ML19225D140

Final Deficiency Rept Re Threads in field-fabricated Anchor Bolts.Incident Determined to Be Nonreportable,Based on Site Dimensional Insp Results

ML19225D140
Person / Time
Site:	South Texas
Issue date:	07/30/1979
From:	HOUSTON LIGHTING & POWER CO.
To:
Shared Package
ML19225D135	List: ML19225D133 ML19225D140
References
NUDOCS 7908070400
Download: ML19225D140 (14)

Text

South Texas Project Electric Generating Station Field-Threaded Anchor Bolts Thread Deficiency Final Report I. SurTncry A. A Synopsis of the Incident Curing a routine surveillance inspection of the installation of the ECC3 accumulator tanks of Unit 2, the anchor bolts were found to have deformed and undersized threads. This condition was evaluated under the guidelines of 10CFR50.55(e) and determined to be a non-reportable incident based upon the results of a test program and analysis. The uniqueness of the thread condition on these specific anchor bolts was investigated, however. This investigation revealed that a large number of anchor bolts had similar thread conditions. The bolts in-volved had been manufactured at the STPEGS site using bar stock. A nonconformance report (NCR) was issued at the site on June 26, 1978, to document the apparent generic problem with site-manufactured anchon bolts.

In order to properly account for each potentially deficient anchor bolt, an intense, detailed investigation was initiated. The 1,100 anchor bolts of various sizes that had been site-manufactured had their physical location marked on appropriate drawings. These draw-ings were then reviewed and 628 of the 1,100 anchor bolts were detennined to support safety-related equipment. The 628 anchor bolts were then subjected to in situ dimensional measurements. The NRC was informed of the incident and activities to this point via telecon on October 27, 1978.

Based on the evaluation of the site dimensional inspecticn results, the incident was determined to be non-reportable under the guidelines of ICCFR50.55(e). However, a " pull" test conducted on ene anchor bolt as par) ,f a strength verification program indicated utilization of im-proper material for the anchor bolt. This condition was reported to the NRC on February 8, 1979. A written report was forwarded to the NRC on Marsh 8,1979, which addressed the anchor bolt improper material defi-ciency. On June 5, 1979, a second written report was sent to the NRC.

The final report addresses in detail the deficient thread condition of the field-fabricated anchor bolts. Although the original intent was to close out the improper material problem in this final report, additional investigations necessitate deferral of a final report on anchor bolt improper material until January 2,1980.

427 l97

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2 B. Postulated Cause of Incident The c.ause of the thread deficiency originated from the use of in-adequate manufacturing controls and was further complicated by inadequate inspection methods and equipment.

C. A Synopsis of the Corrective Action A sample of 12 bolts of various sizes and materials that were manu-factured under the same processes as the suspect bolts was subjected to laboratory examination. Thread acceptance was checked by several techniques including micrometers, the three wire method, and GO-L0 gauges. Thread major diameters were also measured using micrometers.

Bolts were sectioned at the threaded portions for profila determination.

It was found that the measurements of pitch diameter made at the site consistently produced erroneous data, but that measurements of major diameter were correct. It was also determined that the pitch angle was in accordance with the specification. (Refer to Figure 1 fer definitions of terms.) Thus, the tensile areas of all bolts were determined from the measured major diameter. All installed bolts were found to have sufficient tensile area and thread engagement to carry the design loads with an acceptable margin. Therefore, the nonconfor-mance reports were dispositioned: "use-as-is". This disposition was further substantiated by full section load tests on all of the 12 anchor bolt-nut assemblies. All bolts failed in tension at approximately 1.5 to 2 times their specified design load.

D. A Synopsis of Safety Evaluation Since the anchor bolts were determined to meet or exceed all design criteria based upon load capacity, no safety evaluation was conducted.

II. Description of Incident A. Component Identification A large quantity of anchor bolts was manufactured at the STPEGS site by Brown & Root to offset an inadequate supply of vendor supplied anchor bolts. These anchor bolts are used to anchor compcnents to concrete structures throughout the plant. A diagram showing a typical anchor bolt design is shown in Figure 2. The bolt configuration is designed based upon the loading, tensile stress area of the bolt (ANSI Std. Bl.1),

and the yield strength of the bolt. A design allowable strength of 22 ksi was used for ASTM A36 and 48 ksi was used for ASTM A193 Grade B7. These design allowable strengths are used even though the minimum ASTM yield strengths are 36 ksi and 115 ksi for the A36 and A193 Grade B7 materials, respectively.

