Regulatory Guide 1.65
| ML003740228 | |
| Person / Time | |
|---|---|
| Issue date: | 10/31/1973 |
| From: | Office of Nuclear Regulatory Research |
| To: | |
| References | |
| RG-1.65 | |
| Download: ML003740228 (4) | |
October 1973 U.S. ATOMIC ENERGY COMMISSION
REGULATORY GUll DIRECTORATE OF REGULATORY STANDARDS:
REGULATORY GUIDE 1.65 MATERIALS AND INSPECTIONS FOR REACTOR VESSEL CLOSURE STUDS
A. INTRODUCTION
General Deign Criterion 1, "Quality Standards and Records,," of Appendix A, "General Design Criteria for Nuclear Power Plants," to 10 CFR Part 50, "Licensing of Production and Utili(.tion Facilities," requires, in part, that structures, systems, and componenti important to safety be designed, fabricated, erected, and tested to quality standards commensurate with the importance of the safety "unctions to be performed.
Criterion 30, "Quality of. Reactor Coolant Pressure Boundary," of the same appendix requires, in part, that components which are part of the reactor coolant pressure boundary be designed, fabricated, erected, and tested to the highest quality standards practical. In addition, Criterion 31, "Fracture Prevention of Reactor Coolant Pressure Boundary," requires, in part, that the reactor coolant pressure boundary be designed with sufficient margin to assure that when stressed under operating, maintenance, testing, and postulated accident conditions (1) the boundary behaves in a nonbrittle manner and (2) the probability of rapidly propagating fracture is minimized. Appendix B, "Quality Assurance Criteria for Nuclear Power Plants and Fuel Reprocessing Plants," to 10 CFR Part 50 requires, in part, that measures be established for the control of special processing such as heat treating and that proper testing be performed. Section 5O.S5a, "Codes and Standards,"
of 10 CFR Part 50 requires, in part, that pressure vessels which are part of the reactor coolant pressure boundary meet the requirements for Class A or Class 1 vessels set forth in Section HI of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code.
This guide defines materials and testing procedures acceptable to the AEC. Regulatory staff for Implementing these criteria with regard to reactor vessel USAEC REGULATORY GUIDES
Regulatory Guidu we Issued to describe and muks
"sdtable to the Public methods ecmptab. to the AEC Reul*utory staff Irn;planwntlng epvelfIc Pert Of e Commisskn.
inguistlomn.
to deineate actdques mud by the staff In
"
u/aimtlnh spedfc problema or potulated accidents, or to provide ul*tdn* to applicants. Reputatory Guides re not ubstitutes for regulation and cosvslance with ehum k not quired. Methods and eolutiom diffwent f those r
t'Out Wi Suld wl bI ae optle I they jrvide basis for til findings MquWta ID
the Iuenm Or continuence of a erit atosne by the Comnmssion.
Published guides w tll be rv~sed Periodically. a i*pproplate.
t oeccomniod gomanens enid to reflect: new Information or experbince.
closure stud boltinge for light-water-cooiecl reactors. The Advisory Committee -on Reactor Safeguards has been consulted concerning this guide and has concurred in the regulatory position.
B. DISCUSSION
The ASME Boiler and Pressure Vessel Code, Section
11, "Nuclear Power Plant Components," 2 and Section XI, "Rules for Inservice Inspection of Nuclear Reactor Coolant Systems."2 specify certain requirements associated with reactor vessel closure stud bolting. TMl regulatory guide suflplements the requirements of the ASME code.'
High-stiength lowalloy reactor stud bolting may be obtained -through closely controlled quenching tad tempering procedures on grades of 'steel such as.
American Iron anxd'Steel Institute(AISI)4140 and 4340
(Ref. 1). These steels aire apprioved by the ASME as bolting materials and are listed 'uiider Section H,
"Material Specifications." 2 of the ASME Code as SA-540 Grade B-23 and B-24 bar, SA-193 Grade B-7 bar, SA-320 Giade L-43 bar, and SA-194 Grade 7 (nuts for bolting). Additional requirements supplementing these basic materials- specifications are found In SA-614 entitled "Specification- For Special Requirements for Bolting Materials" For' Nuclear and Other Special Applications." 'Proper control of the tempering procedure is necessary to Obtain: the -desired balance of
'Closure stud bolting is defined to include all studs (stud bolts), nuts, and washers used to fasten the pressure vessel head to the pressure vessel.
SCpies may be obtained from the'American Society of Mechanical Engineers, United Engineering Center, 345 East 47th Street, New York, New York 10017.
copwe at pubmwde ukides my be obtained bv request laidlmlinG til
"1004ow desired to the US. Amnc EnerW Cormrublon. Wasngton. D.C. 20546.,
Attention: Director Of Regulatory Stdards. Com ts Snd uguestks for lnl~onents We =ie.eleu encouraepd enid should be gent to thu secretary .
of thw Ccnmo llof. US. Atomic.Eneg Commibsson. Wasngtono. 0.0. 205",
Attention: Chief. Public Prceding. Staff.
.
