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Potential Flooding Common Mode Failure of Service Water Pumps |
On July 18, 2006, V.C. Summer determined that their existing plant configuration did not match the assumptions in the Service Water (SW) Pump House flooding design calculations.
The calculation assumed that a leak developing in the discharge valve bay would cause the water level to rise to the level of the system piping and then stabilize due to leakage out of the valve bay through the area where the piping penetrated the wall. This level is below any electrical equipment and therefore would have no adverse impact on the system. However, the area around the piping where it was assumed the water would escape was found to be sealed with a fire barrier material. Since the water would not be able to escape, it would continue to rise until it impacted the pump discharge valve (motor operated valve). This would potentially render the Service Water train inoperable.
A second issue with the design calculation was discovered in that it assumed the pump discharge bays for the other two pumps were physically isolated from each other. This is not the case. The discharge piping for the three pumps is cross-connected via a line which penetrates all three bays. The area between the cross connect piping and the bay walls was not sealed. Any leakage in one bay could be communicated to all three bays. This resulted in the potential for a leak in any of the bays to possibly impact the operability of both trains of Service Water.
Subsequent to this initial determination, on August 10, 2006, Summer performed a more rigorous evaluation of the subject scenario utilizing existing calculations and analyses. According to the licensee, any flood or spray event in the Service Water Pump House operating floor or any one discharge valve pit will eventually propagate to all three pits through the open areas around the pipe penetrations. The pits will fill up to the pipe level and would conceivably continue to fill to the operating floor level. The discharge motor-operated valve and the control panel above the floor of each valve pit in this scenario would be submerged and would become disabled. The control panels would be shorted to ground, and would fail the discharge motor operated valves in the as-is open position which would not affect the service water supply of either train.
Since the motor operated valves fail as-is, this subject scenario would not result in the loss of both trains of service water.
Event date: Tue, 18-07-2006