B. Source and Extent of Incident Curing a routine surveillance ir.spection, Quality Control discovered an improper fit on the Unit 2 ECCS accumulator tank anchor bolts. Measurement W I90

3 of the thread dimensions indicated the threads were smaller than had been specified. This condition was determined to be potentially reportable and the NRC was notified. However, testing and analysis demonstrated that an adequate safety margin was maintained.

Discovery of the deficiency in the accumulator tank anchor bolts prompted Quality Centrol to inve:tigate the condition of all anchor bolts manufactured at th? site. Three work orders of site-manufactured anchor bolts were selected at random and their pitch and major diameters measured. As a result of these measurements, it was determined that an unsatisfactory condition existed and an NCR was issued. B&R Construction and Quality Assurance identified the locations, appli-cations, and dimensional meas ,rements of anchor bolts manufactured under site work orders. The bolts were identified and their locations indicated on a set of drawings. From these drawings Engineering identi-fied bolts which were safety-related. Site Quality Control was then furnished a list of work orders from which safety-related bolts were manufactured. The list determined which anchor bolts required thread inspection. Each work order of anchor bolts was individually disposi-tiened.

C. Date and Means by Which Incident Information was Obtained The accumulator tank anchor boit deficiency was identified by NCR on June 20, 1978. This led to the in,'stigation described above and an NCP against all anchor bolts was issued on June 26, 1978.

D. Unusual Circumstances Fabrication of anchor bolts at the site was not anticipated in the original planning for the South Texas Project. Initially, all anchor bolts were to be procured from vendors. When production delays at the vendors' facilities resulted in too few bolts being delivered at the site to allow work to continue as scheduled, a decision was made to manufacture some bolts at the site. At that time, however, equip-ment at the site included only some adjustable thread cutting dies and no thread measuring equipment. Requisitions for better thread cutting tools and thread ring gauges were denied on the basis that fabrication of anchor bolts at the site was only a temporary measure. However, the number of anchor bolts manufactured on-site increased rapidly.

Eventually, new requests for threading tools and thread ring gauges were made. These requests were approved and the equipment has now been provided at the site.

E. Status of Construction at the Time of the Incident By the time of the incident, the Site Construction organization had manufactured 1,100 anchor bolts of various sizes and several configu-rations. Of the 1,100 bolts, 628 have been identified as being used in safety-related applications. A majority of the safety-related bolts had already been installed when the deficiency was identified.

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4 F. Procedures in Effect to Avoid Incident The anchor balts in question were manufactured in accordance with the directione included in work orders which referenced drawings which contained the required thread measurements. The threads were inspected using a nut of the proper size and thread form which had been procured from a bolt vendor. This method proved to be in-adequate.

III. Correct 1ve Action The thread configuration of these questionable site-manufactured bolts was determined to be satisfactory. The mechanism for arriving at this position is described belcw.

A. Site Measurements The problem was init ially identified as an out-of-tolerance condition on threads for embedded anchor bolts for the Unit 2 ECCS accumulator tanks. An investiga W :1 of the cause of this condition made a total of 1,100 site-manufvtu ed bolts suspect. A random check of some of these bolts revealed some cases of unsatisfactory conditions or both major and pitch diameter measurements. A set of design drawings was then marked-up by the site personnel showing the location of suspect anchor bolt locations. These drawings were then reviewed by Engineer-ing and thosa anchor bolt loca+ ions where failure could affect safety were identified. The major and pitch diameters of these bolts were then measured and reported to Engineering. A small percentage of the measurements were out-of-tolerance on pitch diameter. An analysis was made of these measurements and enough inconsistency was found to warrant further examination.

B. Laboratory Exaaination A total of 12 anchor bolts was delivered from the site to the Brown &

Root Materials Engineering Laboratory #or examination. These bolts were of various dimensions and materir :s and had been manufactured by the same process as the suspect bolts. The bolt sizes and materials are identified in Tables I and II.

The first task was to determine the thread dimensions using standard methods and tools. Bolt threads were verified using micrometers, pitch gauges, GO-L0 gauges and the three wire method. The bolts were then sectioned through the thread areas and measurements ob-tained using an optical comparator. The results are shown in Table I.

All bolts were within tolerance in major diameter. Five bolts, how-ever, were not within tolerance for the pitch diameter. The .ceasure-ments were out-of-tolerance less than 0.012 inch. The thread pitch was found to be within tolerance on all bolts. During these tests, it was deternined that the site measurements of major diameter could be duplicated but the measurements of pitch diameter could not. The pitch diameter measurements reported by the site were consistently lower than those measured at the laboratory.

