05000282/FIN-2012005-11
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Finding | |
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Title | Licensee-Identified Violation |
Description | Technical Specification 3.8.1 requires that two diesel generators capable of supplying the onsite 4 kV safeguards distribution system be operable when the reactor is operating in Modes 1, 2, 3 or 4. With one diesel generator inoperable, Limiting Condition for Operation (LCO) 3.8.1.b requires that the diesel generator be returned to service within 14 days. With both diesel generators inoperable, LCO 3.8.1.e requires that one diesel generator be restored to an operable status within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />. Contrary to the above, on June 25 and July 9, 2012, the D5 and D6 diesel generators were not restored to an operable status within two hours of removing a concrete trench which served as a barrier to protect the diesel generators from the impact of an internal flood. Per Procedure 5AWI 8.9.0, Internal Flooding Drainage Control, the concrete trench cover must be in place to support diesel generator operability whenever there is a possibility of a HELB in the Unit 2 turbine building. The inspectors performed a significance screening of this finding using the guidance provided in IMC 0609, Significance Determination Process, Appendix A, The Significance Determination Process for Findings At-Power. In accordance with Exhibit 2, Mitigating Systems Screening Questions, the inspectors answered Yes to the screening question Does the finding represent a loss of system and/or function? since there was the potential for the D5 and D6 emergency power sources to be rendered unavailable. The exposure time for the performance deficiency was the period of time that the flood barrier was missing, which was the 25-day period from June 25 to July 20, 2012. In order to affect an increase in plant risk, the D5 and D6 diesel generators would have to be rendered unavailable by some type of flooding event concurrent with a loss of offsite power (LOOP) event. This scenario was assumed to occur during a seismic event which causes a LOOP along with certain pipe breaks. Such pipe breaks can result from direct seismic failures of the piping itself or indirectly from seismic-induced HELB events that in turn break other piping in the turbine building. The NRC performed a detailed SDP analysis for a separate turbine building flooding issue that bounds this issue. On May 27, 2010, the NRC issued Inspection Report 05000282/2010010; 05000306/2010010 (EA-10-070; ML101470607). This inspection report contained the NRC\'s preliminary risk analysis to assess the impact on both units due to the failure to ensure that engineered safety features, including the diesel generators, were not adversely affected by events that cause turbine building flooding. A subset of that analysis is relevant for this current performance deficiency; namely, the seismic-induced failure of piping for Unit 2. 47 Enclosure The NRC later completed its final risk analysis based, in part, on the licensee\'s analysis from a report titled Turbine Building HELB/Internal Flooding Significance Determination Process, which included a Main Report and seven Addendums (dated June 25, 2010). In Addendum 7, Seismic Analysis and Quantification, the licensee performed a detailed SDP analysis of seismic initiating events. The senior reactor analyst (SRA) used that part of the licensees seismic analysis to assess the risk for this current performance deficiency. The resultant seismic-induced flood risk increase for Unit 2 was 1.98E-6 for an entire year, which equated to a risk increase of 1.4E-7/yr for the 25-day exposure period. This value is conservative since it included non-LOOP as well as LOOP events. Since only 18 percent of the Unit 2 sequences involved LOOP flooding scenarios, the SRA determined that the change in core damage frequency (ACDF) for this finding would be approximately 2.5E-8/yr. The dominant sequence involved a station blackout with pipe breaks associated with these pieces of equipment: generator hydrogen cooler, generator exciter cooler, hydrogen seal oil unit cooler, condensate pump motor unit coolers, miscellaneous small piping, and multiple fire protection |
Site: | Prairie Island |
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Report | IR 05000282/2012005 Section 4OA7 |
Date counted | Dec 31, 2012 (2012Q4) |
Type: | NCV: Green |
cornerstone | Mitigating Systems |
Identified by: | Licensee-identified |
Inspection Procedure: | |
Inspectors (proximate) | D Mcneil K Riemer T Bilik C Moore M Phalen R Baker K Stoedter D Passehl J Laughlin K Barclay M Jones M Learn J Beavers D Oliver |
INPO aspect | |
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Finding - Prairie Island - IR 05000282/2012005 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Prairie Island) @ 2012Q4
Self-Identified List (Prairie Island)
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