05000529/LER-2008-003

LER-2008-003, Technical Specification - Limiting Condition for Operation 3.0.3 for Greater Than 1 Hour

Docket Number
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
5292008003R00 - NRC Website

All times are Mountain Standard Time (MST) and approximate unless otherwise indicated.

1. REPORTING REQUIREMENT(S):

This LER is being submitted pursuant to 10 CFR 50.73(a)(2)(i)(B) to report a condition prohibited entered Limiting Condition for Operation (LCO) 3.0.3 and remained in LCO 3.0.3 for greater than 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />.

TS LCO 3.1.5, Control Element Assembly (CEA) Alignment, (EIIS: AA), does not have a condition for multiple CEAs within a group that have less than two OPERABLE CEA position indicators. This condition requires entry into LCO 3.0.3, which requires action be initiated within 1-hour to place the unit in MODE 3 within 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br />.

2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):

Class 1E 120 VAC Instrument (EllS: EF) Power System (PN) The PN system consists of four redundant independent sub-systems each consisting of an inverter (EIIS: INVT) that converts 125 VDC (EIIS Code: EJ) supply to 120 VAC, a transfer switch (EIIS: ASU), a backup voltage regulator (regulating transformer) (EIIS: XFMR) and a 120 VAC distribution panel (EIIS: PL). In Units 2 and 3, the transfer switch will automatically transfer loads from the inverter to the voltage regulator during a faulted condition on the inverter. In Unit 1, the transfer switch is manually actuated to transfer loads. Each of the four sub-systems provides class lE 120 VAC power to one of the four channels of the reactor protection system (EIIS: JC) and engineered safety features actuation system instrumentation and controls (EIIS:

JE).

CEA Position Indication Each CEA has three separate position indications. A CEA pulse counter (EIIS: CTR) determines the CEA position by providing a 0.75 inch position change for each step change signal received from the plant computer but does not directly measure CEA position. Actual movement in CEA position is indicated on the Reed Switch Position Transmitters (RSPTs) (EIIS: ZT) which feed indication to CEA position processors. Two RSPTs provide CEA position indication by using reed switches located every 1.5 inches which are activated by a magnet assembly located at the top of the CEA extension shaft.

The 120 VAC class1E channel C instrumentation and control distribution panel 2E-PNC-D27 provides power for one of the two RSPTs (RSPT channel 2) and the pulse counter for 67 CEAs.

The remaining 22 CEAs receive power for their respective pulse counters and channel 2 RSPTs from 2E-PND-D28. RSPT1 position indications are powered from the A and B class 1E distribution panels.

3. INITIAL PLANT CONDITIONS:

On September 15, 2008, Unit 2 was in MODE 1 at 100 percent power. There were no other structures, systems, or components that were inoperable at the time of the event that contributed to the event.

4. EVENT DESCRIPTION:

On September 15, 2008, at 04:22, a transient on class1E Inverter 2E-PNC-N13 resulted in the automatic transfer of the power supply for the 120 VAC class1E channel C instrumentation and control distribution panel 2E-PNC-D27 from the inverter to the alternate power supply, Class lE 480 / 120 VAC voltage regulator 2E-PNC-V27. The unit entered LCO 3.8.7 (Inverters-Operating) Condition A for the inoperable inverter, which required restoration of the inverter to OPERABLE within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.

The subsequent inverter troubleshooting plan required distribution panel 2E-PNC-D27 to be de­ energized which requires entry into LCO 3.0.3 because de-energizing the panel results in only one of the two required CEA positions indicators for 67 of the 89 CEAs being OPERABLE. De­ energizing the panel also requires entry into LCO 3.8.9 (Distribution Systems-Operating) Condition B for the inoperable instrument bus. This LCO action required restoration of the bus to OPERABLE within two hours.

At 00:03 on September 16, 2008, APS operating staff de-energized panel 2E-PNC-D27 to perform the required troubleshooting and the unit entered LCOs 3.0.3 and 3.8.9 Condition B.

At 00:37, power was restored to panel 2E-PNC-D27 from its backup voltage regulator supply and the unit exited LCO 3.8.9 condition B. Following restoration of power, additional time was required to reset CEA pulse counter position indications and to perform surveillance testing to verify two CEA position indications per CEA were OPERABLE. After these actions were completed, the unit exited LCO 3.0.3 at 01:31 (one hour and 28 minutes after the LCO 3.0.3 entry).

At 04:22 the unit entered LCO 3.8.7 Condition B to be in Mode 3 in six hours because the 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> completion time for Condition A had elapsed.

