05000298/LER-2012-001
| Cooper Nuclear Station | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2982012001R00 - NRC Website | |
PLANT STATUS
Cooper Nuclear Station (CNS) was in Mode 1, Power Operation, at 100 percent power at the time the condition was discovered on March 7, 2012.
BACKGROUND
The direct current (DC) electrical power system [El IS:El] provides the alternating current (AC) emergency power system [EllS:EK] with control power. It also provides both motive and control power to selected safety related equipment. There is an additional 125 Volt (V) battery charger [EllS:BYC] and an additional 250 V battery charger which can be used as backups to supply either division if the normal battery charger is lost. The backup chargers can be supplied from either division to maintain proper divisional separation. Each battery [EllS:BTRY] has adequate storage capacity to carry the required load continuously for approximately 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />. Each station service battery charger has sufficient capacity to restore the battery from the design minimum charge to its fully charged state within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> while supplying normal steady state loads. The DC electrical power system provides normal and emergency DC electrical power for the Diesel Generators (El IS:DG), emergency auxiliaries, and control and switching.
As stated in the Updated Safety Analysis Report (USAR) for CNS, each battery charger shall have adequate capacity to restore its battery to full charge from a totally discharged condition while carrying the normal station steady state DC load. The 125/250 V batteries shall provide power for maintaining the plant in a safe hot shutdown condition in the event Control Room operation is prevented by fire and the Alternate Shutdown System is used. Once the DG is on- line, it will power battery chargers in order to maintain station batteries in a fully charged state.
Battery charger capability requirements are based on the design capacity of the chargers. The battery charger supply is required to be based on the largest combined demands of the various steady state loads and the charging capacity to restore the battery from the design minimum charged state to the fully charged state, irrespective of the status of the unit during these demand occurrences. The minimum required amperes and duration ensures that these requirements can be satisfied.
EVENT DESCRIPTION
On March 7, 2012, CNS discovered that the fuse ratings associated with the "C" battery charger disconnect switches may prevent the "C" battery chargers from fulfilling the USAR and Technical Specification (TS) Bases function. The fuses and fuse disconnects for "A" and "B" battery chargers are sized at 300 amperes, while the fuses and fuse disconnects for the "C" battery chargers are sized at 200 amperes. Subsequently, Operations declared the "C" battery chargers inoperable on March 8, 2012.
The "C" battery chargers were proposed to be added to the plant in 1987. The purpose of this was to increase the reliability of the DC system for both the 125 and 250 VDC systems. The 125 VDC "C" battery charger was installed in 1988 and was to be used as a spare to either "A" battery charger or "B" battery charger. The 250 VDC "C" battery charger was approved and installed in 1989. A USAR change was submitted and approved to update the 125 VDC and 250 VDC battery and charger specifications. Both design changes for the installation of the "C" battery chargers specified that the fuses be 200 amperes.
After the "C" battery chargers were installed, the "A" and "B" battery chargers had a design modification to change their existing breakers to fuses. The "C" battery chargers already had fuses; as such, the sizing for their fuses was not reviewed at that time.
BASIS FOR REPORT
CNS determined that this condition was reportable on March 26, 2012, as a past operation or a condition prohibited by TS per 10 CFR 50.73(a)(2)(i)(B). Although the "C" battery chargers were not in service at the time of discovery of this condition, they had been within the past 3 years.
When in service substituting for an inoperable divisional battery charger, "C" battery chargers are relied on to satisfy TS Limiting Condition of Operation 3.8.4. The 250 VDC "C" battery charger was not in service longer than the TS required actions. However, the 125 VDC "C" battery charger was in service longer than the TS required actions allowed, which was a condition prohibited by TS.
SAFETY SIGNIFICANCE
The safety significance associated with "C" battery chargers' fuse protection non-conformance is low based on the "C" battery chargers remained capable of supplying required DC loads for worst case Anticipated Operational Transients and Accident Loadings; and the time when the "C" battery chargers have been utilized have averaged less than 1.5 percent per year.
CAUSE
CNS has determined the root cause of this condition to be the design preparation, review and approval process when the original design documents were prepared was not sufficiently rigorous to detect the fuse sizing error for the "C" battery chargers. This was a latent design error.
CORRECTIVE ACTION
To prevent recurrence of this condition, design evaluations and any necessary design changes to the size of the fuses in the "C" battery chargers needed to meet USAR and TS requirements will be implemented.
PREVIOUS EVENTS
A condition report was written in 2005 questioning why the "C" battery chargers have smaller fuses than the "A" and "B" battery chargers. The evaluation determined there is adequate margin in the fuse time-current characteristics relative to demand during a design bases Loss of Coolant Accident event. This evaluation did not assess whether the 200 amp fuses met the operational requirements specified in the USAR and the TS Bases.