ML112650136
| ML112650136 | |
| Person / Time | |
|---|---|
| Site: | Wolf Creek  |
| Issue date: | 09/22/2011 |
| From: | Randy Hall Plant Licensing Branch IV |
| To: | Hooper D Wolf Creek |
| Hall, J R, NRR/DORL/LPL4, 301-415-4032 | |
| References | |
| TAC ME4996 | |
| Download: ML112650136 (3) | |
Text
REQUEST FOR ADDITIONAL INFORMATION WOLF CREEK NUCLEAR OPERATING CORPORATION WOLF CREEK GENERATING STATION DOCKET NO. 50-482 LICENSE AMENDMENT REQUEST RELATING TO THE ADDITION OF A NEW ANALYTICAL METHODOLOGY FOR THE BEST-ESTIMATE LARGE BREAK LOSS-OF-COOLANT ACCIDENT TAC NUMBER ME4996 By letter dated November 4, 2010 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML1032002090), Wolf Creek Nuclear Operating Corporation (WCNOC, the licensee), submitted a License Amendment Request (LAR) for a change to the Technical Specifications (TSs) for the Wolf Creek Generating Station (WCGS). Specifically, WCNOC requested a revision to TS 5.6.5, Core Operating Limits Report (COLR), to replace the existing best estimate large break loss-of-coolant accident (LOCA) analysis methodology with a methodology based on the NRC-approved topical report WCAP-16009-P-A, Realistic Large Break LOCA Evaluation Methodology Using Automated Statistical Treatment of Uncertainty Method (ASTRUM).
The NRC staff has reviewed the subject LAR and has determined that the additional information requested below is needed to complete its review. This information is necessary to enable the staff to determine whether the approved methods of WCAP-16009-P-A can be acceptably applied to the plant-specific LOCA analyses for WCGS, and whether the proposed changes comply with the requirements of 10 CFR 50.46, Acceptance criteria for emergency core cooling systems for light-water nuclear power reactors.
- 1. Please provide a description and the results of the evaluation done against the conditions and limitations stated in the staffs safety evaluation report (SER) on the ASTRUM methodology in WCAP-16009-P-A with respect to the Wolf Creek plant-specific adaptation of the ASTRUM methodology. Also, identify any deviations and describe their impact on safe plant operation
- 2. With respect to the analysis employed in the plant-specific adaptation of the ASTRUM evaluation model, please provide the following:
- a. A clarification to show that the plant-specific adaption of the ASTRUM evaluation model for WCGS is still within the approved limitations and conditions stated in the NRC staffs SER for WCAP-16009-P-A;
- b. the plant nodalization scheme for WCGS;
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- c. the results from the 4 circumferential node analysis; and
- d. a clarification and basis for the apparent increase in the number of circumferential noding stacks in the downcomer region from 4 to 12. Based on the staffs experience in reviewing similar applications, it appears that the licensee may have increased the number of circumferential noding stacks.
- 3. With respect to the peak cladding temperatures (PCTs) shown in Figure 1 of the LAR, please discuss the reasons why the PCTs corresponding to the double-ended guillotine break are significantly higher than those resulting from the split break for breaks around same effective break sizes (in the range of CDxAbreak/ACL between 1.7 and 2.1).
- 4. Please explain the physical meaning and cause of the negative values for the hot assembly vapor flow rate between 10 and 30 seconds after the break as shown in Figure 8 in the LAR.
- 5. Please include the date of approval for WCAP-16009-P-A in the proposed TS 5.6.5.b.7 (Revision 0, January 2005, as approved by the NRC staff in a Safety Evaluation dated November 5, 2004). Please also specify which parameters listed in TS 5.6.5.a are supported by TS 5.6.5.b.7.
- 6. Please provide the uncertainty applied to the decay heat curve for the limiting large break LOCA.
- 7. The Idlechik Handbook provides a recommended expression for pressure loss coefficients along a curved channel. Please explain the basis for the assumptions used in the calculation for the k-factor and why the Idlechik Handbook expression was not used in that calculation. Also, provide the values of the lateral k-factors used for the downcomer lateral flow paths for the plant.
- 8. Please provide the method used to compute the azimuthal lateral k-factors and the values used in the plant calculations. The staff notes that the Idlechik Handbook reference for calculating k-factors presents a method to compute k-factors in annuli of various radii. Please provide the results of a k-factor study for the lateral flow paths in the downcomer, if one was performed.
- 9. Please describe the azimuthal nodalization and results from the approved best-estimate WCOBRA/TRAC model. Also provide the results of other nodalization studies applied to the azimuthal detail in the downcomer (other than the four and 12 azimuthal node studies). Please also show the impact of time step on the PCT for the worst case downcomer boiling calculation.
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- 10. Please provide a detailed analysis of the impact of the lateral k-factor values on PCT during downcomer boiling following a large-break LOCA. The NRC staff completed a sensitivity study on downcomer boiling and the effect of lateral k-factor on this phenomenon. The case with zero lateral k-factor in the downcomer cross flow paths joining the azimuthal cells resulted in a 400 degree (°F) reduction in peak clad temperature. This was due to the maximization of mixing between the downcomer azimuthal cells which severely limited downcomer boiling. The cold water entering the downcomer during the long-term cooling phase readily mixed into the adjacent downcomer volumes and thereby reduced boiling, the extent of core uncovery and resulting clad temperatures. ECCS bypass and liquid sweep-out that dominate the very early portion of the event (the first 100-200 seconds) do not prevail during the longer term when the downcomer fills with liquid and vapor velocities are no longer high enough to entrain and sweep out the injected liquid.