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Rev 2 to White Paper, Fastener Strength Analysis,Nuclear Safety Concern 93-11
	ML20082C388
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Site:	San Onofre
Issue date:	03/18/1995
From:	Oashu R, Ramsey M; SOUTHERN CALIFORNIA EDISON CO.
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{{#Wiki_filter:' A WHITE PAPER FASTENER STRENGTH ANALYSIS REVISIONjTWO NUCLEARLSAFETY: CONCERN 93-11 SAN ONOFRE NUCLEARLGENERATINGLSTATION? i. bR AD 00 61 P PDR

j '. i A WHITE PAPER FASTENER STRENGTH ANALYSIS Revision Two j l NUCLEAR SAFETY CONCERN 93-11 PREPARED BY: 0& 3-le-W Senior Root Cause Engineer [ ~ Michael B. Ramsey' Date Safety Engineering Nuclear Oversight Division CONCURRENCE: Sl 1 GL ' QC Dr. Riyad Qasiiu ' Date Engineering Supervisor Analysis Group Nuclear Engineering Design Organization This paper has been independently reviewed and results concurred with by Roger F. Reedy, of Reed. m ;,ciates. Mr. Reedy has been involved in the design and 3 construction of nuclear components using bolting since 1956, and is currently Chairman of the ASME Section III Subcommittee.

P l < t - I A WHITE PAPER l FASTENER STRENGTH ANALYSIS Revision Two NUCLEAR SAFETY CONCERN 93-II TABLE OF CONTENTS EXECUTIVE

SUMMARY

3 l l BACKGROUND...... 6 l NUCLEAR SAFETY CONCERN.. 7 l RETURN TO STOCK INSPECTIONS... 7 l NUCLEAR SAFETY CONCERN... .8 SPECIFIC BIN INSPECTIONS... 8 RESPONSE TO NUCLEAR S AFETY CONCERN.. 8 l WAREHOUSE SAMPLE INSPECTIONS.. 8 FASTENER STRENGTH ANALYSIS.. .. I 1 WORST CASE EXTERNAL THREADS.... 13 THREAD STRENGTH CALCULATIONS FOR EXTERNAL THREADS.... 14 ASME CODE SERVICE APPLICATIONS... 16 WORST OBSERVED INTERNAL THREADS.. 18 THREAD STRENGTH CALCULATIONS FOR INTERNAL THREADS.. 19 EVALUATION OF SERVICE APPLICATIONS.... 20 INDEPENDENT TESTING........ . 21 GENERIC APPLICABILITY OF RFf.3ULTS...... 22 MAXIMUM HYPOTHETICAL DEVIATION.. 22 POTENTIAL FOR FASTENER LOOSENING.. 23 VIBRATION... 23 MATERIAL RELAXATION.... . 24 MATERIAL FATIGUE.. 25 SYSTEM LEAKAGE MONITORING...... 25 FASTENER EXPERIENCE EVALUATION...... . 26 l l l 1

l 9 i l TABLE OF CONTENTS Continued l l TECHNICAL CONCLUSIONS.. 27 l ADDENDUM TO FASTENER STRENGTH ANALYSIS.. 28 l COMBINED-CASE FASTENER STRENGTH ANALYSIS.. . 29 l BACKGROUND . 29 l CALCULATION.,... 34 l l FASTENER STRENGTH

SUMMARY

46 l

SUMMARY

.48 l l ATTACHMENT 1.. .50 LICENSING POSITION.. .51 NRC POSITION.. 57 ADDENDUM TO ATTACHMENT 1.. 62 l l ASME Code Interpretation of Fastener Inspection Requirements.. 63 l l ATTACHMENT 2 Statistical Evaluation.. 66 ATTACHMENT 3 Reedy Associates Letter.. 94 ATTACHMENT 5 l Statistical Evaluation of NRC Inspection Data..... 99 [ l 1 l Revision Bars Indicate Changes Associated with Revision Two l l l 2

1 A WHITE PAPER FASTENER STRENGTH ANALYSIS Revision Two 4 NUCLEAR SAFETY CONCERN 93-11 J EXECUTIVE

SUMMARY

This paper has been prepared to address specific concerns about the ability of Safety-Related 3 fasteners to perform their function at the San Onofre Nuclear Generating Station. A recent upgrade in fastener dimensional inspection technology has revealed that previous inspection . methods may have allowed minor deviations from specified tolerances for a specific fastener thread characteristic, to pass receipt inspections. A'saaple of fasteners from Warehouse stock was inspected in response to this information. A number offasteners were found to pass the. l previous inspection methods, but were found to be vut-of-tolerance with the new inspection methods. Strength calculations were performed on the items with the greatest observed deviations from tolerances. Minor strength reductions were noted. Comparison of the reductions in thread strength to ASME Code design requirements were made and it was demonstrated that the strength reductions were on the order of 3 to 6%, with the Code strength margin being on the order of 200 to 300%. Independent Laboratory analysis and testing w'as performed which correlated closely with these calculated results. It was further calculated that thread attributes would have to be grossly out of tolerance (approximately 15 times the greatest observed deviation) for the threads to fail at installation. Threads this grossly out-of-tolerance would have failed previous inspection methods and would fail visual examination at receipt inspection o-4 during installation. This evaluation clearly demonstrates the significant strength margin inherent in ASME Code fasteners. In addition to the strength margin of the individual fastener, installation configurations j would require multiple fastener failures prior to joint failure. The nature of the design of fastener l preload requires the maximum fastener load to occur at installation, thereby identifying fasteners which may fail at the time ofinstallation instead ofin-service. If a fastener should loosen due to i loss of preload, leakage would occur beforejoint failure. This paper discusses the various leakage monitoring and corrective action systems in place at SONGS, including an effective Root Cause evaluation program. 1 In addition to the fastener strength analysis, this paper documents a review of nuclear industry experience with fasteners, No documented cases of fastener failure due to dimensional non-l 1 conformance were identified at SONGS or elsewhere in the industry. This information is consistent with NRC and EPRI findings summarized in Generic Letter 91-17, " Bolting i ' Degradation or Failure in Nuclear Power Plants", which closed Generic Safcty issue 29 on the same subject. i 7 4 3 ( 1

] i lb A White Paper-FastenerStrength Analysis, Revision Two ' i This paper does not attempt to encompass the current national controversy with respect to j System 21 vs System 22 dimensional acceptance of fasteners, nor is it intended to provide a d statistical dimensional characterization of all fasteners in the SONGS Warehouse. Nor does this paper analyze the effect of variation of all 14 characteristics of thread form, many of which are not measurable by even SONGS current improved inspection technology, but require implementation )- of System 23 level inspections. - What this paper does clearly demonstrate is that' current industry I practice provides adequate fastener strength and reliability for the safe operation'of nuclear power plants. The uncenainties revealed by advanced dimensional inspection technologies play an i important role for applications having relatively small safety factors (such as aerospace), but do not have a direct link to fastener failure in heavy industrial applications where strength margins l l are orders of magnitude higher. In conclusion, plant nuclear safety is not compromised by current j . industry fastener inspection practices. ) Revision Two of this paper has been prepared to include the analysis of data obtained during NRC l [ j inspection of fasteners at SONGS. Also included in this revision is an evaluation of a " combined. I j case" which calculates the effect on fastener strength for hypothetical conditions where both l t ] internal and externally threaded fasteners would exhibit the maximum (proportionally equivalent) l l j out-of-tolerance conditions from this study. The hypothetical nature of the combined-case l l ~ analysis is required since the maximum observed out-of-tolerance conditions for internal and l externally threaded fasteners in this study, occurred in items of different sizes. Combining them l l requires evaluating proportionally similar out-of-tolerance conditions for the same size items. l l The results of this combined-case evaluation revealed that when fastener ultimate tensile strength. l l l is not considered, the ultimate thread strength of the threaded connection would be reduced by l t 10.45% due to the combined out-of-tolerance pitch diameter conditions. Thread strength of this l combined-case was shown to exceed maximum fastener preload specified for ASME Section III, ( { Class 1 service by 240%, and Class 2 and 3 service by 337%. Therefore, it was concluded that l i there is no impact on the ability of the combined-case threaded fastener condition to withstand ' l maximum applied loadings associated with service conditions at SONGSc l l The NRC inspection of fasteners involved obtaining a sample ofitems from the SONGS l i Warehouse stock utilizing a classic random sampling method and the CGI Sampling Procedure. l j] 519 items from 44 material codes were inspected using the Johnson Indicating Gage process and l the Go/No-Go gages. The sample included some items which had been excluded from the White l j Paper sample including items of a small nominal diameter (less than 1/2") and also set screws. l Analysis of the inspection data for this sample revealed the following conclusions': l I I c i " Statistical Evaluation ofNRC Audit Data for SONGS Fastener Stock", Tetra i Engineering Group, INC. Report 94-SCE-005, Authors: Dr. Frank J. Berte', 3 David S. Moelling PE, and Fredrick C. Anderson PE. (Attachment 5) 4 4 e i ~,. - ,-.-r-v-. r ..,.,,.....,,_,.m_,_ r.,m_ p.,. .,m

A White Paper-FastenerStrength Analysis, Revision Two 1) The NRC data is consistent with the White Paper data. This confirms that the sampling l procedures performed for the White Paper data set were sufficiently random to allow valid l statistical results to be drawn. l l 2) The limits computed from the White Paper data bound the pitch diameter deviations l in the NRC Data set. l l l 3) The NRC data shows similar behavior in bin-to-bin and supplier-to-supplier variation as l the White Paper Data set. This confirms previous conclusions that there is no systematic l l supplier-related trends in thread deviations. l l 4) NRC data for smaller items not considered by the White Paper show a slightly wider range l of deviation than the larger items. Otherwise, they are consistent with observations for the l White Paper data. l l 5) Thread Strength Margins for the smaller items are adequate, even with the slightly larger l deviations from the ANSI B-1.1 thread limits. l l 6) Thread Strength Margins for the larger items are consistent with those computed for the l White Paper data. These demonstrate large margins to ASME Code service l requirements. l l The data and evaluations included in Revision Two of this paper support the previous conclusion l that plant nuclear safety is not compromised by current industry fastener inspection practices. l Additionally included in Revision 2 of this report is a recent ASME Code Interpretation which I provides clarification of fastener inspection requirements. The NRC has reviewed and concurred I with this Interpretation. l 5

A White Paper-FastenerStrength Analysis, Revision Two BACKGROUND ] In early 1993, the systematic dimensional overcheck of fastener threadform during Receipt Inspection at SONGS was upgraded from the use of Fixed Limit (Go/No-Go) Gages, to the Johnson Indicating Gage process which provides a direct digital readout of thread attributes for comparison with the appropriate criteria. The Johnson Gage system was found to provide improved inspection accuracy through the measurement of specific thread characteristics, and faster inspection times when compared with the Go/No-Go gages. Since its first issuance in 1924, the ANSI Standard for threaded fasteners, B-1.1, has provided dimensional criteria for standardized thread forms. The dimensions and tolerances have been presented to the 1/10,000 of an inch, and have remained constant over the years for the standard thread forms. Early dimensional inspection methodology.was unable to assure exact fastener. thread conformance to the fine tolerances stated in the standard. Modern fastener industry practice has included the usage ofindicating gages for the setup and maintenance of the manufacturing machining processes, with the fixed limit gages providing final dimensional acceptance. In 1978, framework for the use ofindicating gages for final dimensional acceptance I of threaded fasteners was provided with the issuance of ANSI B-1.3. This standard provided definition of System 21 inspection requirements (which are met with the Go/No-Go gaging), and i System 22 inspection requirements (which include the measurement ofPitch Diameter and can be met with an indicating gage system). Licensing requirements for the final dimensional inspection of safety related fasteners dedicated for use at SONGS are tabulated in Attachment 1. Briefly stated, all safety-related pressure boundary fasteners are required to meet System 21 requirements, except for class 3 fit bolting which must meet System 22 requirements. These are requirements for final dimensional inspection by manufacturers and are reflected in the respective Procurement Engineering Packages (PEPS). SONGS has elected to exceed these requirements through the use of Johnson indicating gages to overcheck the System 22 attribute of thread pitch diameter at Receipt Inspection. The usage of the Johnson Gage system at SONGS was initiated for a twofold purpose, improved inspection productivity and the trending of supplier performance. An independent report by Nuclear Oversight on the usage of the Johnson Gaging system in October 1993, stated the following advantages over the previous system ofinspection: 1 e Increased accuracy in measurement of thread attributes e Fasterinspection process e Minimum gage wear e Less frequent and easier calibration Direct readout of thread dimensions precludes dispute with suppliers over accuracy of e Go/No-Go gages i 6 l l

. ~. _ _ _ i i i A White Paper-FastenerStrength Analysis, Revision Two The Johnson Gage system at use in the Commercial Grade Item Laboratory for Quality Control Receipt Inspection is connected to a computer database into which results of each inspection are entered. The computer can then process the information and produce a Supplier Quality Index for each supplier. From this information, it can be determined if a specific supplier is providing ) consistent bolting product, which is a good indicator that the supplier is utilizing statistical j process controls. Analysis of the inspection data has allowed Procurement Engineering to recommend consistent high-quality suppliers over the less consistent suppliers. l 1 i The nuclear industry is governed by construction standards which routinely provide a margin in j the range of 200 to 300% of ultimate strength in bolting materials over the maximum operating service load. Nuclear industry oversight organizations and interest groups routinely provide information on industry equipment failure and trend perfonnance of specific categories of j components, such as fasteners. Most recently, the NRC issued Generic Letter 91-17, " Bolting i Degradation or Failure in Nuclear Power Plants", which closed Generic Safety Issue 29, on the 1 same subject, based on studies conducted by EPRI, MPC and AIF which analyzed many fastener ) i failures. In closing this issue, the NRC found no evidence to indicate that failures were directly i attributable to dimensionally nonconforming fasteners. Thus, careful evaluation of cumulated 1 i nuclear operating experience has shown that no safety issues exist with current industry fastener inspection practices. 1 j NUCLEAR SAFETY CONCERN i Return To Stock Inspections In December of 1993, the inspection of threaded fasteners with the Johnson Gage system was j expanded from an overcheck during Quality Control Receipt Inspection, to the inspection of fasteners which had previously been examined and accepted with Go/No-Go gages. 1 Bolting which had been issued for plant use, but not used, was subjected to restocking inspection with the Johnson Gage equipment. Inspection revealed several fasteners with j dimensionally out-of-tolerance conditions. Two Warehouse Nonconformance Reports (WNCR) were written on 12/8 and 12/13/93 to document the inspection results. One l WNCR documented the failure of threaded rod to pass the Go/No-Go inspection. The l potential existed that out-of-tolerance fasteners had been installed in the plant. 2 1 NOTE: Subsequent review of work documents revealed that the subject items failing the 4 Go/No-Go inspection had not been installed in safety-related systems, but had 2 been used as construction and maintenance aids, and on non-safety related i equipment. No plant NCRs were required to be written. i a 7 4 i

A White Paper-Fastener Strength Analysis, Revision Two Initiation of Nuclear Safety Concern A Nuclear Safety Concern was issued on 12/27/93 stating the potential for non-conforming fasteners to exist in the warehouse. Specific Bin Insoections Ir, response to specific information from the Submitter, samples of eight warehouse stock bins w ere inspected with Johnson Gage instruments, revealing dimensionally out-of-tolerance fasteners in tivee of the bins. WNCRs were written to document the observed conditions. Response to Nuclear Safety Concern The identification of these out-of-tolerance conditions was not unanticipated due to the nature of the two different (fixed limit and indicating) gaging systems. It was decided that a logical response to the Nuclear Safety Concern would include obtaining some examples of fasteners which had failed the System 22 inspection, but passed the System 21 test. The items falling in this category could then be analyzed for impact on thread strength due to the out-of-tolerance thread conditions and this reduced thread strength could be compared to the ASME Code requirements for thread strength. WAREHOUSE SAMPl,E INSPECTION In order to obtain specific engineering data to address the Nuclear Safety Concern, it was decided I to obtain additional examples ofitems from Warehouse stock which would pass the System 21 inspection method of Go/No-Go gaging, but indicate out-of-tolerance when examined with the Johnson Gages. From the entire Warehouse population of fasteners, sample Material Codes were identified; and from these, sample fasteners were chosen for inspection. \\ A Sample Plan was devised which utilized the existing Receipt Inspection sampling e procedure to the greatest extent practical. A computer listing of all fastener Material Codes, including item descriptions, was obtained from responsible Warehouse personnel. Obvious items which were not subject to the concern were eliminated from the listing (washers, metal screws, set screws). To further defme the test sample, bolting smaller than 1/2 inch diameter was omitted. Additionally, items which had been previously examined with Johnson Gaging were eliminated from the listing. This left 286 fastener i Material Codes subject to sample inspection. From Table 2 of Procedure S0123-XXXII-2.5, " Sampling Program for Assessing, e Es'.imating, and Reporting Commercial Grade Item Quality", a sample size of 32 Material Codes was chosen from Table 2. It should be noted that 32 is the maximum sample size specified in the procedure. Given that a sample of 32 Material Codes needed to be chosen, from the 286, it was decided that a systematic sampling plan would be utilized in which every ninth Material Code from the listing would be chosen and would provide an 8

\\ i l A White Paper-FastenerStrength Analysis, Revision Two adequate assurance of randomness. The fasteners had been sorted by item name alphabetically. Thus, every ninth Material Code would provide various samples of each item type. Systematic sampling plans for this type application, have been shown to be essentially equivalent to simple random sampling. If a Material Code item proved 2 uninspectable due to unavailability ofJohnson Gage segments, the next Material Code, of the 286, would be chosen. i The bins associated with the 32 Material Codes were provided in their entirety to e Receiving Quality ControlInspectors. Fasteners were obtained from each Material Code in accordance with procedure SO123-XXXII-2.5, which specified a sample size, based on the total number of fasteners in each Material Code, and assured randomness in the - sample selection. Samples were then examined with the Johnson Gages. j Sample inspections were performed in accordance with routine requirements for fastener e j Receipt Inspection. The Johnson Gage thread attribute of Functional Diameter, and the j ANSI B-1.1 characteristic of thread Pitch Diameter were measured for each sample j fastener. Thread Functional Diameter is a measurement specific to the Johnson Gage j system and provides a measure to verify that thread conditions are between the maximum and minimum material condition requirements. Thread Pitch Diameter is measured by - l l point contact on essentially one thread at a time. Thread Pitch Diameter is the key 1 element in the calculation of thread shear area, which is discussed later in this paper, and ] relates directly to the shear strength of threads. The measurement of Pitch Diameter is i consistent with the requirements of System 22 as specified in ANSI B-1.3. i i o Sample Data was obtained and evaluated. The 32 sample Material Codes contained 1542 fasteners, of which 356 were inspected using the Johnson Gaging system. A total of 96 fasteners were found to be out-of-tolerance when compared to the requirements of 7 ANSI B-1.1. This represents a 27% rate of fasteners which indicated out-of-tolerance. j The out-of-tolerance items were also checked with Go/No-Go gages. All but one of the out-of-tolerance items were found to be acceptable when checked with Go/No-Go gages. l Many of the out-of-tolerance readings were only out by a few 1/10,000ths of an inch, and i several were reported by the responsible inspector to be slightly out of round, with the worst deviation being the recorded value. Also noted from the review of the data were 4 repeated readings of the same fastener which included differences which were not } averaged, the largest out-of-tolerance reading was recorded and subsequently used in this analysis. The statistical analysis included as Attachment 2 provides numerical distributions of the pitch diameter data from this sample inspection. a l Sampling Techniques, Third Edition, W.G. Cochran, Wiley, New York,1977 2 i 9 t a m r

