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Grappling with nuclear waste.

Grappling with Nuclear Waste

The Waste Isolation Pilot Plant (WIPP) being constructed in the bedded salt formations near Carlsbad, N.M., is intended to demonstrate the safe disposal of transuranic wastes generated by U.S. defense programs. Both contact- and remote-handled transuranic wastes will be handled at WIPP; this article is only concerned with the latter, which will be placed in horizontal boreholes.

Since all deposited transuranic waste must be retrievable during the initial five-year operating period, the boreholes have steel sleeves that withstand salt lithostatic pressure to ease retrieval. If retrieval is not required during this demonstration period, waste deposited later does not have to be retrievable.

The horizontal emplacement and retrieval equipment was designed and fabricated especially for the waste packages. The equipment had to meet many design, operational, and safety requirements. In addition to the structural sturdines and power to support, position, and align massive loads (up to 35 tonnes) with a high degree of precision, it needs a sophisticated control system that permits foolproof operation in only one unique sequence. For the most part, these objectives were achieved using relatively simple, standard power transmission components, such as mechanical screw jacks, shaft rails and associated roller bearings, and hydraulic cylinders.

The surface facilities at the WIPP include a receiving yard, where remote-handled waste packages arrive in shipping casks by truck or rail, and a waste-handling building, where the waste packages are unloaded and transferred into a facility cask. A hoist lowers the cask underground.

The underground facility includes a waste shaft station, where the cask is transferred from the shaft conveyance to an underground transporter, access drifts to the emplacement area, and the emplacement rooms. Each emplacement room is 10 m wide by 4 m high by 91 m long; the most restrictive access drift between the waste shaft station and emplacement area is 6 m wide by 3.5 m high.

Waste packages are placed in horizontal boreholes in the salt pillars. The borehole centerlines are located approximately 1.6 m above the floor. A nominal spacing of 2.4 m between boreholes is used.

When empty, the facility cask (Figure 1) weighs about 30 tonnes. The cask is provided with two shield valves, which are actuated by electric motors, and has a 0.7-m-diameter bore. Pneumatically operated spring-loaded lockpins on the cask lock the valve gates closed to prevent unintentional opening of a shield valve with a waste package in the cask.

Design Requirements

The following are the design requirements for the emplacement and retrieval equipment. The complexity of the equipment was dictated by the required capability of retrieval. Interface. The equipment must: . Interface with a loaded, horizontally oriented facility cask. . Have overall dimensions that permit transportation through the underground access drifts and allow operation in the emplacement rooms. . Fit into the hoist, either as a unit or as dismantled subassemblies, without exceeding its payload limit. Function. The equipment must: . Place a remote-handled waste package--0.65 m diameter, 3.1 m long, 3600 kg, 100 Rem/hr maximum surface dose rate--and a shield plug into a horizontal borehole. . Retrieve the shild plug and waste package from sleeved boreholes. All waste packages have a standard lifting pintle design compatible with the surface facility handling equipment. . Include a steel sleeve designed to withstand a maximum salt lithostatic pressure of 15 MPa. . Limit the equipment surface dose rate during emplacement and retrieval to a maximum of 100 mRem/hr. After emplacement of the waste package and shield plug, the maximum surface dose rate in the emplacement room is limited to 5 mRem/hr. . Operate underground in a ventilated area in the presence of salt dust and humidity; the nominal ambient temperature is 30 degrees C, normal relative humidity less than 60 percent. . Allow emplacement of two remote-handled waste packages in an eight-hour shift.

The phenomenon of creep is particularly significant in salt. Creep data recorded at WIPP during the mining of the access drifts and experimental rooms show the salt walls close several centimeters within months of excavation. The creep rate is accelerated by the thermal output of the waste packages. Thus, the horizontal boreholes in which waste packages are emplaced could, over time, close in around the package, rendering retrieval difficult, possibly even requiring the use of salt overcoring equipment. Therefore, the steel sleeves used during the demonstration phase were designed to withstand the maximum anticipated salt lithostatic pressure.

