ML19291C056
| ML19291C056 | |
| Person / Time | |
|---|---|
| Issue date: | 04/25/1979 |
| From: | Knight J, Mattu R Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19291C055 | List: |
| References | |
| REF-GTECI-B-06, REF-GTECI-PI, REF-GTECI-RV, TASK-B-06, TASK-B-6, TASK-OR NUDOCS 8001110207 | |
| Download: ML19291C056 (24) | |
Text
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Task B-6 Loads, Load Conbinations and Stress Limits Lead NRR Organization:
Division of Systems Safety (DSS)
Lead Supervisor: James P. Knight, A/D for Engineering, DSS Task Manager:
R. K. Mattu, Mechanical Engineering Branch, DSS Applicability:
Light Water Reactors Projected Completion Date:
Septenb er,1981 1734 208 8 001110 b
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DESCRIPTION OF PROBLEM In the analysis of structures, systems, and components important to safety, the NRC has required the combine. ion of structural / mechanical respcnses due to various accident loads and loads caused by natural phenomena, particularly earthquakes. This requirement flows from 10 CFR Part 50, Appendix A, General Design Criterion 2 which was issued in 1971 and calls for an appropriate conbination of the above loads to be reflected in the design bases of safety equipment. The requirement has been implemented in various ways both within the NRC and the nuclear i ndustry.
The loads due to postulated accidents and natural seismic phenomena often yield dynamic responses of short duration and rapidly varying amplitude in the structures and components exposed to the loads.
These loads usually have no physical time phased relationship in the accident analysis either because the loads are random in nature or because the loads have simply been postulated to occur together (e.g., LOCA and SSE) without a known or defined coupling. Lacking a 1734 209 L
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physical basis for relating some of the loads in question, they have been required to be corrbined for design purposes 4.s the licensing process according to the absolute or linear sum methodology, f.e., sumation of peak structural responses due to each of the individual loads. This approach may lead to overly conservative design requirements for certain plant systems and may result in more rigid systems which is not beneficial when designing for thermal stresses which are present in normal day to day operation.
The requirement in GDC-2, to a large degree, was intended to provide a measure of margin in the dynamic strength of equipnent important to safety. These margins are nonuniform since the portion of equipment strength available to accomodate seismic or accident loadings varies widely for equipment within a plant and from plant to plant.
9 The load corrbination requirement in GDC-2 was also intended to provided defense in depth, i.e., to protect against the very low probability event of a severe accident (e.g., LOCA) being caused by a severe 1734 210
natural phenomenon (e.g., SSE), even though the plant is required to be designed to prevent a LOCA being caused by an SSE and the conbined event is not defined in GDC-2 as a design basis accident. The considera-tion and selection of dynamic events like SSE and large LOCA occurring simultaneously / concurrently was originally largely a matter of regulatory philosophy for containment design. Requirements to consider other dynamic events acting concurrently has been based on judgement which tends towards conservatism due to an absence of data on which to base better founded decisions. Present technology probably af fyds better means for specifying and measuring margin in the dynamic strength of safety equipnent.
2.
PLAN FOR PROBLEM RESOLUTION A.
Present Activities The staff has already modified certain of its design requirements for conbining loads and is concentrating on the easier question of how to conbine loads, not to the more difficult and long tenn questions of 1734 211
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'whether' to contine them.
Ibe technical work to develop an adequate design bases on whether to conbine dynamic loads will require co'siderable developnental effort since little experimental evidence is now available for analysis verification.
The methods to be used in the conbination of loads is being addressed in a nunber of current staff technical activities. These activities include consideration of the cod)ination of various accident loads (e.g., LOCA discharge loads plus suppression pool dynamic loads),
conbination of accident loads and earthquake loads (SSE), and contina-tion of operational loads (e.g., SRV discharge) with anticipated earth-quake loads (OBE). A DSS / DOR working group issued a report NUREG-0484,
" Methodology for Conbining Dynamic Retponses."
In the report the working group recomended the use of Square Root of the Sum of the 1734 212
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- 5-Squares (SRSS) method of conbining short duration rapidly varying dynamic responses to SSE and LOCA loads for piping systems, components and supports within the Reactor Coolant Pressure Boundary (RCPB).
During the course of a recent review of an application for an Operating License, the staff concluded that SRSS method is appropriate for conbina-tion of LOCA and SSE responses for all ASME Class 1, 2, and 3 systems.
Work is also underway to extend the SRSS methodology to other systems, components and structures, and to other dynamic load conbinations (e.g.,
Westinghouse has filed two topical reports on this general stbject area. WCAP 9283 entitled, " Integrity of the Primary Systems of W Nuclear Plants During Postulated Seismic Events" deals with the question of whether to conbine LOCA and SSE loads; WCAP 9279 entitled "Conbination of SSE and LOCA Responses from Faulted Condition Evaluation of Nuclear Power Plants" treats load conbination methodology.
