05000255/LER-2008-007
| Palisades Nuclear Plant | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 2552008007R01 - NRC Website | |
unresolved issue related to potentially non-conservative setpoints for the low suction pressure trip (LSPT) of the auxiliary feedwater (AFW) pumps [P; BA]. The potentially non-conservative setpoints, which were first identified on February 13, 2006, could have existed at certain Condensate Storage Tank (CST) [TK] levels and AFW pump flow rates.
The CST is the normal suction source of the AFW pumps. In the extreme unlikelihood of a tornado (probability of 6.51 E-8 events per year), a tornado-generated missile could have caused a rupture near the bottom of the CST. The rupture may have allowed rapid draining of the CST, without completely emptying the tank. Subsequent automatic operation of the AFW pumps after a plant trip could have caused the onset of vortexing within the CST, leading to air entrainment in the auxiliary feedwater suction piping and pumps. This entrained air could have rendered the AFW pumps inoperable. Consequently, the AFW system may not have been capable of supplying the steam generators with Lake Michigan water, as outlined in the Palisades Nuclear Plant design basis.
This condition is being reported in accordance with 10 CFR 50.73(a)(2)(v)(D) as a condition that could have prevented the fulfillment of the safety function of a system needed to mitigate the consequences of an accident.
CAUSE OF THE EVENT
The CST provides the primary source of water to the AFW pumps. The AFW system is designed to provide a supply of feedwater to the steam generators during start-up operations and to remove primary system sensible heat during initial stages of shutdown operations.
In the original plant design, the CST and the AFW system were not intended to have a safety function. Therefore, the CST was not designed with barriers to protect it from tornado-generated missiles. However, in the early 1980's, the AFW system was upgraded from non-safety-related system to a safety-related system as a result of lessons learned from the accident at Three Mile Island.
After the upgrade, Palisades still did not rely on the CST to provide a safety-related source of water for the AFW pumps. Instead, Palisades credited the two alternate sources of water, the fire water system, and the service water system. Both sources draw water from Lake Michigan. Plant operators can manually align these sources of safety-related water to the suction piping of the AFW pumps.
RIRC FORM 366A (9-7007) In 1982, the NRC reviewed the capability of the plant structures at Palisades to withstand tornado wind loads and tornado missile strikes per the Systematic Evaluation Program (SEP). In this evaluation, the NRC concluded that the CST and several other plant structures were not designed to resist tornado wind loads and were vulnerable to tornado missile strikes. However, the NRC concluded in the integrated SEP assessment that any damage to the CST would not adversely affect the shutdown capability of the plant due to the availability of backup water supplies. The AFW pumps were adequately protected against the loss of the CST and the loss of net positive suction head by LSPT devices. These devices protect the AFW pumps from a loss of net positive suction head.
In February 2006, a NRC resident inspector identified a potential error in the calculation used to I ensure the AFW pumps are removed from service by the LSPT. In a scenario where a tornado missile strikes the CST near the bottom and causes a loss of inventory, the CST is postulated to drain down to a point just below the level of vortex formation. Assuming the tornado also causes a plant trip, the AFW pumps would sequentially start on a low steam generator level signal.
When an AFW pump starts, the level at which the LSPT actuates is down in the CST discharge piping. As the AFW pump speeds up, the LSPT actuation level travels upward toward the CST due to the increase in friction head loss as flow velocity increases. A vortex could form in the CST before the LSPT level reaches the CST level. The three AFW pumps could fail due to vortex induced air entrainment before they are tripped by the LSPT.
The setpoints for the AFW LSPT were determined in a calculation that subtracted the friction head loss from the CST level static head. Although this was an appropriate formula, the flow rates used to determine friction head loss were based on a design basis AFW flow rate and may not be bounding under other circumstances in which AFW flow could be less than the design basis flow rate. This resulted in a calculated LSPT setpoint that was lower than that needed to protect the AFW pumps in a postulated tornado missile scenario.
In addition, follow-up reviews of other AFW evaluations were performed in March 2006. An evaluation to validate that an adequate water volume exists in the AFW suction piping, to ensure an air slug does not enter the pump before it has stopped after the LSPT, had a potential non conservative assumption. The coastdown time was assumed to be 3.5 seconds for all three AFW pumps. However, while observing the Technical Specification surveillance test of the turbine-driven AFW pump, the NRC resident inspector identified the pump required 23 seconds to coast down, which means that more water would be pumped than assumed and the air slug could reach the AFW pump. As a result, in March 2006, a compensatory measure was established to ensure at least one AFW pump would remain available if the scenario described above occurred.
On October 14, 2008, after completing reviews of evaluations performed by ENO, the NRC closed the unresolved issue related to potentially non-conservative setpoints for the low suction pressure trip of the AFW pumps, which was first identified on February 13, 2006. On October 21, 2008, ENO determined that this represented a reportable condition.
CORRECTIVE ACTIONS
To address the issue, in March 2006, ENO implemented a compensatory measure to disable the automatic start of one, 100-percent capacity, AFW pump (P-8C) in the event of a tornado watch, warning, or sighting. Guidance for performing the compensatory measure was established in Off-Normal Procedure (ONP), ONP-12, "Acts of Nature." This is physically accomplished by placing the pump handswitch in the "off" position. With manual control of the AFW pump, plant operators can respond to a loss of the CST inventory by manually aligning service water from Lake Michigan to the suction of the AFW pump P-8C. This ensures that at least one AFW pump would remain available to supply water to the steam generators.
On December 9, 2008, as a final corrective action, construction of a tornado missile wall / barrier was completed on the west side of the CST. This barrier, together with other surrounding structures, provides a level of protection for the CST from certain tornado missiles. The compensatory measure was suspended at that time because it was no longer needed.
ASSESSMENT OF SAFETY CONSEQUENCES
The event consequences are considered to be of very low safety significance. The basis for this conclusion is the extreme unlikelihood of a tornado missile strike near the bottom of the CST, rapid draining of the CST without completely emptying the tank, and failure of the operator to provide an alternate makeup source.
PREVIOUS SIMILAR EVENTS
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