NSD-NRC-97-4933, Responds to RAI Re Basis for Use of 20% Critical Damping in Evaluation of Fuel Assembly Seismic Response

Responds to RAI Re Basis for Use of 20% Critical Damping in Evaluation of Fuel Assembly Seismic Response

ML20133G195
Person / Time
Site:	05200003
Issue date:	01/08/1997
From:	Mcintyre B WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
NSD-NRC-97-4933, NUDOCS 9701150214
Download: ML20133G195 (5)

Text

'

m

, e Westinghouse Energy Systems Ba 355 Electric Corporatlon Pittsburgh Pennsylvania 152340355 NSD-NRC-97-4933 DCP/NRC0700 Docket No.: STN-52-003 January 8,1997 Document Control Iksk U. S. Nuclear Regulatory Commission Washington, DC 2055 ATTINTION: T. R. QUAY i

SUBJECF: RESPONSE TO RAls ON FUEL ASSEMBLY DAMPING '

l Dear Mr. Quay-Attached is a response to two requests for additional information about the basis for the use of 20 percent critical damping in evaluation of fuel assembly seismic response. The response includes infbrmation on testing and use of higher damping values previously accepted by the NRC.

"Ihis action completes actions by Westinghouse on these requests and will permit the NRC staff to prrpare input to the AP600 Final Safety Evaluation Report.

If you have any questions please contact Donald A. Lindgren at (412) 374-4856.

B~/r Brian McIntyre, Manager Advanced Plant Safety and Licensing )V

/jml attachment l

cc: W. HufTman - NRC 4

150032 9701150214 970108 PDR ADOCK 05200003 A PDR

Attachement to NSD-NRC-97-4933 )

l RAI 210.213, NRC Ixtter Dated April 10, 1996 - l In a response to DSER open item 3.7.1-3, letter NTD-NRC-95-4460, Westinghouse provided an evaluation of fuel assembly damping. 'Ihe evaluation concluded that a uniform 10 percent of critical damping value is used for all modes higher than the fundamental mode and 20 percent of critical damping is used for the fundamental mode A stafTreview of the fuel assemble test msults under in-core conditions found that a best curve fit in a widely scattered database was used to justify the 20 percent of critical damping value for the fundamental mode, 'lhe stafTconsiders this method to be nonconsenative. In order to ensure adequate margin to bound data uncertainty, a 10 pen:ent of critical damping value also is considered by the staff to appear more appropriate.

Westinghouse is requested to use a uniform 10 percent of critical damping value for all modes of vibrations for the fuel assemblies in its seismic dynamic analysis and to resise Table 3.7-1 accordingly.

RAl# 210.226 - 20 percent damping value for fuel assemblies

'lhe staff found that in Table 3.7-1 of the SSAR, the damping value assigned for the fuel assemblies is 20 percent of critical damping. Westinghouse was requested to provide a basis that justifies the use of this damping value. 'Ihis issue was originally a part of Open item 3.7.1-3, however, it has been reassigned as an open issue in Section 3.9.5.

During the design review meeting on May 10,1995, Westinghouse presented a response to the open item in a separate proprietary and non-proprietary attachment to a letter to NRC, NTD-NRC-95-4460, dated May 10,1995. Westinghouse's evaluation of fuel assembly damping values by analysis and l testing was provided. In the response, Westinghouse states that as a result of combined effects of l inter-fuel assembly rubbing and scraping, fuel md and grid spacer relative motions and frictional i ihrees, and fluid-structure interactions in a closely packed reactor core, damping value increases as 1 amplitude of vibmtion increases. 'Ihe fuel assemblies are stmetumily flexible with low fundamental frequency, and a large amplitude response to postulated seismic loads is expected. Westinghouse's evaluation concludes that a uniform 10 percent of critical damping value is used for all modes higher  ;

than the fundamental mode, and use 20 percent of critical damping value for the fundamental mode to l account for additional hydrodynamic efTects. Ilowever, a subsequent stafTreview of fuel assembly test l results under in-core conditions found that, as shown in Figure 3 of the response, a best-fit cun'e in a widely scattered database was used to justify the 20 percent of critical damping value for the fundamental mode, which appears nonconservative. In order to ensure adequate nnrgin to bound data ,

uncertainty, a 10 percent of critical damping value also for the fundamental mode appears more I appropriate.  !

Westinghouse is requested to use a uniform 10 percent of critical damping value for all modes of vibration for the fuel assemblies in its seismic dyrrunic analysis and to revise the SSAR Table 3.7-1 accordingly.

