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y W. R. GALL, P.E.Engineerim Comultenta

P.O. Box 334Oak Ridge, Ter.nence 37830

(615) 482-4875

November 16, 1983ttr. E. Igne, Staff Engineer

Advisory Committee on Reactor SafeguardsU.S. Nuclear Regulatory CoraissionWashington, D.C. 20555 |Dear Mr. Igne: |

Subject: EPRI/AIF/MPC Bolting Seminar November 2-4, 1983

Transmitted with this letter is a copy of the agenda and handoutsfromtheEPRI/AIF/MPCSeminaronBoltingDegradationorFailureinNuclear Plants, held in Knoxville on November 2 - 4, 1983. I havenumbered the papers in the agenda and marked all the pages in thehandouts with their paper number for reference purposes.

'Ihe Seminar included: 1. A general summary of bolting incidentsthat have occurred in nuclear plants from 1964 to 1983, with their basiccausative factors and NRC and owner corrective actions. I believe thatall of this information has been reported to A.C.R.S. previously.

2. Summaries of AIF/MPC and EPRI prograns dealing with the boltingissue. 3. A report by Richard Vollmer on the NRC views of the bolting

issue.Some principal topics of discussion were:

1. Stress Corrosion Cracking.

2. Boric Acid Corrosion.3. Bolting Preload Control.

4. ASME and AISC Code requirements and shortcomin6s.

Stress Corrosion Cracking has occurred in nearly all applicationsof bolting, internal and external bolting in the reactor coolant systemand component support bolting. Contributing factors seem to be

1. High stress level, usually due to poor cont.rol of boltin6preload.

2. Material hardness and strength too hi h, due either to poor6control of heat treatment or to use of inappropriate materials.

3. Preexistin6 flaws.4. Use of MoS, as thread lubricant5. Presence of moisture and possibly some chlorides in the

*environment.Paper No.13 lists the materials which have failed due to stress

corrosion cracking. All of those listed except Vascomax 250 are includedin the ASME Code or applicable Code cases for the applications in which'

they were used. Iascomax 250, a maraging steel of 260 kai tensile and255 kai yield, is described in paper No. 8.

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Boric Acid Corrosion has occurred where bolting materials oflow corrosion resistance were exposed to leakage of borated water.Paper No. 5 describes an incident of boric acid corrosion ofreactor coolant pump closure bolts. 'Ihe material was SA-193 Grade B7steel.

EPRI efforts toward resolution of the bolting issue are summarizedin Paper No. 14. Fourteen projects initiated by EPRI since 1980 arediscussed, including fracture mechanics procedures for evaluation ofcomponent supports, a computer data base on bolting and boltingfailures, a study of bolting problems, tools, and practices, anddevelopment of NDE methods for inspection of bolts. Eighteenreports on completed work under 9 of these projects are completed orunder review. These projects are handicapped by lack of funds,partly because the pipe cracking problem has preempted some of theavailable funds.

The AIF/MPC program to address the bolting issue is presentedin Paper No.13 This program was transmitted to NRC by letter ofJuly 19,1983, from Richard M. Eckert to Richard H. Vollmer. Underthis program the bolting issue is classified into four groups byapplication and apparent cause: Group I, Pressure Boundary - BoratedWater Corrosion; Croup II, Pressure Boundary - Stress Corrosion Cracking;Croup III, Internals Bolting - Fatigue and Stress Corrosion Cracking;and Group IV, Component and Embedment Bolting - Stress Corrosion Cracking.

Of a total of 36 activities in the program, 14 are complete orunder review, 12 are started or ongoin6, and 10 are not started.

Two tasks included in this program are: 1) General Specificationfor Nuclear Requirements and Recommendations to Improve Procurementand Inspection Requirements, and 2) Recommendations to ASME on ASMECode Requirements for Bolting.

