ML17252A447

Report of Abnormal Occurrence 50-249/75-31 for Dresden, Unit 3, Regarding Cracks in Control Rod Drive Collet Housings

ML17252A447
Person / Time
Site:	Dresden
Issue date:	07/03/1975
From:	Stephenson B Commonwealth Edison Co
To:	James Keppler NRC/RGN-III
References
BBS 409-75 50-249/75-31
Download: ML17252A447 (3)

Text

e Common~h Edison One First NaiJllrPlaza, Chicago, Illinois Address Reply to: Post Office Box 767 Chicago, Illinois 60690 BBS Ltr. #409-75

References:

1)

• Regulator-Y Guide *1.16 *Rev~

• 1 Appendix A

2) Notification of Region III of*u. s. Nucl.ear Regulatory Commission

.Telephone: Mr. P. Johnson, *osoo hours on June 2 ~

~elegram: Mr. *J. Keppler~ 1145 hours0.0133 days <br />0.318 hours <br />0.00189 weeks <br />4.356725e-4 months <br /> on Jun.ea~~ ~-.0~/

~ ~

3) Drawing. Number . .  : *' / . oo, .. r..;,;

USNRC .

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Report Number: 50-2§9/1975-31 __"] JUL l 1 *n75 ,_

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Report Date: July 3, 1975 (;_....\ u.s. ,._.~.~-"--.~=',',~ r v  :, j A/" Mail S ion Occurrence Date: June 26, 1975 . . °Vl')'!--:-- '

. 0) I I Facility: Dresden Nuclear* Power Station*, _Morr.is,* Illinois:. 6o450 * ---"--------

IDENTIFICATION OF OCCURRENCE

• Qn June 26~ 1975 cracks were discovered on the*collet housings of control rod drives 984, 883, 1032, 1099.

CONDITIONS PRIOR TO OCCURRENCE Unit-3 was shut down in a refueling outage.

DF.SCRIPI'ION OF OCCURRENCE At approximately 0900 hours0.0104 days <br />0.25 hours <br />0.00149 weeks <br />3.4245e-4 months <br /> on June 26, 1975, while overhauling control rod drive 98~, a maintenance foreman noticed a crack in the collet housing short tube.

Mr. James G. Keppler e

• July 3, 1975 The crack was detectable visually, and was confirmed by means of a dye penetrant test. Commonwealth Edison's Operational Analysis Department (OAD) wa~

contacted to evaluate the crack on #984:,. as -well *as examine* control rod drives

---as};- -1032, and 1099, -which were available "for scrutiny. A Uevel II inspector confirmed that cracks were present in the collet housing of each of the four control rod drive mechanisms~ In each case, the cracks occurred in the collet "

housing short tube below the water ports, in the' area of increased wall thickness.

General Electric*s Nuclear Energy Division was also contacted on June 26, 1975.

GE examined their test control rod drive mechanisms for cracks similar to those experienced on Unit-3. Cracks were found that were nearly identical to those on the four control rod drives. A problem evaluation has been established by GE to study this problem and determine all possible effects.

Between June 27 and 30, 1975, several ~ore control rodGdrive mechB.ni.sms were examined for _cracks m the collet housingsarea. As of June 30, GE Level II inspectors have examined 18 mechanisms using the dye penetrant test. Of

• these 18 mechanisms, 11 have displayed some indication.of cracking.

DESIGNATION OF APPARENT CAUSE' OF OCCURRENCE Gex:ieral. Electric has. observed similar **cracks on *test drive mechanisms that have been scram-:-cycled 2000 times, and:more severe.cracking on mechanisms scram-cycled m:ore "t;han 4000 times. GE was aware that cracking !:iad occurred, but assumed the problem was associated with the abnormally high number of scram cycles performed. There was no '

indication that crackilig would. develop within the expected lifetime limit of 200 scrams. 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 normal-drive flow. *At present, General Electric, the Oper-ational Analysis Department, and Argonne National Laborat6ry are conducting independent metal_lurgical studies to determine the cause -of the cracking. It has not been

• definitely determined that the cracks are due to thermal stress.

ANALYSIS OF OCCURRENCE As demonstrated by 2000 and 4000 scram cycle tests conducted by GE, the probability of a total collet housing failure is quite remote. The collet housl.ng is not a pressure barrier, but acts as a restraint to contain the collet assembly, experiencing a force of approximatelyt600 psi during withdrawal, and considerably less during scrama and insertions. This stress is vastly less than the yield strength of the collet housing metal. If the collet housing wer,e to fail, the possibility exists that the collet barrel and spring could jam the coll et fingers, reducing the scram speed or pr_even:t:ing the drive. from being

_.inser-ted.- However-, the* probability o-f *a D:imiber of drives failing simultaneously is extremely unlikely. Should such an imp~ble failure occur, highly localized core damage could result from abnormal rod patterns and power levels.

The standby liquid control system would be avail_able to reduce reactivity and main-tain the reactor in a shutdown condition. All radioactivity would be contained either within the reactor vessel or the standby gas<btreatment system. Th.ere would be no danger to plant personnel or the public*

Mr. James;:-,G. Keppler-CORR&;TIVE ACTION

• July 3, _1975

<tmmediate corrective action has been to initiate analyses of the cracked material.

future actions will be determined by the*outcotne of 'the studies now in progress.

~---,'New housing assemblies have been received. tO replace the cracked assemblies.

A follow-up report will be submitted as soon as a definitive cause can be established.

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

The control rod drive mechanism is manufactured by the General Electric Company.

This drive mechanism is the same type used on all boiling water reactors manufactured by General Electric since 1967.

• e enson Superintendent

~. " *.

BBS:smp File/NRC