05000528/LER-2004-007
| Event date: | |
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| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 5282004007R00 - NRC Website | |
1. REPORTING REQUIREMENT(S):
This LER 528/2004-007-00 is being submitted to report a condition which was prohibited by the plant's Operating License. This report applies to Units 1, 2, and 3. Palo Verde's Operating License states in section 2G, "Except as otherwise provided in the Technical Specifications or the Environmental Protection Plan, APS shall report any violations of the requirements contained in Section 2.0 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 30 days in accordance with the procedures described in 10 CFR 50.73(b), (c) and (e).
Specifically, during a review of historical operating data, Engineering concluded that the maximum Palo Verde specific calorimetric error was approximately 38.76 MW in Units 1 and 3 and 39.90 MW in Unit 2 or approximately 1 per cent (1 %). The error resulted in core power levels above the Operating License Condition 2.C(1) of 3876 MW thermal for Units 1 and 3 and 3990 MW thermal in Unit 2 while the ultrasonic flow measurement (UFM) instruments were in service.
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The current external UFM devices (Caldon LEFM 8300) are mounted to the feedwater pipe using an external mounting fixture. The UFM uses ultrasonic energy to determine the flow rate using time of flight acoustic measurements. The mounting fixture is used to precisely locate ultrasonic transducers on the pipe's surface. Each acoustic path sends ultrasonic energy from one transducer through the pipe and fluid to the other transducer. There are two orthogonal measurement planes for each pipe. The measurements from the two planes are averaged to determine the pipe average flow and temperature. On each plane, four measurement paths are installed (total of eight paths per pipe). Measurement paths are either cross path transducer pairs or diagonal path transducer pairs. The UFM calculates fluid temperature and density using the cross path and measures the axial fluid velocity using the diametric paths. The axial velocity is multiplied by the pipe's internal cross-sectional area and the profile factor in order to determine volumetric flow. The volumetric flow is multiplied by the fluid's density in order to determine the mass flow.
The UFM instruments are used to provide a more precise feedwater flow by providing a measurement that is equivalent to the uncertainty limits of the current calorimetric measurement (venturis) and the two percent limit of 10 CFR 50 Appendix K. The advantage of the UFM in this application is that it is not prone to fouling, a condition which causes the venturi to overstate flow and results in overstated calorimetric power.
3. INITIAL PLANT CONDITIONS:
On July 14, 2004, Palo Verde Units 1, and 3 were in Mode 1 (POWER OPERATION), operating at approximately 100 percent power, Unit 2 was in Mode 3 (HOT STANDBY) at zero percent power. There were no major structures, systems, or components that were inoperable at the start of the event that contributed to the event. There were no failures that rendered a train of a safety system inoperable and no failures of components with multiple functions were involved.
4. EVENT DESCRIPTION:
On July 14, 2004, during a review of historical operating data, Engineering concluded that the maximum Palo Verde specific calorimetric error was approximately 38.76 MW in Units 1 and 3 and 39.90 MW in Unit 2 or approximately 1 per cent (1 %). The error resulted in core power levels above the Operating License limits of 3876 MW thermal for Units 1 and 3 and 3990 MW thermal in Unit 2 while the ultrasonic flow measurement (UFM) instrument was in service. (Note: vendor (Caldon) designation of the ultrasonic flow measurement instrument is LEFM.) The non-conservative feedwater flow input to the secondary calorimetric calculation had been in place in Units 1 and 3 since 1999 and in Unit 2 since 2000. This review and its results were based on a letter received from the vendor (Caldon) stating there maybe an error in the flow measurement.
5. ASSESSMENT-OF SAFETY CONSEQUENCES:
Palo Verde's safety analyses bound an operating power level of 102%. The power level during the time frame in which the UFMs were in service did not exceed the analyzed value.
The event did not result in any challenges to the fission product barriers or result in any release of radioactive materials. Therefore, there were no adverse safety consequences or implications as a result of this event.. This event did not adversely affect the safe operation of the plant or health and safety of the public.
Plant operation at 101% licensed maximum power level would not have prevented the fulfillment of any safety function and would not result in a safety system functional failure as defined by 10CFR50.73(a)(2)(v).
6. CAUSE OF THE EVENT:
The cause of the event as described in the Vendor's (Caldon) Engineering Report dated July 2004 is as follows:
"A reappraisal of the calibrations of the LEFM external meters at Palo Verde has been performed because of experience at Watts Bar 1 and River Bend 2. This experience indicates that the meter factors (velocity profile factor) selected on the basis of model tests may be non-conservative in hydraulic geometries where:
(a) The LEFM is 15 or more pipe diameters downstream of the closest hydraulic feature (e.g., a bend), and (b) Non planar features such as another bend are further upstream.
The hydraulic geometries of the UFM installations at Palo Verde meet these conditions. More specifically, at Palo Verde the UFMs are in the individual steam generator leads, about 15 diameters downstream of a U-bend. This geometry was modeled in the tests to establish the meter factor for the Palo Verde UFMs.
However, the test model failed to include a non-planar feed about 10 diameters ■ upstream of the U-bend. The absence of the non-planar feature in the test model has the potential to lead to a non-conservatism in the meter factor established therein.
Potential sources of error in an external UFM fall into several categories: acoustic errors, dimensional errors, time measurement errors and hydraulic profile (velocity profile factor) errors. All of the UFMs at Palo Verde have been checked by Caldon personnel over the years. These checks have confirmed that acoustic and time measurement errors are within their design basis (about 0.4% and 0.25%, respectively). That the dimensions embedded in the LEFM calibration are correct is confirmed by a comparison of the LEFM temperature indications over the years with the feedwater RTDs. The UFM temperature computation, like the flow computation, relies on an accurate measurement of the internal diameter of the feedwater pipe. With the exception of a two year period for one RTD on Unit 3, these measurements agree to better than 1 degree F. In sum, the aggregate errors in the UFMs from all sources except meter factor are unlikely to exceed ± 0.5% and they are likely to be random among units. It is therefore concluded that the error in the UFM is in all probability due to an error in its velocity profile factor, because of the absence of the non-planar feature in the calibration tests for Palo Verde. A non-conservative bias in the 1.0% range is consistent with prior experience for errors of this kind.
No unusual characteristics of the work location (e.g., noise, heat, poor lighting) directly contributed to this event. No procedural errors contributed to this event.
7.� CORRECTIVE ACTIONS:
Initial corrective actions were to reduce power levels in Units 1 and 3 to 96.5% power, take the UFMs out of service by switching feedwater flow data input to the secondary calorimetric to the venturis, and return to full power. Unit 2 which was in Mode 3, switched to venturi inputs prior to power operations.
New software from the vendor, Caldon, that includes newly developed velocity profile factors (the old profile factor was non-conservative) and the associated uncertainties analysis, is being reviewed and a plan that includes more intense validation and monitoring is being developed for placing the UFMs back into service.
No date for placing the UFMs back into service is available at this time.
8. PREVIOUS SIMILAR EVENTS:
In the past three years, no previous similar events were reported.