ML063000449
| ML063000449 | |
| Person / Time | |
|---|---|
| Issue date: | 10/27/2006 |
| From: | Dubouchet A Office of Nuclear Regulatory Research |
| To: | |
| DuBouchet, A. EEMB/DE, 415-2785 | |
| References | |
| DG-1168 RG-1.124, Rev 2 | |
| Download: ML063000449 (10) | |
Text
1 Copies may be obtained from the American Society of Mechanical Engineers, Three Park Avenue, New York, NY 10016-5990; phone (212) 591-8500; fax (212) 591-8501; http://www.asme.org/.
This regulatory guide is being issued in draft form to involve the public in the early stages of the development of a regulatory position in this area. It has not received staff review or approval and does not represent an official NRC staff position.
Public comments are being solicited on this draft guide (including any implementation schedule) and its associated regulatory analysis or value/impact statement. Comments should be accompanied by appropriate supporting data. Written comments may be submitted to the Rules and Directives Branch, Office of Administration, U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001.
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U.S. NUCLEAR REGULATORY COMMISSION October 2006 OFFICE OF NUCLEAR REGULATORY RESEARCH Division 1 DRAFT REGULATORY GUIDE
Contact:
A.V. du Bouchet (301) 415-2785 DRAFT REGULATORY GUIDE DG-1168 (Proposed Revision 2 of Regulatory Guide 1.124, dated January 1978)
SERVICE LIMITS AND LOADING COMBINATIONS FOR CLASS I LINEAR-TYPE COMPONENT SUPPORTS A. INTRODUCTION General Design Criterion 2, Design Bases for Protection Against Natural Phenomena, of Appendix A, General Design Criteria for Nuclear Power Plants, to Title 10, Part 50, Domestic Licensing of Production and Utilization Facilities, of the Code of Federal Regulations (10 CFR Part 50) requires that the design bases for structures, systems, and components important to safety reflect appropriate combinations of the effects of normal and accident conditions with the effects of natural phenomena such as earthquakes. The failure of members designed to support safety-related components could jeopardize the ability of the supported component to perform its safety function.
This guide delineates acceptable levels of service limits and appropriate combinations of loadings associated with normal operation, postulated accidents, and specified seismic events for the design of Class 1 linear-type component supports, as defined in Subsection NF of Section III of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code.1 This guide applies to light-water-cooled reactors.
DG-1168, Page 2 The NRC issues regulatory guides to describe to the public methods that the staff considers acceptable for use in implementing specific parts of the agencys regulations, to explain techniques that the staff uses in evaluating specific problems or postulated accidents, and to provide guidance to applicants.
Regulatory guides are not substitutes for regulations, and compliance with regulatory guides is not required.
The NRC issues regulatory guides in draft form to solicit public comment and involve the public in developing the agencys regulatory positions. Draft regulatory guides have not received complete staff review and, therefore, they do not represent official NRC staff positions.
This regulatory guide contains information collections that are covered by the requirements of 10 CFR Part 50 which the Office of Management and Budget (OMB) approved under OMB control number 3150-0011. The NRC may neither conduct nor sponsor, and a person is not required to respond to, an information collection request or requirement unless the requesting document displays a currently valid OMB control number.
B. DISCUSSION
Background
Load-bearing members classified as component supports are essential to the safety of nuclear power plants because they hold components in place during the loadings associated with normal and upset plant conditions under the stress of specified seismic events, thereby permitting system components to function properly. Load-bearing members also prevent excessive component movement during the loadings associated with emergency and faulted plant conditions combined with the specified seismic event, thus helping to mitigate the consequences of system damage. Component supports are deformation-sensitive because large deformations can significantly change the stress distribution in the support system and its supported components.
To provide uniform requirements for construction, the component supports should, as a minimum, have the same ASME Code classification as that of the supported components. This guide delineates levels of service limits and loading combinations, in addition to supplementary criteria, for ASME Code Class 1 linear-type component supports, as defined by NF-1213 of Section III. This guide does not address snubbers.
