ML20083Q239
| ML20083Q239 | |
| Person / Time | |
|---|---|
| Issue date: | 01/31/1983 |
| From: | Koo W Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| NUREG-0943, NUREG-943, NUDOCS 8302250112 | |
| Download: ML20083Q239 (30) | |
Text
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NUREG-0943 Threaded-Fastener Experience in Nuclear Power Plants U.S. Nucisar Regulatory Commission Offico of Nuclear Reactor Regulation W. H. Koo i.
R882EBRis"
- 2 2
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NOTICE Availability of Reference Materiais Cited in NRC Publications Most documents cited in N RC publications will be available from one of the following sources:
- 1. The NRC Public Document Room,1717 H Street, N.W.
Washington, DC 20555
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Referenced documents available for inspection and copying for a fee from the NRC Public Docu-ment Room include NRC correspondence and internal NRC memoranda; NRC Office of Inspection and Enforcemen* bulletins, circulars, information notices, inspection and investigation notices; Licensee Event Reports; vendor reports and correspondence; Commission papers; and applicant and licensee documents and correspondence.
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NUREG-0943 Threaded-Fastener Experience in Nuclear Power Plants Minuscript Completed: January 1983 Data Published: January 1983 W. H. Koo Divi; ion of Licensing Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Wmhington, D.C. 20665
ABSTRACT This report identifies 44 incidents of threaded-fastener degradation and failure in nuclear power plants from October 1964 to March 1982.
It provides an overview of some of the threaded-fastener problems that have occurred since 1964.
Safety implications of these incidents are discussed, and short-term regulatory actions and ongoing long-term regulatory actions are described.
Information included in this report represents the current NRC staff under-standing of each issue.
TABLE OF CONTENTS P. age ABSTRACT......................
iii
- ACKNOWLEDGEMENTS...........................
vii 1 INTRODUCTION...........................
1-1 2 DESCRIPTION OF THREADED-FASTENER PROBLEMS.............
2-1 2.1 Types of Threaded-Fastener Degradation and Failures.
2-1 2.2 Threaded-Fastener Experience in Nuclear Power Plants.....
2-1 2.2.3 Stress Corrosion...................
2-2 2.2.1.1 Reactor Coolant Pressure Boundary......
2-2 2.2.1.2 Component Supports..............
2-3 2.2.1.3 Component Internals.............
2-3 2.2.1.4 Valves....................
2-4 2.2.2 Fatigue........................
2-4 2.2.3 Borated-Water Corrosion................
2-4 2.2.4 Erosion-Corrosion........
2-5 2.2.5 Other Threeded-Fastener Degradation and Failures.......................
2-5 2.2.6 Summary........................
2-5 3 SAFETY IMPLICATIONS........................
3-1 4 SHORT-TERM REGULATORY ACTIONS....................
4-1 5 LONG-TERM F.EGULATORY ACTIONS...................
5-1 6 REFERENCES............................
6-1 4
v
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l FIGURE PaSe i
2.1 Number of Threaded-Fastener Incidents, October 1964 to March 1982...........................
2-7 LIST OF TABLES i-2.1. Incidents of Stress Corrosion of Threaded. Fasteners.......
2-8 2.2 Incidents of Fatigue of Threaded Fasteners...........
2-11 2.3 Incidents of Borated Water Corrosion of Threaded Fasteners.......
2.....................
2.4 Incidents of Erosion-Corrosion of Threaded Fasteners......
2-13 I
2.5 Incidents of Other Types of Degradation of Threaded Fasteners............................
2-14 3.1 Summary of Degraded Threaded-Fastener Incidents Involving Reactor Coolant Pressure Boundary (RCPB).......
3-2 3.2 Summary of Degraded Threaded-Fastener Incidents Involving Component Supports.................. 3 3.3 Summary of Degraded. Threaded-Fastener Incidents Involving Component Internals..................
3-3 I
vi s
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ACKNOWLEDGEMENTS The. author wishes to thank K. Wichman, W. Hazelton, D. Sellers, H. Conrad, and B. Turovlin of the Office of Nuclear Reactor Regulation, J. Collins of the-Office of Inspection and Enforcement, and ll. Vander Molen of the Division of Safety Technology for their input to this report.
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THREADED-FASTENER EXPERIENCE IN NUCLEAR POWER PLANTS 1
INTRODUCTION Numeroys threaded fasteners, for example, bolts, studs, and capscrews, are used in a nuclear power plant. The most important applications are those constituting cn integral part of the reactor coolant pressure boundary, such as pressure-retaining closures in reactor vessels, pressurizers, reactor coolant pumps, and steam generators.
In recent years, an increasing number of incidents of degraded threaded fasteners have been reported in both operating reactors and reactors under construction.
A large number of reported degraded threaded-fastener incidents involve the reactor coolant pressure boundary and major component supports.
Altnough these incidents have not resulted in an immediate safety concern in regard to the requirements of General Design Criterion 14 of Appendix A to 10 CFR 50 (Title 10 of the Code of Federal Regulations), they do reflect an undesirable level of degradation of the reactor coolant pressure boundary in operating nuclear power plants and they impair the structural integrity of component supports.
The scope of this report is limited in that there is no intent to describe each event in detail and the coverage of the incidents is not exhaustive. The staff has investigated only relatively significant events that occurred during this period. This report covers a total of 44 degraded threaded-fastener incidents reported by the licensees of operating nuclear power plants and the applicants of plants still under construction during the period from October 1964 to March 1982.
