ML20249B815
| ML20249B815 | |
| Person / Time | |
|---|---|
| Site: | Wolf Creek  |
| Issue date: | 06/18/1998 |
| From: | Muench R WOLF CREEK NUCLEAR OPERATING CORP. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| ET-98-0046, ET-98-46, GL-97-04, GL-97-4, NUDOCS 9806240258 | |
| Download: ML20249B815 (4) | |
Text
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W$LF CREEK NUCLEAR OPERATING CORPORATION Richard A. Muench Vice Presdent Engineenng JUN 181998 ET 98-0046 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Mail Station Pl-137 Washington, D. C. 20555
Reference:
- 1) NRC Generic Letter 97-04, " Assurance of Sufficient Net Positive Suction Head for Emergency Core Cooling and Containment Heat Removal," dated October 7, 1997
- 3) Letter dated May 19, 1998 to 0. L. Maynard from USNRC
Subject:
Docket No. 50-482: Response to Request for Additional Information (RAI) Related to Generic Letter 97-04 Gentlemen:
Attached is Wolf Creek Nuclear Operating Corporation's (WCNOC) response to Reference 3, NRC request for additional information (RAI). The RAI was based on Reference 2, WCNOC' s response to NRC Generic Letter 97-04, " Assurance of Sufficient Net Positive Suetion Head (NPSH) for Emergency Core Cooling And Containment Heat Removal" (Reference 1).
WCNOC has evaluated the questions and has reviewed plant information to address the RAI. Based on the reviews completed to develop this response, WCNOC continues to have reasonable assurance that there is sufficient NPSH for emergency core cooling and containment heat removal pumps.
If you have any questions concerning this response, please contact me at (316) 364-8831, extension 4034, or Mr. Michael J. Angus, at extension 4077.
Very truly yours, 9806240258 980618 *
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PDR ADOCK 05000402 P PDR Richard A. Muench }1}
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cc: W. D. Johnson (NRC), w/a E. W. Merschoff (NRC), w/a J. F. Ringwald (NRC), w/a K. M. Thomas (NRC), w/a P.O. Box 411/ Burhngton, KS 66839 / Phone: (316) 364-8831 An Equal Opportunity Ernployer WF/HCNET
STATE OF KANSAS )
) SS COUNTY OF COFFEY )
Richard A. Muench, of lawful age, being first duly sworn upon oath says that he is Vice President Engineering of Wolf Creek Nuclear Operating Corporation; that he has read the foregoing document ar.d knows the content thereof; that he has executed that came for and on behalf of said Corporation with full power and authority to do so; and that the facts therein stated are true and correct to the best of his knowledge, information and belief.
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Richard A. Muench VicePr[esident Engineering SUBSCRIBED and sworn to before me this day of Ml d , 1996.
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Attachment to ET 98-0046 Page 1 of 2 Response to Request for Additional Information (RAI) Regarding Net Positive Suction Head (NPSH) for Emergency Core Cooling and Containment Heat Removal Dumps Question 1. What is ' the maximum sump temperature assumed in the not positive suction head (NPSH) analyses?
Responses A sump temperature of 212'F was used in determining the available NPSH for the Residual' Heat Removal (RHR) pumps when they take their suction from the containment recirculating sumps. The sump temperature is calculated to be approximately 263*F at the onset of the RHR pumps' switchover to the containment recirculating sumps following a postulated large break LOCA.
Utilizing a lower sump temperature of 212*F is a more conservative approach in the available NPSH analyses since it results in higher frictional losses in the suction piping due to increased fluid viscosity and density, thereby reducing available NPSH for the pumps.
As indicated in Reference 2, a saturated sump model was used in the available NPSH analysis. This model assumes containment pressure is equal to the recirculating sump water vapor pressure (regardless of the water temperature);
hence, no credit was taken for the containment overpressure.
Question 2: In response to question 2, it is stated that "during the review of the calculations for adequacy, weaknesses were identified. However, the net impact on the values being reported is insignificant as it relates to available NPSH versus required NPSH." Describe the type of weaknesses identified. Will these weaknesses be corrected in the NPSH analyses?
