ENS 44109
ENS Event | |
|---|---|
17:52 Mar 31, 2008 | |
| Title | Automatic Reactor Trip On Turbine Trip During Maintenance Activities |
| Event Description | Event: At 1352 hours0.0156 days <br />0.376 hours <br />0.00224 weeks <br />5.14436e-4 months <br />, Unit 2 experienced a Reactor Trip due to a turbine trip. The indicated cause was low condenser vacuum. The exact cause is still under investigation but is believed to be related to a maintenance procedure in progress on the condenser vacuum instrumentation. Post-trip response was normal. Auxiliary power transferred to the start-up source (switchyard) as expected. Main Feedwater was not affected so there was no demand for Emergency Feedwater. A second High Pressure Injection pump was manually started per procedure to maintain Pressurizer level indication on scale. This is a routine action to compensate for post-trip RCS temperature and volume changes.
In an apparently unrelated event, Keowee Hydro Units (KHU) 1 and 2 were shutdown from commercial operation at approximately 1425 hours0.0165 days <br />0.396 hours <br />0.00236 weeks <br />5.422125e-4 months <br />. During the shutdown, the KHU 1 output breaker failed to open as expected and KHU 1 was manually locked out. The lockout removed both the Overhead and Underground Power Paths from service, making on-site emergency power unavailable to all three Oconee units. Per Tech Specs, a gas turbine unit at Lee Steam Station was started and used to energize the Oconee Standby Bus at 1518 hours0.0176 days <br />0.422 hours <br />0.00251 weeks <br />5.77599e-4 months <br />. Initial Safety Significance: There is little or no safety significance to the Unit trip. The subsequent KHU 1 lockout removed both the Overhead and Underground Power Paths from service, making on-site emergency power unavailable until the Lee gas turbine was aligned. There was no demand for the Keowee emergency power function during either event. Corrective Action(s): Investigations are in progress as to the cause of the Reactor Trip and the KHU 1 lock out. Alignment of KHU 2 to the underground is currently on hold, pending evaluation of the problem which led to the KHU 1 lockout. All control rods fully inserted with decay heat being removed via the turbine bypass valves. The licensee notified the NRC Resident Inspector.
The licensee is retracting a portion of the original report identified under 10CFR50.72(b)(3)(v)(D) based on the following: Event: UPDATE: At 1352 hours0.0156 days <br />0.376 hours <br />0.00224 weeks <br />5.14436e-4 months <br />, Unit 2 experienced a Reactor Trip due to a turbine trip. The indicated cause was low condenser vacuum. The exact cause is still under investigation but is believed to be related to a maintenance procedure in progress on the condenser vacuum instrumentation. Post-trip response was normal. Auxiliary power transferred to the start-up source (switchyard) as expected. Main Feedwater was not affected so there was no demand for Emergency Feedwater. A second High Pressure Injection pump was manually started per procedure to maintain Pressurizer level indication on scale. This is a routine action to compensate for post-trip RCS temperature and volume changes. In an unrelated event, Keowee Hydro Units (KHU) 1 and 2 were shutdown from commercial operation at approximately 1425 hours0.0165 days <br />0.396 hours <br />0.00236 weeks <br />5.422125e-4 months <br />. During the shutdown, the KHU 1 output breaker, ACB-1, failed to open as expected and KHU 1 was manually locked out. The lockout removed KHU 1 and the Overhead Power Path from service due to the failed ACB. Because KHU 1 was the unit aligned to the Underground Power Path, Operations declared that path inoperable also. This condition was initially reported as making on-site emergency power unavailable to all three Oconee units. Per Tech Specs, a gas turbine unit at Lee Steam Station was started and used to energize the Oconee Standby Bus at 1518 hours0.0176 days <br />0.422 hours <br />0.00251 weeks <br />5.77599e-4 months <br />. In this alignment, the Lee gas turbine provides the on-site emergency power function. A design feature allows a KHU to automatically align to the Underground Path when the unit originally aligned to that path has been locked out, the overhead path is locked out and an emergency start signal exists. Keowee Operations confirmed that during the lockout event the required lockout signals were present on KHU 1 such that KHU 2 would have aligned to the Underground Power Path if an emergency start demand had occurred. As a result, the Underground Power Path remained available during this event and there was no loss of safety function. Therefore the portion of the event related to 10CFR50.72(b)(3)(v)(D) is RETRACTED. The Underground Power Path was administratively inoperable because a surveillance (SR 3.8.1.3) to verify operability of KHU 2 to the underground path, required per TS 3.8.1 Condition C.1, could not be performed. The surveillance procedure utilizes a normal start signal, which was inhibited by the lockout on the overhead path. The surveillance procedure does not include provisions for using an emergency start signal. At 1906 hours0.0221 days <br />0.529 hours <br />0.00315 weeks <br />7.25233e-4 months <br />, after the overhead lockout had been reset, KHU 2 successfully completed an Operability test aligned to the Underground Power Path. Investigation determined that the failure of ACB-1, the output breaker for KHU 1, was due to a terminal strip sliding link in the trip circuit being in an intermediate position. It was damaged during repair so the entire sliding link block was replaced. The unit was successfully tested connected to each power path and was declared Operable on 4-1-08 at approximately 0600 hours0.00694 days <br />0.167 hours <br />9.920635e-4 weeks <br />2.283e-4 months <br />. At 0930 hours0.0108 days <br />0.258 hours <br />0.00154 weeks <br />3.53865e-4 months <br /> on 4-1-08 the standby bus was disconnected from Lee. Initial Safety Significance: There is little or no safety significance to the Unit trip. The subsequent KHU 1 lockout removed the Overhead Power Path from service. The Lee gas turbine was aligned to energize the standby bus. Operations and Engineering subsequently confirmed that on-site emergency power remained available via KHU 2 and the Underground Power Path. There was no demand for the Keowee emergency power function during either event. Corrective Action(s): Investigation as to the cause of the Reactor Trip continues. Unit 2 is now on-line at low power and is in the process of returning to full power. ACB-1, the output breaker for KHU 1, terminal strip sliding link has been repaired/replaced; both KHUs have been tested and declared operable; the Lee gas turbine has been secured. The licensee informed the NRC Resident Inspector. Notified R2DO (Evans). |
| Where | |
|---|---|
Oconee  South Carolina (NRC Region 2) | |
| Reporting | |
| 10 CFR 50.72(b)(2)(iv)(B), RPS Actuation 10 CFR 50.72(b)(3)(iv)(A), System Actuation | |
| Time - Person (Reporting Time:+-0.68 h-0.0283 days <br />-0.00405 weeks <br />-9.31464e-4 months <br />) | |
| Opened: | Richard Todd 17:11 Mar 31, 2008 |
| NRC Officer: | Steve Sandin |
| Last Updated: | Apr 2, 2008 |
| 44109 - NRC Website
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Unit 2 | |
|---|---|
| Reactor critical | Critical |
| Scram | A/R |
| Before | Power Operation (100 %) |
| After | Hot Standby (0 %) |
Oconee with 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(b)(3)(iv)(A), System Actuation | |
WEEKMONTHYEARENS 571592024-06-03T04:51:0003 June 2024 04:51:00
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