This information is derived from pertinent licensee event reports, reportable occurrence reports, operating reactor event memoranda, failure analysis reports, and other relevant documents.
This report was initiated as a result of the Executive Director for Operations response (Dircks, 1981) to R. F. Fraley's memorandum dated October 20, 1981 regarding the safety concern of threaded-fastener failures in nuclear power plants.
It provides a perspective and an overview of threaded-fastener prob-lems in operating nuclear power plants and describes st ort-term regulatory actions and ongoing long-term regulatory actions addressing this problem.
1-1
i l
2 DESCRIPTION OF THREADED-FASTENER PROBLEMS 2.1 Types of Threaded-Fastener Degradation and Failures Degradation and_ failures of threaded fasteners described in this report are the loss of the integrity of threaded fasteners, including bolts, studs and capscrews, resulting from any mechanical, chemical, or electrochemical causes.
The principal types or modes of threaded-fastener degradation and failures discussed are defined below:
(1) Stress Corrosion Stress corrosion resulting from the simu1%neous presence of tensile stress and a specific hostile environment causes cracking, or failure of material.
In this report all cracking that occurs in a hostile environment under steady-state tensile ioad is classified as " stress-corrosion cracking." Stress-corrosion cracking is a concern because it can occur below the design stress limits.
(2) Fatigue Failure of material under repeated cyclic loads is defined as fatigue failure.
Corrosion fatigue is the reduction of fatigue resistance caused by the pres-ence of a corrosive or hostile environment.
This report does not distinguish between fatigue and corrosion fatigue because fatigue test data involving hostile environments are not available.
(3) Borated-Water Corrosion Borated-water corrosion is the uniform attack by borated water (by a chemical or electrochemical reaction) on material over a large exposed area.
(4) Erosion-Corrosion Erosion-corrosion is the corrosion reaction accelerated by a high-velocity medium and abrasion.
It is characterized by the tppearance of grooves, gul-lics, waves, holes, or valleys and usually exhibits a directional pattern.
Corrosion attack by high-velocity wet steam is defined as wire drawing.
2.2 Threaded-Fastener Experience in Nuclear Power Plants A total of 44 degraded threaded-fastener incidents are covered in this report.
Three incidents occurred in boiling water reactors (BWRs) and the rest in presscrized water reactors (PWRs).
Figure 2.1 is a schematic presentation of the number of incidents reported in each year from October 1964 to March 1982.
Figure 2.1 shows that the number of reported degraded threaded-fastener inci-dents has increased in the last 2 years.
This is apparent even when only the incidents pertaining to the reactor coolant pressure boundary are consid. red.
2-1
l I
l i
A review of these incidents shows that the principal types or modes of threaded-i fastener fai ure and degradation are stress corrosion, fatigue, borated-water corrosion, and erosion-corrosion.
2.2.1 Stress Corrosion For'all the incidents involving threaded fasteners covered in this report, stress corrosion was the most common cause of failure.
A total of 19 incidents attributed to strees corrosion are listed in Ta'le 2.1.
Relevant available information, such as material of parts, contributing factors, and the correc-tive action taken, is also tabulated.
Only one threaded-fastener incident (Lacrosse) was reported in a BWR; all the cther incidents occurred in PWRs.
2.2.1.1 Reactor Coolant Pressure Boundary Table 2.1 lists 6 incidents associated with the cracking of primary pressure boundary closure studs:
the failure of steam generator manway studs in San Onofre Unit 1 (1977), Arkansas Unit 1 (1978 and 1981), Oconee Unit 3 (1980),
and Maine Yankee (1982), and the failure of reactor vessel closure studs in Lacrosse (1970).
During the fifth refueling outage at San Onofre Unit 1 in 1977, selected components of the reactor primary coolant system were subjected to an inser-vice inspection.
Cracks were visually identified in eight steam generctor.
manway studs. The cause of the cracking was not determined.
Two steam generator manway studs were sheared off at the base of the nut at Arkansas Unit 1 in 1978 during reinstallation of the lower manway cover follow-ing tube plugging in steam generator A.
Visual' inspection indicated that s crack of 90% of the stud diameter had existed before retensioning.
Ultrasonic test (UT) inspection of other studs of both steam generator upper and lower manways did not show any crack indications.
The cause of the stud failure was not determined because meaningful failure analysis could not be performed on the excessively deteriorated fracture surface of the failed studs.
During a scheduled steam generator inspection in Oconee Unit 3 in 1980, cracking in 9 of 64 upper and lower manway studs was identified by visual and UT examina-tion. At Arkansas Unit 1 during an outage as a result of a steam generator tube leak in 1980, UT inspection identified three lower manway studs with crack indications on steam generator A.
The steam generator manway stud failures at Arkansas Unit 1 and Oconee Unit 3 were attributed to the use of thread lubricant containing molybdenum disulfide.
Molybdenum disulfide decomposes at high tem-peratures to form corrosive hydrogen sulfide, which caused the accelerated cracking of the closure studs.
Experiments performed at Brookhaven National Laboratory have shown a pronounced embrittling effect on carbon steel and low alloy steel when the material is in contact with molybdenum disulfide in a steam environment.
During a routine disassembly of steam generator #2 primary manway cP.sure in Maine Yankee in 1982, 6 of 20 manway studs failed and another 5 Gowed crack indications.
The studs were exposed to leaking borated water and Furmanite, a sealing compound containing leachable sulfur, fluorine, and chlorine, which are known promoters of stress-corrosion cracking.
