ML20084N925

Forwards Summary Rept of Investigatory Work Performed on Dresden 3 Control Rod Drive Collet Housing Cracks, as Followup to AO 50-249/1975-31.W/o Encl

ML20084N925
Person / Time
Site:	Dresden
Issue date:	07/30/1975
From:	Stephenson B COMMONWEALTH EDISON CO.
To:	James Keppler NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
Shared Package
ML20084N873	List: ML20084N914 ML20084N925 ML20084N947
References
463-75, NUDOCS 8306090113
Download: ML20084N925 (1)

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SUBJECT:

Follow-up Report to Abnormal Occurrence Entitled " Cracks In Control Rod Drive Collet Housings" (Report No. 50-249/1975-31)

Report No. 50-249/1975-31A

Dear Mr. Xeppler:

Please find attached the' "Sumary Report of the Investigatory Work Performed on Dresden 3 Control Rod Drive Collet Housing Cracks."

This report is submitted in accordance with the commitment specified under " Corrective Action" of Report No. 50-249/1975-31.

Sincerely,

.B. Stephenson Superintendent

• Dresden Nuclear Power Station BBS:TJK:aw cc: File /NRC Enc.

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Mr. James G. Keppler, Regional Director #'

Directorate of Regulatory Operation-Region III '

U. S. Nuclear Regulatory Co==ission N 799 Roosevelt Road s.

Glen Ellyn, Illinois 60137 - 7

SUEJdET: REPORT OF AE!CR"."JL CCCURREICE PER SECTION 6.6.A 0F THE TECH'!! CAL SPECIFICATICNS CRACKS IN CChTROL ROD DRI'.'E COLLE.T HCUSE!GS P.eferences: 1) Regulatory Guide 1.16 Rev. 1 Appendix A-

2) Notification of Region III of U. S. Nuclear Regulatory Cnmission Telephone: Mr. P. Johnson, 0800 hours0.00926 days <br />0.222 hours <br />0.00132 weeks <br />3.044e-4 months <br /> on June 27, 1975 Telegram: Fr. J. Keppler, 11h5 hours on June 27, 1975 -

3) Drawing Number Report Number: 50-249/1975-31 Report Date: July 3, 1975 Occurrence Date: June 26, 1975 Facility: Dresden Nuclear Power Station, Morris, Illinois 60450 IDS.?IFICATION OF OCCURREICE On June 26, 1975 cracks were discovered on the collet housings of control rod drives 984, 883, 1032, 1099 CONDITICNS PRIOR TO OCCURRE!CE Unit-3 was shut down in a reftteling outage.

DESCRIF"!CN OF OCCURRE!CE l _.

At approximately 0900 r.ours on June 26, 1975, while overhauling control rod drive 984, a c:aintenance foreman notited a crack in the collet housing short tube.

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The crack was dete-table visually, and was confirmed by means ,of a dye penetrant test. Coc=cnwealth Ediscn's Operational Analysis Department (CAD) was contacted to evaluate the crack en fr?34, as well as examinc control rod drives

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863 1032, and 1099, which were available for scrutiny. A Levet II inspector I confirced that cracks were present in the collet housing of each of the four control rod drive =echanis=s. In each case, the cracks occ'urred in the collet housing short tube below the water ports, in the area of increased wall thickness.

General Electric's Nuclear Energ/ Division was also contacted on June 26, 1975 GE exa-ined their test centrol rod d-ive mechanis=s for cracks similar to those experienced en Unit-3 Cracks were f;und that were nearly identical to those on the four control rod drives. A problem evaluation has been established by GE to study this proble

and deterrine all possible effects.

Between June 27 and 30, 1975, several more centrol rod driite mechanisms were exHned for cracks in the collet housing area. As of June 30, GE Level II inspectors have exa.ined 18 =echanisms using the dye penetrant test. Of these 18 =echanisms, 11 have displayed some indication of cracking.

_ . DESIG'IATION OF APPARE"I CAUSE CF CCCUPRE!CE General Electric has observed similar cracks on test drive mechanisms that have been scra=-cycled 2000 times, and more severe cracking on mechanisms scram-cycled

= ore than 4000 times. GE was aware that cracking had occurred, but assumed the proble= was ascociated with the abnor= ally high number of scram cycles performed. There was no indication that cracking would develop within the expected lifetime limit of 2C0 scrats. The cause of the cracking appears to be related to

- temperature cycles the drive experiences during a reactor scram. Alternately, if cooling water is lost or restricted, a thermal cycle will occur when the drive is operated with nor=al drive ficw. At present, General Electric, the Oper-

- ational Analysis Department, and Argenne National Laboratory are conducting independent metallurgical studies to determine the cause of the cracking. It has not been definitely deter =ined that the cracks are due to thermal stress.

ANALYSIS OF O'CU 3EC E As de=onstrated by 2000 and 4000 scram cycle tests conducted by GE, the probability of a total collet housing failure is quite re=ote. The collet housing is not a pressure barrier, but acts as a restraint to contain the collet assembly, experiencing a force of approximately 600 psi during withdrawal, and considerably less during scra=s and insertions. This stress is vastly less than the yield strength of the collet housing metal. If the collet housing were to fail, the possibility exists that the collet barrel and spring could jam the collet fingers, reducing the scram speed or preventing the drive from being inserted. However, the probability of a nu=ber of drives failing simultaneously is extre=ely unlikely. Should such an i=p'.:ccible failure occur, highly localized core da= age could result from abnormal rod patterns and power levels.

_ The standby liquid control system would be available to reduce reactivity and main-1 tain the reactor in a shutdown condition. All radioactivity would be contained p either within the reactor vessel or the standby gas treatment system. There would, u be no danger to plant personnel or the public.

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July 3, 1975 Mr. Jam'es G. Keppl .

CCRRECTIVE ACTIC':

C..::ediate corrective action has been to initiate analyses of the cracked r..aterial.

Future acticns will be determined by the outcome of the studies now in prodress.

New housin.; asse=blies have been received to replace the cracked assemblies.

A fcilow-up repcrt will be submitted as socn as a definitive cause can be established.

FAILURE DATA There have been no known failures of collet housings to date.

The control red drive =echanism is manufactured by the General Electric Company.

Tbis drive mechanism is the sa e type used on all boiling water reactors manufactured by General Electric since 1967

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