05000335/LER-2012-005
| St. Lucie | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3352012005R01 - NRC Website | |
Description of the Event
On June 2, 2012 at 1935 St. Lucie Unit 1 reactor automatically tripped from normal full power operation due to a loss of load following a failure of the turbine control system (TCS) [CON:IT] No automatic safety system actuations were required and none occurred. The reactor coolant system (RCS) heat removal was maintained with main feedwater (MFW) and steam bypass to the condenser.
Cause
The reactor trip on turbine trip was caused by loss of communications of the Drop 3 and Drop 53 controllers in the Ovation turbine control system (TCS) The direct cause for the failure of the primary controller (Drop 53) was an intermittent fault in the Remote Node Controller (RNC) or the Media Attachment Unit (MAU) [DCC:IT] The back up controller (Drop 3) failed due to a failed fiber optic (FO) cable field connector [CON:IT] the TCS upgrade.
The root cause of this event was ineffective design control for
Analysis of the Event
The primary function of the recently installed TCS is to control the proper speed and load of the turbine. The TCS consists of five controller cabinets each containing redundant controllers identified as "Drops." Failed Drops 3 and 53 are redundant Drops in one of the five controller cabinets. Prior to the event, Unit 1 was in Mode 1 and stable at 100% reactor power with no evolutions in progress or abnormal plant indications. At 1453, Drop 53 failed and transferred to Drop 3. At 1935, Drop 3 failed with no back-up causing the turbine / reactor trip.
Contributing factors to the ineffective design control for the TCS upgrade included weakness in the troubleshooting procedure, alarm response procedure, training on the new plant modification, workmanship and post maintenance testing. As a result, the unit was started up with TCS hardware deficiencies and a limited capability for responding to system malfunctions.
This licensee event report is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an event or condition that resulted in manual or automatic actuation of the reactor protection system including reactor scram or reactor trip.
Safety Significance
A risk assessment associated with Unit 1 automatic reactor trip event due to failure of TCS was developed. The conditional core damage probability (CCDP) and conditional large early release probability (CLERP) values were evaluated for the stated event and were found to be significantly below the thresholds required by RG-1.174 for the risk to be "Small", where CCDP is below 1.0E-06 and CLERP is below 1.0E-07.
Therefore, it is concluded that this event had no effect on the health and safety of the public.
Prompt Corrective Action A11 questionable control modules and the controllers were replaced.
Corrective Actions
The corrective actions below have been entered into the site corrective action program. Any changes to the actions will be managed under the corrective action program.
1. Additional guidance has been added to the annunciator response procedure for responding to TCS alarms.
2. A new normal operating procedure was developed and implemented for the TCS.
3. Additional TCS training is being provided to Operations, Engineering, and Maintenance.
4. New maintenance procedures were developed and implemented for the TCS.
5. The design control process is being revised to require risk based reviews for post maintenance test sequencing following risk significant plant modifications.
Similar Events A search of the corrective action database for three years was performed and identified no issues that were related to the faults identified with this event.
Failed Component(e) The primary controller (Drop 53) failed due to a fault in the Remote Node Controller or the Media Attachment Unit. The back up controller (Drop 3) failed due to a failed fiber optic cable field connector.
Manufacture: Model:
Emerson Electric Company 1C31203G01, Ovation RNC Electronics Module Ovation RNC Personality Module Ovation MAU Electronics Module Ovation MAU Personality Module 1C31204G01, 1C31179G02, 1C31181G02,