05000251/LER-2014-002
| Turkey Point Unit 4 | |
| Event date: | 5-25-2014 |
|---|---|
| Report date: | 7-24-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2512014002R00 - NRC Website | |
DESCRIPTION OF THE EVENT
On May 25, 2014, at approximately 0536, Unit 4 experienced an automatic turbine trip and resultant reactor trip from approximately 20% reactor power due to main condenser low vacuum. At the time of the trip, operators were preparing to manually trip the reactor in accordance with procedure 4-GOP-103, Power Operation to Hot Standby. The Unit 4 downpower was a planned shutdown to support an unrelated equipment issue. Reactor power was approximately 20% and condenser vacuum was 27.9 inches with the Condenser Hogging Jet Air Ejector in service.
main steam system to the Unit 3 auxiliary steam supply system. This required aligning auxiliary steam from Unit 3 to supply the Unit 4 Steam Jet Air Ejectors, Unit 4 Gland Sealing Steam and the Unit 4 Condenser Hogging Jet Air Ejector. The supply pressure to the gland sealing steam regulating valve from the main steam system is 950 psig. The supply pressure to the gland sealing steam regulating valve from the auxiliary steam system is 250 psig. When the main steam supply was isolated, the gland sealing steam system could not maintain adequate sealing steam pressure. Consequently, air was introduced into the condenser through the turbine glands resulting in a rapid lowering of condenser vacuum. The low condenser vacuum trip set point was reached in less than 60 seconds. Although control room personnel noted the lowering of vacuum, the automatic trip occurred before the operator could initiate a manual trip. Operations personnel entered procedure 4-EOP-E-0, Reactor Trip or Safety Injection, and transitioned to procedure 4-E0P-ES-0.1, Reactor Trip Response.
In response to the trip all control rods fully inserted, and all systems responded as designed. The unit transitioned to Mode 3.
The NRC Operations Center was notified by Event Notification No. 50140 at approximately 0737 on May 25, 2014 in accordance with 10 CFR 50.72(b)(2)(iv)(B).
This event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) as "...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." The Reactor Protection System [JC] automatically actuated during the event and is included in the systems listed in paragraph (a)(2)(iv)(B).
CAUSE OF THE EVENT
The root cause of the event was that operations personnel did not adequately address the integrated system status as part of the decision making process used to realign the steam supply to the gland sealing steam system, thus making the decision to transfer the gland sealing system from the main steam system to the auxiliary steam system prior to tripping the reactor.
A contributing cause of the event was that procedure 4-GOP-103 allowed the steam supply realignment to be performed while the unit was on-line.
APPROVED BY OMB: NO. 3150-0104 EXPIRES: 01/31/2017 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />.
Reported lessons learned are incorporated into the licensing process and fed back to industry.
Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by intemet e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
Gland sealing steam is supplied by either the turbine cylinder heat system, main steam or auxiliary steam systems depending on turbine loading. The gland sealing steam pressure is normally maintained in the range of 3 to 5 psig. The turbine cylinder heating system provides sealing steam to the Low Pressure (LP) Turbine Glands when reactor power is greater than 20%. At reduced turbine loading, the turbine cylinder heating system does not supply sufficient pressure; therefore, either the main steam system or the auxiliary steam system provides sealing steam to the LP Turbine Glands via the gland steam supply regulator.
When Unit 4 was at 20% reactor power, the turbine cylinder heat system was operating on the bypass valve affecting the ability of the gland sealing steam system to compensate for changes in turbine loading.
Additionally, condenser vacuum was at 27.9 inches Hg, which is approximately one inch lower than normal due to condenser air in-leakage. Normally, condenser vacuum should have been approximately 29 inches Hg. This created a reduced margin to the condenser low vacuum trip setpoint of 24.5 inches Hg at 20% reactor power. With the Condenser Hogging Jet Air Ejector in service to compensate for the condenser air in-leakage, an additional load was placed on the steam supply system causing a longer period for the auxiliary steam supply control valve to restore pressure. When the steam supply to the gland sealing system was realigned from the main steam system to the auxiliary steam system, decreasing the supply pressure to the gland sealing steam regulating valve, air was introduced into the condenser through the turbine glands resulting in the lowering of condenser vacuum.
The decision to re-align the steam supply while the unit was on-line did not account for the Unit 4 power history and its associated effect on RCS temperature, current status of the gland sealing steam system, the turbine cylinder heat system, the main condenser air in-leakage, the impact to the auxiliary steam system from having the Main Condenser Hogging Jet Air Ejector in service, main condenser vacuum at the time, and the higher condenser low vacuum trip setpoint.
ANALYSIS OF SAFETY SIGNIFICANCE
At the time of the event, Unit 4 was at approximately 20% power level. Decreasing condenser vacuum caused a turbine trip which resulted in the automatic reactor trip. Plant response to the decrease in condenser vacuum and reactor trip was as expected. All control rods fully inserted. All systems responded as designed. The unit transitioned to Mode 3. As a result, the safety significance of the event is considered low.
CORRECTIVE ACTIONS
Corrective actions are documented in AR 1967899 and include:
- Procedures 3/4-GOP-103, Power Operation to Hot Standby, will be modified to provide specific direction that steam supply to the gland sealing steam system cannot be transferred from the main steam system to the auxiliary steam system with a unit in Mode 1 or 2.
- Training will be provided to all licensed operators to demonstrate the integrated system response aspect of risk-based decision making.
APPROVED BY OMB: NO. 3150.0104 EXPIRES: 01/31/2017 Reported lessons learned are incorporated into the licensing process and fed back to industry.
Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch(T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by intemet e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
component function identifier (if appropriate)]. Condition Report 1967899 was initiated due to this event.
FAILED COMPONENTS IDENTIFIED: None.
PREVIOUS SIMILAR EVENTS:
Turkey Point Unit 3 had a similar event, Licensee Event Report 00050250/2013-002-00, "Automatic Reactor Trip due to Low Condenser Vacuum." The cause of the Unit 3 event was not the same as the cause of the May 25, 2014, Unit 4 event; therefore, the corrective actions could not have prevented it.