The use of sleeves in the boreholes assures that sufficient clearance will be available around the waste package and shield plug for retrieval. The salt lithostatic pressures on the outside of the sleeve could still shift the sleeve and cause it to tilt from its initial horizontal position. The equipment, therefore, must locate the new orientation of the sleeve and align itself with the sleeve axis. The approach that was selected used an alignment fixture that, when installed on the sleeve, provides the planes of reference and a means for precisely aligning the waste transfer equipment. This approach permitted the development of one set of equipment for both emplacement and retrieval. Transportation and Handling. The transportation of the emplacement and retrieval equipment from the surface to the underground facility using the waste hoist represented another important consideration. The cage space and payload of the hoist are limited, as are the facilities available underground for handling and assembling heavy mechanical equipment. It was therefore essential that the equipment be designed for easy dismantling and reassembly. The waste transfer equipment, the only assembly that requires dismantling, consists of three modules that are completely separated by loosening a few fasteners and electrical connectors.

Transporting the assembled equipment from the underground waste shaft station to the emplacement area presented another design challenge. Low access drifts and right-angled intersections precluded the use of a conventional tractor-and-trailer unit. The equipment was designed to be transported from one emplacement room or location to another with a forklift truck. Operating Envelope. Special consideration was given to minimizing the equipment's operating envelope in the emplacement room. The limited dimensions of the room and the relatively large height of the cask required a design that was structurally adequate for carrying the loadings while maintaining a low profile for installing the cask. The use of standard power transmission components contributed to minimizing the equipment operating envelope while providing the necessary structural strength and motive power. Safety. The safety of operating personnel, prevention of inadvertent radiation exposure, and the safe and correct sequential operation of the equipment were paramount design considerations. The approach was to develop an emplacement and retrieval system that, barring a catastrophic collapse of the mine floor, made it virtually impossible for operating personnel to be inadvertently exposed as a result of postulated equipment failures. A redundant instrumentation and control system, implementing a sophisticated interlock logic integrated with the cask operation, assures that the operations are performed only in the intended sequence.

Special features are provided for recovery from abnormal operating conditions, including manual overrides for all drives and leveling systems, features for manual release of the package grapple, and retraction of the transfer mechanism. Materials. The salt dust that pervades the underground mine is not corrosive in the absence of water. Hence, with appropriate painting, carbon steel was used wherever possible as the structural material.

The Equipment

The sleeve (Figure 3) is a 5.4-m-long carbon steel cylinder with a 0.7-m-diameter bore. Its wall is 8 cm thick, except at the front end, where it is 18 cm thick. This additional thickness is required to limit the dose rate as the waste package is transferred from the facility cask into the borehole or vice versa. The rear end of the sleeve is closed with a steel plate to prevent ingress of salt debris.

A semicircular bracket is welded near the front end of the sleeve. The front face of this bracket is perpendicular to the sleeve axis. The alignment fixture is inserted over the sleeve and bolted to the bracket to obtain the proper orientation. Alignment Fixture. This fixture (Figure 4) provides the planes of reference and a means for aligning the waste transfer machine with respect to the sleeve or the unsleeved borehole.

The fixture used with a sleeved borehole has an L-shaped configuration and is a welded steel structure. The vertical leg has an opening to allow it to pass over the sleeve. The holes provided for bolting the alignment fixture to the sleeve are slotted to permit circumferential adjustment.

The base plate of the fixture has two alignment pins, which are located so that when the waste transfer machine is installed on the fixture, the axes of the machine and the sleeve are in the same plane (though not necessarily parallel). Two wedge-shaped bars, welded to the top of the fixture's base plate, engage with matching V-blocks on the waste transfer machine. This eases alignment and prevents the waste transfer machine from accidentally touching other components on the alignment fixture.

A leveling system consisting of three 18-tonne electric-motor-driven mechanical screw jacks is provided for the alignment and support of the fixture. The screw jacks are provided with special swivel leveler feet to independently bear on a potentially nonuniform, sloping floor. The feet have a large bearing area to distribute the load.

Electronic tilt sensors monitor the transverse and longitudinal tilt angles of the alignment fixture. These sensors, with a tilt sensor mounted on the waste transfer machine, are used to align the equipment. Proximity sensors measure the correct engagement of the facility cask with respect to the sleeve.

When the alignment fixture is used for emplacement operations in unsleeved boreholes, a shield collar is bolted to it. The shield collar performs the same function as the front portion of the sleeve in limiting the surface dose rate. Waste Transfer Machine. This machine (Figure 2) supports the facility cask, aligns it with respect to the borehole, positions it against the sleeve or shield collar, and emplaces and retrieves the waste package and shield plug.

The leveling platform, a structural steel frame, has a stepped down front end, which reduces the overall height of the waste transfer machine. Two alignment arms welded near the front end have the bushings and V-blocks that engage with the pins and wedges on the alignment fixture. An electric-motor-driven 32-tonne mechanical screw jack is provided near the rear end of the leveling platform. Once the waste transfer machine is installed on the alignment fixture and the facility cask is set on it, the only operation required to align the axes of the cask and sleeve or unsleeved borehole is to lower or raise the screw jack.

