ENS 41436

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v t e ENS Event
18:00 Feb 23, 2005
Title	Potential Vulnerability with Alternate Shutdown System (Asds) Isolation Design.
Event Description	The licensee provided via facsimile the following report: During an extent of condition review of the corrective actions associated (with) Event Notification #41374, the Monticello Nuclear Generating Plant (MNGP) engineering staff made the following discovery. On February 22, 2005 at 12:00 hours, MNGP discovered a potential vulnerability with Alternate Shutdown System (ASDS) isolation design which could result in Bus 16 being locked out in the event of a Control Room or Cable Spreading Room fire. The Monticello Appendix R Safe Shutdown Analysis for Control Room/Cable Spreading Room fire assumes a loss of control of Division I and II equipment from the Control Room, however, safe shutdown is achieved remotely from the ASDS panel. ASDS design is such that a Control Room/Cable Spreading Room fire would not impede the ability to safely shutdown and maintain the plant in a shutdown condition. Contrary to the ASDS design, it was discovered that an un-isolated metering circuit from the 1AR transformer could result in Bus 16 being locked out in the event of a Control Room or Cable Spreading Room fire. The bus lockout relay from the 1AR transformer is not isolated by the ASDS transfer switches, therefore, this condition could result in failure of Bus 16 to re-energize during the implementation of the Shutdown Outside Control Room procedure. Since the Bus 16 feeder breaker from the 1AR transformer is not required at this time, it has been isolated from the safeguards bus to preclude occurrences of this event. The event is being reported as a potential loss of safety function (10CFR50.72(b)(v)(A,B and D) and as a degraded or unanalyzed condition (10CFR50.72(b)(3)(ii)(B)). The licensee informed NRC Resident Inspector.

ENS 41436 +/-
Where
Monticello Minnesota (NRC Region 3)
Reporting

10 CFR 50.72(b)(3)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition 10 CFR 50.72(b)(3)(iv)(A), System Actuation 10 CFR 50.72(b)(3)(v)(A), Loss of Safety Function - Shutdown the Reactor
LER:	05000263/LER-2005-001
Time - Person (Reporting Time:+1.33 h0.0554 days <br />0.00792 weeks <br />0.00182 months <br />)
Opened:	William Stang 19:20 Feb 23, 2005
NRC Officer:	John Knoke
Last Updated:	Feb 23, 2005
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v • d • e

v t e Emergency Notification System
Monticello with 10 CFR 50.72(b)(3)(v)(A), Loss of Safety Function - Shutdown the Reactor, 10 CFR 50.72(b)(3)(v)(B), Loss of Safety Function - Remove Residual Heat, 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition, 10 CFR 50.72(b)(3)(iv)(A), System Actuation
WEEKMONTHYEAR2024-06-28T06:10:00 [Table view]
[ENS list for Monticello]

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