Tjw quides ameIssud in thle foflomrict Van broad slvilelone:
1. Poww Reactors
2. Resar ehd 4Tat Reactors
3. Fuels; end Maltriall Fecilities
4. Environmental end liting L. Iaterlat end lant Protection I. Products
"7.
Tranportatlon I. Occ0lt=loe Heatth
9. Antitrust Review
1
0. General DE
mechanical properties;
The above-mentioned, stud materials ivhen tenipered to a maximum tensile strength of. 170'ksi!(Ref. 2) are relatively immune to stress corrosion cracking (SCC). Above this strength level the alloy becomes increasingly susceptible to SCC.
Therefore,. design conservatism should be exercised in determining the sizing of the studs so that the minimum specified strength level of the material selected will not result in a measured ultimate tensile strength exceeding
170 ksl.
Use of martensitic'stainless steels such as the
11-13% chromium grades should be avoided for reactor stud bolting applications. These steels (Ref. 3) require more closely controlled conditioning parameters than do carbon and low-alloy steels. Small variations in heat treatment can cause large increases in hardness and tensile strength with a corresponding decrease in corrosion resistance and fracture toughness. The tensile properties of this class of material are extremely sensitive to tempering temperatures in the 900-1200°F
range and somewhat sensitive to variations In the austenitizing treatment. Another shortcoming of this material is temper embrittlement which may result during cooling through the 750-1O00F temperature
"range. Certain tempering treatments can also produce alloy composiiona gradients within the structhre which can strongly affect service life because of a reduction in _
strength and resistance to corrosion. Although martensitic stainless steels are more resistant to general corrosion, they are less resistant to stress corrosion cracking than are carbon or low-alloy steels.
It Is important that bolting material possess adequate toughness throughout the reactor operating cycle. For high-strength large-diameter bolting& unusual care must be taken to assure adequate fracture toughness, Control of the tempering procedure is very important for this purpose. Fracture toughness for the AISI 4340 steel as measured by energy absorption can be moderately increased through appropriate metallurgical practices. Reictor vessel closure studs and nuts should have a Charpy V energy of 45 ft-lb or greater (Ref. 4).,
Thread roots of bolts are areas of high stress""
concentration and are preferential sites for crack initiation. It Is therefore necessary for the bolting material to possess adequate toughness so that failure will not Initiate in the stud thread configuration and that if cracks are initiated, they can be readily detected by inservice inspection before they reach critical size.
Suppliers of nuclear steam systems for pressurized water reactors recommend that the vessel closure stud bolts be removed prior to raising the water level during refueling or other, operations involving vessel head removal.
The same suppliers also provide a.d recommend the use of seal plugs for insertion into the pressure vessel flange stud holes to protect against corrosion and contamination following stud removal.
Provided the, seal plugs are properly used and the stud K.
bolting;Is maintained -in an area which is free from corrosion and contamination, the above recommended procedure provides adequate protection for the stud bolting following head removal. This; procedure also..
permits the inservice inspection to be performed on the bolting when it has been removed from the pressure vessel.
The inservice inspection as specified in Section XI,
"Rules: for Inservice Inspection of Nuclear Reactor Coolant Systems," of the ASME Code should be supplemented with regard to examination of the closure stud bolting. Visual and surface examinations (Ref. 5)
may fall to reveal unacceptable defects, especially if the studs are examined in an untensioned condition. For these types of examinations, the reliability may be increased by examining the studs in a stud-tensioning fixture.
Volumetric examination by conventional ultrasonic techniques may be difficult to perform, and the results may be inconclusive because of the chamfered ends, the recess for the lug wrench, and the threaded surfaces on the -stud bolts' A technique 3 has been developed in which a transducer is lowered into the stud bolt.center hole and an ultrasonic radial scan is used
- for' the 'Ultrasonic examination. Such a radial scan.;,
technique could be an improvement over the axial scan ultrasonic method and could increase the confidence In the inservice inspection for the determination of flaws In the thread region of the stud bolts.
Many. different plating materials are- available to protect reactor stud bolts and nuts from galling (Ref. 6)
and corrosion; however, in . certain applications, the plating can be more detrimental than helpful.
Metal-plated bolts can be susceptible to fracture of the plating in the root of the thread after a short period of bolt and nut engagement. Moisture accumulation in the area of coating discontinuities can cause corrosion. The electrolytic plating process may produce hydrogen which can become entrapped in the parent metal structure and cause embrittlement.
A potential combination of hydrogen in the base metal, natural notches in the bolt thread, moisture in the environment, Sand high stresses in the material is an ideal condition for cracking. Metallic coatings can be prone to seizing between the bolts and nuts and make bolt disassembly difficult. Examination of failed stud bolts in the LaCrosse Boiling Water Reactor (LACBWR) and Yankee Rowe has shown severe galling associated with silver-plated stud bolts and also severe corrosion damage in the thread root of the stud bolts.
Replacement stud bolts in Yankee Rowe which had a
manganese phosphate surface treatment in
"A study being conducted at the Southwest Research Institute concerning the ultrasonic testing of the LaCrosse Boiling Water Reactor pressure vessel closure stud bolting.
