ML20214P979

Requests Approval to Reinstall Core Boring Machine Over Reactor Vessel to Perform Addl Drilling Operations to Facilitate Defueling.B&W TMI-2 In-Core Nozzle Evaluation Encl.Fee Paid

ML20214P979
Person / Time
Site:	Three Mile Island
Issue date:	09/19/1986
From:	Standerfer F GENERAL PUBLIC UTILITIES CORP.
To:	Travers W Office of Nuclear Reactor Regulation
Shared Package
ML20214P980	List: ML20214P979 ML20214P989
References
4410-86-L-0162, 4410-86-L-162, NUDOCS 8609240125
Download: ML20214P979 (4)

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GPU Nuclear Corporation Nuclear o

=eme:r8o s Middletown, Pennsylvania 17057 0191 717 944 7621 TELEX 84 2386 Writer's Direct Dial Number:

(717) 948-8461 4410-86-L-0162 Document ID 0107P September 19, 1986 TMI-2 Cleanup Project Directorate Attn: Dr. W. D. Travers Director US Nuclear Regulatory Commission c/o Three Mile Island Nuclear Station Middletown, PA 17057

Dear Dr. Travers:

Three Mile Island Nuclear Station, Unit 2 (TMI-2)

Operating License No. DPR-73 Docket No. 50-320 Core Bore Operations The purpose of this letter is to request NRC approval of the GPU Nuclear proposal to reinstall the core boring machine over the Reactor Vessel (RV) to perform additional drilling operations into the core region to facilitate defueling. The drilling operations will be performed on the corium monolith in the core region above the lower Core Support Assembly (CSA). The drill will be administratively controlled by procedure to avoid the lower end l

fittings (for protection of the drill bit) and physically controlled by drill l

string length to prevent the drill from encountering the lower flow l distributor. In general, safety considerations associated with this proposed activity are bounded by the analysis presented in Reference 1. However, in order for this operation to be effective, it is necessary to be able to drill into the corium monolith at any location within the available drilling diameter of the cote boring machine without being restricted by incore

' instrument locations. Previous discussions on this subject raised concerns that the core boring drill could impart downward forces onto an incore instrument without cutting it. The incore instrument could then, after buckling, impart a horizontal force to the top of the incore nozzle. This I

horizontal force could cause a bending moment to develop at the incore nozzle I weld. It was postulated that this moment would exceed the recommended moment limit of 1400 in-lb de mloped in Reference 2.

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ADOCK 05000320 PDR ',) DOT ()[/ g gJ,fN7 GPU Nuclear Corporation is a subsidiary of the General Public Utilities Corporation

Dr. Travers September 19, 1986 4410-86-L-0162 Recent examinations of the TMI-2 core and support structure afforded by the Lower Head Core Stratification (Core Bore) Program have provided further insight into the condition of the lower CSA, the incore instrument nozzles and the RV lower head. In general, little damage was evident. Based on these observations, GPU Nuclear requested B&W to review the RV lower head structural

integrity analysis presented in Reference 2. Based on a review of the video coverage of the Core Bore Program, along with environmental and metallurgical-considerations, B&W reassessed the structural integrity of the RV lower head, particularly the Inconel 600 weld between the incore nozzle and the RV lower 1 head. These new findings are addressed in the attached B&W report entitled, "TMI-2 Incore Nozzle Evaluation," dated September 15, 1986 (Reference 3).

Based on the evaluations presented in Reference 3, it is reasonable to conclude that the Inconel 600 welds are not degraded and have maintained their original metallurgical properties. The structural analysis demonstrates that the load carrying capabilities of an Inconel 600 weld with its original metallurgical properties would be, after applying a safety factor of two (2):

i 113,500 pounds in tension, 51,500 in-lbs in bending and 48,500 in-lbs in

torsion. Using these allowable load limits, it can be shown that any axial or horizontal load imparted by an incore assembly cannot damage the incore nozzle 4 weld.

i Tests performed have shown that an incore instrument string will fail in

! tension and physically separate when pulled axially at approximately 4000 pounds. This represents the ultimate strength of the incore instrument. The

