05000483/LER-2008-007
| Callaway Plant Unit 1 | |
| Event date: | 12-12-2008 |
|---|---|
| Report date: | 02-06-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| 4832008007R00 - NRC Website | |
I.� DESCRIPTION OF THE REPORTABLE EVENT
A. REPORTABLE EVENT CLASSIFICATION
10CFR50.73(a)(2)(iv)(A) requires reporting of any event or condition that resulted in a manual or automatic actuation of any of the systems listed in 10CFR50.73(a)(2)(iv)(B). The systems listed below are relevant to this LER:
(1) Reactor protection system (RPS) including: reactor scram or reactor trip.
(6) PWR auxiliary or emergency feedwater system.
The reactor trip is considered invalid because the plant condition was such that there was not a high neutron flux present in the reactor. The reactor trip signal that was received was the result of a maintenance activity in progress at the time.
B. PLANT OPERATING CONDITIONS PRIOR TO THE EVENT
Prior to the event, the plant was in MODE 3 with the reactor coolant system at approximately 2235 psig and 557 degrees F. Steam generator water levels were being maintained by the feedwater system using the startup feedwater pump. The reactor trip breakers were closed and the control rods inserted in the reactor.
C. STATUS OF STRUCTURES, SYSTEMS OR COMPONENTS THAT WERE INOPERABLE AT THE
START OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT
Intermediate range (IR) nuclear instrument (NI) (IEEE system IG) channel 36 was being taken out of service for card replacement.
D. NARRATIVE SUMMARY OF THE EVENT, INCLUDING DATES AND APPROXIMATE TIMES
An Intermediate Range High Flux reactor trip signal was generated unexpectedly while personnel were replacing a circuit card associated with a bistable card (IEEE system JC) for SEN0036 (IEEE system IG component RE). The following events took place starting at approximately 10:29 am on December 12, 2008:
10:29� Maintenance technicians were authorized to start a plant job to replace a bistable card for intermediate range neutron level channel SEN0036.
10:31� SENI0036A, intermediate range channel 36 neutron level indicator, was taken out of service for maintenance.
10:43� Reactor tripped on intermediate range high flux. (Note: Subsequently, a Feedwater Isolation Signal (FWIS) was generated and feedwater flow to all four steam generators was isolated because reactor coolant system temperature was less than 564 degrees Fahrenheit and the reactor trip breakers were open.) 10:55C Control room operators completed initial monitoring of all critical safety functions. All parameters were green.
10:57C Control room operators manually started motor driven auxiliary feed water pumps (IEEE system BA) to maintain steam generator water level. (For reporting purposes, this was considered a manual actuation of the auxiliary feedwater system).
11:00C Control room operators determined that the intermediate range high flux reactor trip was a result of the removal of Intermediate Range NI N-36 Control Power fuses while performing work to replace a bistable card for SEN0036.
11:30C Control room operators transitioned from emergency operating procedures to normal operating procedures.
12:25C Control room operators restored feedwater flow to generators via the startup feed pump through the main feed regulating bypass valves.
E. METHOD OF DISCOVERY OF EACH COMPONENT, SYSTEM FAILURE, OR PROCEDURAL ERROR
The problem with the work instructions was discovered as a result of the reactor trip signal.
II.C EVENT DRIVEN INFORMATION
A. SAFETY SYSTEMS THAT RESPONDED
All systems functioned as expected (per design) in response to the invalid reactor trip signal.
B. DURATION OF SAFETY SYSTEM INOPERABILITY
Not applicable C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT.
At the time the reactor trip signal was generated, all of the safety related equipment required for existing plant conditions (MODE 3) was operable. The reactor trip signal initiated a feedwater isolation signal which automatically closed the feedwater isolation valves and isolated flow to all four steam generators. Control room operators manually started the two motor-driven auxiliary feedwater pumps and stabilized the plant.
Since all of the safety related equipment was available and working at the time the trip signal was generated, any design basis accident postulated for the existing plant condition (MODE 3) could be adequately addressed by either plant equipment or Operations personnel. Operator response to the feedwater isolation trip signal ensured that the plant was maintained in a safe shutdown condition. Starting the motor-driven auxiliary feedwater pumps ensured removal of residual decay heat from the core. There was no concern during this event of any release of radioactive material from the core or reactor coolant system. All available safety functions and responses were performed adequately to ensure no major or minor events would occur. Safe shutdown conditions were maintained for the duration of the event.
III. CAUSE(S) OF THE EVENT AND CORRECTIVE ACTION(S)
Brief Explanation of the Design:
The Source Range (SR) and Intermediate Range (IR) instruments use a bypass switch in the Nuclear Instrumentation System (NIS) for the associated reactor trip bistables. The SR High Level trip and the IR High Level trip are the only examples of this within the NIS. The TRIP BYPASS switch is located on the front panel of the associated NI drawer. With the TRIP BYPASS switch in the BYPASS position, 118-VAC control power energizes trip bypass relays in the solid state protection system (SSPS). The trip bypass relay, when energized, will bypass the trip function of the associated trip relay. Additionally, when reactor power is greater than 10%, the Power Range (PR) NI's enable a manual block in SSPS that can be used to latch an inhibit signal to block this ER High Level trip signal downstream of the BYPASS relay. This will block the IR High Level trip regardless of whether the BYPASS relay is energized.
For the maintenance activity to be performed on the SEN0036 IR instrument channel, Step 1.0 of the Job Work Instructions directed the LEVEL TRIP switch to be placed in the BYPASS position per plant procedure ISL-SE-00N36 revision 22, step 6.1.5. Step 1.1 of the Job Work Instructions stated "REMOVE Control Fuse." This step removed the 118 VAC control power to the energized BYPASS relay, thereby removing the BYPASS condition.
Causes and Corrective Actions:
The intermediate range high flux reactor trip was a result of removing the intermediate range nuclear instrumentation N-36 control power fuses while performing work to replace a bistable card for SEN0036. When the fuses were pulled, the bypass signal from the NI drawer was removed.
A step was included in the work document to remove the control power fuses. Maintenance personnel did not understand that a relay was energized to enable the IR HI Flux Reactor Trip signal bypass and that 118-VAC control power feeds this relay in SSPS. This was the root cause of the event. The desired practice is to de-energize equipment, where practical, prior to circuit card replacement in order to protect electronic circuits. Therefore, to replace the bistable circuit board, it was decided to completely de-energize the Nuclear Instrumentation (NI) drawer. When the control power fuses were pulled, the relay de-energized, thus opening the bypass relay contacts and satisfying the logic to generate the Intermediate Range High Flux Reactor Trip Signal.
Corrective actions include the following:
1) Labels will be installed on the control power fuses for both trains of the SR and IR Nuclear Instrumentation to flag that pulling them could have significant impact on the plant.
2) Instrumentation and Controls personnel will receive additional training on the IR and SR Nuclear Instrumentation, covering an overview on systems function and operation. The training will also provide an overview on how the system interacts with SSPS permissive relays.
IV. PREVIOUS SIMILAR EVENTS
None of the same type.
V. ADDITIONAL INFORMATION
None