05000325/LER-2006-005
| Brunswick Steam Electric Plant (Bsep) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3252006005R00 - NRC Website | |
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
Introduction At 1232 hours0.0143 days <br />0.342 hours <br />0.00204 weeks <br />4.68776e-4 months <br /> on August 6, 2006, the Control Room Emergency Ventilation (CREV) system [VI] unexpectedly realigned to the chlorine protection mode. Control room annunciation indicated that the actuation was due to high chlorine in the control room air intake. Upon further investigation it was confirmed that, although no chlorine was present, chlorine detectors 1-X-AT-2997, 2-X-AT-2997, and 2-X-AT-2997-1 were tripped. The chlorine isolation signal was removed by disabling the chlorine detectors. Operability of the radiation and smoke protection modes of the CREV system was restored and the system placed in its normal standby alignment at 1255 hours0.0145 days <br />0.349 hours <br />0.00208 weeks <br />4.775275e-4 months <br /> on August 6, 2006.
Subsequent investigations determined that unexpected actuation of the chlorine detectors and realignment of the CREV system was due to sensitivity of the chlorine detectors to Nitrogen Dioxide (NO2), a constituent of diesel exhaust fumes. These detectors were installed by a modification, implemented on February 3, 2006, to replace obsolete detectors in the chlorine detection system.
This failure mode has the potential to render the CREV system inoperable as a result of Emergency Diesel Generator (EDG) [EK] operation in response to a Loss of Offsite Power/Loss of Coolant Accident (LOOP/LOCA) event. Therefore, this event is being reported in accordance with 10 CFR 50.73(a)(2)(v)(D) as an event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident. Additionally, the condition existed since implementation of the modification on February 3, 2006. Therefore, the event is also being reported in accordance with 10 CFR 50.73(a)(2)(i)(B) as operation prohibited by the plants' Technical Specifications.
Event Description
Initial Conditions At the time the condition was identified, Unit 1 and Unit 2 were both in Mode 1, at approximately 100 percent of rated thermal power.
Discussion The CREV system is required to provide protection to the operators for three types of events: a radiation event, up to and including a Design Basis Accident; a toxic gas event (i.e., complete rupture of the chlorine tank car, or a slow leak lasting for an extended period of time); and an external smoke event.
To provide radiation protection, the CREV system automatically switches to the radiation/smoke protection mode of operation to prevent infiltration of contaminated air into the control room. A system of dampers isolates the control room, and a part of the recirculated air is routed through either of the two filter Event Description (continued) subsystems. Outside air is taken in at the normal ventilation intake and is mixed with the recirculated air before being passed through one of the CREV subsystems for removal of airborne radioactive particles.
In the event of a chlorine release, the CREV system enters a full recirculation mode (i.e., chlorine protection mode), with no outdoor air intake. The emergency filtration trains do not start, since they do not effectively remove chlorine and may be damaged by the presence of chlorine. Protection for chlorine gas "overrides" any concurrent, ongoing, or subsequent radiation or smoke initiation signals. The override design offers protection to operations personnel in the control room against potentially fatal chlorine gas releases. This protection is required any time the chlorine tank car is within the exclusion area. Operation of the CREV system in the chlorine protection mode renders the system inoperable in the radiation and smoke protection modes.
On August 1, 2006, a spurious actuation of two chlorine detectors caused the CREV system to realign to the chlorine protection mode. As a result, the chlorine protection mode of the CREV system was declared inoperable, Technical Requirements Manual (TRM) 3.19, "CREV System - Chlorine Protection Mode," was entered, and the chlorine tank car was removed from the exclusion area. The chlorine detectors remained in service to facilitate troubleshooting activities.
Subsequently, on August 6, 2006, at 1139 hours0.0132 days <br />0.316 hours <br />0.00188 weeks <br />4.333895e-4 months <br />, EDG 2 was started for a routine monthly load run. At 1232 hours0.0143 days <br />0.342 hours <br />0.00204 weeks <br />4.68776e-4 months <br />, the CREV system unexpectedly realigned to the chlorine protection mode. Control room annunciation indicated that the actuation was due to high chlorine in the control room air intake. Upon further investigation, it was confirmed that, although no chlorine was present, chlorine detectors 1-X-AT 2997, 2-X-AT-2997, and 2-X-AT-2997-1 were tripped. The chlorine isolation signal was removed by removing power to the tripped detectors. Operability of the radiation and smoke protection modes of the CREV system was restored and the system placed in its normal standby alignment at 1255 hours0.0145 days <br />0.349 hours <br />0.00208 weeks <br />4.775275e-4 months <br /> on August 6, 2006.
Engineering has evaluated the cause of the August 6, 2006, actuation and determined that the chlorine detectors tripped due to the presence of diesel exhaust. On February 3, 2006, a modification was implemented to replace obsolete detectors in the chlorine detection system with new detectors of a different design. A review of vendor documentation for the new chlorine detectors identified a number of gases, besides chlorine, to which the sensors are sensitive. Specifically, the detectors have the same sensitivity to Nitrogen Dioxide (NO2), a constituent of diesel exhaust fumes, as to chlorine. Operating EDG 2 at fully loaded conditions resulted in sufficient concentration NO2 in the control room air intake to trip the detectors.
Inoperability of the radiation protection mode of the CREV system requires entry into Technical Specification (TS) 3.7.3, "Control Room Emergency Ventilation System," Condition B. In addition, inoperability of the smoke protection mode of the CREV system requires entry into Condition B of Corrective Actions (continued) the CREV system to be restored to operable status. The chlorine tank car will not be brought within the exclusion area unless the chlorine protection mode of CREV is restored to operable status.
- Specification 252-100, "Chlorine Gas Monitors," will be revised to ensure that design of the chlorine detection subsystem includes consideration of the potential for inadvertent actuation of the detectors due to cross-contamination. This revision is scheduled to be completed by December 13, 2006.
Previous Similar Events
A review of LERs and corrective action program condition reports for the past three years identified the following similar events.
- LER 1-2006-003, dated June 7, 2006, documents a condition where the CREV system was declared inoperable due to a design deficiency introduced as a result of the February 3, 2006, modification to replace obsolete detectors in the chlorine detection system. The root cause of the event reported in approval. The corrective actions associated with LER 1-2006-003 could not have reasonably been expected to prevent the condition reported in this LER.
- LER 1-2006-001, dated March 9, 2006, documents a condition where the CREV system was declared inoperable when the 2B Control Building instrument air compressor failed and the CREV system shutdown due to loss of control air. The root cause of this event was determined to be ineffective condition monitoring of compressor oil pressure to detect degradation of the compressor.
The corrective actions associated with LER 1-2006-001 could not have reasonably been expected to prevent the condition reported in this LER.
- LER 1-2005-004, dated July 11, 2005, documents a condition where the CREV and Control Room AC systems were declared inoperable when electrical power was lost to bus El, making the 2B Compressor inoperable. The 2A Compressor should have automatically started, but did not start due to a wire lug that had broken and interrupted the control power circuit. The corrective actions associated with LER 1-2005-004 could not have reasonably been expected to prevent the condition reported in this LER.
Commitments No regulatory commitments are contained in this report.