A White Paper-FastenerStrength Analysis, Revision Two 3 Independent dimensional analysis of a sample of allthread fasteners was performed for e comparison with data obtained during the sample inspection. The methodology and results of this testing are documented in Safety Engineering Failure Analysis Report _ FAR-94-005. The independent dimensional data, obtained using the Three Wire method and supermicrometer, was compared to the readings obtained using the Johnson Gages at the SONGS CGI laboratory. The majority of the readings showed differences between the Johnson Gage and Three Wire Method to vary between.1 and.5 mil (mil = 1/1,000") i with the two greatest deviations being 1 and t.5 mil. The differences in the readings could be attributed to the nature of the inspection teNaues, which c asure essentially one portion of one thread, and physical differences betwech ;,indual threads. { 4 The results of the Warehouse Sample Inspection revealed that'over 99% of the inspected l fasteners passed the System 21 requirements. This level of acceptance is the best that can be t 3 expected given the statistical nature of receipt inspection techniques. The fact that only one item failed inspection, of all inspected, validates previous receipt inspection processes as effective in meeting the previous inspection requirements. 1 It should be noted that a subsequent evaluation by an independent statistical analysis firm has shown the systematic sample taken in this evaluation to be essentially equivalent to a simple random sample. The data analyzed in this White Paper was shown to be conservative when compared with the 95% / 95% confidence / probability bounds of a much larger population of thread dimensional values compiled during receipt inspection :rver the period of 8 7-93 to 7-94. i i 't 1 4 J l 1 l ' Statistical Evaluation of SCE Fastener Strength Analysis White Paper Data Base, t Tetra Engineering Group, Dr. Frank Berte', Dr. Peter S. Jackson, David S. Moelling PE, August 5,1994 (Attachment 1) I 10 4 d 1 J 9 .y m ,-e +w-.-,,.- .4e me.-

i. A White Paper-Fastener Strength Analysis, Revision Two FASTENER STRENGTH ANALYSIS The variations in Pitch Diameter will be analyzed for their impact on thread strength, and this will be compared to the ASME Code margins inherent in threaded fastener design. The internally threaded fasteners (nuts) and externally threaded fasteners e bolts / studs /allthread) from the above Warehouse sample with the greatest deviations from the pitch diameter tolerances specified in ANSI B-1.1, were designated as Worst Case examples for the purposes of this evaluation. This does not guarantee that these items have greater deviations than any fastener which may exist in the Warehouse, due to the statistical nature of sample inspections. These Worst Case fasteners have been shown to contain dimensionally out-of-tolerance conditions which are rejectable by System 22 inspection, and will be analyzed in this report for the purpose of responding to the specific Nuclear Safety Concern. i e These Worst Case conditions were analyzed for the amount of reduction in thread strength which would result from the measured out-of-tolerance conditions. The readings for fastener Pitch Diameter were significant because Pitch Diameter relates directly to thread shear area. Fastener strength is affected by the thread shear area and the shear strength of the fastener material. Therefore, a reduction in fastener Pitch Diameter could affect the strength capacity of the threadedjoint. While it is recognized that variation of other thread attributes, such as thread angle, taper, e lead and helical deviation may also have an impact on thread strength, these attributes are not subject to current inspection methods and their effect is not evaluated in this report. 5 4 d i l l 11 i

i 'A White Paper-Fastener Strength Analvsis, Revision Two The following formulas were utilized in the development of the Fastener Strength Analysis *: l Shear Strength Of External Threads = 0.5S * (Ass) i 7 Shear Strength Of Internal Threads = 0.5S *(Ass) r l Where: l Sr = Ultimate Tensile Strength of Fastener Material l Asu = Minimum Thread Shear Area for Internal Threads l Ass = Minimum Thread Shear Area for External Threads 1 Agg = x (1/P) *L *D1g d (1/P) + 0.57735 (D2s - D1gg)] g 2 1 Ag= x(1/P)*L *DM (1/P) + 0.57735 (Dg-D2xg)] g 2 Where: x = 3.14159 ~ 1/P = Number ofThreads per Inch I, = Length of Engagement D. = Minimum Major Diameter of External Thread Da = Maximum Minor Diameter ofInternal Thread De = Maximum. Pitch Diameter ofInternal Thread D, = Minimum Pitch Diameter ofExternal Thread 2 4 - ANSI B-1.1, Unified Inch Screw Threads.1989 Edition Introduction to the Design and Behavior of Bolted Joints. John H. Bickford, Marcel Dekker, Inc.1990. Analysis and Design of Threaded Assemblie_s, E.M. Alexander, SAE,1977 12

1 A White Paper-FastenerStrength Analysis, Revision Two Worst Case External Threads: 1 Item

Description:

All Thread Stud. 5/8"dia-11 (UNC) by 36" length Material Code: 305-05606 RSO#: 2063-93 Supplier: NOVA Inc. Material Specification: ASME SA193 Grade B7 Heat Code: 8099572 Thread Functional Size Inspection was performed Satisfactorily Thread Pitch Diameter Inspection was shown to be Out-of-Tolerance. Pitch Diameter Reading: 0.5554 inches Pitch Diameter Range: 0.5644 (max) to 0.5589 (min)laches 5 Amount Out-of-Tolerance: 0.0035 inches Ultimate Material Strength (Sr) =.125.000 psi (min)' Length of Thread Engagement (Lg) = 1 Diameter = 0.625 inches Minimum Minor Diameter ofInternal Thread = 0.5270 inches 5 Maximum Minor Diameter ofInternal Thread = (Dim) = 0.5460 inches 5 5 l

l 5

ANSI B-1.1.1989 Edition. Table 3 A. Class _2Afit ' ASME Section III,1977 Edition, Appendix I 13

A White Paper-FastenerStrength Analysis, Revision Two THREAD STRENGTII CALCULATIONS FOR WORST CASE EXTERNAL TIIREAD CONDITION, UTILIZING THE ABOVE EOUATIONS AND DATA: A, = x (1/P)

L
Ulm[2(1/P) + 0.57735 (D2m - D1g)]

g E i CASE A: Maximum Material Conditions for both Internal and External Threads Pitch Diameter = 0.5644" (max) D1m = 0.5270" (min) Ass = 3.14159

11 *.625 *.527 * [1/(2*11) +.57735(.5644.5270)]

0 7632 square inches Thread Ultimate Shear Strength

0.5

0.7632
125,000 = 47.700 counds (min)

CASE B: Minimum Material Conditions for both Internal and External Threads (ANSI B-1.1) Pitch Diameter = 0.5589" (min) Dim = 0.5460" (max) Ass = 3.14159

11 *.625 *.546 * [1/(2*11) +.57735(.5589.5460)]

0.6239 square inches Thread Ultimate Shear Strength

0.5

0.6239
125,000 = 38.992 nounds (min) i 14

A White Paper-FastenerStrength Analysis, Revision Two CASE C: Out-of-Tolerance Conditions from Worst Case Data Pitch Diameter = 0.5554" (actual) D1m = 0.5460" (max) Ass = 3.14159

11 *.625 *.546 * [l/(2*11) +.57735(.5554.5460)]

= 0.6000 square inches Thread Ultimate Shear Strength - 1 0.5

0.6000
125,000 = 37.507_p_qunds (min) i It should be noted that the Worst Case External Thread condition represents only a 3.12%

reduction in thread strength from the "H" case above, which represents the industry standard calculation of thread strength based on the requirements of ANSI B-1.1. An ideal thread condition calculation is presented in case "A" ebove which assumes that both the internal and external threads are at the maximum material limit. Comparing "A" and "B" above provide the i strength reduction over the band from the maximum tolerance to minimum tolerance numbers for both the internal and external threads, indicating a 18.26% reduction in strength across this band. The Worst Case External Thread conditions are shown to have thread strength 21.38% l (i.e.18.26% across the tolerance band plus 3.12% due to out-of-tolerance) lower than the maximum material contact thread condition from case "A". EXTERNAL THREAD CALCULATION

SUMMARY

CONDITIONS DIFFERENCE IN THREAD STRENGTH Case A, Maximum Material contact both Int and Ext } }8.26% Case B, Minimum Matclial contact both Int and Ext } 3.12% Case C, Worst Case Out-of-Tolerance 15 i

A White Paper-FastenerStrength Analysis, Revision Two ASME CODE SERVICE APPLICATIONS: The Worst Case out-of-tolerance thread conditions are shown to create reduced thread shear areas thereby reducing the shear strength of the threads. Maximum stress levels for bolting l materials are specified in Section III of the ASME Code. Design stress levels are required to be lower than these maximum levels for each specified material. Shear stresses increase across the reduced shear areas when design loading is applied to a fastener with out-of-tolerance threads. l The Worst Case out-of-tolerance condition must be evaluated to detennine if the increased shear stresses created by the reduced thread shear area falls within the maximum allowed by the ASME I Code. For the Worst Case out-of-tolerance external thread conditions: From ASME Section III, Appendix I, Table I-1.3 (Class 1) and j Table I-7.3 (Class 2 & 3) for SA193 B7 Bolting : 7 l Design Stress Intensity Value = Sm = 3j! Ksi for Class 1 applications Allowable Stress Value S = ;!S Ksi for Class 2 and 3 applications = From ASME Section III, NB 3230 states that stresses for design conditions be limited to Sm. However, service conditions including preload in bolts may be higher than Sm but that average l stresses shall not exceed two times Sm listed in Table I-1.3. These requirements may also be l applied to Class 2 and 3 bolting' Applied to the Worst Case 5/8"-l1 Stud, the maximum allowable preload would be: ASME Section III Class 1: 2

35 Ksi
0.226 square inches tensile stress area = 15.820 pounds l

ASME Section III Class 2 and 3: l 2

25 Ksi
0.226 square inches tensile stress area = 1L390 pounds l

l When compared to the ultimate Thread Strength for the Worst Case out-of-tolerance conditions calculated above (37,502 pounds), the thread strength is shown to have a margin of 2.37 to one above the maximum Code allowable preload. l Values specified are for 100 F and represent most conservative values 7 8 ASME Section III; NB-3222 and NB 3234. 16

l i f l A White Paper-Fastener Strength Analysis, Revision Two l l When the maximum Code allowable load under Design conditions is applied to the reduced thread shcar area which was calculated for the Worst Case out-of-tolerance conditions: AShE Section III Class 1: Maximum allowable shear stress per ASME Code (NB-3227.2)is 0.6 Sm: 0.6Sm = 0.6

26,800 psi = 16.080 pJi @ 700 degrees F l

Maximum Design Load is limited to Sm times tensile stress area: 35 ksi

0.226 square inches tensile area = 7910 pounds Shear stress is the Maximum Design Load divided by the shear area:

7910 / 0.600 square inches shear area =.13.173 osi Therefore, the calculated shear stress !s less than the maximum allowable shear stress specified in ASME Section III. Similarly for ASME Section III Class 2 and 3: Maximum allowable shear stress per ASME Code (NC-3216.3(b))is.0.6 S: i 0.6 S = 0.6

25,000 psi = 15.000 osi @ 700 degrees F 25 ksi *.226 square inches tensile area = 5656 pounds Max Design Load Shear Stress = 5650 pounds / 0.600 square inches = 9417 psi Therefore, the calculated shear stress is less than the maximum allowable shear stress specified in ASME Section III.

I i 17 j l

A White Paper-Fastener Strength Analysis, Revision Two Worst Case Internal Threads: Item

Description:

Heavy Hex Nut 1/2" - 13 Threads per Inch (UNC) Material Code: 305-04211 RSO#: 2583-92 Supplier: NOVA / Texas Bolt Material Specification: ASME SA194 Grade 2H Heat Code: 1D3716 Thread Functional Size Inspection was shown to be Out-of-Tolerance: Functional Size Reading: 0.4572 inches Functional Size Range: 0.4565 (max) to 0.4500 (min) inches Amount Out-of-Tolerance: 0.0007 inches Thread Pitch Diameter Inspection was shown to be Out-of-Tolerance: Pitch Diameter Reading: 0.4642 inches Pitch Diameter Range: 0.4565 (max) to 0.4500Imin)_mches' Amount Out-of-Tolerance: 0.0077 inches Material Proof Strength (S) = 175.000 osi(min)(ASME Section II) Length of Thread Engagement (Id = 1 Diameter = 0.500 inches j Minimum Major Diameter of External Thread = D = 0.4876 inches' Maximum Major Diameter of External Thread = 0.4985 inches' ' ANSI B-1.1.1989 Edition, Table 3A. Class _2B_ fit 18

f 4 A White Paper-FastenerStrength Analysis Revision Two THREAD STRENGTH CALCULATIONS FOR WORST CASE INTERNAL TIIREAD CONDITION, UTILIZING THE ABOVE EOUATIONS AND DATA: I Ag= x(1/P)*L *DM (1/P) g + 0.57735 (Dg-D2g)] 2 l CASE A: Maximum Material Conditions for both Internal and External Threads Pitch Diameter = 0 4500" (min) D = 0.4985* (max) Ass = 3.14159

13 *.500 *.4985 * [1/(2*13) +.57735(.4985.4500)]

= 0.6765 sauare inches Thread Proof Strength = 0.5

0.6765
175,000 = 59.198 counds (min)

CASE B: Minimum Material Conditions for both Internal and External Threads (ANSI B-1.1) Pitch Diameter = 0.4565" (max) D = 0.4876"(min) Asx = 3.14159

13 *.500 *.4876 * [l/(2*13) +.57735(.4876.4565)]

= 0.5618 square inches Thread Proof Strength = 0.5

0.5618
175,000 = 49.158 pounds (min)

CASE C: Out-of-Tolerance Condidons from Worst Case Data Pitch Diameter = 0.4642" (actual) D = 0.4876" (min) Ass = 3.14159

13 *.500 *.4876 * [1/(2*13) +.57735(.4876.4642)]

= 0.5174 square inches Thread Proof Strength = 0.5

0.5174
175,000 = 45.270 pounds (min)

It should be noted that the Worst Case Internal Thread conditions represent only a 6.5% reduction in thread strength from the "B" case above, which represents the calculation of thread 19 i l

A White Paper-FastenerStrength Analysis, Revision Two strength based on the requirements of ANSI B-1.1. An ideal thread calculation is presented in case "A" above which assumes that both the internal and external threads are at the maximum material limit. The Worst Case Internal Thread conditions are shown to have thread strength 23.5% (i.e.17% due to tolerance band and 6.5% due to out-of-tolerance condition) lower than the ideal thread condition from case "A". INTERNAL THREAD CALCULATION

SUMMARY

l CONDITIONS DIFFERENCE IN THREAD STRENGTH - i Case A, Maximum Material contact both Int and Ext } 17% Case B, Minimum Material contact both Int and Ext } 6.5% l Case C, Worst Case Out-of-Tolerance t EVALUATION OF SERVICE APPLICATIONS: Inherent in the design of threaded fasteners is a strength bias favoring the internally threaded components. The thread stripping areas for internal threads are 1.3 to 1.5 times those for external threads. A typical bolted joint will fail in tension at the root of the external threads. The reduced thread shear area calculated above for the Worst Case out-of-tolerance internal threads would still l exceed the maximum material shear area for corresponding external threads. Worst Case Internal Thread Shear Area = 0.5174 square inches i Maximum Material External Thread Shear Area = 0.4150 square inches Therefore, even with the reduced shear area resulting from out-of-tolerance conditions, the Worst Case out-of-tolerance nut would still be stronger than the corresponding bolt or stud at the maximum material (max P.D.) conditions. t 20 1 i 1

A White Paper-FastenerStrength Analysis, Revision Two INDEPENDENT TESTING Independent dimensional analysis and mechanical testing of a sample of allthread fasteners were performed for comparison with results obtained during the investigation of Nuclear Safety Concern (NSC) 93-11. The methodology and results of this testing are documented in Safety Engineering Failure Analysis Repon FAR-94-005. The independent dimensional data was compared to the readings obtained using the Johnson Gages at the SONGS CGI laboratory. Additionally, mechanical testing of the samples was performed to verify the fastener material mechanical properties and thread stripping strength for comparison with calculated thread strength values, discussed in the White Paper associated with this NSC. In summary, the following conclusions were demonstrated: The independent dimensional examinations generally correlated well with the e Johnson Gage data e The material physical test data corresponded well with that contained on the supplier Certified Material Test Reports The tensile test samples failed at the thread minor diameter cross section at a load very e close to calculated values e The thread stripping strength pull test results were consistent with the calculated thread strength values for the known out-of-tolerance pitch diameter data of the samples e The thread strength, even for out-of-tolerance conditions, was shown to be inherently larger (1.3 to 1.5 Times) than the tensile load bearing capability of the externally threaded fasteners Even though the tested samples were shown to be out-of-tolerance with respect to pitch e diameter, they developed thread strength well in excess of ASME Code requirements The fasteners which were the subject of this test included sample pieces which represent the Worst Case out-of-tolerance example observed during sampling of Warehouse fastener stock. 21

A White Paper. Fastener Strength Analysis, Revision Two GENERIC APPLICABILITY OF RESULTS Calculations have shown that the Worst Case out-of-tolerance conditions can be generalized across all sizes ofinternal and external threaded fasteners. Expressing the out-of-tolerance 4 conditions as a percentage of nominal fastener diameter will produce a relatively constant percentage reduction in thread strength, for an equivalent Worst Case condition, across all sizes of fasteners. Analysis of the most severe service conditions at SONGS was reviewed. High j Temperature ASME Section III Class I service requirements were researched and fastener loading for these applications were found to be below the applicable ASME Code allowances. I Given that the fastener strength reduction calculated from the Worst Case out-of-tolerance conditions (above) left the thread strength with a 2.37 to 1 margin over ASME Code preload i stress allowables, it can be concluded that the Worst Case conditions observed during the sample of Warehouse stock, generalized across all sizes and materials, contain adequate strength margin i for service in the most severe service conditions at SONGS. MAXIMUM HYPOTHETICAL DEVIATION i In an effort to demonstrate the maximum hypothetical impact on fastener strength that could result from out-of-tolerance pitch diameter conditions, calculations were performed to determine 1 the conditions necessary for the threads to actually strip when the maximum Code allowed 4 preload was applied to the fastener. In other words, these are the reduced-thread conditions l where the threads would strip during installation. For comparison purposes, these calculations ] were performed for a fastener of similar nominal size and material as that identified as the Worst Case externally threaded fastener from the Warehouse sample and for the.most severe (ASME Class 1) service conditions. To restate the measured conditions for this 5/8"-11 allthread: l Maximum Pitch Diameter = 0.5644" (ANSI B-1.1) Minimum Pitch Diameter = 0.5589" (ANSI B-1.1) i Measured Pitch Diameter = 0.5554" (Warehouse Sample) j i 0.0035" Out-Of-Tolerance i For Thread Strength Equal to Maximum Design Load Condition: Calculated Pitch Diameter 0.5044" (ASME Class 1) = 0.0545" Out-Of-Tolerance i This out-of-tolerance condition represents a factor 15.6 times worse than the measured Worst Case from this Warehouse sample. A fastener this grossly out-of-tolerance condition would easily 1 fail visual inspection, either at Receipt Inspection, or by the craft at installation; and would l certainly not pass System 21 Go/No-Go inspection. 22 c.

4 4 A White Paper-FastenerStrength Analysis, Revision Two ^ POTENTIAL FOR FASTENER LOOSENING ] Vibrating environments, material relaxation and material fatigue are conditions which may result in the loosening and failure of fasteners. The potential for vibration and material relaxation and fatigue of bolted connections is routinely assessed in the design of equipment and components for operating senice conditions. The following discussion assesses the affect of the measured out-of-tolerance conditions on vibration, material relaxation and fatigue. VIBRATION Vibration has been shown, under certain conditions, to cause the loosening of threaded fasteners. The vibration can overcome the friction forces which act between the faces of the mating interface of a bolted joint and also the friction forces at the face of the nut and/or bolt. If these friction forces, which can be typically 80 - 90% of the torque loading, are overcome or negated by the effects ofvibration, the energy stored in the fastener will be released and the bolt will return to its original length with the inclined plane of the bolt threads pushing the inclined plane of the nut threads out of the way. Vibration is theorized to negate the friction forces by creating a rapid series of small relative motions between the thread mating faces in a direction perpendicular to the friction forces. Vibration loosening is agreed to occur more commonly in fasteners loaded in shear, especially those with vibration forces acting perpendicular to the axis of the fastener. Fasteners loaded in tension are therefore, less susceptible to the effects of vibration. After loss of sufficient preload in a bolted joint susceptible to vibration, the friction forces will be reduced sufficiently to allow the nut to back off. l l Higher initial preload can mitigate the effects of vibration. A higher preload will increase the friction forces between the thread faces making them less susceptible to the small relative motions which negate the friction forces. In many cases, a sufficiently high preload can create a completely vibration-resistant boltedjoint. In other cases, more direct physical means must be - considered to prevent relative motion between the nut and bolt: Utilize locking devices or other form of action to prevent relative motion between the nut e and bolt. Mechanically prevent slippage between bolted joint surfaces loaded in shear to prevent slip e between bolt andjoint surfaces. Utilize fine thread bolting to reduce the helix angle of threads and thereby reduce the e back-off torque caused by the preload. The consideration of vibration as a potential cause of fastener loosening is performed during the design ofindividual components for SONGS. Vendor manuals routinely specify the torque values for bolting, and these values are incorporated into maintenance procedures. These values 23 i I l

i 4 A White Paper-FastenerStrength Analysis, Revision Two conform to those specified by ASME Code for bolt preloads and have been evaluated by the vendor as satisfactory for the service application. The adequacy of these values is also verified by hydrotest of the component and system, which is usually conducted at 1.5 times the Design pressure. If fastener loosening, as evidenced by leakingjoints, is discovered during operation or maintenance, programs are in place to require engineering evaluation of the conditions, and for the specification of corrective measures to prevent recurrence, Corrective measures may include the increase offastener preload or any of the options listed above, as long as they are appropriately documented on design documents and procedures are updated. In the consideration of the susceptibility of the Worst Case out-of-tolerance fastener condition identified during the Warehouse sample inspection to the effects of vibration, it should be recognized that the thread mating area would be slightly reduced from that ofin-tolerance fasteners, but the total clamping forces would remain the same. The friction forces would remain constant for a given preload irrespective of a minor reduction in thread mating area. It was shown above that even with the reduced shear area from the Worst Case out-of-tolerance condition, the threads maintained a significant margin of strength above Code maximum allowable stresses and would therefore, accommodate any preload increase required for specific system or component considerations. MATERIAL RELAXATION Short term relaxation can create a reduction in preload. The most common cause of short term relaxation is thread embedment. The loss ofpreload occurs when tiny high spots on thread surfaces are overcome by pressure from clamping forces. Plastic deformation of the high spots occurs until enough of the total thread surface is loaded to prevent further deformation. Embedment is more common on new parts than on used ones due to the smoothing of thread j surfaces that occurs as fasteners are torqued. Critical SONGS bolting applications require torquing of fasteners. Embedment loss ofpreload was identified through the SONGS Root Cause Program and a successful anti-embedment process was incorporated in applicable procedures as a corrective measure. Fasteners in the Worst Case out-of-tolerance condition would be less susceptible to embedment loss of preload due to the slightly reduced area of thread mating surfaces which would create higher contact forces during torquing. These slightly higher forces would reduce the effects of embedment, for a given torque force, by more effectively smoothing away the slight irregularities which cause the embedment. Long term relaxation can also create a reduction in preload. This creep or stress relaxation involves the slow shedding ofload by a fastener under constant deflection (strain). This process is encouraged by high temperatures. The effects of this relaxation vary for different materials and temperatures and must be considered during the original design of nuclear equipment and systems. The Worst Case out-of-tolerance condition identified in the Warehouse Sample Inspection would not create a condition to further any fasteners susceptibility to this phenomenon. The Worst Case minor reduction in thread shear area did not decrease the fasteners thread shear area below the 24

