ML20217G366
| ML20217G366 | |
| Person / Time | |
|---|---|
| Site: | Byron  |
| Issue date: | 07/30/1997 |
| From: | Hosmer J COMMONWEALTH EDISON CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9708070230 | |
| Download: ML20217G366 (4) | |
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o,num.nwpim ni-n <omiun3 1 ei Opus l' tat t i>,w nm onne. n < osis s ni July 30,1997 U. S. Nuclear Regulatory Commission Washington, D. C. 20555 Attention: Document Control Desk
Subject:
Response To Request For Aoditional Information (RAl) -
Uhlmate lleat Sink Technical Specification Amendment
References:
1)
J. Ilosmer letter (Comed) to USNRC regarding Byron Station UllS Technical Specification Amendment Request, dated May 6,1997 2)
G. Dick letter (NRC) to I Johnson (Comed) regarding Request for AdditionalInformation Related To Revision of Ultimate lleat Sink Technical Specifications, dated June 26,1997 In Reference 1, Comed submitted a request to change the Ultimate lleat Sink Technical Specification 3/4.7.5. Reference 2 is an NRC Request for Additional Information (RAI) concerning the single failure analysis discussed in Reference 1.
Attached find the technical response to the questions in the RAI, if you have any questions regarding this matter, please contact this office, bd[.
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John B. Ilosmer Engineering Vice President Attachment ec: A. B. Beach, Regional Administrator - Region Ill
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G. F. Dick, Jr., Project Manager - NRR S. D. Burgess, Senior Resident Inspector - Byron C. Phillips, Senior Resident Inspector - Braidwood Office of Nuclear Safety - IDNS 9708070230 970730 Cr7 p p o DR ADOCK 05000454 PDR j
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ATTACilMENT REQUEST FOR ADDITIONAL INFORMATION RELATED TO REVISION OF ULTIMATE IIEAT SINK TECilNICAL SPECIFICATIONS FOR BYRON STATION
- 1. -Question: In your basin temperature evaluations for the various scenarios considered, you discuss the single active failure of one of the cooling tower fans. It is not clear to the stafTthat you have considered the worr' ase single active failure during your analyses. Please describe why the single hetive failure of a cooling tower fan is the most limiting single active failure. In your response you should discuss the possibility of single electrical failures (e.g., loss of bus, Breaker trip, loss of diesel generator) that can result in the loss of more than o.s fim.
g Response: Previous analysis for the 1992 Ultimate lleat Sink (UllS) design basis reconstitution and 3/31/92 Technical Specification Arrrndment Request evaluated a variety ofinitial conditions and single active failures. Postulated single active failures analyzed included: 1) Containment Spray (CS) Pump Failure, 2) Essential Service Water Cooling Tower (SXCT) Fan Failure, 3) Emergency Diesel Generator (EDG)
C ilure,4) Essential Service Water (EX) Pump Failure, and 5) Bypass Valve Failure.
a Fcr this Amendment Request the edsting calculations were reviewed to identify the most hmiting scenarios. Three sceurios (one SXCT fan failure scenario and two bypass valve failure scenarios) were revised to ;aclude the new inputs.
These scenarios are moc iimiting because the heat input into the UHS is high while the heat removal capability is minimized. A summary of the afTc;ts of the single failures is
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provided in UFSAR Table 9.2-16. For comparison, the following sesults were obtained in the 1992 revisions of the UllS calculations for the difTerent single failure scenarios-Single Active Failure Initial Basin Temperature Calculated Peak Basin
( F)
Teraperature ( F)
CS Pump 96 96.5 Cooling Tower Fan
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OA Bypasr Valve Fails Or :n 70 91.4 OB Bypass Valve Fails 0,ien 70 i90.9 EDG Failure which preven.s 70 89 S
{ Closure of a Bypass Valve
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3 As discussed in the 3/31/92 UllS Technical Specification Amendment Pequest, failure of an EDG could result in a total of only four SXCT fans being availiuie (assuming two initially Out of Ser ice (OOS) and a loss of two more fans due to the EDG failure). If an EDG were to fail on the accident unit, only one SX pump would be running and the fans of the afTected Reactor Containment Fan Cooler (RCFC) train would remain deenergized and the RCFC discharge check dampers would Jose. The resultant heat removal from containment would be approximately one half of the load calculated for the scenarios where all four RCFCs are available. With approximately half the heat load and four out of eight fans available, the EDG failure scenario is less limiting than the SXCT fan failure scenario with half the full heat load and five out of eight fans available.
