ML19095A554

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Submit Licensee Event Report No. AO-S1-76-08 Re Broken Stud from 2A Steam Generator Girth Strap with Unit 2
ML19095A554
Person / Time
Site: Surry  Dominion icon.png
Issue date: 01/11/1977
From: Brown S, Stallings C
Virginia Electric & Power Co (VEPCO)
To: Moseley N
NRC/RGN-II
References
Serial No. 385 LER 1976-008-00
Download: ML19095A554 (12)
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R~gion *. II.:~ *Su:i:te 818 .. **,_ ,_ ,: . : .* * **DOcket**No. 50-280*,.

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Virgiriia Electric* ari:d -Po~i~r *. Company:: hereby 1;3upriiits .<;1. cQpy pf:. Lic~nsee *

~ven:t'Report_No.".A0_':""81-76-08.* -,:*. ' .. . . . .

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e LICENSEE EVENT REPOR.

CONTROL BLO~K: I l. I I I I [PLEASE PAINT ALL REQUIRED INFORMATION]

1 6 LICENSEE LICENSE EVENT NAME LICENSE NUMBER TYPE TYPE

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! 1 I o30I Io31 I 32 1I REPORT REPORT CATEGORY TYPE SOURCE DOCKET NUMBER EVENT DATE REPORT DATE 7

@TI]coN'T 8

lo I I I lIJ 57 58 59 w 60 I61 ols lo I-lo l2lalol 68 11 lo . I ol z I z I G74I LQ..l 69 75 1:Lo I 6 J1 lz80I EVENT DESCRIPTION lo'21 I With Unit No. 2 at cold shutdown, a broke*n stud from the 2A steam generator girth I 1 7 8 9 . 80 loj3j I strap was discovered while completinq a modification to the 2A steam generator. I 7 89 , 80 4

7 1°1 81 9I Further investigation* to the. llnit Na

mare cracked studs . 80I joj5!8 9!This event. is reportable .per Technical 7 .

Specification 6.6.2a(5). A similar problem I 80 lo!6j !was identified on Unit No. 1 (AO-S1~76-08). I 7 8 9 . 80 PRIME SYSTEM CAUSE COMPONENT COMPONENT CODE CODE COMPONENT CODE SUPPUER MANUFACTURER VIOLATION 7

@El 8 9 Is10I H I []J 11 12 IH I A IN .I G IE !R 17 I ~ IX l 9 l 9 ! 9 47l Ll!J 43 44 48 CAUSE DESCRIPTION

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7 8 9 IStress corrosion cracking caused by degradation of the coating on the stud in I 80 9

1°1 1 I 7 8 9 conjunction with

  • high stresses developed d11ring the roao11factJJring of the st11ds 80 I

II@! !See attached report for additional information. I

7. 8 9 80 FACILITY METHOD OF STATUS  % POWER OTHER STATUS DISCOVERY DISCOVERY DESCRIPTION 7

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9 1010101 10 12 13

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. 80 FORM OF ACTIVITY CONTENT

.* RELEASED OF RELEASE AMOUNT OF ACTIVITY

  • LOCATION OF RELEASE

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N_,_/...,_A 80 PERSONNEL EXPOSURES NUMBER TYPE DESCRIPTION .

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13 . 60 PERSONNEL INJURIES NUMBER DESCRIPTION EE IO I OI 01 ~_:.aN/_A___________________________-,--_ _~

7 8 9 11 12 80 OFFSITE CONSEQUENCES

[lfil. N/A 7 8 9 80 LOSS OR DAMAGE TO FACILITY

. . TYPE . OESCRIPl"ION

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7. 8 9 10 80 PUBLICITY (ill} NA 7 8 9 ,80

. ADDITIONAL. FACTORS

  • Elfil I The heal th and safety of the general public were not affected. I 7 8 9 80

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7 8 9 80 NAME: _ _ _ T.._y--nd_a__l;..;l;......;;;L'""'.'.....;;;.Ba;;::.;u:::.;c=o=m'----------- PHONE: (804) 357-3184

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e LICENSEE EVENT REPORT REPORT NO. AO-Sl-76-08 STRESS CORROSION'CRACKING'OF STUDS FOR STEAM.GENERATOR.RESTRAINT SYSTEM DECEMBER 3, 1976 DOCKET NOS. 50-280

... 50-281 LICENSE NOS. DPR-32 DPR-37 SURRY POr/ER STATION VIRGINIA ELECTRIC AND POWER COMPANY

I. INTRODUCTION In accordance with the requirements of Technical Specification 6.6.

2a(5) for Surry Power Station Operating License Numbers DPR-32 and DPR-37 this report describes a Reportable Occurrence which occurred on October 7, 1976. ,

The Directorate of Regulatory Operations, Region II, was notified October 21, 1976s, The occurrence described herein involved the discovery of cracked Vascomax studs on the upper restraint anchor system for Unit No. 2 Steam Generator. Because a similar problem was identified on Unit No. 1, both occurrences will be discussed in this report.

