ML20135D796

Requests Addl Info Re Use of Wcobra/Trac Large Break LOCA Analysis Computer Code for Assessing Long Term Cooling Performance of AP600

ML20135D796
Person / Time
Site:	05200003
Issue date:	03/04/1997
From:	Huffman W NRC (Affiliation Not Assigned)
To:	Liparulo N WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
References
FACA, NUDOCS 9703050514
Download: ML20135D796 (4)

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UNITED STATES NUCLEAR REGULATORY COMMISSION lk f WASHINGTON, D.C. 20666-0001

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\ . . .. . p# March 4, 1997 I

Mr. Nicholas J. Liparulo, Manager Nuclear Safety and Regulatory Analysis Nuclear and Advanced Technology Division

Westinghouse Electric Corporation

! P.O. Box 355 Pittsburgh, Pennsylvania 15230 l

SUBJECT:

REQUEST FOR ADDITIONAL INFORMATION ON THE WCOBRA/ TRAC OREGON STATE

UNIVERSITY (OSU) LONG TERM COOLING (LTC) FINAL VALIDATION REPORT

Dear Mr. Liparulo:

1 i In support of the AP600 design certificatien review, the Nuclear Regulatory.

Commission (NRC) staff is evaluating the use of the WCOBRA/ TRAC large break l loss-of-coolant-accident (LOCA) analysis computer code for assessing the long '

j term cooling (LTC) performance of the AP600. Westinghouse letter NSD-NRC .

' 4877, dated November 6, 1996, submitted the WCOBRA/ TRAC OSU Long-Term Cooling Final Validation Report, WCAP-14776. The WCOBRA/ TRAC simulation of the OSU

tests uses a " windows" approach; it is the staff's. understanding that this is

the approach for AP600 LTC analyses as well. Due to the amount of computer j time which would be required to run an LTC transient from beginning to end,

! Westinghouse has selected representative windows of the LTC transient to l analyze and demonstrate that a quasi-equilibrium state is reached at the end

of the window consistent with the test results. The staff is in the process l j of reviewing this submittal.

1 The staff's objectives in evaluating the use of WCOBRA/ TRAC for LTC cal-  !

culations are to verify the code's capability in reliably predicting the OSU l test data and, through scaling, to calculate AP600 LTC behavior. Although the i staff may eventually conclude that WCOBRA/ TRAC adequately simulates the OSU '

test data over selected short duration calculational windows, Westinghouse has not yet convinced the staff that the code will predict stable output for the time. periods between the windows or beyond the last window. It is the staff's view that the window widths chosen for the WCOBRA/ TRAC validation analysis of (OSU) test data are too short to conclusively demonstrate the code's capability to predict long term cooling phenomena. Small variations in flow injection or system exit flows at the end of a window could result in sig-nificant deviation between the test data and the calculated reactor vessel water level when the net flow variance is integrated over the unanalyzed LTC time periods. Consequently, based on its review of WCAP-14776 to date, the staff needs additional infors.stion for its evaluation of the adequacy of the window approach in demonstrating that the core remains covered indefinitely during the long term cooling phase of an accident analysis.

Enclosed with this letter are requests for additional information (RAls) which more specifically reflect the staff's concerns as documented above.

9703050514 970304 NRC HLE CENTER CSPY orM' PDR ADOCK 05200003 A ppg l

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Mr. Nicholas J. Liparulo March 4, 1997 If you have any questions regarding this matter, you can contact me at (301) 415-1141.

tSincerely,

, . original signed by:  ;

William C..Huffman, Project Manager Standardization Project Directorate Division of Reactor Program Management Office of Nuclear' Reactor Regulation Docket No.52-003' l

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Enclosure:

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DATE 3 /$97 3.//997' 3/9/97 0FFICIAL RECORD COPY

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j Mr. Nicholas J. Liparulo Docket No.52-003 Westinghouse Electric Corporation AP600 cc: Mr. B. A. McIntyre Mr. Ronald Simard, Director Advanced Plant Safety & Licensing Advanced Reactor Programs Westinghouse Electric Corporation Nuclear Energy Institute 1 Energy Systems Business Unit 1776 Eye Street, N.W. l P.O. Box 355 Suite 300 1 Pittsburgh, PA 15230 Washington, DC 20006-3706 J Ms. Cindy L. Haag Ms. Lynn Connor j Advanced Plant Safety & Licensing Doc-Search Associates ,

Westinghouse Electric Corporation Post Office Box 34 l Energy Systems Business Unit Cabin John, MD 20818 Box 355 Pittsburgh, PA 15230 Mr. James E. Quinn, Projects Manager i LMR and SBWR Programs  !