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5 Next, the same twelve bolt-nut assemblies were subjected to full section load tests until failure occurred. The load was applied to the bolt-nut assembly and the loads at yield and failure were determined. The results are shown in Table II. In each case, the bolt material failed at ultimate strengths higher than the minimum specified for the material. The margin between the bolt design allowables and their test loads varied by a factor of between 1.5 and 2.

C. Field Examination The purpose of this exercise was to provide a correlation between the site data and laboratory data. Selected installed anchor bolts were subjected to thread measurements by Materials Engineering personnel. Thread verification was made using GO-LO gwges, thread micrometers, diameter micrcmeters, three wire gauges, ano ;'tch angle gauges. The results are shown in Table III.

The thread measurements in th. 1 eld verified those made in the laboratory. They also conf'rmed that the previous measur ements of pitch diameter made by site personnel were in error. This fact was further confimed when site p3rsonnel remeasured the threads for several bolts that had previously been measured and, on the second attempt, obtained satisfac, tory results. The nonconfomance reports for these measurements were revised to show only the major diameter measurements whi;h were then used in the analysis. In general, the majority of the bolts were found to have major diameter measurements that met requirements. Of the bolts that did not, the maximum de-viation was less than 0.020 inch.

D. Analysis Based on measured major diameter and the pitch, the tensile areas were calculated for each work order, each group of bolts and each foundaticn pad. The analysis has shown that the stressed area of the bolt averaged 5% below the specified tensile area, which is within the allowable design criteria.

E. Repair None of the installed ancnor bolts were rejected because of thread deficiencies, therefora, no repair was required.

IV. Recurrence Control In order to preclude recurrence of this deficiency, four actions have been taken. These actions are:

A. Purchase of Thread Cutting Tools The new thread dies have a more accurate cutting tool. The re-427 >

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6 sulting improvement in the precision of thread formation will help to prevent recurrence of this deficiency.

B. Purchase of Thread Ring Gauges The use of precise inspection tooling will assure a more accurate measurement of thread dimensions. Similar deficiencies can thus be avoided in the future by the detection of out-of-tolerance threads before they are accepted.

C. Preparaticn and Distribution of Procedure A procedure has been developed and issued for use which describes the thread-cutting process and gives a detailed explanation of the proper method for inspecting threads.

D. Training Program A training program has been initiated in which site personnel are instructed in the proper procedure for fabricating and inspecting threads. This includes the proper use of thread ring gauges and pitch micrometers.

V. Safety Analysis Since anchor bolts were determined to meet or exceed all design criteria based upon capacity, no safety evaluation was conducted.

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9 TABLE I ANCHOR BOLT THREAD DIMENSION DETERMINATION MEASURED ANSI Bl.1 SAMPLE MEASUREMENT METHOD DIMENSION REQUIREMENTS NUMBER LOCATION USED (IN.) CLASS 2A (IN.)

1 OD Micromete- .736 .744 .7353 .7482

(.75 In.) Pitch Dia. Micrometer .679 .680 .6773 .6832 Pitch Dia. 3 Wire (.055In.) .601 .682 .6773 .6832 Minor Dia. Opt. Comp. .6155 .6255 Pitch Pitch Gage 10 TPI (1) 10 1P!

2 OD Micrometer .742 .745 .7353 .7482

(.75 In.) Pitch Dia. Micrometer .680 .6773 .6832 Pitch Dia. 3 Wire (.055In.) .680 .681 .6773 .6832 Minor Dia. Op t. Comp. .6167 .6255 Pitch Pitch Gage 10 TPI 10 TPI 3 OD Micrometer .867 .868 .8592 .8731

(.875 In.) Pitch Dia. Micrometer .796 .797 .7946 .8009 Pitch Dia. 3 Wire (.063In.) .795 .797 .7946 .8009 Minor Dia. Opt. Comp. .7236 .7368 Pitch Pitch Gage 9 TPI 9 TPI 4 OD Micrcmeter .868 .869 .8592 .8731