The cause of leakage was due to 2-2
l an. interference contact between the gasket retainer lip.and vessel cladding which prevented proper compression of the flexitallic gasket during reinstal-lation of the manway cover.
Furmanite was injected into the manway closure to control the leafage when increasing the torque in the studs to hydrotest level niled to stop the leakage.
UT inspection of steam generators #1 and #3 manway studs did not show any indications of cracking.
The cause of the failure of steam generator manway studs at Maine Yankee is currently under study.
During a 1970 refueling outage at Lacrosse, tw reactor vessel closure studs failed in a head removal operation after tensions of 46 ksi and 35 ksi were applied. -The licensee attributed the failures of the two reactor vessel clo-sure studs made of 12% Cr martensite stainless steel mainly to exposure to an aqueous environment while the studs were under load during an outage.
The other contributing factors were ir.. proper heat treatment of the material resulting in a high susceptibility to intergranular stress corrosion and galvanic action caused by localized breakdown of the silver plating on the threads.
2.2.1.2 Component Supports Four incidents occurring at Surry Units 1 and 2 (1975) and Prairie Island Units 1 and 2 (1980) were related to the cracking of threaded fasteners in steam genera-tor supports.
All the threaded fasteners were made of maraging steel (Vasco-max 250).
The exact cause of the failure at Surry Units 1 and 2 (1975) was not determined.
The failure of steam generator column support bolts at Prairie Island Units 1 and 2 was attributed to an excessive pretorque of 1,400 ft-lb.
Laboratory test results have shown that high strength maraging steels and low alloy steels heat treated to high hardness are highly susceptible to stress corrosion, especially under large preload conditions.
Four incidents reported at Ginna (1970), Haddam Neck (1973), Midland Unit 1 (1979), and Palo Verde (1981) were related to failure of imbedded anchor bolts in steam generator supports, reactor vessel skirt flange, and piping restraints.
The exact cause of the failure of steam generator support anchor bolts at Ginna and Haddam Neck was not determined.
Anchor bolt failures at Midland Unit 1 and Palo Verde, which are still under construction, were attributed to excessive hardness as a result of improper heat treatment of the low alloy steel material.
2.2.1.3 Component. Interrals The studs of main steam isolation valve internals made of AISI 4140 material failed at D.C. Cook Unit 1 in 1981.
The failure was attributed to overtorque during installation and the use of thread lubricant containing molybdenum disulfide.
Several broken thermal shield bolts in the reactor vessel internals of Oconee Unit I were observed while a 10 year inservice inspection was being performed during a refueling shutdown in 1981.
Subsequent UT inspection identified 94 of a total of 96 bolts showing crack indications. The results of fa' lure analysis on the broken thermal shield bolts of Oconee Unit 1 had suggested stress cor-rosion as well as fatigue as potential failure mechanisms.
The fracture surfaces of the broken bolts were so severely corroded that it was not possible to deter-mine which of the two failure mechanisms was :esponsible.
The corrective action for Oconee Unit 1 consists of a redesign of the lower thermal shield and use of Inconel X-750 material for bolts and nuts.
2-3
. ~. - -.
I UT inspection of reactor vessel thermal shield bolts of Oconee Unit 2 in 1982 also identi.fied 3 broken bolts and another 24 bolts showing crack indications.
The cause of thermal shield bolt failure in 0conee Unit 2 is currently under-study.
2.2.1.4 Valves One incident involving some valve studs made of stainless steel Type 416 in Rancho,Seco (1980) was attributed to improper heat treatment of the material.
The failure of two bonnet-to-body studs of a 6-in. gate valve in the-spent fuel cooling system during a routine disassembly for maintenance was reported at Maine Ysnkee in 1982.
The studs were made of Type 416 stainless steel and were corroded because they were exposed to boric acid from a small body-to-bonnet leak. The licensee reported that there are about 150 valves with Type 416 stainless steel studs in the plant.
Inspection of 12 such valves did l
not show any evidence of borated-water corrosion.
The licensee has replaced the degraded studs with AISI 4140 low alloy steel studs, and in.the longer term will replace all AISI 4140 studs with studs made of 17-4 PH material.
2.2.2 Fatigue All identified fatigue failures'of threaded fasteners in nuclear power plants have been associated with reactor vessel internals.
Table 2.2 lists three threaded-fastener failures attributed to fatigue, namely, the failure of thermal shield bolts in Big Rock Point and Yankee Rowe and the failure of holddown bolts for the ring shim ir, Palisades. The failure of holddown bolts in Palisades was identified after a broken bolt head was discovered in the steam generator #2 inlet plenum; the cause of the failure was improper pre-torque of the bolts.
The cause of the thermal shield bolt failures in Big Rock Point (1964) and Yankee Rowe (1968) was the flow-induced vibration.
2.2.3 Borated-Water Corrosion i
Borated-water corrosion is the second most numerous type of threaded-fastener-failure or degradation covered in this report.
It occurs only in pressurized water reactors.
A total of 13 incidents resulting from borated-water corrosion are listed in Table E.3.
In alrost every case, the cause of threaded-fastener degradation was corrosive attack by borated water leaking from closure gaskets or seals, and the degraded fasteners were discovered during the process of l
correcting the leakage.
After prolonged exposure to borated water, the affected closure studs or holddown bolts can be corroded sufficiently to impair their load-carrying capability.
