05000306/LER-2010-003
| Prairie Island Nuclear Generating Plant Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 3062010003R00 - NRC Website | |
Prairie Island Nuclear Generating Plant (PINGP) Unit 2 has four Fuel Oil Storage Tanksi (FOSTs) to provide fuel oil (FO) to the D5 and D6 Emergency Diesel Generators2 (EDG). The EDGs and support systems were installed in 1992.
Both FO transfer pumps for D5 (21 and 23) are powered from 480V Bus 211 by the single main supply breaker 211 K. Similarly, both FO transfer pumps for D6 (22 and 24) are powered from 480V Bus 221 by the single main MCC supply breaker 221 K.
Additionally, the D5 FO Transfer Pump auto control circuitry for both D5 pumps is powered from the 125V DC Panel 27 Circuit 2 by a single breaker, 8/FTP/D5. Similarly, the D6 FO Transfer pump control circuitry for both D6 pumps is powered from the 125V DC Panel 28 Circuit 2 by a single breaker, 8/FTP/D6.
The EDGs and support systems were designed to meet the requirements of Regulatory Guide (RG) 1.137, (Fuel Oil Systems for Standby Diesel Generators, Revision 1, October 1979). RG 1.137 endorses the requirement in ANSI N195-1976 that "sufficient on-site oil storage shall be provided to operate the required number of diesel-generators for seven (7) days or the time required to replenish the oil for sources outside the plant site following any limiting design basis event or accident.
In 2004, a Corrective Action Process action request (CAP) was written (CAP 566095) to document the potential common mode failure of 21 and 23 FO Transfer Pumps since both are powered by a common 480 VAC electrical bus. The evaluation of the CAP concluded that manual action would be available to transfer FO out of the FOST if this condition occurred. However, if the FO Transfer Pumps fail, there is no manual line up of the system that would have been able to transfer the FO out of the 21 or 23 FOST. In addition, the CAP did not recognize that the 125 VDC control power for the FO Transfer Pumps was also supplied from a common source.
On May 12, 2010, CAP 01232408 questioned the control power for each set (21 and 23, 22 and 24) of the FO Transfer Pumps came from the same DC power panel via the same breaker. CAP 1232504 was also written on the same day to document a concern that each set of FO Transfer pumps power came from the same 480 VAC bus via a single breaker.
On May 21, 2010, it was determined this condition existed and was reportable. Engineering determined that D5 FOSTs had an adequate volume of FO. However the D6 FOSTs volume was inadequate and D6 was declared inoperable until a TS sufficient volume of FO was transferred.
EMS System Code: DE EMS System Code: EK NRC U.S. NUCLEAR REGULATORY COMMISSIONFORM 366A� LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET
(9 00
EVENT ANALYSIS
Technical Specifications LCO 3.8.3 states "The stored Emergency Diesel Generator (EDG) fuel oil supply shall be within limits". LCO 3.8.3, Condition A requires the Unit 2 Fuel Oil supply to be greater than 75,000 gallons. If less than 75,000 gallons, but more than 65,000 gallons, the specification allows 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> to recover fuel above 75,000 gallons. With less than 65,000 gallons available, the Unit 2 Diesel Generators (both D5 and D6) must be declared inoperable immediately.
The Unit 2 FO system has four Safeguards tanks to maintain the required inventory. Each tank has approximately 30,800 available gallons. With one tank out of service, three fuel oil storage tanks have the available capacity of approximately 90,000 gallons (providing the three available fuel oil storage tanks are full). Two tanks filled to 100% cannot by themselves satisfy the 75,000 gallons as required by TS. It was determined that at least 39,000 gallons of FO were required to be available for each EDG.
On May 21, 2010, with Unit 2 in Mode 3, it was determined this condition was reportable since the Unit 2 FO Transfer Pumps were potentially vulnerable to a common mode failure since:
1.�Both FO transfer pump motors for D5 (D6) were powered from the same 480 VAC bus via a single breaker.
The FO Transfer Pump Auto Control Circuitry for both pumps for D5 (D6) is powered from the same 125 VDC DC Panel via a single breaker.
No equipment actually failed; however if any of the potential failures actually occurred, two FOSTs would have been unavailable and there would have been inadequate FO to meet the TS inventory requirement. The Unit 2 EDGs may not have fulfilled their required mission time; therefore this condition does represent a safety system functional failure. This condition is reportable as a common-cause inoperability of independent trains or channels under 10 CFR 50.73(a)(2)(vii) and as a safety system functional failure under 10 CFT 50.73(a)(2)(v)(A).
SAFETY SIGNIFICANCE
This issue had no nuclear, radiological, industrial, or environmental impact. The FO transfer pumps common mode breakers did not fail so the required TS FO inventory was available. Therefore, this event did not affect the health and safety of the public.
CAUSE
The causal evaluation determined that the apparent cause is inadequate design for the power supplies and control power for the Unit 2 FO transfer pumps.
due to common power supplies or control power for safety related systems was identified in the last three years.