ML20084N646

Followup AO 50-249/75-44A:on 751125,offgas Activity Increased.Ge Insp Revealed Major Defect in Fuel Assembly DD-418.Assembly Removed from Reactor

ML20084N646
Person / Time
Site:	Dresden
Issue date:	03/31/1977
From:	Stephenson B COMMONWEALTH EDISON CO.
To:	James Keppler NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
Shared Package
ML20084N649	List: ML20084N646 ML20084N651 ML20084N654 ML20084N664
References
77-268, NUDOCS 8306080407
Download: ML20084N646 (2)

Text

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) Dresden Commonw Nucleg1r Edison ower St; tion O# 2-Q',/fgg'f.

i - N R.R. #1

'\ / Morris. Illinois 60450 ,

_/ Telephone 815/942-2920 i?//

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BBS Ltr. 877-268 March 31, 1977 /

1. ' N COPY Mr. James G. Keppler, Regional Director  %/a.

l Directorate of Regulatory Operations - Region lli $ g//

U. S. Nuclear Regulatory Commission ,

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799 Roosevelt Road Glen Ellyn, Illinois 60137

SUBJECT:

Follow-up Report to Abnormal Occurrence Report Entitled "Off-Gas increase on Unit-3"

REFERENCE:

Letter f rom B. B. Stephenson to J. G. Keppler (A. O. report no.

50-249/75-44) dated December 11, 1975 REPORT NUMBER: 50-249/75-44A REPORT DATE: March 31, 1977 OCCURRENCE DATE: November 25, 1975 FACILITY: Dresden Nuclear Power Station, Morris, Illinois INTRODUCTION The referenced Abnormal Occurrence report described the Unit-3 off-gas activity increase discovered on November 25, 1975 At that time, the cause of the off-gas activity increase could not be definitively established, al-though limited degradation of the fuel cladding was considered a probable factor. As stated, an investigation was initiated, the results of which were to be submitted in a follow-up letter.

This investigation has been satisfactorily concluded, and the subject follow-up report is being submitted in accordance with the Dresden Nuclear Power Station Technical Specifications, Section 6.6.B.

INVESTIGATION AND RESOLUTION in November, 1976, General Electric and Edison personnel visually in- j spected six fuel assemblies that had been determined to be defective at the end of cycle 4. During this visual inspection, a major defect was discovered in fuel assembly DD-418. The defect, which was confined to fuel rod A-3, was located immediately abovs and below the second fuel assembly spacer from the  !

bottom.

Below the spacer, the defect appeared as a longitudinal crack in the cladding approximately 6 inches long. Above the spacer, the defect consisted of num' e rous longitudinal splits in the cladding, with an approximately inch-long section completely devoid of cladding. There was no evidence of any fuel in this inch-long section.

8306080407 770331 PDR ADOCK 05000249 APR .G 1977 s ena W d&d /9 p

O o INVESTIGATION AND RESOLUTION (Continued)

It is believed that this single defective fuel rod caused most or all of the off-gas activity increase of November 25-26, 1975. The defect apparently originated below the spacer, with the splitting above the spacer resulting from secondary hydriding of the cladding over a period of time. Through-wall hydride formation was then followed by erosion of the exposed UO2 fuel pellet (s)'.

Secondary hydriding has been observed before in the BWR fuel rods at Dresden, but never to the extent evinced by this particular fuel rod.

Since assembly DD-418 was situated in the extreme periphery of the core during cycle 4, operations during that cycle could not have caused the primary defect observed. It appears most probable, therefore, that this defect is related to cycle 3 operations, and in particular to the event of October 31, 1974.

!t is believed that the defect either developed during cycle 3, or was initiated during that cycle, and propagated through the cladding wall during cycle 4 operations. .

Fuel assembly DD-418 has already been permanently removed from the reactor.

No further corrective action is anticipated at this time, hy on u B. Ir. St4phenson Station Superintendent Dresden Nuclear Power Station BBS:JMD:jo i