]
One of the severe cases of borated-water degradation of threaded fasteners reported was the corrosion of a reactor coolant pump closure stud made of low alloy steel (AISI 4140) at Fort Calhoun.
The diameter of the stud was reduced from 3.5 in to 1.1 in.
Among the 13 incidents resulting from borated-water corrosion, 6 incidents that were related to reactor coolant pump closure studs occurred at Fort
.Calhoun (1980) (1981), Calvert Cliffs Units 1 (1980) and 2 (1981), and Oconee Units 2 (1981) and 3 (1981); three incidents that were related to steam generator-manway closure studs occurred at St. Lucie (1977), Arkansas Unit 1 i
2-4
1 (1981) and-Calvert Cliffs Unit 1 (1981); two incidents that were related to pressurizer manway studs occurred at St. Lucie (1978) and Calvert Cliffs i-Unit ~2 (1981); and three incidents that were related to various types of valve studs occurred at Calvert Cliffs Unit 2 (1981), Kewaunee (1981), and D.C. Cook Unit 2 (1981).
1 2.2.4 Erosion-Corrosion One threaded-fastener failure resulting from erosion-corrosion by borated water is given in Table 2.4.
This incident occurred at Zion Unit 1 in 1979 and resulted from leakage from a valve bonnet joint in the chemical and volume control system.
The failure analysis indicated that the failure of the bolts to seal the bonnet was probably caused by " wire drawing" as a result of impropor assembly of the bonnet, t
2.2.5 Other Threaded-Fastener Degradation and Failures Seven degraded threaded-fastener incidents that cannot be classified into the previous categories are listed in Table 2.5.
At Sequoyah Units 1 and 2 in 1977, some steam generator support bolts failed while they were being hammered to ensure proper seating into the helicoils.
Both plants were still under con-struction at that time.
The applicant attributed this premature failure of the support bolts to the presence of quench cracks resulting from improper heat
-treatment of the bolting material during fabrication.
Reactor coolant pump support bolt failures occurred at Waterford in 1981.
(Waterford is still under construction.) The failure was attributed to the improper torquing of the support bolts and to the fact that some bolts were too short.
It was also discovered that the torquing equipment was not properly calibrated.
Some motor holddown bolts on valve limit-torque operators failed at Vermont Yankee (1981) and Pilgrim Unit 1 (1981). The exact cause of these failures is not known.
The licensee believed that the holddown bolts that were' loosened by vibration and were subsequently sheared during operation were the probable cause i
of the failure at Pilgrim Unit 1.
The emergency.feedwater turbine steam inlet bolts at Arkansas Unit 1 (1980) failed because the bolts were made of the wrong material, carbon steel (C-1117),
instead of the originally specified low alloy steel (AISI 4140).
The carbon steel bolts were not strong enough to~ withstand the waterhammers that occurred.
Surry Unit 2 reported the failure of a capscrew in a service water pump impeller in 1981.
The impeller capscrew had been corroded in an aqueous environment ~
thus causing the pump to be inoperable.
The broken car:on steen capscrew was
-replaced with a stainless steel capscrew.
2.2.6 Summary
~0n the basis of the available information concerning the incidents covered in this report, the major causes for threaded-tastener degradation and failures are summarized as follows:
f 2-5
(1) Stress Corrosion i
i (a) borated-water leakage (b) wet or humid environment (c) high preload (d) use of lubricant containing molybdenum disulfide (e) improper heat treatment of material (2) Fatigue (a) ' flow-induced vibration (b) improper preload (3) Borated-Water Corrosion and Erosion-Corrosion (a) borated-water leakage (4) Other Threaded-Fastener Degradation and Failures (a) improper heat treatment of material (b) improper preload (c) wrong material The majority of these threaded-fastener problems are corrosion related, and the staff. believes that they are caused by a combination of any of the following three factors:
(1) presence of hostile environment such as borated water, wet or humid environment, and sulfur or chloride contamination (2) application of high preload (3)' use of material susceptible to stress-corrosion cracking such as high-strength maraging steel and low alloy steels heat treated to high-strength levels.
t 2-6
17 16 16 15
~
14 TOTAL: 44 incidents y
13 gj-Reactor Coolant Pressure Boundary 0
k/
Threaded-Fastener incidents s
5 11 5g 10 j
i 9
3 9-e f
8 (7)
'Sv 7
6' (5) o 5
E 5
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3 3
2 2
(2)
(2) 2 1
1 (1) 1 1
(1)
(1) n
/b b
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1968 1970 1972 I
1978 1M I
1965 1967 1969 1971 1973 177 3g79 1981 Year Figure 2.1 Number of threaded-fastener incidents, October 1964 to March 1982. The numbers in parentheses refer ta reactor coolant pressure boundary threaded-fastener incidents for that year.