Two SXCT fans and two RCFCs are powered from the same 4160 volt bus.
N Therefore, a bus failure has the :,ame effect as a EDG failure. The 4160 volt feed
'J breakers for the SXCT fans do not auto open on a Loss of Offsite Power (LOOP)/EDG load sequence. Thus the breaker remains in its safe position for the accident and its failure is not postulated. Failures of active breakers or switches in the 480 volt SXCT bus would result in the failure of a single fan. Therefore, a single active breaker failure cannot result in the loss of two SXCT fans.
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- 2. Question: In your UHS basin water level evaluation you describe tv o scenarios.
Both of these scenarios identify the failure of one of the makeup pumps as an example of a single active failure. Similar to Item 1 above, please provide information to verify that you have considered the worst case single active failure that could afTect UHS basin level. In your response you should discuss the possibility of electrical failures that may result in the loss of one or more fans, in addition to the loss of one of the makeup pumps (i.e., loss of bus that supplies power to fan (s) and makeup pump).
Response: Dasin volume is a function of the makeup rate versus the SX system inventory losses. Two pumps are normally maintained operable to provide makeup.
The makeup analysis postulates a single active failure of one of the makeup pumps which reduces the makeup capability by 1350 gpm for a failed SX makeup pump or 550 gpm for a failed deep wc!! pump.
Sources ofinventory loss include: 1) evaporation, 2) bl; down,3) drift 4) possible use of the SX system as the backup Auxiliary Feedwater (AF) supply source,5) strainer backwash, and 6) system leakage or pipe breaks. Single active failures which could affect inventory loss eith:r reduce the inventory loss or are bounded by the failure of one of the makeup pumps as follows:
1)
Evaporation is a function of the number of active cooling tower cells, the heat input into the SX system, and the weather. The makeup analysis assumes all tower cells are active (i.e., fans running in high speed with riser valves open),
maximum accident heat load, and weather conditions corresponding to the maximum one day evaporative period. Single active failures which affect the ability of the SXCT to reject heat (i.e., fan failure) would reduce the evaporation rate. Failures which affect the heat load on the tower (i e., EDG k:nlaibyrbwdisgrpit:nsRAI DOC
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failure) would also reduce the evaporation rate. Therefore, an electrical failure which could cause the failure of a fan in addition to a makeup pump w"ild be less limiting than loss of the makeup pump alone.
2)
Blowdown is administratively limited to 300 gpm per train, for a total inventory loss of 600 gpm. The analysis assumes that blowdown is isolated within two hours of an event which challenges SX basin volume. An active failure of a blowdown isolation valve (failure to close) would result in an additional inventory loss of 300 gpm. This is less than the 550 gpm of makeup which is lost when a deep well pump is assumed to fail.
l 3)
Drift is a function of the physical design of the cooling towers and is not affected by active failures.
l 4)
SX supply to the AF system is a function of the Steam Generator (SG) level, the decay heat load, and the cooldown rate. Procedural guidance is in place to l
mitigate AF system failures that result in an increase in AF flow beyond that l
oiscussed in the 5/06/97 submittal. The duration of any increase in AF flow due to an active failure would be short and the increased SX inventory i vs would be bounded by the postulated loss of makeep due to a makeup pump failure that lasts for the duration of the event.
5)
SX strainer backwash is administratively controlled such that backwash is initiated manually with consent of the unit NSO. The NSO would be aware of any event in progress so that backwash should not be initiated during an accident when basin level is challenged. In the unlikely event that backwash was in progress at the start of the accident or w".s inadvertently initiated during an event and the backwash isolation valve failed to close at the end of the backwash cycle (normal cycle time is approximately 3 minutes), operator ac9on would be taken to manually isolate backwash. The additional SX inventory loss until backwash could be isolated is bounded by the postulated loss of makeup due to a makeup pump failure that lasts for the duration of the event The backwash procedure is being enhanced to further clarify operation during events challenging brin level (NTS #454-240-97-055-05).
6)
System leakage or pipe breaks. These are passive failuies and are not postulated concurrent with an accident.
Based on the above discussions, Comed believes that the worst single active failure has been considered in the UHS level evaluation.
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