Protecting and transporting nuclear waste is serious business. The idea of carrying spent nuclear fuel across political boundaries in the U.S. was rejected years ago in favor of storing it locally, but safe handling is still an issue. The radioactive material is isolated long-term in concrete casks, which are stored at independent spent fuel storage installation (ISFSI) sites within the facilities. Moving the spent fuel from the reactor into this storage is an involved process, requiring precise, deliberate motion—with many safety measures taken along the way. The process needs to be accomplished per strict guidelines supplied by the Nuclear Regulatory Commission (NRC).
First, the spent fuel bundles are placed into a multi-purpose canister (MPC) located in a transfer cask, which is a standard means used to remove the spent fuel from the reactor building. The transfer cask is then lifted and mounted on top of a long-term dry storage cask, which is secured below ground level in a concrete-lined pit at a location called a Cask Transfer Facility (CTF). The MPC containing the spent fuel is then transferred into the long-term storage cask. Multiple casks are used during the process because the transfer cask is easier to move in and out of the reactor building. The transfer cask typically weighs between 100 and 125 tons, whereas the long-term dry storage casks contain large amounts of concrete and typically weigh just less than 200 tons when fully loaded. Ultimately, the long-term dry storage cask is transferred from the CTF to the ISFSI concrete “parking lot,” where the radiation emission levels are monitored and the cask is securely protected into perpetuity.
Transporting the heavy casks requires great care. One crane manufacturer, Konecranes Nuclear Equipment and Service, a division of Konecranes Plc of Hyvinkää, Finland, has developed specialized transport systems for this purpose. The Konecranes Vertical Cask Transporter has 205 tons of lifting capacity with redundant protection against dropping its load. Designed to stringent NRC design standards, the VCT meets all of the same requirements as a single-failure-proof crane inside the reactor building. It uses redundant lifting drums and mechanical interlocks to protect against any single-point failures of mechanical equipment.
Controlling the dual lifting drums, operated by hydraulic motors, is a precise task, since the drums must keep the cask level, and the process requires that the casks be moved very slowly.
“We have set 4 ft/min as our maximum vertical travel rate,” said Joe Yustus, Konecranes assistant chief engineer and technical lead on the project.
Each lifting drum is independently controlled, but the motion of the winches needs to be synchronized to ensure that the casks are lifted evenly. This motion requires the precision of a modern closed-loop motion controller to make sure that the drums are synched together, and also to ensure that one drum can move the full rated system load by itself in the case of a failure of the other drum.
The vertical motion of the lifting system is controlled with a joystick. To operate the hydraulics, Mike Waedekin, hydraulic lead for Konecranes, selected proportional valves capable of metering very small amounts of hydraulic oil when required, and SSI (synchronous serial interface) rotary encoders give motor position feedback. To provide feedback on the position of the lifting towers on which the drums are mounted, SSI string potentiometers were affixed to the tower structure. Tracking the position of the lifting hook requires obtaining information from both the rotary encoders and the string pots.
Selecting a motion controller
For help with hydraulics-related issues, Konecranes consults with representatives of fluid power equipment distributor J.M. Grimstad of Muskego, Wis. Grimstad’s engineer recommended that Konecranes use a motion controller from Delta Computer Systems of Battle Ground, Wash.
“The Delta system allows us very fine-tuned control, down to 0.002 of an inch in positioning the hoist,” said Yustus. “We were also impressed by the Delta controller’s ability to easily interface with Allen-Bradley products, and the fact that when you buy a Delta controller, you don’t need to pay extra for the programming software since it is already included.”
The VCT incorporates an Allen-Bradley PLC, which performs supervisory control functions. The PLC communicates with the Delta motion controller to tell it when to move and when to stop. The PLC monitors the system and makes sure that all the right signals are being provided to the motion controller to meet the conditions for starting up a motion operation.
To program the controller, John Kuderski, electrical lead engineer for Konecranes, worked with Grimstad’s engineer, Ryan Behnke.
“We needed an expert that had already done a number of Delta motion controller applications, and Behnke filled the bill,” said Kuderski.
Four hydraulic motors are used in the cask lifting system—two to operate each drum hoist through planetary gearing. Each pair of motors controlling each drum is controlled as a separate motion axis by the Delta RMC150 motion controller. To ensure that the drums are operated in complete synchrony, the motion controller executes a synchronized move operation, a special function that is a field-proven capability of Delta controllers, to establish control of the four drum motors as slaves under the master control of an operator joystick. If the two drums aren’t at the same location before the lifting operation begins, a transition command in a RMCTools software user program is issued to the appropriate motor(s) to bring the drums to the appropriate position before the synchronized move is initiated. During operation, the maximum speed of raising or lowering a cask was less than 1 in./sec, so if the need to stop moving is discovered and commanded, that can be accomplished with practically zero additional cask motion.
The biggest technical issue that concerned the Konecranes and Grimstad team was making sure that the movement was smooth once the axis synchronization was set up. There were brakes on the system, which, when applied, skewed the axes by a small amount.
“We needed to make sure that the transition was made smoothly each time the motion was restarted,” said Yustus.
“I used Delta’s Plot Manager and autotuning tools to tune the two axes,” said Behnke. “With these tools, which Delta provides free with their motion controllers, we were able to tune the motion to an accuracy of two-thousandths of an inch.”
Motion programs were developed to support different modes of operation for raising and lowering the hooks. The options were set via an HMI touchscreen at the transporter’s operator station.
“The Delta motion controller gives the operator significant position control of the hook, which enables easy alignment with the cask,” said Yustus.
The touchscreen also displays hoist position information and the status of the joystick input, which was obtained from the motion controller. The motion controller, the touchscreen operator interface, another touchscreen status display, and the PLC are all connected through an Ethernet/IP link.
As part of the fault protection process, the motion controller looks at the speed of the drums to make sure that there is no free-fall in process. If this should occur, there is a system of brakes that will stop the motion—activated by the controller’s digital I/O. Should a problem occur, the controller provides diagnostic information that can be used for troubleshooting.
The VCT system has been installed at the Enrico Fermi Nuclear Generating Station, on the shore of Lake Erie, near Monroe, Mich.
“The Konecranes system is a great design,” said Dennis Bergmooser, site manager at the Fermi plant. “It’s the only VCT with crane capability, which speeds up our operations, improving safety and saving costs.”
“The Fermi Station has already seen significant improvement in process times after their first dry cask test campaign,” said Yustus.
Typically, the first time a nuclear facility moves spent fuel from pool to ISFSI, it takes about 14 days. The Fermi site completed this in just 7 days, which resulted in an impressive reduction in cumulative radiation exposure to site personal. This low level of radiation exposure was in line with some of the safest waste transfer programs in the nation.
Delta Computer Systems
deltamotion.com
Konecranes
konecranesusa.com
Filed Under: Fluid Power World Magazine Articles
