ML19312E394
| ML19312E394 | |
| Person / Time | |
|---|---|
| Issue date: | 04/07/1980 |
| From: | Chan S Office of Nuclear Reactor Regulation |
| To: | Schauer F Office of Nuclear Reactor Regulation |
| References | |
| TASK-A-40, TASK-OR NUDOCS 8006040367 | |
| Download: ML19312E394 (15) | |
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MEMORANDUM FOR: Franz P. Schauer, Chief.
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SUBJECT:
REVIEW 0F LLL's DRAFT REPORT
" REC 0fmENDED REVISIONS u
TO NRC SEISMIC DESIGN CRITERIA" FEB. TASK 10/ TAP..A-40 >
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In response to.G. Bagchi's memorandum to you'and others dated January-2G 7M 23,1980 I have coupleted the review of the subject report..I find ' - T e p _';,- i
- that some recommendations of this report are based on beesed ' views,
?,'7'SM commerHal aspects, incorrect technHal conceptsi or questionable R
engineering theories and should not be accepted.. However,this report ".- ;;
.% N'S provides' extensive information on the state.cf-the-art to seismic-O ^ ; NM.T. 4 design of nuclear power plants and is very helpful in SRP revisipnsa,,
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The caments and discussions are attached herewith. -
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COMMENTS ON LLL's DRAFT REPORT
" RECOMMENDED REVISIONS TO NRC SEISMIC DESIGN CRITERIA" General Comments 1.
We had made comments during the early drafting stage of the subject report and find that some of our previous comments have not been incor-porated to the final version of the report.
The contractor feels no need to respond to our suggestion that all technical terms that may be unfamiliar to many engineers should be defined and explained.
The purpose of our suggestion is to get a uniform understanding among the staff members on a technical subject, leaving no room for guesswork or ambiguous interpreta-tion. We also suggested that technical justifications, theoretical bases, and analytical proofs be provided to support the recommendations and statements.
Only with correct understanding and adequate background information can we evaluate and make judgment on the recommendations.
Therefore, we suggest a glossary of technical terms be provided as the appendix of the report.
2.
It is undeniable that the original Standard Review Plan, Sections 3.7.1, 3.7.2, and 3.7.3,containssome deficiencies and mistakes which should be eliminated in future revision.
In fact, there were some positions and criteria arbitrarily decided by the state-of-the-art or staff judgments in those days and have been seriously criticized or challenged.
It is also the purpose of this SRP revision to remove such embarrassing articles.
In the portion of the original SRP where no recommended revision is given, we would presume that the contractor (LLL) and his consultants agree with the original SRP in this respect and have technical basis for the agreement.
In other words, we would also like to know the technical basis and opinions of certain areas which have not been revised in the report.
'3.
We agree that the SRP be allowed to be more flexible and not so restrictive as to preclude the use of new and more rational approaches, but we disagree that the flexibility be extended to design criteria and to technical positions that may become wishy-washy, indecisive, and impractical.
To illustrate our point, in Table 4, page 48 of the report, the recommended damping value for vital piping at low stress level is "1 to 2."
We object to this kind of flexibility not only because of its wide range of difference (100%), but also because it incurs tremendous difficulty 60 the reviewer to exercise his judgment as to what number should be used.
In this case we prefer a less flexible approach by specifying a single number.
In allowing more flexibility, the regulatory aspect should also be considered.
4.
The SRP. should r,at hasten to introduce or endorse any new methods based on unproven controversial theories or personal judgment.
NRC should not accept any paper without critical review just 9ecause it has been pub-lished by a well-known technical organization.
Furthermore, NRC should not accept any computer code even though its basic theory and assumptions have been approved, because the computer code may involve programming.-.
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errors and limitations.
Therefore, we regard the references given in the report as for information only and not necessarily be endorsed for its content, and the computer codes quoted in the report as for convenience of discussion only and not necessarily be approved for application. We wor!1d like to see a statement to clarify this position.
5.
We are pleased to see that the SRP Section 3.7.2 has been rewritten to combine Sections 3.7.2 and 3.7.3 into one document and that SRP Section 3.7.3 becomes a new section covering the seismic analysis of special structures, in support to our previous suggestion.