A White Paper-FastenerStrength Analysis, Revision Two point where thread shear strength would be less than fastener tensile strength. Therefore, the dominant effects of any relaxation would occur acroce the tensile area, or body, of the fastener. It should also be noted that the effects of creep and stress relaxation occur largely at temperatures above one half of the material melting temperature (Tm expressed in degrees Kelvin). The highest fastener material temperatures at SONGS are conservatively shown to be less than this value. Thus, the effects of material relaxation would not be exacerbated by the minor thread form deviations noted in the performance of this evaluation. MATERIAL FATIGUE The ASME Code provides requirements for the evaluation of the suitability of bolting and bolting materials for cyclic service, including stress limits and design fatigue curves. Minor reductions in thread shear area would have no impact on the fatigue failure of fasteners. Bolting stresses are concentrated at the root of the thread and any cracking propagating from fatigue, even ifinitiated in the thread material would be expected to propagate through the thread root and across the plane of the minimum tensile area. Samples of stock from lots containing the Worst Case internal and external threaded fasteners were dimensionally examined by independent laboratories. The tensile root stress area for the Worst Case fastener was found to be independent ofmeasured pitch diameter and major diameter readings. A sample of the Worst Case externally threaded stock was machined and destmetively tested to verify material properties stated on the supplier Certified Material Test Report. A tensile pull test of a section of this threaded stock was then performed, and tensile area was calculated from the results. The tensile area was found to be essentially unchanged from the design value and correlated well with the areas calculated from dimensional readings. This testing verified that the tensile areas of Worst Case fasteners remain j essentially unchanged and therefore the stresses within the fastener would be unaffected by the measured out-of-tolerance thread conditions exhibited during sample inspection. The Worst Case out-of-tolerance condition identified in the Warehouse Sample Inspection would not create a condition to further fastener susceptibility to fatigue failure. SYSTEM LEAKAGE MONITORING The above discussions have stated that minor out-of-tolerance conditions, of the sort that could be expected if a fastener were to pass System 21 inspection but fail System 22, would not create conditions to increase fastener susceptibility to loosening or failure. SONGS systems and programs have, however, been designed for early detection, control and the prevention of recurrence of any leakage. SONGS systems are continuously monitored for evidence ofleakage through routine operator rounds and monitoring of primary system inventory balance. Effective . programs exist to correct any identified leakage and ensure corrective actions. 1 25 . ~ -.

A White Paper-FastenerStrength Analysis, Revision Two FASTENER EXPERIENCE EVALUATION Reviews of both site information and industry experience were conducted to understand the causes of any reported fastener failures. Many cases of fastener failures have been analyzed both at SONGS and in the nuclear industry. A search of SONGS maintenance and nonconformance databases identified no conditions of bolting failure due to out-of-tolerance thr eadform. The SONGS Root Cause program has been effective in identifying causes and recommending corrective actions for bolting failures including the recommendation of alternate materials, alternate preload and the anti-embedment process. Review of SONGS Root Cause database and discussions with responsible personnel indicated that bolting failures had been analyzed, however, the causes of the failure were clearly determined to not be due to out-of-tolerance threadform. Fasteners were evaluated which had failed due to tie fcilowing causes: e Stress corrosion cracking of stud material e Losses of preload due to improper (soft) washer material o Corrosion of fasteners Overload and Overtorquing e Cold work induced material embrittlement Thread embedment - special case where fastener loadings and vibration were very high. e Anti-embedment torquing technique recommended by Root Cause Engineering has successfully addressed this condition. The SONGS Root Cause program is sensitive to the potential that threadform may contribute to fastener failure, and utilizes the thread analysis capability of the SONGS CGI lab to investigate any suspect conditions. i l Nuclear industry oversight organizations and interest groups routinely provide information on industry equipment failure and trend performance of specific categories of components, such as l fasteners. A review of databases and reports was conducted with respect to fastener failures. l Most recently, the NRC issued Generic Letter 91-17, " Bolting Degradation or Failure in Nuclear Power Plants", which closed Generic Safety Issue 29, on the same subject, based on studies conducted by EPRI, MPC and AIF which analyzed many fastener failures. In closing this issue, the NRC found no evidene to indicate that failures were directly attributable to dimensionally nonconforming fasteners. Thus, careful evaluation of cumulated nuclear operating experience has shown that no safety issues exist with current industry fastener inspection practices. { l 26

I j e i A White Paper-FastenerStrength Analysis, Revision Two j TECHNICAL CONCLUSIONS The inspection of a statistical sample of Warehouse fastener stock with Johnson Gage equipment 4 has identified dimensionally out-of-tolerance conditions in sample items which were shown to be ) 99% acceptable when inspected with the industry accepted standard of final thread dimensional I inspection, Go/No-Go gages. The out-of-tolerance conditions were analyzed for impact on ultimate thread strength. It was shown that when compared to the ultimate thread strength for the Worst (observed) Case out-of-tolerance conditions a minimum ma 3 n ofsafety of 2.37 to one i i i above the maximum fastener preload remains. Independent testing was perfor.ned which confirmed the calculated expectations on actual out-of-tolerance samples. It can also be concluded that the increased shear stress created by the reduced thread shear area of the Worst Case thread condition are less than the ASME Code stress allowables for the Section III Class 1, l 2 and 3 Design conditions. The effects ofvibration, material relaxation and fatigue were assessed to compare the susceptibility for the Worst Case out-of-tolerance conditions identifed during the j Warehouse Sample Inspection to facilitate fastener loosening or failure from these most common i recognized failure modes. No increase in susceptibility was found during this evaluation. It was also shown that the Worst Case conditions observed during the sample of Warehouse stock, if generalized across all sizes end materials, contain adequate strength margin for service in the most severe conditions at SONGS; and that adequate thread area remains to develop the fiill bolt load without thread stripping and will accommodate any preload adjustments required for specific service conditions. In an effort to demonstrate the maxirnum hypothetical impact on fastener strength that could result from out-of-tolerance pitch diameter conditions, calculations were performed to determine the conditions necessary for the threads to actually strip when the maximum Code allowed preload was applied to the fastener. This value was shown to be 15.6 times greater than the maximum measured deviation from ANSI B-1.1 tolerances observed for external thread in this Warehouse sample. Fasteners this grossly out-of-tolerance would easily fail visual inspection, either at Receipt Inspection or by the craft at installation, and would certainly not pass System 21 Go/No-Go inspection. I 27

A White Paper-FastenerStrength Analysis, Revision Two a I I I I I l ADDENDUM TO FASTENER STRENGTH ANALYSIS l l I I I J l COMBINED-CASE l THREAD STRENGTH ANALYSIS l 1 I I -l l i I I i i I 4 l I I i l I l I I I I I I I I I l 1 28 . ~.

l i~ A White Paper-FastenerStrength Analysis, Revision Two COMBINED-CASE FASTENER STRENGTH ANALYSIS l i I i l BACKGROUND l 4 l This evaluation calculates the combined effect on the strength of a threaded joint comprised of an l } externally threaded fastener, which has been shown to be out-of-tolerance with respect to pitch l diameter, and an internally threaded fastener which is also out-of-tolerance with respect to pitch l i j diameter. The values for pitch diameter utilized in this evaluation will correspond with the l i maximum observed deviations from specified values'." reported in warehouse sample inspections l f associated with this White Paper. The externally threaded fastener to be considered in this l j analysis will be the 5/8" all-thread stock described as " Worst Case" by System 22 inspection, in l [ the body of this report. The internally threaded nut to be considered in this analysis will be a l hypothetical 5/8" nut, with out-of-tolerance conditions proportionally adjusted from the l ) System 22 data for the " Worst Case" 1/2" nut described in the body of this report. Additional l detail has been added to the technical discussion of this evaluation to provide greater clarity to the [ fastener strength evaluation. l l l ] Thread Shear Strength l l Ultimate thread strength is compnsed of the amount of shear area contained in the shear plane l where the internal and external threads mate, and the respective fastener material strengths. l The thread shear area varies proportionally with thread pitch diameter. This is to say that: l I As the internal thread (nut) pitch diameter increases, l j and/or the bolt pitch diameter decreases, l l the nut threads engage less of the bolt threads, l l thus decreasing the thread shear area. l l l As the external thread (bolt) pitch diameter increases, l and/or the the nut pitch diameter decreases, l j the bolt threads will engage further into the nut threads, l l thus increasing the thread shear area. l l l I j The following figures demonstrate these prinicples: l [ l I d I i l l " ASME/ ANSI B-1.1,1989 Edition k 29 i

i e i i i A White Paper. FastenerStrength Analysis, Revision Two 1 l l l l NUT l ~ l - - sh. rw= CmAm L._\\ - _, _l l u_ j l i \\ l i mm1 l i BOLI l 1 l Mishn== Mated.: I 1 (Not to Scale) l [ l 4 l l NET i sheer rimw Caenace Area - l l I f sn,rn _ 3 l l 5 BOLI m ai. m t.ri.i l (Not to Scale) l l Sheatflanc_Widthlatteases With Deeper _IhreadIngagenwat l i l Both internal and external threads have respective maximum and minimum criteria for pitch l diameter. If the minimum value of external thread (bolt) pitch diameter is combined with the l maximum value ofinternal (nut) pitch diameter, the condition known as " minimum material" is l ] created. This means that the threads are engaged to the minimum amount allowed by the l 4 specifications, and create the minimum thread contact and shear areas for thread strength. l l 3 If the maximum value of external thread (bolt) pitch diameter is combined with the minimum l 1 value ofinternal thread (nut) pitch diameter, the condition known as " maximum material" is l created. This means that the threads are engaged to the maximum amount allowed by the l specifications, and create the maximum thread contact and shear areas for ultimate thread l 3 strength. l 4 30 i t

l A White Paper-FastenerStrength Analysis, Revision Two The maximum and minimum material thread conditions referenced above were analyzed in the l body of this report as Cases "A" and "B" of the thread strength calculations for both internal and l external threads. l l In the case to be analyzed in this example, the pitch diameter of the externally threaded item l (allthread) will be below the specified amount, and the pitch diameter of the internally threaded l nut will be above the specified value. This will create a condition ofless thread shear area than l { the minimum material condition. Calculations, from formulas utilized in the body of this report, l will assess the impact on this combined out-of-tolerance thread shear area and also the resulting l efrect on thread ultimate strength. l l I EVAIAJATION l J The determination of the strength of the threads of a threaded joint requires the calculation of the l location of the shear plane in the thread profile, and its width. l i I Fastener Failure Modes { i The shear plane is the location at which the threads would fail or strip when subjected to overload l forces, such as could occur due to overtorquing. The following photograph from Nuclear l j Oversight Failure Analysis Report (FAR) 94-05 demonstrates the nature of the sheai plane after l threads have been stripped during tensile testing: l t l l I I 1 I m I khh l I I I I l 31

d A White Paper-Fastener Strength Analysis, Revision Two Another mode of failure for a bolted joint is tensile failure at the cross-sectional area of the l externally threaded fastener when overloaded. This type of tensile failure is demonstrated by the l following photograph from Nuclear Oversight Failure Analysis Report (FAR) 94-05: l l l l i l 'A l I l l f l l -L L u u o 2ssa r. I i l I l )i I I Calculations and independent testing conducted by Nuclear Oversight and documented in Failure l Analysis Report 94-05, have shown that for an externally threaded fastener, the strength of the l threads, over a length of engagement equal to one nominal diameter, is greater than the tensile l strength ofits cross-sectional area. This strength ratio will be examined later in this combined-l 3 l case example. l l Additionally it can be demonstrated that an internally threaded fastener (nut) made from the same l l material as an externally threaded fastener (bolt / stud) will have 1.3 to 1.5 times" as much thread l shear area, and therefore greater thread strength than that of the bolt or stud (bolt will be used l here for convenience). This is due to the greater diameter of the location of the shear plane in the l ] nut threads over that of the bolt threads, and resulting greater shear area length throughout the l l circumferential length of thread engagement. The fastener design materials at SONG S typically l require the nut material strength to be equal to, or greater than the material strength of the bolt. l Therefore the limiting factor for the design strength of a bolted connection is the tensile strength l of the bolt material at its cross-sectional area, as demonstrated in the photograph above. l l 4 4 " "An Introduction To The Design And Behavior Of Bolted Joints", John H. Bickford, Second Edition 32

N I A White Paper-Fastener Strength Analysis, Revision Two Thread Shear Plane Area l The internal and external threads come in contact in the mating surface contact area over the l length of thread engagement. The graphic figures on Page 30 illustrate this description. The l shear plane of the bolt exists at the inside diameter of the mating surface contact area. The shear l plane of the nut exists at the outside diameter of the mating surface contact area. The greater l radius of the position of the shear plane for the nut, when rotated circumferentially throughout the l length of engagement of the mating surfaces, results in a longer shear plane than the bolt shear l plane (which has essentially the same width). l I This difference in length is between 1.3 to 1.5 times that of the bolt, depending on the width of l the mating surface contact area. The mating surfice contact area tends to be wider for conditions l of greater thread engagement. A wider contact are will result in a greater difference in internal j and external thread shear area length. It is the length of the shear plane, multiplied by its width l which gives the shear area. The shear area (measured in square incher), when factored into the l shear strength properties (measured in pounds per square inch) of the particular fastener material, l gives the maximum force (in pounds) that the thread can withstand prior to shearing or stripping. l Factors such as nut dilation and thread bending may affect the ultimate load capacity of a threaded l joint, however their effect is considered neglible when heavy hex nuts are specified and at loads l i2 less than the maximum preloads allowed by ASME Section III. l l Combined Case Thread Failure Location l From the above, it can be concluded that the greater length of shear plane inherent in a nut, results l in greater ultimate thread shear strength over the length of engagement between the nut and bolt. l 3 For an ideal case which excludes possible failure of the fastener at the tensile cross-section area, I the greater strength inherent in the threads of the nut would result in failure of the bolt threads l when the threaded joint is subjected to overload conditions. The failure of the bolt threads would l 4 occur at the shear plane on the inside diameter of the thread mating surface contact area, which is l the bolt (external thread) shear plane. This type of failure is demonstrated in the photograph on l Page 31. l l The fastener design materials at SONGS typically require the nut material strength to be equal I i2 to, or greater than, the material strength of the external fastener. For this case, the nut material l (SA 194-2H)is stronger than the allthread material (SA 193-B7). This nut material strength is l reflected in greater material strength at the nut shear plane than at the allthread shear plane. l This greater material strength at the nut shear plane further empasizes that the overload thread l failure of this bolted connection would occur at the shear plane of the bolt (external) threads l (excluding consideration of failure at the bolt tensile cross-section area). This assumption will be l verified through the calculation of nut thread strength for this combined-case example, and l comparison with the bolt thread strength. l 4 2 90004, Piping & Material Specification, San Onofre Units 2 and 3. 33 l

t A White Paper-FastenerStrength Analysis, Revision Two CALCULATIONS l l In order to determine the force necessary to cause overload failure of the threads, the thread shear l areas must first be determined. The location that would be most susceptible to overload failure l l must also be determined. Formulas for the determination ofinternal and external thread shear l areas are restated here: l 1 l l Ass = Minimum Thread Shear Area for External Threads : l 1 I A, = n (1/P)*L *D1g y (1/P) + 0.57735 (D2m - D1m)] l g g 2 l s I i I l f Ass = Minimum Thread Sheatr).rea for Internal Threadt l 1 l 1 I A,,y = x(1/P)*L *D g (1/P) + 0.57735 (Dg-D2gy] l g u 2 l I I Where: x = 3.14159 l 1/P = Number of Threads per Inch l 1 I, = Length of Engagement l D. = Minimum Major Diameter of External Thread l Da = Maximum Minor Diameter ofInternal Thread l j De = Maximum Pitch Diameter ofInternal Thread l De = Minimum Pitch Diameter of External Thread l l These equations consider the location of the shear plane on the thread profile, and determine its I width for a specific pitch diameter. This value is multiplied in the equation by the length of l engagement to give the total area of the shear plane. The length of engagement for this example l is given to be that of a heavy-hex nut, which equals one nominal diameter in length. l l For the externally threaded item which was shown to exhibit the greatest deviation from specified I tolerances during warehouse sampling associated with this paper, the following data was obtained l and is restated for this example: l l l 34 i 1

i A White Paper-FastenerStrength Analysis, Revision Two External Threads: l l Item

Description:

All Thread Stud. 5/8"dia-11 (UNC) by 36" length l 1 Material Code: 305-05606 RSO#: 2063-93 Supplier: NOVA Inc. l l Material Specification: ASME SA193 Grade B7 Heat Code: 8099572 l l Thread Functional Size Inspection was performed Satisfactorily l l Thread Pitch Diameter Inspection was shown to be Out-of-Tolerance. l l Pitch Diameter Reading: 0.5554 inches = D. l l Pitch Diameter Range: 0.5644 (max) to 0.5589 (min) inches.83 l l Amount Out-of-Tolerance: 0.0035 inches l l Ultimate Material Strength (Sr) = 125.000 psi (min)" l l Length of Thread Engagement (Iy) = 1 Diameter = 0.625 inches l l l 1-l l l l 1 I I i l 1 1 I I I I l 88 ANSI /AShE B-1.1.1989 Edition. Table 3 A. Class 2A Fit " AShE Section III,1977 Edition, Appendix I 35

l I l l A White Paper-FastenerStrength Analysis, Revision Two l For the internally threaded item which was shown to exhibit the greatest deviation from specified l tolerances during warehouse sampling associated with this paper, the following data was obtained l l and is restated for this example: l l l i l Internal Threads: l l l Item

Description:

Heavy Hex Nut 1/2"- 13 Threads per Inch (UNC) l l. Material Code: 305-04211 RSO#: 2583-92 Supplier; NOVA / Texas Bolt l l. Material Specification: ASME SA194 Grade 2H Heat Code: 1D3716 l' l Thread Functional Size Inspection was shown to be Out-of-Tolerance: l l Functional Size Reading: 0.4572 inches l l i Functional Size Range: 0.4565 (max) to 0.4500 (min) inches [ l Amount Out-of-Tolerance: 0.0007 inches l l Thread Pitch Diameter Inspection was shown to be Out-of-Tolerance: I I Pitch Diameter Reading: 0.4642 inches l l l l Pitch Diameter Range: 0.4565 (max) to 0.4500 (min) inches" l [ l l Amount Out-of-Tolerance: 0.0077 inches l l i Material Proof Strength (S) = 175.000 psi (min)(ASME Section II) l \\ Length of Thread Engagement (Lg) = 1 Diameter = 0 500 inches l ) I Minimum Major Diameter of External Thread = D. = 0.4876 inches l l l Maximum Major Diameter of External Thread = 0.4985 inches l l I I l l " ANSI /ASME B-1.1,1989 Edition, Table 3-A, Class 2A Fit L 36 b

l t l '. A White Paper-FastenerStrength Analysis, Revision Two Pitch Diameter of Hypothetical Nut l In order to perform a combined-case evaluation utilizing the out-of-tolerance conditions exhibited l by 5/8" allthread and the 1/2" nut, the Pitch Diameter readings from the nut must be converted to l a proportionally equivalent out-of-tolerance reading for a hypothetical 5/8" nut. This may be l accomplished by two different methods; by ratio of pitch diameter tolerances for 1/2"-13 internal l threads and those for 5/8"-l1 threads, or by a direct ratio of fastener nominal diameters. l l Tolerance Ratio l Tolerance in this case is the difference between the Maximum and Mir.imum specified pitch l l diameters for each of the two size fasteners. Tolerances for pitch diameter do not vary linearly I with respect to fastener nominal diameter or thread pitch, that is to say that the tolerances for a 1" l fastener are not twice as large as those for a 1/2" fastener. From Table 3 A of ASME B-1.1: l l 5/8"-l1 Class 2B Internal Thread Pitch Diameter Tolerance = 0.0072" l l 1/2"-13 Class 2B Internal Thread Pitch Diameter Tolerance = 0.0065" l l Therefore, for a 1/2"-13 nut pitch diameter out-of-tolerance reading to be proportionally l equivalent to one for a 5/8"-l1 nut, it would have to be multiplied by the ratio of the tolerances I shown above: l I i 5/8"-11 to 1/2"-13 Tolerance Ratio = 0.0072" + 0.0065" = 1.108 l l l Multiplication of the 1/2"-13 nut pitch diameter out-of-tolerance reading by this Tolerance Ratio l will result in a proportionally equivalent out-of-tolerance value for the hypothetical 5/8"-l1 nut to l be analyzed in this combined case: l l 1/2"-13 nut out-of-tolerance reading (from above data) = 0.0077" l l I Multiplying by the Tolerance Ratio: 0.0077" x 1.108 = 0.0085" l l Nominal Diameter Ratio l Developing a proportionally out-of-tolerance pitch diameter for the hypothetical 5/8"-l 1 nut by a l l nominal diameter ratio would involve the following: 1 I Nominal Diameter Ratio = 5/8" + 1/2" 1.25 l = l l 1/2"-13 Nut Pitch Diameter Out-of-Tolerance Amount (from above data) = 0.0077" l l Multiplying by the Nominal Diameter Ratio: 0.0077" x 1.25 = 0.0096" l j l A greater out-of-tolerance condition is created by using the Nominal Diameter Ratio, therefore l 37 i l l i L

a 1 A White Paper-FastenerStrength Analysis, Revision Two 1 this number will be conservatively utilized for the purposes of this example. From this, l the following data may be stated for the hypothetical 5/8"-l I nut to be used in this analysis: l l Item