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Th'e DSS technical assistance contract at Brookhaven National Laboratory has produced a draf t review of the Westinghouse submittal WCAP 9283. The staff believes that both the Westinghouse report and the BNL review significantly advance current understanding of decoupling LOCA and SSE loads.
General Electric Company has filed topical report NEDE 24010-P entitled,
" Technical Bases for the Use of the SRSS Method for Contining Dynamic Loads for Mark II Plants." Similar work has also been submitted by Nuclear Services Corporation, and Engineering Decision Analysis Corporation for the Mark II Owner's Group. Dr. R. Kennedy and Dr.
Nathan Newmark have worked with the Mark II Owner's Group to develop criteria for conbination of responses using SRSS.
Brookhaven National Laboratory under technical assistance contract from DSS is performing sensitivity studies for investigating methodologies of conbining dynamic responses and is evaluating the acceptability of the Kennedy Newmark Criteria for application to the Mark II dynamic response conbinations.
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Plan for Future Activities (1) Evaluation will be perfonned of existing NRC requirements including all current regulation, regulatory guides, branch technical positions, etc. for treating loads and their structural responses in conbination.
(2)
Identify the intent of how each item covered in (1) was meant to be used; (e.g., interpretation of GDC-2) cover scope in tenns of design for system consequences of event conbinations as well as structural design.
(3)
Identify current treatment of environmental and postulated event scenarios and loads in conbination for various systems, fuel, and structures.
(4) Develop rationale for decoupling effects of specific loads now treated in conbination for elimination of overly conservative requirements and the provisions for the need for a more detailed guidance under certain loading circumstances.
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('5 ) Developnent of loads, load cont >inations, and any systems criteria such as redunda.cy or single active failure, for which specific systems, fuel supports, structures have to be designed.
(6) Load Cont >ination Methodology (a) Various methods and limits (b ) Response Cont >1 nation and Load Cont)ination (c) Extend NUREG 0484 (7)
Stress Limits (a)
Relate concept of probability of occurence to stress limit.
(b )
Develop proper choice of service limit (stress / strain) for each load or load / response cont >ination.
(c) Consideration of operability and functional capability (8)
Revise Standard Review Plan, develop Regulatory Guide and make Regulation Changes The results of the study (items (1) thru (7)) will result in a more rational basis for the establishment of regulatory requirements and it may also lead to some relaxation of the current requirements.
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such relaxation would be based on improved knowledge of loads, load c.d)inations and stress limits, it is expected that these relaxations will improve the safety standards.
3.
BASIS FOR CONTINUED OPERATION AND LICENSING PENDING COMPLETION OF TASK Criterion 2 of GDC requires that the design bases for structures, systems, and components shall reflect " appropriate combinations of the effects of the normal and accident conditions with the effects of natural phenomena."
In view of this statement, the question arises as to why operating reactors.which have been designed before GDC-2 was instituted and to load cod)iantions that may not have considered normal and accident conditions together with the effects of natural phenomena) are safe or why their continued operation is acceptable.
There are several conservatisms utilized in the design of operating plants which may provide sufficient safety margin against low probability events such as the combined effect of SSE and LOCA. Some of these conservatisms are:
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(a) Elastic analysis is used in,the design for condition involving SSE and/or iJCA. An app eciable margin would exist if inelastic system and component analysis were utilized.
(b ) The ASME Code service limits are equivalent static limits.
Since SSE and LOCA loads / responses are dynamic in nature, the available margin for dynamic loads may be considerably higher (for ductile materials) than the margin available when the loads are static or quasi-static.
(c) The probability of SSE or LOCA occuring alone is very low. The probability of their simultaneous occurance with peaks conbining is extremely low.
Some of the recent plants that have been licensed have used absolute summation of loads, i.e., using unlikely peak conbinations of various dynamic loads / responses and therty making systems more rigid.
Stresses in these more rigid systems are higher for normal operation than would be the case for less rigid system, although still within code allowable limits. This seemingly extremely conservative philosophy does not 1734 218
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b enhance the reliability for nonnal operation especially when thermal stresses are present and flexibility is desirable.
The purpose of this task is to develop improved and more uniform requirements for dynamic loads, their responses and conbination of responses and to assess the effect of these requirements on plant operation under various conditions. While this task is being completed, the existing criteria, which has been in use, are adequate to assure that continued operation of licensed plants and continued licensing of plants now under review impose no undue hazard to public health and safety.
4.
NRR TECHNICAL ORGANIZATIONS INVOLVED:
A.
Mechanical Engineering Branch, Division of Systems Safety (MEB/ DSS).
MEB/ DSS is responsible for overall program management and the following specific tasks for piping systems, equipment and supports:
(i) Evaluation of current NRC regulation requirements and licensee L
procedures.
