05000423/LER-2014-002
| Millstone Power Station Unit 3 | |
| Event date: | 03-12-2014 |
|---|---|
| Report date: | 05-05-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| Initial Reporting | |
| ENS 49889 | 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| LER closed by | |
| IR 05000245/2016008 (4 January 2017) | |
| 4232014002R00 - NRC Website | |
Reported lessons learned are incorporated into the licensing process and fed back to industry.
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1. EVENT DESCRIPTION
While Millstone Power Station Unit 3 (MPS3) was operating at 100 percent power in Mode 1 on March 12, 2014, operations determined that a concern identified by Engineering as a result of recent industry operating experience (NRC Event 49889) could result in an unanalyzed condition. Engineering's review of the direct current (DC) circuits associated with the main turbine emergency lube oil and main generator emergency seal oil pumps determined the described condition to be applicable to MPS3.
The vulnerability identified includes DC circuits, connected to a 125VDC power source without appropriate rated protective device, and at MPS3 these are the Main Turbine Emergency Lube Oil and Main Generator Emergency Seal Oil pump control and indication circuits. Fire areas affected because of the postulated hot shorts in DC circuits are the Turbine Battery Switchgear Area (TB-3), Cable Spreading (CB-8), Instrument Rack Room (CB-11A and CB-11B), and the Control Room (CB-9).
This condition is reportable pursuant to 10 CFR 50.73(a)(2)(ii)(B) as an unanalyzed condition that significantly degraded plant safety.
2. CAUSE
The cause of the condition was a manufacturer's design error that was installed during plant construction.
3. ASSESSMENT OF SAFETY CONSEQUENCES
Fire areas affected because of the postulated hot shorts in DC circuits in the equipment listed above are the Turbine Battery Switchgear Area (TB-3), Cable Spreading (CB-8), Instrument Rack Room (CB-11A and CB-11B), and the Control Room (CB-9). The zones where the initial fire occurs that could lead to a secondary fire are the Cable Spreading (CB-8), Instrument Rack Room (CB-11A and CB-11B), and the Control Room (CB-9).
Each fire area contains safe shutdown equipment. A primary fire in any area (except the turbine battery area) could cause a secondary fire in a separate safe shutdown fire zone. The secondary fire would be expected to be low energy.
The majority of these fire areas have fire detection installed and either reliable automatic fire suppression systems or a manually initiated CO2 system. The control room is constantly manned by personnel who are trained to extinguish fires. Therefore, any low energy secondary fire will be quickly detected and extinguished. On this basis, the safety consequences of these postulated secondary fires is considered low.
4. CORRECTIVE ACTION
Compensatory fire watches have been implemented for affected areas of the plant. Plant modifications are planned that will protect or isolate the affected circuits leaving the initial fire area.
Millstone Power Station Unit 3 05000423 4. CORRECTIVE ACTION (Continued) Additional corrective actions are being taken in accordance with the Station's corrective action program.
5. PREVIOUS OCCURRENCES
None.
6. ENERGY INDUSTRY IDENTIFICATION SYSTEM (EIIS) CODES 125V DC [EI/EJ] DC Circuits [II]