05000364/LER-1997-001, :on 970113,nuclear Instrumentation Sys Inaccuracies Were Below 50% Power.Caused by Personnel Error. Unit Operating Procedures Have Been Modified to Specify That Nis Must Be Adjusted
| ML20134M230 | |
| Person / Time | |
|---|---|
| Site: | Farley  |
| Issue date: | 02/12/1997 |
| From: | Hill R, Dennis Morey SOUTHERN NUCLEAR OPERATING CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| LER-97-001, LER-97-1, NUDOCS 9702200149 | |
| Download: ML20134M230 (6) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2) 10 CFR 50.73(a)(2)(iv), System Actuation |
| 3641997001R00 - NRC Website | |
text
I Da e M:r:y Sruthern Nucirr Vice President Operating Compa1y Farley Project P0. Box 1295 Birmingham. Alabama 35201 Tel 205 992.5131 SOUTHERN February 12, 1997 gggg j
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Docket No.: 50-364 10 CFR 50.73(a) l U.S. Nuclear Regulatory Commission ATTN: Document Control Desk l
Washington, DC 20555
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Joseph M. Farley Nuclear Plant-Unit 2 Licensee Event Report 97-001-00 Nuclear Instrumentation System Inaccuracies Below 50% Power j
Ladies and Gentlemen:
Attached is Licensee Event Report 97-001-00 (Unit 2). This report is being submitted in accordance with 10 CFR 50.73(a)(2)(i)(B).
Respectfully submitted, (9 V ht-tw Dave Morey MGE/ cit:NISLER01. doc Enclosure cc: Mr. L. A. Reyes, Region 11 Administrator Mr. J. I. Zimmerman, NRR Project Manager Mr. T. M. Ross, Plant Sr. Resident Inspector y
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TITLE (4)
Nuclear Instrumentation System Inaccuracies Below 50% Power l
EVENT DATE (6)
LER NUMBER [6)
REPORT DATE (7)
OTHER FAC4UTIES INVOLVED (8)
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Westinghouse Technical Bulletin ESBU-TB-92-14-R1 describes the potential decalibration effects of j
calorimetric power measurements on the power range nuclear instrumentation (NI) at low power levels. As a result of evaluating the applicability of the bulletin, on January 13,1997, with Unit 2 in mode 1 operating at 100 percent reactor power, it was determined that between January 31,1996 and February 1,1996, Unit 2 had been operated in a condition prohibited by Technical Specifications.
Specifically, Unit 2 had operated with less than the Technical Specifications minimum required power range channels (JC] operable due to channels NI-41 and NI-44 being adjusted in the nc,n-conservative direction while the unit was in a low power condition. Channels N1-41 and NI-44 were functional; 1
however, in the event of an overpower condition, their associated high flux high setpoint reactor trip signal may not have been initiated before thermal power exceeded the assumed design basis safety j
analysis limit of 118% rated thermal power (RTP) by approximately 0.5% RTP. Channels NI-42 and NI-43 remained operable and would have actuated within the safety analysis limit. This event was caused by personnel error in that the NSSS vendor failed to identify the potential magnitude of the instrument uncertainty associated with the secondary side power calorimetric performed at low RTP conditions. Unit operating procedures have been modified to specify that for sustained operation (greater than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />) below 50% RTP, or if power range NI's must be adjusted in accordance with the results of a calorimetric below 50% RTP, then the power range high flux high trip setpoint will be reduced to less than or equal to 85% power.
i NRC Form 306 (446)
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Plant and System Identification
i Westinghouse -- Pressurized Water Reactor Energy Industry Identification System codes are identified in the text as [XX].
Description of Evemi On January 18,1993, Westinghouse issued Technical Bulletin NSD-TB-92-14-R0 (Revision 0),
" Instrumentation Calibration At Reduced Power." This bulletin addressed the issue of potential nuclear instrumentation system (NIS) miscalibration due to the degree of control rod insertion and calorimetric uncertainties when less than 50% rated thermal power (RTP). This bulletin was evaluated for applicability to Farley Nuclear Plara (FNP), and it was determined that practices in effect were adequate and no further action was required.
Subsequently, as a result of Westinghouse's review of Combustion Engineering (CE) InfoBulletin 94-01, " Potential Nonconservative Treatment of Power Measurement Uncertainty," Westinghouse bulletin ESBU-TB-92-14-R1 (Revision 1), "Decalibration Effects of Calorimetric Power Measurements on the NIS High Power Reactor Trip At Power Levels Less Than 70% RTP," was issued on February 6, i
1996 to address a similar issue. Specifically, it was noted that NSD-TB-92-14-R0 was not explicit enough in the limitations on adjustment of the NIS to reflect the potential non-conservative influence of secondary side power calorimetrics performed at low power levels. A review of the recommendations of NSD-TB-92-14-R0 by Westinghouse personnel concluded that the recommendations of NSD-TB-92-14-R0 should be expanded to quantify the concern with regards to potential miscalibration of the nuclear instrumentation system power range channels at low power levels. ESBU-TB-92-14-R1 was issued to provide the expanded recommendations.
The expanded recommendations were evaluated for applicability to FNP.
As a result of the assessment, on January 13, 1997, with Unit 2 in mode 1 operating at 100 percent reactor power, it was determined that between January 31,1996 and Febmary 1,1996, Unit 2 had been operated in a condition prohibited by Technical Specifications. Specifically, Unit 2 had operated with less than the Technical Specifications minimum required NI power range channels [JC] operable due to channels NI-41 and N1-44 being adjusted in the non-conservative l
direction while the unit was in a low power condition.
