05000336/LER-2014-002
| Millstone Power Station Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| 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) | |
| 3362014002R00 - 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 2 (MPS2) 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 system determined the described condition to be applicable to MPS2.
The vulnerability identified includes DC circuits connected to a 125VDC power source without appropriate rated protective device, and at MPS2 these are the Main Turbine Emergency Lube Oil pump control and indication circuits. The following fire areas are affected by the postulated hot shorts in DC circuits: the turbine battery room located in the 31' 6" elevation of the turbine building, the 25' elevation cable vault, the plant equipment operator (PEO) meeting area, and the control room.
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
The following fire areas are affected by the postulated hot shorts in DC circuits listed above: the turbine battery room located in the 31'6" elevation of the turbine building, the 25' elevation cable vault, plant equipment operator (PEO) meeting area, and the control room. The zones where the initial fire occurs that could lead to a secondary fire are the control room, the 25' cable vault, and the PEO meeting area.
Each fire area contains safe shutdown equipment. A primary fire in any area (except the cable vault and the PEO meeting 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 several have reliable automatic fire suppression systems. 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 2 05000336 4. CORRECTIVE ACTION (Continued) Additional corrective actions are being taken in accordance with the Station's corrective action program.
5. PREVIOUS OCCURRENCES
MPS2 LER 2013-003-00.
6. ENERGY INDUSTRY IDENTIFICATION SYSTEM (EIIS) CODES 125V DC [EI/EJ] DC Circuits [II]