An occurrence involving the failure of Vascomax material at Surry Power Station, used in the steam generator lower ring support swivel end coupling_s, was discussed in AO-S2-74-09, dated December 12, 1974.

II.

SUMMARY

OF OCCURRENCE All supports in the reactor coolant system including the steam generator supports, are designed to withstand the design basis earthquake acting simultaneously with an instantaneously applied pipe break.

(The part numbers referenced below a're shown on station drawings 11448-FM-511 Rev. 6 and listed in the "Bill of Material". Refer to Figure 1).

The function of the subject studs (Item No. 7) is to couple together the steam generator .gir.th straps (Items 1 and 2) thereby forming a contin-uous ring. This coupling is accompUshed through two joint flanges (Item

3) on ~ach side of the steam generator. Two vessel girth straps make-up the upper restraint anchor system for each steam generator. Eight studs are required for each joint flange or a total of 32 studs are required for

each anchor system.

  • I These studs are pretensioned across the joint flange spacer block (Item No. 9). 'I'he spacer block serves to reduce bending stresses in the studs.

A total of 9 machined shoe openings (Item No. 6) are welded to each vess*el girth strap (Items 1 and. 2 ). These shoe openings accommodate 9 keys (It.em Nos. 4 and 5) which themselves are fastened by dowels (Item No. 8)

  • to the large upper restraint: casting which is shown on station drawing 11448-FM-51G as Item No. 7 (Figure 2). These key and shoe openings func-tion to allow vertical thermal expansion of the.steam generator within the I

upper restraint casting, but will restrict lateral movement resulting from forces generated following a main steam line break and/or seismic event.

The upper restraint casting is anchored to the containment floor through hydraulic snubbers.

In the event of a main steam pipe break and/or seismic event the shoe openings in the vessel girth straps act against the keys which result in a tangentic:1-l load on the.girth straps. The subject studs are designed to accommodate the maximum t~ngential load resulting from this accident con-dition *. Existing space restrictions and restraint design required a limita-tion on,stud size and quantity which necessitates the use *of .an ultra-high strength bolting material. Vascomax 350 CVM maraged alloy steel is the only material available with the strength requirements of the stud material.

On October 7, 1976, with Unit No. 2 at cold shutdown conditions, a broken Vascomax stud for 2A steam generator girth strap was discovered. The insulation*. had been removed from around the upper restraint in order to loosen the girth strap to complete a modification *to the 2A ste*am

  • generator when the broken stud was found.

Furth~r*investigation of all Vascomax studs on the upper.restraints of the steam generators revealed.more failed studs.

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e* eI III. ANALYSIS OF THE OCCURRENCE Three studs from the Unit No. 2 steam gener~tor upper restraint system were sent to an independent testing laboratory to determine the cause of the stud failure. One of the studs was in two pieces with the nut attached to

. the smaller piece an4 the. other two were intac't.

Visual examination of the threaded areas showed cracks in the threads of the intact studs which were confirmed by flourescerit dye~penetrant exami-nation. The small piece* from the failed stud also had a crack.two to six threads up towards the slot. Examination of the fracture surface showed an apparent "initiator" crack, delineated with a ring of blue oxide, with a crack propagating from this initiator crack. The fracture surface was descaled, *and examined by a Scanning Electronic Microscope (SEM) and the morphology was found to be mixed intergranular and transgranular.

The threaded end from an intact stud was metallographically*examined.

The crack indicated by the visual and PT examination extended from the fourth thread up from the base of the slot, at an angle of approximately 30 degrees from tl:ie horizontal.* The crack had propagated* to within 1/32 inch of the

  • slot. Metallography showed cracks from the main crack; the: morphology was
  • mixed tr~nsgranula; and intergranular~ The threaded end from the othe~ intact stud which showed visual and PT -cracks was immersed in liquid nitrogen and deliberately fractured. The fracture surface showed that the crack had almost propagated completely*through the cross-section.
  • ... These results, together with the k'nownproperties of the 350 *grade niarag-irig steels. and. the en:~i;omnent i~ which these studs operate; indicate that

~ . . . .

