ML20136B735

Forwards Detailed Analysis & Sketch of Refuel Floor Which Displays Features Cited in Text in Response to RAI Re Ieb 96-002, Movement of Dry Storage Casks Over Spent Fuel in Reactor Core or Safety Related Equipment

ML20136B735
Person / Time
Site:	Oyster Creek
Issue date:	02/28/1997
From:	Roche M GENERAL PUBLIC UTILITIES CORP.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
6730-97-2065, IEB-96-002, IEB-96-2, NUDOCS 9703110102
Download: ML20136B735 (5)

Text

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{ GPU Nuclear, Inc.

( U.S. Route #9 South 9

NUCLEAR . Post 0ffice Box 388 l

Forked River, NJ 087310388 Tel 609-9714000 February 28, 1997 l 6730-97-2065 l 1

U.S. Nuclear Regulatory Commission  !

Attention: Document Control Desk Washington D.C. 20555 l

Subject:

Request for AdditionalInformation related to Bulletin 96-02,

" Movement of Dry Storage Casks Over Spent Fuel, Fuel in the Reactor Core, or Safety-Related Equipment" In accordance with the referenced document, GPU Nuclear has considered the potential for dropping or tipping the fuel transfer cask during movement from the spent fuel pool to the area in which activities such as drying, inerting, and final securing of the cask lid are performed. The RAI specified that the analysis should assume the reactor is at power during fuel movement and i GPU Nuclear's analysis makes that assumption.

In evaluating the potential cask drop, GPU Nuclear considered the design of the Oyster Creek Reactor Building crane, the load path used in moving a fuel transfer cask from the Cask Drop Protection System (CDPS) to the area in which the dry shielded canister (DSC) is inerted and welded closed, and the loading and unloading processes. As a result of this review, GPU Nuclear concludes that a cask drop accident in that segment of the load path is not a credible event at  :

Oyster Creek. Attachment 1 to this letter contains our detailed analysis. Attachment 2 is a sketch of the refuel floor which displays features cited in the text. If you have any questions on the analysis please contact Mr. Michael Laggart at (201) 316-7968.

Very truly yours, D f Michael B. Roche I

)

Vice President & Director I '

cc: Administrator, Region 1 NRC Project Manager '

NRC Resident Inspector 9703110102 970228 PDR ADOCK 05000219 ,

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l Attachment 1

. Loading Process

' The loading of the spent fuel assemblies into the dry shielded canister (DSC) takes place

! within the Cask Drop Protection System (CDPS). ARer the DSC is loaded with spent fuel in accordance with the applicable procedure and verified, the lifting yoke is attached to the crane. The DSC shield plug is attached to the yoke and carried along the east-west 1

centerline of the safe load path to the CDPS. The shield plug is placed on the loaded i DSC, the lifting yoke is attached to the cask and the cask is lifted above the CDPS. The i cask will be lifted no more than 6 inches above the top plate of the CDPS in accordance j- with Technical Specification 5.3.1 C.

1 i

The CDPS is permanen.1 alled in the northeast corner of the Oyster Creek spent fuel l pool. The CDPS is a pas.sive system which mitigates the effect of a cask drop on the spent fuel pool structure. (A complete description of the CDPS is contained in Section 9.1 of

. the Oyster Creek FSAR.) The CDPS includes the following features:

' a. A guide structure which properly guides and restrains a falling cask in the event it is dropped into the spent fuel pool.

i 4 b. A hydraulic dashpot in the lower section of the guide structure which j retards the falling cask such that impact loads are kept below acceptable i

values.

l In the event of a cask drop, the CDPS has been designed to slow the rate of fall by i hydraulic pressure. The system is designed to attenuate the forces generated by the l displacement of water and the impact of the cask against the guide tube walls. To achieve l the hydraulic attenuation effect, a base plate is attached to the bottom of the cask. The

) purpose of the base plate is to act as a piston within the dashpot.

A number of postulated cask drop scenarios have been analyzed and are included in the '

Oyster Creek Updated Final Safety Analysis Report. The following types of potential cask drops have been considered: drops onto the top plate of the guide structure, drops over or within the guide cylinder, and impacts with the floor of the dashpot.

Cask drops on the top plate of the guide structure including straight drops such as those caused by a break in the vertical cable of the crane, and eccentric drops such as when a lifting trunion or the lifting yoke fails on one side and then the other, were postulated. In addition, tipping of the cask caused by the cask base plate catching the edge of the top plate and tipping onto the north or east wall of the spent fuel pool was considered.

Furthermore, a potential scenario in which the base plate catches the edge of the top plate and allows the cask to fall into the guide structure was also postulated.

