05000454/FIN-2013004-02
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Finding | |
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| Title | Ultimate Heat Sink Limiting Design Basis Event |
| Description | During inspection activities associated with the review of tornado protection of the UHS, the inspectors identified an Unresolved Item concerning the heat removal requirements of the SXCT. Specifically, the inspectors questioned whether the licensees analysis for a design basis accident on one unit coincident with a shutdown of the non-accident unit was adequate. The inspectors reviewed the UFSAR and supporting analyses to understand the heat load impact on the UHS during a design basis accident on one unit coincident with the safe shutdown of the non-accident unit. The inspectors determined that the licensee assumed that a design basis accident and safe shutdown of the non-accident unit would begin simultaneously. This sequence had the net effect of separating the maximum heat loads for each unit by a span of hours. In particular, the accident unit maximum heat load would be generated almost immediately following the onset of the accident, while the maximum heat load from the non-accident unit would be delayed by some amount of time during the transition from full power operation to a cold shutdown condition. The UHS is shared between units; that is, either unit can transfer heat to the UHS and cause its temperature to rise. Therefore, this separation in time of the maximum heat loads experienced by the accident and non-accident units reduced the overall peak heat load. The inspectors reviewed the list of Condition I events, Normal Operation and Operational Occurrences, described in Chapter 15 of the UFSAR to determine whether any of those events listed were capable of generating concurrent heat loads. The inspectors selected Hot Shutdown (subcritical with residual heat removal system in operation) from the list under item a, Steady State and Shutdown Operations. The inspectors postulated a case where the non-accident unit had just entered Hot Shutdown when an accident occurred on the other unit. The inspectors discussed this postulated scenario with licensee personnel. A preliminary review by the licensee staff concluded that this case may not be bounded by the current analysis. The licensee also concluded that although this case may not have been previously considered, it was also not required to be considered as part of their current licensing basis. The inspectors questioned this position as technical specifications (TSs) included limiting conditions for operation (LCOs) for the UHS that were derived from a design basis event. The licensee entered this issue into their CAP as IR 1524228, NRC Question on the Licensing/Design Bases for the UHS. The inspectors have engaged NRR for clarification regarding the licensees current licensing basis. This issue is an Unresolved Item (URI 05000454/2013004-002; 05000455/2013004-002) 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 Gob Bartlettc Thompson D Szwarc E Duncan J Benjamin J Cassidy J Draper J Mcghee J Robbins |
| 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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