Subsection NF of Section III permits the use of four methods for the design of Class 1 linear-type component supports: (1) linear-elastic analysis, (2) load rating, (3) experimental stress analysis, and (4) limit analysis. For each method, the ASME Code delineates allowable stress or loading limits for various code levels of service limits, as defined by NF-3113 of Section III, so that these limits can be used in conjunction with the resultant loadings or stresses from the appropriate plant conditions.
Because the ASME Code does not specify loading combinations, guidance is required to provide a consistent basis for the design of component supports.
Component supports considered in this guide are located within Seismic Category I structures and, therefore, are assumed to be protected against loadings from natural phenomena (or manmade hazards) other than the specified seismic events. Thus, only the specified seismic events need to be considered in combination with loadings associated with plant conditions to develop appropriate loading combinations.
Loadings caused by any natural phenomena other than seismic events should be considered on a case-by-case basis.
2 Regulatory Guide 1.84, Design, Fabrication, and Materials Code Case Acceptability, ASME Section III, provides guidance for the acceptability of ASME Code,Section III code cases and their revisions, including ASME Code Cases N-71 and N-249. Code cases identified as Conditionally Acceptable Section III Code Cases are acceptable, provided that they are used with the identified limitations or modifications.
DG-1168, Page 3 Design by Linear-Elastic Analysis a.
Sy and Su at Temperature Tables U and Y-1 in Subpart 1 of Part D of Section II and Tables 3, 4, and 5 in the latest accepted versions2 of ASME Code Cases N-71 and N-249 give the relevant material properties when the linear-elastic-analysis method is used to design Class 1 linear-type component supports.
These tables list values at various temperatures for the minimum yield strength Sy and ultimate tensile strength Su. At room temperature, Sy varies from 62 percent to 93 percent of Su for component support materials.
Levels of service limits that are derived from either material property alone might be insufficient to provide a consistent safety margin.Section III recognizes this issue in NF-3322.1(a), which defines the allowable stress in tension on a net section as the lesser of two values, 0.6Sy or 0.5Su.
Although NF-3322.1(a) specifies allowable tensile stress in terms of both Sy and Su, the rest of NF-3320 notes other allowable service limits in terms of Sy only. This does not maintain a consistent design margin for those service limits related only to material properties.
Modifications similar to NF-3322.1(a) should be employed for all those service limits.
b.
Allowable Increase of Service Limits Although NF-3321.1(a) and F-1334 of Section III of the ASME Code permit the increase of allowable stresses under various loading conditions, NF-3321.1(b) limits the increase to less than or equal to two-thirds of the critical buckling stress for compression and compression flange members. NF-3322.1(c) of Section III derives critical buckling stresses with normal design margins. Because buckling prevents shakedown in the load-bearing member, NF-3322.1(c) should be controlling. Also, buckling is the result of the interaction of the geometry of the load-bearing member and its material properties (i.e., elastic modulus E and minimum yield strength Sy).
Because both of these material properties change with temperature, calculation of the critical buckling stresses should use the values of E and Sy of the component support material at temperature.
Tensile and shear stress limits and their nonlinear interaction are used to derive allowable service limits for bolted connections, which also change with the size of the bolt. For this reason, the increases permitted by NF-3321.1(a) and F-1334 of Section III do not directly apply to allowable tensile stresses and allowable shear stresses for bolts and bolted connections.
As specified in F-1335 of Section III, the allowable increase in tensile stress for bolts should not exceed the lesser value of 0.70 Su or Sy, at temperature, and the allowable increase in shear stress for bolts should not exceed the lesser value of 0.42 Su or 0.6 Sy, at temperature.
For the linear-elastic-analysis method, F-1334 permits an increase of tension limits for the Level D service limits by a variable factor that is the lesser of 2 or 1.167Su/Sy if Su >1.2Sy, or 1.4 if Su
- 1.2Sy. Depending on whether the section considered is a net section at pinholes in eyebars, pin-connected plates, or built-up structural members, Ft may assume the lesser value of 0.45Sy or 0.375Su (as recommended by this guide for a net section of pinholes, for example) or the lesser value of 0.6Sy or 0.5Su (for a net section without pinholes, for example).