After completion of corrective maintenance and testing, inverter 2E-PNC-N13 was declared OPERABLE at 08:16 on September 16, 2008 and the unit exited LCO 3.8.7 Conditions A and B.

During the periods while Unit 2 was in LCO 3.0.3 and 3.8.7 Condition B, preparations were initiated to shutdown, but no power reductions were made.

5. ASSESSMENT OF SAFETY CONSEQUENCES:

The event did not result in any challenges to the fission product barriers or result in the release of radioactive materials. The plant remained within safety limits throughout the event. The primary system and secondary pressure boundary limits were not approached and no violations of the specified acceptable fuel design limits (SAFDL) occurred. No Engineered Safety Feature actuations occurred and none were required. There were no adverse nuclear safety consequences 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 event did not result in a transient more severe than those analyzed in the updated Final Safety Analysis Report Chapters 6 and 15.

There were no other failures that rendered a train of a safety system inoperable. The condition would not have prevented the fulfillment of the safety function, and the condition did not result in a safety system functional failure as defined by 10 CFR 50.73(a)(2)(v).

6. CAUSE OF THE EVENT:

The root cause investigation of the automatic transfer of panel 2E-PNC-D27 to its backup voltage regulator supply is in progress. Troubleshooting identified the inverter's DC-DC converter circuit board (+)15 VDC voltage regulator (Silicon General #SG7815ACK) on the DC-DC converter circuit board (Elgar #62813540) did not meet vendor specifications.

If information is subsequently developed that would significantly affect a reader's understanding or perception of this event, a supplement to this LER will be submitted.

7. CORRECTIVE ACTIONS:

The degraded circuit board was replaced and normal power was restored to the distribution panel.

Any additional corrective actions taken as a result of this event will be implemented in accordance with the APS corrective action program.

8. PREVIOUS SIMILAR EVENTS:

Since 2005, five other failures of the class 1E 120 VAC inverters have occurred.

panel 2E-PNA-D25 automatically transferred from the 2E-PNA-N11 inverter to the voltage regulator 2E-PNA-V25. The investigation for this event is included as part of the investigation for the September 16th event reported in this LER. This event did not require entry into LCO 3.0.3 because RSPT2 and pulse counter CEA position indicators remained OPERABLE.

On July 23, 2008, a Unit 2 inverter 2E-PNB-N12 failed and power to the panel 2E-PNB-D26 automatically transferred from the inverter to the voltage regulator 2E-PNB-V26. Trouble­ shooting found a blown A2 bridge fuse, one failed silicon controlled rectifier (SCR), and three degraded SCRs. The SCRs and the fuse were replaced. The SCRs are undergoing laboratory testing to determine the cause of the failure. The respective class bus remained energized throughout the troubleshooting and repair. The corrective actions for this event would not have prevented the conditions reported in this LER.

On April 24, 2008, Unit 1 inverter 1E-PNC-N13 failed and power to the class distribution panel 1E-PNC-D27 was manually transferred from the inverter to the class voltage regulator 1E-PNC- V27. Troubleshooting found a failed (-)15 VDC voltage regulator in the DC-DC converter card.

This event was reported in LER 05000528-2008-002-00 because Unit 1 entered LCO 3.0.3 for longer than one hour on two occasions during troubleshooting and repair. Restoration and verification of CEA position indications precluded exit of the LCO within one hour.

On October 30, 2005, a Unit 1 inverter 1E-PNB-N12 failed and power to the class distribution panel 1E-PNB-D26 was manually transferred from the inverter to the class voltage regulator 1E- PNB-V26. Troubleshooting found internal short circuits inside output filter capacitors that were the result of a manufacturing process defect. This event did not result in an LCO 3.0.3 entry because only RSPT1 position indicators were affected; RSPT2 and pulse counter CEA position indicators remained OPERABLE. This event was reported in LER 05000528-2005-008-01 as a condition prohibited by Technical Specifications because the defective capacitors had been installed for six years. The corrective actions for this event would not have prevented the conditions reported in this LER for the September 16th event.

On May 5, 2005, with the reactor de-fueled, a Unit 2 inverter 2E-PNB-N12 failed and power to the class distribution panel 2E-PNB-D26 automatically transferred from the inverter to the class voltage regulator 2E-PNB-V26. Troubleshooting found the A2 bridge fuse was blown, three SCRs were degraded, and the J4 circuit board was not fully seated. The SCRs and circuit board were replaced. The apparent cause of the failure was the circuit board was not fully seated and caused an intermittent connection. This resulted in the blown fuse. CEA position indicators were not required to be OPERABLE while the reactor was de-fueled. The corrective actions for this event would not have prevented the conditions reported in this LER.