05000499/LER-2006-001
| Docket Numbersequentialmonth Day Year Year Rev Mo Day Yearnumber No N/A | |
| Event date: | 12-04-2006 |
|---|---|
| Report date: | 01-31-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4992006001R00 - NRC Website | |
I. DESCRIPTION OF EVENT
A. REPORTABLE EVENT CLASSIFICATION
This event is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B). Technical Specifications 3.5.2 and 3.6.2.1 require that three trains of ECCS and containment spray be operable. However, an ECCS train and a containment spray train were inoperable longer than the allowed outage time and plant shutdown was not accomplished within the required time. Consequently, STP Unit 2 was in a condition prohibited by Technical Specifications.
B. PLANT OPERATING CONDITIONS PRIOR TO EVENT
STP Unit 2 was in Mode 1 at 100% power.
C. STATUS OF STRUCTURES, SYSTEMS, AND COMPONENTS THAT WERE INOPERABLE
AT THE START OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT
No other inoperable structures, systems, or components contributed to the event.
D. NARRATIVE SUMMARY OF THE EVENT
At 0100 on November 27, 2006, Unit 2 High Head Safety Injection (HHSI) pump 2A of the Emergency Core Cooling System (ECCS) was declared inoperable to perform planned maintenance activities to replace the mechanical seal. This also made the associated Low Head Safety Injection (LHSI) and Containment Spray (CS) systems inoperable.
Access to the mechanical seal required removal of a pump half-coupling. However, the half coupling was found to be stuck. To free the stuck half-coupling, use of hydraulic force was determined to be necessary. The force exerted in freeing the half-coupling apparently damaged the pump internals. The mechanical seal package was replaced and, during reassembly, a spacer connecting the half-coupling could not be reinstalled due to a lack of clearance. The pump casing was filled with water to check the pump shaft for freedom of rotation. However, the pump shaft would not rotate, with the following as probable causes:
- Internal obstruction resulting from application of excessive force to remove the half coupling.
- Internal obstructions resulting from repeated vertical movement of the pump shaft without lubrication to the upper impeller bearings during reassembly, causing galling of the stainless steel bearings.
Consequently, the decision was made to overhaul the pump, including replacement of the rotating element. Actions to restore the pump to operability were expected to require an additional six days beyond the seven-day allowed outage time that expired at 0100 on December 4, 2006. Because this was a first-time evolution for the South Texas Project, Enforcement Discretion was requested from the NRC and was verbally approved by the NRC on December 3, 2006, allowing an additional eight days of operation to complete restoration of the pump.
Operability of the other two Unit 2 HHSI pumps, Train B and Train C, was confirmed by satisfactory test results on December 2, 2006.
The rotating element was shipped to the vendor for disassembly, failure analysis, and refurbishment. Using a replacement rotating element, there continued to be insufficient clearance available to reinstall the spacer. The dimensions of the motor stand, pump shaft, and motor shaft were measured; all dimensions were found to be within the allowed tolerances. However, the combination of multiple deviations from nominal dimensions was sufficient to restrict spacer installation. Machining of selected components enabled the spacer to be installed.
After reassembly, post-maintenance testing was completed satisfactorily, and the HHSI pump and associated components were declared operable at 1119 on December 7, 2006.
E. METHOD OF DISCOVERY OF EACH . COMPONENT FAILURE, SYSTEM FAILURE, OR
PROCEDURAL ERROR
This condition was identified during planned maintenance on the HHSI pump of Unit 2 Train A.
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
The Train A HHSI, LHSI, and CS pumps were declared inoperable at 0100 on November 27, 2006. The allowed outage time of seven days permitted the affected train to be inoperable until 0100 on December 4, 2006. Repairs to the HHSI pump were completed and Train A was declared operable at 1119 on December 7, 2006. The train was inoperable for ten days, ten hours.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT
Technical Specification Requirements:
Technical Specification 3.5.2 requires that three trains of the Emergency Core Cooling System (ECCS) be operable in Modes 1 through 3. With less than this operable, but with at least two HHSI pumps operable, two LHSI pumps and associated Residual Heat Removal (RHR) heat exchangers operable, and sufficient flow paths to accommodate them, the inoperable train is to be restored to operable status within seven days. If not accomplished, the unit is to be in at least hot standby within the next six hours and in hot shutdown within the following six hours.
Technical Specification 3.6.2.1 requires that three independent trains Containment Spray (CS) be operable in Modes 1 through 4. With one CS train inoperable, the inoperable train is to be restored to operable status within seven days or be in at least hot standby within the next six hours. If not restored to operability within the next 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />, the unit is to be in cold shutdown within the following 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />.
Because these components were inoperable longer than allowed under the Technical Specifications without entering the appropriate action statements, this event is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B).
Design Description:
The Train A LHSI pump and the Train A CS pump share a common suction header with the Train A HHSI pump. With this line out of service to support HHSI maintenance, the LHSI and CS pumps are also inoperable. Only Train A of the Unit 2 ECCS and CS systems was affected.
Equipment History:
STPNOC reviewed data covering the preceding three years of inservice test performance, vibration monitoring, motor and pump predictive maintenance, and motor current trends, as well as open work requests associated with the affected pumps. No adverse trends were identified that would have indicated the found condition of the Train A HHSI pump.
Risk Assessment:
The pump was non-functional until December 7, 2006, at 0400, and declared operable on December 7, 2006, at 1119. STPNOC assessed the Unit 2 incremental core damage and large early release probabilities associated with the actual time as a result of continuing to operate while the Train A HHSI was non-functional beyond the allowed outage time. The resulting risk values are as follows:
ICCDP* ICLERP** ' Projected Change for Additional 8 Days 4.44E-07 7.57E-11 Actual Change for Additional 75 Hours 1.7E-07 3.0E-11 Allowable Change (Part 9900) * Incremental Conditional Core Damage Probability ** Incremental Conditional Large Early Release Probability The calculated changes in probabilities demonstrate that the extended period of inoperability had a very small impact on plant risk.
This event resulted in no personnel injuries, no offsite radiological releases, and no damage to other safety-related equipment. The extension was not risk significant and did not result in a net increase in the radiological risk to the public.
III. CAUSE OF THE EVENT
The unexpected condition of the stuck half-coupling was not sufficiently assessed and the potential hazards to the affected components were not identified prior to proceeding to use force in removing the pump half-coupling.
Review by the vendor of the disassembled rotating element found evidence of a pre-existing material condition that may be contributing factor to the pump's adverse condition. This aspect of the event is under review and will be addressed in a supplement to this report.
IV. CORRECTIVE ACTIONS
Affected personnel will be retrained on the Conduct of Maintenance and expectations on tool use.
The Rotating Equipment Certifications of the craftsmen involved have been deactivated, pending remedial training and recertification.
A supplemental report will be submitted to the NRC providing the findings from review of the as found condition of the rotating element.
V. PREVIOUS SIMILAR EVENTS
STPNOC reviewed the work history for mechanical seal replacement on Unit 1 and Unit 2 HHSI pumps and identified no previous adverse experiences. The work history for mechanical seal replacement in LHSI and CS pumps was also reviewed. There had been only one previous instance of a pump half-coupling stuck in an LHSI pump. This was resolved by removing the motor and removal of the half-coupling using a puller.
VI. ADDITIONAL INFORMATION
None.
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