2-7
Table 2.1 Incidents of stress corrosion of threaded fasteners Year Components Materials Contributing Corrective Plants reported and parts of parts factors action Lacrosse (BWR) 1970 Reactor vessel 12% Cr marten-(1) Aqueous environment (1) Replaced with studs made from closure studs sitic stainless during outage A-540-823, Class 4 material (3.5-in diameter) steel (ASTM-A-(2) Improper heat (2) Augmented inservice inspec-437-84R) treatment of tion ultrasonic test material surveillance (3) Galvanic action resulting from silver plating breakdown (4) Pretension Ginna 1970 Steam generator Low alloy steel (1) 160 ksi pretension (1) Replaced with studs made from support anchor (AISI-4140)
(2) Humid / wet borated A-490 material studs (1-3/8-in.
water (2) No pretension diameter) rp Haddam Neck 1973 Steam generator Low alloy steel (1) Pretension (1) 24 of 256 bolts replaced oo support anchor (2) Water leakage (2) Pretension reduced on bolts (2-in.
replaced bolts diameter)
(3) Microswitch installed on all bolts for monitoring Surry 1 1975 Steam generator Maraging steel Replaced with Cd plated support bolts (Vascomax 250)
Vascomax 250 bolts 2
Surry 2 1975 Steam generator Maraging steel Replaced with Cd plated support bolts (Vascomax 250)
Vascomax 250 bolts San Onofre 1 1977 Steam generator Low alloy steel 8 studs replaced manway studs (AISI 4140)
(A193-87)
Midland 1 1979 Reactor vessel low alloy steel (1) Improp9 heat (1) Remaining studs detensioned skirt flange imbed (AIST-4140, treatment.of to 6 ksi anchor studs 4145) saterial (2) Upper lateral support (2-1/2 in.
(2) "_xcessive preload installed on vessel diameter) of 87-92 ksi 4
Table 2.1 (continued)
Year Components Materials' Contributing Corrective Plaats reported and parts of parts factors action Arkansas 1 1978 Steam generator Low alloy steel 2 cracked studs replaced manway closure (AISI 4340) studs 1980 Steam generator Low alloy steel (1) Use of. thread lub-3 cracked studs replaced manway closure (AISI 4340) ricant containing i
studs molybdenum disul-fide l
(2) Preload Oconee 3 1980 Steam generator Low alloy steel (1) Use of thread lub-All studs replaced (threaa manway closure (SA-320, Grade ricant containing lubricant containing molybdenum studs (2-in.-
L-53) molybdenum disel-disulfide was applied) diameter)
.(AISI-4340) fide
.(2) Trapped moisture Prairie 1980 Steam generator Maraging steel Excessive preload (1) Replaced with studs made from
'?
Island 1 column support (Vascomax 250)
(1,400 ft-lb torque) same material
-bolts (1-1/2 in.
(A538 grade B)
(2) Pretension recuced diameter)
Prairie 1980 Steam generator Maraging steel Excessive preload (1) Replaced with studs made from Island 2 column support (Vascomax 250)
(1,400 ft-lb torque) same material bolts (1-1/2-in.
(A538 grade B)
(2) Pretension reduced i
diameter)
~
Rancho Seco 1980 Valve studs Stainless steel Improper heat treatment Type 416
.of material i
(A-193-86)
D.C Cook 1 1981 Main steam Low alloy steel (1) Primary steam isolation valve
-(AISI 4340)
.(2) Possible use of internals - studs thread lubricant i
containing molyb-denes disulfide (3) Possible over-torque 4
1
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4 i
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Table 2'.1 (continued)
Year Components Materials ~
Contributing Corrective 4
Plants reported-and parts of parts factors action 4
Oconee 1 1981 Reactor vessel A-286 stainless (1) Borated water (1) Lower thermal shield
-internals -
steel environment ~
redesigned i
thermal shield (2) Preload of 32 ksi
-(2) Use of Inconel X-750 studs bolts and 32 ksi bending and nuts Oconee 2 1981 Reactor. vessel A-286 stainless (1) Borated-water (1) Lower thermal shield internals -
steel environment redesigned thermal shield (2) Preload of 32. ksi
-(2) Use of Inconel X-750 studs bolts.
and 32 ksi bending and nuts Palo Verde 1981 Piping restraint Low alloy steel Improper heat treatment' imbedded anchor (AISI 4140)(A-of material bolts (1-1/2-in.
354 Grade BD) diameter)
Maine Yankee 1982
. Steam generator Low alloy steel (1) Gasket leakage of 10 failed studs replaced with manway closure (SAS40-824) borated water studs of the same stock s3 y,
studs (1-1/2 in.
(2) Use'of Furmanite c) diameter) sealing compound,
(3) Use of thread lub-ricant containing.
j molybde'.um disulfide (4) Preload of 900 -
)
1,100 ft-lb j
6-in. gate valve Stainless steel Valve bot..)-bonnet (1) Proposed short-term actica -
bonnet-to-body gasket lec,,ge of replace with AISI 4140 (A-studs (5/8-in.
borated water 196-87) studs i-i diameter);
(2) Proposed long-term action -
~?
{
use 17-4 PH studs
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Table 2.2 Incidents of fatigue of threaded fasteners Year Components Materials Contributing Corrective Plants reported and parts
-of parts factors action Big Rock Point 1964 Reactor vessel Type 316 stain-Flow-induced vibration Support and flow pattern modified (BWR) internais -
less steel thermal shield (ASTM A-276) bolts Yankee Rowe 1968 Reactor vessel Type 316 stain-Flow-induced vibration Clamp added to each thermal internals -
less steel shield j" int thermal shield bolts Palisades 1972 Reactor vessel Type 304 stain-Improper torque (1) Broken bolts replaced interna's - hold-less steel (2) Proper torque and clearance down bolts for ring shim (1/2-in.
diameter) i q)
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7 l
9
Table 2.3 Incidents of borated-water corrosion of threaded' fasteners Year Components Materials Contributing Corrective-Plants reported and parts of parts factors-action St. Lucie 1977 Steam generator Low carbon low Manway gasket leakage
-(1) 3 studs replaced manway closure alloy' steel of borated water (2) Gasket replaced studs (1-1/2-in.
