05000282/LER-2010-002

LER-2010-002, Postulated Flooding of Unit 1 Fuel Oil Transfer Pump Motor Starters Could Have Resulted In Reduced Fuel Oil Inventory

Docket Number
Event date:
Report date:
2822010002R00 - NRC Website

During original plant construction, the Unit 1 Diesel Driven Cooling Water Pumps1 (DDCLP) Fuel Oil Storage Tank2 (FOST) fuel oil transfer pump starter motors3 were installed in the lower level of the Plant Screenhouse at the approximately 674 foot elevation. This elevation is below the calculated flood depth in the Plant Screenhouse.

In a postulated internal flood event in the Plant Screenhouse, a common mode failure of the redundant safety equipment (DDCLP fuel oil pump motor starters) could occur. On April 26, 2010, it was determined that this postulated flooding event was reportable.

EVENT ANALYSIS

Historically, Prairie Island Nuclear Generating Plant (PINGP) Unit 1 relied on the total fuel oil stored in six FOSTs to meet the required minimum Technical Specification fuel oil inventory for the DDCLPs and the Unit 1 Emergency Diesel Generators4 (EDG). In 1972 and 1985, the Plant Screenhouse equipment was incorrectly reviewed and the postulated flooding common mode failure of redundant safety related equipment was not identified.

In 1998, further review identified that internal flooding in the Plant Screenhouse could result in water covering the DDCLP FOST transfer pumps' motor starters. The review incorrectly resolved the issue by stating that, "the Diesel Driven Cooling Water Pumps Fuel Oil Day Tanks have capacity for eight hours of cooling water pump operation below the low level alarm point. Also, the DDCLP Fuel Oil Day Tanks can be supplied from the other safety related interconnected fuel oil storage tanks for the Unit 1 Diesel Generators by manual alignment, as per USAR section 2.4.3.5." The resulting common mode failure of redundant safety related equipment was not recognized. This analysis resulted in an October 7, 1998, letter to the NRC to update the 1985 response to Generic Issue 77.

In 2001, a calculation (ENG-ME-203) identified that the "electrical box that starts the pumps that transfer fuel to the Diesel Driven Cooling Water Pumps" would quickly flood in the postulated event that a Circulating Water pipe failed. The consequences would be a loss of ability to use the fuel oil stored in the DDCLPs FOST. It was not recognized that this would cause a common mode failure of redundant safety related equipment and not meet PINGP licensing basis.

On February 8, 2010, with Unit 'I at 100% power, the location of the safety related DDCLP FOST fuel oil transfer pump motor starters was reviewed again. The evaluation recognized the potential for a common mode failure of redundant safety related equipment due to internal flooding. Immediate actions were taken to transfer fuel oil to the unaffected safety related FOSTs to ensure the required fuel oil inventory was available in the four FOSTs not susceptible to this issue. On April 26, 2010, it EllS System Code: BI 2 EllS System Code: DE 3 EllS System Code: EC 4 EllS System Code: EK was determined that the postulated flooding event in the Plant Screenhouse resulting in flooding the Diesel Driven Cooling Water Pumps (DDCLP) FOSTs fuel oil transfer pump's motor starters was reportable. The postulated loss of two FOSTs fuel oil transfer pumps' motor starters could have resulted in the total fuel oil inventory falling below the amount required by Technical Specifications LCO 3.8.3 Condition A.

PINGP had procedures and training in place to transfer fuel oil from other FOSTs to the DDCLP fuel oil Day Tank if the DDCLP FOST transfer pumps were unavailable to operate. A walkthrough was performed on February 10, 2010 to perform the steps required to transfer fuel oil to the DDCLP Day Tanks. This walkthrough was completed in eighty-two minutes under adverse winter conditions. The eighty-two minutes may be compared to the approximately seven hours of fuel available below the low level alarm on the DDCLP Day Tanks. Therefore, this event does not represent a safety system functional failure.

SAFETY SIGNIFICANCE

This issue had no nuclear, radiological, industrial, or environmental impact. Internal flooding of the Plant Screenhouse did not occur, so the fuel oil transfer pumps did not fail. 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 location of the DDCLP FOST fuel oil transfer pumps' motor starters.

A contributing cause is that the site had less than adequate knowledge of the licensing basis for internal flooding.

CORRECTIVE ACTION

Fuel oil was transferred to the 121, 122, 123, and 124 EDG FOSTs to ensure the correct amount of fuel oil was available to meet the requirements of both the DDCLPs and the EDGs.

The Operable but Nonconforming condition of the Unit 1 Fuel Oil System was documented.

Additional Corrective Actions:

1. An Engineering Change (EC-15988), RELOCATE DDCLP FUEL OIL STORAGE TANK (FOST) PUMP MOTOR STARTERS, was created to relocate the motor starters out of the calculated flood zone.

NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION (9-2007) �

CONTINUATION SHEET

2. Procedure H36 (Plant Flooding) was revised to state that the licensing basis included the need to prevent common mode failure of redundant safety related equipment due to internal flooding. The improved knowledge of the licensing basis was shown by the fact that the severity of this issue was correctly determined when it was revisited in 2010.

PREVIOUS SIMILAR EVENTS

Other flooding events are currently under review at PINGP. However a LER search was conducted and no similar LER events at PINGP involving flooded motor starters were identified in the last three years.