05000498/LER-2006-006
| South Texas | |
| Event date: | 12-17-2006 |
|---|---|
| Report date: | 11-08-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4982006006R01 - NRC Website | |
I.�DESCRIPTION OF REPORTABLE EVENT
A. REPORTABLE EVENT CLASSIFICATION
This event is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by Technical Specifications (TS). The TS 3.3.3.6 Table 3.3-10 Action 35 allowed outage time for AFW Flow Indication was exceeded.
B. PLANT OPERATING CONDITIONS PRIOR TO THE EVENT
At the time of discovery, Unit 1 was operating at 100%.
C. STATUS OF STRUCTURES, SYSTEMS, OR COMPONENTS THAT WERE
INOPERABLE AT THE START OF THE EVENT AND THAT CONTRIBUTED TO THE
EVENT
No equipment that was inoperable at the initiation of the event contributed to the event.
D. NARRATIVE SUMMARY OF THE EVENT, INCLUDING DATES AND APPROXIMATE
TIMES
At 0945 on December 15, 2006 with Unit 1 at 100% power, D train distribution panel 1202 (DP1202) generated a voltage transient that resulted in the loss of multiple loads due to fuses clearing and damage to the connected loads. The transient was indicated by ground fault and inverter trouble alarms in the control room. However, following the transient, inverter 1202 was found to be operating normally. An Engineering evaluation determined the inverter was degraded but operable. The loads listed in Section II.0 below were lost from the inverter.
The Operators responded appropriately to the electrical transient and its effects. The Operators were temporarily required to take manual control of pressurizer level and Steam Generator (SG) 1D and SG 1C level.
The most limiting TS actions were due to the inoperable AFW Flow indication with a 72-hour shutdown action required by TS 3.7.1.2 ACTION b and a 48-hour shutdown action required by TS 3.3.3.6 ACTION 35. STPNOC determined that restoration of the QDPS Auxiliary Processing Cabinet (APC) D2 and the AFW flow instrumentation channel would likely require replacement of EEPROM chips that were not available on site. Consequently, STPNOC determined that the function could not be restored prior to the expiration of the allowed outage time. In a teleconference on December 17, 2006, STPNOC requested enforcement discretion to extend the allowed outage time to 2145 on December 18, 2006. NRC granted the enforcement discretion at 0839 on December 17, 2006.
STPNOC found that two of three EEPROM chips located on a QDPS CPU circuit board had been damaged due to the electrical transient. STP replaced the damaged EEPROMs and affected downstream circuit boards. QDPS Cabinet D2 and the functions above were declared operable at 2155 hours0.0249 days <br />0.599 hours <br />0.00356 weeks <br />8.199775e-4 months <br /> on December 17, 2006.
The electrical transient was caused by failure of a capacitor in the C805 capacitor bank in inverter 1202. The capacitor failure was due to a manufacturing defect in the internal capacitor lead. The affected capacitor bank was replaced on December 20, 2006.
E. THE METHOD OF DISCOVERY OF EACH COMPONENT OR SYSTEM FAILURE,
OR PROCEDURAL OR PERSONNEL ERROR
Control Room Operators noted an electrical transient on a Distribution, Panel (DP1202) for the 120VAC distribution system. Visual inspection revealed a failed capacitor in the C805 capacitor bank in inverter 1202 that supplies power to DP1202. Following the electrical transient, the Control Room Operators noted that several electrical loads supplied by DP1202 were no longer in operation. The affected loads included a QDPS cabinet that provides AFW flow indication and control functions for the Train D turbine driven AFW train.
II.� COMPONENT OR SYSTEM FAILURES
A. FAILURE MODE, MECHANISM, AND EFFECTS OF EACH FAILED COMPONENT
The failed C805 capacitor was inspected after replacement of the capacitor bank. The inspection revealed that the failure mode was a short circuit to ground caused by the internal capacitor lead to the capacitor casing. The effect was an over-voltage electrical transient caused the loads identified below to be lost from the inverter. The failure was a Maintenance Rule Functional Failure.
B. CAUSE OF EACH COMPONENT OR SYSTEM FAILURE
The failure of the C805 capacitor was due to a manufacturing defect. Off site failure analysis'of the failed capacitor found that outside internal lead had either been folded incorrectly or the insulation had a pre-existing flaw. In either case the lead was either in touch with the side of the capacitor housing (can) or within close enough proximity to the can that when the insulation broke down it resulted in a short circuit to the can causing the capacitor to fail which resulted in a short circuit to ground.
The cause of the fuse and load failures is a design incompatibility between the 120 volt AC system and the connected loads. The ungrounded system design allows a high voltage bus to ground transient to be created under the conditions that existed when C805 faulted. The ground capacitors and varistors in the loads then coupled the transient to the loads. This resulted in load fuses clearing, varistors failing and power supply damage/failure.
