05000454/LER-2001-001
| Event date: | |
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| Report date: | |
| 4542001001R01 - NRC Website | |
APPROVED BY OMB NO. 3150-0104 EXPIRES 06/30/2001 collection request: 50 hrs. Reported lessons learned are incorporated into the licensing process and fed back to industry. Forward comments regarding burden estimate to the information and Records Management Branch (t-6 03), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, and to the Paperwork Reduction Project (3150-0104), Office Of Management And Budget, Washington, DC 20503. If an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Byron, Unit 1 STN 05000454 (If more space is required, use additional copies of NRC Form 366A)( 17)
A. Plant Conditions Prior to Event:
Event Date/Time: May 15, 2001 / 0700 hours0.0081 days <br />0.194 hours <br />0.00116 weeks <br />2.6635e-4 months <br /> Unit 1 - Mode 1 - Power Operations, Reactor Power - 98% Unit 2 - Mode 1 - Power operations, Reactor Power - 95% Reactor Coolant System [AB] for both Units: Normal operating temperature and pressure No structures, systems or components were inoperable at the start of the event that contributed to the event.
B. Description of Event:
On May 12, 2000, for Unit 2 and May 16, 2000, for Unit 1, Byron Station implemented an ultrasonic flow measurement system (UFMS) to more accurately determine main feedwater (FW) [SJ] mass flowrate. The UFMS directly measures the time the FW travels a known distance in the pipe and then, utilizing a computer program, calculates a reference FW mass flowrate by using manually inputted parameters such as the pipe inside area and a reference FW density. This more precise measurement of FW mass flowrate is then used to calculate FW correction factors (i.e., calibration constants) which are applied to the FW mass flowrate measurements from the installed venturi flow measurement system. This corrected FW mass flowrate is then used as an input into the process computer [ID] generated reactor power calorimetric calculation.
The FW calibration constants are determined in accordance with Byron Station Engineering Procedure, BVP 800-44, "Feedwater Venturi Calibration Unit 1 and 2.
Following the UFMS FW measurement collection period, the calculated UFMS reference data is then inputted into a separate computer spreadsheet tool. This tool performs the final data reduction and determines an actual mass flowrate by correcting for actual FW conditions at the time of the measurement. An external FW density correction is part of this data reduction effort. This density correction is the ratio of the actual density of the FW during the data collection period, based on fluid temperature and pressure as recorded by plant instrumentation, and the reference density which UFMS uses in its computer program to calculate the reference mass flowrate.
(pA0 1 lers1454-2001-001-01.doc) APPROVED BY OMB NO. 3150-0104 EXPIRES 06/30/2001 collection request: 50 hrs. Reported lessons learned are incorporated into the licensing process and fed back to industry. Forward comments regarding burden estimate to the information and Records Management Branch (t-6 f33), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, and to the Paperwork Reduction Project (3150-0104), Office Of Management And Budget, Washington, DC 20503. If an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Byron, Unit 1 STN 05000454 (If more space is required, use additional copies of NRC Form 366A)(17) B. � Description of Event, cont.:
On May 10, 2000, two days prior to the start of UFMS implementation at Byron Station, the UFMS vendor implemented a change to the UFMS computer program.
Instead of manually determining the reference fluid density from the steam tables and inputting it into the UFMS, the reference FW temperature and pressure are entered as input parameters and the UFMS calculates the FW reference density.
This change also modified the UFMS output data format to include the calculated reference density. The Byron Station engineer involved in the UFMS implementation was trained on this change by the UFMS vendor.
As a result of this change, the Byron Station engineer recognized that in addition to the UFMS input changes, the computer spreadsheet tool would need to be adjusted to accept the new output of UFMS. He decided the new UFMS output data files could be made to conform to the current spreadsheet tool by simply moving data columns.
The changes were made and the UFMS implementation occurred as scheduled. The procedure yielded FW calibration constants of 1.69% for Unit 1 and 1.68% for Unit 2. BVP 800-44 was performed several times since this initial implementation in May 2000. The results of these tests were FW calibration constants ranging from 1.35% to 1.8%.
On May 10, 2001, UFMS testing was performed on Byron Station Unit 1 as part of the reactor power uprate implementation. The FW calibration constant calculated (i.e., 2.31%) was larger than expected. Due to the larger than previously calculated constant, the data was sent to corporate engineering and the vendor for review. On May 14, 2001, the vendor noticed a potential discrepancy in the external density correction and contacted the Byron Station engineer. On May 15, 2001, the Byron Station engineer determined the discrepancy with the external density correction was due to an error with the reference density used in the data reduction computer spreadsheet.
An evaluation revealed that when the UFMS changes were performed on May 10, 2000, the reference temperature and pressure inputs into the UFMS were incorrectly verified by the engineer to yield the same reference density previously entered.
Consequently, the reference density entered as a constant in the data reduction spreadsheet tool now differed from the reference density calculated by the UFMS.
This resulted in the external density correction to be in error.
