ML20057C422

Ro:On 930910,noticed Fluctuations on Power Channels Due to Element 574 in Position J-7 Becoming Unlatched for Listed Reasons.Subj Element Replaced.Special Instructions Will Be Provided to Operators on Fuel Handling & Monitoring

ML20057C422
Person / Time
Site:	National Bureau of Standards Reactor
Issue date:	09/22/1993
From:	Rowe J NATIONAL INSTITUTE OF STANDARDS & TECHNOLOGY (FORMERL
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
NUDOCS 9309280351
Download: ML20057C422 (2)

Text

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September 22, 1993 U.S. Nuclear Regulatory Commission l Attention: Document Control Desk Washington, D.C. 20555

Subject:

Unlatched. Element, Docket No. 50-184 j on September 'o, 1993, following refueling the day before, a routine retatr startup was made. After required. stops at'O.1 ,

and 1 MW, reactor power was increased to the next level of 10 MW when the operator noticed fluctuations on the power channels.

Confirming that the fluctuations were real and on all three channels, he immediately reduced power to 1 MW and when the fluctuations continued, shut down the reactor. A check of the entire core revealed that element 574'in position J-7 was >

unlatched. Refueling consists of removing four. elements from the ,

center of the core, moving the remaining -26 elements sequentially towards the center and loading four.new elements on the outside.

'i Element 574 had been in the reactor for one cycle only. Proper .l latching is checked in two ways; the refueling tool is designed i so that it'cannot be removed unless the element is-latched. - This ,

is followed by a flow test of each element. . Starting primary  ;

flow would raise an' unlatched element-above the grid.- ' This is .

detected by lowering the tool to touch the element head and ,

checking the height. In the past, this method has detected any ii unlatched element. Immediately following refueling', all checks-were normal and confirmed by two operators. Following'the .

reactor shut down on September 10, element 574 (which was found to be unlatched) was latched and unlatched in position J-7 several times and functioned normally. It was then transferred to the storage pool for examination. The element was intact but appeared more discolored than normal. There was no release of activity of any kind. It should be noted that all fueling ~

operations are carried out remotely '

A careful analysis of the cause of the unlatching indicated three possibilities:

1. The element was never latched'and was not-detected by flow tests.

2. The-element was only partially latched and.became unlatched, probably as a result of flow.

3. The element became partially unlatched.as a result of flow tests and then unlatched.  :

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The latter is considered most unlikely since the orientation of the t6ol when lowered only rests en the element head and cannot latch-on to it or rotate it. Nevertheless, provisions are being I taken to guard against all three possibilities. 'i l

The decision was made not to reuse the element until a more '

thorough inspection could be made. Other immediate actions taken were to double check all elements and to double check the replacement element. The replacement element was initially' flow tested, after which primary flow was continued for about two hours. The flow was stopped for about 30 minutes, then restarted 1 and the element checked again. Nuclear instrumentation was also l carefully monitored once flow was started to possibly detect any i fuel movement. All checks were normal. A programmed reactor l restart was instituted with an additional stop at 5 MW and j operation at 10 MW for two days ~ prior to increasing power. The'  ;

reactor was restarted on September 16, 1993 and everything was ,

routine and smooth. j Future plans include the following additional measures:  ;

1. Special instructions to operators on fuel handling, i monitoring, and testing and response to instrument i fluctuations.  ;

2. Examination by borescope of the area in position J-7 1 during the next refueling. .

3. Examination of all positions in the upper grid next i year during a major shutdown when the entire' core is  ;

removed. i

4. Construction of a mock up test rig to allow tests of (

various hypotheses, and to facilitate training of new i operators. i once the results of these measures are evaluated, additional steps as necessary will be instituted.

. Si cerely, ,

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/ [/ Michael Rowe i

('- '/J. Chief, Reactor Radiation Division 'l+

i cc: Project Scientist  ;

l U.S. Nuclear Regulatory Commission '

Region 1 475 Allendale Road -

King of Prussia, PA 19406

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Project Manager Non-Power Reactor Directorate U.S. Nuclear Regulatory Commission Washington, D.C. 20555 l

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