The staging platform supports and positions the facility cask and transfer carriage. A system of shaft rails and roller bearings permits movement of the staging platform relative to the leveling platform for placing the cask against the sleeve or shield collar. The mechanical drive system for this is mounted on the leveling platform; it consists of a 4.5-tonne electric-motor-driven screw jack and traveling nut arrangement. The top of the staging platform has guide blocks for positioning the facility cask. The staging platform also supports the mechanical drive system that moves the transfer carriage forward to bear against the facility cask or retracts it for installing the shield plug carriage. A regulated air-supply system on the staging platform provides compressed air for the operation of the facility cask lockpins.

The transfer carriage houses a hydraulically actuated transfer mechanism and provides shielding at the rear end of the cask to limit surface dose rates when the cask shield valves are open. The transfer carriage also houses a completely self-contained hydraulic system for operating the transfer mechanism. A system of shaft rails and roller bearings allows movement of the transfer carriage with respect to the staging platform. The mechanical drive system consists of a 9-tonne electric-motor-driven screw jack and traveling nut arrangement. Proximity sensors on the transfer carriage measure the correct engagement of the carriage with the facility cask.

The transfer mechanism, with the grapple, is used to transfer the waste package from the facility cask into the borehole or vice versa. The mechanism provides the same functions for shield plug emplacement and retrieval. It consists of a five-stage double-acting hydraulic cylinder. The front face of the cylinder bolts to a support plate, which has a system of rollers that support the weight of the transfer mechanism. A reel-type potentiometer measures the position of the transfer mechanism during its travel.

If the hydraulic system malfunctions, the transfer mechanism can be manually retracted from a partially or fully extended position by pulling on two wire ropes attached to the support plate at one end and to a pair of torque reels mounted on the outside of the transfer carriage (Figure 5). The waste package is released from the grapple prior to performing this operation.

The grapple (Figure 6) consists of a pair of traveling nuts on an electric-motor-driven 9-tonne mechanical screw jack provided with left-and right-hand screw threads. Extensions to the traveling nuts act as jaws for the grapple. A pintle detection switch on the grapple detects and stops the transfer mechanism prior to closing the grapple jaws around a waste package or shield plug pintle. The grapple design includes a manual release mechanism (Figures 5 and 6). The back plate of the grapple housing is bolted to the hydraulic cylinder support plate, with a controlled clearance between them. Slots are provided on the back plate so the entire grapple assembly can slide down far enough for the jaws to clear the pintle when a spring-loaded pin holding the grapple to the support plate is pulled out. The pin is secured to a wire rope connected to a torque reel mounted on the outside of the transfer carriage. Shield Plug. The shield plug (Figure 7) limits the borehole surface dose rate to less than 5 mRem/hr after emplacement is complete. It is provided with the same pintle design as the remote-handled waste package. The shield plugs for the WIPP demonstration phase are of all-steel construction; however, the plugs for later emplacements will be concrete with a steel pintle. Shield Plug Carriage. A carriage (Figure 7) supports the shield plug in a horizontal position during emplacement and retrieval operations and aligns the shield plug with the facility cask bore. The shield plug carriage is a welded steel cradle provided with roller bearings that ride on the staging platform shaft rails.

Operating Sequence

The operating sequence for the emplacement of a waste package and shield plug in a sleeved borehole is shown in Figure 8. The same sequence is used for emplacements in unsleeved boreholes, except that the alignment fixture is first installed and visually aligned with the borehole. The reverse sequence is followed for retrieval.

PHOTO : Figure 1. Facility cask.

PHOTO : Figure 2. Horizontal emplacement and retrieval equipment.

PHOTO : Figure 3. Sleeve.

PHOTO : Figure 4. Alignment fixture.

PHOTO : Figure 5. Transfer mechanism retract and grapple release.

PHOTO : Figure 6. Grapple configuration.

PHOTO : Figure 7. Shield plug carriage and shield plug.

PHOTO : Figure 8. Operating sequence of the horizontal emplacement and retrieval equipment.
COPYRIGHT 1989 American Society of Mechanical Engineers
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Title Annotation:Waste Isolation Pilot Plant remote handling equipment
Author:Nair, B.R.
Publication:Mechanical Engineering-CIME
Date:May 1, 1989
Words:2516
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