Description:

Heavy Hex ' Nut S/8" - 11 Threads per Inch (UNC) l I ~ Material Specification: ASME SA194 Grade 2H l 4 l Thread Pitch Diameter Out-of-Tolerance: l I l Pitch Diameter Rcnge: 0.5732 (max) to 0.5660 (min) inches l l Amount Out-of-Tolerance: 0.0096 inches l l ) Pitch Diameter Reading: 0.5828 inches (Hypothetical) l l l Material Proof Strength (S) = 175.000 osi (min)(ASME Section II) l l Length of Thread Engagement (L ) = 1 Diameter = 0.625 inches l t l I Calculation of External Thread Combined-Case Shear Strength l From the preceding discussion, it can be expected that for this combined-case, thread shear failure l from overload forces would occur at the external thread shear plane (excluding possible failure at I the fastener tensile cross-sectional area). The shear area and resulting ultimate thread strength l l for the external threads will be calculated first, and will then be compared to calculations for the l internal thread shear area and thread strength. l i I The combined case thread strength calculation for the combination of the 5/8"-11 allthread and l hypothetical 5/8"-l1 nut described above can be performed using the following equation to I determine the thread shear area in the shear plane of the all thread: l l As3 = Minimum Thread Shear Area for External Threads : l 1 I A, = x (1/P)*L *D1 g d (1/P) + 0.57735 (D2, - D1gg)] l g g 2 l 1 Where: n = 3.14159 l 1/P = Number of Threads per Inch l L = Length ofEngagement l 3 Da = Calculated Minor Diameter ofInternal Thread l D = Actual Pitch Diameter ofExternal Thread l l 38 i

A White Paper-FastenerStrength Analysis, Revision Two The preceding equation calculates the shear area for the specific allthread pitch diameter at the l place in the thread profile where the minor diameter of the nut contacts the allthread. l I i Adiusted Minor Diameter l The routine calculation ofminimum external thread shear area utilizes a Maximum Minor l Diameter of the Internal Thread, or D sm, which is specified in ASME B-1.1. In this .l i combined-case analysis, the value of D sm is conservatively modified to reflect the out-of-l i tolerance pitch diameter condition considered for the hypothetical 5/8"-l1 nut. In the basic form l of the screw thread profile described in ASME B-1.1, the pitch diameter is defined as falling at the - l halfway point in the Height (H) of the sharp V-thread (fundamental triangle). The height of l thread engagement is that of H, minus the losses associated with the crest and root of the thread, j and is equal to 0.625 H. Truncations associated with the crest and root of the thread are j essentially equal therefore, the pitch diameter lies at the halfway point of the height of thread l j engagement, for both internal and external threads. For internal threads, the minor diameter of l ~ the nut is equal to the major diameter at the root of the thrend plus twice the height of thread l engagement (accounting for thread engagement on opposing sides of the nut). Therefore, in l accordance with the basic form of the screw thread, a variation in pitch diameter would have a ' l l corresponding effect on the height of thread engagement equal to 1/2 the variation in pitch l-diameter (assuming a constant major diameter). This resulting variation in thread height, when I considered for opposing positions on the nut diameter, would result in the nut minor diameter l varying an amount twice that of the thread height variation, which would equal the amount of l pitch diameter variation. Therefore we can conclude, for the purposes of this evaluation, that a l given variation outside of specified tolerances for nut pitch diameter will produce an equivalent l variation outside specified tolerances for nut minor diameter This would result in the following: l I 5/8"-l1 Nut Pitch Diameter Calculated Reading ' 0.5828" l = I 5/8"-l1 Nut Pitch Diameter Spec Maximum 0.5732" l = Amount Out-of-Tolerance =- 0.0096" l I I 5/8"-l1 Nut Maximum Minor Diameter ' 2 0.5460" l l = l + Add Equivalent Pitch Diameter Out-of-Tolerance = 0.0096 l j Resultant Adjusted Nut Minor Diameter (Dimx) = 0.5556" l-i l 1 Independent thread dimensional mspection performed in accorda:2ce with Nuclear ) Oversight Division Failure Analysis Report (FAR) 94-05 did not demonstrate a linear correllation between pitch diameter variations and major (minor) diameter readings. Therefore, the maximum Minor Diameter is being conservatively utilized in this i calculation to result in an out-of-tolerance Adjusted Minor Diameter for this example. 39 i m + " ~ - " ~ ' ~ ' ' - ' " '~'

A White Paper-FastenerStrength Analysis, Revision Two Applying this value to the above equation for external thread shear area: l l I Ag= x(1/P)*Lg*D1g (1/P) + 0.57735 (D2, - D1m)] 2 l I l D2m = Allthread Pitch Diameter = 0.5554" (Page 35) l l Dim = Nut Adjusted MinorDiameter = 0.5556" (Page 39) l l l Ass = 3.14159

11
0.625
0.5556 [ 1/(2*11) +.57735(0.5554 - 0.5556)]

l j l A33 = 0.5441 in2 l l I Factoring this combined-case external thread shear area into the equation te determine thread l shear strength: l l I Shear Strength Of External Threads = 0.55,.* (A ) g l I I Where: 1 Sr = Ultimate Tensile Strength of Allthread Material l = 125,000 psi (ASME Section II) l l A = Calculated Thread Shear Area for External Threads l 33 t l I ] Shear Strength of Allthread Threads = 0.5

125,000 psi
0.5441 in l

2 l j = 34.006 pounds force l l Thus, the external threads of the 5/8"-l1 allthread could be expected to fail when subjected to a l load in excess of approximately 34,000 pounds. l l l l l l 40

l l, y A White Paper-Fa&ner Strength Analysis, Reision Two i Calculation ofInternal Thread Combined-Case Shear Strength l In order to verify the location of expected t'aread failure, the shear area and resulting ultimate l thread strength for the internal nut threads will be calculated and compared to the allthread l 1 external thread shear area and thread strength. The combined-case thread strength calculation for l the combination of the 5/8"-l1 allthread and hypothetical 5/8"-l1 nut can be performed using the l following equation to determine the thread shear area in the shear plane of the nut threads: l l Asu = Thread Shear Area for Internal Threads: l l 1 I Ag= x(1/P)*L *Dg (1/P) g + 0.57735 (Dg-D2y] l 2 g I Where: n . = 3.14159 l 1/P = Number of Threads perInch l I = Length of Engagement l s D. = Calculated Major Diameter ofExternal Thread l Du = Calculated Pitch Diameter ofInternal Thread l l I This equation calculates the shear area for the pitch diameter of the hypothetical nut, at the place l in the thread profile where the major diameter of the allthread contacts the nut. I l The routine calculation of minimum internal thread shear area utilizes a Minimum Major Diameter l of the External Thread, or D , which is specified in ASME B-1.1. In this combined-case l analysis utilizing the above equation, the value of D is conservatively modified to reflect the l e out-of-tolerance pitch diameter condition of the 5/8"-l1 allthread. In the basic form of the l screw thread profile desenoed in ASME B-1,1, the pitch diameter is defined as falling at the l halfway point in the Height (H) of the sharp V-thread (fundamental triangle). The height of l thread engagement is that of H, minus the losses associated with the crest and root of the thread, l l and is equal to 0.625H. Truncations associated with the crest and root of the thread are l j essentially equal and therefore the pitch diameter also lies at the halfway point of the height of l 1 thread engagement, for both internal and external threads. For external threads, the major i diameter of the fastener is equal to the minor diameter at the root of the thread plus twice the l height of thread engagement (accounting for thread engagement on opposing sides of the bolt). l Therefore, in accordance with the basic form of the screw thread, a variation in pitch diameter l would have a corresponding effect on the height of thread engagement equal to 1/ 2 the variation i in pitch diameter (assuming a constant minor diameter). This resulting variation in thread height, -l when considered for opposing positions on the allthread diameter, would result in the allthread l major diameter varying an amount equal to twice that of the thread engagement height variation, l which would equal the amount of pitch diameter variation. l I Therefore we can conclude, for the purposes of this evaluation, that a given variation below l 41

4 A White Paper-FastenerStrength Analysis, Revision Two specified tolerances for allthread pitch diameter will produce an equal variation in major diameter l below specified values." This would result in the following: l 1 5/8"-l1 Allthread Pitch Diameter Minimum 0.5589" l = l 5/8"-i1 Allthread Pitch Diameter Reading 0.5554" l = l Amount Out-of-Tolerance 0.0035" l = l I Applying this amount of variation in pitch diameter, outside of specified tolerances to adjust the l Major Diameter of the allthread: l l 5/8"-l1 Allthread Minimum Major Diameter 0.6113" ] = l Subtract Equivalent Pitch Diameter Out-of-Tolerance = 0.0035 l_ l Resultant Adjusted Allthread Major Diameter (D.) = 0.6078" l l 1 Applying this value to the equation for internal thread shear area: l l 1 I Ag= x(1/P)*L *Dy (1/P) g + 0.57735 (Dm-D2d] l 2 l D2m = Nut Pitch Diameter 0.5828" (Page 38) l = 1 D. = Allthread Major Diameter = 0.6078" (Above) l l Asy 3.14159

11
0.625
0.6078 [ 1/(2*11) +.57735( 0.6078 0.5828) l

= l l 2 Asy = 0.7862 _ in l l " Independent thread dimensional inspection performed in accordance with Nuclear Oversight Division Failure Analysis Report (FAR) 94-05 did not demonstrate a linear correllation between pitch diameter variations and major diameter readings. Therefore, the minimum Major Diameter is being conservatively utilized in this calculation to result in an out-of-tolerance Adjusted Major Diameter for this example. 42

A White Paper-FastenerStrength Analysis, Revision Two It should be noted that the combined-case nut shear area of 0.7862 in is 1.45 times the value of l 2 the combined-case allthread shear area of 0.5441 in, and is 1.26 times the minimum mat: rial I 2 external thread shear area of 0.6239 in (Case B). l 2 I Factoring this combined-case internal thread shear area into the equation to determine thread l shear strength: l l 0.5 5 * ( A.) l Shear Strength of Internal Threads = 7 g l Where: l l Ac Minimum Thread Shear Area for Internal Threads l Sr = ProofLoad Strength ofNut Material l l l 1 Shear Strength ofInternal Threads 2 0.5

175,000 psi
0.7862 in

= I 68.793 pounds l = l Thus, the proofload that the combined-case nut threads could withstand without yielding would l be 68,793 pounds, or approximately twice the ultimate strength of the combined-case allthread l external threads. From this information, we can conclude that if fastener tensile failure is not l considered, the combined-case threaded joint would fail when subjected to an overload condition I of approximately 34,400 pounds and would fail at the shear plane of the allthread material. I l I Externally Threaded Fastener Tensile Streneth j l The ultimate tensile strength of the externally threaded fastener (in pounds) equals the ultimate I strength of the fastener material (in pounds per square inch) multiplied by the fastener tensile l cross-sectional area (in square inches). For the SA 193-B7 allthread, the ultimate material l strength is specified in ASME Section Il as being a minimum of 125,000 psi. The ASME Code l tensile area is based on the root area, which for the 5/8"-l1 thread is 0.202 square inches. l The calculation of tensile strength: l l 125,000 psi x 0.202 in2 2121Q pounds (min). l = l The value of 25,250 pounds tensile strength is less than the combined-case external thread l strength and therefore overload of the threaded joint would result in failure of the allthread in j tension, not in stripped threads. It should be noted that the combined-case out-of-tolerance l thread strength (34,006) is 1.35 times (135%) stronger than the fastener ultimate tensile strength. l 43 l l l l

2 A White Paper-FastenerStrength Analysis, Revision Two a ComDarison Of Results l For purposes of comparison, it would be useful,to restate the external thread strength calculation ( results which were developed in the body of this report. l I CASE A: Maximum Material Conditions for both Internal and External Threads l i I j Pitch Diameter = 0.5644" (max) Dluu = 0.5270" (min) l I i A33 = 3.14159

11 *.625 *.527 * [1/(2*11) +.57735(.5644.5270)]

l = 0.7632 square inches l l j Thread Ultimate Shear Strength = l l 0.5

0.7632
125,000 = 47.700 counds (min) l i

l I j CASE B: Minimum Material Conditions for both Internal and External Threads l I Pitch Diameter = 0.5589" (min) D1ua = 0.5460" (max) l 4 I A33 = 3.14159

11 *.625 *.546 * [l/(2*11) +.57735(.5589.5460)]

l = 0.6239 square inches l l Thread Ultimate Shear Strength = l l 0.5

0.6239
125,000 = 38.992 pounds (min) l l

CASE C: Out-of-Tolerance Conditions from " Worst Case" Data l l Pitch Diameter = 0.5554" (actual) Dluu = 0.5460" (max) l l A = 3.14159

11 *.625 *.546 * [1/(2*11) +.57735(.5554.5460)]

l 33 = 0.6000 square inches l l Thread Ultimate Shear Strength = l l 0.5

0.6000
125,000 = 37.502 nounds (min)

{ I t I Addition of the Combined-Case evaluation to these results would provide a Case D for l comparison: l 44

A White Paper-FastenerStrength Analysis, Revision Two CASE D: Combined-Case Internal and External Thread Out-of-Tolerance (Hypothetical) l l Pitch Diameter = 0.5554" (Actual) De = 0.5556 (Adjusted) l I A 3.14159

11
0.625
0.5556 [ 1/(2*11) +.57735(0.5554 - 0.5556)]

l = 33 l 2 A33 = 0.5441 in .l Thread Ultimate Shear Strength = l l. 1 0.5

0.5441
125,000 = 34.006 counds (min)

.l l t Addition of the fastener Tensile Strength evaluation to these results provides a Case E for l comparison: l l CASE E: Fastener Tensile Strength l l Root Area = 0.202 in2. 2 125,000 psi x 0.202 in 25,250 pounds (min) l = Tensile Area = 0.226 in2 125,000 psi x 0.226 in2 28,250 pounds (min) l = l l 1 Addition of the maximum Section III allowable preloads for Class 1,2 and 3 applications l provides Cases F and G for comparison: l l CASE F: Maximum Preload for Section III, Class 1 Applications l I c l ASME Section III Class 1: l l Root Area 2

35,000 psi
0.202 in = 14.140 pounds l

2 l Tensile Area 2

35,000 psi
0.226 in = 15.820 pounds l

t 2 I l l l CASE G: Maximum Preload for Section III,88 Class 2 and 3 Applications l I l l ASME Section III Class 2 and 3: l Root Area 2

25,000 psi
0.202 in = 10.100 pounds l

2 Tensile Area 2

25,000 psi
0.226 in = 11.300 pounds l

2 l 8' Strength based on Root Area and Tensile Area are shown here for comparison. t Root areas are used for ASME analysis at SONGS. 45 I l I .l

A White Paper-FastenerStrength Analysis, Revision Two l FASTENER STRENGTH CALCULATION

SUMMARY

l l l CONDITIONS STRENGTH l-I l Thread Strennth: l Case A, l Maximum Thread Material Contact Area 47.700 pounds l (Int and Ext) { l Case B, l Minimum Thread Material Contact Area 38.992 pounds [ (Int and Ext) l I { Case C, l Worst Case Thread Out-of-Tolerance 37.502 pounds (Ext) l l Case D, l l Combined-Case Thread Out-of-Tolerance 34.006 pounds I (Int and Ext) l 1 Tensile leads: l Case E, l Ultimate Allthread Tensile Strength 25,250 pounds l (Root Area) l l Case F, l Maximum Section III Preload 14.140 pounds l l Class 1 l l (Root Area) l l Case G, l l Maximum Section III Preload 10.100 pounds l Class 2 and 3 l (Root Area) l l 1 I 46 l

k A White Paper-Fastener Strength Analysis, Revision Two STRENGTH ANALYSIS RESULTS l I The Combined-Case thread strength analysis for both internal and external thread out-of-tolerance l conditions reveals that ultimate thread strength of the bolted connection is reduced 7.33% more ' I than the case where only the external fastener was considered to be out-of-tolerance (Case C). l This results in the Combined-Case evaluation indicated that the ultimate thread strength of the i bolted connection is reduced a total of 10.45% from the Minimum Material condition (Case B). l l When the thread strength of the Combined-Case bolted connection is compared to the maximum I allowable preload for thejoint specified per ASME Section III Class I service (14,140 pounds), l the ultimate thread strength of the Combined-Case connection is found to be 2.40 times (240%) l stronger. When compared to the maximum allowable preload for ASME Section III, Class 2 and l 3 service (10,100 pounds) the ultimate thread strength of the Combined-Case connection is found l l to be 3.37 times (337%) stronger. This clearly demonstrates the significant strength margin l l inherent in ASME Code fastener design. l l l l l l I I l l l l l l l l l l l 1 l I l I i It should be noted that for purposes of comparison in this report, that the fastener 2 tensile area (0.226 in ) was previously utilized in obtaining this value to produce a conservative comparison of thread tensile load to thread strength. 47

l 'e A White Paper-FastenerStrength Analysis, Revision Two I

SUMMARY

l l This evaluation has conservatively calculated the effect on fastener strength when an externally l threaded fastener, which has been shown to be out-of-tolerance with respect to pitch diameter,. 1 is combined with an internally threaded fastener which is also out-of-tolerance with respect to l pitch diameter. The values for pitch diameter utilized in this evaluation corresponded with the l maximum observed deviations from specified values reponed in warehouse sample inspections l i associated with this White Paper. l I Comparison of the reductions in thread strength to ASME Code design requirements were made l and it was demonstrated that the combined-case thread shear strength reductions were on the ( order of 10.45%, with the Code strength margin being on the order of 200 to 300%. The shear l stresses across the combined-case external thread shear area which would result from the l j maximum allowable preloads were compared with AShE Section III allowables and were verified l l to be less than the Code maximums for Class 1,2 and 3 service. This evaluation clearly l j demonstrates the significant strength margin inherent in ASME Code fasteners. In addition to the l l strength margin of the individual fastener, installation configurations would require multiple l l fastener failures prior tojoint failure. The nature of the design of fastener preload requires the I maximum fastener load to occur at installation, thereby identifying fasteners which may fail at the l l time ofinstallation instead ofin-service. If a fastener should loosen due to loss of preload, l leakage would occur beforejoint failure. This paper discusses the various leakage monitoring and l corrective action systems in place at SONGS, including an effective Root Cause evaluation l t program. l l It is therefore concluded that an unlikely combination ofinternal and external fasteners l demonstrating out-of-tolerance conditions proportionally equal to the maximum observed l deviations noted in this paper would have no impact on the bolted connections ability to withstand l 1 design loads and fulfill a safety function. The results of this evaluation does not change l conclusions stated in this White Paper that plant nuclear safety is not compromised by current l industry fastener inspection practices. l l l l l l I I I I I I 48

l A White Paper-Fastener Strength Analysis, Revision Two I The following discussion is provided in response to specific comments: l l OFFSET THREADS l l A condition where the externally threaded fastener may not concentrically penetrate the internal l fastener may result from circumstances where the contact faces of the nut or bolt head are not 1 perpendicular, or where the fasteners may be bent. In this type ofcase the self-centering nature of l the inclined thread faces across the thread contact surface area respond to loading of the fastener l i by inducing bending moments across the tensile cross-sections of the fastener. Stresses associated l with these bending moments have been shown to affect fastener fatigue life, relaxation - loss of l preload, and preload control. l l Issues associated with this type of condition do not routinely include thread profile or pitch l diameter compliance, but rather center on flange face perpendicularity, nut and bolt head l perpendicularity, and fastener straightness. For Safety-Related applications, these conditions are l considered during the Procurement Engineering and Receipt Inspection processes at SONGS. l l l l 49 r