The new Section 3.7.3 is well written, contains background information of special structures, We do nc' k>"a technical comments yet. However, only two types of special stre
.es, buried pipes and above ground vertical tanks, are, included in the new section.
We suggest this section be enlarged to contain the following structures:
Category I dams, l
masonry walls, spent fuel pools and storage racks, radwaste systems and housing structures.
6.
It appears that part of this report is written from a commercial point of view.
In the discussion of direct solution techniques, it states i
" Economics generally require models to be two dimensional." (p. 26) On i
design codes:
"The major question then is economic: What is the optimum 6
cost of a structure (optimum target design requirements) considering initial cost, repair or replacement costs, and the loss of availability of the facility?" (p. 62) Our primary concern is public safety, and the design criteria should not be dictated by the economics of plant design.
We request deletion of such comments.
Technical Discussions 1.
(pp. 6,11) The performance specification adopted as the basis for the recommendations in the report has not been clearly described.
We do not understand what is meant by "the required value of seismic response" and why will the probability of exceeding this value be about 10 1? We also would like to know the basis of assuming an annual t.rubability ;f exceedance on the order of 10 3 for SSE peak accelerations.
khat does " annual probability of exceedance" mean? How does this performance specification affect the recommendations ensuits? What is a "' erf ormance specification"?
p What is a " procedural specifi' cation"? Why should che ultimate goal be pe'rformance specifications and not procedural specifications?
2.
(p. 13) We like to clarify the meaning of '" site pecific spectra" before getting into further discussion.
A few years ago there were discussions in engineering circles about overconservatism involved in using site-independent spectra, such as R.G. 1.60 spectra where the zero period acceleration is anchored to a g-value determined by seismic zoning and. _. _
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The shape of response spectra, an indication of amplification property of the ground motion at site, is usually flatter and lower for a site-dependent than a site-independent one because of less band width of frequency.
The site-independent spectra covers generally the conditions of the area of the continential U.S. and should be made more conservative, while the site-dependent spectra covers the vicinity of a specific site and is more definite and realistic.
The terms site-independent and site-dependent berome later evolved into site general and site specific.
Site specific spectra for a specific plant sit,e, can be developed from available ground motion data related to the site, and the use of site specific spectra in lieu of site-independent spectra can usually avoid the penalty imposed by the broad-band characteristics.
R.G. 1.60 are site-independent spectra.
We are confused by the. statement "to recommend replacement of the existing R.G. 1.60 response spectra with the more site specific response spectra recommended by Newmark and Hall" and by the recommendation "The current definition of ground design response spectra contained in R.G. 1.60 should be replaced with the ground motion response spectra recommended by Newmark and Hall in NUREG/CR-0098.
Amplification factors corresponding to the mean plus one standard deviation (MSD) should be used."
We do not know how the contractor defines " site specific response spectra,"
but to us the above statement can be interpreted in two distinctively different ways:
(1) Abolish the site-independent approach completely.
All plants must use the site specific approach for seismic analysis including development of their own site specific spectra.
Each plant should have a unique set of custom-made ground response spectra.
(2) Keep the option of choosing the site-independent or site specific approaches, but the site independent R.G. 1.60 spectra must be replaced by the "more site specific" spectra.
The recommendation according to the first interpretation is in contra-diction with the recommended policy that SRP should be more flexible and is therefore unacceptable.
The second interpretation is even more puzzling because it defeats its own logic by using site specific spectra for the site-independent approach.
Furthermore, there are few available techniques of generating site specific spectra for a particular site and the technique proposed by Newmark-Hall in NUREG/CR-0098 is still debatable.
3.
The design ground motion, whether represented by response spectra or by time history, is a design criterion of the seismic environment irrespective of how the plant structures will ultimately be situated or where they are founded.
The design motion is usually defined in the -free field and at the level of ground surface. The variation of motion with soil depth under the ground surface obeys the law of nature which we do not quite understand, but we know that the calculated motions depend very much on.
6 the assumed model of soil media.
It is generally believed that the free field ground motion decreases somewhat with the depth, less with the half-space model and more with the shear beam / soil column model.
Unfortunately there are not enough reliable experimental results to support any theories or assumptions.
We disagree with the statement "while it was generally agreed that a reduction in acceleration is justified because of embedment, the amount of reduction that should be allowed....
