05000382/LER-2002-006
| Waterford Steam Electric Station, Unit 3 | |
| Event date: | |
|---|---|
| Report date: | |
| 3822002006R00 - NRC Website | |
REPORTABLE OCCURRENCE
Operating License NPF-38 Condition 2.C.1 authorizes the operation of the facility at reactor core thermal power levels not in excess of 3390 megawatts thermal (MWt). On May 9, 2002, a preliminary investigation was completed. The investigation began on April 20, 2002 to evaluate identified discrepancies between the three available reactor core calorimetric thermal power indications calculated by the Core Operating Limits Supervisory System (COLSS) [JC] during the plant approach to 100% power following completion of Refueling Outage 11. The preliminary investigation concluded that the Leading Edge Flow meter (LEFM) CheckPlus ultrasonic flow meter, installed during RF11, identified an approximate 1.9% non-conservative deviation in power measurement when compared with Main Steam venturi flow meter power indication (MSBSCAL), which was used as the primary indication of reactor power over 95%. This 1.9% non-conservative deviation was likely the aggregate of several biases factored into the MSBSCAL indication and secondary side component degradation that occurred during the period February 1995 through 1998. The 1.9% bias has remained essentially constant since 1998 to the present.
Comparison of this 1.9% bias factored into the MSBSCAL indication to the accepted power measurement uncertainty of 1.68% for the MSBSCAL power measurement indicates the plant was operating outside the acceptable measurement uncertainty range by approximately 0.22%. Based on this information, it was conservatively determined that Waterford 3 may have operated at steady state power levels as much as 1.9% in excess of the 100% licensed power limit during the period from approximately February 1995 to plant shutdown for Refueling Outage 11 on March 22, 2002.
Even though the investigation to determine the actual root cause of this condition is ongoing, this condition is being reported as a potential violation of License Condition 2.C.1, "Maximum Power Level." Pursuant to Waterford 3 License Condition 2.F, this potential violation requires a 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> NRC notification followed by a written follow-up report within 30 days in accordance with the requirements described in 10CFR50.73(b), (c), and (e).
INITIAL CONDITIONS
At the time of discovery on May 9, 2002, Waterford 3 was operating in Mode 1 at 99.9% reactor power using the highest indicated (most conservative) power indication. There were no major systems, structures, or components that were inoperable at the time of discovery that contributed to the condition.
EVENT DESCRIPTION
Background Information The Core Operating Limit Supervisory System (COLSS) consists of process instrumentation and algorithms implemented on the Plant Monitoring Computer (PMC). COLSS is used to corroborate and calibrate the CPCs to maximize plant power and ensure the plant operating limits are protected through continuous monitoring of the Technical Specifications limiting conditions for operations (LCOs) on the following:
1. Linear Heat Rate (LHR) 2. Margin to Departure from Nuclear Boiling Ratio (DNBR) 3. Total Core Power 4. Azimuthal Tilt 5. Axial Shape Index COLSS measures the core thermal power based on three methods - turbine first stage pressure, primary calorimetric, and secondary calorimetric - and uses it to determine margins to the power operating limits (POLs) on DNBR and LHR as well as the margin to the licensed power limit (LPL).
The COLSS secondary calorimetric power is the standard by which the other power values are calibrated. Plant secondary calorimetric power is calculated in COLSS by the following general equation.
Power = (Main steam mass flow)(main steam enthalpy) + (Blowdown mass flow)(Blowdown enthalpy) - (Feedwater mass flow)(feedwater enthalpy) + (primary coolant system credits/losses) Prior to 1995, COLSS calculated main steam mass flow by measuring feedwater flow and deducting measured blowdown flow. This method is known in COLSS as "FWBSCAL," which is the feedwater venturi flow meter based power calculation.
In 1994, the feedwater venturis were suspected of fouling, potentially indicating higher feedwater mass flow than actual and resulting in a higher reactor power indication than actual. To counteract the suspected feedwater venturi fouling, a main steam venturi flow meter based power calculation (MSBSCAL) was installed in COLSS. MSBSCAL used measured main steam mass flow and blowdown mass flow to calculate feedwater mass flow. The MSBSCAL algorithm used mass flow constants that were consistent with feedwater mass flow measurements obtained from temporarily installed externally mounted ultrasonic flow meters in late 1994. The resulting MSBSCAL indicated plant power was lower than the FWBSCAL indicated plant power by around 0.7 to 0.8%.