A draft of the proposed specification has been submitted to ASTMCommittee F16 for consideration as an AS'IM specification. The contentswere not described in detail, but Paper No. 22 indicates that objectives

|will be to 1) specify increased performance requirements and uniformity,with special attention to metallurgical integrity, 2) increase Pampling'

requirements, 3) establish tighter definitions for production lots andrequire lots to be identified throughout the supply system, and 4)require markin6 to indicate acceptance for nuclear service.

It in hoped that the specification will be ba31otted in ASTM in 1934and be available in 1985 That may be optimistic.

The only indication of the contents of the recommendations to ASMEis that they will include comments on pretensioning of both pressureboundary and structural bolting. Although the ASME Code does limitmaximum stress in pressure boundary bolting with pretension included,it does not give detailed methods for controlling the pretension.

Bolting Preload Control is discussed in Papers 12, 18, 19 and 20.For pressure boundary bolting in the secondary system, only the

simplest preload methods are used, slug 6 n6 wrenches, impact wrenches,1

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torque tables, and few instructions or procedures.For primary coolant pressure boundary bolting,preload control

ranges from manual torque wrenches to direct tension indicators andReactor pressure vessel headin some cases multiple bolt tensioners.hly engineered and are assembled using direct.tensioners,closures are hi6

and written procedures.'Ihe ASME Code provides guidance for determinin6 minimum bolt loads

required to seal flanged joints in both Class 1 and Class 2 systemsar.d prescribes maximum limits for stresses in the bolts due to theHowever,combination of preload thernal expansion and pressure.

the amount of preload are not given,detailed methods for controllon6and the maximum stress levels are not based on consideration of the

susceptibility to stress corrosion cracking.materialThe objectives of improving preload control are 1) to achieve

sufficient tightness to prevent leakage, and 2) to prevent overstressingTo achieve both of these objectives, it is necessary tothe bolts.

attain as nearly as possible uniform tension in all bolts in a joint.Otherwise some bolts will be overstressed and others will not be tightenough to prevent leakage.

The preload control methods available are torque control, directtension indicators, hydraulic bolt tensioners, and bolt heaters.

Papers 18 and 19 discuss the difficulties of attaining uniformbolt tension around a flanged joint. concluding that it is practicallyimpossible by tightening one bolt at a time regardless of the sequence

Results of a test procedure are illustrated in Figures 5that is used. Although the method used was torque, the conclus.and 6 of Paper No.19ion would be the same for other methods where one bolt is tightened at

Problems with torque control are mostly due to variability ofa time.thread friction which affects reproducibility of predicted bolt tension.Lubrication improves reproducibility, and molybdenum disulfide has beenused extensively as a thread lubricant, but .here are indications that

breaks down at come temperatures to form H S which results in stress2MoScorrosion cracking of the bolting material. Some NSS suppliers and2

and using other lubricants orutilities are banning the use of MoS 2other methods of preload control. A four-iteration procedure was tried

After tightening and loosening eachfor steam 6enerator manway clocures.bolt four times acceptable consistency of torque vs tension was obtained.However, because of the radiation level at the steam generator manwaylocation, this method requires increased manpower to assemble the closure.

An al3-stud tensioning device was described which uses hydraulictensioners on all studs in a joint simultaneously,with a common source ofhydraulic pressure.

For struct1ral boltinn, the ASME Code permits use of torque wrenches," turn of nut", and direct tension indicators for tightening, but it doesThe AISCnot specify the amount of bolt tension that should be applied.Manual of Steel Construction, however, does specify a minimum bolt tens-,

ion of 70% of the ultimate tensile strength of the bolt, and defines the|

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amount of nut rotation required to achieve that tension in boltsmade of ASTM A325 and A490 steel startin6 from the snug tight position.AIGC does not permit the use of torque methods to control the bolt tens-ion. 'Ihe maximum bolt diameter covered by the AISC turn of nut methodis li inches, and the maximum bolt length is 12 diameters. Since moststructural joints are metal-to-metal joints, the turn of nut method isprobably the most practical one for control of initial bolt tension,provided the AISC definition of " snug tight" can be consistentlyestablished. Users of the ASME Code must refer to the AISC Manual forthe definition of " snug tight" and the amount of rotation to use, orestablish their own standards by experiment.

