05000298/FIN-2007011-08
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Finding | |
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| Title | Fuel Oil Storage Tank Required Submergence to Prevent Votexing and Available Volume Are Marginal Without Accounting for Instrument Uncertainties |
| Description | As a result of the team\'s questions regarding the available net positive suction head and vortexing potential in the diesel fuel oil systems, the licensee prepared Calculation NEDC 07-090, which was approved December 4, 2007 (subsequent to the team\'s departure from the site). The final results of Calculation NEDC 07-090 concluded that there was adequate available net positive suction head and vortexing would not occur in both the fuel oil storage and the day tanks. The team reviewed the calculation and did not identify any significant issues with respect to the calculation of net positive suction head since there was ample margin between the available and required net positive suction head. The team, however, found that the margin to avoid vortexing for the fuel oil storage tanks was approximately 0.323 inches. This is a concern with the team since this amount of margin is very low and may not have considered instrument uncertainties. The team determined this issue was an unresolved item. The team requested the licensee to demonstrate how instrument uncertainty is accounted for in various engineering calculations and the basis for assumptions regarding the accounting for differences between water and fuel oil. Specifically, the calculations reviewed were: NEDC 07-090, DG Fuel Transfer Pump Submergence Requirement, Revision 0 NEDC 97-012, Emergency Diesel Generator Fuel Oil On-Site Storage Technical Specification Requirements, Revision 2 NEDC 87-052,Emergency Diesel Generator Storage Tank Fuel Capacities, Revision 4 The description stated in Engineering Calculation 07-090,DG Fuel Transfer Pump Submergence Requirements, Revision 0, is to determined the submergence requirements for the diesel generator fuel oil transfer pumps in the diesel generator fuel oil storage tanks and also determined the submergence requirements to prevent vortex formation at the diesel generator diesel oil pump suction inlets in the diesel generator diesel oil day tanks and to provide adequate inlet pressure to the diesel generator diesel oil engine driven and booster fuel pumps. Calculation NEDC 07-090 concluded that the available net positive suction head was adequate and vortexing would not occur in the diesel generator fuel oil storage tanks or fuel oil day tanks. The findings suggest there are no issues with the net positive suction head results for fuel tanks, since there are several feet of margin available in both. The team however, had the following concerns regarding minimum margin: a) In regards to fuel oil storage tank inventory, vortexing may have a more significant impact since there is a minimal margin for the fuel oil storage tanks between the 8 feet-4.0 inches or 49,768 gallons. For a 7-day run technical specifications require 49,500 gallons, this is only a margin of 268 gallons. b) Part of the calculation conclusion indicated that the submergence criteria used in the calculation to prevent vortex formation are based on Froude number correlations developed from testing performed with water instead of diesel fuel. They also state that submergence required to prevent vortex formation with diesel fuel will be higher than water and, therefore, a 20 percent allowance was added to the critical submergence values calculated. The conclusion in the Calculation NEDC 07-090 states, The critical submergence values calculated herein would still remain below the actual submergence at the minimum tank level even with this conservative 20 percent factor added. This assumption of a 20 percent allowance for difference between water and fuel was not substantiated for the fuel oil tanks. Given this assumption, the margin to vortexing is reduced from 1.54 to 0.323 inches. c) Since margins are minimal, instrument uncertainty considerations become very important and it is not obvious how instrument uncertainties are considered in the licensees fuel oil calculations. Specifically, in the conclusions found in Calculation NEDC 07-090 and in the revision to Calculation NEDC 87-052 . The team identified that Revision 4 to Calculation NEDC 87-052, which is used to calculate fuel oil tank capacities, deleted the reference to Calculation NEDC 91-198, Diesel Generator Fuel Oil Tank Level instruments Accuracy, Revision 0. The reason given for this deletion was not clear. d) Discrepancies between calculations that evaluate fuel oil storage tank volumes. Specifically, considering the fuel storage tanks, Calculation NEDC 97-012, Emergency Diesel Generator Fuel Oil On-site Storage Technical Specification Requirements, concluded that the minimum administrative limit based on 8 feet- 4.0 inches plus 8.5 inches (for instrument uncertainty) is 9 feet 0-1/2 inches. However Calculation NEDC 87-052, Emergency Diesel Generator Storage Tank Fuel Capacities, Revision 4, stated that at 8 feet-4 inches the calculated volume is 49,768 gallons. The required volume for a 7-day run is 49,500 gallons. Following the telephone exit on December 12, 2007, the licensee identified two additional issues concerning the diesel fuel oil day tanks and the storage tanks, which are documented in Condition Reports CNS-2007-08590 and CNS-2007-8682, respectively. These issues are related to the licensees failure to account for vortexing impact on available fuel oil volume, and not considering the impact of instrument uncertainties on measuring the fuel oil storage tank volumes. Additional analysis is needed from the licensee to determine whether instrument uncertainly has been adequately accounted for and if adequate margin exists to ensure usable volume remains above the minimum submergence limits in the emergency diesel generator system fuel oil tanks. Therefore, this issue is considered as an unresolved item pending additional information from the licensee and a final review of this analysis by the NRC staff. The licensee has written Condition Report CNS-2007-08482 to address the team\'s questions regarding Calculation NEDC 07-09. (URI 05000298/2007011-08) |
| Site: | Cooper  |
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| Report | IR 05000298/2007011 Section 4OA5 |
| Date counted | Dec 31, 2007 (2007Q4) |
| Type: | URI: |
| cornerstone | No Cornerstone |
| Identified by: | NRC identified |
| Inspection Procedure: | |
| Inspectors (proximate) | T Farnholtz S Garchow M Young M Yeminy N Greca N Okonkwol Ellershaws Graves R Latta |
| INPO aspect | |
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Finding - Cooper - IR 05000298/2007011 | |||||||||||||||||||||||||||||||||
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Finding List (Cooper) @ 2007Q4
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