A White Paper-FastenerStrength Analysis, Revision Two ATTACHMENT 1 LICENSING POSITION i FASTENER FINAL DIMENSIONAL ACCEPTANCE INSPECTION 1 REGULATORY ANALYSIS ] A. Introduction 4 The purpose of this presentation is to provide a regulatory analysis regarding the use of fasteners at the San Onofre Nuclear Generating Station (SONGS). When threaded fasteners are manufactured, there are numerous properties (size, diameter, thread pitch, helix angle, etc.) of thread form which, depending on the application, may or may not be important. A listing of all possible properties is contained in ANSI B1.1, " Unified Inch Screw Threads". Once it has been determined which properties are important from a design engineering standpoint, selection of a gaging system is made by the design engineer to ascertain thread form acceptability. ANSI B1.3, " Screw Thread Gaging Systems for Dimensional Acceptability", lists four gaging methods [ System 21, System 21 A, System 22, and System 23]. Each of the methods evaluates certain screw thread characteristics. 4 ANSI B1.3, Screw Thread Gaging Systems for Dimensional Acceptability -Inch and i Metric Screw Threads, states in part: "4(b) The difference between gaging systems is the level ofinspection deemed necessary to satisfy that dimensional conformance has been achieved. The following gaging systems descdbe four accountable levels of dimensional inspection..." i ~4(b)(1) System 21. Provides ofinterchangeable assembly with functionalsize control at the maximum materiallimits within the length of standard gaging elements, and also controlof the characteristics identified a NOT GO functional diameters." "6(d) Relationship of Gaging Systems to Product Screw Thread Acceptability. (1) Product screw threads acceptable to System 23 are acceptable where System ' 22 and 21 are specified. The reverse is not necessarily true. (2) Product screws acceptable to System 22 are acceptable where System 21 is specified. The reverse is not necessarily true." 50 4 e r - r

l'. l A White Paper-FastenerStrength Analysis, Revision Two l l B. Current Licensing Basis for Fasteners l This section discusses the legal, binding requirements imposed upon SONGS by the U.S. Nuclear Regulatory Commission (NRC). There are two aspects to the current licensing basis for fasteners: (1) NRC regulations do referencellink l ASME and ANSI standards which provide, by fastener material type and application, what receipt acceptance method is acceptable; and (2) NRC regulations do referencellink ASME and ANSI standards which state that it is up to the design engineer to determine what, of the many individual thread form parameters which can be evaluated, should be evaluated for receipt acceptance. 1. Fastener Requirements - CODE CASES SONGS was issued an Operating License by the NRC in 1982. License Condition 2.C states: "This license shall be deemed to contain and is subject to the conditions specifiedin the Commission's regulations set for1h in 10 CFR ChapterI..." 10 CFR 50.55a, Codes and standards, states in part: "Each operating license for a boiling or pressurized water-cooled nuclear power facility is subject to the conditions in paragraphs (f) and (g) of this section and each construction permit for a utilization facility is subject to l the following conditions in addition to those specifiedin Sec. 50.55.,." "... (a)(2) Systems and components of boiling and pressurized water-cooled nuclearpower reactors must meet the requirements of the ASME Boiler and Pressure Vessel Code specified in paragraphs (b), (c), (d), (e), (f), and (g) of this section... " " (c) Reactor coolant pressure boundary. (1) Components which are part of the reactor coolant pressure boundary must meet the requirements for Class 1 components in Section III of the ASME Boiler and Pressure j Vessel Code, except as provided in paragraphs (c)(2), (c)(3), and (c)(4) of r this section..." " (d) Quality Group B components. (1) For a nuclearpowerplant whose application for a construction permit is docketed after May 14,1984 components classified Quality Group B must meet the requirements for Class 2 Components in Section III of the ASME Boiler and Pressure Vessel Code... " 51

.= i A White Paper-FastenerStrength Analysis, Revision Two " (e) Quality Group C components. (1) For a nuclearpowerplant whose application for a construction permit is docketed after May 14,1984 components classified Quality Group C must meet the requirements for - Class 3 components in Section III of the ASME Boiler and Pressure VesselCode. 1 i Section lli of the ASME Boiler and Pressure Vessel Code, Subpart NA-1220, Materials, states in part: " Materials are manufactured to an SA, SB, or SFA specr6 cation or any other material spect6 cation permitted by this section.' Such material shall i be manufactured and certi6 edin accordance with the requirements of this Section.. " There are two categories of fasteners: fasteners manufactured to the A, B, or F specification; and fasteners manufactured to unique specifications which are - produced on a case-by-case application. Materia!s manufactured to an A, B, or F specification have specific characteristics depending on the type and application of the fastener. t There are specific individual ASME A, B, F standards for each. The attached Table 1 delineates the fastener type and controlling ASME standard number. Within each ASME standard, is a specific reference to a controlling ANSI standard which specifies thread form. Table 1 also contains a column for each ASME standard, which has its corresponding ANSI standard. 52

l l Table 1-Thread Acceptance Criteria Associated With ASTM Fastener Material Specifications ASTM Material Type ASTM ANSI ANSI Thread Gaging Number Standard Acceptability Requirements Alloy Steel and Stainless Steel Bolting A 193 18.2.1 System 21 Materials for High-Temperature Service Carbon and Alloy Steel Nuts for Bolts for A 194 18.2.2 System 21 High-Pressure and High-Temperature Service Carbon Steel Bolts and Studs, 60,000 psi A 307 18.2.1 System 21 Tensile Strength Alloy Steel Bolting Materials for A 320 18.2.1 System 21 Low-Temperature Service High-Strength Bolts for Structural Steel A 325 18.2.1 System 21 Joints Quenched and Tempered Alloy Steel Bolts, A 354 18.2.1 System 21 Studs, and Other Externally Threaded Fasteners Alloy Steel Turbine-Type Bolting Material A 437 18.2.1 System 21 Specially Heat Treated for High-Temperature Service Quenched and Tempered Steel Bolts and Studs A 449 18.2.1 System 21 Bolting Materials, High Temperature, 50 to 120 Ksi [345 to 827 MPa] Yield Strength, with' A 453 18.2.1 System 21 Expansion Coefficients Comparable to Austenitic Steels Heat-Treated Steel Structural Bolts, 150 Ksi A 490 18.2.1 System 21 Minimum Tensile Strength carbon and A11 mv Steel Nuts A 563 18.2_2 System 21 53

.m ._......_._m. _m

m.. _ _ _.. _ _. ~... _ _...

e ASIDI Material Type - Continued ASTM ANSI ANSI Thread Gaging Number Standard Acceptability Requirements Alloy Steel Socket-Head Cap Screws A 574 18.3 System 22 High-Strength Nonheaded Steel Bolts and Studs A 687 N/A Not Addressed Nonferrous Nuts for General Use F 467 18.2.2 System 21 Nonferrous Bolts, Hex Cap Screws, and Studs F 468 18.2.1 System 21 for General Use Carbon and Alloy Steel Externally Threaded F 568 18.2.3 Not Addressed Metric Fasteners Stainless Steel Bolts, Hex Cap Screws, and F 593 18.2.1 System 21 Studs Stainless Steel Nuts F 594 18.2.2 System 21 Alloy Steel Socket Button and Flat F 835 18.3 system 22 Countersunk Head Cap Screws Stainlesr Steel Socket Head Cap Screws F 837 18.3 system 22 Stainless Steel Socket Button and Flat F 879 18.3 system 22 Countersunk Head Cap Screws Stainless Steel Socket-Set Screws F 880 18.3 system 22 Alloy Steel Socket Set Screws F 912 18.3 system 22 54

I '. A White Paper-Fastener Strength Analysis, Revision Two Within each ANSI standard is a specific reference to the gaging system which is acceptable for use in determining thread acceptance. Table 1 also contains a j column for each ANSI standard, which has its corresponding gaging system i specified. Of the 23 categories,15 categories specify System 21,6 specify System 22, and 2 are not addressed. 7 I An example of this would Specification SA-193, Standard Specification for Alloy ] Steel and Stainless Steel Bolting Materials for High Temperature Service (note: SA-193 is for RCS bolting applications, similar cross references to ANSI B18.2.1 j and ANSI B1.1 exist for other bolting materials and applications], which states in part: 3- ) "11.1 Allbolts, studs, stud bolts, and accompanying nuts, unless otherwise specifiedin the purchase order shall be threadedin accordance i with the American National Standard for Screw Threads (ANSI B1.1), i Class 2A fit,..." t "13.3...Unless otherwise specified in the purchase order, the Heavy Hex ) Screws Series should be used... for sizes not covered in the Heavy Hex f Screws Series in ANSI B16.2.1..." ANSI B18.2.1, " Square and Hex Bolts and Screws - Inch Series", states in the notes to Tables on Threads: i 1 "Accentabilltv of screw threads shall be determined based on Svstem 21. ANSIB1.3 Screw Thread Gaaina Svstems for ) Dimensional Accentabilitv." l l 2. Fastener Requirements - ENGINEERING 4 ANSI B1.3 states in part: l l "5.a Screw threads of threaded products are defined by the applicable thread document.. 5(b) the gaging system used to inspect the screw thread of a j threaded product shall be as specified in the product standard, procurement drawing, or purchase inquire." I 55

A White Paper-FastenerStrength AnalysisJerision Two The ANSI B1.1, Sections 5(a and b), authorize product standards and purchase - documents, which are created by the design engineer, to define which thread characteristics are important and select the appropriate gaging system to j ascertain thread conformance. lf the thread characteristics are undefined, then j j ' the program defaults to System 21. i The American Society of Mechanical Engineers, in a letter from 4 l Mr. Kurt Wessely, Director, ASME Codes and Standards, to ) Senator Joseph Lieberman, dated June 10,1994, states in part: 1. i "As the result of a hearing before the Senate Subcommittee on Antitrust l and Monopolies, ASME agreed to publish an array of gaging systems and describe the attributes of each system. From this array of gaging j systems, it is intended that the engineer or related scientist who is i i designing, fabricating orinspecting equipment will select the system that l addresses the need. The selection of the gaging system is intended to be j made by the user of the threaded product based on the intended i application of the threaded fastener. The ASME Standard does not l. recommend one gaging system over the others. " 4 j Accordingly, it is recognized that design engineers do ng.t have to specify thread i conformance and thread acceptance via gaging system to every dimensional i limit listed in ASME B1.1. It may be appropriate for some gaging to be made by j System 22 (as indicated in Table 1), however, for the majority of threaded 4' fasteners, System 21 is the acceptable method. [ NEDO has endorsed the practice of using system 21 as the method of choice. J. i, I i l I i a 3 56 1 l i

O A White Paper-FastenerStrength Analysis. Revision Two i C. NRC POSITION

References:

1. U. S. Nuclear Regulatory Commission, NUREG-1349,

" Compilation of Fastener Testing Data Received in Response to NRC Compliance Bulletin 87-02, June 1989 On November 6,1987, the NRC issued Bulletin 87-02, " Fastener Testing to Determine Conformance with Applicable Material Specifications." The bulletin was issued so that the NRC staff could gather data to determine whether fasteners obtained from suppliers and/or manufacturers meet the mechanical and chemical specifications stipulated in the procurement documents. Based on the results, the NRC concluded that nonconforming fasteners do not seem to represent a s;gnificant safety hazard to the nuclear industry (Reference 1).

References:

2. Letter from Stanley P. Johnson to Ivan Selin (NRC),

l dated March 8,1994. l

3. Letter from William T. Russell (NRC) to Stanley P. Johnson, dated March 25,1994.

in response to the Johnson Gage Company's initial alleged concerns outlined in Reference 2, the NRC indicated (Reference 3) I that "the NRC staff does not consider System 21 or the use of go-no-go gauges to be inappropriate (" flawed") for accepting certain fastener threads.. " The NRC further stated "that, although System 22 may be an improvement over System 21, there is not sufficient basis to make its use a requirement for NRC licensees." In summary, the NRC noted that "the NRC staff has not found evidence that failures due to dimensionally nonconforming fasteners are occurring and therefore, does not consider it to be a safety concern."

References:

l

4. NRC Memorandum from Brian W. Sharon to l

Ashok C.Thadani, " Meeting with NIST Regarding Gauging of i Threaded Fasteners," dated May 5,1994. l 57 l l l

i A White Paper-FastenerStrength Analysis, Revision Two i' . 5. Letter from Richard Jackson (NIST) to l James A. Davis (NRC), dated March 10,1994.' i j A meeting (Reference 4) between members of the NRC staff and the j. National Institute of Standards and Technology (NIST) was held on [ May 4,1994, to obtain clarification of letters NIST has written j over the years (e.g., Reference 5) regarding the unacceptability l 'of System 21 for ensuring threaded fasteners meet the tolerance i specifications in ANSI B1.1. NIST had gone on record to state: 4 ) " System 21 (plug and ring) acceptance methods do not assure - i dimensional conformance with material limits specified in ASME B1 1...." The senior official from NIST at the meeting (Richard Jackson) I i was asked why their letters only stated that use of System 21 would not ensure compliance with the dimensional tolerances of j ANSI B1.1, but were silent on the fact that neither would System i 22 ensure compliance with all of the dimensional tolerances in i the ANSI standard. NIST was also asked what the purpose of was for making such a statement, since NIST did not imply or state j that this meant the System 21 was considered unacceptable or 1-would result in fastener failures. In response, Mr. Jackson j agreed that NIST would write a letter (Reference 6) to the NRC j clarifying their position that failure of threaded fasteners to i meet the dimensional tolerance of ANSI B1.1 does not necessarily_ l imply that an unsafe condition will result from their use. They l also agreed to state that the acceptability of the gauging system 4 j used to accept threaded fasteners is the responsibility of the user of the fasteners. Mr. Sharon (NRR) concluded that the Division of Engineering was ' preparing a technical report that will document their assessment of this issue. The analysis will demonstrate why fasteners that meet System 21 but do not meet System 22 gauging tolerances are - considered acceptable by the staff from a structural standpoint, that operational data does not support Mr. Johnson's implication that fastener failures pose a threat to Nuclear Power safety, that risk assessments show that the risk of core melt from threaded fastener failures is extremely low; and finally, that redundancy and ample structural safety margins in the design of commercial nuclear plants do not result in situations in which 58

A White Paper-FastenerStrength Analysis, Revision Two single fastener performance is critical. D. NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY (NIST) POSITION

References:

6. Letter from Richard Jackson (NIST) to Brian W. Sharon (NRC), dated May 19,1994.

)

7. Draft Handbook, " Fasteners and Metals,"

Stiefel, S. W., U. S. Department of Commerce, NIST, dated February 1993 i As indicated above, NIST had gone on record to state: i " System 21 (plug and ring) acceptance methods do not assure dimensional conformance with material limits specified in ASME B1.1...." in response to the NRC's request for clarification, as indicated above, NIST responded (Reference 6) by stating that "we [NIST] l are not similarly able to make any definitive statements j [ assurance of dimensional conformance] about Systems 22 and 23. Unfortunately, there is not enough data to support such conclusion... Thus, in matters of product performance, we defer to industry standards committees and government regulatory agencies, and view the responsibility for any safety issues 4 associated with choice of fastener or gauging method to res'. solely with the user." To assist in complying with Public Law 101-592, " Fastener Quality i Act," the Department of Commerce has drafted a handbook for establishing an labortitcry accreditation program (Reference 7). This publication was drafted by the NIST, but appears to contradict their statements above in that this handbook endorses System 21. Specifically, under the Section titled " Gaging } requirements," NIST states that "The System 21 gaging system and i its derivatives allow the test laboratory to choose among .specified types of screw thread gages and measurement equipment for use in determining the required thread characteristics." 59

A White Paper-FastenerStrength Analysis, Revision Two E. AMERICAN SOCIETY OF MECHANICAL ENGINEERS POSITION References-

8. Letter from William T. Russell (NRC) to Walter R. Mikesell (ASME), dated June 1,1994.

9. Letter from Stanley P. Johnson to Ivan Selin (NRC), dated April 12,1994.

In a letter (Reference 8) to the ASME Chairman in regards to additional concerns submitted to the NRC Chairman by Mr. Johnson (Reference 9), the NRC staff indicated that they " concluded an initial review of this issue and has determined that there is no immediate safety concern. However, the NRC staff believes that there is an inconsistency in the existing standards in that the i required gaging systems cannot assure that all the tolerances will be satisfied." In order for the NRC to address Mr. Johnson's concerns, the NRC requested ASME to address 1) what is the significance of the tolerances given in the ANSI B1.1 tables when gaging systems specified do not guarantee conformance, and

2) the safety significance with fasteners failure to meet ANSI B1.1 tolerances but are found to be acceptable by the various gaging systems specified in ASME B1.2.

References:

10. Letter from M. R. Green (ASME) to William T. Russell (NRC), dated June 10,1994.

11. Letter from Kurt Wessely (ASME) to Senator Joseph I. Lieberman (U. S. Senate), dated June 10,1994.

12. Letter from Senator Joseph I. Lieberman to Kurt Wessely (ASME), dated May 20,1994.

ASME responded (Reference 10) to the NRC's request above by forwarding a copy of their response (Reference 11) to similar concems raised by Senator Lieberman (Reference 12). ASME concluded that "The ASME Standard does not recommend one gaging system over the others. Qualified engineers can be relied upon to make the proper selection of gaging systems based upon i application.... ASME has no information that compliance with any ASME B1 Screw Thread Standard has cau, sed any unsafe condition". 60

A White Paper-FastenerStrength Analysis, Revision Two F. INDUSTRIAL FASTENERS INSTITUTE POSITION

Reference:

13. Recommendations for Fastener Thread Acceptability, Industrial Fasteners Institute (IFI).

IFl recommendations for fastener thread inspections (Reference 13) conclude that the most practical thread measuring systems currently in existence which should be used are ANSI /ASME B1.3M-1986 System 21 (Method A) for all internal and extemal threads, except Class 3A external threads, where System 22 (Method B) is applicable. This position is interpreted to be in basis agreement with FED-STD-H28/20A September,1987, " Inspection Methods for Acceptability of UN, UNR, UNJ, M and MJ Screw Threads." l l l I l \\ l 61

A White Paper-FastenerStrength Analysis, Revision Two l I I I I I ADDENDUM TO ATTACHMENT 1 I l l l ASME CODE INTERPRETATION I OF FASTENER INSPECTION REOUIREMENTS l l l I I i l l l l l l l l l 62

i Question (1): Tables NB/NC/ND/NE-3132-1 identify standards B18.2.1, B18.2.2, and B18.3 j for bolting and Bl.1 for threads. ANSI standards B18.2.1, B18.2.2, and B18.3 also identify dimensions and tolerances for threads. B18.2.1 and B18.2.2 provide for the use of System 21 (ASME Bl.3M) gaging and B18.3 provides for the use j of System 22 (ASME B1.3M). Does Section III require that the dimensions and tolerances of B18.2.1, B18.2.2, and B18.3, and System 22 gaging, be used for threads on bolting produced to the requirements of ASME material specifications? 5 Reply (1): No. Section III requires that the provisions of the ASME bolting material specifications be met, for thread dimensions, tolerances, and gaging systems. Section III provides for the use of these material specifications with System 21 gaging or other systems designated by the purchaser. Question (2): Does Section III require the use of B18.3 and the associated tolerances for bolting l other than socket cap, shoulder, and set screws? Reply (2): No. Question (3): Does Section III permit use of System 21 Go/Not-Go gaging for acceptance of threads used in Class 1,2,3, and MC components? t Reply (3): Yes. [ Question (4): Is it a requirement of Section III that thread dimensions be measured to verify ~ they are within the tolerances specified in ASME Bl.17 Reply (4): No. Question (5): Does Section III accept the use of thread gages for bolting, such as Go/Not Go t i gages, that do not vedfy compliance of all dimensions and tolerances given in ASME Bl.17 Reply (5): Yes. Question (6): Does use of System 21 Go/Not-Go gages for Section III bolting ensure the-dimensional conformance to the ASME bolting standards identified in Tables NB/NC/ND/NE-3132-1 to the degree necessary to be consistent with the design philosophy and criteria of Section III? Reply (6): Yes. September 26.1994 eenwuntesp.nre j

i i The Amerless Seelett et i seesand mensues IIschemisel Engineers l Tel.212-7tH5ml 346 East 47B Sted

Fat 21 N I H 501 liew York, NY 10017 Ocsober (,1994 i

i 1 i Mr.Keiet R.Wiran a 01D4 j U1 Nacher RegnIntory t'a==3-6 Washtspan. Dr.20555 i 1 j

Subject:

Seaba m, Dt sion I,Tatdc NB/NC/ND/NE-3132-1; Design '11erced Osgleg *, " Pile: N19410 near Mr.wkhma.: Our understandhg of the quesdom h your hasar of T, '_2 5, IpH and our soply ass as blious: Question (1): Tables NB/NC/ND/NE.3132-1 Ider tandards B1811. B1822 and 5383 kr baadag and I Bl.1 lbr threads. ANSI mandards ..t. B1822, and B18.3 aho identify dimcasions and j soleraarm fbr stucads. B1811 and. 22 pimide ser the use of Sysacus 21 (ASME B1.3b0 1 gaging and B18.3 provides for the uso d3ystem 22 (ASME B13M). Does Section M sequhu j that the diwat <== and tolerancw of B182.1, B1822, and BIE3, and Symns 22 gaging, be i j used br threads on bolting prodacmd to the.y.;is of ASME matertal Waad l j Empty (1): No. Section III requhes that the provisions of the A5ME boldng material,. r-d-; be 1 met, for thread dh*iaa=. ri = =.and gaging systemt h*ian II1provides fbr the use of j these material g _-"" d--- with Symes 21 pging or other systems designated by the j perchaser. j j Qasstlam CD: Does wh E rcqahe the use of B183 and the maneimmd tolerances hr bokhg other thea socket cap *=1 der, and set actesti i f hh Ne k j Question O): Does Secoon DI persk ase of System 21 Oo/Not<fo gaging for -- ; E - ofihreads used j h Clas 1,2,3 and MC -pame===7 i j Empy Ok