One type of corrective action that has been taken for structuralbolting where stress corrosion cracking has occurred is to reduce theinitial bolt tension to a very low level in anchor bolts which do notexperience operation loads in tension. This was done for some steamgenerator anchor bolts and some reactor vessel support skirt anchorbolts. It is possible that this action may introduce the possibilityof the nuts loosening due to temperature cycling or vibration. Somemethod of keying or cottering the nuts to the bolts should be used toprevent loosaning.

ASME and AISC Code requirements and shortcomings are discussedin Paper No.12. Some of these have been reviewed above.Pressure boundary bolting and component support bolting for all classerare covered by the rules of the ASME code, Section III, Division 1.However the rules for Classes 2, 3, and MC are similar to those in

Section VIII, Division 1, and the rules for bolting of component supports

are similar to those Sn the AISC Manual for Steel Construction. Sothe comments in Paper 12 referring to those other standards applypartly to Section III. The ASME Code design rules include upper limitsfor bolt stresses including preload, but the Code does not specifya

installation requirements for controlling the preload stresses, exceptto indicate acceptable methods and to require that preload requirementsbe included in the Design Specifications.

Bolting of Reactor Internals is discussed in Papers 6 and 7Applications were upper and lower core barrel and thermal shield bolts.Materials affected were X750 and A-286, both of which are acceptablefor reactor internals ur. der Subsection NG of the ASME Code.

A-286 failures were attriouted to environmentally assisted inter-

grannular cracking. No failures occurred in bolts at moderate to lowstresses.

X750 guide tube support pina failed due to stress corrosion cracking.Studies showed that better stress corrosion characteristics resultedwhen the materi:0 wan solution annealed at 2000 F prior to aging at

1300 F. The pins were also redesigned to reduce stress concentrationn.and average stressen were reduced.

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Proposed Actions. The following list includes some of the possibleactions discussed or mentioned, which a::y be taken to alleviate thebolting problem. Some of these actions have already been taken bysome licensees, and some of them may require further development beforethey can be implemented.

1. Improve control of preload by use of written procedures,instrumentation, and better quality assurance measures during installat-ion.

2. Provide in-service inspection methods and schedules for bolting.

3 Limit or ban use of molybdenum disulfide as thread lubricant.4. Provide receiving inspection of all new and replacement bolts to

assure compliance with specified requirements, especially hardness limitsand flaws.

5 Improve quality assurance measures and procedures in heat treat-ment, testing, receiving, and installation of bolts and nuts.

6. Where bolts may be exposed to borated water, use corrosionresistant bolting materials, provide leak and moisture detection atbolted joints, and remove insulation from around bolts.

Evaluate high stren6th bolting materials (yield strength > 150 ksi)7for susceptibility to stress corrosion cracking by linear elastic fracturemechanics methods as outlined in Paper No.16 of this Seminar. Wherenecessary, change materials or reduce stress levels to provide acceptableconditions.

8. Develope improved procedures for ultrasonic examination of highstren6th bolting.

9 Prepare new nuclear bolting specification coverin6 materials,'

fabrication procedures, and receiving inspection.

10. Propose revisions to the ASME Code to limit and control preloadstress levels in bolting in both design and installation.

11. Reduce bolting preloaa stress levels in applications whereoperating conditions do not impose tensile loading or the bolts, asdescribed in Paper No. 8 for steam generator anchor bolts.

Richard Vollmer, in presenting the NRC perspective on the boltingissue, indicated that KRC does not contemplate any regulatory action onthe boltin6 lbsue at present, but expects to issue the NRC boltingpocition in the Summer of 1984.

Short term regulatory actions required of licensees by IE SulletinNo. 82-02 should be suificient until completion of Generic Bolting IssueB-29

Very truly,yours,[. , /\ . /b(.(W. R. Gall, Consultant

Attachment: Seminar handouts.CC: P. G. Shewmon (w/o attachment)

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