ML19210A183

Abnormal Occurrence 50-289/75-39:on 751112,control Rod 4 in Group 7 Dropped Into Core,Resulting in Asymmetrical Rod Signal & Automatic Power Reduction.Caused by Failure of Stator Winding.Stator Winding Replaced & Tested

ML19210A183
Person / Time
Site:	Three Mile Island
Issue date:	11/21/1975
From:	Arnold R METROPOLITAN EDISON CO.
To:
References
GQL-1741, NUDOCS 7910240905
Download: ML19210A183 (4)

Text

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FILE: INCIDENT REPORT FIL:

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11-12-75 re inoperable control rods r.,

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3 0gulat0Ty.__DOCxet ukzco ur MER DR MOGM33 _ .: "Fiic METROPOLITAN EDISON COMPANY PCST OFFICE BOX 542 READING, PENNSYLVANIA 19603 TELEPHONE 215 - 929 3601 Ncvember 21, 1975 GQL 17h1 L'irector of Nuclear Reactor Regulation U. S. Nucleer Pegulatory Cc==ission 4ashington, D.C. 20555

Dear Sir:

Docket No. 50-289 Operating License No. DPR-50 In accordance v.'.th the Technical Specifications of our Three Mile Island Nuclear Station Unit 1 (TMI-1), we are reporting the folleving abner =al CCCurrence. --

(1) Report Number: A0 50-289/75-39 g (2a) Report Date: November 21, 1975 (2b) Occurrence Date: November 12, 1975 5 - ~.

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(3) Facility: Three Mile Island Nuclear Statien Unit 1 k (L) Identificatien of Occurrence: g Y c> -2

Title:

Incperable Control Reds During Pcver Operation Type: An abnormal cecurrence as defined by the Technical Specifica'. lens, paragraph 1.eb, in that more than ene control red became incperable during pcVer operation which constitutes a violatien of Technical Specificatien 3.5.2.2.a.

(5) conditions Prior to occurrence:

Pcver: Core: 100%

Elec. 8h5 MWe (Grcss) 0 RC Flev: 138 x 10 lb/hr.

RC Pressure: 2150 psig g]{ }79 RC Temp.. 579 F 13356

PR"R Level: 2h2 in.

PRZR Temp.. 650 ?

(6) Description of Occurrence:

On UcVe=ber 12, 1975, and during 100% power operation, Centrol Rod h in Group 7 dropped into the ccre frc= approximately the 155 vithdrawn positien due to stator vinding failure. The drcpped rod resulted in an asyn=etric rod signal, thereby, initiating an autc atic pcver reduction

( runback) . The runback resulted in Group 6 and Grcup 7 rods being inserted.

As Group 7 was moving in, Group 7 Rod 5 became misaligned from the Group 7 average position by = ore than the allevable nine (9) inches and was therefore declared incperable. Group 7 Rod 5 did not move in with the rencining rods in the Group becruse of a temporary system lineup which was being used to minimize motion on Red 5 due to a suspected stator vinding prcblem. In this temporary lineup, all Group 7 rods except Rod 5 vere ccnnected to auxiliary pcVer while Red 5 remained on normal power.

The operator assumed manual centrol at about 70% power to =inimize the feed-water oscillations that develcped during the aute=atic runback. The plant was stabilized and power was manually reduced to 60% power within approximately twenty (20) minutes after Grcup 7 Red h was dropped. Subsequently, an attempt was made to exercise Group 7 Red L. The rod cculd not be removed and therefore became the seccnd rod in Group 7 to be declared inoperable. Since operatien with =cre than one (1) inoperable rod in the regulating rod banks is a violation of Technical Specification 3 5 2.2.a, an orderly shutdevn was cc==enced without =cving Rod 5 The generater was subsequently taken off line approximately one (1) hour and twenty (20)=inutes after Grcup 7 Rod h ves dropped.

During the inspection for blown fuses on Grcup 7 Rod h, a blown .use var also cbserved on Group 5 Rod 12. This rod dropped when Group 5 insertion began during the power reduction.

(7) Designation cf Apparent Cause of Cecurrence:

The cause of this occurrence has been determined to be material in that the droppeu .'4 resulted frct a failed stator vinding. A problem of failed stator vind1.is has been experienced due to a possible material prcblem.

Group 7 Red h winding failure may hava been due to a missing upper "0" ring where the stator assembly seals to the Control Rod Drive motor tube. The missing "0" ring may have alleved moisture to enter the stater cavity resulting in vinding failure.

(8) Analysis of Occurrence:

It has been determined that this cccurrence did not constitute a threat to the health and safety of the public in that:

a. in the event of a manual cr aute=atic trip of the reactor, the affected rods would have drcpped into the ccre and, 1476 i80

b. an orderly reactor shutdcvn was cc==enced i==ediately after the two rods were declared ineperable and ec=pleted within apprcximately cne (1) hour.

(9) Corrective Actien:

In additicn to the i==ediate action described abcVe, icng tern ecrrective actions are as follows:

a. The failed stator vinding was replaced and the remaining stators were tested to scsure that there were no apparent weak vindings.

b. A maintenance procedure change vill be implemented to assure inspection cf both upper and lever "0" rings on the stator assembly during the installation of stators.

(10) Failure Data:

Control rod drive stator vindings h pole, 6 phase,15a reluctance stepping motor.

Diamond Fover Specialist / Royal Industries Group i Rod h: 703h60-1-263 120-J-3hl_lRev. L Serial No. 95 Group 7 Rod 5: 703h60-1-lTh 120-J-3ki-Rev. L Serial No. R-70-063 Group 5 Rod 12: 703h60-1-29 R0-J-3hl-Rev. H Serial No. R-70-C63 Similar Cccurrences: A0 SC-289/75-39 Sincerely, f

1 . - ,

/

R. C. Arncid Vice President RCA:JMC:tas cc: Office of Inspecticn & Enforcement, Regien 1 File: 20.1.1 / T.7.3.5.1 1476 'iBI