I

j l

1 Attachement to NSD-NRC-97-4933

Response

1he data in Figure 3 of the Westinghouse letter NID-NRC-95-4460, dated May 10,1996 was developed from neutron noise data. The amplitude of fuel assembly vibration for neutronnoise ,

data is very small compared to the assembly to assembly gap. The amplitude of motion for a l fuel assembly during a safe shutdown canhquake is much larger with amplitudes on the order l of the total accumulated gaps between the fuel assemblies. The data from Figure 3 is not i directly applicable to damping of fuel assemblies during a safe shutdown earthquake. The i figure is useful to show that at the low frequencies of interest, the tmnd, even for very small amplitudes, suppons use of higher damping fbr faulted conditions. l The data available, shown in Figure 1 and 2 of this response, supports the use of 20 percent damping in representative fluid conditions. Test data indicates that damping in still water is '

significantly higher than damping in air and that damping in flowing water is higher than l damping in still water. Figure 1 compares tests in still water and air and uses small to l nuierate amplitudes and demonstrates the efTect on damping of water versus air Figue 2 shows that for moderate amplitudes in flowing water with velocities typical of the water velocities in a reactor core, damping for modemte amplitudes is well in excess of 20 pement.

As outlined above, when the appropriate information is used, that is test data with moderate to l hirge fuel assembly amplitudes and flowing water, the use of 20 percent damping for the  :

fundamental nule in evaluation of AP600 fuel assembly dynamic response is supported by j test data. The use of 20 percent damping is consenntive. l l

The use of 20 percent damping is consistent with evaluations for Westinghouse designed fuel in openning nuclear power plants. For example, the seismic mevaluations for San Onofre Unit 1 Dated April 18,1975 and April 29,1977 use a value of 25 percent damping for the fuel assembly. These discussion of damping values in these reevaluations referenced WCAP-8236 (Proprietary) and WCAP-8288 (Non-proprietary), entitled " Safety Analysis of the 17 x 17 Fuel Assembly fbr Combined Seismic and less-of-Coolant Accident." In a letter dated June 12, 1974 from the AEC to Romano Sahntori, Manager Nuclear Safety Department, Westinghouse Electric Corpomtion the following statement was included "The test results presented in WCAP-8236 and its Addendum, provide an acceptable basis for demonstrating reliability of the Westinghouse 17 x 17 fuel assemblies, with either seven grid spacers or eight grid spacers, to withstand the combined effects of the loss-of coolant accident and safe-shutdown canhquake loading."

In summary the infbrmation in presented in the attached figures and previous actions by the NRC. provide the basis for the use of 20 percent critical damping fbr evaluation of the fuel assemblies in the AP600.

2

Figure 1 17x17 8-Grid Fuel Assembly Mechanical Test Lateral Vibrational Amplitude venus Damping 0.25 ,

i i  ! i I I  !

i  : l j  ;

In sbilwater 0.20 - - - - - - - - * - - - - - - - - - - - --

u I  !

.E ,

I 1

c. ' '

E i i i i Q 0.15 m

,-- - - l -- - - -~ d - -- --- I-- - --

< f I

.M i i  ;

i '

-15 1 i i U

l m i w In air ;

O e 0.10 -- -- , ; -a - - - - - - - - - - - - - - - - - <

t y n OO ,

O g l l oO i i i

0.05 - -- --

---

V-----+----------f-------

O ' i i i

i i  ?

i i ' ' i 0.00 '

0 0.1 0.2 0.3 0.4 0.5 Amplitude - Single Peak (in.)

NLV Cyrs '

<~

Figure 2 Fuel Assembly Damping versus Flow Velocity

~

60 a E E Moderate Amplitude adi O to 50 _O AmpliMde -

-O N----. O.___

O

.5 m OC Q E a a 0 O

E  % 0 0 40 -----------------E------

m Q O

- 0 0 8 0 0o OO 3 0 - - -- - - - - - - - -- - -- e - -i - - - - - - - - - - - - -

i2

- i -

U e g l 0  ;  ;

" -----i i-- -- -

20 - - ---

~

5 o

a m ogC i i-m w en= ,

u U BE  :  :

A 10 --------------------l-l-

i  !

0 0 5 10 15 20 25 30 Flow Velocity, ft/sec ra &wm .

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - _ _ _ . - _ _ _ - - - _. _ _ _