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(ii) Develop load / response combiantion methodology for piping systems, i
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equipnent and supports.
(iii) Develop a rationale for decou;, ling o' specific loads / responses now treated in conbination and develop listing of loads and load conbinations including all system transients required for the design of each system.
(iv) Determine proper service limits for each load / response or for each load / response conbination.
(v) Extend NUREG 0484.
(vi) Prepare a final report describing findings of studies and conclusions.
(vii) Coordinate with Mechanical Engineering Research Branch (MERB/RS1) the findings of their studies on ' Nuclear Power Plant Design Load Conbination Research' and ' Seismic Safety Margin Research Program.'
(viii) Develop Standard Review Plan, Regulatory Guide and proposed Regulation changes.
Manpower Estimates:
10.0 man months in 1979, 11 man months in U80, 9 man months in 1981.
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B.
Structural Engineering Branch, Division of Systems Safety (SEB/ DSS)
SEB/ DSS is responsible for the following specific ti,ks for PWR and BWR containment and structures:
(i)
Evaluation of current NRC regulaiton requirements and licensing procedures.
(ii)
Develop load / response combination methodology.
(iii) Develop a rational for decoupling specific loads / responses which are currently treated in cod)ination. Develop a listing of loads and load combinations required for design of structures.
(iv) Determine proper load factors and service limits for each load or load / response combination.
(v) Coordinate with MEB/ DSS in writing NUREG 0484 Revision.
(vi) Prepare a final report describing findings of studies initiated by SER and the conclusions.
(vii) Coordinate with MEB/ DSS in revising Standard Review Plan (SRP),
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develop Regulatory Guide (R.G) and meia regulation changes.
Manpower Estimates: 8 man months in 1979, 9 man months in 1980, and 7 man months in 1981.
C.
Materials Engineering Branch, Division of Systems Safety (MTEB/ DSS).
MTEB/ DSS is responsible for the following specific tasks:
(1) To determine what size of an undetected flaw in the reactor coolant system under SSE excitation could lead to a large LOCA. Extend the task to include other high energy lines (e.g., main steam line, feedwate' line).
(ii) Develop a rationale for decoupling SSE and LOCA load / response conbination.
(iii) Coordinate with MEB/ DSS to define piping stresses for determining critical flaw sizes under the operational and SSE loads.
(iv) Coordinate with Metallurgy and Materials Research Branch (MMRB/RES) the findings of their studies on ' Mechanism and Probability of Pipe Failure' and 'Large LOCA Induced by Seismic Crack Growth' and 'J-R Curve Testing of Practor Pressure Vessel and Primary Piping Steels.'
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P (v) Coordinate with MEB/ DSS to revise SRP, develop R.G., and make regt.lation changes.
Manpower Estimates:
3 man months in 1979, 2 man months in 1980, and 2 man months in 1981.
D.
Core Performance Branch, Division of Systems Safety (CPB/ DSS)
CPB/ DSS is responsible for the following tasks:
(i) Develop fuel damage criteria for an SSE.
(ii) Develop fuel damage criteria for LOCA.
(iii) Develop fuel damage criteria for conbination of seismic and LOCA loads / responses.
(iv) Modi fy S.R.P., Section 4.2.
Manpower Estimates:
3 man months in 1979, and 1 man month in 1981.
E.
Reactor Systems Branch, Division of Systems Safety (RSB/ DSS)
(1) RSB/ DSS is responsible for evaluating system consequences for decoupling the dynamic loads effects of SSE and LOCA.
(ii) For the design load cont)inations identified in A(if f), develop 1734 223 z
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a listing of these systems which are required to operate and the systems criteria (e.g., redundancy, single active failure) which are applicable for such systems.
Manpower Estimates: 1 man month in 1979,1 man month in 1980.
and 1 man month in 1981.
F.
Auxiliary Systems Branch ( ASB/ DSS). For the design load cocbinations identified in A(iii), ASB/ DSS will coordinate with RSB/ DSS the task of developing a list of those systems which are required to operate and the systems criteria (e.g., redundancy, single active f ailure) which are applicable for such systems.
Manpower Estimates:
1 man month in 19p79,1 man month in 1980, and 1 man month in 1981 G.
Systematic Evaluation Program Branch, Division of Operating Reactors (SEPB/ DOR)
SEPB/ DOR will evaluate the effects of decoupling Seismic and LOCA load /
responses on reactor coolant system and safe shutdown system of nuclear power plants involved in SEP program.
Manpower Estimates:
1 man month in 1979 and 1 man month in 1980.
5.
TECHNICAL ASSISTANCE Contractor Amount Program Wjectives FY 79 FY 80 l-1734 224 k-A.
Brookhaven National Lab.