Response
The type of weaknesses in the subject calculations are grouped and summarized as follows:
A) The available NPSH (NPSHa) for the Safety Injection Pumps (SIPS) and the Centrifugal Charging Pumps (CCPs) during the injection phase was calculated in the original analysis based on Refueling Water Storage Tank (RWST) level corresponding to elevation 2005.4 feet. The nominal RWST lo-lo-2 setpoint is at elevation 2007.25 feet. The level used in the calculations should be j
the nominal RWST lo-lo-2 setpoint, minus calculated instrument error (3.2%
of 513 inch level span'). The corresponding value, therefore, should have been 2005.88 feet (2007.25 - 1. 37 in) ) . Tne lower RWST water elevation used in the original calculation results in a calculated NPSHa that is smaller than predicted NPSHa (i.e., more conservative).
B) The pump suction header pressure drop analyses for the SIP and CCP during the injection phase -were based on a calculation that was later revised; however, the results of the revised pressure drop calculation were not included in the NPSHa calculations described in Reference 2. The pressure drops were reduced in the revised calculation which, in this case, would yield a larger NPSHa, C) In the case of NPSHa for the Containment Spray Pumps (CSPs) during the recirculation phase of operation, there is an inconsistency between the containment flooding analysis and the CSP NPSH 3 analysis for the containment recirculating sump water level. Based on the identified inconsistency, the calculated FPSH 4 during recirculation is lowered by approximately 7 inches. This 7 inch error is well within the available margin of 6.7 feet (23.2' - 16.5') for the CSP NPSHa; hence, there is an insignificant impact on the CSP NPSH A analysis and no impact on the safe operation of the subject pumps.
D) As indicated in Reference 2, the RHR pump NPSHa is approximately 21.0 feet at 4800 gpm. .However, the original RHR pump NPSH analysis states that the required NPSH at the 4300 gpm flow rate-is 21.3 feet. Our review of the
Attachment to ET 98-0046 page 2 of 2 pump performance curve confirms that the required NPSH, at 4,800 gpm, is approximately 21.0 feet as reported in Reference 2.
E) The original RHR pump NPSH analysis used the pump discharge center line as
. . the reference for determining the available static head. Standard thermo-hydraulic methodology practices use the eye of the pump's impeller (datum line) as the reference for this application. The datum line for the RHR pump is located 1.6 inches below the discharge centerline. This correction will result in an additional NPSHx of 1.6 inches.
F) Frictional losses at the RHR pump suction piping were calculated based on a flow rate of 4,800 gpm. During the normal recirculation mode alignment, RHR pump "A" provides suction to the CCPs and injection to cold legs 1 and 2, and RHR pump "B" pro / ides suc; ion to the SIPS and injection to cold legs 3 and 4. A recent analysis indicates that during the recirculation phase of ECCS operation, the maximum possible flow rate for one RHR pump can be as high as 4,880 gpm. This flowrate is based on a bounding system configuration where one RHR pump feeds two CCPs and two SIPS and also provides RCS injection through two cold legs. The effect of the additional RHR flow is estimated to cause an increase of 0.117 psi (or 3.28 inches at 212'F) in the suction piping frictional losses. The available NPSH decreases by 3.28 inches (from 21.9 feet to 21.6 feet). The RHR pump required NPSH (NPSHa) at the flow rate of 4,880 gpm taken from the pump performance curve, is approximately 21.2 feet. In this case, the NPSHa continues to exceed the NPSHg and therefore, safe operation of the RHR pumps is assured.
WCNOC is evaluating and integrating, through the WCNOC Corrective Action Program, Refueling Water Storage Tank issues that have been identified through internal sources and recent inspection activities. This integrated evaluation will determine the corrective actions needed to address weaknesses in the NPSH analysis.
Question 3: Do these weaknesses affect RHRPs NPSH analyses, given that it was reported that there is 0.9 foot margin between NPSHa and NPSHa?
Response
The issues identified in the original RHR Pump NPSH analyses discussed in Items b), E), and F) result in a total calculated decrease in NPSH margin for the RHR Pumps from 0.9 feet to 0.5 feet. However, significant conservatism exists in the original RHR Pump NPSH calculation. For example, the calculation conservatively calculates entrance losses and pump suction frictional losses. The total calculated frictional losses were then increased by an additional 20%. This conservatism in the original calculation results in an additional NPSH margin of 1.4 feet (at 212*F) .
In conclusion, although the weaknesses identified in the original RHR analysis decrease the calculated NPSH margin, significant conservatism remains in the analysis, thereby ensuring safe and effective operation of the RHR pumps during the analyzed scenarios.