05000456/FIN-2012005-06
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Finding | |
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| Title | Concerns with the Bases for the Acceptability of GOTHIC for Void Transport Prediction |
| Description | The inspectors identified an URI regarding the use of the software GOTHIC to justify the acceptability of a design bases change, which incorporated gas voids in the suction piping from the containment emergency sump into the design of the plant. The licensee identified unventable sections at the suction piping from the containment emergency sump downstream of the Unit 1 and Unit 2 SI8811 and CS009A valves. As a result, the licensee evaluated the impact of a maximum potential void size into their licensing and design bases. The licensee justified the maximum potential void size through the use of the software GOTHIC. However, the inspectors noted instances where the basis of GOTHIC as a void assessment tool was questionable. Specifically, the licensee used WCAP-16631-NP, Testing and Evaluation of Gas Transport to the Suction of ECCS [Emergency Core Cooling System] Pumps, to demonstrate that GOTHIC could acceptably predict quantitative void transport behavior. WCAP-16631-NP documented tests that were conducted by Westinghouse to study the transport of a gas void through a piping system. As discussed in NRC Inspection Report 05000456/2011002; 05000457/2011002, the inspectors noted several differences between test and actual plant configurations and conditions that could impact the overall gas void assessment results as follows: a) The difference between test and plant pressures was not considered in assessing a void decrease in the vertical test section. The pressure range used during the test was significantly lower than the typical range in nuclear power plants. This effect would be insignificant in a nuclear power plant due to the higher pressures. Therefore, the inspectors questioned if the void fraction change observed during testing would be analogous in a nuclear power plant. b) Two phase fluid flow test data typically exhibited significant scatter. This was addressed by running many duplicate tests and carefully examining the test results. However, NRR stated in ML090150637, Forthcoming Meeting with the Nuclear Energy Institute (NEI) to Discuss NRC Generic Letter 2008-01, that this effort was not fully successful and some of the conclusions were not adequately supported by the test data due to data scatter. For example, this effort did not address allowance for uncertainty and the effect of actual plant pressures in contrast to test pressures. c) The inspectors questioned if the test report adequately considered a water fall effect (also known as hydraulic jump ) when the upper part of the vertical pipe was voided. Specifically, the inspectors questioned if the pipe length used for the test was representative of the limiting conditions of a plant. The inspectors were concerned that such an effect could propel air further down in the pipe than would be predicted using a single dimensional Froude number and would be of concern if the vertical pipe length was significantly less than the pipe used for the test. d) The use of an average of pipe slopes to determine an equivalent pipe length associated with an elbow with a void reduction of 20 percent was debatable. For example, the average slope of -0.055 was obtained from slopes of -0.333, -0.15, and -0.0883. In addition, as discussed above, the 20 percent factor did not consider the pressures that would be typically present in nuclear power plants. Although the basis for this void assessment tool was questionable, the inspectors noted that the licensee used significant conservatisms when assessing the void sizes at these locations. Consequently, it was determined, with assistance from NRR, that there was reasonable assurance that these unventable locations did not represent an adverse condition pending further assessment of GOTHIC. This issue is an URI pending further NRC review of the use of GOTHIC to justify the acceptability of the design bases change, which incorporated the potential unventable voids in the suction piping from the containment emergency sump into the UFSAR and determination of further NRC actions to resolve the issue (URI 05000456/2012005-06; 05000457/2012005-06, Concerns with the Bases for the Acceptability of GOTHIC for Void Transport Prediction). |
| Site: | Braidwood  |
|---|---|
| Report | IR 05000456/2012005 Section 4OA5 |
| Date counted | Dec 31, 2012 (2012Q4) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | |
| Inspectors (proximate) | A Garmoe E Duncan J Benjamin J Corujo Sandin J Laughlin J Neurauter J Steffes M Perry N Feliz Adorno R Ng T Gom Holmbergm Perry N Feliz Adorno R Ng T Go V Meghani A Garmoe B Bartlett C Moore C Zoia D Szwarc E Duncan J Benjamin J Jandovitz J Laughlin J Robbins |
| INPO aspect | |
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Finding - Braidwood - IR 05000456/2012005 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Braidwood) @ 2012Q4
Self-Identified List (Braidwood)
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