ENS 47637
ENS Event | |
|---|---|
06:00 Feb 3, 2012 | |
| Title | |
| Event Description | VOLUNTARY REPORT - DESIGN VULNERABILITY IN 4.16kV BUS UNDER-VOLTAGE SCHEME
On January 30, 2012, a design vulnerability was discovered at Byron and Braidwood stations in the Engineered Safety Feature 4.16kV bus under-voltage protection scheme for Braidwood Station Units 1 and 2. Specifically a voltage unbalance created by an open circuit of either the A or C phase from the offsite grid to the System Auxiliary Transformers (SAT) is not designed to actuate the protective relays on the 4.16kV safety bus that provides isolation from the offsite grid and the automatic start and loading of the emergency onsite diesel generators. Two under-voltage relays are provided on each 4.16kV safety bus, which are combined in a two out of two logic to generate a loss of power signal. The relays are sensing voltage between two phases (i.e., A&B and B&C). An open circuit condition on the C phase or the A phase would not satisfy the two out of two logic. This condition results in both 4.16kV safety buses remaining energized with a bus undervoltage situation and results in equipment protective devices actuating from over-current conditions. This configuration is a non-conforming condition in that the design of the under-voltage relays and logic was intended to identify degraded grid conditions, not loss of a single phase. With an open circuit on the A or C phase from the grid to the SATs, during normal operations, operators have to diagnose the condition and manually isolate safety buses from offsite power which would automatically start and load the emergency diesel generators. During a design basis event concurrent with an open circuit on A or C phase from the grid to the SATs, analysis performed to date indicates that starting of the ECCS loads would have caused the bus voltage to decrease sufficiently to actuate the under-voltage protective relays and restore cooling with emergency onsite power without challenging fuel design limits. The 4.16kV safety bus under-voltage protection scheme at Byron and Braidwood is believed to be a typical industry design. This design issue is being evaluated at the other Exelon stations. The results of this evaluation will be shared with the NRC. Therefore, this condition is being reported as a voluntary notification due to its potential generic industry applicability." The licensee notified the NRC Resident Inspector. |
| Where | |
|---|---|
Braidwood  Illinois (NRC Region 3) | |
| Reporting | |
| Other Unspec Reqmnt | |
| Time - Person (Reporting Time:+16.17 h0.674 days <br />0.0962 weeks <br />0.0221 months <br />) | |
| Opened: | Mike Lindeman 22:10 Feb 3, 2012 |
| NRC Officer: | Donald Norwood |
| Last Updated: | Feb 3, 2012 |
| 47637 - NRC Website | |
Unit 1 | |
|---|---|
| Reactor critical | Critical |
| Scram | No |
| Before | Power Operation (100 %) |
| After | Power Operation (100 %) |
Unit 2 | |
|---|---|
| Reactor critical | Critical |
| Scram | No |
| Before | Power Operation (100 %) |
| After | Power Operation (100 %) |
WEEKMONTHYEARENS 528022017-06-12T14:14:00012 June 2017 14:14:00
[Table view]Other Unspec Reqmnt, 10 CFR 50.72(b)(2)(xi), Notification to Government Agency or News Release Offsite Notification for Discharge of Circulating Water ENS 476372012-02-03T06:00:0003 February 2012 06:00:00 Other Unspec Reqmnt ENS 467122011-03-30T20:38:00030 March 2011 20:38:00 Other Unspec Reqmnt Discovery of After-The-Fact Emergency Condition (Unusual Event) ENS 421872005-12-02T23:00:0002 December 2005 23:00:00 Other Unspec Reqmnt Reactor Power Operation in Excess of Operating License Condition ENS 405592004-03-01T22:00:0001 March 2004 22:00:00 Other Unspec Reqmnt 24-Hour Condition of License Report Involving Potential Violation of Maximum Power Level ENS 401302003-09-03T12:30:0003 September 2003 12:30:00 Other Unspec Reqmnt 24-Hour Report Required by License Condition -- Potential Violation of Maximum Power Level ENS 401232003-08-31T06:32:00031 August 2003 06:32:00 Other Unspec Reqmnt 24-Hour Condition of License Report Involving Potential Violation of Maximum Power Level 2017-06-12T14:14:00 | |