DG-1168, Page 4 Design by Load Rating NF-3380 of Section III specifies the qualification of linear-type supports to Service Level A, B, and C limits, using load-rating criteria. F-1334.8 specifies the qualification of linear-type supports to Service Level D limits using load rating criteria. This guide provides additional guidance for determination of the Service Level D load rating.
Design by Experimental Stress Analysis Although II-1430 in Appendix II to Section III defines the test collapse load for the experimental stress analysis method, the various levels of service limits for experimental stress analysis are not delineated.
The method described in this guide remedies this deficiency.
Large Deformation The design of component supports is an integral part of the design of the system and its components.
A complete and consistent design is possible only when the interaction between the system, component, and component support is properly considered. When all three are evaluated on an elastic basis, the interaction is usually valid because individual deformations are small. However, if the design process uses plastic analysis methods, large deformations may occur that would result in substantially different stress distributions.
When component supports are designed for loadings associated with the faulted plant conditions, Appendix F to Section III of the ASME Code permits the use of plastic analysis methods in certain acceptable combinations for all three elements. The selection of these acceptable combinations assumes that component supports are more deformation sensitive (i.e., their deformation in general will have a large effect on the stress distribution in the system and its components). Because large deformations always affect the stress distribution, care should be exercised even when using the plastic analysis method in the methodology combination approved in Appendix F. This is especially important for identifying buckling or instability problems when the change of geometry should be considered to avoid erroneous results.
Function of Supported System In selecting the level of service limits for different loading combinations, the decision should take into account the function of the supported system. To ensure that systems will operate properly regardless of plant condition if their normal function is to prevent or mitigate the consequences of events associated with an emergency or faulted plant condition [e.g., the function of the emergency core cooling system (ECCS) during faulted plant conditions], use of the Level A or B service limits specified in Subsection NF of the ASME Code Section III (or other justifiable limits provided by the Code) is appropriate.
Because NF-3320 derived all equations from American Institute of Steel Construction (AISC) rules and many AISC compression equations have built-in constants based on mechanical properties of steel at room temperature, it would be imprudent to use these equations indiscriminately for all NF sections and the latest accepted version of ASME Code Cases N-71 and N-249 involving materials at all temperatures. For materials other than steel and/or for working temperatures substantially different from room temperature, these equations need to be rederived with the appropriate material properties.
DG-1168, Page 5 Deformation Limits Because component supports are deformation-sensitive load-bearing elements, satisfying the service limits of Section III will not automatically ensure their proper function. If specified by the Code design specification, deformation limits might be the controlling criterion. However, if a particular plant condition does not require the function of a component support, the stresses or loads resulting from the loading combinations under that plant condition do not need to satisfy the design limits for the plant condition.
Definitions Design Condition. The loading condition defined by NF-3112 of Section III of the ASME Boiler and Pressure Vessel Code.
Plant Conditions. Operating conditions of the plant categorized as normal, upset, emergency, and faulted plant conditions.
Normal Plant Conditions. Those operating conditions that occur in the course of system startup, operation, hot standby, refueling, and shutdown, with the exception of upset, emergency, or faulted plant conditions.
Upset Plant Conditions. Those deviations from the normal plant condition that have a high probability of occurrence.
Emergency Plant Conditions. Those operating conditions that have a low probability of occurrence.
Faulted Plant Conditions. Those operating conditions associated with postulated events of extremely low probability.
Service Limits. Stress limits for the design of component supports, defined by Subsection NF of Section III of the ASME Boiler and Pressure Vessel Code.
Levels of Service Limits. Four levels of service limits A, B, C, and D defined by Section III of the ASME Boiler and Pressure Vessel Code for the design of loadings associated with different plant conditions for components and component supports in nuclear power plants.