Yes, i

i 4 i l }

14/04/94 09:52 FAI til 708 8501 AsuE ms oca 1 Questles(@ la k a requirement of Secatos M that thread W be usammed to sesi$ they ame idibla the soberances specified la ASME Bl.17 5 f Rag % (& No. t Quesilea(5): Docs Section m accept the um or skised says sur boldng such as Go/Not4e ynys thee de act verify compEancs of aR dhocasions and solerances ghes in A3RdB Bl.17 i i Esgh(5): Yes. Question (@ Docs thC une of Syissas 21 Go/Not4o gages ter sectium m bolties assare shs h conformance to the ASMB baldag standards identitled in Tables MB/NC/ND/NE 31321 to i j the degree permary to te osaducat wtrk the design phBogophy and criteria of h Mt 4 Beg %(@ Yes. 3 l t M j WA D. S. Pombs seastary,em m m.. _ \\ i i i i 1 i + t t l 1 1 1 i i .d S ~ J

A White Paper-FastenerStrength Analysis, Revision Two ATTACHMENT 2 STATISTICAL VALIDATION OF SCE FASTENER STRENGTH ANALYSIS WHITE PAPER DATABASE BY TETRA ENGINEERING GROUP August 5,1994 67

4 9 Statistical Validation of SCE Fastener Strength Analysis White Paper Data Base 4 Tetra Engineering Group, Inc. Report 94-SCE-003 August 5,1994 Authors Dr. Frank J. Berte' Dr. Peter S. Jackson, PE Mr. David S. Moelling, PE L

~. -. - -.. ~.. - Riport No. 94-SCE-OO3 P gs 1 I August 5,1994 1 EXECUTIVE

SUMMARY

The White Paper ("A White Paper, Fastener Strength Analysis, Nuclear Safety Concem 93-11" Reference 1) was reviewed by Tetra Engineering Group to with regard to statistical and sampling concems. The White Paper was written to - demonstrate that there is a high degree of confidence that fasteners now inservice in the San Onofre Nuclear Generating Station (SONGS) and in the SONGS Warehouse possess adequate margin with respect to ASME code and i design requirements. This was addressed in the White Paper by selecting a sample of warehouse stock fasteners, inspecting them with Johnson Indicating gages to provide accurate measurements of thread pitch diameter, and assessing the impact of the worst observed deviations on fastener strength. Several concerns were raised with regard to the sampling and analysis of warehouse stock. We have restated those concems as follows i

1. Was the sampling procedure used to select items from warehouse stock structured to al!ow valid statistical conclusions regarding the probability /ccr fidence level of " worst case" deviations in pitch diameter? (i.e. was -

it a " random" sample) 2.Was the stock in the warehouse procured and inspected in such a way that the ~ current warehouse stock is representative of fasteners taken from stock? (i.e. does it appear that accepted fasteners are produced and accepted to a consistent quality level?)

3. Based upon the samples drawn, can it be shown hat the fastener stock has adequate strength margin at a high probability / confidence level?

We examined the White Paper sampling procedure, the data obtained as well as the past year's incoming receipt inspection data to address these questions. With regard to Question 1, the samplir.g procedure used was essentially a systematic sampling plan in nature. Provided the ordering of samples by material code was random with respect to dimensional characteristics, systematic sampling is essentially equivalent to a random sampling. Since there is some order to the assignment of material codes by alloy, size, thread pitch. etc., it is not possible to positively determine randomness in sample order. The sampling procedure for individual lots was properly employed to draw random samples. The statement in the White Paper that the use of an attributes based lot sampling plan with a 95% confidence limit implies the same confidence level in the limiting thread measurement is not correct. That said, a comparison of the White Paper data with the recent receipt inspection data leads us to believe that conclusions drawn from the White Paper Data would be consistent with data drawn using a truly random sample. We recommend that a limited re-sample be made to confirm this conclusion. TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

Riptrt No. 94-SCE-003 P;gs 2 August 5,1994 With regard to Question 2, the data indicates that the fasteners procured to SYSTEM 21 have been manufactured so that the average thread dimensions (in this case pitch diameter) are maintained within or close to ANSI B1.1 tolerances. While individual items may exceed ANSI B1.1 tolerances, over a number of items the average tends to lie within or close to the tolerance bands for external { thread items. For intemal thread items (nuts) it appears that the suppliers have j allowed the production lots to trend slightly in direction of greater assembability, and a greater number are somewhat outside of the ANSI B1.1 tolerances. In all cases production tolerances appear to be in statistical control with no observable trends with regard to item type or size. From this we conclude that the dimensional quality of fasteners has been consistent at least over the period covered by the supplied data (1992-1994). Thus fasteners drawn from warehouse stock and installed in the plant should have the same dimensional characteristics as the warehouse stock. With regard to Question 3, we conclude that the White Paper data would be similar to data obtained from a truly random sample. Based on this conclusion we determined the limiting out-of-tolerance condition for intemal and for external thread items. These conditions were determined ouch that for an item drawn at random from any Material Code bin in the warehouse there would be a 95% probability that the item's Pitch Dimension would be more conservative that the limiting condition. The distributions of dimensional deviations obtained from the data were used to confirm that the strength margin calculations in the White Paper were consistent with the data. The average fastener has negligible difference from the nominal fastener strength, and the " limiting worst case" fasteners have only minor loss of strength margin. TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

R port Ns. 94-SCE-O'03 P&gn 3 August 5,1994 INTRODUCTION The White Paper ("A White Paper, Fastener Strength Analysis, Nuclear Safety Concem 93-11" Reference 1) was reviewed by Tetra Engineering Group to with regard to statistical and sampling concems. The White Paper was written to - demonstrate that there is a high degree of confidence that fasteners now ) inservice in the San Onofre Nuclear Generating Station (SONGS) and in the SONGS Warehouse possess adequate margin with respect to ASME code and design requirements. This was addressed in the White Paper by selecting a sample of warehouse stock fasteners, inspecting them with Johnson Indicating gages to provide accurate measurements of thread pitch diameter, and assessing the impact of the worst observed deviations on fastener strength. Several concems were raised with regard to the sampling and analysis of warehouse stock. We have restated those concems as follows;

1. Was the sampling procedure used to select items from warehouse stock structured to allow valid statistical conclusions regarding the probability / confidence level of " worst case" deviations in pitch diameter? (i.e. was it a " random" sample) 2.Was the stock in the warehouse procured and inspected in such a way that the current warehouse stock is representative of fasteners taken from stock.? (i.e.

does it appear that accepted fasteners are produced and accepted to a consistent quality level?)

3. Based upon the samples drawn, can it be shown that the fastener stock has adequate strength margin at a high probability / confidence level?

j We examined the White Paper sampling procedure, the data obtained as well as the past yea'r's incoming receipt inspection data to address these questions. l TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

t Rrport No. 94-SCE-003 Pags 4 i August 5,1994 REVIEW OF WHITE PAPER SAMPLING PLAN The White Paper sampling plan can be summarized as follows: l

1. All safety-related fastener Material Codes'(for sampling purposes these constitute lots) were identified.

2. Material Codes not inspected to SYSTEM 21 or SYSTEM 22 (Ref. 2) were deleted as were all items with diameter less than 0.5".

l

3. Material Codes previously inspected with indicating gages were also deleted from the population.

4. The remaining Material Codes (286) were ordered by general item type (bolt, nut, all-thread, etc.)

5. Every ninth Material Code was selected from the list for sample selection.

This resulted in 32 Material Codes (lots) with at least one of each general item type.

6. From each of these 32 lots, samples were drawn using the sampling procedure of Reference 3. This is an attributes based lot acceptance sampling plan that imposes random selection of samples. The number of samples ranges from 4 to 32 and depends on the lot size, in this case the number of items in the l

warehouse bin. This resulted in 356 items drawn. l

7. These item were inspected by indicating gages to determine the thread pitch diameter (PD) as defined by ANSI /ASME B1.1 (Ref. 7). Some items (long all l

thread) were measured in more than one location so the total number of i measurements was 425. i The first observation we made with regard to this sampling procedure was that it was developed in an ad-hoc manner using attribute based acceptance sampling plans as guidance. For this type of problem where obtaining an accurate estimate of population characteristics is the goal, a survey sampling plan should have been used. The reason for using a formal, theoretically defined plan is that the precision (confidence) of the results can be computed. We then looked to see if the White Paper sampling procedure could be matched to any standard survey sampling plan. l l The WP plan is of the same type as what are called " systematic sampling plans". In a standard systematic sampling plan (Ref. 4), the N units of a population are numbered from 1 to N in some order. To select a sample of n units, a unit is ( drawn at random from the first k units and then every k units the~. Jfter until n l units are drawn. This type is called an every kth systematic sa 9f!e. Systematic sampling is easy to implement, but suffers from the e awback that - l TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

Rip:rt No. 94-SCE-003 - PIga 5 August 5,1994 the precision can vary depending on any underlying structure in the dataset. If the ordering of the data is essentially random, so that the measured statistic is not correlated with the ordering variable, "there will then be no trend or. stratification in [the measured statistic) as we proceed along the file and no correlation between neighboring values. In this situation we would expect systematic sampling to be essentially equivalent to simple random sampling and to have the same variance." (Reference 4). This situation would be true if there were no correlation between material code and the dimensional characteristics of the fasteners. A review of some of the material code listings shows some degree of trend in MC number with item diameter, length, thread pitch, etc. Although an examination of the data (discussed later) does not indicate any correlation of these with pitch diameter deviations, it would require a detailed analysis to prove this definitively. We would recJmmend a limited random resampling to confirm randomness in the - data order. l The second stage of the sampling process uses the procedure of Reference 3. It produces an acceptably random selection of items from the lot. The White Paper (Page 10) makes the following statement with regard to the precision of the sampling plan: i "Following the statistical logic that provides a 95% confidence level to lots / batches found acceptable using this sampling program, it can be shown that the same confidence level can be applied to the band of readings found during j this specific warehouse sample. In short, the sampling plan is designed to provide a 95% confidence level tW there are no greater out-of-tolerance conditions in the warehouse stor n those identified in this sample." { e This statement is not correct. tes acceptance plans such as that of Reference 3 are developed using nypergeometric probability comptdeticas or the equivalent. Relating the confidence levels to tolerance bounds on specific diameters may or may not be true depending on the details of the plan. As 1 described later, the observed " worst case" out-of-tolerance items in fact are essentially 95%/95% probability / confidence bounds, but this is not due to the 1 design of the sampling procedure as taken from Reference 3. For the purposes of further statistical analysis, we refined the goal with regard to warehouse stock. A 95% confidence level on the largest out-of-tolerance condition with respect to the entire warehouse stock could be affected by the relative stock numbers of individual items (for example lots of nuts and few large - bolts). Since items from any safety-related material code could be installed in the plant, we formulated the following goal: l For any item, drawn from the stock in any Material Code, there will be a 95% probability at a 95% confidence that the deviation of the items Pitch Diameter from nominal will be less than the computed limiting values. TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

I t j RIport No. 94-SCE-003 Pags 6 August 5,1994 l REVIEW OF AVAILABLE DATA l White' Paper l Inspection data from the White Paper (Reference 5) was supplied in EXCEL { spreadsheet format. Information was supplied for 425 individual items in 32 j separate material codes. Five of these material codes are used for both internal- + thread (nuts) and extemal threads (all thread) items. These were split into separate groups for the purpose of this analysis. Each item had the following information provided. j Sequence number from White Paper inspection e Material Code (SCE) l Supplier l Material Grade e i j Intemal/Extemal Thread Identification e l Material Specification Nominal Pitch Diameter (inches) Measured Pitch Diameter (inches) Maximum Pitch Diameter from ANSI B1.1 (Reference 2) e Minimum Pitch Diameter from ANSI B1.1. (Reference 2) J RSO Number (SCE) e Description of item i Recent inspection Samples A set of inspection data from new material receipt inspections (Reference 6) was 4 supplied in EXCEL spreadsheet format. This data covered inspections from 7/15/93 to 7/15/94. The data included all inspections for those fastener types covered by the White Paper, but excluding re-inspection or re-stocking inspections. It includes all lots inspected including those rejected by receipt inspections. 2089 Individual items were inspected from 65 separate material - codes. Only one material code was common between this data set and the White Paper sample data set. Each item had the following information provided. . Sequence number Material Code (SCE) Supplier Number e Material Grade e Intemal/Extemal Thread identification e Material Specification e TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070 ~

\\ R port No. 94-SCE-003 P:gs 7 { August 5,1994 Nominal Pitch Diameter (inches) ] e Measured Pitch Diameter (inches) Maximum Pitch Diameter from ANSI B1.1 (Reference 2) Minimum Pitch Diameter from ANSI B1.1. (Reference 2) RSO Number (SCE) e RSO Revision (SCE) Requester e Test Type Test Lab Report Number Description of item 4 Purchase Order Number Supplier Name Statistics Used Strengtn is one of the primary features important to the end use of the fasteners. Fastener pitch diameter is the dimension which has the primary impact on fastener thread strength. Thus the thread characteristic of interest is the Pitch Diameter (PD). The deviation of actual PD (PD.) with respect to the nominal PD (PD n) and the acceptance band about the Nominal PD is the population characteristic of interest. To assess this the following statistic was computed for each item in the two data sets: RAPD = (PD n-PD ) /(Max PD - Min PD) where: Max PD = Maximum Allowed PD from ANSI B1.1 Min PD = Minimum Allowed PD from ANSI B1.1 This represents the deviation of the items PD from nominal PD as a fraction of the allowed tolerance band. Values of RAPD within -0.5 to +0.5 thus lie within the allowed band. Note that in a% plots of this deviation there are three reference lines given: Nominal = 0 (No deviation from Nominal PD) Min = 0.5 (Measured PD is at the Minimum Allowed value by ANSI B1.1) Max = -0.5 (Measured PD is at the Maximum Allowed Value by ANSI B1.1) TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

4 R: port No. 94 SCE-003 - P:gs 8 j August 5,1994 CHARACTERISTICS OF DATA Homogeneity of PD data i To assess any set of data in a statistical method, certain key assumptions of randomness and homogeneity of the data set must be examined. Since both the White Paper data and the Recent inspection Data represent mixtures of item 3 l types and sources, the assumption of homogeneity with respect to RAPD was examined. This is a prerequisite to determining if the effect of ordering semples i by material code in the White Paper Sample resulted in non-random samples. j Internal / External thread Intemal Thread (Nuts) and Extemal Thread (bolts, studs, threaded rods) items are manufactured in different manners. To check for homogeneity of RAPD between intemal and extemal thread items, histograms were prepared for all intemal thread and all external thread items in both data sets and compared. l White Paper Data l Figure 1 shows the Internal Threads from the WP Data Set: j Figure 1 j Distribution of Relative PD Deviation White Paper Internal Thread items 50: } t i 40-1 i j 30-1 4 20 10 Std. Dev =.35 i Mean =.51 I u. 0 .19 .10 N = 178.00 -1.93 -1.64 -1.35 -1.06 .77 -A8 -1.78 -1.49 -1.21 .92 .63 .34 .05 .24 Relative PD i TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070 1 1

j RIport No. 94-SCE-003 P:gs 9 i August 5,1994 Figure 2 shows the External Threads from the WP Data Set. ~ 4 Figure 2 ) i Distribution of Relative PD Deviation { White Paper External Thread items j 50 1 40 30 l 20 g E i s 10 Std. Dev =.26 { N N = 247.00 Mean =.22 I u0 iggg,j4>b'b>b42%v4%, 4 l Relative PD i 1 1 J l These represent a large number of different item types and suppliers but it is clear that the two groups are statistically different. The Internal Thread items tend to have deviations in the direction of larger than nominal pitch diameters and the External thread items tend to have deviations in the direction of smaller than nominal pitch diameters. k )- TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070 1 4

Rrpsrt No. 94-SCE-003 Pegs 10 August 5,1994 Recent inspection Data Figure 3 shows the Internal Threads from the Recent inspection Data. J Figure 3 F Distribution of Relative PD Deviation Recent Acceptance Data Icternal Thread items 30C' 20G 1 00 Std. Dev =.33 5 e t[L Mean =.41 i 0, N = 1598.00 I gggggg/jWWW4W'd@@@ l 1 l Relative PD 1 i f f 4 i i 1 1 l TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

R; port No. 94-SCE-003 Paga 11 - August 5,1994 Figure 4 shows the Extemal Threads from the Recent inspection Data, l Figure 4 t a i Distribution of Relative PD Deviation Recent Acceptance Data Extemal Thread items 140 120 100 80 i 60 D g 40 s Std. Dev =.25 { 20 Mean =.20 LL. O. N = 491.00 1 .75 .39 .03 .33 .69 1.05 .57 .21 .15 .51 .87 Relative PD The distributions of the Recent inspection data show similar populations for the Intemal and External Thread items as the White Paper data. The presence of larger extreme values in the Recent inspection data is related to the inclusion of alllots, notjust those that passed acceptance tests. Comparison of the overall means ano standard deviations for the White Paper and Recent inspection t Results show reasonable consistency. The Table 1 illustrates this: 4 i 1 3 TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070 I i

Riport No. 94-SCE-003 Pign 12 l August 5,1994 i I Table 1 i Comparison of White Paper and Recent Receipt Inspection Data t Extemal Thread Summary Statistics - Relative PD (Dimensionless) Statistic White Paper Recent Receipt j Mean PD Deviation 0.22 0.20 j j Std. Dev. of PD Deviation 0.26 0.25 b l Intemal Thread Summary Statistics - Relative PD (Dimensionless) j Statistic Wh'te Paper Recent Receipt 1 Mean PD Deviation -0.51 -0.41 j Std. Dev. of PD Deviation 0.35 0.33 Material Code The key factor to be examined for homogeneity of PD deviation is the individual material codes. It is important as like items are installed in many plant applications (for example several bolts are installed in a single flange or coupling would be of the same material code). Boxplots were used to rapidly compare the statistics of individual material codes. These are provided in the Appendix. There is significant difference between many material codes in both the extemal and intemal data. The intamal thread MC to MC variation appears to be larger t than the extemal thread MC to MC variation. ~ l 1 ) TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

[ *, . RIport No. 94-SCE-OO3 Pags 13

August 5,1994 i

} Supplier Differences between Suppliers in a systematic way would be a source of } significant non-randomness in the samples. Again boxplots were used to provide'a rapid comparison. These are supplied in the Appendix. By looking at ~ 1' various material code data from a single supplier we found considerable. ~ deviation between material codes. The variability in deviations between material l codes is a good indication of a lack of systematic effect of a supplier. The data for a single supplier was also examined for any trend with item size as indicated by nominal PD. There was no obvious systematic trend giving further evidence - 4 for randomess in the Material Codes. ] i i i The data shows that is no indication of a significant systematic supplier effect i and that variations are very likely to be production lot to production lot variations l ( rather than supplier to supplier in nature. 1 i 4 i Control of Pitch Diameter l The observed deviations of Pitch Diameter in both the White Paper and the I l Recent inspection data indicate that the fastener manufacturers are in general i achieving a good state of statistical control. For extemal thread items is appears J j that most suppliers are attempting to control the mean PD to the ANSI B1.1 1 j tolerances. A similar situation is found with the intemal thread items except that j a wider range of deviation is observed in the direction of greater assembility. l-This allows a reasonable expectation that future shipment from these suppliers j will show a similar behavior as some measure of statistical control is in place. The variability in any particular item seems to be due to production lot to j production lot variations. Figures 9,10, and 11 show this clearly. Production j from a single supplier varies from MC to MC but does not depend on item size - for example. e i I 1 3 ) lI TETRA ENGINEERING GROUP, INC SIMSBURY CT 06070 L a . n

RIport No. 94-SCE-003 Pag'a 14 August 5,1994 COMPUTATION OF LIMITING OUT-OF-TOLERANCE VALUES Assuming that the WP data is essentially random in order, limiting out of-tolerance values can be computed. As is clear from the data, internal and extemal thread items must be treated separately. It does not appear that item type or dimensions have systematic effect, nor that the item supplier has a strong non-random effect. Clearly there is both a between-lots and a within-lot (material code) effect. This is possibly due to the particular sequence of j production lots going into a material code bin at any particular time. Because of the varying sample sizes resulting from the use of the Reference 3 procedure, sample size weighted estimates of these effects must be used. For the estimation of the lot-to-lot variation the following procedure is used (both for internal and extemal thread groups); )

1. Compute the mean and variance of RAPD for each material code lot.

2. Compute the weighted mean and variance of the lot averages (means) as.

n,x a,* x= %-.-w= h-h 2 k-I where: mean value of lot i l xi = 1 standard deviation oflot i = ai k number of lots = (Reference 9).