Channels NI-41 and NI-44 were functional; however, in the event of an overpower condition, their associated high flux high i
setpoint reactor trip signal may not have been initiated before thermal power exceeded the l
assumed design basis safety analysis limit of i18% RTP by approximately 0.5% RTP. Channels NI-42 and NI-43 remained operable and would have actuated within the safety analysis limit.
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V6 TE,I' @f ",0llllE Joseph M. Farley Nuclear Plant - Unit 2 i
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FNP Technical Specifications require NIS calibration adjustments whenever the absolute difference between the NIS and the secondary side calorimetric is greater than 2% when above i
15% RTP. This adjustment can be non-conservative if power range channels are adjusted in a l
l downward direction during low power conditions. As required by Technical Specifications, channels NI-41 and NI-44 mre adjusted downward to match the calorimetric power. However, increased instrument uncertainties associated with secondary side power calorimetric j
measurements at low power levels resulted in a non-conservative calibration adjustment. The primary contributor to the increased instmment uncertainty for a secondary side power calorimetric measurement at low power is the determination of feedwater flow.
l In response to ESBU-TB-92-14-R1, Westinghouse was requested to determine the power level where the Technical Specifications calorimetric power measurement uncertainty of 2% RTP is j
valid specific to FNP, based on the FNP calorimetric measurement procedure and instrumentation uncertainties. It was determined that the uncertainties associated with the FNP secondary power l
calorimetric could result in errors greater than 2% for calorimetrics performed at less than 50%
l power. In addition, Westinghouse was requested to assess the impact on recent operation. As j
pan of this assessment, the FNP staff reviewed the past two years of operation of both units to identify specific instances where low power (i.e., less than 50% RTP) NI calibration adjustments based on secondary side calorimetrics were performed. FNP identified 2 occurrences on Unit I and 5 occurrences on Unit 2 where one or more power range channels were adjusted downward to match a secondary side calorimetric.
Calculations utilizing best estimate uncertainties (i.e., realistic power calorimetric uncedainty and radial power redistribution effects) along with a potential channel calibration gain error were performed. Based on these calculations, there were no instances on Unit I when a NI channel's high flux high setpoint would have exceeded 118% RTP. However, there was one occurrence identified on Unit 2 where thermal power could have reached 118.5% RTP prior to power range channels NI-41 and NI-44 providing a reactor trip signal. This condition existed following the NI calibration adjustments on January 31, 1996. Specifically, on January 30, 1996, with Unit 2 operating in mode 1 at 100 percent reactor power, the unit was ramped to 15% reactor power to perform a chemistry flush of the steam generators. On January 31, 1996, a secondary side calorimetric was performed at approximately 32% RTP, and all NIs were adjusted downward.
This adjustment resulted in the potential for the high flux high setpoint reactor trip of power range l
cham els NI-41 and NI-44 to exceed the assumed design basis safety analysis limit. Subsequent channel calibration adjustments made on February 1,1996 at approximately 51% RTP returned the affected power range channels to within design basis safety analysi; hmits. The unit was returned to 100% reactor power on February 2,1996.
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Cause of Event
l Personnel error, in that prior to issuance of Westinghouse Technical Bulletin ESBU-TB-92 R1, the NSSS vendor failed to identify the potential magnitude of the instrument uncertainty associated with the secondary side power calorimetric perfonned at low RTP conditions. A contributing cause to this event is that industry standard Technical Specifications require adjustments of nuclear instrumentation whenever the indicated power and the calorimetric power differ by greater than 2% RTP. At low power this can result in a non-conservative channel calibration.
Safety Assessment
For the one occurrence identified when two Unit 2 channels (NI-41 and NI-44) could have allowed thermal power to reach 118.5% RTP before tripping, the remaining two channels would have tripped below the assumed safety analysis limit of 118% RTP. Therefore, the 2 out of 4 reactor trip logic would have been satisfied prior to exceeding 118% RTP. In addition, the power range positive rate, the over-power-delta-temperature and the over-temperature-delta-temperature reactor trip functions remained operable during this time frame.
The power range high neutron flux high trip function is credited for primary protection in the Rod j
Ejection and Rod Control Cluster Assembly Withdrawal At Power analyses. An assessment of these analyses, assuming a safety analysis limit of 118.5% RTP, determined that applicable safety analyses acceptance criteria would not be exceeded. Therefore, in the event of a failure of either NI-42 or NI-43 and the reactor trip did not occur until 118.5% RTP, the transients for which this trip function is credited would not have been significantly impacted.
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Corrective Action
Unit operating procedures have been modified to specify that for sustained operation (greater than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />) below 50% RTP, or if power range NIs must be adjusted in accordance with the results of a calorimetric below 50% RTP, then the power range high flux high trip setpoint will be i
reduced to less than or equal to 85% power.
FNP will evaluate the need for a Technical Specifications change based on the results of the Westinghouse Owners Group program MUHP-3034, " Redefinition of Power Level for NIS High Trip Setpoint Adjustment."
Lessons learned from this event have been discussed with appropriate NSSS vendor personnel.
Additional Information
The following LER was previously submitted on the subject ofinaccuracies potentially preventing proper operation of nuclear instrumentation:
LER 364/89-009 Inaccurate Feedwater Flow Indication Could Have Prevented Proper Operation of the Power Range Nuclear Instrumenttien and j
the Over-Temperature-Delta-Temperature Loops.
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