05000315/LER-2002-005
| Donald C. Cook Nuclear Plant Unit 1 | |
| Event date: | 06-14-2002 |
|---|---|
| Report date: | 08-13-2002 |
| 3152002005R00 - NRC Website | |
17. TEXT (If more space is required, use additional copies of NRC Form (366A) Conditions Prior to Event Unit 1 — Mode 1, 88 percent power Unit 2 — Mode 1, 100 percent power
Description of Event
At 1443 hrs on June 14, 2002, Unit 1 was manually tripped following the trip of the East main feedwater (FW) [El IS:SJ] pump (MFP) [EIIS:P]. The East MFP tripped due to a loss of main feed pump condenser vacuum caused by an influx of debris (zebra mussel shells, silt, sand, and sticks) following the start of the #13 circulating water (CW) pump.
The post-trip investigation indicated that all control rods fully inserted. The three auxiliary feedwater (AFW) [EIIS:BA] pumps automatically started as expected. The reactor coolant system (RCS) [El IS:AB] cooled down below no-load Tavg (541 degrees Fahrenheit) for approximately 70 minutes. The excessive cooldown was attributed to: 1) the large amount of feedwater flow from the operation of the three AFW pumps; and 2) the reactor core had limited power history and therefore little decay heat. The turbine-driven AFW pump and other secondary loads were manually secured in accordance with plant procedures to stabilize RCS temperature.
The four steam generator (SG) stop valves [EIIS:V] drifted partially closed and had to be manually reopened by the operations staff. This occurred only once for the #11 and #12 SG stop valves, but #13 and #14 continued to drift closed at regular intervals.
The "AB" reserve feed [EIIS:EA] transformer tap changer did not lower Train "B" bus voltages to the expected level, resulting in a standing alarm in the control room due to Train "B" bus over voltage. However, the 4-kilovolt (kV) and 600-volt busses voltages remained within the operable range.
In accordance with 10 CFR 50.72 (b)(2)(iv)(B), a four-hour ENS notification (Event No. 38993) was made to the NRC on June 14, 2002, at 1751 hours0.0203 days <br />0.486 hours <br />0.0029 weeks <br />6.662555e-4 months <br /> for a condition that resulted in an actuation of the reactor protection system (RPS) when the reactor is critical. As such, this Licensee Event Report (LER) is being submitted in accordance with the requirements of 10 CFR 50.73 (a)(2)(iv)(A) for a condition that resulted in an actuation of the RPS system.
Cause of Event
The cause of this event was the transport of debris into the East MFP turbine condenser [EllS:COND] upon the start of the #13 CW pump. The debris is thought to have accumulated at the #13 CW pump suction in the forebay [EIIS: NN] and in the CW tunnel behind the #13 CW pump discharge valve (1-WMO-13) as a result of back-leakage past the valve with the #11 and #12 CW pumps operating.
A contributing factor was the closure of 12-WMO-30 (the center lake water intake valve) a few days prior to the start of the #13 CW pump. With 12-WM0-30 closed, higher velocities in the North and South intake tunnels occur, resulting in the potential for increased debris transport into the forebay.
Analysis of Event
At the time of this event, the center lake water intake tunnel had been isolated (via valve 12-WM0-30) for several days.
In addition, two of the three Unit 1 CW pumps were running and all four Unit 2 CW pumps were in service, but the #13 17. TEXT (if more space is required, use additional copies of NRC Form (366A) September 2001 ESW silt/sand intrusion event (LER 316/2001-003), having valve 12-WMO-30 closed with the units at power results in higher velocities in the North and South lake water intake tunnels, with a potential for increased debris transport into the forebay and the CW system. During the recent Unit 1 refueling outage, the #13 CW pump discharge valve (1-WMO-13) was identified to have a damaged seal [EllS:SEAL]. As a result, some back-leakage through valve 1-WM0-13 from CW pumps #11 and #12 occurred, allowing the formation of a new debris pile in the CW tunnel just downstream of valve 1-WMO-13.
The geometry of the #13 CW pump discharge tunnel is such that with CW pump #13 not operating, system flow can deposit debris in the tunnel. The configuration of the #13 CW pump discharge tunnel and FW pump turbine condenser inlet piping can direct debris passing through the #13 CW pump towards the inlet lines of the Unit 1 FW pump turbine condensers. This vulnerability exists whether a local debris pile accumulates on the discharge side of 1-WMO-13 or at the base of the #13 CW pump.
A review of the CW, ESW and NESW system performance was performed following the trip. The ESW and NESW pump strainers [EIIS:STR] limit the vulnerability of these systems to debris intrusion. The post-trip investigation concluded that no adverse pressure drops were identified for the ESW pump strainers, the EDG lube oil coolers, the main condensers, or the NESW components.
With respect to the main FW system, the system has one active accident mitigation function, namely feedwater isolation.
However, this event had no impact on the ability of the main FW system to perform its safety function. In addition, the CW system has no safety or accident mitigation function.
Based on the discussion above, the safety significance of this event was minimal since plant procedures and operator training provided sufficient direction for control room personnel to shut down the plant and maintain it in a safe shutdown condition. There was no impact on the health and safety of the public as a result of this event.
Corrective Actions
The Unit 1 East and West MFP turbine condenser tubes and waterboxes were cleaned on June 14, 2002. The debris removed from the waterboxes consisted mostly of zebra mussels, a small percentage of sticks, and sand/silt.
A post-trip walkdown of the ESW system did not identify any high-pressure differential or low flows on the equipment serviced by the ESW system.
CNP will take actions to mitigate the effects of debris on CW pump startups with the unit on line. Specifically, precautions will be added to the CW system operating procedure to identify the potential vulnerability for debris intrusion associated with starting CW pumps with the unit on-line. The procedures will also include guidance for use of MFP turbine condenser waterbox lancing when starting a CW pump with a unit on-line.
Previous Similar Events
vacuum. This occurred while securing the #21 CW pump from service to troubleshoot a spurious low-low voltage alarm on the 4-kV bus. The cause of this event was attributed to zebra mussels within the circulating water system that temporarily blocked cooling flow to the MFP condensers causing a MFP trip on low vacuum and a subsequent reactor trip on low feed flow coincident with low steam generator level. The corrective actions associated with this LER would not have prevented this 2002 event from occurring since the actions taken in 1994 only addressed the use of biocide chemicals for zebra mussel control (on a yearly basis) in the forebay and intake tunnels.
2002 005 00 17. TEXT (if more space is required, use additional copies of NRC For ► (366A) loss of vacuum). As a result, Unit 1 was manually tripped. The cause of this event was due to the low circulating water flow and fouled condenser tube sheets. The debris that was removed from the tube sheets consisted of metallic flakes determined to have originated from the inside of the vertical circulating water line leading to the FW pump turbine condenser. Some zebra mussels and sand were also present. The extended shutdown of Unit 1 and 2 allowed corrosion products (i.e., metallic flakes) to develop on the vertical portion of the lines leading to the condensers.