(.875 In.) Pitch Dia. Micrometer .797 .798 .7946 .8009 Pitch Dia. 3 Wire (.063In.) .797 .798 .7946 .8009 Minor Dia. Opt. Comp. .7228 .7368 Pitch Pitch Gage 9 TPI 9 TPI 5 OD Hicrometer .993 .995 .9830 .9980 (1.000 In.) Pitch Dia. Micrometer .912 .913 .9100 .9168 Pitch Dia. 3 Wire (.072In.) .914 .915 .9100 .9168 Minor Dia. Opt. Comp. .8283 .8446 Pitch Pitch Gage 8 TPI 8 TPI 6 OD Micrometer .994 .995 .9830 .9980 (1.000 In.) Pitch Dia. Micrometer .902 .912* .9100 .9168 Pitch Dia. 3 Wire (.072In.) .902 .915* .9100 .9168 Minor Dia. Opt. Comp. .8303 .8446 Pitch Pitch Gage 8 TPI 8 TPI 7 OD Micrometer 1.234-1.236 1.2329-1.2479 (1.250 In.) Pitch Dia. Micrometer 1.159-1.160 1.1597-1.1667 Pitch Dia. 3 Wire (.072In.) 1.159-1.162 1.1597-1.1667 Minor Dia. Opt. Comp. 1.0804 1.0945 Pitch Pitch Gage 8 TPI 8 TPI 427 205

10 TABLE I (Continued)

MEASURED ANSI Bl.1 SAMPLE MEASUREMENT METHOD DIMENSION REQUIREMENTS NUMBER LOCATION USED (IN.) CLASS 2A (IN.)

8 OD Micrometer 1.234-1.236 1.2329-1.2479 (1.250 In.) Pitch Dia. Micrometer 1.158 1.1597-1.1567 9 itch Dia. 3 Wire (.072In.) 1.159-1.160 1.1597-1.1667 Minor Dia. Opt. Comp. 1.0856 1.0945 Pitch Pitch Gage 8 TPI 8 TPI 9 CD Micrometer 1.479-1.482 1.4828-1.4978 (1.500 In.) Pitch Dia. Micrometer 1.402-1.405* 1.4093-1.4166 Pitch Dia. 3 Wire (.072In.) 1.403-1.405* 1.4093-1.4166 Minor Dia. Opt. Comp. 1.3234 1.3444 Pitch Pitch Gage 8 TPI 8 TPI 10 CD Micrometer 1.489-1.494 1.4828-1.4978 (1.500 In.) Pitch Dia. Micrometer 1.405-1.416* 1.4093-1.4166 Pitch Dia. 3 Wire (.072In.) 1.402-1.416* 1.4093-1.A166 Minor Dia. Opt. Comp. 1.3101 1.3444 Pitch Pitch Gage 8 TPI 8 TPI 11 00 Micrometer 1.483-1.488 1.4828-1.4978 (1. 500 In. ) Pitch Dia. Micrometer 1.402-1.412* 1.4093-1.4166 Pitch Dia. 3 Wire (.072In.) 1.400-1.413* 1.4093-1.4130 Minor Dia. Opt. Comp. 1.3288 1.3444 Pitch Pitch Gage 8 TPI 3 TPI 12 OD Micrometer 1.476-1.486 1.4828-1.4978 (1.500 In.) Pitch Dia. Micrometer 1.391-1.407* 1.4093-1.4166 Pitch Dia. 3 Wire (.072In.) 1.393-1.409* 1.4093-1.4130 Minor Dia. Opt. Comp. 1.3221 1.3444 Pitch Pitch Gage 8 TPI 8 TPI

• These five Pitch diameter dimensions deviate fro ' ANSI specification.

Note: TPI - Threads Per Inch 42'7 206

11 TABLE II FULL SECTION LOAD TEST RESULTS ASTf1 NOMINAL THREADS BOLT APPR0X. LOAD LOAD SAMPLE MATL. SIZE PER AREA (1) AT YIELD AT UTS NUMBER SPEC. ( IN. ) INCH (50.IN.) (2)(LB.) FAILUREll b) (PSI)

1. A36 .750 10 .3344 14400 22930 68570

2. A193B7 .750 10 .3344 -

48220 144198

3. A36 .875 9 4617 '0960 c 32870 71193 4 A193B7 .875 9 4617 55700 64600 139917

5. A193B7 1.000 8 .6058 -

83900 138494

6. A36 1.000 . .6058 24770 42410 70006

7. A36 1.250 8 .9998 42200 66100 66113

8. Al93B7 1.250 8 .9998 117700 135500 135527

9. A36 1.500 8 1.4920 69000 112600 75469

10. A36 1.500 8 1.4920 70500 112400 75335

11. A36 1.500 8 1.4920 69100 113700 76205

12. A36 1.500 8 1.4920 70100 112300 75268 NOTES:

(1) The stressed area A of a bolt is computed from As = .7854 (0 .9 43)2 where D is the nominal bolt diameter and N represents the thread per inch.

