05000317/LER-2005-003
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. Calvert Cliffs 05000 318 | |
| Event date: | 09-12-2005 |
|---|---|
| Report date: | 12-14-2005 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3172005003R01 - NRC Website | |
I. DESCRIPTION OF EVENT
Chemical tracer testing indicated that main feedwater flow venturi cOrrectionfactors, calculated using ultrasonic flow meters (UFMs), may have been set non-conservatively low. Preliminery results from testing performed on August 18 — 19, 2005 were received froni1he vendor oni September 12, 2005. Final vendor approved results and root cause were not received within 60 days of discovery of this condition, but are now complete. The initial testing results were corroborated by the vendor as part of their root cause analysis. Immediate corrective actions included returning main feedwater flow correction factors on Unit 1 and Unif2 to (1). This action effectively removed the UFMs from service. The non-conservative correction factors were installed on July 22, 2003 (Unit 1) and July 8, 2003 (Unit 2).
The maximum authorized steady-state reactor core power levels, per; Unit 1 and Unit 2 Operating License Conditions 2.C.(1), are not to exceed 2700 MWth. Unit 1 oPerated at a maximum power level of 2711 MWth [100.4 percent rated thermal power (RTP)] from July 22j 2003 until the time of discovery (September 12, 2005). Unit 2 operated at a maximum poWer level of 720 MWth (100.74 percent RTP) from July 8, 2003 until the same time of discoVery. The maximum analyzed steady-state reactor core power levels, including uncertainties, are 102 percent of RTP or 2754 MWth.
II. CAUSE OF EVENT
1 The non-conservative main feed flow correction factors were installed based on the use of a vendor supplied and approved methodology, in accordance with site prOce'duret. The root cause for the event was a failure to consider data, within the design basis document, (Topical Report), indicating that piping/component configuration could produce flow distortions farther downstream than analyzed. A contributing cause was weak oversight by the vendor in pq4,' which would have allowed an earlier opportunity to analyze the piping configuration in ac , � with vendor procedures and identify errant meter readings.
The vendor root cause analysis concluded that design input contained In ttie Topical Report was inadequate. The vendor's design review of the Calvert Cliffs Nuclear 'Povve1(:-Plant (CCNPP) feedwater piping/component configuration was based solely on the specified criteria for meter installation contained in the Topical Report. The report failed to consider Oher data showing that the piping/component configuration could produce flow distortions farther dpVmstream than analyzed, and failed to clarify an aspect of the >15 UD (length/diametc0 criteria that flow entering the 90-degree elbow must itself be stable. As a result, the design review did not adequately address placement of the upstream meter.
Engineering personnel did not realize that the piping/component configuration' upstream of the Loop 12 upstream UFM was influencing the meter and leading to an erroneous correction factor.
Flow at this meter did not meet stable flow criteria cited by the Topical Report as a requirement for meter mounting location.
f' The root cause and contributing cause were both related to human performance. The failure to consider critical data within the design basis document can be attributed tb Work practices that did not focus sufficient attention on all areas of the evaluation. Weak oversight by the vendor allowing an earlier opportunity to identify the problem to be missed can be attributed to insufficient self-evaluation and command and control.
III.� ANALYSIS OF EVENT This event is reportable in accordance with the following:
10 CFR 50.73(a)(2)(i)(B); "Any operation or condition which was prohibited by the plant's Technical Specifications.
Calvert Cliffs Nuclear Power Plant (CCNPP) Technical Specifications, Limiting Condition for Operation (LCO) 3.2.5 for Axial Shape Index (ASI) requires that ASI shall be maintained within the limits specified in the Core Operating Limits Report (COLR). This Technical Specification is applicable when in Mode 1 with thermal power greater than 20 percent. Axial Shape Index shall be restored to within COLR limits within two hours if this LCO is not met. The corresponding COLR Section 3.2.5, requires ASI to be maintained within the limits of COLR Figure 3.2.5 when the Better Axial Shape Selection System (BASSS) is inoperable.
Operability of the BASSS is typically only challenged when the plant computer fails. Two periods were identified for Unit 2 where the non-conservative main feedwater flow correction factors were installed and the plant computer was down longer than two hours while the ASI LCO was applicable. The Unit 2 BASSS was inoperable for greater than 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> on July 24, 2003 (one occurrence of 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> 30 minutes, and one additional occurrence of 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> 2 minutes) and February 14, 2005 (14 hours1.62037e-4 days <br />0.00389 hours <br />2.314815e-5 weeks <br />5.327e-6 months <br /> 17 minutes).
During periods of BASSS inoperability, COLR Figure 3.2.5 establishes acceptable and unacceptable regions for operation within limits of Peripheral ASI (x-coordinates) and Fraction of Maximum Allowable Thermal Power (y-coordinates). The maximum value on COLR Figure 3.2.5 within the acceptable operation region has a corresponding maximum allowable thermal power value (y-coordinate) of 1.000. Operation in excess of 100 percent RTP is, therefore, in the unacceptable operation region of the curve and prohibited by Technical Specification 3.2.5 with BASSS inoperable. Although the non-conservative main feedwater flow correction factors were installed for over two years, the total amount of time in excess of the LCO Required Action Completion Time for the ASI Technical Specification (3.2.5) was less than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> for each Unit.
Engineering evaluation has determined that operation during the period when BASSS was inoperable in excess of the Technical Specification LCO Required Action Completion Time did not result in exceeding any design limits.
An evaluation of the impact on core damage frequency was performed for an extended period of operation at 101.1 percent RTP. This evaluation is bounding for both Unit 1 and Unit 2. The assessment determined an increase in annual core damage frequency of less than 1.0E-6/year.
IV. CORRECTIVE ACTIONS
A. All main feedwater flow correction factor inputs to the plant computer were restored to a value of 1.000 to effectively remove the UFMs from service.
B. The vendor has initiated measures to further evaluate and revise, as necessary, design basis documentation to further clarify flow stability requirements based on lessons learned at CCNPP.
C. Calvert Cliffs Nuclear Power Plant will verify the UFM's are installed in an area of suitable flow and in-situ calibration correction factors are accurate prior to returning the UFM's to service.
D. The vendor has initiated measures to ensure that oversight of future installations of their UFM system is sufficient to prevent recurrence of the conditions identified in this report.
V.�ADDITIONAL INFORMATION A. Component Identification IEEE 803�IEEE 805 Component� EIIS Function 'System ID Feedwater Flow Venturi� Fl� SJ Plant Computer� CPU� ID
B. Previous Occurrences
No other previous similar events have occurred within the past three years at CCNPP.