ML20217F926
| ML20217F926 | |
| Person / Time | |
|---|---|
| Site: | Byron  |
| Issue date: | 10/03/1997 |
| From: | Hosmer J COMMONWEALTH EDISON CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9710090102 | |
| Download: ML20217F926 (3) | |
Text
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g October 3,1997 U. S. Nuclear Regulatory Commission Washington, D. C, 20555 Attention: Document Control Desk
Subject:
Byron Station Response to Review of Preliminary Accident Sequence Pwcursor Analysis of Operatior.at Event at Byron Station, Unit 1
Reference:
" Review of Preliminary Accident Sequence Precursor Analysis of Operational Event at Byron Station, Unit 1," G. F. Dick (NRC) to I. M.
Johnson (Comed), dated September 4,1997.
In the reference, Comed was requested to review the enclosed preliminary report and to provide any comments to the NRC on the accuracy of the information contained in that report within 30 days of receipt of the letter. The attachment provides Byron Station's comments on the referenced report.
If no have any questions regarding this matter, please contact this office.
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l John B, llosmer
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Vice Presidem 0
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A. B. Beach, Regional Administrator-RIII G, F. Dick, Jr., Byron Project Manager-NRR S. D. Burgess, Senior Resident Inspector-Byron Office of Nuclear Safety-IDNS 9710090102 971003 PDR ADOCK 05000454 p
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ATTACllMENT A Comed Review of Preliminary ASP Analysis For Operational Event at Byron Station, Unit 1 Comed personnel have reviewed the Preliminary Accident Sequence Precursor Analysis of the Operational Event at Byron Station, Unit 1 on May 23,1996 (reference). This analysis was prepared by Oak Ridge National Laboratory (ORNL) personnel under contract with the NRC. The preliminary analysis contained in the reference indicates that this condition may be an accHent precursor for 1996. The NRC's preliminary analysis indicates that the Conditional Core Damage Probability (CCDP) for this event is 2.5x10'. Comed's undcratanding of the criteria for determining that the May 23,1996 event for Byron Unit 1 represents an accident precursor for 1996 is the determination that the CCDP exceeds 1.0x10*.
The following represents Comed's review comments on the reference report:
1 Event Descrintign The event description accurately describes the initial conditions for the plant i
and the sequence of events starting with the loss of ofTsite power (LOOP) for Unit 1. Note, however, even though station personnel made a conscious decision to not cross-tie the Unit i E3F buses with Unit 2, the capability to cross-tie the buses existed from the beginning of the event, The decision to not implement the ESF bus cross-tie was made by station management considering the acceptable running condition of both Unit 1 Emergency Diesel Generators t
(EDGs) and the lack of an immediate root cause as to why offsite power was lost on Unit 1. In hindsight, it would have been fully acceptable and achievable to cross tie the Unit 1 to Unit 2 ESF buses immediately after the initiating event, if needed, due to a failure of the Unit 1 EDGs to run and load onto the ESF buses.
Additional Event Related Information In the first paragraph of this section, it should be noted that there are two (2)
Unit Auxiliary Transformers (UATs) for each unit, along with corresponding two (2) Station Auxiliary Transformers (SATs). The second sentence in paragraph one of the section should read, "During power operation, non ESF buses (including two of the four G.9 kV buses that provide power to the reactor coolant pumps and other large electrical loads) are usually aligned to the tica UATs u:hich ate fed from the output of the main generator, The other two 6.9 kV buses are usually fed from the SATs."
In paragraoh three (3), sentence five (5), the actual interlock setpoint pressure is c
337 psig instead of ~360 psig at which the motor-operated valves for the RHR suction lines are allowed to open. This provides additional operating margin to ensure that overpressurization does not occur in the RHR system piping.
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ATTACHMENT A MitdtMag Assumptions
. In general, the discussion of the modeling and/or assumptions for the Unit 1 event on May 23,1996 is reasonable. The following exceptions and/or clarifications are noted:
te use of failure to start and failure to run probabilities based on the 29 hours3.356481e-4 days <br />0.00806 hours <br />4.794974e-5 weeks <br />1.10345e-5 months <br /> of EDG operation is extremely conservative in that Comed has determined that offsite power was available via either of the ESF Unit 1 to Unit 2 bus cross ties, if needed. The Byron emergency procedures would have allowed for the cross tie to be established in an expedited manner. The determination to supply AC power to Unit I for 29 hours3.356481e-4 days <br />0.00806 hours <br />4.794974e-5 weeks <br />1.10345e-5 months <br /> from the EDGs was a conscious management decision based on the acceptable operating condition of the EDGs and the uncertainty of the root cause for the LOOP (i.e. to limit any potential propagation of a fault in the AC system from Unit 1 to Unit 2).
The AC RECOV branch calculation for failure to recover AC power appears to be overly conservative in that the potential failure of the ESP cross-tie breakers was modeled at (3x10*
- 2 breakers) or 6 x 10*. Note that offsite power to Unit I can be established via either ESF bus cross-tie. Therefore, it appears that the l
CCDP model sums the failure-to-operate probability when, in fact, either ESF bus cross-tie is fully capable of supplying the necessary AC power to maintain the umt in a safe shutdown condition. It appears that the net affect of this
- assumption results in a total CCDP value (2.5 x 10*) that is somewhat higher than it should be.
Additlpnal Comments No other comments were identified that warrant your consideration before finalizing the analysis. Should you have any questions pertaining to this review, please contact Brad Adams at Byron Station phone number 1815-234-5441 x2979.
Reference:
" Review of Preliminary Accident Sequence Precursor Analysis of Operational Event at Byron Station, Unit 1," G, F. Dick (NRC) to I. M. Johnson (Comed), dated September 4,1997.
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