05000499/LER-2013-004
| South Texas Unit 2 | |
| Event date: | 12-19-2013 |
|---|---|
| Report date: | 03-20-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4992013004R01 - NRC Website | |
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I. DESCRIPTION OF EVENT
A. REPORTABLE EVENT CLASSIFICATION
This event is reportable pursuant to 10 CFR 50.73(a)(2)(iv)(A), any event or condition that resulted in the valid manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B) that was not part of a pre-planned sequence during testing or reactor operation. The listed system is the Main Steam Isolation Valves (MSIVs).
B. PLANT OPERATING CONDITIONS PRIOR TO EVENT
Unit 2 was operating in Mode 3 at 0% power.
C. STATUS OF STRUCTURES, SYSTEMS, AND COMPONENTS THAT WERE INOPERABLE AT THE
START OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT
There were no structures, systems, or components that were inoperable at the start of the event that contributed to the event.
D. NARRATIVE SUMMARY OF THE EVENT
On December 19, 2013 at approximately 2200 hours0.0255 days <br />0.611 hours <br />0.00364 weeks <br />8.371e-4 months <br />, while in Mode 3 preparing the Unit 2 secondary plant for startup, a repeat failure of the non-safety bearing oil lift piping occurred preventing placing the turbine on the turning gear. After approximately 30 minutes with no turbine rotation, efforts to place the Main Turbine back on the turning gear had failed so the associated annunciator response procedure guided the staff to secure main condenser vacuum and turbine gland seal steam to minimize turbine rotor bow.
The notes in the annunciator response procedure state that the procedure used to secure main condenser vacuum and turbine gland seal steam depends on the type of unit shutdown. Normal shutdown would require the use of normal operating procedures and the off-normal procedure for secondary plant stabilization, and its associated Conditional Information Page (CIP) for the "Condenser Unavailable" condition, would be used following a reactor trip.
An additional note in annunciator response procedure states that the main turbine should to be rotated at least 180 degrees every 15 minutes if possible. The situation was considered urgent because 30 minutes had already elapsed while attempting to restore the turning gear.
This situation did not qualify as a normal shutdown nor as a reactor trip, however all crew members agreed that the actions needed to be performed promptly and the CIP for "Condenser Unavailable" was followed. The Unit Supervisor gave the CIP procedural direction to the crew to isolate the Main Steam Lines (MSL). The Reactor Operator repeated the direction back and turned the Main Steam Isolation Actuation switch to close the MSIVs.
The Control Room staff closed all four MSIVs at once using the Main Steam Isolation (MSI) Actuation switch, rather than closing each valve individually, due to the urgency to prevent damage to the main turbine.
South Texas Unit 2 05000499 It should be noted that the off-normal procedure states "ENSURE MSIVs and MSIBs are closed", but does not specifically call for the use of the Main Steam Isolation Actuation switch.
By using the Main Steam Isolation Actuation switch instead of the individual valve switches, the main steam valves close signal was processed through the solid state protection system (SSPS), which is an Engineered Safety Feature (ESF) actuation and is reportable.
E. METHOD OF DISCOVERY
The manual actuation of the MSIVs via the SSPS was self-revealing when the action was performed in the Control Room.
II. EVENT-DRIVEN INFORMATION
A. SAFETY SYSTEMS THAT RESPONDED
The MSIVs closed in response to the manual actuation of the Main Steam Isolation circuitry. No other safety systems were affected by this event.
B. DURATION OF SAFETY SYSTEM INOPERABILITY
No safety systems were inoperable as a result of this event.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT
The event did not have an adverse effect on the health and safety of the public.
A risk assessment was performed for this event. At the time of the event, the plant was in Mode 3, preparing the Unit 2 secondary plant for startup. The plant had not been critical for almost a month, which means decay heat was low. There was no initiating event or impact to the capability of mitigating systems or other plant equipment to perform required safety functions. Therefore, there is no incremental core damage or large early release risk associated with this event.
CAUSE OF THE EVENT
The cause of the event was unspecific guidance in the off normal procedure for secondary plant stabilization.
The urgency in this event was not from a design basis accident but from a need to protect major plant equipment. The failure of the bearing oil lift piping placed the Control Room Staff in a situation that required prompt action to prevent equipment damage, and the Staff made a decision to use the MSI Actuation switch as a result of the unspecific written guidance to ensure MSIVs and Main Steam Isolation Bypass valves (MSIBs) are closed.
IV. CORRECTIVE ACTIONS
The off normal procedure for secondary plant stabilization will be revised to provide specific direction for the switches to use for closing the MSIVs.
V. PREVIOUS SIMILAR EVENTS
There have been no similar reportable events at STP within the last three years that have occurred for the same reason as this event.
VI. ADDITIONAL INFORMATION
None