05000321/FIN-2009006-07
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Finding | |
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| Title | Postulated Early Transfer of Non-Safety Buses |
| Description | The team identified an Unresolved Item (URI) regarding the licensees calculation that evaluated the adverse effects of a postulated early transfer of non-safety buses to a start-up transformer that energizes safety-related buses. Unit 2 utilizes three safety-related buses (2E, 2F, and 2G) to energize emergency core cooling system equipment. Safety-related buses 2F and 2G are normally connected to the same winding of start-up transformer 2D. Hatch utilizes a fast transfer scheme that allows the same winding of the start-up transformer 2D to energize non-safety bus 2D following a main generator trip. The fast transfer results in non-safety bus 2D and safety-related buses 2F and 2G being energized from the same winding of start-up transformer 2D.For a main generator trip thats caused by high drywell pressure or low reactor water level, the fast transfer of non-safety bus 2D occurs several seconds after the large ECCS motors have block started onto the safety-related buses 2F and 2G. If a single failure is postulated, the fast transfer would be concurrent with ECCS block motor starting. A postulated single failure could be caused by the faults to the following components: UAT relaying, normal supply breaker circuits, or main generator tripping scheme. The effect of the fast transfer occurring with a postulated single failure is that the electrical power for two trains of safety-related equipment would be adversely affected. The licensee analyzed the vulnerability described above in Calculation SENH 92-133,Bus Transfer Study, Rev. 1 and determined that if a failure as described above occurred, safety-related bus voltage would dip to approximately 48% of 4160V for 1.09seconds during motor starting. Calculation SENH 92-133 showed that large motors would start, but the team had additional questions regarding modeling techniques used for systems experiencing voltage dips greater than 30%. In addition, the team had questions regarding why the calculation did not evaluate the effect of the voltage dip on other safety related equipment connected to the system. This item is unresolved pending the NRCs review of Calculation SENH 92-133 to determine the adequacy of the methodology, and the NRCs review to determine if the postulated scenario is within the Hatch licensing bases. The issue is applicable to both Units 1 and 2. (URI 05000321/2009006-07 and 05000366/2009006-07, Postulated Early Transfer of Non-Safety Buses |
| Site: | Hatch  |
|---|---|
| Report | IR 05000321/2009006 Section 1R21 |
| Date counted | Sep 30, 2009 (2009Q3) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.21 |
| Inspectors (proximate) | C Smith C Baron D Jones G Skinner R Lewis J Hamman B Desai R Williams J Eargle A Senguptas Shaefferd Jones E Morris D Hardage |
| INPO aspect | |
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Finding - Hatch - IR 05000321/2009006 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Hatch) @ 2009Q3
Self-Identified List (Hatch)
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