05000454/FIN-2013004-01
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Finding | |
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| Title | 10 CFR 50.59 Evaluation Affecting Tornado Analysis |
| Description | During inspection activities associated with the review of tornado protection of the UHS, the inspectors identified an Unresolved Item associated with the implementation of Engineering Change (EC) 385829, Tornado Missile Design Basis for the Essential Service Water Cooling Towers, that potentially departed from an approved method of evaluation described in the FSAR (as updated). In response to 2007 NCV 05000454/2007004-003; 05000455/2007004- 003, Discrepancies with Tornado Analysis, the licensee completed EC 385829, Tornado Missile Design Basis for the Essential Service Water Cooling Towers, dated February 9, 2012. This analysis reviewed the design standards and design requirements for tornado protection of structures, systems, or components (SSCs). The licensee concluded that deficiencies identified in the violation could be addressed analytically; therefore, no physical changes to the system would be required. The licensee adopted an approach that required two of the eight UHS fans to be available for use following a tornado. The inspectors questioned this change as it differed from the description found in the original safety evaluation. Specifically, Section 9.2.5, Ultimate Heat Sink, of Safety Evaluation Report Related to the Operation of Byron Station, Units 1 and 2, dated February 1982, contained the following information regarding tornado protection of the UHS: The UHS function is not affected by tornado missiles as discussed further in Section 3.5.2 of this SER [Safety Evaluation Report]. Thus, the requirements of GDC 2 and the guidelines of Regulatory Guides 1.27, Position C.2, regarding UHS protection against natural phenomena, and 1.29 are met. Also, Section 3.5.2, Structures, Systems, and Components to be Protected from Externally Generated Missiles, of Safety Evaluation Report Related to the Operation of Byron Station, Units 1 and 2, dated February 1982, stated, in part: Each unit has one essential service water cooling tower composed of four cells which serves as the ultimate heat sink. The towers are concrete structures designed to withstand tornado-missile impact. However, exposed piping on the towers, and the cooling tower fans and fan motor drives located on top of the towers are not protected from tornado-generated missiles. The applicant has committed (Tramm letter dated January 2, 1982) to provide protection for all piping external to the missile-proof cooling tower walls. The applicant has also provided the results of an analysis (Tramm letter dated January 2, 1982) which shows that in the event of a failure of all the essential service water cooling tower fans, the essential service water system temperature can be maintained within acceptable limits for proper operation of safety-related equipment served in both units with the towers functioning strictly in a natural draft cooling mode and makeup available from the tornado-missile protected onsite wells. The staff concurs that the applicant has satisfactorily demonstrated the availability of the essential service water system and ultimate heat sink in the event of postulated tornado missiles. Thus, the design of the essential service water cooling towers meets the guidelines of Regulatory Guide 1.27, Ultimate Heat Sink for Nuclear Power Plants, with respect to missile protection. The inspectors also identified information in UFSAR Section 3.5.4.1 that indicated that the UHS was expected to be able to meet its design requirements without operating fans. The following first appeared in a January 2, 1982, licensee response to questions by NRC staff regarding UHS performance. These words, or similar words, continued to appear through Revision 13 of the UFSAR. An analysis of cooling tower capacity without fans has been made. Using the most conservative design conditions, it is predicted if the plant is shut down under non-LOCA conditions with loss of offsite power, the temperature of the service water supplied to the plant will not exceed 110F. Although this exceeds the normal maximum temperature of 100F, no adverse impact on safety equipment will result. In December 2012, the UFSAR was updated to Revision 14. The following language in Revision 14 was selected to be in alignment with the methodology outlined in EC 385829. An analysis of the UHS cooling capability for a tornado missile event has been made. The analysis was performed using service water cooling tower performance curves generated using the method described in UFSAR Section 9.2.5.3.1.1.2 and the time dependent two cooling tower model described in UFSAR Section 9.2.5.3.1.1.3. The following inputs and assumptions were used in the analysis: a. A single tornado generated missile is assumed to disable two essential service water cooling tower (SXCT) fans. Concurrent with the missile impact, a LOOP [Loss of Offsite Power] and an electrical failure is assumed to occur that results in the loss of power to two additional SXCT fans. Additionally up to two SXCT fans are assumed to be initially out of service... The inspectors determined that the method of analysis used in the original safety analysis only relied on equipment that was protected from a tornado to evaluate the designs capability to mitigate the consequences of a tornado. As the fans, fan motors, and fan drives were not protected, they were not relied upon. As stated in the original SER, the design complied with the GDCs, due to the designs ability to maintain service water temperatures within design limits while operating strictly in a natural draft cooling mode. The inspectors questioned whether the licensees revised approach which credited equipment that was not protected from a tornado to mitigate the consequences of a tornado constituted a change in a method of evaluation as described in the UFSAR and associated safety analyses. The licensee entered this issue into their CAP as IR 1546621, Inadequate 50.59 for EC 385829 (SXCT Tornado Missiles). The inspectors have engaged the Office of Nuclear Reactor Regulation (NRR) for clarification regarding the licensees current licensing basis. This issue is an Unresolved Item (URI 05000454/2013004-001; 05000455/2013004-001) pending a response from NRR and the completion of additional reviews. |
| Site: | Byron  |
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| Report | IR 05000454/2013004 Section 1R18 |
| Date counted | Sep 30, 2013 (2013Q3) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.18 |
| Inspectors (proximate) | B Bartlett C Thompson E Duncan J Cassidy J Draper J Rivera J Robbins L Carson T Goc Edwardsc Thompson C Zoia E Duncan J Cassidy J Draper J Mcghee L Smith |
| INPO aspect | |
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Finding - Byron - IR 05000454/2013004 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Byron) @ 2013Q3
Self-Identified List (Byron)
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