05000277/LER-2005-001
| Docket Number Sequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | |
|---|---|
| Report date: | |
| Initial Reporting | |
| ENS 41832 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 2772005001R00 - 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. Weekly Main Turbine (EIIS: TRB) Mechanical Trip Valve (EIIS: V) testing was in progress.
Description of the Event
On 7/10/05, at approximately 0318 hours0.00368 days <br />0.0883 hours <br />5.257936e-4 weeks <br />1.20999e-4 months <br />, a Unit 2 automatic scram occurred as a result of a Main Turbine Stop Valve closure signal. The Main Turbine Stop valves had closed as a result of a Main Turbine trip signal generated during performance of the weekly Main Turbine Mechanical Trip Valve routine test. The Main Turbine trip signal was generated when a failed test was being aborted and did not reflect any actual adverse Main Turbine condition that required the Main Turbine to be tripped.
As a result of the automatic scram, the Reactor Vessel water level 3 set point was reached as expected. This resulted in Primary Containment Isolation System (PCIS) Group II and HI isolations. The PCIS (EIIS: JM) Group II and III isolations resulted in the closure of valves in various systems including the Reactor Building Ventilation system (EIIS: VA), the Containment Atmospheric Control / Containment Atmospheric Dilution systems (EIIS: BB), the Reactor Water Cleanup sytem (EIIS: CE) and other containment penetrating process lines. The Standby Gas Treatment system (EIIS: BH) also actuated as expected on the Group III PCIS isolation.
Also, as expected for an automatic scram involving closure of the Main Turbine Stop Valves, a Reactor Vessel high pressure condition occurred resulting in automatic operation of the C, D, and E Main Steam Safety Relief Valves (SRVs) (EIIS: RV) and initiation of Alternate Rod Insertion (EIIS: AA). The Recirculation Pump Motors (EIIS: AD) also tripped as a result of the closure of the Main Turbine Stop Valves as designed. All control rods properly inserted and there were no safety significant anomalies involved with the plant equipment response to the event.
The scram and ARI initiation were reset by approximately 0327 hours0.00378 days <br />0.0908 hours <br />5.406746e-4 weeks <br />1.244235e-4 months <br />. The PCIS Group H and III isolations were reset by approximately 0335 hours0.00388 days <br />0.0931 hours <br />5.539021e-4 weeks <br />1.274675e-4 months <br /> and the normal Reactor Building ventilation was restored by approximately 0355 hours0.00411 days <br />0.0986 hours <br />5.869709e-4 weeks <br />1.350775e-4 months <br />.
As required by 10CFR 50.72, NRC prompt notifications were completed on 7/10/05 at approximately 0642 hours0.00743 days <br />0.178 hours <br />0.00106 weeks <br />2.44281e-4 months <br /> to report the automatic scram and PCIS isolations (Event Notification#41832). This report is being submitted pursuant to 10CFR50.73 (a)(2)(iv)(A) due to valid actuations of the Reactor Protection System and the Primary Containment Isolation System.
Analysis of the Event
There were no actual safety consequences associated with this event. The normal heat removal path (i.e.
feedwater / condenser) was maintained during the event. All control rods properly inserted and there were no safety significant anomalies involved with safety equipment response to the event. It was determined that safety systems responded appropriately for this event. This event is bounded by the non-limiting event entitled 'Turbine Trip from High Power with Bypass' described in Updated Final Safety Analysis Report (UFSAR) section 14.5.1.2.1. As noted in this UFSAR section, it is expected that the SRVs would open for a short time to relieve the pressure increase caused by the transient.
Analysis of the Event (continued) The Main Turbine trip signal was generated during performance of the Main Turbine Mechanical Trip Valve routine test. This test verifies that the Main Turbine Mechanical Trip Valve (MTV) is functioning and is performed weekly with the Main Turbine in service. The primary purpose of the MTV is to remove the Main Turbine emergency trip system hydraulic pressure in the event of a Main Turbine over-speed condition, thereby resulting in a Main Turbine Stop and Control Valve closure (i.e. main Turbine trip). The MTV is mechanically operated by the Main Turbine over-speed trip mechanism. The test is credited in the Updated Final Safety Analysis Report (UFSAR) section 11.4 to minimize the likelihood of a Main Turbine failure that could generate missiles. The Main Turbine trip signal was generated when a failed test was being aborted and did not reflect any adverse Main Turbine condition that actually required the Main Turbine to be tripped.
This event is not considered as risk significant.
Cause of the Event
The cause of the event is due to shortcomings in human performance when aborting the Main Turbine MTV routine test being performed on 07/10/05.
While performing the test, the test acceptance criteria had not been met. This was a result of not receiving indication lights that the MTV had been actuated by the Main Turbine over speed mechanism when a test oil signal was applied during the test. When the indication was not received, the Reactor Operator (licensed, utility) performing the test received direction from the Control Room Supervisor (licensed, utility) to abort the test. Because there are two indications of MTV actuation, it was incorrectly assumed by the Control Room Supervisor (CRS) that the test oil signal had not been received by the over speed mechanism and the MTV actuation did not occur. The CRS did not request any additional review concerning the method of aborting the test. The steps in the routine test involving the resetting of the test oil signal were assumed to be not required and the test was aborted by placing the trip system back in service. However, the over speed mechanism and MTV had actuated as a result of the test oil signal. When the MTV was placed back into service, the Main Turbine tripped.
The failure of both MTV indications appears to be related to a relay card (General Electric, 125 VDC Relay Board, Part No. 114D6063G0003)(EllS: RLY) associated with the MTV indicating lights. Further assessment of the card is being performed in accordance with the Corrective Action Program.
NRC FORM 366AU.S. NUCLEAR REGULATORY COMMISSION (1.2001) FACILITY NAME (1) DOCKET (2) LER NUMBER (6) PAGE (3)
Corrective Actions
The CRS has been remediated and reevaluated, additional corrective actions will be taken prior to resumption of full control room duties.
A standing order was issued to licensed operators communicating management expectations regarding human performance practices.
An assessment of Operations personnel concerning their knowledge of expected human performance behaviors will be performed.
A case study was developed covering the details of this event for use in continuing training.
The relay card and MTV switch were replaced by 7/11/05 and MTV indication was tested to ensure reliable operation.
Previous Similar Occurrences There were no previous LERs identified involving Main Turbine trips during testing.