Self-Propelled Omni Directional Drive Transporters

Improve NUHOMS type ISFSI Installations

For nuclear plants that are using, or intend to use, the NUHOMS^® system¨ for spent fuel storage, a new self-propelled transporter technology has emerged that dramatically improves the movement of spent fuel into ISFSI facilities.  These new all-directional travel transporters maneuver well in much smaller spaces, allowing the horizontal storage modules (HSM) to be closer together.  The wide alleys that are necessary today for maneuvering trailer-mounted transfer cask skids can be made much narrower in new facilities in order to gain much needed additional storage space. In this case, when the transporter carried cask transfer skid approaches an alley that runs between storage modules, rather than turn into the alley, the transporter can simply stop and rotate its axles 90^o and drive sideways down the alley. 

As illustrated in the above artistic license photo, the alleys need only be slightly wider than the combined overall length of the transfer skid and dry storage canister.  If the transporter is driven straight down the alley, it can be stopped where the centerline of the transporter intersects the centerline of the storage cell. At that point, the operator selects the ROTATE function and the transporter rotates ninety degrees around its center point to align with the HSM cell centerlines.

As indicated by the manager of ISFSI Planning at one major nuclear plant “These new transporters can open an entirely new era for us and for the entire nuclear industry.  Safety and reliability are the primary issues of concern within the existing trailer systems.  We have visited two major industrial manufacturing plants that are using almost identical transporters, and we are entirely convinced that self-propelled transporters are the way to go.  Our only hurdle now is getting though our budget approval system.”

Multiaxle SPMTs (Self-Propelled Modular Transporters) of the type produced by well-known European manufacturers like Scheuerle, Goldhofer, and others have been used in the nuclear industry for years, to move reactor vessel loads often weighing well over a thousand tons.  What is new is this miniaturized IN-PLANT version of that technology.  The designation IP-SPMT refers to self-propelled transporters that have relatively small diameter solid urethane wheels attached to on-center rotation axle assemblies, allow an in-plant transporter to be considerably smaller yet have the same or greater load capacity as their European counterparts.

These new transporters replace the conventional and often problematic trailer and tractor approach.  Overloaded tires along with bent and broken steering linkages from attempting to turn the wheels while the trailer is not moving are an ongoing problem that, until now, had no alternative.  Self-propelled transporters introduce a new era of safety and maneuverability to NUHOMS storage applications that has simply never before been available.

Typically, these omnidirectional-travel, all-wheel drive, IP-SPMT transporters will have twelve or more independent on-center rotation axle assemblies.  Each axle is trunnion mounted to a robust vertical lifting center column. The axles tilt laterally and have two power-driven, wide-faced, urethane wheels. The suspension’s tilt and fluid vertical lift combines to assure fully equalized loading over irregularities and unevenness of operating surfaces.

While there is an increased wheel loading with solid rather than pneumatic tires, asphalt roadways can be resurfaced with a newly formulated asphalt mix that will accommodate the harder tires without creating ruts in the surfaces. For this application oversize tires are used (18" dia. x 12" face width as opposed to the normal 12" dia. x 6” face width).

Each on-center rotation axle has electronically controlled fluid equalizing suspension and provides ten inches of independent vertical lift. The suspension accommodates both vertical lift for self-loading and the suspension’s load sharing compliance across all axle assemblies.  By independently driving each wheel, the omni directional IP-SPMT travel is accomplished by precisely varying the speed and direction of each axle’s wheel motor.

An onboard diesel-driven generator supplies the transporter’s AC 480V, 3Ph electric power.  This on-board power also serves the transfer cask ram’s built-in hydraulic power unit. To enhance operation during fuel loading and during precision alignment at the ISFSI, the engine can be shut down and a shore power extension cord plugged in for near silent operation.

The interface with the transfer cask skid requires modifying the skid base to accommodate the raised engine and control compartment on the transporter. The skid no longer needs X-Y slide mechanism. The precision alignment is now all within the functionality of the transporter - not in the inefficient and often highly problematic slide mechanisms and overstressed steering linkages used now. That, combined with the transporter’s very low deck height of only 24”, provides enough space in the revised skid’s base structure to tuck the telescoping ram’s hydraulic power unit inside.  With the HPU built-in, the cask transfer skid is now fully self-contained, receiving its power from a plug-in connection into the transporter’s power supply.

In addition to the task of transporting casks from the fuel building to the ISFSI, one of the transporter’s primary benefits involves effortless precision alignment.   The transporter’s ability to smoothly align and then make minute directional changes in the X, Y, and Z planes for final alignment brings a degree of operating capability never before available for loads in these weight ranges.  The biggest job doesn’t involve the transporter maneuvering, but the setting up the optical devices that confirm the alignment.   

For instance, assume the transporter is roughly aligned with the centerline of the storage cell as it is moving forward in a “creep speed” mode into final alignment and has lifted to roughly match the height of the cell center as well.  When a check with the optical verification instruments indicates it must move a tiny amount laterally. The operator will select LATERAL travel. To avoid unwanted movement, or “tire squirm,” the axles rotate 90-degrees, the transporter’s computer will only allow two of the twelve axles to power rotate in place at a time. This sequential axle rotation assures that the transporter is locked in place on the prior alignment until all of the axles have been rotated.   The transporter is then released to make the actual adjustment sideways.

With sideways adjustment confirmed, all of the axles repeat the sequence to return to forward travel.   Horizontal pitch to reach exact level travel too, is simply a fingertip control function of the transporter’s independent suspension. The operator has precise control in adjusting tilt fore and aft to reach exact vertical and level alignment. 

Once final alignment is optically confirmed, four threaded “snugging” jacks will be spun down to contact footpads placed under each jack.  The snugging jacks simply assure that the alignment remains solid through the payload transfer.

Adapting the self-loading transporter technology to replace current trailer systems will result in a major array of operational improvements.  The modified cask transfer skid will now be designed with solid external legs that allow the transporter to lower and set the skid legs down on the operating surface. The transporter can lower further and disengage, then drive out from under the skid for use on other heavy haul projects in and around the plant.  The transporter is no longer simply dedicated to only one function.

Though capable of carrying much more, the twelve-axle ISFSI transporters will be derated to 155-ton gross load capacity for asphalt surface operation.  The interface between the transporter and the skid will have two alignment pin locations. The transporter is designed to “center” directly under the center-of-gravity of the combined weight of the transporter, the skid, and the loaded cask.   This will assure that the downloading pressure on every axle is the same.  The equalizing suspension assures that the loading remains unchanging while traveling over irregular surfaces in route to the ISFSI.  Upon reaching the ISFSI area, the transporter will lower the legs of the cask transfer skid to surface contact, then further lower to clear the alignment pins. The transporter will effortlessly reposition itself under a second set of pins that put the leading edge of the transporter back far enough to clear the HSM wall as the casks move into the docking position. 

Additional information and operational videos can be obtained by visiting www.wheelift.com or contact Mel Terry, at mel.terry@wheelift.com.

************