(SA-540-824) diameter)'
1978 Pressurizer man-Low carbon low Manway leakage of'
'S corroded studs replaced way closure. studs alloy steel borated water (SA-540-B24)
Calvert 1980 Reactor coolant Low alloy steel.Possible gasket leak-27 studs replaced Cliffs 1 pump closure studs age of borated water 1980 Steam generator Low alloy steel Gasket leakage of 11' studs replaced manway studs borated water Fort Calhoun 1980 Reactor coolant Low alloy steel Flexitallic flange 9 studs replaced 3,j, pump closure studs (AISI 4140) gasket leakage to
-(3-1/2-in.
(SA-193-87) diameter) 1981 Reactor coolant Low alloy steel Corroded studs replaced pump. closure studs (AISI 4140)
(3-1/2-in.
(SA-193-B7) diameter)
Arkansas 1 1981 Steam generator Low alloy steel Closure gasket leak-Corroded studs replaced manway closure age of borated water studs.
Calvert 1981 Reactor' coolant Low alloy steel Possible gasket leak-12 studs replaced Cliffs 2 pump closure studs age of borated water 1981' Pressurizer manway Low alloy steel Seal leakage of bor-2 studs replaced studs ated water
)
Table 2.3 (continued)
Year Components Materials Contributing Corrective Plants reported and parts of parts factors action D.C. Cook 2 1981 Check valve bonnet Low alloy steel Valve body-to-bonnet All 12 studs replaced bolts (AISI 4140) gasket leakage of (A-193-87) borated water Kewaunee 1981 8-in. motor-Low alloy steel Valve body-to-bonnet Corroded studs replaced operated valve gasket leakage of con-body-to-bonnet centrated (12%) borated studs water Oconee 2 1981 Reactor coolant low alloy steel Closure gasket leakage I stud replaced pump closure studs of borated water Oconee 3 1981 Reactor coolant Low alloy steel Closure gasket leakage 1 stud replaced pump closure studs of borated water I
m Table 2.4 Incidents of erosion-corrosion of threaded fasteners Year Components Materials Contributing Corrective Plant reported and parts of parts factors action Zion 1 1979 Chemical and Low alloy steel Valve gasket leakage (1) Degraded bolts replaced 1
volume control (AISI 4140) of borated water (2) Valve bonnet reassembled system valve bolts (A193-B7) 1 0
d Table 2.5 Incidents of other types of degradation of threaded fasteners Year Components Materials Contributing Corrective Plants reported and parts of parts factors action
^
Sequoyah 1 1977 Steam generator Quench cracks Bolts replaced support bolts (1-1/2-in.
diameter)
Sequoyah 2
.1977 Steam generator Quench cracks Bolts replaced support bolts (1-1/2-in.
diameter)
Arkansas 1 1980 Emergency feed-
. Carbon steel (1) Wrong material All bolts. replaced with low
. C-1117)
(2) Waterhammer alloy steel (AISI 4140) bolts
(
water turbine steam inlet bolts Pilgrim 1 1981 Valve limit-torque Bolts replaced (BWR) operator motor holddown bolts no fg Surry 2 1981 Service water. pump Carbon steel (1) Broken capscrew replaced impeller capscrew (2) All impeller capscrews to be replaced with stainless steel capscrews
. Vermont Yankee 1981 Valve limit-torque 4 mounting bolts. replaced operator motor mounting bolts Waterford 1981 Reactor coolant A-490 alloy (1) Improper torque (1) Failed bolts and short bolts pump. support bolts steel (2) Some bolts too replaced short (2) Bolts retorqued with cali -
brated torque equipment (3) Quality assurance plan for bolting improved i
J
3 SAFETY -IMPLICATIONS Most of the incidents reported in Tables 2.1 through 2.5 were discovered during refueling outages, scheduled inservice inspections, or maintenance /
repair outages.
As a result, such reported incidents have as yet had no impact on public health and safety.
Threaded-fastener failures discovered during normal operation have not challenged the integrity of plant engineered safety features.
In spite of limited safety consequences to date, many inci-dents involved threaded fasteners that constitute an integral part of the reactor coolant pressure boundary.
As shown in Table 3.1, a total of 19 of the reported 44 incidents involved reactor coolant pressure boundary applica-tions.
Degradation and failure of such threaded fasteners constitute a poten-tial loss of integrity of the reactor coolant pressure boundary and could lead to malfunction or failure of the affected components in contravention to the requirements of General Design Criterion 14.
In the extreme case, a loss-of-coolant accident (LOCA) could occur if extensive threaded-fastener failures in a pressure-retaining closure are not detected.
As shown in Table 3.2, a total of 11 threaded-fastener incidents involved component supports.
Failure of such threaded fasteners will not impair the normal operation of the plant; however, under the extreme loads associated with a LOCA or earthquake, extensive failures of support or anchor threaded fasteners can result in component uplift and possible failure.
For threaded-fastener incidents involving component internals, the major safety concerns are (1) the degradation of the component performance and (2) the effect of loose parts on the safe operation of the plant when the physically separated internal bolts or studs are not captured.
A total of seven threaded-fastener incidents involving component internals are listed in Table 3.3.
Although those seven incidents did not result in any problem with component performance or plant operation, they do constitute a potential safety concern.
The safety implications discussed above are a cause for concern, especially because of the increased number of reported threaded-fastener incidents in recent years as shown in Figure 2.1.
