05000313/LER-2012-002
| Arkansas Nuclear One - Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3132012002R00 - NRC Website | |
A. Plant Status At the time of the event, ANO-1 was at approximately 100% power.
B. Event Description
On 2/15/2012 at 0300 (CST), Arkansas Nuclear One - Unit 1 (ANO-1) Control Room Operators discovered that the CV-2673 B Start Up Valve controller Hand/Auto (H/A) station ICC-0004 [JA][XIK], and CV-2672 B Low Load Valve controller H/A station, ICC-0006 [JA][XIK], on Control Room panel CO3 [PL] had no light indication for the "auto" or "hand" mode. The light bulbs were replaced for the auto and hand indication, but the modules still had no auto or hand mode indication.
Normal operation of the startup and low load control valves is dependent upon the plant power level. At low power operation with the loop feedwater demand less than 50 percent, feedwater flow is controlled by modulation of the startup and low load control valves with the main feedwater block valve closed and the feedwater pumps operating to maintain 70 psid across the block valves. The valve positioning is sequenced so that when the startup control valve is approximately 90 percent open, the low load control valve will begin to open. When the loop feedwater demand reaches approximately 50 percent, the main block valves open and the startup and low load control valves have no effect on feedwater flow. In the event of a main feedwater line or main steam line break, the start up valves and low load valves are designed to provide automatic steam generator isolation capability that is redundant to Main Feedwater Isolation Valve (MFIV) isolation. This feature was added by Design Change Package 91-1008, "Integrated Control System (ICS) Backup to Main Feedwater (MFW) Isolation" to address a postulated failure of MFIV CV-2630 [SJ][ISV] or CV-2680 [SJ][ISV].
The Control Room staff implemented applicable Technical Specification (TS) requirements at the time of discovery (2/15/2012 at 0300) as follows:
TS 3.7.3: Main Feedwater Isolation Valves (MFIVs), Main Feedwater Block Valves, Low Load Feedwater Control Valves and Startup Feedwater Control Valves:
- Condition C was satisfied with the inoperable Low Load Feedwater Control Valve closed.
- Condition D was satisfied with the inoperable Startup Feedwater Control Valve closed.
- Condition E (two feedwater control valves in the same flow path inoperable) requires feedwater flow to the affected steam generator be isolated within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />; otherwise, Condition F must be entered.
- Condition F requires the unit 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 4 within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.
were exited at 1323 on 02/15/2012. Plant computer points indicated that both H/A stations had previously transferred from auto to hand with no operator action at 1924 on 2/14/2012, resulting in the subject valves being inoperable for a total of 18 hours2.083333e-4 days <br />0.005 hours <br />2.97619e-5 weeks <br />6.849e-6 months <br />, a Condition Prohibited by TSs.
C. Event Cause The apparent cause was determined to be a latent design change error that resulted in the 24 vDC auxiliary relay C46 5310 [JA][RLY] being utilized in a 48 vDC service application.
Historical research revealed that 24 vDC auxiliary relay module C46 5310 and similar relay C46 533 [JA][RLY] were installed in 1990 in accordance with Design Change Package 88-1096, "OTSG Level Setpoints & ICS Known Safe States." The misapplication of the relay at this higher voltage resulted in overheating of a dropping resistor which resulted in its value reduction, and subsequently the opening of a fuse in the 24 vDC feed for ICS transfer relay module C46 5311 [JA][RLY]. This subject resistor is in series with the relay indicating lamp, which is illuminated when the relay is energized.
D. Corrective Actions
The subject resistor was eliminated from the affected circuit by removing the relay position indicating light, and the as left condition of relay modules C46 5310 and C46 533 were verified to be within allowable operating conditions. The proposed future corrective action will develop an engineering modification package and install ICS 48 vDC Auxiliary relays in C46 5310 and C46 533 locations, and is being tracked under the ANO Corrective Action Process.
E. Safety Significance Evaluation In original plant design, the MFIVs provide the automatic isolation of main feedwater to the steam generators in the event of a main steam line or main feedwater line break. The main feedwater startup valves and low load valves were subsequently modified to provide automatic steam generator isolation capability that is redundant to MFIV isolation. This feature was added by Design Change Package 91-1008, "ICS Backup to MFW Isolation" to address a postulated failure of MFIV CV-2630 or CV-2680. The resistor and relay failure described in this report impacted the ability of the startup and low load valves to provide automatic isolation redundancy of the "B" feedwater train in the event of a main steam line or MFW line break; however, the MFW safety related function was maintained with the "B" Train MFIV being fully capable of MFW automatic isolation. Additionally, Operators maintained the ability to manually close the startup and low load flow paths from the Control Room during the subject time period. Based on these facts, this event is considered to have no significant safety implications.
This event is reported as a condition prohibited by TSs pursuant to 10 CFR 50.73(a)(2)(i)(B).
G. Additional Information
No similar events have been reported in the last three years.
Energy Industry Identification System (EllS) codes and component codes are identified in the text of this report as p0q.