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(Managed by
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$90K
$50K To study methodology of conbining dynamic e
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Contractor Amount Program Q)jectives FY 79 FY 80 B.
Brookhaven National Lab (Managed by MEB/ DSS
$55K Evaluation of Mark II SRSS Load Combination Criteria and to investi-gate the acceptability of Kennedy /Newmark Criteria C.
Teledyne Engrg.
Services (Managed by MEB/ DSS)
$35K Effect of faulted condition load combinations on normal operations.
D.
Naval Research L ab.
(Managed by MTEB/ DSS)
$43K
$40K Assess available experimental data and systematically evaluate to determine stable flaw size in piping.
E.
To be se1*cted (Managed by MTEB/ DSS) 335K
$40K Perform tearing stability analyses for LWR piping to determine under what load and material conditions stW)1e crack extension will be assured.
F.
To be selected l734 22b (Managed by SEB/ DSS)
$150K
$150K Perform reliability estimates for different seismic Category I structures subjected to various safety signi-ficant events.
,e.
Contractor Amount Program Objectives FY 79 FY 80 G.
To be selected (Managed by SEB/ DSS)
$50K
$50K Develop methods for load combinations for structures and detent.ine the significance of each load in the load combina-tions through probabilistic approach.
H.
To be selected (Managed by SEB/ DSS)
$75K Determine load factors for each load in factored load cod)in-ations by probabilistic methods.
6.
ASSISTANCE REQUIREMENT FROM OTHER NRC 0FFICES A.
Office of Standards Development, Division of Engineering Standards, Structures and Components Standards Branch (SCSB/SD).
SCSB/SD will specify the original item of treating loads / responses in combination for each of the current regulations, regulatory guides, and branch positions on the sd> ject.
SCSB/SD will coordinate with MEB the task of developing an appropriate Regulatory Guide and changes in the Regulations that might be needed to complete this task action plan.
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. e B.
O'ffice of Nuclear Regulatory Research, Division of Reactor Safety Researt n, P;chanical Engineering Research Branch, (MERB/RES). MERB/RES is funding a major program on evaluating and developing load combination criteria for the design of commercial nuclear power plants. The project contains three major tasks, viz (1) Assess the contribution to safety resulting from the requirement to design for simultaneous large LOCA and SSE ar.d the cost incurred due to this requirement, (2) Assess the proba-bility of a LOCA induced directly or indirectly by a range of earthquakes; and (3) Evaluate and recommend generic techniques and standards for cod)ining dynamic loads. The Metallurgy and Materials Research Branch (MMRB/RES) is assisting MERB/RES in completing this program and is funding the development of fracture mechanics methodology for evaluating piping integrity. MEB/ DSS will coordinate with MERB/RES to provide input to the NRR task action plan (B-6). MTEB/ DSS wil coordinate with MMRB/RES to provide input to task action plan B-6.
C.
ACRS The ACRS Subcommittee on Load Combination has been established and this task will be coordinated with the committee as the task progresses.
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INTERACTION WITH OUTSIDE ORGANIZATIONS A.
Mark II Owner's Group This is an "ad hoc" organization of utilities constructing Mark II BWR facilities. They have engaged G.E., Engineering Design Analysis Corporation, Nuclear Services Corporation, Dr. Robert Kennedy and Dr. Nathan Newmark for resolution of generic dynamic load / response combination methodology and to work with NRC in establishing an acceptance criteria (Ref: NEDE 24010, Supplement 1 and 2).
B.
General Electric Company G.E. has filed topical report NEDE-24010-P entitled " Technical Bases for the Use of the SRSS Method for Combining Dynamic Loads for Mark II Plants." The topical is being reviewed by NRC staff.
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C.
Westinghouse Electric Corporation Westinghouse has filed two topical repo-cs, (1) WCAP 9779 entitled,
" Cont >ination of SSE and LOCA Responses for Faulted Condition Evaluation of Nuclear Power Plants" which treats load cont >ination methodology and (2) WCAP 9283 entitled, " Integrity of the Primary Piping Systems of W Nuclear Plants During Postulated Seismic Events" which deals with whether to cont >ine LOCA and SSE loads.
The staff is reviewing these reports with the help of BNL.
D.
Atomic Industrial Forum Inc. ( AIF)
The AIF Comittee on Reactor Licensing and Safety has formed an "Ad Hoc" comittee on Load Cont)inations. The committee will be looking at generic, long-range load / response cont >inations related issues. They may develop a technical standard dealing with the general load cont >ination/ stress limit issue and the necessary value impact for the Standard.
If developed, the Standard may be endorsed in the Regulatory Guide Series.
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American Society of Mechanical Engineers (ASME)
Interaction is required with the Task Force on D*,namic Loads of Section III of the ASME Boiler and Pressure Yessel Consnittee to ensure consistency of staff requirements with the ASME Code requirements when they are develo ped.
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