3. Compute the weighted variance of the within lot variation as:

TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

RIport No. 94-SCE-OO3 P:ga 15 August 5,1994 "*,"(((n,-1)1,2 j n, - k (Reference 8)

4. Combine the variances as; c '., = a Lu + o '.,

5. Estimate the upper and lower limits as:

U = 5 + Ko w L=i-Ko_, where K is the tolerance factor for a population proportion P of a normal distribution (Reference 8) with n=k lots and P=0.90 (two sided limits) and a significance level of 0.05 (95% confidence). Limiting External Threads Applying the calculation to the White Paper External thread data results in the following values: K 2.244 (K Factor) = X 0.208 (Weighted Lot Mean) = 0.2439 ' (Lot-to-Lot Variation) otot.co. tot = = 0.1652 (Within Lot Variation) cte otots = 0.2945 (Total Variation) 0.8689 (Upper 95% Limit) ULes = Llos = -0.4528 (Lower 95% Limit) The minimum material condition is defined by the ULes value. Thus for any external thread item, there is a 95% probability its Pitch Diameter will be greater than the nominal PD minus 87% of the ANSI B1.1 Tolerance value. TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

4 R; port No. 94-SCE-003 Paga 16 j* August 5,1994 4 Limiting Internal Threads Applying the calculation to the White Paper internal thread data results in the following values: i K 2.529 (K Factor) = -0.274 (We ighted Lot Mean) X = = 0.2737 (Lot 'o-Lot Variation) otoi.i to 0.2396 (Witha Lot Variation) = ciot etot.i 0.3638 (Total Variation) = = 0.646 (Upper 95% Limit) ULes } LLes -1.194 (Lower 95% Limit) = i The minimum material condition is defined by the LLes value. Thus for any i intemal thread item, there is a 95% probability its Pitch Diameter will be less than the nominal PD plus 119.4% of the ANSI B1.1 Tolerance value. ) i ) i i t E i .t 1 1 TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070 +

Riport No. 94 SCE-003 PIga 17 l* August 5,1994 ASSESSMENT OF IMPACT ON FASTENER STRENGTH To examine the impact of the observed dimensional deviations on the strength of a typical fastener installation the example in the White Paper was used. This computation (page 4 of Reference 1) is an all thread stud 5/8" diameter - 11 threads per inch with a matching nut. A Monte Carlo simulation was prepared using the same formulations as in the White Paper. The exception is that the distributions of Intemal and Extemal Thread deviations were used instead of the worst case values. By simulating the analysis, the upper and lower limits on thread strength corresponding to 95% probability limits can be estimated. The formulations used were: Ass = x (1/ P)(LE)D1 max [0.5(1/ P) + 0.57735(D2 min-D1 max) Where: Ass = Minimum Thread Shear Area for External Threads P = Thread Pitch (inches) LE = Length Engaged (inches) D1 max = Maximum Minor Diameter of intemal Thread D2 min = Minimum Pitch Diameter of Extemal Thread The Shear Strength of the Threads is then: l Thread Strength = 0.5

St
Ass i

where. St = Ultimate Tensile Strength of the Bolt Material. i l For the simulation the values used were: l P = 0.09091 inches LE 0.625 inches = Dimax = 0.5460 inches (Minimum Material Condition) D2 min Normal Distribution from WP Data = St 125 Ksi = l l The noimal distribution had the parameters: i l TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

Rhport No. 94-SCE-003 Pigs 18 August 5,1994 0.5648 inches Mean = S.D. = 0.00^>0 inches - The basic thread shear area computation is an input to the design load computation. The applied shear stress at Maximum Design Load is given by: Applied Shear Stress = Maximum Design Load / Shear Area From the White Paper Example (page 7) the Maximum Design Load is 7.91 Ksi and the Maximum allowable shear stress (0.6Sm) is 16 Ksi. The Monte Carlo Simulation then computes the distribution of the ratio; Rcode = Maximum Allowable Shear Stress / Applied Shear Stress at Design Load. The formulations were implemented in an Excel spreadsheet and the simulations run using the @ Risk Monte Carlo Simulation Package. One thousand simulation trials were run in the simulation which were sufficient to produce good convergence. The results are shown in the following tables: Table 2-Basic Thread Strength: Percentile Simulation - Thread White Paper Worst Case - Strength Thread Strength 5% 38,667 pounds 37,502 pounds 50 % 40,820 pounds Not Computed 95 % 42,971 pounds 47,700 pounds The 5% percentile is the limit of interest and it is seen that the White Paper ' Worst Case" is conservative. Since the limiting values computed from the sample statistics are normalized and do not depend on item type or dimensions, it can be expected that similar results would be obtained for other item types. The range of the predicted thread strength is also consistent with the White Paper results. TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

l l Report No. 94-SCE-003 Page 19 August 5,1994 i Table 3-Applied Shear Stress at Maximum Load for ASME Section lli Class 1 Percentile Simulation - Shear Stress White Paper Worst Case l Shear Stress l' 5% 11,500 pounds Not Computed 50% 12,120 pounds Not Computed l 95 % 12,780 pounds 13,173 pounds i } The margin to the code limits is very good. Figure 5, shows the distribution of j the ratio of Maximum Allowable Shear Stress / Applied Shear Stress at Design Load. l Figure 5 i l Distribution of ASME Stress Limit to Applied Stres ,3-i l l l l i p l l a ._______.1. ______2__ l R j i o f B A l l B l l .!______...l......... L I l T L--------- em- --------- -- Y l Bham .. - um u. ,a e L, su is ,a 'alues i i KSl I l_ The key percentiles are shown in the following table. !i i 1 j TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070 f (

Rsport No. 94-SCE-003 Paga 20 August 5,1994 - Table 6 - Load Capability t Percentile Simulation - White Paper Worst Case 1 5% 1.25 (125 %) 1.19 (119 %) 50 % 1.32 (132 %) Not Computed 95 % 1.47 (147%) Not Computed Thus for items with adverse thread dimensions at the 95% probability /95% confidence level (approximately) there is still a 25% margin to the code allowable l limits. This confirms the conclusions of the White Paper bounding computations. It would be expected that computations for other items would show similar results. l l l l l l TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

i J Report No. 94-SCE-003 Pzgs 21 August 5,1994 i RECOMMENDATIONS Development of Revised Acceptance Plan for PD Based upon the behavior of the data for both data sets, it should be possible to Implement a revised acceptance plan based upon indicating gage measurement of Pitch Diameter (as well as other dimensional features). A strength based tolerance on PD can be established which should be only a modest increase over the ANSI B1.1 tolerances. The current festener suppliers should be able i to provide acceptable product with little increas ) in lot rejection over the current i SYSTEM 21 plan. I 4 l k l 4 j i TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

I R; port No. 94-SCE-003 P:gs 22 August 5,1994 1 REFERENCES

1. "A White Paper, Fastener Strength Analysis, Nuclear Safety Concern 93-11",

Michael B. Ramsey, May 6,1994.

2. ASME/ ANSI B1.3M-1993 " Screw Thread Gaging Systems for Dimensional Acceptability - Inch and Metric Screw Threads"

3. " Sampling Program for Assessing, Estimating and Reporting Commercial Grade item Quality", Procedure SO123-XXXll-2.5, Revision 1.

4. _Samoling Technioues. Third Edition, W. G. Cochran, Wiley, New York,1977.

5. Transmittal of White Paper Sample Data, Disk and Letter, Michael Ramsey to Frank Berte', July 24,1994.

6. Transmittal of Recent inspection Sample Data, Disk and Letter, David Ovitz to Frank Berte', July 22,1994.

7. ASME/ ANSI B1.1-1989, Unified Inch Screw Threads.

8. Statistics Manual, E.L. Crow et. al, Dover, New York,1960.

9. Statistical Theory with Engineering Aeolications. A. Hald, Wiley, New York, 1952.

TETRA ENGINEERING GROUP,INC. SIMSBURY CT 06070

RIport No. 94-SCE-003 Pzg3 23 August 5,1994 APPENDIX - MATERIAL CODE AND SUPPLIER VARIATION .l Boxplots I Boxplots show a number of summary statistics on a single plot. The horizontal line in the box is the median. The lower boundary of the box is the 25th percentile and the upper limit of the box is the 75th percentile. The largest and 1 smallest observed data lying within 1.5 box lengths of the median are shown by the lines from the box (" whiskers"). Outliers and extreme values (greater than j 1.5 box lengths away) are shown by asterisks and circles. 4 From a boxplot much information can be drawn quickly. The median shows the center of the data. The length of the box shows the spread of the data. If the median is not in the center of the box the data is skewed. The length of the distribution tails are visible in the " whiskers" and outliers. J Relative PD Deviation by Material Code i The variation of pitch diameter deviation between material codes is seen in these boxplots. i 1 i i 1 1 TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

RIport No. 94-SCE-003 Pags 24 August 5,1994 Boxplot of Relative PD Deviation vs MC WP ExternalThread items 1." 1.0 I

a;. ee k s.z.= 5 s 2 P.,.*dP*5

~ 0.0 i O e ~~ m 2 m v ifu m g CL .v y ) .$-1.G 16 i N =10 5 5 5 10 7 5 1010 9 7 39 4, 1212 3 32101010 8 20 4 i l Material Code ] Boxplot of Relative PD Deviation by MC i WP Internal Thread Data j 1.5 1.0 . 5-i 00 j .er .:ss. -,-- ^ T-8 m n E -- ~ 4 ge g ~~ um ,g-1.0 1 ^ I -1.5 h-2.0 N = iO 8 31 8 i8 5 i0 i4 7 i8 i0 iO 5 20 250837 275366 30504213050440305048925094202512622i 3 i 260276 3050311305042930504763050908251234125880361 l l Material Code l TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070 I 4 -. ~., .-,-,.n..

l R; port No. 94dCE-003 Page 25 l

August 5,1994 Boxplot of Relative PD Deviation by MC Recent Acceptance Data Internal Thread items 2-l 1:

1-A A, -- _ o 0-l g v.er M o e -- - j g.3 m l 1-l A l fh S e ! -2 N= 5 f2 3 f5 8 f5 485 7'33 i f1 h 14 51 5 1'48 i i0 % 'o % 's J MATERIAL l Relative PD Deviation by Supplier l The variation in pitch diameter deviation between material codes for a single supplier is shown in these plots. Boxplot of Relative PD vs. MC WP Data, External Thread, Supplier: Cardinal 1.0-00 ~O1 z 1 g .5-g ,g -1.0 L W 1.5-k 2.0 Na f0 i0 i f 4 i O f2 i i0 i i0 f4 250837 3050164 3050560 3050602 3050689 2509420E 2588036E l 3050093 3050184 3050573 3050615 3051073 2512341E Material Code l TETRA ENGINEERING GROUP, INC SIMSBURY CT 06070 l

4 R: port No. 94-SCE-003 Prga 26 August 5,1994 I Boxplot of Relative PD vs. Nominal PD WP Data, External Thread, Supplier: Cardinal 1.5 1.0 < 5 o g 0.0 I 6 .5 - g j 2 -1.0 < -1.5< (E -2.0 N. 6 12 f2 1'2 25 2'O 3 .45 .56 .80 .82 .91 1.16 1.26 l Nominal item Pitch Dimension (Inches) l l This plot shows a single material code data by supplier. These are 36" lengths of all thread and several measurements were taken along a single item. Thus the data for each supplier is a single item. l Boxplot of Relative PD vs. Supplier MC 2509420 External Thread, WP Data 1.5 - 1.0 - .5 - 0.0 .5 ^ -1.0< -1.5 - -2.0. No 2 k 4 } A&G Cardinal Tx Bolt Supplier These plots show similar data for Recerit inspections data, i i TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

R: port No. 94-SCE-003 P,rgi 27 August 5,1994 Boxplot of Relative PD Deviation by Supplier Recent Acceptance Data External Thread item 2: 1< ~ R 1- ",r"- 0-i ,.3 l 3 -2< h -2 N= i2 28 87 23 4 b 3370 9897 12437 12659 28648 33649 Supplier Code Boxplot of Relative PD Deviation by Supplier Recent Acceptance Data Internal Thread items 2-1< 1: 0-G 0 E M i E g -- CL g h -2< b s -2 l N= 468 1'23 42 1'OS 8'48 9897 12437 12659 18171 40250 l Supplier Code t TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

i t e i A White Paper-FastenerStrength Analysis, Revision Two a i i 1 l ATTACHMENT 3 l l 4 4 COMMENTS ON SAN ONOFRE NUCLEAR GENERATING STATION BOLTING i i l BY l REEDY ASSOCIATES,INC. May 2,1994 i f J 96 --w. e

9EEDY$550CIATES,lMC. ENGINEERING MANAGEMENT CONSULTANTS May 2,1994 SCE-94-008 Mr. Michael B. Ramsey Senior Engineer Souther California Edison San Onofre Nuclear Generating Station Post Office Box 128 San Clemente, CA 92674-0128

Dear Mike,

Enclosed are my comments regarding the San Onofre Nuclear Generating Station bolting. If you have any questions, please feel free to give me a call. Very truly yours, oger ee RFR\\n Enc. 15951 LOS GATOS BLVD., SUITE 1

LOS GATOS, CALIFORNIA 95032 * (408) 356-6300

9EEDYrfl550CIATES,1NC. ( ^. ENGINEERING MANAGEMENT CONSULTANTS COMMENTS ON SAN ONOFRE NUCLEAR GENERATING STATION BOLTING Nuclear power plants are designed by Registered Professional Engineers trained and qualified to design pressure retaining equipment and the associated bolting. The pressure retaining components in these plants are designed and constructed to strict requirements of the ASME (American Society of Mechanical Engineers) Boiler and Pressure Vessel Code, Section III for Nuclear Components. The requirement to meet the provisions of the ASME Section III Code is mandated by the U.S. Federal Regulations 10CFR50.55a. Bolting for pressure components (including all nuclear components in the piping systems) must comply with Section III of the ASME Code. I have reviewed and evaluated the system used by SCE (Southern California Edison) at their San Onofre Plant to inspect, evaluate, and accept or reject bolting and I am firmly convinced that the SCE inspection program for bolting fully complies with the Federal Regulations and the ASME Code. In fact, the bolting inspection program exceeds both Federal and ASME Code requirements. Although the act of bolting items together appears to be a simple operation (almost everyone has assembled nuts and bolts at some time in their life), there are significant engineering principles involved in the design of bolted connections. It is the responsibility of the design engineer to consider these principles in the design of the bolted equipment and to establish or approve appropriate tolerances and acceptance criteria. The nuclear plant must be constructed and maintained to the tolerances and acceptance criteria selected by the responsible engineer. l For pressure retaining components (piping, pumps, valves, and pressure vessels), bolting is pre-stressed. That is, during assembly, the bolts are tightened in such a manner that system pressure and seismic loads will not increase the stress in the bolts. This operation is known as prestressing the bolts. That means that the worst condition ofloading occurs when the workmen first tighten the bolts. If the bolts don't fail during this tightening as prestressing, they will not fail in service due to dimensional variations. The prestressing operation is a good test for any bolting because any significant problems associated with the bolting will show up during the initial bolting of the items rather than during plant operation. In the design of bolting, the ASME Code requires a design factor (safety factor) of 4 or

5. This means that bolts shouldn't fail until they are loaded to a level 4 or 5 times the design load. There has been more than 50 years of experience using these design factors, May 2,1994

\\sce\\ bolting - 1 of 3 - 15951 LOS GATOS BLVD., SUITE 1

LOS GATOS, CAUFORNIA 95032 * (408) 356-6300

j and bolting failures caused by dimensional variations has not been a problem in the - l piping industry. It should also be noted that bolting in pressure piping is redundant. That is, failure of one bolt will never cause failure of the connection because there are many other bolts to take up any additional load caused by the failure of a single bolt. The ASME bolting standards allow the design engineer to select from a series.of acceptable tolerances. The most common.way of assuring. tolerances for pressure retaining applications is to allow the "Go/No Go" gage system. This is known as System

21. This system of tolercnces is fully acceptable for ASME Code applications and is used extensively in Indusay, including the U.S. Navy nuclear reactor submarine i

program. The ANSI /ASME Bl.2-1983 Standard " Gages and Gaging for Unified Inch Screw Threads" states: i " Product threads accepted by a gage of one type may be verified by l other types. It is possible, however, that parts which are near a limit l may be accepted by one type and rejected by.another. Also, it is possible for two individual limit gages of the same type to be at opposite extremes of the gage tolerances permitted,'and borderline product threads - accepted by one gage could be rejected by another. For these reasons, - l-a product screw thread is considered acceptable when la passes a test by I ' gly of the permissible' gages.in ANSI Bl.3 for the gaging system j specified,'provided the gages being used are within the tolerances l specified in this Standard." [ Emphasis added.] ^ j! He manufacturer of the bolts has the responsibility for meeting the bolting tolerances specified by the purchaser. The user of the bolts can re-check the bolt dimensions if it is felt necessary. Normal nuclear industry practice is that sometimes this tolerance verification is performed, sometimes it is not. De tolerance verification is not a i requirement of any regulations or Code. Tolerance Systems 21,22, and 23 are not used l l to determine the structural adequacy of the bolts. As stated in the ASME Bl.3M-1992 Standard (4b), which defines the Systems, " Screw Thread Gaging Systems for Dimensional Acceptability - Inch and Metric Screw nreads," paragraph 4(b), "The - difference between gaging systems is the level of inspection deemed necessary to satisfy l that dimensional conformance has been achieved...." Note that the Standard implies there is no strength criteria associated with any of these j inspection systems. His is the correct implication. The bolting gaging systems are for inspection purposes only. The engineer designing the bolting determines which system is appropriate to the design, and inspectors must assure that the engineer's tolerances are met. The issue at San Onofre centers around whether fasteners which pass System 21 requirements, but exhibit minor dimensional out-of-tolerance conditions when examined l May 2,1994 \\sce\\ bolting - 2 of 3 -

1 + - i. 1-l to System 22 requirements, will fail in service. The increase in measurement accuracy identified by System 22 or 23 inspections has no significant effect on the strength of the bolting. Herefore, "Go/No Go" gages are appropriate for inspecting ASME Code l bolting. i i j SCE has decided to screen bolting suppliers by using closer tolerances by using special gages which are more accurate, measure major characteristics and can check bolts much - ,] P more quickly. These special gages are good, but are many times more accurate than j necessary. If the readings from the special gage show deviations from either System 22 or 23 tolerances, the bolting is still acceptable if within the tolerances specified by, or j acceptable to, the responsible engineer. l It is my professional opinion that the SCE quality program meets the ASME Code and 4 } the Federal Regulations. Neither the ASME Code nor the Federal Regulations, nor any j heavy industry bolting standards require the use of the new "high tech" gages and System 22 or 23 inspections. Deir use is far beyond requirements and industry practice. The l i increase in dimensional accuracy afforded by measuring thread attributes by System 22 l or 23 has no significant effect on the strength of the bolting, is much less than the design l i-safety factors inherent in Code applications and therefore has no bearing on nuclear l l safety. He analysis provided in the SONGS White Paper adequately demonstrates these r i conclusions by the specific sample inrMetion results. The issues regarding "high i i rejection rates", " don't meet standards" ud "out-of-toleran'ce" regarding the SCE boltin j inspection program, are absolutely untrue. i I .E. t i r C fornia #27562 I-i i b 1 I i i ) I ( i 1 i i ) j I 1 l j 1 1 4 i i / 1 - May 2,1904 \\sce\\ bolting - 3 of 3 - i 1 .j

A White Paper-FastenerStrength Analysis, Revision Two l I I I I I ATTACHMENT 4 l 1 I I I i STATISTICAL ANALYSIS OF NRC AUDIT DATA I FOR SONGS FASTENER STOCK l l 1 1 I I TETRA ENGINEERING GROUP l November 17,1994 i i I l I I I 101

o Statistical Analysis of NRC Audit Data for SONGS Fastener Stock Tetra Engineering Group, Inc. Report 94-SCE-005 November 17,1994 Authors Dr. Frank J. Berte' Mr. David S. Moelling, PE Mr. Frederick C. Anderson, PE i i O

REPORT NO. 94 SCE 005 1 Executive Summary A random sample of SCE warehouse fastener stock was obtained and inspected l as requested by a NRC team. This random sample included 44 bins, with a total of 519 items. The intent was to provide an independent sample of the warehouse stock. This sample included small fasteners (nominal diameter less than 0.5") which were excluded from previous samples used to prepare the SCE White Paper on Fastener Strength. The intent of this report is to compare the results of dimensional measurements of the NRC data set with the data used for the White Paper. The following conclusions can be drawn from the NRC sample data.

1. The NRC data is consistent with the White Paper Data. This confirms that the sampling procedures performed for the White Paper data set were sufficiently random to allow valid statistical results to be drawn. The limits computed from the White Paper Data bound the pitch diameter deviations in the NRC Data set.

2. The NRC data shows similar behavior in bin-to-bin and supplier-to-supplier variation as the White Paper Data set. This confirms the conclusion th it there is no systematic supplier related trends in thread deviations.

3. NRC data for smaller items not considered by the White Paper show a slightly wider range of deviation than the larger items. Otherwise they are consistent with the observations for the White Paper Data.

