05000499/LER-2005-003
| South Texas | |
| Event date: | 03-31-2004 |
|---|---|
| Report date: | 5-10-2005 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 4992005003R00 - NRC Website | |
I.�DESCRIPTION OF EVENT
A. REPORTABLE EVENT CLASSIFICATION
This event is reportable pursuant to 10CFR50.73(a)(2)(i)(B). The South Texas Project has determined that the Unit 2 Cold Overpressurization Mitigation System (COMS) was inoperable without compensatory actions required by Technical Specifications. The requirements of Technical Specification 3.4.9.3 were not met in that a Reactor Coolant System vent of at least two square inches was not provided within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> following removal of power to COMS actuation circuitry.
This event is also reportable pursuant to 10CFR50.73(a)(2)(vii). This condition made two independent trains inoperable in a single system designed to mitigate the consequences of an accident. Reportability was determined on March 11, 2005.
B. PLANT OPERATING CONDITIONS PRIOR TO EVENT
South Texas Project Unit 2 was in Mode 5.
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 additional inoperable structures, systems, or components that contributed to the event.
D. NARRATIVE SUMMARY OF THE EVENT
A modification affecting the Main Steam Isolation valves was implemented in Unit 1 during refueling outage 1RE12. Implementation of the modification required that two of the Solid State Protection System actuation trains be de-energized. On March 7, 2005, during preparation for the outage, review of the schedule led to a question about operability of the COMS during solid plant operation. The response at the time was that operability would not be affected while the unit was under solid plant conditions. However, further discussion came to the conclusion that de-energizing both of the Solid State Protection System trains would make COMS inoperable.
Documentation of the effect on COMS from loss of power to the actuation cabinets is a seldom used vendor control-wiring diagram. This had not been documented in any procedure or guideline used during the process of implementing the modification. The Unit 1 outage schedule for 1RE12 was rearranged so that COMS operability was no longer a Technical Specification issue. Consequently, there was no violation of Technical Specification requirements by Unit 1.
This led to review of the experience for its applicability to Unit 2.
A modification affecting the Feedwater Isolation Valves was implemented in Unit 2 during refueling outage 2RE10 in March 2004. While Unit 2 was in Modes 5 and 6, power was removed from the Solid State Protection System actuation trains for the modification to be performed. It was not realized that de-energizing the two SSPS actuation cabinets would make COMS inoperable. Power was removed at 1922 on March 31, 2004 and not restored until 1115 on April 2, 2004. Technical Specification 3.4.9.3 requires that either two pressurizer power operated relief valves be operable, or that the Reactor Coolant System be depressurized using a vent of at least two square inches. With both relief valves inoperable, venting through the 2.0 square inch vent is required within the subsequent 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />. Power was not restored for approximately 39 hours4.513889e-4 days <br />0.0108 hours <br />6.448413e-5 weeks <br />1.48395e-5 months <br /> 53 minutes, exceeding the time allowed by the Technical Specifications.
This was determined to be reportable on March 11, 2005.
E.� METHOD OF DISCOVERY OF EACH COMPONENT FAILURE, SYSTEM FAILURE, OR
PROCEDURAL ERROR
This condition was identified by reviewing Unit 2 records after the fact following discovery of the potential for the problem during preparations for the Unit 1 outage.
II. EVENT-DRIVEN INFORMATION
A. SAFETY SYSTEMS THAT RESPONDED
No safety systems were required to respond during this event.
B. DURATION OF SAFETY SYSTEM INOPERABILITY
Power was removed from the Solid State Protection System actuation cabinets at 1922 on March 31, 2004 and not restored until 1115 on April 2, 2004. Technical Specification 3.4.9.3 requires that either two pressurizer power-operated relief valves be operable, or that the Reactor Coolant System be depressurized using a vent of at least two square inches. With both relief valves inoperable, venting through the 2.0 square inch vent is to be accomplished within the next 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />. Power was not restored for approximately 39 hours4.513889e-4 days <br />0.0108 hours <br />6.448413e-5 weeks <br />1.48395e-5 months <br /> 53 minutes, exceeding the time allowed under the Technical Specifications.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT
This event is significant because it placed the South Texas Project in a condition outside the operating configuration allowed by Technical Specifications. Inadvertently disabling the automatic Cold Overpressurization Mitigation System function during 2RE10 represented a potential challenge to the RCS overpressure protection function. Using a conservative bounding calculation, the increase in risk imposed by the inoperable components was 1.4E 08. If this increase is conservatively assumed to lead directly to a core damage event, the calculated increase in core damage frequency was below the significance determination lower bound of 1E-06.
Defense-in-depth capability was reduced during solid plant operation with both actuation trains disabled. However, because two trains of Residual Heat Removal (RHR) were available with the suction valves open, redundant pressure control functions were available to mitigate an over-pressurized condition via the RHR pump discharge relief valves.
III. CAUSE OF THE EVENT
The root cause of this event was that the operational impact on COMS of de-energizing the 'A' and 'B' SSPS actuation cabinets for maintenance was not recognized. Detailed information regarding which equipment/components would be affected was not readily available in a usable format for review.
IV. CORRECTIVE ACTIONS
For corrective action, a load list will be developed for each of the Solid State Protection System actuation cabinets identifying the affected components and their state when the cabinet is de energized. This information will be included in the applicable operating procedure. As a compensatory action until the corrective action is completed, the system engineer will be contacted to confirm the extent of impact on plant equipment/components prior to implementation of scheduled work activities that include de-energizing SSPS equipment.
V. PREVIOUS SIMILAR EVENTS
None.
VI. ADDITIONAL INFORMATION
None.