In February 1995 the plant installed MSBSCAL and used this secondary plant calorimetric power measurement as the basis for power operation at plant power over 95%.
During refueling outage 11, Caldon LEFM CheckPlus ultrasonic flow meters (UFMs) were installed on the feedwater piping. These units have been documented in various topical reports to be more accurate than venturi based flow instrumentation. The output from these meters was incorporated into COLSS as USBSCAL. USBSCAL uses the more accurately measured UFM based feedwater flow to calculate main steam mass flow.
Event
On April 20, 2002 as the plant was approaching 100% power following completion of Refueling Outage 11, the USBSCAL, FWBSCAL, and MSBSCAL power indications were not in agreement.
The initial installation testing of the ultrasonic feedwater flow measuring system was also in progress. The expectation was that the USBSCAL power indication would be equivalent to the MSBSCAL power indication. However, the USBSCAL power indication was reading closer to the FWBSCAL power indication. As shown on Attachment 1, the non-conservative bias between MSBSCAL and USBSCAL was approximately 1.9%, with MSBSCAL reading lower than USBSCAL.
The reactor power increase was terminated at approximately 94% and an assessment of reactor core power indications was performed. The plant shifted from using MSBSCAL as the measure of thermal power for power operation at plant power over 95%, to COLSS calculated FWBSCAL, the highest and most conservative indication of reactor core power. Power ascension was recommenced and stopped at approximately 99.5% pending resolution to the discrepancies between COLSS power indications.
An investigation was begun to determine the cause of the larger than expected differences between steam flow and feed flow derived power indications. An evaluation was conducted by the root cause determination team and preliminary findings concluded that the approximate 1.9% non- conservative deviation was likely the aggregate of several biases factored into the MSBSCAL indication and secondary side component degradation since February 1995.
Comparison of this 1.9% bias factored into the MSBSCAL indication to the accepted power measurement uncertainty of 1.68% for the MSBSCAL power measurement indicates the plant was operating outside the acceptable measurement uncertainty range by approximately 0.22%. Based on this information and receipt of the LEFM CheckPlus feedwater flow meter validation of accuracy from Caldon on May 9, 2002, Waterford 3 determined it may have operated at steady state power levels by as much as 1.9% in excess of the 100% licensed power limit since approximately February 1995 to plant shutdown for Refueling Outage 11 on March 22, 2002.
Even though the investigation is ongoing to determine the actual root cause, this condition is being reported as a potential violation of License Condition 2.C.1, "Maximum Power Level." Pursuant to Waterford 3 License Condition 2.F, this potential violation requires a 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> NRC notification followed by a written follow-up report within 30 days in accordance with the requirements described in 10CFR50.73(b), (c), and (e).
CAUSAL FACTORS
As part of the continuing investigation into this condition, actions are being considered to identify the extent of secondary side equipment degradation and the contribution it had on the approximate 1.9% non-conservative deviation. These actions include: (1) a moisture carryover performance test within the next several months; and (2) plans to visually inspect the Main Steam and Feedwater venturis during Refuel Outage 12 scheduled for October 2003.
However, the root cause determination did conclude that the cause of the discrepant power indications was most likely the result of changes or effects on the mass flow inputs to the COLSS power calculations. More specifically, it is suspected that any one or a combination of the following are causing the power indication discrepancies:
- Main steam venturi output is low as a result of venturi damage or COLSS main steam mass flow constants are incorrect.
- Feedwater venturi output has increased as a result of fouling.
- Moisture carryover has increased significantly above the value assumed in the power calculation.
