05000457/LER-2001-001
| Event date: | 05-19-2001 |
|---|---|
| Report date: | 07-17-2001 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation |
| 4572001001R00 - NRC Website | |
Plant Operating Conditions Before The Event:
Unit: 2 � Event Date: 5/19/2001 � Event Time: 0406 hours0.0047 days <br />0.113 hours <br />6.712963e-4 weeks <br />1.54483e-4 months <br /> MODE: 1 � Reactor Power: 100.0 percent Reactor Coolant System [AB] Temperature: 580 degrees F, Pressure: 2235 psig B. � Description of Event:
The Unit 2 system auxiliary transformer [EB] was out of service at the beginning of the event. This is the only equipment that was inoperable at the beginning of the event that contributed to the event.
On the afternoon shift of Friday 5/18/01 with Unit 2 at full power, Operations was in the process of removing the Unit 2 system auxiliary transformers (SATs) from service. The 6.9kv buses (258 and 259) [EA] that are normally powered by the Unit 2 SATs were transferred to the Unit 2 unit auxiliary transformers (UATs) [EA] to support the SAT outage. The Unit 2 safety related 4kv ESF buses [EB] that are normally powered by the Unit 2 SAT's were transferred to the Unit 1 SATs.
On Saturday 05/19/01, the SAT outage Project Manager (PM), an off-shift Senior Reactor Operator (SRO), led the midnight shift operations crew in a pre-job briefing. The Shift Manager, the Unit 2 Supervisor, both Unit 2 Nuclear Station Operators (NSOs), and four extra non-licensed operators participated in the pre- job briefing. The PM stressed the importance of operating the correct potential transformer fuse doors during the pre-job briefing. The Field Supervisor (FS) and the Work Execution Center (WEC) Supervisor did not participate in the pre-job brief. The FS was in the field investigating a problem on Unit 1 at the time of the pre-job briefing. Shift supervision failed to recognize the need to have the FS present at the briefing. This contributed to the improper command and control of the SAT outage evolution.
The non-licensed operators assigned to the task had not performed this evolution previously and did not have experience working with potential transformer fuses.
Because of the inexperience of the non-licensed operators performing this evolution, the PM directed the FS to observe the potential transformer fuse removal. Although the FS was not present at the pre-job briefing, the FS was experienced in this task and had observed performance of this task in the past.
When the non-licensed operators were ready to remove the first set of potential transformer fuses, they contacted the FS for further direction and guidance per the pre-job briefing instructions. The FS explained and supervised removal of the potential transformer fuses for bus 241 SAT feed potential transformer.
Within the fuse cabinet, there is an upper and a lower door. One door is for the bus potential transformer, the other is for the SAT feed potential transformers.
The potential transformer fuse removal on bus 241 and bus 242 required the upper door labeled "SAT FEED POTENTIAL TRANSFORMERS" to be opened. Prior to beginning Nuclear Station Operator (NSO) contacted the FS for a problem not related to the Unit 2 SAT outage. Before leaving the area, the FS questioned the non-licensed operators to ensure they were comfortable with continuing the fuse removal evolution in his absence. The non-licensed operators answered in the affirmative and the FS left the area. The non-licensed operators successfully removed the bus 242 potential transformer fuses. The non-licensed operators then proceeded to the 6.9kv switchgear room to continue with bus 258 potential transformer fuse removal.
The non-licensed operators identified the correct external cubicle door for 6.9kv bus 258 using concurrent verification. A third non-licensed operator already wearing protective gear from another evolution joined the group to complete the fuse removal evolution. This non-licensed operator had attended the pre-job briefing.
The non-licensed operator who had the procedure for the task directed the non- licensed operator in protective gear to open the upper door for the SAT feed to bus 258 primary potential transformer fuses. This direction was incorrect for the 6.9kv buses and was not what was stated in the procedure. The procedure stated to pull open the lower door labeled "SAT FEED POTENTIAL TRANSFORMERS" for 6.9kv bus 258. The labeling on the correct door exactly matched the procedure step that was in progress. Neither of the other two non-licensed operators read the procedure step. One assumed that the 6.9kv bus SAT feed potential transformer fuses would be contained in the same location as the previous two sets of 4kv bus SAT feed potential transformer fuses which had just been pulled. The non- licensed operator in protective gear had not been present for the previous fuse removal, and he assumed that the procedure had been read correctly.
