05000461/LER-2006-002

LER-2006-002, Turbine Bypass Function Lost Due to Circuit Card Maintenance Frequency

Docket Number
Event date:	08-17-2006
Report date:	11-10-2006
4612006002R01 - NRC Website

PLANT OPERATING CONDITIONS PRIOR TO THE EVENT

Unit: 1� Event Date: 8/17/06 Event Time: 10:44 AM Central Daylight Time Mode: 1 (Power Operation)� Reactor Power: 96 percent

DESCRIPTION OF EVENT

On August 17, 2006, Clinton Power Station was operating in Mode 1 at about 96 percent power.

At 1044 hours0.0121 days <br />0.29 hours <br />0.00173 weeks <br />3.97242e-4 months <br />, operators in the Main Control Room received a Main Condenser [SG] low vacuum alarm [ALM]. Operators responded by verifying condenser vacuum was normal. Initial troubleshooting in response to the alarm identified that a false low condenser vacuum signal was present, inhibiting the Main Turbine bypass [JI] [SB] valves [V] from being able to open upon receipt of a demand.

At 1110 hours0.0128 days <br />0.308 hours <br />0.00184 weeks <br />4.22355e-4 months <br />, operators entered the actions of Technical Specifications (TS) 3.7.6, Main Turbine Bypass System, Condition 'A'. TS 3.7.6 requires the Main Turbine bypass valves to be operable when Reactor Thermal Power is greater than or equal to 21.6 percent. When the Main Turbine Bypass System is not operable per Condition 'A', Required Action A.1 is to restore the system to operable status within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />.

A troubleshooting team investigating the condition determined that a lead could be lifted to remove the low vacuum inhibit logic input and restore the Main Turbine Bypass valve function. At 1212 hours0.014 days <br />0.337 hours <br />0.002 weeks <br />4.61166e-4 months <br />, the lead was lifted and the low vacuum light [IL] went out as expected. At 1215 hours0.0141 days <br />0.338 hours <br />0.00201 weeks <br />4.623075e-4 months <br />, the low vacuum inhibit was reset, restoring the bypass valves to operable status before the TS 2-hour action completion time expired.

The Main Turbine Bypass System is designed to control steam pressure when reactor steam generation exceeds turbine requirements during unit startup, sudden load reduction, and cooldown, and allows excess steam flow from the reactor to go to the condenser without going through the turbine.

Two alarm/trip units (electronic circuit cards) [RLY] are arranged in a 1-out-of-2 logic to inhibit the Main Turbine bypass valve opening function when Main Condenser vacuum is too low for steam to be admitted. The alarm/trip units are part of two independent instrument loops beginning with pressure sensing transmitters [PT] and ending in computer points, and alarm/trip units.

Troubleshooting isolated the cause of this event to one circuit card failure, the Main Condenser Pressure Trip Unit (electronic circuit card).

The circuit card that failed was remcved from the system and was sent to the card manufacturer for failure analysis to identify the cause for the circuit card failure. The remaining redundant circuit card is providing the necessary function to inhibit the steam bypass function upon a loss of condenser vacuum.

Issue Report 520922 was initiated to perform a cause evaluation of this event and to identify corrective action.

No automatic or manually initiated safety system responses were necessary to place the plant in a safe and stable condition. No other inoperable equipment or components directly affected this event.

CAUSE OF EVENT

The circuit card that failed was returned to the manufacturer for failure analysis. The manufacturer determined the most likely cause for the failure was a leaky electrolytic capacitor.

At the time of the event, the card power supply apparently shutdown or stalled, reducing voltage until the output relay de-energized. When the relay de-energized the relay contacts closed and the Main Turbine Bypass System was inhibited.

The cause of this event is the failure the electrolytic capacitor on the Main Condenser Pressure Trip Unit (electronic circuit card) due to the Performance Centered Maintenance (PCM) template not specifying the appropriate replacement interval for Moore Industries model Direct Current Alarm (DCA) cards. The circuit card replacement frequency in the PCM template for Moore model DCA cards is 30 years, and is in direct contrast to a PCM template created for power supplies, which addresses electrolytic capacitor aging and has a replacement frequency of 7.5 years.

SAFETY ANALYSIS

This event is reportable under the provisions of 10 CFR 50.73 (a)(2)(v)(D) as an event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident. Events that are reportable under 10 CFR 50.73 (a)(2)(v)(D) are considered to be safety system functional failures.

A failure of the turbine bypass system function is within the design basis of the plant as analyzed in Chapter 15 of the Clinton Updated Safety Analysis Report.

CORRECTIVE ACTION

Immediate corrective action for this event was lifting a lead to restore the Main Turbine Bypass System function to an operable condition. The circuit card that failed will be replaced in the future at a time when plant conditions support the replacement.

The PCM Template for Moore Industries Cards Model DCA will be revised to an appropriate replacement frequency and preventive maintenance tasks will be created for affected critical/vital Moore DCA cards.

PREVIOUS OCCURRENCES

None

COMPONENT FAILURE DATA

Manufacturer: Moore Industries Nomenclature: Main Condenser Pressure Trip Unit (electronic circuit card) Manufacturer Model Number: DCA/4-20MA/DX1X4/45VDC/-AD-DPST The following table identifies commitments made in this document. (Any other actions discussed in the submittal represent intended or planned actions. They are described to the NRC for the NRC's information and are not regulatory commitments.)

COMMITMENT TYPE

COMMITMENT ONE-TIME ACTION Programmatic (Yes/No) (Yes/No) This document has no regulatory commitments