05000323/LER-2002-004
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| 3232002004R00 - NRC Website | |
I. Plant Conditions
Unit 2 was in Mode 1 operating at 20 percent reactor power. Unit 1 was in Mode 5 (Cold Shutdown) for a generator rotor replacement.
II. Description of Problem
A. Background
The circulating water system [KE] provides a continuous saltwater supply to the main condenser [GOND], condensate cooler [CLR], service water cooling system [KG], and intake cooling system EKE]. The saltwater enters the cooling water intake structure by passing through bar racks and then through traveling screen assemblies. Each unit has two single-stage circulating water pumps (CWP) and each CWP has three traveling screens. The bar racks and traveling screens prevent debris and sea life from entering the circulating water system and restricting flow through the main condenser.
The screens for the CWPs are operated either in manual or automatic.
When in manual, the screens are controlled by the operator and can be operated in slow or high speed.
Due to a past history of kelp loading as a result of Pacific storms, PG&E has implemented weather-monitoring systems, and developed extensive models to help predict the potential impact to the plant. In addition to the monitoring systems and models, PG&E has procedures governing the proper response to any high swell condition, or other weather related warnings.
Section 6.4.10 of operating procedure OP 0-28, "Intake Management" states that, "a reduction to less than or equal to 25 percent power should be considered in the event a high swell condition is predicted. Operation with the Unit well below the P-9 permissive will allow a turbine [TA] trip to be performed simultaneously with a controlled, rapid shutdown of the reactor [AC], thereby avoiding a reactor trip.
Operating procedure OP L-4, "Normal Operation at Power" provides instructions for power maneuvers between 20 percent and 100 percent reactor power.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) 5 0 Diablo Canyon Unit 2 0 01010 3 2 3 2002 0 0 1 4 1 0 3 I OF I 7 Abnormal operating procedure OP AP-7, "Degraded Condenser" gives operators specific guidance in the event of increasing traveling screen differential pressure, including a reduced load shutdown.
B. � Event Description A high swell warning was issued on November 7, 2002 at 1200 PST to allow adequate time to ensure the proper measures were taken as per operating procedure OP 0-28, "Intake Management.
On November 7, 2002, following procedure OP 0-28, and OP L-4, operators had ramped Unit 2 reactor power from 100 percent to 20 percent in anticipation of a significant storm and associated high swells and ocean debris. The possibility of losing the circulating water pumps [KE] as a result of the storm was anticipated, and procedures specify maintaining reactor power below 25 percent during these conditions.
On November 8, 2002 at approximately 0630 PST, the Unit 2 control room operator received an alarm indicating a high differential pressure, over 70 inches, across the intake screens [KE]. Procedure OP AP-7, Section C, dictates that if the difference in pressure across the screen is 70 inches or more, the operator is to perform a reduced load shutdown.
This procedure includes the following actions:
- Trip the turbine
- Insert control rods [AA] to shut down the reactor
- Trip the running CWP
- Close the MSIV's [SB][ISV]
- Trip the MFW [SJ] Pps.
- Perform Attachment 4.1, Hot Condenser Cooldown Instructions
- Refer to OP L-5 for completion of shutdown activities The Unit 2 shift foreman then initiated a rapid manual shutdown of the reactor. At 0643 PST, the control banks were fully inserted into the reactor. Mode 3 was declared at 0643 PST.
At 1300 PST, the shift manager made an 8-hour non-emergency notification to the NRC in accordance with 10 CFR 50.72(b)(3)(iv), for valid manual actuation of the auxiliary feed water (AFW) pumps in conjunction with the manual shut down.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION 2 FACILITY NAME (1) DOCKET NUMBER (2) 0 151 0 0 3 2002 LER NUMBER (6) 0 1 0 4 0 1 0 4 0 1 3 OF 7 C. Inoperable Structures, Systems, or Components that Contributed to the
Event
None.
D. Other Systems or Secondary Functions Affected
None. The auxiliary saltwater (ASW) [BS] system was not affected by the debris and swells, due primarily to the lower flow-rate and lower velocity through the ASW traveling screens.
E. Method of Discovery
Licensed plant operators following abnormal procedure OP AP-7, manually started the AFW pumps. The high differential pressure across the traveling screens was immediately known to licensed plant operators due to alarms and indications received in the control room.
F. Operator Actions
Operators performed the actions specified in abnormal procedure OP AP-7, Section C "perform reduced load shutdown.
G. Safety System Responses
As per design, the two standby containment fan coolers [EK][FAN] and the standby ASW pump automatically started on the transfer to start up power.
