L-PI-06-046, Cancellation of Licensee Event Report (LER) 1-05-01, Discovery of Single Failure Vulnerability of Unit 1 Safeguards Buses

Cancellation of Licensee Event Report (LER) 1-05-01, Discovery of Single Failure Vulnerability of Unit 1 Safeguards Buses

ML061640447
Person / Time
Site:	Prairie Island
Issue date:	06/09/2006
From:	Thomas J. Palmisano Nuclear Management Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
L-PI-06-046 LER 01-005-01
Download: ML061640447 (2)

Text

N Commttted to Nucl Prairie Island Nuclear Generating Plant Operated by Nuclear Management Company, LLC L-PI-06-046 10 CFR 50.73 U S Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Prairie Island Nuclear Generating Plant Unit 1 Docket 50-282 License No. DPR-42 Cancellation of Licensee Event Report (LER) 1-05-01, Discoverv of Single Failure Vulnerabilitv of Unit 1 Safeguards Buses

Reference:

1) LER 1-05-01, April 4, 2005, (ML050960556).

Reference 1 transmitted the LER for a condition discovered in the Prairie Island Unit 1 safeguards 4 kV buses. At the time the LER was submitted, Nuclear Management Company, LLC (NMC) postulated that the Unit 1 safeguards bus 15 and bus 16 did have a single failure vulnerability due to current transformer (CT) circuits for the source breakers for these buses from the CT11 (Cooling Tower 11 transformer) source. The CT circuits are tied together with a secondary CT on the common wires which feeds metering equipment. Thus, because NMC believed the condition violated the single failure criterion for redundant components, the condition was reported pursuant to 10 CFR 50.73(a)(2)(ii)(B).

Subsequent to the transmittal of the LER, NMC conducted additional analysis that has concluded that a common mode failure resulting in a simultaneous lockout of both Bus 15 and Bus 16 due to the common metering circuit was not a credible event in the as-found condition. To demonstrate that a dual lockout of bus 15 and bus 16 is not credible under hot short conditions, the analysis showed that the required magnitude of the fire induced fault is much greater than the available fault current in any of the associated circuits. To demonstrate that a dual bus lockout due to an open circuit under non-fire induced conditions is not credible, the analysis showed that the resultant imbalance current will be less than the trip setpoint of the affected relays.

Therefore, because subsequent analysis has concluded that bus 15 and bus 16 were not vulnerable to a potential single failure, the as-found condition is no longer reportable and NMC hereby cancels LER 1-05-01. Notwithstanding cancellation of the LER, NMC 1717 Wakonade Drive East Welch, Minnesota 55089-9642 Telephone: 651.388.1 121

Document Control Desk Page 2 will still pursue corrective actions to address this condition via the site corrective action program.

Summaw of Commitments This letter contains no new commitments and no revisions to existing commitments.

~ h o m a sJ. Palmisano Site Vice President, Prairie Island Nuclear Generating Plant Nuclear Management Company, LLC cc: Administrator, Region Ill, USNRC Project Manager, Prairie Island, USNRC Resident Inspector, Prairie Island, USNRC