05000528/LER-2005-008
| Palo Verde Nuclear Generating Station Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 5282005008R01 - NRC Website | |
1. REPORTING REQUIREMENT(S):
Arizona Public Service (APS) is reporting this condition pursuant to 10 CFR 50.73(a)(2)(i)(B) for failing to meet a Technical Specification (TS) requirement.
TS LCO 3.8.7 requires both A Train and B Train inverters to be OPERABLE in Modes 1, 2, 3 and 4. With one Train INOPERABLE, the effected inverter must be restored within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
If the inverter is not restored to OPERABLE within the allowed time, then actions are required to be in Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and Mode 5 within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. Unit 1 was de-fueled when a defective capacitor failed in the inverter, therefore; it was assumed TS 3.8.7 was not applicable. However, the capacitor (EIIS Code: CAP) was installed approximately 6 years earlier and an engineering evaluation could not support operability of the inverter (EIIS Code:
INVT), therefore, it is considered to have been INOPERABLE, and reportable as a TS violation.
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
Class 1E 120VAC Instrument (EIIS Code: EF) Power System The system consists of four redundant independent sub-systems each consisting of an inverter that converts 125 V DC (EIIS Code: EJ) supply to 120 VAC, automatic and/or manual transfer switch, (manual transfer switches for unit #1; static transfer switch and manual bypass disconnect switches for units 2 and 3) a backup voltage regulator (regulating transformer) and a 120 V AC distribution panel. Each of the four sub-systems provides Class IE 120 V AC power supply to one of the four redundant channels of the reactor protection system, and engineered safety features actuation system instrumentation and controls which are electrically and physically isolated from each other.
3. INITIAL PLANT CONDITIONS:
On October 30, 2005, at approximately 12:26 Mountain Standard Time (MST), Unit 1 was de fueled for a Steam Generator Replacement and Refueling outage.
There were no other major structures, systems, or components that were inoperable at the start of the event that contributed to the event.
4. EVENT DESCRIPTION:
On 10/30/2005, at approximately 12:26 MST, Unit 1 was de-fueled for an outage when 1EPNBD26 malfunctioned causing the following actuations in the Control Room: Train B CPIAS, FBEVAS and CREFAS. The initial indication was an undervoltage on inverter 1EPNBN12. The inverter output voltage was noted to be oscillating from OVAC to approximately 50VAC. The nominal inverter output voltage is 120VAC. During preparations to down power the inverter a blown fuse alarm indication came in. During troubleshooting activities, eight of the fifteen AC output filter capacitors in the C3/C4 capacitor banks were identified as having their oil fill/pressure relief plugs missing.
5. ASSESSMENT OF SAFETY CONSEQUENCES:
Unit 1 was de-fueled at the time of the inverter failure. The inverter failure resulted in a loss of distribution panel 1EPNBD26 which resulted in actuations of Train "B" CPIAS, FBEVAS, and CREFAS. No TS LCO was entered as a result of the inverter failure.
The inverter normal and post accident loads are essentially the same. The subject inverter did perform its function up until the time it failed.
All equipment and systems assumed in UFSAR, Chapter 15 were functional and performed as required.
The event did not result in any challenges to the fission product barriers or result in the release of radioactive materials. Therefore, there were no adverse safety consequences or implications as a result of this event and the event did not adversely affect the safe operation of the plant or health and safety of the public.
The condition did not prevent the fulfillment of any safety function and did not result in a safety system functional failure as defined by 10 CFR 50.73(a)(2)(v) because it did perform its function up until the time of failure.
6. CAUSE OF THE EVENT:
The direct cause of the inverter failure was internal shorts on the output filter capacitors.
The root cause for the capacitor failure was the lack of "Dielektrol-VI Fluid" (oil) which was caused by a manufacturing process error. The affected capacitors were procured commercial grade and dedicated by Palo Verde for safety related use. Therefore, the root cause of the capacitor failure is attributed to weakness in the procurement control process at Palo Verde. Specifically, the procurement process and procedures allowed material classification upgrades without first assigning a new part number. Additionally, the impact review did not consider capacitors that were issued to the field, but not installed in the plant. As a result of the classification change, eight capacitors that were previously classified as "safety related low significant" and issued to the field, were later returned to the warehouse and accepted as "safety related commercial grade" without receiving the appropriate re-qualification dedication. These eight capacitors were eventually reissued to the field and installed in the failed inverter.
7. CORRECTIVE ACTIONS:
An equipment root cause of failure was preformed on the failed inverter and inspections to identify the extent of the condition were completed in all three Units. No other inverters were found with a similar condition as the failed inverter. Additionally, the warehouse stock of similar capacitors were inspected and verified not to be affected.
Since this issue identified a problem with the commercial grade dedication program, the procedure which provides the direction for procurement specification requirements was revised. Additionally, a corrective action item was generated to conduct a briefing for warehouse personnel explaining how changes to the quality classification of a component may set up a potential for errors to be made when material is later returned to the warehouse for restock while using an incorrect inspection plan.
8. PREVIOUS SIMILAR EVENTS:
No similar condition has been reported in the past three years.
9. ADDITIONAL INFORMATION:
None.