The magnitude'of allowable reduction ranged from 25 to 40% of the design ground response spectrum, frequ'ency by frequency...." (p. 29), for the following reasons:
(1) That the general agreement about a reduction in acceleration is not a fact.
There are oppositions to this issue.
(2) A design engineer should not in any way change or modify the design conditions, design loads, or design criteria which should be irrespective of how the plant structures will ultimately be situated, founded, or embedded.
(3) The reduction or amplification of acceleration response at locations is calculated from given design data and based on certain theories and assumptions.
To put a limit on computed results seems to be a safeguard against computational mistakes, wrong theories, incorrect assumptions, and other human errors.
The concern is quality assurance, not engineering.
(4) The values of 25% and 40% reduction are arbitrary and judgmental.
They have no theoretical or scientific basis.
The statement that i
the Japanese have limited the embedment reduction effect to a maxi-mum reduction of 25% of the ground response spectrum is questionable.
The Japanese built their nuclear power plants on rocks and thus the soil-structural interaction effect is negligible.
How do the Japanese utilize the 25% reduction of ground design response spectrum is not understandable.
1 4.
Referring to the recommendation:
"Both real and synthetic time histories are acceptable for the design and analysis of nuclear power plant systems, subsystems, and components." (p. 20) We need a clear definition of.
l synthetic time history, a detailed description of methods and procedures of generating and synthesizing such time history, and its acceptance criteria.
5.
The statement "If proper account is taken of the seismic wave amplifi-cation properties of a site in specifying the free-field motion,...."
(p.19), does not clarify the SRP position as it claims. What proper account is has not been specified.
The discussion of seismic wave amplification properties is vague and involves controversial subjects such as deconvolution and. eduction of surface motion.
6.
It is also desirable for the SRP revision. Lo simplify requirements and to settle controversies.
The recommendation on use of time histories (p. 20),
V is just doing the opposite.
Regarding the approximate nature of the analysis and the uncertainties involved in the problem, such tedious and complicated procedures as requiring seven or more time histories are really uncalled for.
The requirement of proper scaling for frequency content, amplitude.and emergency content is confusing, controversial, and incomprehensible.
7.
The concept of using field data and experimental records to verify analytical techniques (p. 22) is absurd. While field data and experiments had been used to verify theories and assumptions, but never considered to be the basis to check the mathematics or analytical technigdE.-
8.
Chapter II on soil-structure interaction fails to distinguish clearly the different stages of analysis such as the modeling assumption (e g.,
half-space, finite boundaries of soil medium), discretization methods (e.g., finite element, lumped parameter, finite difference), and analytical techniques (e.g., direct solution, substructure method).
The discussion mixes them all together and is therefore confusing and inconceivable.
9.
In statics the substructure solution technique is theoretically equivalent to the direct solutian technique.
But in dynamics, it is at best an approximation of the latter.
In principle, the substructure technique is acceptable as an alternate to the direct solution technique, but the i
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acceptance criteria depend very much on the detailed formulation and its theoretical basis.
Since the recommendation does not specify the theories or methods of substructures (p. 27), the acceptance of this technique should be on a case-by-case basis.
The report also pushes for the use of frequency-domain technique on the substructure solution and imposes the deconvolution procedure on soil-structure interaction analysis (Appendix B,
- p. 12-16).
This biased attitude should be avoided.
- 10. We feel the requirement of investigating the effect of structure-to-structure interaction should be deleted from the SRP (p. 30).
- First, there is no evidence that the structure-to-structure interaction is significant, at least its significance is not in the same magnitude of soil-structure interaction.
Secondly, the state-of-the-art does not warrant j
such analysis, i.e., there is at present no acceptable method of doing the analysis.
This is a good topic for research projects and it is premature to include in SRP.
- 11. We disagree that the flexible side boundaries effect on overall structural response is considered to be of secondary importance (p. 30). We have observed on one occasion, by comparing the half-space model and the shear-beam model of soil medium, a difference of 400% in overall structural response which can hardly be called of secondary importance.
The discrepancy is caused by different assumptions of side boundarie:.
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12.
The report has committed a serious mistake of mixing response spectrum with power or other forcing function spectra which we have pointe't out fu previous meetings.