Operating-Basis Earthquake (OBE). Seismic event defined in Appendix A to 10 CFR Part 100, Reactor Site Criteria.
Safe-Shutdown Earthquake (SSE). Seismic event defined in Appendix A to 10 CFR Part 100.
Specified Seismic Events. Operating-basis earthquake (OBE) and safe-shutdown earthquake (SSE),
as defined above.
System Mechanical Loadings. The static and dynamic loadings developed by the system operating parametersincluding deadweight, pressure, and other external loadingsand effects resulting from constraints of free-end movements, but excluding effects resulting from thermal and peak stresses generated within the component support.
Ultimate Tensile Strength. Material property based on the engineering stress-strain relationship.
Critical Buckling Strength. The strength at which lateral displacements start to develop simultaneously with in-plane or axial deformation.
3 ASME Boiler and Pressure Vessel Code,Section III, Division I, 2001 Edition through the 2003 Addenda.
4 If the function of a component support is not required during a plant condition, satisfaction of the design limits of the support for that plant condition is not needed, provided excessive deflection or failure of the support will not result in the loss of function of any other safety-related system.
DG-1168, Page 6 C. REGULATORY POSITION The construction of ASME Code3 Class 1 linear-type component supports, excluding snubbers, which this guide does not address, should follow the rules of Subsection NF of Section III as supplemented by the following stipulations:4 1.
The classification of component supports should, as a minimum, be the same as that of the supported components.
2.
The ASME Code Level A and B service limits for component supports designed by linear-elastic analysis, which are related to Sy, should meet the appropriate stress limits of Subsection NF of Section III of the ASME Code, but should not exceed the limit specified when the value of 5/6 Su is substituted for Sy. Examples are shown below in Regulatory Positions 2a, 2b, and 2c:
a.
The tensile stress limit Ft for a net section, as specified in NF-3322.1(a)(1) of Section III, should be the lesser of two values, 0.6Sy or 0.5Su, at temperature. For net sections at pinholes in eyebars, pin-connected plates, or built-up structural members, Ft as specified in NF-3322.1(a)(2) should be the lesser of two values, 0.45Sy or 0.375Su, at temperature.
b.
The shear stress limit Fv for a gross section, as specified in NF-3322.1(b)(1) of Section III, should be the lesser of two values, 0.4Sy or 0.33Su, at temperature.
c.
The bending stress limit Fb resulting from tension and bending in structural members, as specified in NF-3320, should be (1) the lesser value of 0.66 Sy or 0.55 Su, at temperature, for compact sections, (2) the lesser value of 0.75 Sy or 0.63 Su, at temperature, for doubly symmetrical members with bending about the minor axis, and (3) the lesser value of 0.6 Sy or 0.5 Su, at temperature, for box-type flexural members and miscellaneous members.
Many of the limits and equations for compression strength specified in NF-3320 have built-in constants based on Youngs Modulus of 29,000 kips per square inch (Ksi).
For materials with Youngs Modulus at working temperatures substantially different from 29,000 Ksi, these constants need to be rederived with the appropriate Youngs Modulus unless the conservatism of using these constants as specified is demonstrated.
3.
Component supports designed by linear-elastic analysis may increase their Level A or B service limits according to the provisions of NF-3321.1(a) of Section III of the ASME Code.
F-1334 permits an increase of Level A or B service limits for Level D service limits by the lesser factor of 2 or 1.167Su/Sy if Su > 1.2Sy, or 1.4 if Su # 1.2Sy, where Sy and Su are component-support material properties at temperature.
However, all increases (i.e., those allowed by NF-3321.1(a) and F-1334) should always be subject to the limits in NF-3321.1(b). Material properties at temperature should be used to calculate the critical buckling strengths defined by NF-3321.1(b). As specified in F-1335, the allowable increase in tensile stress for bolts should not exceed the lesser value of 0.70 Su or Sy, at temperature, and the allowable increase in shear stress for bolts should not exceed the lesser value of 0.42 Su or 0.6 Sy, at temperature.