' yield strength of the incore in compression would be less than this figure;

therefore, the maximum axial load an incore instrument could support cannot

< exceed 4000 pounds. The calculated axial load needed to Duckle an incore

! instrument string in the space between the incore guide tube and incore nozzle

! is approximately 600 to 900 pounds. The unsupported length in this location is approximately 9 inches. As the load on the incore instrument increases

after buckling, the mid-span of the buckled column would come to rest against t the side of the incore instrument guide tube, imparting a horizontal load on ,

l the incore nozzle. By applying a downward force from the Core Boring Machine of 4000 pounds (i.e., the maximum axial load before the incore instrument i fails in compression) the buckled incore instrument would impart a 1900 pound l

horizontal load to the top of the incore nozzle. The allowable bending moment of the B&W report is 51,500 in-lb which corresponds to a 4292 pound horizontal force applied at the top-of the nozzle. In summary, the maximum horizontal l load that can be applied to the top of the nozzle, (1900 lbs), is less than f

the recommended limit (4292 lbs). Therefore, the maximum axial loads which l could be imparted to the incore instrument cannot overload the incore nozzle.

l l

The above evaluation is relevant only if the incore instruments are intact.

I As indicated in Reference 2, it was originally calculated that the nozzle weld area of the incore nozzles reached temperatures of 20550F to 21670F. At this temperature, the remaining nozzle above the weld would have been between 32200F to 40960F. The temperature difference is due to the large heat sink provided by the RV wall. Thus, in order for an Inconel 600 nozzle weld to l

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r Dr. Travers September 19, 1986 4410-86-L-0162 reach a temperature near its melting point (2540oF), the incore nozzle and instrument string would have to have melted. Therefore, a bending moment could not be transmitted through an incore nozzle to a degraded weld. In other words, if the weld is degraded, there is no incore instrument string above the nozzle to which loads can be applied. Similarly, if the incore nozzle is intact, the nozzle weld can accept loads similar to the original design basis.

Based on the above discussions, a failure of an incore nozzle weld due to the core boring of the corium monolith is highly unlikely. However, the capabilities do exist to mitigate the consequences of an incore nozzle weld failure and the resultant leakage. As demonstrated in Reference 4, sufficient capability exists to maintain the TMI-2 core covered with borated water for an indefinite period in the event of a complete loss of an incore instrument nozzle which would result in a 125 gallon per minute leak.

In conclusion, the loads imparted to an incore nozzle weld will remain below those minimum loads calculated to be necessary to cause a failure of an incore nozzle weld that exhibits original metallurgical properties. However, if a weld is degraded, it is expected that the remaining incore nozzle would have melted. In addition, the consequences of the failure of an incore nozzle have been previously evaluated. Therefore, this proposed activity does nnt represent an unreviewed safety question since it does not create consequences or increase the probability of an accident previously evaluated, create the possibility of an accident of a different type than previously evaluated, or reduce the margin of safety as defined in the Technical Specifications. Thus, the proposed activity can be performed without undue risk to the health and safety of the public.

Per the requirements of 10 CFR 170, an application fee of $150.00 is enclosed.

Sincerely, 3 weeJ , o )

F. R. Standerfer

' Vice President / Director, TMI-2 FRS/CJD/eml Attachment Enclosed: GPU Nuclear Corp. Check No. 00025904

4410-86-L-0162 ,

REFERENCES )

1. Core Stratification Sample Acquisition Safety Evaluation Report, Revision

'4, GPU Nuclear letter 4410-86-L-0101 dated June 11, 1986.

2. Evaluation of the Structural Integrity of the TMI-2 Reactor Vessel Lower Head Final Report, June 1985, B&W 77-1154826-00.

3. TMI-2 Incore Nozzle Evaluation, September 15, 1986, B&W 51-1165539-00.

4. Extended Core Stratification Sample Acquisition Activity, GPU Nuclear letter 4410-86-L-0122 dated July 11, 1986.

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