ML19340C832

RO 80-25,Revision 1:on 800904,during Investigations Re Generic Technical Activity A-12,CVM Steam Generator Mounting Bolts Found Defective.Caused by Stress Corrosion Cracking. New Steam Generator Mounting Bolts Installed & Tensioned

ML19340C832
Person / Time
Site:	Prairie Island
Issue date:	12/03/1980
From:	Mayer L NORTHERN STATES POWER CO.
To:	James Keppler NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
References
RO-80-25, NUDOCS 8012170640
Download: ML19340C832 (6)

Text

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p .sB NSED NORTHERN STATES P O W E Fv COMPANY M f N N E A PO LI S. MIN N E SCTA SS409 December 3,1980 Mr J G Keppler Office of Inspection & Enforcement ,

U S Nuclear Regulatory Commission 799 Roosevelt Road Glen Ellyn, IL 60137

Dear Mr Keppler:

PRAIRIE ISLAND NUCLEAR GENERATING PLANT Docket No. 50-282 License No. DPR-42 Supplement to Licensee Event Report 80-25 (Rev 1)

Defective Steam Generator Bolting -- ...

Attached are three copies of a followup report containing additional information related to Licensee Event Report 80-25.

Yours ve ry t ruly, 0*

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.s L 0 Mayer, PE Manager of Nuclear Suppo:t Services ti LOM/DHM/bd cc: Director, IE, USNRC (40)  ;

l Director, MIPC, USNRC (3) r-NRC Resident Inspector .3 g

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J W Ferman l

Attachment

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80121706.40 g DEC 101980

i Attachment December 3, 1980 Director, IE III, USNRC 1

NORTHERN STATES POWER COMPANY PRAIRIE ISLAND NUCLEAR GENERATING PLANT R0-80-25 Supplemental Report Defective Steam Generator Bolting 1

Identification of Occurrence De fe ct iva Vascomax 250 CVM steam generator mounting bolts. Twenty eight of the 48 mounting bolts on #11 and #12 Steam Generators were found defective.

As a result, Unit 2 was shutdown for mounting bolt inspection; this inspec-tion revealed three defective bolts in #22 Steam Generator.

Conditions Prior to Occurrence Unit I was in refueling shutdown and Unit 2 was at 100% steady-state power.

Method of Discovery and Description 1 On September 4, 1980 maintenance personnel began to detension the Unit 1 Vascomax 250 CVM Steam Generator mounting bolts. During investigations initiated by the NRC related to Generic Technical Activity A-12 we learned that the bolts were installed with a pre-load. The bolts were to be l detensioned from approximately 1550 ft lbs to 115 ft lbs to eliminate this i

j condition. The firs t two bolts on which detensioning was attempted f ailed l

just below the head at the thread relief.

Ultrasonic testing of the remaining 46 mounting bolts on the Unit I steen gene ra t ors indicated 26 additional bolts with 'no, or weak, back reflection.

During bolt removal and replacement 25 of these bolts failed. One bolt was cracked below the head at the thread relief. Of the 28 defective bolts,1,4 were found in each generator.

On the evening of September 4, 1980 Unit 2 was shut down to inspect the mounting bolts in #21 and #22 steam generators. On #22 steam generator three mounting bolts were found to be defective.

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December 3, 1980 Page Designation of the Apparent Cause of the Occurrence It is known that maraging high strength steels are highly susceptible to stress corrosion cracking in the presence of moisture, high tensile stress, and elevated temperature. Elements that contribute to stress corrosion cracking were found to exist: *

1) The thread relief located below the bolt head created a stress riser.

2) The bolts when installed during construction were torqued to approximately 1550 ft 1bs.

3) Westinghouse's metallurgical investigation revealed the presence of zine and chlorine. -

4) Approximately 50 percent of the support adapter holes did not line up with the holes in the support so some binding was present.

5) It appears a bottoming tap may not have been utilized on all the steam generator support bolt holes. This may have resulted in stress levels in the bolts higher than originally cons ide red . .

6) There was evidence of valve leakage in the vicinity of some bolting which provided a source of moisture.

Westinghouse R & D Center performed visual metallographic and f ractographic examination of the cracks, chemical evaluation of bolt material and crack deposits, and sechanical property tests. The results of the Westing-house findings are as follows:

1) The cracks were circumferential and were initiated at the outer f surf ace around the thread relief below the bolt head where a stress

' concentration due to the head-to-shank radius is likely to be present.

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2) The fractures contained points of multiple origin along the outer circumference of the thread relief region. The fractures were relatively flat, although highly irregular in a few cases. ,

3) In almost all cases, cracking was extensive with the apparent service fracture covering in excess of 90% of the bolt section. The l

l fractures were he3vily oxidized in most cases.

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4) Micro structural studies of the bolt material by light optic and l

electron transmission microscopopy showed no apparent abnormalities in the micro structure.

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5) Scanning electron fractography of the fractured surf aces revealed that fracture occurred primarily by transgranular cleavage, except for small islands of dimpled rupture where the bolt was fractured during removal, t

December 3, 1980 Page 6) Chemical evaluations of the fracture surf aces by EDAX analysis revealed, in addition to the principal alloying elements, the presence of zine and chlorine. Both elements are known to lower the stress corrosion resistance of the bolt material in the presence of moisture.

