The Remote Recovery and Handling of a Cesium Source

Michael Prather, Robert Fogle, and Casandra Robinson
Westinghouse Savannah River Company
Aiken, SC 29808

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The Savannah River Site (SRS), a Department of Energy (DOE) facility operated under contract by the Westinghouse Savannah River Company, was an isotope production facility that produced nuclear materials for defense and peacetime applications. Since the end of the Cold War, SRS has been involved in stabilizing its nuclear materials and remediating the environment. The site’s Health Physics Technology group is responsible for calibrating the radiological detection instrumentation used on site. Their Low Scatter Irradiator (LSI) room is used to calibrate some of these instruments. The LSI is a vendor designed and installed system which uses pneumatics to transfer radioactive calibration sources from a shielded storage carousel to a calibration receiver. This is accomplished through a transport tube located within the LSI calibration room. It was thought that the maturity of the technology used to transfer the sources was such that they could not become lodged. However, during a routine calibration, a 100-curie cesium source became lodged in the transport tube at the calibration receiver. Because of the extremely high radiation rate in the room, a remote controlled vehicle, custom designed tooling, and multiple cameras were required to inspect, remove, and relocate the source from the transport tube to a shielded cask.

The calibration receiver consists of a 1-1/2-inch diameter tube terminated with a screw cap. A set screw was used to prevent the cap from unintentionally rotating off. The initial phase of the recovery process required that a visual inspection of the receiver tube be performed to determine the orientation of the set screw since it was out-of-view of the room’s cameras. An OAO remote controlled vehicle, equipped with a manipulator arm and parallel jaw gripper was selected to perform the initial inspection. The inspection revealed that the set screw was in an advantageous position for removal.

Next, a mockup of the calibration receiver was fabricated and custom tooling was designed to remove the set screw, unscrew the cap, and recover the source. The set screw removal tool was basically a hex head ball driver coupled to a small electric motor. The cap removal tool was an aluminum cup, filled with a two-part adhesive, and driven by an air motor. The recovery tool was a nylon sock attached to an aluminum 2-inch diameter hose coupling. Upon recovery, the source was relocated to a shielded cask.

To determine why the source became lodged, a visual inspection of the source was performed. A source retrieval tool was developed to remove the source from the cask. The inspection revealed a missing set screw. It is believed that the set screw became trapped between the source and transfer tube causing the source to lodge in the calibration receiver. This paper will discuss the successful recovery of a highly radioactive cesium source using a remote controlled vehicle and custom designed tooling.

The Health Physics Technology Group is responsible for the calibration of much of the portable radiological detection equipment used daily in support of site radiological work. Normal protocol requires that all equipment be calibrated routinely to ensure accurate count rate measurements. Methods of calibration vary, but one method used by Health Physics personnel, utilizes a radiological source of known radioactive intensity for the calibration of equipment. During this process, the source and the radiological detection equipment to be calibrated are merged together at a predetermined distance where the calibration process is completed.

In the past, the source was stored in a shielded source holder located below ground level for safe storage and then hoisted to a calibration position above ground level during the calibration process. A winching mechanism was used to lift the source from the storage position to an awaiting instrument staged at the calibration position. Once the calibration procedure was completed, the source was returned to the storage position, a new instrument was staged, and the process repeated. This system proved to be adequate, but after many mechanical problems with the hoisting mechanism, a more dependable and automated system was purchased and installed.

The Low Scatter Irradiator (LSI) Model N40, as shown in Figure 1-1, is a vendor designed and installed system for irradiating radiation detection equipment with gamma radiation or neutron radiation depending on the source used for calibration. (The Low Scatter Irradiator Model N40 is a product of Hopewell Designs Inc., Alpharetta, GA. 30201) The LSI has been a very dependable system and has worked successfully for several years without incident. It uses a pneumatic transport system for moving the source from a shielded storage position to the calibration position. Four linear positioning tracks located on ground level move the radiological instruments into place for completion of the calibration process.

Figure 1-1 Low Scatter Irradiator (Model N40)

The source is encapsulated in a welded stainless steel container and then placed within a source holder. The source holder is made of aluminum approximately 2-1/2 inches long and 1-inch in diameter and is externally ringed near each end for maintaining a seal with the transport tubing, as shown in Figure 1-2. The source holder has a threaded removable cap secured with set screws located on two sides of the cap.