This concern is further compounded by the fact that the UT methods used in inservice inspection programs are not sensitive enough to detect initial cracking in the threaded fasteners result-ing from stress corrosion and fatigue without the development and use of special techniques. The UT sensitivity as required by the calibration stand-ard in Section V of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code," is not high enough to detect the critical crack size of low alloy high-strength steel material, which is typically less than 0.10 in.
Furthermore, for detecting threaded-fastener degradation by borated-water corrosion or erosion-corrosion, visual examination at this time is the only reliable method to discover such degradation; in almost all cases dis-assembly of the component is necessary so that the threaded fasteners can be seen.
Therefore, degradation by borated-water corrosion or erosion-corrosion of such visually inaccessible threaded fasteners can be left undetected when there is no clear evidence of leakage in the surrounding area.
Under the present inservice inspection progreas, visual inspection is not a mandatory requirement 3-1
and UT inspection is not required on pressure-retaining bolts or studs with diameters of less than 2 in. Maine Yankee steam generator manway studs, for example, have diameters of less than 2 in.
Because each bolt, stud, or capscrew in a component has a unique purpose or function, the loss of service of any particular part threatens the design basis of the component.
In turn, this icssens the safety margin for the plant.
Table 3.1 Summary of degraded threaded-fastener incidents involving reactor coolant pressure boundary (RCPB)
Degtaded RCPB No. of threaded reported Year Reactor Mode of fasteners incidents Plants repoited vendor
- failure **
San Onofre 1 1977 W
SC manway closure St. Lucie 1 1977 CE BC studs Arkansas 1 1978 B&W SC 1980 Calvert Cliffs 1 1980 CE BC Oconee 3 1980 B&W SC Arkansas 1 1981 B&W BC Maine Yankee 1982 CE SC Reactor coolant 5
Calvert Cliffs 1 1980 CE BC pump closure studs Ft. Calhoun 1980 CE BC Calvert Cliffs 2 1981 CE BC Oconee 2 1981 B&W BC Oconee 3 1981 B&W BC Pressurizer manway 2
St. Lucie 1
.1978 CE BC closure studs Calvert Cliffs 2 1981 CE BC Reactor vessel 1
Lacrosse 1970 AC SC closure studs Chemical and volume 1
Zion 1 1979 W
Erosion-corrosion control system isolation valve bolts Safety injection 1
Calvert Cliffs 2 1981 CE BC check valve studs Check valve studs 1
D.C. Cook 2 1981 W
- W = Westinghouse; CE = Combustion Engineering; B&W = Babcock & Wilcox; AC = Allis-Chalmers.
- SC = stress corrosion; BC = borated-water corrosion.
NOTE: Total number of plants - 13; total number of incidents - 19.
3-2
Table 3.2 Summary of degraded threaded-fastener incidents involving components supports Degraded threaded fasteners No. of Year in component supports plants Plant reported Steam generator support bolts 6
Surry 1 1974 Surry 2 1974
~
Sequoyah 1 1977 Sequoyah 2 1977 Prairie Island 1 1980 Prairie Island 2 1980 Steam _ generator support imbedded 2
Ginna 1970 anchor studs Haddam Neck 1973 Reactor vessel imbedded anchor 1
Midland 1979 stude Piping restraint imbedded anchor 1
Palo Verde 1981 bolts Reacto*
it pump support 1
Waterford 1981 bolts NOTE:
Total number of plants - 11; total number of incidents - 11.
Table 3.3 Summary of degraded threaded-fastener incidents involving component internals Degraded threaded fasteners Nc. of Year in component internals plants Plants reported Reactor vessel invernals -
4 Big Rock Point 1968 thermal shield bolts Yankee Rowe 1968 Oconee 1 1981 Oconee 2 1982 Reactor vessel internals -
1 Palisades 1972 holddown bolts for ring shim i
D.C. Cook 1 1981 internals - studs Service water pump internals --
1 Surry 2 1981 impeller capscrew NOTE:
Total number of plants - 7; total number of incidents - 7.
3-3
4 SHORT-TERM REGULATORY ACTIONS Office of Inspection and Enforcement Bulletin No. 82-02, dated June 2,1982, requires that the following actions be taken by PWR licensees.
These actions apply to reactor coolant pressure boundary closures and could significantly reduce both the number and severity of threaded-fastener failures.
l (1) Licensees should provide quality control for sealant compounds and fastener lubricants to ensure proper selection, procurement, and application to minimize fastener susceptibility to stress-corrosion-cracking environments.
r (2) Licensees should develop maintenance procedures detailing the instructions for removal (detorque) and treatment (cleaning-handling) of fasteners, as well as detailed tensioning techniques during assembly and disassembly of closure seal systems.
(3) Licensees should clean and visually irispect studs or bolts of manway closure assemblies and should perform magnetic particle or dye penetrant (for nonmagnetic material) examinations during each outage in which the closure seal is removed for equipment inspection / maintenance.
b 4-1
t 5
LONG-TERM REGULATORY ACTIONS The following sections summarize the long-term regulatory actions in progress at this time:
(1) Wbrk Sponsored by Materials Engineering Branch, Division of Engineering NUREG/CR-2467 entitled " Lower-Bound K Values for Bolting Materials - A ISCC Literature Study" was issued in February 1982.