1 the ~rack propagates bra stress corr~sion.mecha~ism.. Although, the maraging steels have higher fracture toughness and greater resistance to-stress corro-sion cracking than other steel heat treated to comparable strengths, the 350-

.. e grade maraging steel is ve~y susceptible to stress corrosion cracking. The presence of the branching cracks, together with a lack of striat~ons vis"".

ible in the SEM, indicate that fatigue is not the propagation mechanism.

Stress corrosion cracks were induced by the degradation of the protective coating (Heresite) combined with the stress intensification of the thread root developed during manufacture. The degradation of the (Heresite) coating on these studs was attributed to the high temperature in the con-.

fined space between the insulation and the steam generator.

IV *. CORRECTIVE ACTION TO PREVENT RECURRENCE

1. The replacement studs are H; in.-12UNF-2A Vascomax 18% Ni, maraging grade 350, 326,000 psi minimum yield strength, with nickel cadmium coating.

Stud t4reads were rolled P!ior to maraging. The design of the replace-ment studs and nuts has been modified to minimize stress concentration during manufacture and a coating was added to protect the studs from the environment (Refer to Figure 3). The* following modifications to the stud have been made:

a. The stud neck diameter hasbeen reduced to the diameter of the thread root to allow a greater proportion of the strain to be taken up in the neck of the stud. This change relieves the strain in .the threaded parts of the stud.
b. The thread crests and roots have been rounded and the stud has been polished to .a fine finish, (a 32 RMS finish or better) to minimize points at which stresses can concentrate *

. .' c. The stud threads, were rolled rather than cut to lessen. residual stress concentration from fabrication.

d. The studs were given a protective coating. The coating is a diffused

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.. - eI nickel cadmium plating performed to Aerospace Material Specification AMS-2416E. The first coat consists of nickel, electrodeposited from a solution of nickel sulfamate solution to a thickness of 0.0002" - 0.0004" followed by a coat of cadmium, electrodeposited fr0m a solution of cadmium cyanide to a thickness of 0.0002" - 0.0004".

The total coating thickness is approximately .0005" to allow proper thread fit.

e. After plating, the two coatings were diffused by a heating process to 360 degrees F for at least 30 minutes.
2. The replacement nuts are 1\ in.-12UNF-3B Vascomax 18% Ni, maraging grade 250, 150,000 psi minimum yield strength~ with Helicoil inserts. Th~

following modifications to the nuts were made: (Refer to Figure 4)

a. The_ nuts are 50% deeper than standard castellated nuts to provide greater thread surface.
b. The nuts have *a groove machined in the mating surface concentric with the axis of the nut. Th~s groove makes the region of the nut more flexible and acts to more equally apportion loads to the t,hreads.

This eliminates.a high load concentration on the first few threads of*

.*nut engagement.

c. The nuts have Helicoil inserts. The inserts also provide a better distribution of thread loading.
3. The studs were installed with a moderate preload stress level below the critical crack size for stress corrosion crack porpagation for this .material.
4. All studs (192) and .nuts (384) were replaced on'*the*support rings of**the.

generators on both units.

V. ANALYSIS AND EVALUATION OF SAFETY*IMPLICATIONS OF THE OCCURRENCE The upper restraint anchor system discussed herein does not carry any

operational loads and .do not support the steam generator. They serve only to restrain the steam generator in the improbable event of a pipe break of a feedwater, steam or reactor coolant system pipe simultaneously with a design basis seismic event as defined in the Final Safety Analysis Report.

The con'ditions described above did not exist and there was no requirement to utilize the upper restraint anchor system. Therefore, there were no safety implications associated with this occurrence.

VI. CONCLUSIONS

1. From the preliminary data received from an independent testing laboratory, it has been concluded that the degradation of the studs was caused by stress corrosion cracking.
2. The stress corrosion cracks were induced by the degradation of the pro-tective coating (heresite) combined with the stress intensification at the thread root developed during manufacture and the excessive preload stresses applied during initial installa.tion.
3. The design of, the replacement studs and nuts has been modified to minimize stress concentration during manu.facture. A nickel and cadmium coating ha.s been specified* to protect the studs from the .environment.
4. The studs were installed with a moderate preload stress that is below the critical crack size for the Vascomax 350.
5. The occurrence described herein did not affect the health or safety of the general public.
6. The occurren~e described herein did not affect the .safe operation of the station ...
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