A response spectrum, as defined in Regulatory Guide 1.60, means a plot of the maximum response (acceleration, velocity, or displacement) of a family idealized single degree-of-freedom damped oscillators as a function of natural frequencies (or periods) of the oscillators to a specified vibratory motion input at their supports.
The abscissa of the respo..se spectrum represents clearly the natural fre-quencies of the oscillators and not the forcing frequencies of the input motion. While discussing a response spectrum, the report states:
For time history analysis, artificial earthquakes are generated by adjusting the power spectrum and using random phases.
These notions must have spectra which match in some general sense the design spectra for all frequencies of interest." (Appendix 8, p. 2) Another instance of mix-up is, "Since the actual wave content of the carthquake is not known, it would be logical to require that this value be no less than 0.5 or 0.6 (notice that the reduction in the response spectra is smaller than in the transfer function)." (Appendix B, p. 6) We like to know which recom-mendations are based on this erroneous judgment and what credibility should be given to those recommendations.
13.
We do not believe that a Regulatory Guide or Standard Review Plan should include requirements of computation 11 science such as:
conditions on mesh size and boundaries for discrete models, time step for numerical integration, frequency increment, interpolation procedures, and frequency _ _
i range for solutions in the frequency domain, etc. (Appendix B, p. 6)
These limitations are based on the theories of error propagation, numerical stability, order of precision, and other considerations in numerical analysis.
It may be desirable to include these in a designer's handbook or computer program user's manual, but surely is inappropriate for a regulatory document.
14.
The recommened new Section 3.7.3 of SRP which deals exclusively with special structures offers valuable information and interesting insight of many structural problems. We feel that the scope of this section should be widened and more time should be dedicated to study and s rcussions.
We withhold our technical comments in this area until further study.
15.
The recommended damping values in Table 4 (p. 48), are on the high side for high stress level.
These values are based on the Newmark-Hall report NUREG/CR-0098 (1978) which is in turn based on their early findings in 1969.
Thesevaluesarecrudeandhighlyjudgmental,andhavebeenmeticu-lously considered during tha draft of the original 1973 Regulatory Guide 1.61.
Therefore, it presents no new evidence or results of new study and therefore provides no justification for the change.
Furthermore, specifying a range of numbers instead of one single number, thus leaving the decision i
to the reviewer,is not a good regulatory procedure.
- 16. Wave passage effects are resulting torsion and rocki.g effects, concern of inelastic behavior of structures, effect of embedments may all be j
considered in dynamic analysis and calculation of structural responses.
These items should not be used as pretexts or justifications to modify the design seismic environments or design ground motions.
17.
The idea of direct generation of floor response spectra is not new.
Although there are some published papers dealing with this subject, they have not been c: stically reviewed and generally lack rigorous theoretical basis and general application.
Until an acceptable method for direct generation of floor response spectra is developed and recognized, the recommendation, "The Standard Review Plan" should give equal weight to the use of both time-history analysis methods and direct solution methods for the generation of in-structure response spectra (p. 51) seems point-less.
Therefore, specific direct solution methods should be recommended with theoretical justifications and technical discussions as alternates to time-history analysis methods.
18.
More detailed explanation and specific meaning should be provided for the term "probabilistically generated in-structure response spectra."
(p. 52)
Also, the Standard Review Plan should accept, not encourage (p. 52).
19.
It is understood that in the licensing process tests of confirmatory or qualifying nature may be required for each plant, but tests of a studying or research nature for generic issues or general information are not justified.
In the recommended list of testings (pp. 63-64), one cannot tell which tests are required for_ licensing.
If so, testing procedures,
instrumentation requirements, setup information, theoretical background, and acceptance criteria should be given in detail.
Tests to verify design methodology or to gain information on material properties should not be required.
20.
In the original SRP Sections 3.7.2 and 3.7.3, the items " basis for selection of frequencies" and " analysis procedure for damping" have been critized as technically incorrect and baseless.
The validity of decoupling criteria for subsystems in " procedures used for analytical mooeling" has also been challenged.
Since there are no comments nor recommended revisions on these subjects, we would presume that these items have Faen agreed upon or endorsed and we like to know the basis of acceptance.
If these items have been overlooked, we would ilke to know the latest comments and suggestions.
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