05000341/LER-1997-001, :on 970206,error in Mass Flow Conversion Algorithm in Heat Methodology for Calculating Core Thermal Power Discovered.Caused by Deficiency in Calculation Software.Calculation Will Be Revised
| ML20136H096 | |
| Person / Time | |
|---|---|
| Site: | Fermi  |
| Issue date: | 03/10/1997 |
| From: | Jaworsky M DETROIT EDISON CO. |
| To: | |
| Shared Package | |
| ML20136H046 | List: |
| References | |
| LER-97-001, LER-97-1, NUDOCS 9703190049 | |
| Download: ML20136H096 (5) | |
| Event date: | |
|---|---|
| Report date: | |
| 3411997001R00 - NRC Website | |
text
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d LICENSEE EVENT REPORT (LER)
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THS REPORT IS AUBMrTTLD PUkbumHT TO THE REQUIREMEM5 OF 10 CFR 01; 4
PUWEH LhEL 10 CFR X
OTHER-ViolattOn of Ucense Condition 2.C 0l0l0 (Specify in Abstract below and in text, NRC Form 366A)
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AREA Coot Mari Jaworsky - Compliance Engineer 313 586-1427 COMPLETE ONE LINE FOR EACH COMPONENT F AILURE DESCRIBED IN THIS REPORT (13)
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On Febmary 6,1997 during a review of the Reactor Water Cleanup (RWCU) system inputs to the reactor heat balance calculation, an error was discovered in the mass flow conversion algorithm used by the Process Computer for RWCU flow. It was determined that this resulted in a nonconservative error in the heat balance calculation. Due to this error, it is possible that Fermi 2 exceeded its licensed power limit of 3292 megawatts thermal (MWt) on one or more occasions during Cycle 1 and 3293 MWt on one or more occasions during Cycles 2 and 3 by approximately 0.6 MWt. The current licensed power limit of 3430 MWt has not been exceeded as a result of this error because of administrative limits already in place.
Based on the low order of magnitude of the error and conservatism inherent in power levels used for safety analyses, this condition did not result in any adverse impact on the health and safety of the public.
The mass flow conversion algorithm will be corrected and the procedure for the manual heat balance calculation and the software specification will be revised prior to startup from the present outage to include the appropriate algorithm. The software and instrumentation interface in the Process Computer will be evaluated for similar problems.
9703190049 970310 PDR ADOCK 05000341 S
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128 degrees Fahrenheit Description of the Event A.
Background
Section 2.C.(1) of the Fermi 2 Operating License states: " DECO is authorized to operate the facility at reactor core power levels not in excess of 3430 megawatts thermal (100 percent power) in accordance with the conditions specified herein and in Attachment 1 [Preoperational Test, Startup Tests and Other Items] to this license..." Prior to the third refueling outage, Fermi 2 was authorized to operate at 3292 megawatts thermal under the original operating license, and at 3293 megawatts thermal for the second and third reactor core cycles. The original 3292 megawatt rating was a typographical error in the operating license.
Section 2.F. of the Fermi 2 Operating License states: "Except as otherwise provided in the Technical Specifications or Environmental Protection Plan, DECO shall report any violations of the requirements contained in Section 2.C. of this license in the following manner: initial notification shall be made within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> to the NRC Operations Center via the Emergency Notification System with written follow-up within thirty days in accordance with the procedures described in 10 CFR 50.73(b), (c) and (e)."
B.
Event Description
On February 6,1997 during a review of the Reactor Water Cleanup (RWCU) [CE] system j
inputs to the reactor heat balance calculation, an error was determined to exist in the calculation of RWCU flow. This error resulted in a nonconservative error in the heat balance calculation. This flow input to the Process Computer [ID] is one of the computer points that are density compensated to provide a mass flow rate within a balance of plant (BOP) computer code that runs once per minute. When this code was examined, it was determined that the density compensation appeared to be applied incorrectly since it uses a correction proportional to density rather than the square root of density. When this formula i:. used, an error is introduced in the computed mass flow rate. In most cases, the reference temperature is near the operating temperature, and therefore, the error is negligible.
However, in the case of the RWCU flow, a reference temperature of 126 degrees Fahrenheit is used where the operating process temperature is approximately 535 degrees Fahrenheit.
i As a result of spurious actuations of the RWCU differential flow isolation circuitry early in plant operation, the RWCU inlet and outlet flows were calibrated to a reference temperature
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l*l5 0 0 1 nun of 126 degrees Fahrenheit instead of the RWCU inlet process temperature of about 535 degrees Fahrenheit. Thus, using the incorrect application of the density compensation combined with the difference between the reference and process temperature results in a nonconservative error in the Reactor Heat Balance. Further investigation revealed that the Control Rod Drive [AA] tlow input to the Reactor Heat Balance was also in error by a minimal amount. Therefore, the total non-conservative error in the Reactor Heat Balance is approximately 0.6 MWt.
Due to these errors, it is possible that Fermi 2 exceeded its licensed power limit of 3292 MWt on one or more occasions during Cycle I and 3293 MWt on one or more occasions during Cycles 2 and 3 by approximately 0.6 MWt. The current licensed power limit of 3430 MWt has not been exceeded as a result of this error because of administrative limits already in place. An ENS notification of this event was made on February 6,1997.