Surge protection for the EEPROMs is provided at the AC input to the QDPS cabinet.
Testing of the surge suppressors found they were undamaged. The most probable cause of the QDPS EEPROM damage was the same design incompatibility that caused the other loads to fail.
C. SYSTEMS OR SECONDARY FUNCTIONS THAT WERE AFFECTED BY FAILURE
OF COMPONENTS WITH MULTIPLE FUNCTIONS
The loads that were lost from the inverter and their associated TS included the following:
a) Isolation Relay Cabinet b) Steam Generator 1D Power Operated Relief Valve (PORV) servo amplifier (TS 3.3.5.1, TS 3.7.1.6 for inoperable PORV) c) QDPS APC D1
- Inoperable Channel 2 for Steam Generator Narrow Range Level (TS 3.3.1, Table 3.3-1 Item 14 — ACTION 6, TS 3.3.2, Table 3.3-3 Item 5.b — ACTION 20).
- Inoperable Channel 2 input to automatic Steam Generator (SG) Water Level Control for SG 1D and SG 1C — required manual control of SG level (no TS)
- Inoperable RCS T-hot for loop 2 (TS 3.3.1, Table 3.3-1 Item 8 & 9 — ACTION 6) d) QDPS APC D2
- Inoperable AFW flow indication (TS 3.3.3.6)
- Inoperable automatic AFW flow control for Train D AFW (TD AFW, TS 3.7.1.2) e) Nuclear Instrument System (NIS) control and instrument power to panel 5Z111ZCP011
D. FAILED COMPONENT INFORMATION
The C805 capacitor and the EEPROMs are the primary component failures. The information on the components is coded in Block 13 above.
III.�ANALYSIS OF THE EVENT
A. SAFETY SYSTEM RESPONSES THAT OCCURRED
No safety system responses were required or occurred.
B. DURATION OF SAFETY SYSTEM TRAIN INOPERABILITY
The AFW flow instrumentation was inoperable from 0945 on December 15, 2006 until 2155 December 17, 2006. STP Unit 1 was in TS 3.3.3.6 Action 35 and TS 3.7.1.2 Action b for the same duration.
C. SAFETY CONSEQUENCES AND IMPLICATIONS
The event did not have an adverse affect on the health and safety of the public. The incremental conditional core damage probability and the incremental conditional large early release probability for the 36 hour4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> extension to the allowed outage time requested in the enforcement discretion were 8.0E-09 and 1.0E-12, respectively.
A detailed assessment is provided in STPNOC's documentation of the enforcement discretion request dated December 18, 2006 (NOC-AE-06002095, ML063620440).
The NRC approved the request as described in Notice of Enforcement Discretion for South Texas Project Nuclear Operating Company Regarding South Texas Project, Unit 1 [ ML063540518, TAC NO. MD3829, NOED NO. 06-4-002].
IV. CAUSE OF THE EVENT
The electrical transient that initiated the event was caused by the failure of the C805 capacitor.
V. CORRECTIVE ACTIONS
- STPNOC replaced the damaged EEPROMs and affected downstream circuit boards.
- The affected capacitor bank was replaced on December 20, 2006 with a bank that was x-rayed prior to installation to confirm acceptability.
- The corresponding capacitor banks for the inverters on the other channels will be replaced as needed. Capacitor banks to be used for replacement will be x-rayed prior to installation to confirm manufacturing flaws like the one that caused the subject capacitor failure are not present.
- STPNOC is evaluating design modifications to the inverter that would better protect downstream loads.
STPNOC submitted a TS change application prior to the event that proposed to change the allowed outage time for the AFW Flow indication channel from 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> to 30 days (ML062830032). The application was approved on June 13, 2007 in Amendments 177 & 164. The new allowed outage time will provide adequate time to restore the affected AFW Flow channel to operable status in order to preclude a similar TS violation and associated reportable condition in the future.
VI.� PREVIOUS SIMILAR EVENTS A similar event occurred at STP in December 2005. The initiating cause of the event was a failed capacitor in the same inverter. STPNOC performed a failure analysis of that capacitor and determiried that an internal lead in the capacitor had shorted to ground. STPNOC notified the inverter vendor (Ametek) and the capacitor vendor (Aerovox) of our findings.
Aerovox subsequently notified STPNOC that changes had recently been made in their manufacturing process to reduce the potential for internal faults. STPNOC replaced all capacitors in all Unit 1 Ametek inverters with newly designed capacitors in the October 2006 refueling outage. The December 2005 event did not result in damage to the EEPROMs because the input line surge suppressor protected the EEPROMS.