ML20246E761
| ML20246E761 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 08/21/1989 |
| From: | Stewart W VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 89-619, NUDOCS 8908290288 | |
| Download: ML20246E761 (2) | |
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e VIRGINIA ELECTRIC AND POWER COMPANY RICHMOND, VIRGINIA 23261 August 21, 1989 United States Nuclear Regulatory Commission Serial No.89-619 Attention: Document Control Desk NO/DLDM:jmj R3 Washington, D.C. 20555 Docket Nos.
50-338 50-339 License Nos. NPF-4 NPF-7 Gentlemen:
i VIRGINIA ELECTRIC AND POWER COMPANY NORTH ANNA POWER STATION UNITS 1 AND 2 LARGE BREAK LOCA ANALYSIS INPUT ERROR in accordance with'the requirements of 10CFR50.72, Virginia Electric and Power Company notified the NRC on August 14,1989 that an error was discovered in the current North Anna Large Break LOCA analysis. This issue was also discussed in conference calls with NRR and Region 11 staff on August 15 and 16,1989. This letter is intended to document the information discussed at that time and our corrective action schedule.
The error found in the North Anna Large Break Loss of Cooling Accident (LBLOCA) analysis for the 18% steam generator tube plugging case was an incorrect initial value for the average enthalpy of the pressurizer steam and water volume. 'The incorrect value appears to have been a human error made during generation of the code input parameters. The error resulted in the code model of the pressurizer being completely full of saturated water at the initiation of the transient. The additional energy released when this fluid is expelled into the containment during the blowdown calculation produces greater calculated containment pressure. Because of the physical phenomena governing core reflooding, this resulted in an increase in the reflooding rate. This results in a lower predicted peak clad temperature (PCT). The analysis was rerun with the affected input corrected. Using the revised enthalpy, the resulting PCT was greater than the previously analyzed PCT and also greater than the 2200 F limit in 10CFR50.46.
The same LBLOCA analysis which currently uses an assumed uniform steam generator plugging level of 18% applies to both North Anna Units 1 and 2. It has been determined that Unit 2 with less than 6% plugging in the most restrictive steam l
generator has suffic!snt margins and can be shown to be below the LOCA limits of 10CFR50.46 with no restrictions on current operating conditions. However, Unit 1 with 13.1% plugging in the most restrictive steam generator, does not possess as much margin.
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it was judged prudent to first conduct the necessary analyses and institute any
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associated oporating restrictions to ensure that the LBLOCA results for Unit 1 remained within acceptable limits. Our preliminary assessment indicated that several analysis evolutions would be required to fully evaluate and resolve the situation.
A reanalysis of the North Anna LBLOCA case (assum'ing 18% steam generator I
plugging) has been performed with the following operating restrictions:
- 1) the maximum heat flux hot channel factor, FO(z), (at rated thermal power) is reduced from the current value of 2.19 to 2.15,2) the containment bulk air temperature lower limit is raised from 86 to 98 F, and 3) the minimum Refueling Water Storage Tank (RWST) temperature is raised from 40 F to 42 F.
As a result of these restrictions, the f 3 analyzed peak clad temperature meets the acceptance criteria of 10CFR50.46.
North Anna Unit 1 has implemented administrative controls to assure adherence to these operating restrictions. In addition, the Unit 1 core surveillance report (subrnitted on April 28,1989, and revised on June 20, 1989) was verified to remain applicable and the design FG(z) predicted for the remainder of Unit 1 cycle 8 operation was determined to be less than 2.15.
Virginia Electric and Power Company is currently pursuing a more detailed review of the LOCA analysis to justify eliminating the operating restrictions and restore the analysis conditions to fully support the current Technical Specifications. This analysis requires iterative calculations with revised input parameters until acceptable conditions are identified. In the event the additional analysis can support only a partial recovery to the approved Technical Specification limits, appropriate changes to the Technice! Specifications will be proposed. Elimination of the operating restrictions or proposed Technical Specification changes is currently expected to be achievable by September 29,1989.
Finally, we have reviewed the corresponding LOCA analyses for Surry and have determined that the initial pressurizer enthalpy is input correctly for that model.
If you have any questions or require additional information, please contact us immediately.
Very truly yours, (KL0 u>%d W. L. Stewart Senior Vice President - Power cc:
United States Nuclear Regulatory Commission Region ll 101 Marietta Street, N.W.
Suite 2900 Atlanta, GA 30323 Mr. J. L. Caldwell NRC Senior Resident inspector North Anna Power Station
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