heading, allowing for the tornado path width.
The analysis method is an improval computerized version of that presented by Mr. John Teles at the 1980 American Power Conference, with certain analytical refinements relating to the path lengths and compass headings of tornadoes.
7 Y
A. W. Schneider, Jr.
System Planning Department Approved:
Other Approaches de attempted to show that restoring of f site power to Unit I within the steam generator boil dry time, or even within the core uncovery time, a credible event.
is one transformer f ailure, the mostThough we certainly have good f eelings that follow ing likely event, the operator can restore the other system aux transformer within a sufficiently short time period.
Similarly, if a Cherry Valley Line outage occurs, we are reasonably certain that station personnel can isolate the fault and restore power to the system aux transformers.
Unf ortunately we have not compiled sufficient data to support our claim.
We are continuing to search out data that will provide reasonable reliability numbers f or operator action following either a transformer failure or a line outage.
Until such time we cannot assume that offsite power is restored.
- mnh 0984b*
/