(p: 0 1 leis 454-2001-001 -01.doc) APPROVED BY OMB NO. 3150-0104 EXPIRES 06/30/2001 collection request: 50 hrs. Reported lessons learned are incorporated into the licensing process and fed back to industry. Forward comments regarding burden estimate to the information and Records Management Branch (t-6 f33), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, and to the Paperwork Reduction Project (3150-0104), Office Of Management And Budget, Washington, DC 20503. If an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Byron, Unit 1 STN 05000454 (If more space is required, use additional copies of NRC Form 366A)(17) B. Description of Event, cont.:
The reference FW temperature and pressure inputs were 438.5°F and 1000 psia, respectively. This correlated to a UFMS calculated density of 52.23 lb/cu ft.
To obtain the same reference density as the data reduction computer spreadsheet (i.e., 52.3 lb/cu ft), a FW temperature of 437.5°F should have been used. This one-degree difference in FW temperature resulted in incorrect FW calibration constants and a 0.12% non-conservative calculation of reactor power.
Although Unit 1 was at 98.2% reactor power at the time of discovery, this error was present since the original Byron Station implementation of the UFMS in May 2000. Though not specifically verified, there appear to have been numerous instances when both Units operated above 99.88% reactor power for an 8-hour period. This exceeded NRC guidance for determining the operating license maximum power level and consequently is reportable to the NRC in accordance with license requirement 2.F for Unit 1 and license condition 2.G for Unit 2.
Following discovery of this error, a complete validation of the entire FW venturi calibration process was performed. This validation discovered one additional error in an input parameter used in the UFMS. An instantaneous thermal expansion coefficient should have been used instead of an average thermal expansion coefficient. This error was minor and had an insignificant effect on the FW calibration constant calculation. This error had also been in place since the UFMS implementation in May 2000.
C. Cause of Event:
The cause of the density correction and thermal expansion coefficient errors was human performance related. A formal engineering calculation process was not utilized due to a lack of clear management expectations and detailed guidance to recognize when a formal calculation is required. Had a formal calculation been performed, the appropriate approval process would have been utilized, thereby minimizing the chance of error.
(00 Ilers 454-2001-001-01.doc) APPROVED BY OMB NO. 3150-0104 EXPIRES 06/30/2001 collection request: 50 hrs. Reported lessons learned are incorporated into the licensing process and fed back to industry. Forward comments regarding burden estimate to the information and Records Management Branch (t-6 f33), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, and to the Paperwork Reduction Project (3150-0104), Office Of Management And Budget, Washington, DC 20503. If an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Byron, Unit 1 STN 05000454 (If more space is required, use additional copies of NRC Form 366A)(17)
D. Safety Analysis:
The safety analyses contained in the Update Final Safety Analysis Report for Byron Station contains an allowance of two percent rated thermal power to account for calorimetric program uncertainty. This allowance is provided to bound the periods of time when actual reactor power is greater than indicated calorimetric power. An evaluation of the calorimetric instrument uncertainty calculation determined that the error induced by the density and thermal expansion factor errors did not result in exceeding the 2% RTP allowance contained in the safety analysis. Therefore, the assumptions assumed in the safety analysis remained bounded. The overall risk significance of the 0.12% error is minimal.
E. Corrective Actions:
An immediate administrative reactor power limit was established on Unit 1 to preclude operation above 100% with the current error in the feedwater flow calibration coefficients. There was no administrative reactor power limit required for Unit 2 because, as a result of the power uprate plan, the current feedwater calibration factors were conservatively reset to 1.00. Following the validation and correction of the errors, feedwater flow calibration coefficient testing was performed on both Byron Station units and the administrative reactor power limit on Unit 1 was removed.
The corrective action to prevent recurrence is:
Byron Engineering will develop guidance, establish expectations, and perform training on the proper circumstances for a formal calculation.
Additional corrective actions to be taken include:
The Feedwater Venturi Calibration procedure will be revised to include a step to insure the spreadsheet (tool) and UFMS program are validated together.
Documentation of all critical input parameters to the UFMS will be obtained from the vendor and all critical parameters will be confirmed to be correct and in the appropriate procedure.
(p:O I lers 454-2001-001-0 I .doc) APPROVED BY OMB NO. 3150.0104 EXPIRES 06/30/2001 collection request: 50 hrs. Reported lessons learned are incorporated into the licensing process and fed back to industry. Forward comments regarding burden estimate to the information and Records Management Branch (t-6 f33), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, and to the Paperwork Reduction Project (3150-0104), Office Of Management And Budget, Washington, DC 20503. If an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Byron, Unit 1 STN 05000454 (If more space is required, use additional copies of NRC Form 366A)(17) E. Corrective Actions, cont.:
A formal calculation for each UFMS installation will be created.
Other spreadsheets or calculational tools that support Operational License applications will be identified and reviewed to insure inputs are validated to support such calculations.
The software control procedure will be revised to expand the scope of software control to include not only technical review of the software, but also to address impact review of the associated software change.
F. Previous Occurrence:
Discrepancy, dated February 27, 1998. Corrective actions in this LER would not have been expected to prevent the occurrence of this current event.
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