7) Tensile testing of a sound bolt resulted in a yield strength of 260.9 Ksi.

8) Hardness readings taken on a sectioned bolt head revealed Rockwell "Rc" numbers ranging from 48 to 53.

9) The fracture toughness tests revealed a room temperature value of 98.9 Ksi. ,

10) The results of the chemical analysis of the bolt material showed that the chemistry is within the specification requirement for 18% Ni (250) Vascomax Material.

11) Estimates of the load to which the bolts were subjected to during service showed that cracking occurred at a substantially lower stress level (60 to 80 Ksi) than the load bearing capacity of the bolt material (yield stress of 260 Ksi).

The following dif ferences existed between the Unit 1 & 2 steam generator mounting bolts:

1) On Unit 1, the steam generator mounting bolts are located under the mirror insulation and on Unit 2 they are outside the insulation.

This results in a temperature dif ference between Unit 1 and Unit 2 mounting bolts of approximately 150 F.

2) The thread relief for the bolts for the Unit 2 mounting bolts is located approximately 1/2" away from the bolt head instead of just below the bolt head in the Unit 1 mounting bolts.

3) The initial torque data shows the Unit 1 bolts were torqued to approximately 1550 ft 1bs and the Unit 2 bolts were torqued to approximately 1400 ft 1bs.

4) The Unit 2 mounting bolts were supplied with washers and the Unit 1 bolts were not.

5) The mounting bolts. on Unit 2 broke loose for detensioning at approximately 2500 f t 1bs. r,me of the bolts on Unit 1 could not be detensioned at loads esi imated to be in excess of 4000 f t lbs and had to be burned out.

$ December 3, 1980 Page Analysis of Occurrence As a result of the steam generator mounting bolt f ailures, no radioactive material was released, no other equipment was af fected, no adminis trative controle were violated, no extra personnel exposure occurred (except for the bolt replacement), and normal plant operation was not affected.

Fluor Power Services performed an evaluation of the saf ety margin available due to the degraded bolt ing. The evaluation indicated that the Vascomax 250 CVM bolts reach a maximum allowable stress in the SSE, taking into account dead we igh t , pressure, and thermal considerations, of 11.8 percent of the specified bolt minimum yield strength. On #11 and #12 Steam Generators one bolt for each steam generator was unavailable for shear load, due to failure in the Heli Coil. Fourteen bolts for each steam generator were unavailable' to assume tension load. Based on the as-found condition of the

. S/G mounting bolts, the remaining 14 bolts would have held the steam generator during an SSE.

Fluor Power Services safety evaluation indicated that the Vascomax 250 CVM bolts reach a maximum allowable stress during the design basis LOCA, taking into account dead weight, pressure, and thermal considerations, of approx-imately 41.4 percent of the specified bolt minimum yield strength. The remaining bolts, 96 percent available for shear load and 41.7 percent available for tensile loads, may have prevented support f ailure during the

design basis LOCA condition and would have been adequate for less severe LOCA conditions.

The re f o re , the consequences from the standpoint of the public health and safety were minimal.

NOTE: Westinghouse and Fluor Power Services are working on-reanalysis of the loads the mounting bolts actually see. It appears the actual bolt loadings during the SSE and LOCA conditions will be considerable sma lle r .

On #22 Steam Generator the three defective bolts were located in one support pad. Westinghouse analysis indicated it was acceptable to operate

• l with the remaining three Vascomax 250CVM bolts in that pad. To increase l

the safety margin on that pad, a bolt was removed f rom two other legs and i

installed in that pad. All other Unit 2 bolts were de-tensioned and the I

unit was returned to service.

Corrective Action l

l The following actions were taken in Unit I to prevent recurrence of mounting bolt failures:

1) The new steam generator mounting bolts were installed and tensioned to 50 f t lbs with a maximum specified tension of 115 f t lbs.

2) The mouating holes in the' steam generator were cleaned with low l

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halogen or sulfur material.

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December 3,1980 Page 5

3) The bolts were installed with a certified lubricant with independent chemical contamination tests.

4) The exposed bolt head and surrounding area were seal coated with a high temperature silicone paint.

5) The steam generator support adapter plates bolt holes were machined so the adapter holes will line up with the steam generator support holes .

6) The steam generator support holes were retapped. as necessary with a bottoming tap.

7) The new bolts have rolled threads in lieu of machined threads.

8) The thread relief on the new bolts is very gradual and located approximately one inch from the bolt head.

9) The bolts will continue to be ultrasonically inspected in accordance with our inservice inspection program.

As a result of the above modifications, Prairie Island's steam generator mounting bolts should now be classified in Group III in NUREG-0577.

Failure Data

Description:

1 1/2" - 12 UNF-2A Vascomax 250 CVM steam generator mounting bolts Material Manufacturer: VASCO (a Teledyne Company)

Machined By: Welding and Steel Fabrication Co., Inc.

Future Action During the 1981 refueling of Unit 2 the Vascomax 250 CVM steam generator mounting bolts will be replaced with the new improved Vascomax 250 CVM bolts as described above. The bolts that are removed will be examined for signs of stress corrosion cracks.

I Prepared by: George Lenertz, Production Engineer Prairie Island Nuclear Generating Plant I

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NRC FO M 366 U. S. NUCLE AR REGULATORY COMMISSION (7 77)

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