05000413/LER-2001-002
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| Event date: | |
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| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 4132001002R00 - NRC Website | |
Background
This event is being reported under 10CFR50.73(a)(2)(i)(B), any operation or condition which was prohibited by the plant's Technical Specifications, and 10CFR50.73(a)(2)(v)(D), any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.
The Control Room Area Chilled Water System (CRACWS) [EIIS:KM] provides temperature control for the control room and the control room area. This function is accomplished by providing chilled water through the cooling coils of the air handling unit supply fans. The CRACWS consists of two independent and redundant trains with each train consisting of a chiller package, chilled water pump, and air handling units with cooling coils.
For the duration of this event, Catawba Units 1 and 2 were operating in Mode 1 at approximately 100 percent power. With the exception of A-train chiller maintenance, no systems, structures, or components were out of service that had any significant effect on the event.
Event Description (dates and approximate times) 5-2-01 � A-train chiller was removed from service to 0330 � clean the heat exchanger tubes, replace the heat exchanger divider plates, and test the differential pressure switch that provides input to the chiller start logic indicating the chilled water pump is running.
5-3-01 0101 A-train chiller maintenance activities were completed and several differential pressure switch tests had been conducted - two successful tests and 2 unsuccessful tests.
Personnel performing the testing concluded it was necessary to replace the switch and testing was stopped.
While A-train chiller was still out of service, the B-train chiller automatically tripped due to a high motor temperature indication. This left the control room with no cooling and temperature began to increase. After evaluating B-train chiller, it was determined that it would be faster to place A-train chiller in service.
0128 � Plant personnel were dispatched to restart the A-train chiller. The focus for the technicians' task was to start a chiller with any actions necessary to provide control room cooling.
These technicians also participated in the earlier differential pressure switch tests and were aware that the switch had required manual manipulation to successfully operate.
Therefore, the technicians assisted the switch contacts to start the chiller and provide control room cooling.
The engineer was aware that the technicians operated the switch prior to the chilled water pump start but was unaware of any switch manipulation after the start signal. The engineer did not communicate this information.
The Operations Shift Manager (OSM) was not aware of the manual switch manipulation - before or after the start signal. The OSM believed that the system started without assistance and the unassisted start would be information in determining system operability.
A-train chiller was started at 0128.
0145 � The OSM initiated a conference call with senior plant managers to discuss A-train CRACWS operability. The engineer participated in the call and, when asked if the A-train CRACWS was assisted for the start, the engineer did not communicate that the technicians manually operated the switch. The participants in the call understood, at that time, that the chiller had started without assistance and would restart under accident conditions. The OSM, with management concurrence, concluded A-train chiller was operable.
0155 � A-train CRACWS was declared operable and the unit shutdown preparations were stopped.
B-train CRACWS remained out of service and inoperable.
0514 � Maintenance personnel discovered a bad motor temperature module[EIIS:MO] in the B-train chiller and repaired B-train chiller.
0733 � B-train chiller was started.
0751 � B-train chiller was declared operable.
0830 � Based on information provided that the A-train chiller was started with manual assistance and the chiller may not have been capable of manually being started from the control room or capable of automatically starting during an accident, the A-train chiller was then returned to inoperable status.
5-4-01 0200 � The A-train chiller was started successfully without manual assistance and with no maintenance activities.
1043 � A-train chiller restored to operable status.
The chiller start logic was temporarily changed to remove the switch signal input.
Causal Factors A root cause investigation was initiated to evaluate the incorrect declaration of CRACWS operability, based on inaccurate information, that � resulted in not initiating a dual unit shutdown within one hour of entering TS 3.0.3. The Root Cause Team determined the incorrect decision was based on the over reliance on one supporting piece of information.
This event is a recurring event. This event is a human performance related event.
The motor temperature module failure is an EPIX reportable equipment failure associated with this event. Reference Catawba Nuclear Station Unit 2 EPIX report 276.
Corrective Actions
Immediate 1. A-train CRACWS was placed in service to provide control room cooling.
Subsequent 1. A-train and B-train CRACWS were restored to operable status.
Planned 1. Senior plant management will provide additional expectations to the appropriate management staff, including the OSMs. These expectations will include the emphasis of a checklist designed to assist managers when they make time critical and risk significant decisions.
2. The plant administrative procedure will be revised to more formalize management conference calls when conducted outside normal working hours.
3. Engineering will evaluate the control room chiller equipment and, based on the study, will initiate appropriate changes to the system to improve the chiller reliability.
Safety Analysis
The safety significance of this event is low because the A-train chiller was started within 30 minutes to maintain the control room temperature within the design limits. Control room temperature was less than the design limit of 90 degrees at all times. Although the manual start of the chiller did not occur, the A-train chiller could have been manually restarted with local switch manipulation, if necessary, prior to the B- train chiller being restored operable at 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />, 50 minutes into the event.
Additionally, the A-train chiller automatic start capability was successfully demonstrated at 0200 hours0.00231 days <br />0.0556 hours <br />3.306878e-4 weeks <br />7.61e-5 months <br /> 5/4/01 with no manual assistance or intervening maintenance activity. The possibility exists that the A- train chiller may have been capable of performing its design function and fully operable for the approximately 25 hours2.893519e-4 days <br />0.00694 hours <br />4.133598e-5 weeks <br />9.5125e-6 months <br /> since it was considered inoperable at 0128 hours0.00148 days <br />0.0356 hours <br />2.116402e-4 weeks <br />4.8704e-5 months <br /> 5/3/01.
The Probabilistic Risk Assessment (PRA) evaluated, qualitatively, the significance of the control area ventilation system and determined the system to be of low importance. On this basis the PRA does not include the CRACWS in the model and no significant impact on core damage frequency is expected.
The Control Room Area Ventilation System (CRAVS) was maintained operable throughout this event. Control room habitability was assured by maintaining the control room pressurization fans, filters, and automatic initiation features available. In the event of a Loss of Coolant Accident at this time, the CRAVS could have operated for the duration of the event with a high level of reliability thereby maintaining exposure to the operators within regulatory limits. Therefore this event is not significant relative to control room dose consequences.
The health and safety of the public were not affected by this event. There were no radiological events or consequences associated with this event.
Although the safety impact of this event was minimal, this condition met the reporting criteria of 10 CFR 50.73(a)(2)(v) and therefore will be recorded under the NRC Performance Indicators for both units as a Safety System Functional Failure.