Figure 1-2 Source Holder

The source is stored approximately 10 ft. underground in a shielded storage receiver and is moved from the storage receiver to the calibration receiver by air pressure through a 1-1/2-inch aluminum transport tube approximately 30 ft. long. The top portion of the transport tube acts as the calibration receiver and is terminated with a threaded tube cap secured by a single set screw. A vacuum cup located on the bottom of the tube cap holds the source holder in position during the calibration process. Once completed, the source is returned to the storage receiver. Pneumatic transport system features include: 1) short transport time, which improves counting time accuracy; 2) inherent safety (if power is lost, the source drops to the storage receiver); and 3) simple design, which increases reliability.

The LSI is a semi-automated system, allowing the operator to control the operation from a control room located outside of the LSI calibration room. In normal operation, the operator sets up the radiological equipment on the linear positioning track tables, the LSI calibration room is secured and the operator initiates the system for calibration. High gamma rates do not allow personnel access while the calibration process is in progress.

During a routine calibration task, a 100-curie Cesium 137 source became lodged in the transport tube. Due to high personnel exposure rates of 450-500R/hr @ 30 cm, all work was suspended with no admittance into the LSI room until the source could be returned to a safe position. With the system down, work scheduling was impacted daily and work backlog was steadily increasing.

The selection of the LSI system was based on its simple and problem free design. The design premise of the source transfer to and from the "home" position was to allow the source to move freely without the possibility of becoming lodged in the transport tube. With this, it was desirable for the vendor and WSRC to determine why the source had become lodged and what improvements could be made to prevent similar problems in the future.

At this point, several options were considered for either dislodging or retrieving the source while maintaining personnel dose rates ALARA (As Low As Reasonably Achievable). Manipulations to the transport tube would have been very difficult due to the shielding requirements needed to reduce personnel dose. The LSI intentionally uses aluminum floor grating for supporting low scatter design for neutron measurements. Load limitations of the aluminum grating would not allow the amount of shielding needed to significantly reduce dose. Given these constraints and the hazardous environment, remote recovery and handling of the cesium source was the first option of choice. This paper will discuss the use of a remote controlled vehicle, custom designed tooling, and multiple cameras for the inspection, removal and relocation of the cesium source from the LSI transport tube to a shielded cask.

A team was quickly gathered to discuss and plan the work evolution and possible solutions needed to overcome the complexity and technical challenges of recovering and handling the source remotely, as well as, meeting site safety and ALARA requirements and aggressive schedule demands. In summary, the recovery of the source would require that a remote device enter the LSI calibration room, remove the set screw that secures the tube cap to the transport tube, remove the threaded tube cap, recover the source and then move the source to a shielded cask.

Initially, removal of the set screw securing the tube cap was crucial for the planning of all subsequent tasks associated with the recovery effort. An inspection was needed for determining the accessibility of the set screw given the structural layout of the LSI system and components. Within one day of the initial request, an OAO remote controlled, battery powered, vehicle was selected and deployed to perform the inspection. (The OAO Remote Vehicle is a product of OAO Corporation Robotics Division, Maryland 21754) The OAO is a remote vehicle equipped with a manipulator arm, parallel jaw gripper and two video cameras used for various remote applications. The vehicle uses skid steering for navigation and is controlled by either radio or tether. The tracks are steel with rubber feet attached for positive traction on different types of terrain. There was some concern of how the tracks would respond on the aluminum-grating floor, but initial testing proved that the vehicle could move across the floor without difficulty.

The inspection revealed that the set screw was in an accessible position for removal, but custom tooling and additional cameras would be needed to supplement the various functions of the remote controlled vehicle for performing the tasks of set screw removal, tube cap removal, source recovery, and source relocation.

The "Set Screw Removal Tool", as shown in Figure 2-1, is comprised of a hex head ball driver coupled to a small electric motor housed within a piece of aluminum tubing. The ball driver shank is coupled to the motor with a helical beam coupling that also provided the compliance needed as the set screw was removed.

Figure 2-1 Set Screw Removal Tool

A second tool was developed for removing the transport tube cap. The "Cap Removal Tool" is a tapered aluminum cup filled with a two-part adhesive, and driven by a pneumatic motor for unscrewing the cap. A pneumatic slide provides the lateral movement and compliance needed for removing the cap. See Figure 2-2.

Figure 2-2 Cap Removal Tool

The "Sock Recovery Tool" is a nylon sock attached to a 2-inch aluminum hose coupling, as shown in Figure 2-3. The tool was designed to be grasped by the parallel jaw gripper, then positioned over the end of the transport tube for retrieving the source as it is driven out the end of the transport tube by a second dummy source. Once recovered, the "Sock Recovery Tool" would be lowered into the shielded cask for storage.