It was based on the work per-formed by Lawrence Livermore National Laboratory and documents the available test data, in the form of K values versus yield strength, on various low ISCC alloy steels, maraging steels, and stainless steel in water, aqueous chloride, aqueous sulfide, and other environments.
The staff and the NRC contractor, Brookhaven National Laboratory, will use this report to prepare an NRC position on the actions that should be taken to prevent stress-corrosion cracking in threaded fasteners and fastener materials.
A contract pertaining to evaluating and establishing requirements for threaded-fastener application was recently placed (March 1982) with Brookhaven National Laboratory (B;4L).
The objective is to obtain information leading to a regula-tory position en material selection, installation, and inspection of threaded fasteners and threaded-fastener material used in water reactors.
The BNL study is expected to be completed by May 1983.
NRC will issue a report on this subject on the basis of that input.
This report will serve as the principal vehicle for evaluating the safety significance of threaded-fastener degradation and failures and for developing and implementing new and improved regulatory requirements applicable to threaded fasteners.
(2) Work Sponsored by Chemical Engineering Branch, Division of Engineering Brookhaven National Laboratory completed NUREG/CR-2827 entitled " Boric Acid Corrosion of Ferritic Reactor Components" and dated July 1982.
This report summarizes the material degradation experience resulting from boric acid cor-rosion at seven nuclear power plants (Fort Calhoun, Calvert Cliffs Units 1 and 2, Oconee Units 2 and 3, Kewaunee, and Zion Unit 1) and was based on the review of applicable licensing event reports and other relevant documents.
This report also reviewed the available data on corrosion rates of various low alloy steels in H 80, H B0 K-OH, and H 80 -LiOH solutions and showed that a corrosion rate 3 3 3 3 3 3 of at least 112 mils / year can be attained at 212* F.
The BNL report is being used in licensing reviews and operating reactor licensing _
actions to establish the basis for analysis of the effects of borated-water cor-rosion on carbon and low alloy steel components, including threaded fasteners.
(3) Prioritization by the Division _of Safety Technology (DST)
The roblem of threaded-fastener degradation or failure in nuclear power plants has been designated as Generic Issue 29 in draft NUREG-0933, '!Prioritization of 5-1
\\
NMM Generic-Issues" (to la published). The Division of Engineering has been assigned the responsibility of developing a task action plan to reso.<e this generic issue.
I 5-2
a 6
REFERENCES Code of Federal Regulations, Title 10 " Energy" (includes General Design Criteria).
American Society of Mechanical Engineers (ASME), " Boiler and Pressure Vessel Code,"Section V, " Nondestructive Examination."
Dircks, W.
J., NRC, memorandum to R. F. Fraley, ACRS,
Subject:
Bolt Failures in Nuclear Power Plants, December 2, 1981.
Fraley, R. F., ACRS, memorandum to W. J. Dircks, NRC,
Subject:
Bolt Failures in Nuclear Powar Plants, October 20, 1981.
I U.S. Nuclear Regulatory Commission, NUREG/CR-2467, " Lower-Bound K Values ISCC for Bolting Materials - A Literature Study," Lawrence Livermore National Laboratory, February 1982.
--, NUREG/CR-2827, " Boric Acid Corrosion of Ferritic Reactor Components,"
Brookhaven National Laboratory, July 1982.
--, Office of Inspection and Enforcement (IE) Bulletin No. 82-02, " Degradation of Threaded Fasteners in the Reactor Coolant Pressure Boundary of PWR Plants," June 2, 1982.
6-1
W.S. IlluCLEAR RESULATORY CCMielMION BIBLIOGRAPHIC DATA SHEET NUREG-0943 i
- 4. T4TLE AN D SUSTITLE d4dd Vodume Na,if enterresel
- 2. (teare alm &/
l l
- 3. RECIPIENT *S ACCESSION NO.
Threaded Fastener Experience in Nuclear Power Plants
- 7. AUTHOR (S)
- 6. DATE REPORT COMPLETED ~
MONTH l YEAR William H. Koo January 1983
- 9. PE RFORMING ORGAN 12ATION NAME AND MAILING ADDRESS (lactude lip Codel DATE REPORT ISSUED MONTH l YEAR Office of Nuclear Reactor Regulation January 1983 Division of Licensing s.(te e===>
U.S. Nuclear Regulatory Comission Washington D.C.
20555
- s. (te.e m-es
- 12. SPONSORING ORGANIZATION NAME AND MAILING ADDRFSS (include Ip Codel
~
p
- 11. FIN No.
- 13. TYPE OF REPORT PE RIOD COVE RE D (/nclusrye dears)
Technical Report
- 15. SUPPLEMENTARY NOTES
- 14. (teare Wmkt
- 16. ABSTR ACT #00 words or less)
This report identifies 44 incidents of threaded-fastener degradation and failure in nuclear power plants.from October 1964 to March 1982, It provides an overview of some of the threaded-fastener problems that have occurred since 1964. Safety implications of these incidents are discussed, and short-term regulatory actions and ongoing long-term regulatory actions are described. Information included in this report represents the current NRC staff understanding of each issue.
- 17. KEY WORDS AND DOCUMENT ANALYSIS 17a DESCRIPTORS Threaded Fastener Stress corrosion Borated-wate corrosion Fatigue Erosion-corrosion 17b IDENTIFIERS!OPEN-ENDE D TLRMS
- 18. AVAILABILITY STATEMENT 19 QUgggr es re,orr) 21 NO. OF PAGES Unlimited 2o SgUggS rga,s,ws 22 PRICE
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