III. � Cause of the Problem A. � Immediate Cause The immediate cause of the manual start of the motor-driven AFW pumps was the procedural guidance in abnormal procedure OP AP-7. In the event of high differential pressure across the circulating water traveling screens, the procedure directs operators to take actions that will prevent equipment damage, including tripping the turbine, and shutting down the CWPs. Without CWPs, the condenser could be damaged from continued steam input. Therefore, the procedure directs operators to terminate steam flow to the condenser by closing the main steam isolation valves and tripping the turbine-driven main feedwater pumps. In order to LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) 0 15 010 0 0 4 0 Diablo Canyon Unit 2 0 3 1 2 1 3 2002 0 5 I OF 7 maintain reactor coolant system heat removal without main feedwater, the procedure directs operators to start the motor-driven AFW pumps prior to tripping the main feed water pumps.
B. R Root Cause The root cause of the event was the sudden heavy debris loading on the circulating water system traveling screens during a period of high seas that dislodged plant life from the ocean floor.
IV. Assessment of Safety Consequences
Previously analyzed Final Safety Analysis Report Update, Chapter 15, Condition II events, envelope a manual turbine trip from 20 percent power. The 10 percent steam dump valves and the pressurizer controlled the reactor coolant temperature and pressure in accordance with plant design and licensing basis.
Therefore, the health and safety of the public were not adversely affected by this event.
The event was not evaluated using the criteria defined in the NRC's Significance Determination Process because the transient is accounted for in the performance indicator program as an unplanned power change.
V. Corrective Actions
A. Immediate Corrective Actions
Plant personnel reviewed the plant equipment response following the manual reactor shutdown and confirmed that the equipment operated within design requirements. Plant management initiated an event response plan to establish a controlled restart of plant systems following confirmation of decreased storm activity.
B. Corrective Actions to Prevent Recurrence
PG&E cannot entirely eliminate the potential need to expedite manual reactor shutdowns when high seas dislodge plant life from the ocean floor resulting in heavy debris loading on the traveling screens. Therefore, there are no corrective actions to prevent recurrence.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION 5 2 FACILITY NAME (1) Diablo Canyon Unit 2 0 DOCKET NUMBER (2) 0 0 3 2002 LER NUMBER (6) 4 0 1 0 6 3 1 OF 7 VI. � Additional Information
A. Failed Components
None.
B. Previous Similar Events
Traveling Screens During a Pacific Ocean Storm," reported a dual unit manual reactor trip on October 28, 1999. With Unit 1 at 50 percent power, and Unit 2 at 30 percent power, operators initiated manual reactor trips due to high differential pressure across the circulating water traveling screens caused by debris loading due to heavy swells from a major Pacific storm.
Circulating Water System During a Pacific Ocean Storm," reported a Unit 2 manual reactor trip from 100 percent power on December 1, 1998.
Operators initiated a manual reactor trip due to high differential pressure across the circulating water traveling screens caused by debris loading due to heavy swells from a major Pacific storm. Corrective actions included revising operating orders for storm conditions, improving circulating water system indication in the control room, and providing additional training to operators regarding lessons learned from this event.
Traveling Screens," reported a Unit 1 manual reactor trip from 50 percent power on December 13, 1995. Operators initiated a manual reactor trip due to high differential pressure across the circulating water traveling screens caused by debris loading due to heavy swells from a major Pacific storm. Corrective actions included adding more screen rakes to the traveling screens to improve debris removal efficiency, and additional operator training.
Damage to Traveling Screens," reported a Unit 2 manual reactor trip from 40 percent power on September 23, 1995. Operators initiated a manual reactor trip when the circulating water traveling screens stopped due to high differential pressure across the traveling screens caused by debris loading due to heavy swells from a major Pacific storm. Corrective actions for this event included installation of new kelp rakes and revision of OP AP-7 to reduce or prevent damage to the traveling screens.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION DOCKET NUMBER (2) 5 FACILITY NAME (1) Diablo Canyon Unit 2 0 01010 3 12 1 3 2002 LER NUMBER (6) 4 _._ 0 1 0 7 I OF 7 0 0 Cavitation as a Result of Intake Screen Fouling," reported a Unit 2 manual reactor trip from 35 percent power on December 19, 1994. Operators tripped Unit 2 due to CWP cavitation caused by intake screen fouling.
Corrective actions included installing an audible alarm at the intake that activates if a traveling screen drive fails, installing an alarm in the control room for high condenser differential pressure, revising OP AP-7 to include guidance on condenser fouling, revising the operator rounds sheets to provide guidance on monitoring performance of the intake screen wash system, and installing a computer program that enables control room operators to monitor and trend condenser differential pressures.
PG&E cannot entirely eliminate the potential need to expedite manual shutdown when high seas and dislodged plant life from the ocean bottom result in heavy debris loading on the traveling screens. However, the corrective actions taken for the above previous events helped mitigate the effects of the storm. For example, development and use of extensive storm prediction models, combined with a proceduralized intake debris management strategy, were successful in protecting the circulating water traveling screens while at the same time avoiding the need for initiation of a manual reactor trip.