Mr. M. D. Beaumont GE Nuclear Energy j Nuclear and Advanced Technology Division 175 Curtner Avenue, M/C 165  ;

Westinghouse Electric Corporation San Jose, CA 95125 I One Montrose Metro l 11921 Rockville Pike Mr. Robert H. Buchholz '

I Suite 350 GE Nuclear Energy i Rockville, MD 20852 175 Curtner Avenue, MC-781 l San Jose, CA 95125 Mr. Sterling Franks  :

U.S. Department of Energy Barton Z. Cowan, Esq. '

NE-50 Eckert Seamans Cherin & Mellott 1S901 Germantown Road 600 Grant Street 42nd Floor Germantown, MD 20874 Pittsburgh, PA 15219 Mr. S. M. Modro Mr. Ed Rodwell, Manager Nuclear Systems Analysis Technologies PWR Design Certification Lockheed Idaho Technologies

• Company Electric Power Research Institute Post Office Box 1625 3412 Hillview Avenue Idaho Falls, ID 83415 Palo Alto, CA 94303 Mr. Frank A. Ross Mr. Charles Thompson, Nuclear Engineer U.S. Department of Energy, NE-42 AP600 Certification 4 Office of LWR Safety and Technology NE-50 19901 Germantown Road 19901 Germantown Road Germantown', MD 20874 Germantown, MD 20874 2

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l AP600 REQUEST FOR ADDITIONAL.INFORMATION 440.589 The trends in the WC0CRA/ TRAC analyses of the OSU 58-10 test show an t

overprediction of injection flow and an underprod'ction of exit flow.

. Extrapolation of these trends would lead to a cenclusion that the calculated reactor vessel water level will not fit the test data well beyond the end of the window. Specifically, Figures 5..-3 to 5.1-10 show that the injection l flow rate is higher than the corresponding measured value. Figures 5.1-11 and l 5.1-12 indicate that the injected water temperature is considerably lower.

! Figures 5.1-16 and 5.1-18 indicate that ADS 1-3 and ADS 4-1 flows are lower. '

i If these trends continue, the calculated total injected water would be much

! greater than the test data shows and the calculated total boil off and water

exiting the RCS would be much less than the test data. This would result in an erroneously high value for the calculated reactor vessel water level j sometime beyond the end of the window. Westinghouse should consider extending

the. window width (by 3 to 4 times the current length of 1000 seconds) to demonstrate that the eventual trend stabilizes and balances with the final i vessel water level consistent with the test data.

440.590 Page 5-34, 5th paragraph states that the discrepancy shown in the upper plenum

calculated pressure (Fig. 5.2-2) is due to the uncertainty of the measured i pressure values. However, uncertainty cannot account for a one sided dif-i forence unless some special instruments have been used. Please account for j the above statement and the discrepancy of measured to calculated values.

! 440.591 The calculational window for 58-10 sump injection flow (Figures 5.2-5 and 5.2-l 6) stops at 14,500 seconds. This window does not envelop a significant change

! in the sump injection flow shown in the OSU test data which occurs when the

i. sump injection valves open. Since the sump isolation valves opened at 14,800 i seconds, the staff believes the window should continued to at least 16,000 j seconds to give WCOBRA/ TRAC the opportunity to calculate full sump injection

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flow.

440.592 Westinghouse assessment of the OSU 58-12 test states on page 5-66 that "..the i sump flow was inhibited in this calculation". Why was this done? How is the l comparison of sump flow rates meaningful with this manipulation of the cal-j i

culation? Could another test be substituted for this test without inhibiting sump flow?. What is unique in this test for the WCOBRA/ TRAC long term cooling l validation?

4 440.593 In Figure 6-8, the measured and calculated ratios of the inflow and outflow .

are a function of time. How would these ratios change if the calculations were performed for longer time intervals.

(Note: there is a typo in the title of Figure 5.4-17) 1 Enclosure

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