Cause of the Event
The most probable cause of this event was that the section of the vendor BOP equipment performance calculations software specification did not properly account for density compensation of the volumetric flow. Neglecting this volume flow density correction is not a concern provided the reference temperature and pressure are close to the process temperature and pressure.
i As a result of spurious actuations of the RWCU differential flow isolation circuitry early in plant operation, the RWCU inlet and outlet flows were calibrated to a reference temperature of 126 degrees Fahrenheit instead of the RWCU inlet process temperature of about 535 degrees
' Fahrenheit. This was contrary to an apparent assumption of the specification that the reference temperature would be close to the operating temperature of the loop. If the reference temperature j
had not been changed from 535 degrees Fahrenheit to 126 degrees Fahrenheit to account for RWCU leakages, the error would have been insignificant. The approximate 0.6 MWt error is, therefore, primarily based on this temperature difference.
- - A contributing factor to this event was incomplete corrective action taken in 1983. During Preoperational testing it was identified that this change to the algorithm was needed. Although a correction was incorporated into the software at the time, the site BOP equipment performance calculations software specification was not corrected to reflect this change in the code.
Subsequently when a new version of the system software was installed in 1985, the code was restored to match the site specification and the same error was re-introduced.
Analysis of the Event
The design basis Loss of Coolant Accident (LOCA), design basis Containment, and Transient Analyses incorporate a two percent power level measurement uncertainty which equates to 66 j
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MWt. Historically, the maximum heat balance calculation uncertainty due to instrument inaccuracies when using the Process Computer was 1.85 percent or approximately 61 MWt. This results in an approximate five MWt margin in the two percent uncertainty assumption. This remaining margin is sufficient to absorb both the approximate 0.6 MWt nonconservative error from this event and the additional approximate four MWt nonconservative errors previously discussed in Licensee Event Reports, LER 95-008 and LER 96-013. Because the maximum magnitude of the effective mass flow conversion error is bounded by the available margin in the allowed two percent uncertainty, the assumptions used for the safety analysis were not exceeded. As a result, previous operation with this error was within the bounds of the design basis LOCA, Containment, and Transient Analyses as described in the Updated Final Safety Analysis Report (UFSAR).
When using the manual heat balance calculation method, the maximum uncertainty due to instmment inaccuracies is 2.3 percent versus the 1.85 percent assumed for the Process Computer calculation. Since this exceeds the two percent power uncertainty requirement, a derate of about 0.4 percent power has historically been required and administered through procedure for the manual heat balance calculation. This derate may not have provided suflicient margin to ensure that Fermi did not exceed 102 percent of the licensed power level ifit was calculated by the manual heat balance. However, the derated power level that was implemented was typically more than 0.4 percent. Therefore, it is not likely that reactor power was maintained that close to the required derated power level limit such that an approximate 0.6 MWt error could have exceeded the assumptions used for the safety analysis. As a result, previous operation with this error was within the bounds of the design basis LOCA, Containment, and Transient Analyses as described in the UFSAR.
Therefore, the health and safety of the public were not adversely affected by this event.
Corrective Actions
The following actions will be taken prior to plant startup:
The mass flow conversion algorithm will be corrected.
e The procedure for the manual heat balance calculation will be revised.
The software specification will be corrected to include the appropriate algorithm.
Additionally, the sof1 ware and instrumentation interface in the Process Computer will be evaluated for similar problems. This evaluation will be completed by December 17,1997. Should this evaluation uncover any additional errors in the heat balance calculation, a supplement to this LER will be submitted.
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AdditionalInformation i
A.
Failed Components
. None.
B.
Previous LERs on Similar Problems
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LER 95-008 On December 13,1995, Detroit Edison discovered a nonconservative omission in the heat balance methodology for calculating core thermal power. Control rod drive flow that is directed to the -
3 Reactor Recirculation Pumps for seal flow contributes approximately four gallons per minute of j
cold water to the primary system. The impact on the heat balance calculation was that calculated l
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core power was approximately one MWt lower than actual power. Administrative controls were implemented to limit core thermal power to 3429 MWt, which will ensure that the current licensed power limit of 3430 MWt will not be exceeded. This LER is similar in that the heat balance calculation was impacted in a nonconservative manner. However, in LER 95-008 the reason for this error was a deficiency in the heat balance methodology. In the current LER it is due to an error i
in the calculation ofRWCU flow.
I LER 96-013 On October 4,1996, Detroit Edison determined that a nonconservative bias existed in the heat i
balance methodology for calculating core thermal power. During performance of a Preventative r
Maintenance (PM) event, a discrepancy in the "as found" calibration data between the Recirculation Pump.B Motor Power Wattmeter and the associated Process Computer Point was discovered.
Initial investigation showed that the wattmeter was scaled for 0.0 to 8.0 megawatts full-scale and l
i the Process Computer Point was effectively scaled for a 0.0 to 10.6 megawatt range full-scale. The i
impact on the heat balance calculation was that calculated core thermal power could have been up i
to approximately three MWt lower than actual power, at the highest Reactor Recirculation Pump.
speeds. Based on the low order of magnitude of the bias and conservatism inherent in power levels j
used for safety analyses, this condition did not result in any adverse impact on the health and safety i
of the public. The discrepancy between the Reactor Recirculation Pump Motor Power Wattmeters l
and the Process Computer Points was corrected. This LER is similar in that the heat balance l
calculation was impacted in a nonconservative manner. However, in LER 96-013 the reason for the i
event was a Process Computer input scaling error. In the current LER it is due to an error in the i
calculation of RWCU flow.
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