The historical trend data suggests an initial non-conservative bias was installed into the MSBSCAL calculation in February 1995 due to what appears to be inaccurate mass flow constants obtained from a temporary externally mounted ultrasonic flow meter installed in late 1994. At the time, this ultrasonic flow meter device was considered more accurate than the feedwater venturi, which was suspected of being fouled. The external ultrasonic flow meter seemed to confirm the fouling, so these new mass flow constants were used in the MSBSCAL power calculation. In addition to the 1995 installed bias, a slow continuing decrease in indicated steam mass flow through 1998 was also experienced and most probably caused by degradation in the steam flow venturi. Possible mechanisms for this degradation are as follows:
- Erosion of the venturis due to higher than expected moisture carryover
- Chemical erosion of the venturis
- Inaccurate indication due to higher moisture carryover than assumed in the COLSS algorithm.
The higher moisture carryover mechanism (greater than 1%) could account for the deviation in power indications and result in a higher indicated power measurement than actual. The COLSS secondary calorimetric calculations assume a value for moisture carry-over of 0.2%. If moisture carry-over were greater than assumed, the feedwater venturis would indicate an increase in feedwater flow, without an equivalent increase in steam flow due to the volumetric flow measurement from the main steam venturis. A large amount of carry-over would result in very little change in steam volume, but a significant increase in steam density and a significant decrease in steam enthalpy. The net result would be an increase in indicated power using feedwater based secondary calorimetric, without a change in the main steam venturi based calorimetric power measurement.
The LER will be revised if the results of the actions to determine the root cause significantly change the significance, implications, or consequences of this event as outlined in the revision criteria of NUREG 1022.
CORRECTIVE ACTIONS
The plant power will be monitored using the more accurate power measurement from the newly installed Leading Edge Flow meter (LEFM) CheckPlus ultrasonic flow meter (USBSCAL).
Actions were also identified and listed below to prevent recurrence of the deviation between the main steam and feedwater derived power indications.
1. The Main Steam COLSS calibration factor was revised to agree with the LEFM prior to use as a backup to the LEFM. This action will also be performed at the start of every cycle based upon trending of the calibration factors.
2. COLSS MS Venturi constants were recalculated prior to use as a backup to the LEFM.
3. Actions to develop a program to trend both the calibration factors and the deviations of MS and FW correction factors.
SAFETY SIGNIFICANCE
The Waterford 3 safety analysis assumes reactor power is 102% (3458 MWt) at the initiation of postulated accidents. There have been two instances in which the indicated "instantaneous" secondary calorimetric power, biased conservatively by 1.9%, has actually exceeded 102% for a short time period. The 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> average power, also conservatively biased by 1.9% through the time period in question, remained below 101.9%. The instances in which the biased "instantaneous" secondary calorimetric power exceeded 102% were due to decreases in feedwater temperature.
This type of transient is evaluated in FSAR Chapter 15.1, increase in heat removal by the secondary plant. Thus, since the analyzed power of 102% was not exceeded on a steady state basis and the transients where power did exceed 102% for a short time are bounded by the existing Chapter 15 safety analyses, the safety significance of this occurrence is minimal.
This event is not considered a Safety System Functional Failure (SSFF).
SIMILAR EVENTS
As a result of this event, CR-2002-0824 was initiated.
Another condition involving possible operation of the plant in excess of 100% licensed power limit was reported in LER 96-013-00. In that event, a power mismatch of approximately 1.5% was identified between MSBSCAL and FWBSCAL. Preliminary investigation into the condition under Condition Report CR-1996-1299 indicated that MSBSCAL may have been indicating less than actual thermal power by approximately 0.4%.
On November 21, 1996, a Notice of Cancellation of LER-1996-013-00 was issued based on subsequent root cause analysis results that concluded Waterford 3 did not operate in excess of 100% licensed power limit. Although the #2 Steam Generator flow transmitter was found out of calibration, the overall impact on the thermal power calculation was still within allowable uncertainties. This was attributed to other process parameters used in the calculation being well within their calibration limits. Steam flow measurement was therefore determined to be functioning properly. It was concluded that MSBSCAL functioned properly during the period of gradual divergence of MSBSCAL and FWBSCAL reactor power indications.
ADDITIONAL INFORMATION
Energy Industry Identification System (EIIS) codes are identified in the text within brackets [ ].