Figure 2-3 Sock Recovery Tool and Shielded Cask

All tooling was developed, fabricated, tested and installed on the OAO remote vehicle for remote deployment. See Figure 2-4. In addition, a mock up of the transport tube and cap assembly was fabricated for pre-job training and final testing of all equipment. Additional cameras were added to the vehicle for providing critical camera views needed to accomplish the many remote tasks. A control station was developed for control of the vehicle and custom tooling, as well as, a video console for selection of the various camera views.

Figure 2-4 OAO Remote Vehicle with Tooling, Cameras and Lights

Within two weeks from the initial request, the remote vehicle was deployed to recover the source. The first step was to remove the #8-32 set screw. Positioning the vehicle for the precision needed to remove the set screw was difficult. Feelers and lights were added to the "Set Screw Removal Tool" housing for providing feedback for the depth perception needed to do this remote operation. The "Set Screw Removal Tool" successfully removed the set screw without incident. Next, the manipulator arm was repositioned for removal of the tube cap. The linear slide was activated for positioning the "Cap Removal Tool" and the pneumatic motor energized for unscrewing the cap. The cap was successfully removed and a brief inspection of the cap vacuum cup was done with a camera mounted on the vehicle.

Next, an inspection of the inside of the transport tube was done to confirm the presence of the source and to obtain any information that might be helpful in determining why the source became lodged. The source was located near the top of the tube as suspected, but there was no apparent evidence of what could have caused the source to lodge within the transport tube. At this point, the vehicle was returned to the control room and the "Sock Recovery Tool" was installed for the recovery of the source. A string was attached to the tool for future recovery of the source from the shielded cask.

Once the tooling was installed and the applicable cameras adjusted, the vehicle was returned to the LSI room for positioning of the recovery tool. The "Sock Recovery Tool" was inverted and carefully positioned over the end of the transport tube. The LSI pneumatic system was used to drive a second dummy source holder up the transport tube thus driving the lodged source into the "Sock Recovery Tool". After the two unsuccessful attempts, the third attempt drove the cesium source into the sock successfully. The sock was moved off of the transport tube and inverted for transfer to the shielded cask. Once at the cask, the sock was lowered into the cask while operators covered the cask opening with lead brick shielding. At this point, the source was safely stored and admittance into the LSI room was approved.

To further investigate the root cause of the lodged source, the customer requested a visual inspection of the source. A "Source Retrieval Tool" was fabricated for recovering the source from the shielded cask. The tool was grasped by the parallel grippers and then rotated for winding up the string attached to the "Sock Recovery Tool". Once the source was recovered from the shielded cask, the sock was transferred to a staging area for a remote camera inspection, as shown in Figure 2-5. The vehicle manipulator grippers assisted in holding the source for careful examination. The source was placed back into the sock and returned to the shielded cask until all information could be reviewed and a path forward determined.

As the sock was inverted and dumped onto the staging platform, a dummy source holder, a cesium source holder and a set screw were found on the platform. The inspection revealed a missing set screw on the cesium source holder. It is believed that over repetitive use, the set screw eventually loosened, fell out and lodged itself between the source holder and the inner diameter of the transport tube.

Figure 2-5 Remote Camera Inspection of Source Holder

After careful review, the customer determined that the vendor would do all repair work to the source holders to prevent future problems. The final step was to return the source back to the LSI transport system using the remote vehicle. The vehicle was deployed to recover the source from the shielded cask and return it to a transport tube staging platform. Once in position, the sock was inverted and the source was dumped onto the platform. The vehicle gripper was used to grasp the source and place it into the transport tube. Free from obstruction, the source returned to the underground shielded storage receiver and the LSI room was now safe for entry and future vendor repair.

In conclusion, the source was recovered and safely stored within the shielded cask and later returned to the LSI transport system without incident. Robotics coupled with custom tooling provided a method for completing the work within a very hazardous environment. Thorough planning and innovative solutions, provided the tools necessary for overcoming the many obstacles encountered during the evolution of the program. Aggressive schedule demands were met without compromising the safety and success of the many tasks needed to complete the job. ALARA goals were achieved by maintaining personnel exposure to less than 92mRem whole body dose for the entire job. The success of the remote recovery of the cesium source further validates the feasibility of robotic handling of hazardous materials.

Hopewell Designs, Inc.
Alpharetta, GA. 30201
www.hopewelldesigns.com

OAO Corporation/Robotics Division
9639 Doctor Perry Road Suite 209 S
Ijamsville, Maryland 21754
Oaorobot@erols.com

The author would like to thank the team members, the Health Physics Technology group and the Hopewell Design, Inc. for all the help and dedication to complete a very successful program.