ML20058M832

Discusses Spent Fuel Racks Poison Surveillance Coupon Boraflex Degradation.Visual Exam of Remaining Surveillance Coupons Revealed Similar Situation Existed in All Coupon Samples

ML20058M832
Person / Time
Site:	Millstone
Issue date:	08/07/1990
From:	Mroczka E NORTHEAST NUCLEAR ENERGY CO., NORTHEAST UTILITIES
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
CON-FIN-B-13605 NUDOCS 9008130083
Download: ML20058M832 (2)

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. o.nor.i Ome.. . seio n sir t. semn. connecticut OR ONNECTICUT 06m-oNo August 7, 1990 Docket No. 50-336 B13605 L Re: Boraflex Degradation U.S. Nur, lear Regulatory Commission Attentson: Document Control Desk Washington, DC 20555 Gentlemen:

Millstone Unit No. 2 Spent Fuel Racks Poison Surveillance Coupon Borsflex Dearadation On July 27, 1990, while preparing Millstone Unit No. 2 poison surveillance coupon #5 for routine examination, Northeast Nuclear Energy Company-(NNECO) observed that the boraflex material in the area of the vent hole was missing.

A visual examina. tion of the remaining surveillance coupons revealed a similar situation existed in all the coupon samples. This was identified _to the NRC Resident inspector on July 30, 1990.

NNECO's inittal assesment was that the deterioratiw mcchanisms were probably '

due to a comoination of radiation exposure and eros son induced by flow and gas 1 eneration exiting at the vent hole. Additionally, :he erosion was probably imited to only the . vent location.

was delivered to Combustio1 Engineering on July 30,  !

The 1990subject coupon for a more (#5)d examination, specifically the removal of the stain-detaile less steel shell encasements, so as to permit inspection of the entire boraflex sample.

- This issue was the subject of a conference call with the NRC Staff on August 1, 1990 in which NNECO explained the ci.cumstances and provided a  !

preliminary assessment of the deterioration mechanisms. NNECO also pr vided a i short-term action plan that conservatively addressed these observations (i.e.,

maintaining the scent fuel pool boron concentration greater than 1720 ppm and restriction o' the loading pattern to a checker board configuration). These actions were at the time deemed prudent prior to receiving engineering 4 information from Combustion Engineering.  !

On August 1,1990, Combustion Engineering reported that the boraflex mattrial was missing only in the immediate proximity of the vent hole and the remaining coupon appeared to be relatively intact and undamaged. Further testing and examinations are ongoing. Combustion Engineering's assessment based upon visual inspection of the material in the area under question was that the damage .to the boraflex is due to flow-induced erosion, 90 130083 900807 $

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U.S. Nuclear Regulatory Commission B13605/Page 2 August 7, 1990 On August 3,1990, photographs of the subject boraflex coupons were presented to an independent consultant who has extensive experience in inspections and evaluation of boraflex material. NNEC0 was informed that several utilities have experienced erosion of the t'oraflex coupons in areas that are exposed to flow currents in the rack region of the spent fuel pool. In the consultant's opinion, based upon the photographs, the damage to the coupon was due to flow-induced crosion.

NNECO's determination, based upon the visual inspection of the surveillance coupon and utt,ity experiences, is that the deterioration of tha boraflex at the vent hole location is due to the accelerated radir.tio of the exposed boraflex in the coupon coupled with erosion induced by flow currents in the rack region of the spent fuel pool. Accelerated radiation surveillance has the sample coupons exposed to the most reactive discharged spent fuel on a cycle basis as opposed to long-term surveillance that accounts for fuel age and decay.

NNECO's conclusion is that this deterioration experienced in the surveillance coupon does not affect the calculated K of the spent fuel racks and does not violate the Technical Specificati8tffrequirement of K i .95. The conclusion is based upon the fact that the vent hole in the' kent fuel racks is above the active fuel region and, if the erosion exists at the vent hole location in the racks, it does not affect the current qualification to store spent fuel. Therefore, no restrictions need to be instituted with respect to storage of fuel in the spent fuel racks such as alternate checker board storage patterns or maintaining high soluble boron concentrations.

NNECO intends to continue to monitor the situation and collect additional intelligence on the deterioration mechanisms being experienced to further i Lupport our conclusion. Our efforts include continuation of the coupon surveillance program and visual inspection of the vent holes in a ,

representative sample of the spent fuel racks.

NNECO trusts that the i,rormation in this submittal, most of which was provided to the NRC Staff in a conference call on August 6,1990, is useful. ,

Should you require any additional information, please contact us.

Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY

/

E. J. Myoczka /

Senior Vice President i

cc: T. T. Martin, Region I Administrator G. S. Vissing, NRC Project Manager, Millstone Unit No. 2 ,

P. Habighorst, Resident Inspector, Millstone Unit No. 2 W. J. Raymond, Senior Resident Inspector, Millstone Unit Nos.1, 2, and 3

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