05000483/LER-2007-002
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. | |
| Event date: | 03-09-2007 |
|---|---|
| Report date: | 05-08-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4832007002R00 - NRC Website | |
I. DESCRIPTION OF THE REPORTABLE EVENT
A. REPORTABLE EVENT CLASSIFICATION
10CFR50.73(a)(2)(iv)(A) — Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B) of this section (1) Reactor protection system (RPS) including: reactor scram or reactor trip.
(6) PWR auxiliary or emergency feedwater system.
B. PLANT OPERATING CONDITIONS PRIOR TO THE EVENT
MODE 1 at approximately 25-30 percent power with a rapid power reduction in progress due to a main condenser tube leak and subsequent high condenser sodium levels.
C. STATUS OF STRUCTURES, SYSTEMS OR COMPONENTS THAT WERE INOPERABLE AT THE
START OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT
Valve AEFCV0570, "C" Main Feedwater Regulating Bypass Valve (MFRBV) was isolated for scheduled maintenance when the event started.
D. NARRATIVE SUMMARY OF THE EVENT, INCLUDING DATES AND APPROXIMATE TIMES
On March 9, 2007, while operating at 100 percent power, Callaway Plant experienced a main condenser tube leak. At 0824, high sodium levels were indicated in the main steam condenser, requiring power reduction to less than five percent power until all Action Levels for condenser sodium were exited. The on-shift Operations crew began reducing power to enter MODE 3 in accordance with procedural guidance. During the transition from the Main Feedwater Regulation Valves (MFRV) to the Main Feedwater Regulation Bypass Valves (MFRBV), problems were experienced controlling water level in the "C" steam generator (S/G). At 1043, with both the MFRV and MFRBV indicating closed and the "C" S/G level still increasing, the reactor was manually tripped.
The "C" MFRBV was isolated for scheduled maintenance when the event started. Upon completion of the work, the valve was retested and returned to service at 0928 while the rapid power reduction was in progress.
The Balance of Plant (BOP) Operator reported the "C" MFRBV did not respond correctly during the transition from the MFRV to the MFRBV on the "C" steam generator. Indications were no change in feedwater flow was noted for corresponding change in valve demand in either open and closed directions. Feedwater flow changed to a value greater than expected causing steam generator level to rise outside the normal operating band. Feedwater flow to the "C" steam generator indicated about 1.3E6 lbs/hr and level in the "C" S/G continued to increase, while both the "C" MFRV and MFRBV indicated closed. The BOP Operator reported the condition and at 10:43 the Control Room Supervisor directed the Reactor Operator to trip the reactor due to loss of level control for "C" steam generator.
(At this point it should be noted that the actual problem was a failed I/P positioner on MFRV AEFCV0530).
After the manual reactor trip all rods indicated fully inserted except for rod K10. The digital rod position indication (DRPI) for control rod K10 failed during the trip. Control rod K10 was believed to be fully inserted, but was treated as indeterminate. Rod K10 was assumed as not inserted when the shutdown margin calculation was performed and the plant conservatively implemented the actions of FSAR specification 16.1.3.1. Troubleshooting showed that the problem with was with the DRPI for control rod K10, therefore, all rods were fully inserted after the reactor trip.
E. METHOD OF DISCOVERY OF EACH COMPONENT, SYSTEM FAILURE, OR PROCEDURAL ERROR
During the power reduction, "C" S/G level was increasing with both the MFRV and MFRBV indicating closed. Subsequent investigation indicated that the positioner for the "C" MFRV had failed. A field walkdown of AEFCV0530, following the plant trip, identified that the positioner was exhibiting abnormal indications. Observation of the LCD display indicated that "ERROR 12" was being displayed on the positioner. The positioner was periodically reinitializing on approximately 5 second intervals. It was also identified that AEFCV0530 was off its closed seat as identified by a yellow positioning pin being approximately one inch above the close limit switch actuating arm.
The rod K10 DRPI malfunction was discovered after the reactor trip, when a general warning annunciator was received for the digital position rod indication for control rod K10.
II.� EVENT DRIVEN INFORMATION
A. SAFETY SYSTEMS THAT RESPONDED
All safety systems functioned as designed with the exception of the digital rod position indication for control rod K10 which failed during the trip due to an existing problem. Control rod K10 was believed to be fully inserted, but was being treated as indeterminate. Main feedwater isolation and motor-driven auxiliary feedwater actuations occurred as expected. Both trains of the emergency diesel generators and the offsite power supplies were available.
B. DURATION OF SAFETY SYSTEM INOPERABILITY
Duration of the indication problem for control rod K10 was from the time of the reactor trip on March 9, 2007, through completion of post maintenance testing on the digital rod position system for control rod K10 on March 11, 2007. The duration was 1 day 13 hours1.50463e-4 days <br />0.00361 hours <br />2.149471e-5 weeks <br />4.9465e-6 months <br />.
The safety function for the MFRV and MFRBV is to close on a feedwater isolation signal. The safety function occurred when the reactor was tripped on March 9, 2007. There was not a safety system inoperability associated with the MFRV or MFRBV.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT.
The "C" MFRV, AEFCV0530, was able to perform its nuclear safety function to close on a Feedwater Isolation Signal (FWIS) during this event because this safeguards signal bypasses the valve positioner.
From a review of plant data after the reactor trip, the MFRV fully closed when the FWIS was generated.
This event was evaluated with the Callaway PRA model. The evaluation determined that the conditional core damage probability (CCDP) of the event was less than 1E-6; therefore, this event was of very low risk significance. Use of the PRA model to evaluate the event provides for a comprehensive, quantitative assessment of the potential safety consequences and implications of the event, including the consideration of alternative conditions beyond those analyzed in the FSAR.
III. CAUSE(S) OF THE EVENT AND CORRECTIVE ACTION(S)
During the downpower, at approximately 30% power, the positioner to AEFCV0530, "C" SG MFRV, failed such that the valve would not change position on demand. The programmed failure response is 'Fail as is'.
This is supported by the fact that the feedwater flow could not be reduced below 1.3E6 lbs/hr. An as-found AOV Diagnostic test was performed that determined the positioner had failed. The positioner could be made to intermittently operate by lightly tapping on it. The positioner was sent to an independent laboratory for analysis. The root cause of the event was attributed to a defective or damaged feedback shaft gear tooth in the positioner. The gear tooth damage was most likely due to a manufacturing defect or a problem during the initial factory assembly of the positioner.
The positioner has been replaced with a like kind positioner. The functionality and operability of the MFRV were verified through the performance of diagnostic and surveillance testing prior to returning the unit to service. An extent of condition review has been completed with no additional deficiencies identified.
The cause of the digital rod position indication problem for control rod K10 was a bent pin on a cable connector. The bent pin was straightened. Post maintenance testing was completed satisfactorily.
IV. PREVIOUS SIMILAR EVENTS
A reactor trip due to a MFRV positioner failure is not a recurrence for Callaway.
V. ADDITIONAL INFORMATION
The system and component codes listed below are from the IEEE Standard 805-1984 and IEEE Standard 803A-1984 respectively.
AEFCV0530, Steam Generator C Main Feedwater Regulating Valve, IEEE System: SJ, Feedwater Component: FCV, Valve, Control, Flow Positioner on AEFCV00530, System: SJ, Feedwater, Component: CPOS, positioner Manufacturer:
ABB, Model: TZID-C Digital Rod Position Indication, System: AA, Control Rod Drive Component: CON, connector