5 System mechanical loadings include all non-self-limiting loadings and the effects resulting from constraints of free-end displacements, but not the effects resulting from thermal or peak stresses generated within the component support.
6 Because component supports are deformation sensitive in the performance of their service requirements, satisfying these criteria does not ensure that their functional requirements will be fulfilled. Any deformation limits specified by the design specification may be controlling and should be satisfied.
7 Because the design of component supports is an integral part of the design of the system and the component, the designer should make sure that methods used for the analysis of the system, component, and component support are compatible. The designer of component supports should consider large deformations in the system or components.
DG-1168, Page 7 If the increased service limit for stress range by NF-3321.1(a) is more than 2Sy or Su, its limit should be the lesser of two values, 2Sy or Su, unless a shakedown analysis justifies it.
4.
The limits in Regulatory Positions 4a - 4d should apply to the design of component supports subjected to the combined loadings of system mechanical loadings5 associated with (1) either the ASME Code design condition or the normal or upset plant conditions, and (2) the vibratory motion of the OBE.5, 6 a.
Component supports designed by using the linear-elastic-analysis method should not exceed the stress limits of NF-3320 of Section III and Regulatory Position 2 (above).
b.
Component supports designed by using the load-rating method should not exceed the Service Level A or B load rating of NF-3382 of Section III.
c.
The lower bound test collapse load determined by NF-3340 and adjusted according to the provision of NF-3341.1(a) of Section III should not be less than that required to support a factored load equal to 1.7 times those of the Service Level A and B limits for component supports designed by the limit analysis method.
d.
Component supports designed by using the experimental stress analysis method should not exceed the test collapse load determined by II-1400 of Section III divided by 1.7.
5.
The limits in Regulatory Positions 5a - 5d should apply to the design of the component supports subjected to the system mechanical loadings associated with the emergency plant condition, except when the normal function of the supported system is to prevent or mitigate the consequences of events associated with the emergency plant condition (Regulatory Position 7 then applies).6, 7 a.
Component supports designed by using the linear-elastic-analysis method should not exceed the stress limits of NF-3320 and Regulatory Positions 2 and 3, increased according to the provisions of NF-3321.1(a) of Section III and Regulatory Position 3.
b.
Component supports designed by the load-rating method should not exceed the Service Level C load rating of NF-3382.2 of Section III.
c.
The lower bound test collapse load determined by NF-3340 and adjusted according to the provision of NF-3341.1(a) of Section III should not be less than that required to support a factored load equal to 1.3 times that of the Service Level C limit for component supports designed by the limit analysis method.
d.
Component supports designed by using the experimental stress analysis method should not exceed the test collapse load determined by II-1400 of Section III divided by 1.3.
DG-1168, Page 8 6.
The limits in Regulatory Positions 6a - 6d should apply to the design of component supports subjected to the combined loadings of (1) the system mechanical loadings associated with the normal plant condition, (2) the vibratory motion of the SSE, and (3) the dynamic system loadings associated with the faulted plant condition, except when the normal function of the supported system is to prevent or mitigate the consequences of events associated with the faulted plant condition (Regulatory Position 7 then applies).
a.
Component supports designed by using the linear-elastic-analysis method should not exceed the stress limits of NF-3320 of Section III and Regulatory Position 2 of this guide, increased according to the provisions of F-1334 of Section III and Regulatory Position 3.
b.
Component supports designed by using the load-rating method should not exceed the lesser value of TL x 2Fall/Su* or TL x 0.7 Su/Su*, where TL, Su, and Su* are defined in F-1332.7 of Section III and Fall is the allowable stress value defined in NF-3382.1.
c.
Component supports designed by using the limit analysis method should not exceed the lower bound test collapse load determined by NF-3340, adjusted according to the provision of F-1334.6(a).
d.
Component supports designed by using the experimental stress analysis method should not exceed the test collapse load determined by II-1400, adjusted according to the provision of F-1334.6(c).
7.
