05000389/LER-2009-002
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| 3892009002R00 - NRC Website | |
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)
Description of the Event
April 1, 2009, St. Lucie Unit 2 experienced a high ingress of algae. All traveling water screens (TWS) [EIIS:SCN], circulating, and intake cooling water debris filters [EIIS:FLT] were operating continuously to control differential pressures across the filters and downstream equipment caused by the debris. Steps were taken to down power the Unit to secure the 2A1 circulating water pump (CWP) from service. During this time it was identified that the differential pressure on the 2A2 circulating water debris filter screens (DFS) [EIIS:SCN] had exceeded the administrative limits of 200 inches of water. Given the circumstances and the degrading conditions, the decision was made to manually trip the reactor while at 92% power. Subsequently, the auxiliary feedwater actuation system (AFAS) automatically initiated on low steam generator level. All systems functioned normally, the plant was stabilized at normal operating temperature and pressure and a non-emergency notification was made to the NRC.
Cause of the Event
The loss of vacuum was caused by a greater than expected influx of a unique strain of algae which challenged the debris filter screening system and weir pit design.
As the ingress of algae began to accumulate on the screen wash pumps (SWP) strainers, the 2A and 2B SWP were taken out of service to backwash the strainers to alleviate the accumulation. The screen wash header flow was isolated to all Unit 2 traveling water screens while SS-21-2B strainer was backwashed. During this time debris loading on all Unit 2 traveling water screens began to accumulate to severe levels.
Changes to the original design of the weir pit for Units 1 and 2 contributed to the recirculation of removed debris from the traveling water screens to re-circulate back to the intake canal. The traveling screen debris flow starts in the trash trough and then flows into the trash box for removal. At the time of the ingress the trash box had filled to the spillway level with debris. Without the capability to accumulate debris in the trash box, debris flows directly into the trash pit. A basket strainer is often used to aid in the removal of debris from the trash pit but was not installed during this event.
The design of the weir pit has two 28" x 16" window openings that function to equalize the water level in the intake canal and the pit. The recent influx of algae was "neutrally-buoyant", unlike previous influx of debris, and was distributed evenly throughout the water column. This characteristic allowed the debris to recirculate back through the window openings in the weir pit and be pulled back into the traveling screens.
St. Lucie has experienced various unit down powers, equipment failures, and high ingress of sea grass events that have impacted electric generation and system reliability at the station over the past several years. Most sea grass and jelly fish events at St. Lucie station occurred during the mid to late summer season. The FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Unit 2 manual plant trip on April 1, 2009 is an outlier to this trend.
Analysis of the Event
This event is reportable under the requirements of 10 CFR 50.72(b) (2) (iv) (A) due to manual reactor protection system (RPS) actuation, and under 10 CFR 50.72(b) (3) (iv) (A) due to PWR auxiliary feedwater system actuation (AFSA).
The plant response to the reactor trip resulted in all control rods being fully inserted, no power operated relief valves (PORVs) opened, the reactor coolant system (RCS) heat removal was maintained with main feedwater and steam bypass control systems, and the auxiliary feedwater actuation system (AFAS) was automatically initiated on low steam generator level. All systems functioned normally, and the plant was stabilized at normal operating temperature and pressure.
Prior to the trip, reactor power had been reduced to 94% in order to secure the 2A1 CWP. It was then identified that 2A2 circulating water debris filter differential pressure was above administrative limits of 200 inches water. While the station was preparing to reduce circulating water flow on the 2A2 circulating water pump, the unit began losing condenser vacuum and the reactor was manually tripped at 92% power.
All systems functioned normally, and the plant was stabilized at normal operating temperature and pressure.
In previous site evaluations and in St. Lucie's original response to SOER 07-2, "Intake Cooling Water Blockage", it had been determined that most sea grass and jelly fish events at the St. Lucie Plant occurred during the mid to late summer season.
Marine life macro-fouling across the southeastern Florida coast is expected to increase due to higher nutrient solutions being discharged from various sources across the coast.
Analysis of Safety Significance All safety related systems functioned as designed. The event was bounded by the analysis described in Section 15.2.3,"Loss of Condenser Vacuum". The actual plant response was more conservative than that described in the analysis given the reactor and turbine were tripped manually before reaching automatic set points, the plant was not at full power when the events occurred and a complete loss of condenser cooling did not occur, thus the steam bypass control system was able to remove heat from the steam generators. Given the response of the plant and actions taken, the health and safety of the public
- was not affected by this event.
Corrective Actions
Corrective actions taken include a daily Staff Biologists' analysis of biota entering the intake canal with reports provided to the Shift Manager and actions to review weir design, preventive maintenance pit cleaning and maintenance practices, and ' � /- development of a procedure for Intake Intrusion Mitigation for monitoring, measuring" and combating of intake intrusion events.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Similar Events St. Lucie has experienced similar plant events as a result of intake sea grass and jelly fish intrusion and/or related equipment failures. In previous site evaluations and in PSL's original response to SOER 07-2, "Intake Cooling Water Blockage", it had been determined that most sea grass and jelly fish events at the station occurred during the mid to late summer season. Additionally, the type and amount of sea grass experienced during this event had different characteristics which created additional cleaning challenges not previous experienced.
A review of nuclear industry events also identified several similar failures associated with traveling water screens and intake system blockage/intrusion resulting from algae and intake debris.
Failed Components
NA