05000390/FIN-2017007-03
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Finding | |
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| Title | Potential Failure to Address Environmental Qualification of Brand-Rex Cables |
| Description | The NRC opened a URI to review the adequacy of the licensees justification for the qualified life of Brand-Rex cables, documented in EQ binder WBNEQ-CABL-050. Description: The inspectors reviewed qualification binder WBNEQ-CABL-050 for Brand-Rex cables. Tab C of the binder contained WBNAPS2-089, EQ Calculation for Brand-Rex Cable Insulation, which determined the qualified life and evaluated the operability during design basis accidents for materials used in Brand-Rex cable insulation for plant service environments. Tab D of the binder documented three cable qualification test reports, each of these specified that the qualification standard of record was IEEE 323-1974, IEEE Standard for Qualifying Class 1E Equipment for Nuclear Power Generating Stations, in accordance with the Updated Final Safety Analysis Report (UFSAR). Tab D also contained two Arrhenius plots obtained from the cable vendor. The first plot was the originally specified plot of record and used XLPO/XLPE insulation breakdown as the end of life failure mechanism. A second plot was later supplied by letter dated March 10, 1986, at the request of the licensee. This plot used XLPO/XLPE insulation elongation to break as the end of life failure mechanism. Both plots used temperatures at 113oC, 121oC, 136oC, and 150oC, and indicated that a single test sample was evaluated at each temperature periodically until the end of life was identified. From these documents, the inspectors determined that the licensee qualified the cables by a combination of type test and analysis supported by type test data in accordance with IEEE 323-1974. The inspectors reviewed the data from these plots and determined they had activation energies of 1.37eV for the insulation breakdown plot and 1.48eV for the elongation to break plot. The qualification binder did not use data from the first plot, and eliminated the 150oC test data point from the second plot. The result was an activation energy of 2.13eV, which indicated a 40-year life for the insulation. The criteria for analysis in IEEE 323-1974, Section 6.5, specified, in part, that for extrapolation, the modes of failure produced under intensified or accelerated environmental or other influences shall be the same as those predicted under the required service conditions. The inspectors believe that insulation breakdown was a known predicted failure mechanism. Section 6.5 also specified, in part, that a valid mathematical model is required and it shall be based upon established principles and verifiable test data. The inspectors determined that the licensee extrapolated an invalid mathematical model by using an invalid treatment of the test data. In the licensees development of the mathematical model for this XLPE/XLPO insulation, they removed test data that closely agreed with published 150oC test data, and they used test data that differed from published 136oC test data. When asked, the licensee could not adequately justify, verify, or validate that their treatment of the test data met 10 CFR 50.49 requirements. The inspectors determined that this treatment altered the extrapolation of the mathematical model and produced a 2.13eV activation energy that differed from published activation energies for this type of insulation. Because of this treatment and the lack of verification, validation, and justification, the inspectors believe that the licensees mathematical model did not meet the specifications for analysis in IEE 323-1974. The standard IEEE 323-1974, Section 6.5.4, specified that, the qualified life shall be based upon the known limits of extrapolation of the time dependent environmental effects if an accelerated aging test was used to determine the mathematical model, which required that the certainty of the statistical analysis bound the qualified life of the components for qualification. The limits of extrapolation was described as confidence limits in IEEE 101-1972, IEEE Guide for the Statistical Analysis of Thermal Life Test Data, which is referenced in IEEE 323-1974, Section 6.3.3, as one of the established principles at the time of these Arrhenius extrapolations. The IEEE 101-1972 standard is also referenced in NUREG 0588, Interim Staff Position on Environmental Qualification of Safety-Related Electrical Equipment, (ADAMS Accession No. ML031480402) as an established principle in the resolutions to questions about aging in Section 4. The inspectors believe that the licensee did not account for uncertainties, which must be quantified in their extrapolations in accordance with established principles and NUREG 0588. In addition, established principles in consensus standards specified that the certainty of the extrapolations are strongly dependent on the data for the various test temperatures, spaced at a minimum of 10oC apart, and the total number of samples for each test temperature. For the uncertainties, either fewer test temperatures or fewer samples could produce larger uncertainties, and thus wider confidence limits that could reduce the qualified life. The inspectors noted that the Arrhenius plot used by the licensee at 113oC and 121oC were separated by only 8oC, which was less than required minimum for accurate extrapolations. The inspectors noted that the licensees extrapolations could contain wide confidence limits based on the limited test temperatures and samples. In response to these concerns, the licensee provided NUREG-0847, Supplement No. 15, Safety Evaluation Report related to the operation of Watts Bar Nuclear Plant Units 1 and 2, Appendix DD, page 36, dated June 1995, which documented an audit of the licensees cable binder WBNEQ-CABL-050 by the NRC and their contracted audit team, Idaho National Engineering Laboratory (INEL), which concluded in March 1995. The inspectors noted that this binder had been periodically updated since the audit, including the addition of calculation, WBNAPS2-089, EQ Calculation for Brand-Rex Cable Insulation. Further review of the EQ binder reviewed by the NRC/INEL audit is needed to determine whether the conclusions were based on information similar to that which is included in the current binder. This URI is opened to determine if a performance deficiency exists and it is identified as URI 05000390/2017007-03 and 05000391/2017007-03, Potential Failure to Address Environmental Qualification of Brand-Rex Cables. |
| Site: | Watts Bar  |
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| Report | IR 05000390/2017007 Section 1R21 |
| Date counted | Jun 30, 2017 (2017Q2) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.21 |
| Inspectors (proximate) | E Stamm T Fanelli T Su J Bartley |
| INPO aspect | |
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Finding - Watts Bar - IR 05000390/2017007 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Watts Bar) @ 2017Q2
Self-Identified List (Watts Bar)
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