The limits in Regulatory Position 4 or other justifiable limits provided by the ASME Code should apply to the design of component supports in systems for which the normal function is to prevent or mitigate the consequences of events associated with an emergency or faulted plant condition. The design specification should define these limits, which will be stated in the preliminary and final safety analysis reports (PSAR, FSAR), so that the function of the supported system will be maintained when it is subjected to the loading combinations described in Regulatory Positions 5 and 6.
D. IMPLEMENTATION The purpose of this section is to provide information to applicants and licensees regarding the NRC staffs plans for using this draft regulatory guide. No backfitting is intended or approved in connection with its issuance.
The NRC has issued this draft guide to encourage public participation in its development.
Except in those cases in which an applicant or licensee proposes or has previously established an acceptable alternative method for complying with specified portions of the NRCs regulations, the methods to be described in the active guide will reflect public comments and will be used in evaluating (1) submittals in connection with applications for construction permits, standard plant design certifications, operating licenses, early site permits, and combined licenses; and (2) submittals from operating reactor licensees who voluntarily propose to initiate system modifications if there is a clear nexus between the proposed modifications and the subject for which guidance is provided herein.
DG-1168, Page 9 REGULATORY ANALYSIS 1.
Statement of the Problem The NRC issued Revision 1 of Regulatory Guide 1.124, Service Limits and Loading Combinations for Class 1 Linear-Type Component Supports, in January 1978 to document service limits and loading combinations for the design of Class 1 linear-type component supports in accordance with the requirements of Section III, Division I, Subsection NF of the ASME Boiler and Pressure Vessel Code, 1974 Edition, including the 1976 Winter Addenda.
The NRC staff updated draft Revision 2 of the regulatory guide to incorporate the revisions documented in the 2001 Edition through the 2003 Addenda of the ASME Code, as permitted by 10 CFR 50.55a(b)(1), revised on January 1, 2006. The staff also revised the draft revision of the regulatory guide to delete guidance supplanted by current Code requirements.
2.
Objective This regulatory action updates the NRCs guidance with respect to the design of Class 1 linear-type component supports. The revised regulatory guide will continue to provide guidance to applicants and licensees on the acceptable levels of service limits and appropriate loading combinations for the design of Class 1 linear-type component supports. Use of the regulatory guide as an adjunct to the ASME Code requirements will give applicants and licensees additional assurance that the design of Class 1 linear-type component supports is being implemented conservatively.
3.
Alternative Approaches The NRC staff considered the following alternative approaches to the problem of outdated guidance regarding the design of Class 1 linear-type component supports:
(1)
Do not revise Regulatory Guide 1.124.
(2)
Update Regulatory Guide 1.124.
3.1 Alternative 1: Do Not Revise Regulatory Guide 1.124 Under this alternative, the NRC would not revise this guidance, and licensees would continue to use the original version of this regulatory guide. This alternative is considered the baseline or no action alternative and, as such, involves no value/impact considerations.
3.2 Alternative 2: Update Regulatory Guide 1.124 Under this alternative, the NRC would update Regulatory Guide 1.124 to incorporate revisions documented in the newer edition and addenda of the ASME Code and to delete guidance supplanted by current Code requirements.
The benefit of this action is to provide additional assurance to applicants and licensees that the design of Class 1 linear-type component supports is being implemented conservatively.
The costs to the NRC would be the one-time, relatively small expense of issuing the revised regulatory guide, and applicants and licensees would incur little or no cost.
DG-1168, Page 10 4.
Conclusion On the basis of this regulatory analysis, the staff recommends that the NRC revise Regulatory Guide 1.124. The staff concludes that the proposed action will provide additional assurance to applicants and licensees that the design of Class 1 linear-type component supports is being implemented conservatively.
BACKFIT ANALYSIS This draft revision to the regulatory guide provides licensees and applicants with updated guidance that the NRC staff considers acceptable for the design of Class 1 linear-type component supports. The application of this regulatory guide is voluntary. Licensees may continue to use the original version of this regulatory guide if they so choose. No backfit, as defined in 10 CFR 50.109, Backfitting, is either intended or implied.