(2) Load at yield determined by the halt of the pointer method.

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12 TABLE III FIELD MEASUREMENTS WORK METHOD MEASURED ORDER ME ASUR EMENT USED DIMENSION 836 el (1.375) 0.0. Micrometer 1.359-1.362 Pitch Dia. 3 Wire (.072) 1.286 Pitch Dia. Micrometer 1.282-1.285 Pitch Pitch Gage 8 TPI

1. 2(1.500) 0.D. Micrometer 1.475-1.481 Pitch Dia. 3 Wire (.072) 1.403 Pitch Dia. Micrometer i.402-1.415 Pitch Pitch Gage 8 TPI

1. 3 (1.500) 0.0. Micrometer 1.475-1.476 Pitch Dia. 3 Wire (.072) 1.396 Pitch Dia. Micrometer 1.393-1.396 Pitch Pitch Gage 8 TPI 1257

1. 1 (1.000) 0.D. Micrometer .976 .986 Pitch Dia. 3 Wire (.072) .908 Pitch Dia. Micrometer .906 .916 Pitch Pitch Gage 8 TPI

1. 2 (1.000) 0.D. Micrometer .969 .985 Pitch Dia. 3 Wire (.072) .908 Pitch Dia. Micrometer .906 .914 Pitch Pitch Gage 8 TPI

1. 3 (1.000) 0.D. Micrometer .981 .990 Pitch Dia. 3 Wire (.072) .911 Pitch Dia. Micrometer .903 .909 Pitch Pitch Gage 8 TPI 1219

1. 1 (1.375) 0.D. Micrometer 1.375-1.378 Pitch Dia. 3 Wire (.072) 1.290 Pitch Dia. Micrometer 1.290-1.298 Piten Pitch Gage 8 TPI 427 208

13 TABLE III (Continudd )

WORK METHOD MEASURED ORDER MEASUREMENT USED DIMENSICN

1. 2 (1.375) 0.D. Micrometer 1.310-1.379 Pitch Dia. 3 Wire (.072) 1.285 Pitch Dia. Micrometer 1.283-1.287 Pitch Pitch Gage 8 TPI

1. 3 (1.375) 0.D. Micrometer 1.359-1.371 Pitch Dia. 3 Wire (.072) 1.284 Pitch Dia. Micrometer 1.283 '.286 Pitch Pitcr. aage 8 TPI 1110

1. 1 (1.500) 0.0. Micrometer 1.486-1.490 Pitch Dia. 3 Wire (.072) 1.411 Pitch Dia. Micrometer 1.408-1.410 Pitch Pitch Gage 8 TPI

1. 2 (1.500) 0.D. Micrometer 1.486-1.488 Pitch Dia. 3 Wire (.072) 1.412 Pitch Dia. Micrometer 1.408-1.409 Pitch Pitch Gage 8 TPI

1. 3 (1.500) 0.D. Micrometer 1.488-1.492 Pitch Dia. 3 Wire (.072) 1.413 Pitch Dia. Micrometer 1.410-1.411 Pitch Pitch Gage 8 TPI 1373

1. 1 (.875) 0.D. Mic rometer .859 .862 PC ;h Dia. 3 Wire (.072) .789 Pitch Dia. Microneter .787 .788 Pitch Pitch Gage 9 TPI

1. 2 (.875) C.D. ficrometer .855 .862 Pitch Dia. , Wire (.072) .790 Pitch Dia. Micrometer .787- 788 Pitch Pitch Gage 9 TPI v3 (.875) 0.D. Micrometer .861 .862 Pitch Dia. 3 Wire (.072) .789 Pitch Dia. Micrometer .787 .789 Pitch Pitch Gage 9 TPI 427 209

14 TASLF III (Continued)

WORK NETH00 MEASURED GRDER MEASUREMENT LSED DIMENSION 1342

1. 1 (1.500) 0.D. Micrometer 1.478-1.496 Pitch Dia. 3 Wire (.072) 1.419 Pitch Dia. Micrometer 1.412-1.413 Pitch Pitch Gage 8 TPI

1. 2 (1.500) 0.D. Micrometer 1.489-1.497 Pitch Dia. 3 Wire (.072) 1.445 Pitch Dia. Micrometer 1.412-1.431 Pitch Pitch Gage 8 TPI

1. 3 (1.500) 0.D. Micrometer 1.486-1.491 Pitch Dia. 3 Wire (.072) 1.414 Pitch Dia. Micrometer 1.410-1.411 Pitch Pitch Gage 8 TPI 427 2i0