4. Thread Strength Margins for the smaller items are adequate even with the slightly larger deviations from the ANSI B1.1 thread limits.

5. Thread Strength Margins for the larger items are consistent with those computed for the White Paper Data. These demonstrate large margins to ASME code service requirements.

TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

REPORT NO. 94 SCE 005 2 l I l Introduction A random sample of SCE warehouse fastener stock was obtained and inspected i as requested by an NRC team. This random sample included 44 bins, with a total of 519 items. The intent was to provide an independent sample of the warehouse stock. This sample included small fasteners (nominal diameter less than 0.5") which were excluded from previous samples used to prepare the SCE l White Paper on Fastener Strength. The intent of this report is to compare the results of dimensional measurements of the NRC data set with the data used for the White Paper. l Review of NRC Sampled Data l l The NRC requested an audit sample of 44 randomly selected bins on September I 13,1994. Random samples were drawn from these bins giving a total of 519 items (Reference 1). Of these 188 were of nominal size (diameter) of 0.5" or greater and 331 were of nominal size less than 0.5". Of the set of 0.5" and greater,7 had no measured data due to the fact that no Johnson Gage Segments were available in this size for System 22 Pitch Diameter measurements, this left 181 fasteners with measured data. The set of 0.5" and greater diameter is directly comparable with the set of data used for the White Paper (Reference 2) which excluded iten's smaller than this. The analysis of the smaller diameter data set was performed in the same manner as was done for the White Paper data set (Reference 3) and is reported in Appendix 1. The data l shows the same large bin-to-bin (Material Code) variation seen in the White l Paper data, and the same lack of systematic trend with supplier as seen in the White Paper data. The boxplots illustrating these effects are provided in Appendix 2. i l l l l l l TETRA ENGINEERING GROUP. INC. SIMSBURY CT 06070

REPORT NO. 94.SCE 005 3 l Comparison of NRC Sampled Data with White l Paper Data l External Threads I l There were 43 external thread items of nominal size greater than or equal to 0.5" l in the NRC sampled data. These were drawn from nine material codes. The l basic statistics for these items are given in Table 1. l Table 1 Relative Pitch Diameter Deviation Statistics Statistic NRC Samole Wtlite Paper Samole Raw Mean 0.1658 0.22 Weighted Mean 0.2153 0.208 Raw Standard Deviation 0.2606 0.260 Weighted Standard 0.2945 0.2870 Deviation Since the NRC Data Set covered fewer material odes than the White Paper data set, the effects of bin-to-bin variation have a larger impact on the computed raw means and standard deviations. Weighted Means and Standard Deviations account for this effect and a comparison of these shows the two samples are in good agreement. The distribution of this data is shown in Figure 1. 1 I l l l TETRA ENGINEERING GROUP, INC, SIMSBURY CT 06070 l

I REPORT NO. 94 SCE-005 4 Figure 1 l I l Relative PD Deviant;n NRC Inspection Internal Thread items Nominal Diameter >= 0.5" 20 l 10- ~ Std. Dev =.27 iRI N = 138.00 Mean =. 39 01 ii l ? ?q ?,, ?q ie, is ie, is i i 4 o 4 s is, Qs% Relative Deviation Another comparison is the fraction of samples from the NRC data lying outside of the upper and lower limits computed from the White Paper Data (Reference 3). Table 2 shows this comparison. Table 2 Comparison of NRC Data Quantiles with White Paper Limits Limit White Paner Value % of NRC Data Exceedino Upper 95% limit 0.8689 0% Lower 95% limit -0.4528 0% The computed limits clearly bound the NRC data set. This clearly supports the applicability of the conclusions of the White Paper to the Warehouse stock. Internal Threads There were 138 internal thread items of nominal size greater than or equal to 0.5"in the NRC sampled data. These were drawn from 12 material codes. The basic statistics for these items are given in Table 3. YETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

~ REPORT NO. : 94-SCE 005 5 Table 3 Relative Pitch Diameter Deviation Statistics Statistic NRC Samole White Paner Samole Raw Mean -0.3873 -0.51 Weighted Mean -0.553 -0.27 Raw Standard Deviation 0.2714 0.35 l Weighted Standard Deviation 0.2765 0.364 l i The White Paper data set had 178 items from 12 material codes. The White Paper data showed a greater spread and larger average deviation than the NRC data. l l The distribution of this data is shown in Figure 2. { Figure 2 Histogram of Relative PD Deviation i NRC Data External Thread items Nominal Dia. >=0.5 14 12-10-8 6-4- Std. Dev =.26 2-Mean =.17 l 0 N = 43.00 .38 .25 .13 0,00 d3 .25 .38 .50 .63 Relative Deviation Table 4 shows the fraction of samples from the NRC data lying outside of the upper and lower limits computed from the White Paper Data (Reference 3). TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

i le REPORT NO. 94 SCE 005 6 Table 4 Comparison of NRC Data Quantiles with White Paper Limits LIDst White Pacer Value % of NRC Data Exceeding Upper 95% limit 0.646 0% Lower 95% limit -1.194 1% The NRC data set is consistent with the limits computed from the White Paper Data. This clearly supports the applicability of the conclusions of the White l Paper to the Warehouse stock. i 1 1 Assessment of Fastener Strength l The impact of the observed dimensional deviations on the strength of fasteners greater than (or equal to) 0.5 inches in diameter was examined by performing the White paper strength calculations for one typical fastener. The 5/8 all thread stud - material code 30505606, Class 2a, was selected a representative fastener. A Monte Carlo simulation was prepared using the distributions of internal and external thread deviations for fasteners greater than or equal to,0.5 i inches in diameter. One thousand simulation trials were performed to generate the distributions. From this simulation, upper and lower limits on thread strength corresponding to 95% probability limits were estimated. Since the NRC data set is bounded by the White Paper Data, the White Paper data was used for the strength assessment. The formulations used were: Ass = n(1/P)(LE)Dimax[0.5(1/P) + 0.57735(D2 min - D1 max)] Where: Ass Minimum Thread Shear Area for External Threads = P Thread Pitch (inches) = LE = Length Engaged (inches) D1 max = Maximum Minor Diameter of Intemal Thread D2 min = Minimum Pitch Diameter of Extemal Thread 'ETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

l REPORT NO. 94-SCE 005 - 7 The shear strength of the external thread (TSS) is then given by: TSS = 0.5(St)(Ass) Where: t Minimum Ultimate Tensile Strength of the Bolt Material St = For the simulation the values used were: P 0.0909 inches = LE 0.625 inches = Dimax= 0.6113 D2 min = Normal Distribution from White Paper Data Set St = 125000 psi l The Normal Distribution had the parameters: Mean = 0.5628 l S.D. = 0.0022 l I The thread shear area (Ass) and the thread shear strength were calculated. in addition, the ratio of the Thread Shear Strength to the Maximum Preload was determined using the following formula: TSS/MP = TSS/(0.7854(D-{0.9743/(1/P)}) (2Sm)] Where: j D = 0.625 inches (Nominal Diameter) Sm 25000 psi (Code Allowable Stress - Class 2 and 3) = l l 1 TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

1 REPORT NO. 94 SCE 005 8 i. i i l Figure 3 i e 1 i, .m. - ~ 6 .m m ~ H. 1;g>D,istri,.b... tto. rtofhTh.w a...uStren.mgth.. we:MaxtP_rel.oa,d,? i a-~ww -~ o u to u o re M n g - n w-r ..u-n y,a e y \\ w- > gn;g l C '

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l REPORT NO. 94.SCE 005 9 Conclusions The following conclusions can be drawn from the NRC sample data.

1. The NRC data is consistent with the White Paper Data. This confirms that the sampling procedures performed for the Whito Paper data set were sufficiently random to allow valid statistical results to be drawn. The limits computed from the White Paper Data bound the pitch diameter deviations in the NRC Data set.

2. The NRC data shows similar behavior in bin-to-bin and supplier-to-supplier variation as the White Paper Data set. This confirms the conclusion that there is no systematic supplier related trends in thread deviations.

(

3. NRC data for smaller items not considered by the White Paper show a wider l

range of deviation than the larger items. Otherwise they are consistent with the observations for the White Paper Data. l

4. Thread Strength Margins for the smaller items are adequate even with the l

slightly larger deviations from the ANSI B1.1 thread limits. l

5. Thread Strength Margins for the larger items are consistent with those computed for the White Paper Data.

References

1. Electronic Transmittal, M. Ramsey (SCE) to F. Berte' (Tetra),10/3/94.

2. "A White Paper; Fastener Strength Analysis Nuclear Safety Concem 93-11",

M.B. Ramsey, (SCE), 5/6/94.

3. " Statistical Validation of SCE Fastener Strength Analysis White Paper Data Base", Tetra Engineering Group Report,94-SCE-003,8/5/94.

1 TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070 . - ~.

4' ]. REPORT NO. 04-SCE 005 10 APPENDIX 1 i NRC Sample Data Set Fasteners Less than 0.5 Inches in Diameter i j Characteristics of Data Homogeneity of PD Data The NRC Sample data set of items less than 0.5 inches in diameter contains a mixture of item types and sources. The assumption of homogeneity with respect to the relative deviation in Pitch Diameter (RAPD) was examined for both i internal and external threads. The RAPD is defined as the nominal value minus j the measured value. To check for homogeneity of RAPD between internal and j external thread items, histograms were prepared for all internal and external l items less than 0.5 inches in diameter and compared. i j Figure 1 shows the distribution of internal threads from the NRC Sample data set j items less than 0.5 inches in diameter. ~ Figure A-1 Distribution of Relative PD Deviation l NRC Sample Data Intemal Threads <0.5 Inch 12-i 10-8- g 6-h 4-I Std. Dev =.41 l { 2-Mean =.59 i LL 0 N = 68.00 % 5 $ Ij & % & b 4 6 'b @ 'e W M er 1 i Relative PD i i i TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

REPORT NO. 94-SCE-005 11 Figure A-2 shows the distribution of external threads from the NRC Sample data set items less than 0.5 inches in diameter Figure A-2 Distribution of Relative PD Deviation NRC Sample Data Extemal Threads < 0.5 inch 80-60-40-Std. Dev =.36 3 { 20 Mean =.32 N = 283.00 1.L 0 qVdr 9pJpj@W64rMe@@+9%#e* y Relative PD A large number of different item types and suppliers are represented in the sample population for both internal and external threads. Examination of the two figures clearly shows that the internal and external thread types are statistically different. The internal thread items tend to have deviations in the direction of larger than nominal pitch diameters and the external thread items tend to have deviations in the direction of smaller than nominal pitch diameters. Table A-1 shows a comparison between the internal and external thread item statistics. Table A-1 Statistic External Thread laternal Thread Mean PD Deviation .32 .59 Std. Dev. of PD .36 .41 Deviation Material Code A key factor to be examined for homogeneity of PD deviation is the individual material codes. Homogeneity within a material code is important since many plant applications would require multiple items from the same material code. For example, several of the same type bolt installed in a single flange. Boxplots TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070 .l

J REPORT NO. 94 SCE 005 12 were used to rapidly compare statistics of the individual material codes. Figures A-3 and A-4 show the relative PD deviation as a function of material code for internal and external thread items respectively. There are significant differences between many material codes for both the external and internal thread types. The external thread type has a wider range of variation than the internal thread type, although this may be due to the larger external thread population. Figure A-3 Box Plot of Relative Deviation By Material Code NRC Sample Data Intemal breads <0.5 Inch .5 i 0.0-m I o .5-g Q. ] g -1.0-l $-1.5-I $-2.0 N= 8 31 2'9 2512200 2655322 30504054 Material Code i l 1 TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

~ l REPORT NO. 94 SCE-005 13 Figure A-4 Box Plot of Relative Deviation By Material Code NRC Sample Data Extemal lhreads <0.5 Inch 2.0-1.5-T E 5:3 _j i ,z g g o 0.0-m a ] g .5-4= 4 fo t b i7 io io b io b io io 9 5 4 4 26 i1 i8 i7 i8 i8 h 7 Material Code Supplier Variations in thread items from a given supplier and variations between suppliers were examined using boxplots. Figure A-5 is the boxplot for internal thread items by supplier. Figure A-6 is the boxplot for extemal thread items by supplier. A significant variation between suppliers is observed for both thread types. However, the variation in material code for a given supplier would argue against any systematic effect of supplier. The conclusion is that there is no significant systematic supplier effect and that variations are very likely to be on a production lot basis. J 1 i TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

I REPORT NO. 94-SCE 005 14 ] Figure A-5 j i Box Plot of Relative Deviation By Supplier ] NRC Sample Data Intemal Threads <0.5 Inch .5 0.0-i 1 5 l l d ;3 g -1.0-l a .9 -1.5-O j % -2.0. 14 16 8 16 i N= 14 i 2430 9897 12659 14857 18171 i SUPPLIER i j FigureA-6 j i j Box Plot of Relative PD Deviation By Supplier 4' NRC Sample Data Extemal Threads <0.5 inch 2.0-j j 1.5-i 1.0-l k 5- - M. 9 0.0-N l 4 %g -1.0 j N= 7 94 116 4 31 11 6117 9897 12659 14857 18171 27794 SUPPLIER i l TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070 l

I REPORT NO. 94-SCE 005 15 Computation of Limiting Out of Tolerance Values The limiting out of tolerance values were computed for both external and internal thread types using the methodology provided in Reference 3. Table A-2 provides the results of those computations. Table A-2 Statistic External Threads Internal Threads K (Statistical Factor) 2.244 8.38 X (Weighted Lot 0.445 -0.686 Mean) ate to te(Lot to Lot 0.0614 0.1874 Variation) ote (Within Lot 0.0776 0.0818 Variation) o re.i (Total 0.1390 0.2692 Variation) ULg5(Upper 95% 0.757 1.570 Limit) LLg5 (Lower 95% 0.1335 -2.942 Limit) The minimum material condition is defined by the LLg5 value. Thus for any internal thread item, there is a 95% probability that its pitch diameter will be less than the nominal PD plus 294.2% of the ANSI B1.1 Tolerance value. For external threads the limiting condition is defined by the ULg5 value. Similarly, there is a 95% probability that any external thread item will have a pitch diameter which is greater than the nominal PD minus 75.7% of the ANSI B1.1 Tolerance i value. Assessment ofImpact on Fastener Strength The impact of the observed dimensional deviations on the strength of fasteners less than 0.5 inches in diameter was examined by performing the White paper strength calculations for one typical fastener. The 3/8 Cap Screw - material number 2734045, Class 3a, was selected a representative f,,tener. A Monte Carlo simulation was prepared using the distributions of internal and external thread deviations for fasteners less than 0.5 inches in diameter. One thousand simulation trials were performed to generate the distributions. From this simulation, upper and lower limits on thread strength corresponding to 95% probability limits were estimated. TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

REPORT NO. 94 SCE-005 16 The formulations used were: Ass = Tr(1/P)(LE)D1 max [0.5(1/P) + 0.57735(D2 min - Dimax)] Where: Minimum Thread Shear Area for External Threads Ass = Thread Pitch (inches) P = Length Engaged (inches) LE = D1 max = Maximum Minor Diameter of Internal Thread D2 min = Minimum Pitch Diameter of External Thread The shear strength of the external thread (TSS) is then given by: TSS = 0.5(St)(Ass) Where: Minimum Ultimate Tensile Strength of the Bolt Material St = For the simulation the values used were: 0.0625 inches P = 0.375 inches LE = D1 max = 0.3387 D2 min = Normal Distribution from NRC Sample Data Set 125000 psi St = The Normal Distribution had the parameters: Mean = 0.445327 S.D. 0.508586 = This distribution is shown graphically in figure A-7. Figure A NRC Sample Data Set - 3/8 Cap Scntw Data 1 External Thread Minim' ni Pitch Dli.ineter.(d2' min); u E ll0f5 }m& Nm e Pk.0.16----- '-- 2-2-- E!Ein % em pn, PMW4 M3 k g]& BM O,12- -- -- -i- - b----; 9 Af7;,% l l l l @ '3% 0;0I - - '--- '- ---?- E h!$ r r lg;NN: h*h gg g',g4 l ['@Tjdh QD - --9" b- -, bY 0 I -- pr#REng{gg.3133-53I.-. ~i j QW4M;337.31340.3d!b 2.31325M328 334.3l .I/$"h'c EW@#ptTYP TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

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i REPORT NO. 94 SCE-005 17 i

s The thread shear area (Ass) and the thread shear strength were calculated for the 3/8 Cap Screw data from the NRC Sample data set. In addition, the ratio of the Thread Shear Strength to the Maximum Preload was determined using the i following formula: l TSS/MP = TSS/[0.7854(D-{0.9743/(1/P)})*(2Sm)) Where: 0.375 inches (Nominal Diameter) j D = 25000 psi (Code Allowable Stress - Class 2 & 3) Sm = { Figure A-8 and A-9 show the distributions of thread shear strength and thread j shear area respectively. I Figure A-B L .m.,~.e _ibutiFrEf6r;T_Kr_edd_;Sh,Wa.T._StrengtE.%.;.m_% Diitr %.d0.13; -.~~ ._m 22 leis 1 x:stm &:pa j R,.sTS8"11 E .nWWr:(, j W O. F---- " - - ~ Rmtypy rpm!.; oWup w. 3 1

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1 REPORT NO, 94 SCE-005 19 Table 3 - NRC Sample Data Set - 3/8 Cap Screw Data Percentile Thread Shear Ratio Strength (lbs) Thread Shear to Max Preload 5% 10067.33 2.598 50% 10749.95 2.775 } 95% 11402.42 2.943 j The results show that even at the 5% percentile level the thread shear strength is considerable higher than the maximum bolt preload. l i I s i i i i N l ? i 4 J i ) i TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070 4

REPORT NO. 94-SCE 005 20 Appendix 2 NRC Data Set - Nominal Diameter > 0.5" Relative PD Deviation NRC Inspection Internal Thread items Nominal Diameter >= 0.5" 20 ~ 10-Std. Dev =.27 Mean =.39 RI N = 138.00 0 m i i i ~9 ?p, ?,, ?g e i i>, ie, is O Q, G, 4s% 4 s o g Relative Deviation Relative PD Variation vs Material Code NRC Inspection internal Thread items Nominal Diameter >= 0.5" .5 1 R 0.0 - l . T' l .g -6 , ~ _,_ I h _2.2-i 4, o i m. o L 1.0 ' ~ .1.5 - ~ k -2.0 Ne i i8 f6 i f0 i f0 '2 (0 f0 f0 (0 i 2508620 2510246 2602449 2766871 30504286 30504807 2510105 2552602 2753853 30504088 30504369 30504864 Material Code TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

I e REPORT NO. 94-SCE-005 21 PD Variation vs. Supplier Internal Thread items NRC Data Nominal PD >= 0.5" .5 0.0 - ~ - -' ,h l i i I y .5 - i -1.0< j.i.3 h -2.0 No 4e 42 i 31 4 / 2 6117 9897 12437 12659 16532 18171 27794 SUPPLIER Histogram of Relative PD Deviation NRC Data External Thread items Nominal Dia. >=0.5 14 12 10 E' 6-4 Std. Dev =.26 2' Mean =.17 .18 .25 .i3 0.'00 .13 .25 .38 .50 .63 1 Relative Deviation j TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

I 1 l' REPORT NO. 94 SCE 005 22 Variation of PD vs. Sup'w NRC Data External Thread items Nominal Dia. >=0.5" .8 .61 l l .4 < t l .2 < i ~ .0 2i

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.67 11 11 1 N= 20 6117.00 9897.00 12659.00 27794.00 SUPPLIER Variation of PD Va. Material Code NRC Data External Thread items Nominal Dia. >=0.5" .8 .6 ' .4 < .2 ~ O .0 l .2 < k .4' 1 i .6 ~. i ,o a 2508620.00 2510246.00 2704534.00 30500854.00 30506000.00 2510105.00 2609345.00 2750628.00 30501670.00 MATERIAL TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070

REPORT NO. 94-SCE-005 23 4 NRC Sample Data internal Thread itcms Nominal Diameter > 0.5" 5 00 E! l I~ 1,-- m l e, _ W l. ~ 56 581 1: 7 i 1 i .5 1, y' m, 1 o -1.0 < o 1.5' ~- k -2.0 to 16 4 10 4 10 32 10 to 10 10 No 4 2508620 2510246 2602449 2766871 30504286 30504807 2510105 2552602 2753853 30504088 30504369 305& K A MATERIAL NRC Sample Data Extemal Thread items Nominal Diameter > 0.5" .8' .6 < l .4 < .b .2 < l .0< } =.2 < E .4 < .6 Na I i i i 1 i i 1'O A 2508620 2510246 2704534 30500854 30506000 2510105 2609345 2750628 30501670 MATERIAL l l l l l I TETRA ENGINEERING GROUP, INC. SIMSBURY CT 06070 0 l}}

ML20082C388

Contents

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SUMMARY

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0 7632 square inches Thread Ultimate Shear Strength

0.6239 square inches Thread Ultimate Shear Strength

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Dear Mike,

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ML20082C388

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0 7632 square inches Thread Ultimate Shear Strength

0.6239 square inches Thread Ultimate Shear Strength

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Dear Mike,

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