Seve'ral types of cask drops over or within the guide cylinder were evaluated. These include a straight fall within the cylinder, a straight fall with the cask tipped at the maximum angle ofinclination and eccentric drops resulting in the cask impacting the side of the guide cylinder during its descent.

The impact of a falling cask on the dashpot floor was considered both with the cask centerline vertical and at the maximum angle ofinclination permitted by the guide structure.

The NRC evaluation of the CDPS, in a March 1977 SER, concluded that it is adequate for the prevention of cask tip accidents and that the dashpot structure and the fuel pool stmeture are adequate for the loads imposed during postulated cask tip accidents. GPU - .

Nuclear believes that the occurrence of a scenario in which a fuel transfer cask is dropped or tipped while in or over the CDPS is a very low probability event. In the unlikely event of such an occurrence, however, the features of the CDPS are sufficient to mitigate the effects.

l Horizontal Movement i 1

A series of modifications were recently made to the reactor building crane to enhance its  ;

safety and reliability. Among the enhancements was the installation of variable frequency '

drive (VFD) controllers which provide very smooth and precise speed control along with ,

torque limitation. In addition, a remote control system was added which permits  !

l operation of the crane from the refuel floor. These enhancements, along with procedural controls limiting the speed ofload movement, will permit the operator to stop the load smoothly and quickly if a problem were to occur.

To preclude the accidental drop of a fuel transfer cask during horizontal movement on the refueling floor (reactor building elevation 119') engineered systems have been implemented including the installation and use of a redundant support system. This redundant support system is referred to as the Fixed-Link Support System (FLSS) and is designed for use with the NUHOMS@ fuel transfer cask. The FLSS consists of two vertical support arms that are attached to and hang down from the main reactor building crane trolley. During fuel transfer operations, the FLSS is attached to the yoke of the transfer cask. In the unlikely event of a crane system failure (such as a failure of the hoist rope), the FLSS system will completely support the cask. The FLSS is designed to provide this protection to the transfer cask and its contents, up to a maximum design load

. of 100 tons. The FLSS was load tested to 200% ofits design load in the shop and 125%

cfits design load after installation in the reactor building. Therefore, GPU Nuclear ,

believes that a cask drop during lateral movement while the transfer cask is attached to the FLSS is not a credible event.

1

' Dralitina. Inerting and Sealin.g The FLSS is not designed for, and therefore is not engaged during, vertical movement of

, the fuel transfer cask. At only two points during the cask move .1ent from the CDPS to the area in which the cask tid is secured, is the transfer cask disconnected from the FLSS.

In the first instance, the cask is over the CDPS and the analyses cited above are bounding.

In the second instance, the FLSS is disconnected in preparation for draining,'inerting and permanent sealing of the lid. During that activity, which takes place in the north east corner of the refuel floor, an energy absorbing crush pad is placed directly beneath the fuel transfer cask prior to disconnecting the FLSS.

i Although the fuel transfer cask will only be disconnected from the FLSS for a short duration, a crush pad fabricated from an aluminum honeycomb material was developed to 4 ensure protection against a cask drop event. If a cask drop event were to occur, the crush

- pad is designed to absorb the energy of the cask by plastic deformation of the honeycomb.

The crush pad is also designed to maintain the stability of the cask during a drop event.

l The reaction loads on the floor will be controlled (based on design calculations and full-

scale materials tests) and held below the load capacity of the floor. The crush pad is designed to provide protection against a cask drop for a 100 ton cask, dropped from a height of up to three (3) inches, with a safety factor of two The FLSS is designed such

that the bottom of the base plate will clear the edge of the crush pad by 0.5 inches. To disconnect the FLSS requires that the cask be lifted approximately 1.0 inches. Therefore, 4

the maximum height of the cask will be approximately 1.5 inches above the crush pad, providing an additional safety factor of two.

Upon completion of preparing the cask for transport, the yoke is attached to the hook, the yoke is then attached to the cask. The cask is lifted approximately 1.5 inches and the FLSS is installed, the cask is then transported south along the safe load path.

?

l Conclusion i

l. In summary, while the fuel transfer cask is in or directly above the CDPS, the features of

! that structure will mitigate the effects of a postulated cask drop. During lateral movement of the fuel transfer cask, the Fixed-Link Support System precludes the dropping of the cask and/or catching on the edge of the cmsh pad. Enhancements to the reactor building ,

crane, along with procedural controls on the speed ofload movement, will ensure precise l control over cask movement. Finally, the crush pad in the north east corner of the refuel l floor will protect the integrity of the floor and stabilize the cask in the unlikely event of a I cask drop in that location. For all these reasons, GPU Nuclear believes that an accident in which a loaded fuel transfer cask is dropped or tipped over prior to the point that the lid is permanently secured is not a credible event at Oyster Creek.

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