05000325/LER-2012-005
| Brunswick Steam Electric Plant (Bsep), Unit 1 | |
| Event date: | 08-29-2012 |
|---|---|
| Report date: | 10-29-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| Initial Reporting | |
| ENS 48253 | 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| 3252012005R00 - NRC Website | |
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
Introduction Initial Conditions At the time of the event, Unit 1 and Unit 2 were in Mode 1, at approximately 100 percent of rated thermal power (RTP). Emergency Diesel Generator (EDG) [EK] No. 2 was inoperable for planned maintenance.
Offsite power and the remaining EDGs were operable.
Reportability Criteria This condition affected the ability to locally control EDG 2. Local control of EDG 2 is credited in the safe shutdown analysis. As such, this event is being reported in accordance with 10 CFR 50.73(a)(2)(ii)(B) as an unanalyzed condition that significantly degraded plant safety. The NRC was initially notified of this event on August 30, 2012 (i.e., Event Number 48253). Due to the shared configuration of the onsite AC Electrical Distribution System [EB], this event is applicable to both Units 1 and 2.
Event Description
On August 28, 2012, during planned maintenance on EDG 2, a post-maintenance continuity testing associated with the Alternate Safety Shutdown (ASSD) switch on EDG 2 revealed unexpected results when the switch was taken to the LOCAL position. During this work, Maintenance was verifying model numbers on ASSD switch 2-DG2-SS-A1 installed in the EDG 2 local panel. Verifying this number required the removal of the switch cover. Due to the congested wiring within this switch, the work order required testing of the contact wiring once the cover was reinstalled. The post-maintenance test consisted of continuity checks across remote terminal points in the diesel control panel with the switch in both the NORMAL and LOCAL positions. When continuity checks were made across the 2-2C contact with the switch in the LOCAL position (i.e., contacts open), the switch appeared to be closed. After subsequent troubleshooting, a wire was found to be incorrectly installed between terminal points TBF-6 and TBB-64 that essentially jumpered around the switch contacts.
An additional wiring error was discovered associated with the EDG 2 speed switches. The negative leg of the EDG speed switches were found to be wired upstream of the ASSD switch. However, the incorrect jumper noted above electrically connected the speed switches to the correct location. If the jumper above had not been installed, the speed switches, which are essential to EDG operation, would have been inoperable when the ASSD switch was placed in the LOCAL position.
The erroneous wiring in the field was not reflected on the control wiring diagrams or interconnection wiring diagrams.
At 2134 hours0.0247 days <br />0.593 hours <br />0.00353 weeks <br />8.11987e-4 months <br /> Eastern Daylight Time (EDT) on August 29, 2012, it was concluded that the condition may impact the ability of EDG 2 to perform its intended ASSD function. In the event of a fire, an induced fault could potentially affect the ability to locally control EDG 2. Local control of EDG 2 is credited in the safe shutdown analysis. This condition did not affect the Technical Specification operability of EDG 2 and it remained fully capable of performing its intended safety function.
Event Description (continued) The EDG 2 control circuit was re-wired to match existing approved design. EDG 2 was returned to operable status on September 1, 2012. Subsequent inspections were performed and it was confirmed that similar wiring errors did not exist on EDGs 1, 3, or 4.
Event Cause The cause for the incorrect wiring is a historical error. Therefore, no root cause was determined.
A historical review identified no maintenance or engineering change activities that introduced the wiring discrepancies. The original installation of ASSD switch 2-DG2-SS-A1 occurred in 1979.
EDG2 ASSD functionality is tested in accordance with plant procedure OPT-12.12.L, "Diesel Generator 2 Local Control Operability Test." This test demonstrates that the control and indication for EDG 2 can be isolated from the control room and controlled from their respective local control stations. However, OPT-12.12.L does not check the continuity of the contacts for each switch to determine if there is an issue with any individual contact or current path. Thus, if only one contact opens, as long as it breaks the circuit connection to power or ground, the test will be completed satisfactorily. As such, the test does not demonstrate proper isolation from control room circuitry in the event of a fire induced fault.
Safety Assessment The actual safety significance of this condition is considered minimal. This condition did not affect the TS operability of EDG 2 and it remained fully capable of performing its intended design basis accident response functions. No actual fire induced trip condition occurred.
From a probabilistic risk standpoint, the number of ignition sources capable of causing a fire scenario potentially affecting the cables is very low. Additionally, the frequency of a fire scenario occurrence in the ignition source is very low. The combination provides a very low potential for a scenario which would require the ASSD function. The impact on the consequences of an event is conservatively considered medium. Therefore the overall significance of the condition is low.
Corrective Actions
The following corrective actions were completed.
- The EDG 2 control circuit was re-wired to match existing approved design.
- The control circuitry for EDGs 1, 3, or 4 was inspected and no similar wiring errors were identified.
The following additional actions are planned.
- EDG local control operability test procedures (i.e., OPT-12.11.L, OPT-12.12.L, OPT-12.13.L, and OPT-12.14.L, for EDGs 1,2, 3, and 4, respectively) will be revised to require testing to verify the functionality of individual ASSD switch contacts. These revisions are currently scheduled to be completed by March 14, 2013.
Previous Similar Events
A review of LERs and corrective action program condition reports identified the following previous similar occurrence.
- LER 1-2008-006, Supplement 1, dated July 30, 2009, reported a plant modification that was installed on the four EDGs, which impacted the ability of EDG Nos. 2, 3, and 4 to perform their intended ASSD function (i.e., local control of EDG 1 is not credited in the ASSD analysis). The root causes for the event were a latent organizational weakness existed where the EDG control wire circuitry maintained the same wire segment number on either side of the ASSD key-switches, and a lack of sufficient rigor by Engineering in performance of their responsibilities during activities associated with modification of control logic circuitry.
The corrective actions from the LER 1-2008-006 event could not have been expected to identify this discrepancy. The corrective actions from the LER 1-2008-006 were focused on providing adequate barriers to prevent the latent organizational weakness associated with control wire labeling from resulting in development of an inadequate Engineering Change. To date, the corrective actions from LER 1-2008-006 have been effective in this respect.
Commitments No regulatory commitments are contained in this report.