05000277/LER-2001-002
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2772001002R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 2 was in Mode 1 and operating at approximately 100% rated thermal power when the event occurred. There were no structures, systems or components out of service that contributed to this event.
Description of the Event
On evening of June 30, 2001, during shift turnover, the operating crew noticed erratic indications on the main turbine control valve servos, pressure set, and various EHC indicators (EIIS:TA). No turbine control valve movement was noted at this time. I&C was subsequently contacted to develop a troubleshooting plan.
The plan was developed, approved, and a pre-job brief was conducted.
The initial step in troubleshooting involved obtaining readings from the -22 Volt power supply. The I&C technician noticed that the as-found voltage reading was -22.05 Volts instead of the expected -22.00 Volts. Also, a 59 milliVolt ripple was detected instead of the expected 17 milliVolt ripple. On July 1, 2001, at approximately 0206 hours0.00238 days <br />0.0572 hours <br />3.406085e-4 weeks <br />7.8383e-5 months <br />, shortly after getting the as-found readings, the -22.00 Volt power supply failed (EIIS:JJ). The power supply failure resulted in a Unit 2 Main Turbine trip and caused the reactor to automatically scram on a turbine control valve fast closure signal.
As a result of the main turbine trip and reactor scram, reactor pressure increased to approximately 1140 psig and reactor water level decreased to approximately -13 inches. This resulted in four safety relief valves lifting initially to control reactor pressure and PCIS group II and III isolations occurring, as expected. Reactor pressure was subsequently controlled by the main turbine bypass valves. Additionally, both Reactor Recirculation pumps received a trip signal when reactor pressure exceeded 1106 psig. No Emergency Core Cooling actuations occurred due to this event. All other systems responded as expected for the given plant conditions. The operating crew stabilized the plant per applicable procedures.
Subsequent to the event, the -22 Volt power supply was replaced with a new power supply and all parameters indicated normally. The failed power supply was quarantined for further analysis.
This report is being submitted pursuant to 10CFR50.73(a)(2)(iv)(A) due to a valid actuation of specified systems. The specific systems being reported included a valid actuation of the Reactor Protection System due to a main turbine trip and a valid primary containment isolation of PCIS group II and III valves due to low reactor water level.
Analysis of the Event
A main turbine trip from high power with bypass valves available is analyzed in the UFSAR as an abnormal operational transient (UFSAR Section 14.5.1.2.1).
According to the USFAR analysis, the closure of the main turbine stop valves should generate an automatic scram signal and the EHC system should generate a signal to open the main turbine bypass valves. Additionally, the analysis states that reactor vessel safety relief valves open for a short period of time as a result of the initial pressure transient. No fuel damage or unacceptable system stress result from this transient.
After the main turbine trip on July 1, 2001, all systems responded as expected for the transient. The reactor scrammed immediately, four safety relief valves lifted for a short period of time as a result of the initial pressure transient, and main turbine bypass subsequently opened to control reactor pressure. The maximum reactor steam dome pressure was 1140 psig and the minimum reactor water level was -13 inches. Reactor water level was restored to the normal band and controlled by the reactor feedwater system.
All mitigating systems operated as expected during this transient.
This event is not a significant operational event. The event was previously analyzed to result in no fuel damage or unacceptable system stress and all mitigated systems responded properly. Additionally, this event resulted in no operations outside the design basis, no major deficiencies having potential generic safety implications, no loss of fuel or barrier integrity, no loss of safety function, no adverse generic implications, no unexplained system interactions, no safety related equipment failures, and no crew performance concerns.
Cause of the Event
The cause of the main turbine trip was a failure of an Electro-Hydraulic Control System power supply, which caused a closure of the main turbine control valves. This resulted in a valid reactor scram signal and a valid primary containment isolation signal.
Subsequent failure analysis of the power supply identified a design deficiency in that a resistor rated at 1 Watt was used in an application with a demand of approximately 0.97 Watts. The resistor overheated, which changed the ohm value, and resulted in both the normal and back-up power supplies indicating erroneously. Both power supplies were replaced and tested satisfactorily.
Corrective Action Completed The failed power supply was replaced and tested satisfactorily.
Corrective Actions Planned None Previous Similar Occurrences None