1.65-2
combination with MoS2 as a lubricant demonstrated superior resistance to galling when compared to the original silver-plated stud bolts. The manganese phosphate surface treatment provides only mifd
,protection against corrosion and. contaminants; and therefore the stud bolts require additional -protection during venting, filling, flooding, or unusual exposure of the reactor vessel.
Because of the Items mentioned In the above discussion and In order to pin the necessary assurance that the reactor pressure boundary will operate 1i safe manner, some of the additional material ad nispection requirements as referenced in this guide are more stringent than those stated in Section mI of the ASME
Code and those of Appendix G, "Fracture Toughness Requirements," to 10 CFR Part 50.
Ca REGULATORY POSITION
I.
Bolting Materials a.
Reactor vessel closure stud bolting should be fabricated from materials which have adequate toughness throughout the life cycle of the reactor. Stud bolting should meet the requirements set forth in Subsection NB, ."Requirements for Class 1
"rpr ponents," Section 1I of the ASE Code.. Bolting.
, materials should meet the requirements of one .of the following ASME specifications:
(1) SA-540 Grade B-23 and B-24 bar (AISI
4340).
(2) SA-193 Grade B-7 bar (AISI 4140, 4142,
4145).
(3) SA-194 Grade 7 (nuts for bolting) (AISI
4140,4142,4145).
(4) SA-320 Grade L.43 bar (AISI 4340).
b. The requirements of the specification In paragraph C.Ia. should be supplemented by the following:
(1) The maximum measured ultimate tensile strength of the stud bolting material should not exceed
170 ksi.
(2) Charpy V impact testing should be performed according to ASME SA-370, "Methods and Definitions for Mechanical Testing of Steel Products,"
and to be acceptable, .the results must satisfy the requirements of Paragraph IV.A.4. of Appendix G to 10
CFR Part 50.
In case a test falls, one retest may be conducted according to subsubartidle NB-2350 of Section EIl, ASME Code.
(3) Stud bolting should not be metal-plated unless it has been demonstrated that the plating will not degrade the quality of the material in any significant way (e.g.,.corrion, H embrittlement) or reduce the quality, of results attainable by the .Various required inspection procedures. The stud bolting may have a manganese phosphate (or other acceptable) surface
"treatment.
Lubricants for" -the stud bolting are permissible p.ovided they are stable at ,operating temperatures and are compatible with the bolting and vessel materials and the surrounding environment.
2. Inspection. Ike nondestructive examination of the stud bolfs and nuts phouldbe performed acording to subsub'article' NB2580 of Section MI of the ASME code as supplemented by the following:
a.
The stud bolts and nuts should be ultrasonically examined after final heat treatment and prior to threading.
b. The ultrasonic examination (paragraph NB-2584) should be conducted according to ASME
Specification SA-388, "Ultrasonic Examination of Heavy Steel Forgings."
c.
The calibration standard used to establish the first back reflection for the ultrasonic testing should be based on good sound representative material. To assure that the material is representative, the selection of the standard should be based on a preliminary ultrasonic examination of a number of 'specimens (a minimum of three per standard).
d. The magnetic particle or liquid penetrant examination (paragraph NB.2583) should be performed on the studs and nuts after final heat treatment and threading.
e. The requirements of paragraph N&2585 should be applied to allclosure stud bolts and nuts.'
3. Protection Against Corrosion. During venting and filling of the pressure vessel and while the head is removed, the stud bolts and stud bolt holes in the vessel flange should be adequately protected from corrosion and contamination.
4. Ilservice Inspection. The inser ice examination of the pressure vessel stud bolting should be performed in accordance with the requirements of Section XI of the ASME Code as supplemented by the following:
a.
The inservice inspection should include a.
surface examination which should be in accordance with paragraph NB-2545 or NB-2546 of Section III of the ASME Code. For this inspection the studs should be removed from the pressure vessel.
b. Selection of the bolting material for each required Inservice inspection should be based on a representative sample and on a reasonable geometric distribution.
1.65-3
SREFERENCES
1. Aerospace Structural Metals Handbook, Code 1206, Vol. 1, "Ferrous Alloys," Syracuse University Press.
2. J. H.: Gross, "Th1 Effective Utilization of Yield Strength," Transactions of the ASMB, Paper No.
71.PVP-.1 1.
3.
"ASM Metals Handbook," Vol.2, p. 24 5-248 . 1964.
4. "PVRC
Recommendationsi n -Toughness Requirements for- Ferriti6 :Materials, Welding Research Council Bulletin No. 175, p. 14-16, August 1972.
5. A. G. Pickett and IL C. Burghard, Jr., "An Investigation of the Failure of LACBWR Pressure Vessel Closure Studs," Southwest Research Institute SWRI-2154-20, December 1971.
6. R. L Scott and P. H. Harley, "Failure of Threaded Fittings and Fasteners at Nuclear Facilities,'
Nuclear Safety, Vol. 13, No. 1, Jan.Feb. 1972.
K.ý
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