The non-licensed operator in the protective gear, using 3-way communication, repeated back the instructions, pointed to the upper door and asked if the upper door was the correct component to manipulate. The non-licensed operator with the procedure in-hand replied that it was the correct door. The non-licensed operator in protective gear opened the upper door on verbal direction and caused an under voltage on bus 258 which tripped the 2C reactor coolant pump (RCP)[AB]. The three non-licensed operators heard the 2C RCP breaker open and observed that the indicating light had changed from red to green. At this time, the Unit 2 reactor automatically tripped due to loss of flow in the 2C reactor coolant loop with reactor power greater than 30%. All control rods inserted and all appropriate safety systems operated as designed.
The reactor trip caused a trip of the Unit 2 main turbine/generator [TB]. Due to the alignment of the non-safety related buses to the Unit 2 UATs and with the Unit 2 SATs de-energized, all non-safety related 4kv and 6.9kv buses de- energized. The remaining three RCPs tripped along with the circulating water pumps [SG], the main feedwater pumps [SJ] and the condensate/condensate booster pumps [SD]. The Unit 2 primary plant was placed on natural circulation cooling with the secondary heat removal from the steam generator power operated relief valves. The auxiliary feedwater [BA] pumps started on the RCP bus under voltage signal and provided feed flow to the steam generators. The final result was a loss of all non-safety related off-site power to Unit 2. The Unit 2 safety related 4kv ESF buses were cross-tied to Unit 1 and remained energized throughout the event. The Unit 2 diesel generators [EK] were subsequently manually started and buses 241 and 242 were transferred to the 2A and 2B Diesel Generators.
C. Cause of Event:
The reactor tripped when the incorrect door was opened on bus 258 for the removal of bus 258 potential transformer fuses. Opening the incorrect door caused bus 258 to de-energize which caused the 2C RCP to trip. The trip of the 2C RCP caused the reactor to trip on loss of flow. The reactor trip caused the main generator to trip which then resulted in all non-safety related 4kv and 6.9kv busses de-energizing.
The first root cause for this event was the failure of non-licensed operators to perform concurrent verification of the SAT primary potential transformer fuse door.
The second root cause for this event was improper command and control of the critical steps within the evolution.
D. Safety Consequences:
The reactor protection system responded normally following a loss of flow to one reactor coolant loop, and the reactor was placed in a sub-critical state. Due to the existing electrical lineup, the event resulted in loss of forced circulation through the reactor coolant system and a loss of main condenser vacuum. Core cooling was maintained by operation on natural circulation with secondary heat sink provided from the steam generator power operated relief valves and auxiliary feedwater. Natural circulation is a design feature of the plant and is discussed in the Updated Final Safety Analysis Report (UFSAR). As discussed in the UFSAR, natural circulation flow in the RCS following a loss of forced coolant flow is sufficient to remove residual heat from the core.
There were no safety consequences as result of the event as no margins were exceeded. All safety related equipment required to mitigate the consequences of this event was available following the reactor trip, and operated as designed.
The event did not result in a Safety System Functional Failure.
E. Corrective Actions:
The first root cause for the event was the failure of the non-licensed operators to perform concurrent verification as required by station procedures for the SAT feed potential transformer fuse door. The corrective action to prevent recurrence of this root cause was counseling and personnel discipline in accordance with station policy. An additional corrective action was a station stand down. During the stand down, the Site Vice President and Station Manager reviewed the event and the human error prevention aspects that failed during the evolution with station employees.
The second root cause for the event was improper command and control of the critical steps within the evolution. The corrective action to prevent recurrence of this root cause was the issuance of an Operations Department Standing Order, "Command and Control Standard for Execution of Critical Steps." This standard has been issued at all Exelon Nuclear sites. This standard states the requirements to screen all activities for risk impact, to identify critical steps that require direct oversight and to obtain authorization from the Main Control Room prior to performing critical steps. An additional corrective action was counseling and personnel discipline in accordance with station policy.
F. Previous Occurrences:
There have been no similar occurrences at Braidwood Station.
G. � Component Failure Data:
Manufacturer � Nomenclature � Model � Mfg. Part Number N/A � N/A � N/A � N/A