ML19339B964
| ML19339B964 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 11/05/1980 |
| From: | Sylvia B VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | Harold Denton, Youngblood B Office of Nuclear Reactor Regulation |
| References | |
| 892, NUDOCS 8011100509 | |
| Download: ML19339B964 (4) | |
Text
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VIlf GINIA E r.ucruis.
.x n l'ow n:ic Cox iwxy R ucunown,VIHGIN IA 20061
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.., u ; WI November 5, 1980 C33 !IV 10 il 9 15 Mr. Harold R. Denton, Director Serial No. 392" '
ERVICES Office of Nuclear Reactor Regulation 50/RMB:jmj
ACH Attn:
Mr. B. Joe Youngblood, Chief Docket No. 50-339 Licensing Branch No. 3 License No. NPF-7 Division of Licensing U.S. Nuclear Regulatory Commission Washing ton, D.C.
20555
Dear Mr. Denton:
MULTIPLE STRUCTURE ARS CONCERN NORTH ANNA UNIT NO. 2 A meeting was held between Stone and Webster, Vepco, and members of the NRC staf f on September 22, 1936 to discuss the results of the independent verifi-cation by the NRC of the pipe stress analysis performed by Stone and Uebster on the North Anna Unit No. 2 Low Head Safety Injection Pump discharge piping.
During the meeting, the NRC staff requested the following additional informa-tion that could not be adequately addressed during the meeting:
1.
The ef f ect of containment growth, due to post-accident pressurization and thermal expansion, on the operability of the Low Head Saf ety Injection (LHSI) pumps.
2.
A discussion of the conservatisms used in the generation and use of the Amplified Response Spectra (ARS) curves.
3.
A discussion of the relationship of a plot of the station ground response spectra to Curve No. 9, Reactor Containment Mat Response 0 el. 204 ft., supplied to you in our letter Serial No. 616 dated July 11, 1930.
The purpose of this letter is to respond to item No. 2 above concerning conservatisms used in the generation of the ARS curves.
The response to item No. 1 was included in our letter Serial No. 849 dated October 18, 1980.
The response to item No. 3 will be addressed in a separate letter. to this letter discusses the many conservatisms inherent in the seismic design of North Anna Power Station piping systems. The attachment lists conservatisms in the dynamic analysis performed including a discussion of thbtA-EE MJ, PJeak broadening, conservatism in the pipe stress analysis and allowable stresses, and a discussion of the scismic resistance of piping in power plants.
8017vnn N
ill' VikOINIA ELECTRIC AND POWEH COMPAW TO Mr. Harold R. Denton, Director 2
If you have any questions or require any additional information, please Contact US.
Very truly yours.
llH /~
s);.
B. R. Sylvia Manager - Nuclear Operations and Maintenance Attachment cc:
Mr. Robert A. Clark, Chief Operating Reactors Branch No. 3 Division of Licensing Washington, D.C.
20555 Mr. James P. O'Reilly, Director Office of Inspection and Enforcement Region II Atlanta, Georgia 30303
V Attachment I NORTIl ANNA 1 & 2 CONSERVATISMS IN SEISMIC ANALYSIS OF PIPING DYNAMIC ANALYSIS A.
Structures 1.
Ground response spectra for North Anna is broad band and nearly coincides with Regulatory Guide 1.60 Spectra.
This is conser-vat ive, since ground response for actual earthquakes is not broad band.
2.
Damping values assigned to subgrade and structures are low, two percent and five percent for reinforced concrete depending upon stress level. These damping values are conservative.
B.
Piping 1.
Damping values for North Anna piping are low; 0.5 percent for the Operating Basis Earthquake (OBE) and one percent for the Design Basis Earthquake (DBE), as compared to one percent for the OBE and two percent f or the DBE (piping uith diameter 12 inch or less) as permitted by Regulatory Guide 1.61.
2.
The maximum responses in three orthogonal directions f rom three orthogonal directions of earthquake motions are combined by modified Square Root of Sum of Squares (SRSS). This is conserva-tive when compared to three dimensional time history analysis results.
V 2
C.
ARS Peak Broadening As noted in the North Anna FSAR, a i 15 percent peak broadening is conservative f or rock f ounded structures. The table in FSAR page 3.7.10 shows that a t 15 percent change in the subgrade shear modulus results in a change of less than 15 percent in the frequencies of the contain-ment building. Thus a piping system with a frequency that lies near the edge of i 15 percent broaden peak is not likely ta have a response equal to the ARS peak.
Furt he rmore, except f or certain cases A small change in the piping model is not expected to result in a large change in the piping response. The case where a significant change occurs is characterized by the following:
1 a.
The change in the piping model affects a frequency that lies very close to the edge of a large discontinuous increase in the peak spread ARS curve.
b.
The f requency change shif ts the mode onto the spread ARS peak, which increases the calculated modal response.
c.
The increased modal response is a large contributor to the total response.
Cases in which a small piping model change can result in a significantly increased total calculated response are rare because of the coincidental prior conditions that must exist as detailed above.
It should be noted that the sensitivity is a result of an introduced conservatism, i.e., a discontinuity due to peak spreading.
It is not expected that a small change in the piping model will result in a significantly dif f erent response to an actual earthquake.
V o
3 An additional conservatism induced by the ef fect of ARS peak broadening is that all modes with f requencies within the broadening peak are assumed to have responses equal to ARS peak acceleration.
In reality, only one mode can have a response equal to ARS peak acceleration.
PIPE STRESS ANALYSIS AND ALLOWABLE STRESSES 1.
Allowable stresses are based on minimum strength determined fron static tests. Actual material strength is generally higher.
2.
Stress intensification factors (SIF) are determined from fatigue testing with large number of cyclic loadings. Use of the same SIF under seismic load (low cycle) is conservative.
3.
For the North Anna containment building, ARS for DBE are similar or lower in magnitude to that of ABS for OBE.
This is due to difference in structural and component damping values. DBE stress allowables, however, are 50 percent (Classes 2 and 3) to 100 percent (Class 1) higher than OBE stress allowable. Thus the upset case (OBE) is a more restrictive one with regard to piping.
4 4.
Stress limits of 3.0 S for Class 1 and 1.8 Sh for Classes 2 and 3 a
piping are conservative. NUREG/CR-0261 dated July 1978 established that piping remains functional until primary stress exceed 1.5 to 2.0 yield strength (S ).
As an example for SA-106 Grade B steel (1.5S /
y y
1.8S ) ranges from 1.4 at 650 F to 1.94 at 100 F.
h 5.
Design pressure is generally used for North Anna in calculating stresses when only maximum operating pressure needs to be used.
Design temperature is generally used for North Anna to determine allowable stresses when only maximum operating temperature needs to be used.
V 4
PIPE STRESS ANALYSIS AND ALLOWABLE STRESSES (Continued) 6.
Pipe and structural support menbers are selected from standard available secticas and, consequently, have generally greater area properties than the minimum required by analysis.
7.
Redundant systems are analyzed separately.
Even identically designed redundant systems do not experience the same seismic excitation due to different counting locations with structural filtering effects.
Thus, simultaneous failure of redundant systers due to earthquake is unlikely.
SEISMIC RESISTANCE OF PIPING IN POWER PLANTS Case histories from power plants that have experienced earthquake have demon-strated the inherent seismic resistance of power plant piping even when they are not specifically designed to withstand earthquakes. A comprehensive discussion is given in a report by Dr. R. L. Cloud included as Appendix F to
" Report on the Reanalysis of Safety-Related Piping Systems, Surry Power Station, Unit 1," dated June 5,1979.
,