Duratek Verifies Safety of Radioactive Waste Container with ALGOR Mechanical Event Simulation

	In order to transport a radiation-contaminated glovebox (like the one shown at top) within the U.S. Department of Energy Hanford Site, Duratek, Inc. designed, analyzed and manufactured an Industrial Packaging Type 2 (IP-2) container (shown at bottom left and right). The container shielded people and the environment from radiation exposure during transport to the onsite Environmental Restoration Disposal Facility landfill where it was buried safely.
	ALGOR Mechanical Event Simulation (MES) results showed the stress distribution as well as the plastic deformation in the finite element model of the IP-2 glovebox container due to being dropped two feet onto one of its upper corners.
	A sliced display of the container allowed viewing of the stresses and displacement on the critical internal sealing surfaces. The area of highest stress on the sealing surfaces was right behind the corner of impact. Duratek determined that even this localized peak stress was insignificant because the sealing surface displacement would not cause a breach of the gasket seal. Thus, the analysis results indicated containment was maintained and the design successfully passed the drop test.
	Jeff Scott, P.E. of Duratek used ALGOR MES software to simulate a drop test of the IP-2 glovebox container in order to demonstrate compliance with Federal regulatory requirements for structural strength.

When a safe container was needed for the disposal of radioactive waste from a facility on the U.S. Department of Energy (DOE) Hanford Site in southeastern Washington state, ALGOR Mechanical Event Simulation (MES) software was chosen to verify the container's structural integrity quickly and economically. As part of decommissioning the inactive nuclear facility, a contaminated apparatus called a glovebox (a steel isolation chamber with built-in gloves that allow personnel to remotely manipulate radioactive materials) needed to be removed and transported to the onsite Environmental Restoration Disposal Facility (ERDF) landfill and then buried safely without exposing people or the environment to harmful radiation.

The responsibility for providing safe removal, transport and disposal of the radioactive glovebox was handled by Duratek, Inc. of Richland, Washington, a member of the DOE-contracted Hanford Site cleanup team and a leading provider of radioactive waste disposal solutions. Duratek designed an Industrial Packaging Type 2 (IP-2) container that had to meet strict regulatory safety standards for shielding, containment and structural strength.

Duratek used ALGOR finite element analysis (FEA) software to verify the IP-2 glovebox container's structural integrity including simulation of a two-foot drop test as specified in the Code of Federal Regulations (49 CFR 173.465, "Type A Packaging Tests"). "The Code of Federal Regulations allows one to demonstrate that Federal requirements are met either by physical testing or analysis," said Jeff Scott, P.E. of Duratek. "In this case, we verified the IP-2 container analytically with ALGOR in lieu of doing a physical test. If we had done an actual drop test – bought the material, fabricated prototypes, performed preliminary tests and then contracted out the final testing – it would have been very time-consuming and expensive. Using MES to demonstrate compliance with Federal regulations saved significant time and money."

Applying FEA to Radioactive Waste Disposal

Since 1986, Duratek has been one of the nation's leading full-service environmental remediation contractors. Clients include commercial nuclear power plants, the DOE, the U.S. Department of Defense (DOD), universities, hospitals and laboratories. At the DOE Hanford Site, Duratek provides management and professional services for several key functions including onsite waste characterization, waste verification, waste packaging, shipping waste, overall transportation logistics and engineering and testing of radioactive waste packages.

"We use ALGOR FEA for design verification including nonlinear simulations such as drop tests," said Scott. "We chose ALGOR's MES software because it combines motion and stress analysis, which lets us study motion and its results such as impact, buckling and permanent deformation."

For the IP-2 glovebox container, Duratek designed a rectangular steel structure that was approximately 22 feet long, 6 feet wide and 12 feet tall and weighed 27,000 pounds when filled. It included structural reinforcements and a sealing mechanism sufficient to meet Federal transport regulations. After delivery to the ERDF landfill, grout was poured into the container to fill the void space around the glovebox and then the container was buried.

"During the design of this container, we used ALGOR software to perform several types of analyses," explained Scott. "We performed buckling and linear static stress analyses on different parts of the box to verify structural adequacy. Additionally, we simulated a stacking test, which is a Federal requirement for IP-2 certified containers. This test placed a compressive load on top of the container equal to five times its maximum weight for 24 hours. Another requirement is a two-foot drop test. Regulations specify that the container must be dropped in the worst orientation onto a hard, unyielding surface. Of these, the drop test was the most challenging requirement to meet."

Simulating the Drop Test

The objective of the ALGOR MES analysis was to determine if the IP-2 glovebox container maintained containment after a two-foot drop in its weakest orientation. Duratek decided to drop the container on a top corner because the gasket seal, located on the inside top edge of the container near the lid, was the most vulnerable containment barrier. For containment to be breached, the two gasket sealing surfaces would have to move apart by 0.1875 inches.

Scott created a CAD solid model of the IP-2 glovebox container using Autodesk Inventor. Then, he and a colleague used MES to simulate the drop test. "We simplified the Inventor model for FEA by removing small, complex features and keeping only the features that were necessary for the simulation," said Scott. "Then, we used ALGOR's InCAD technology to capture the CAD solid model from Autodesk Inventor into ALGOR. It was very easy to use. In the Inventor pull-down menu, there was an option to select ALGOR and mesh. All I had to do was click it and the complete model opened in ALGOR's FEMPRO interface."

The finite element mesh was generated using 8-node brick elements. "While working on this project, I consulted ALGOR's technical support service," said Scott, "and one suggestion I got was to use options more appropriate for thin cross-sections, which would ensure that the mesh of the walls had at least two elements through the thickness." Additionally, mesh refinement tools were used to make the mesh finer in the corner area of impact. "We specified the refinement points at the corner, including the stiffeners, and then the mesh was regenerated with additional, smaller elements in that area, which helped to achieve a convergent solution and gave more accurate results."

The model was positioned with one of the top corners at the impact plane and an initial velocity was defined for the container equal to the velocity it would have had after freely dropping two feet (136 inches per second). "This modeling technique shortened the run time because it eliminated the need to solve for the container dropping through the air," explained Scott.

The event duration (0.0409 seconds) and capture rate (0.0001 seconds) were defined with standard gravity as the only loading on the model. "Mechanical Event Simulation differs from traditional FEA in that MES typically doesn't require any constraints, forces or pressures to be defined," noted Scott. "It simulates the effects of gravity under the physics of the Laws of Motion."

Material properties for steel (ASTM A572) were defined for the container by selecting from a built-in library. The mass density was customized to simulate the gross transportation weight of the IP-2 container (27,000 lbs). An initial analysis run indicated the need to adjust analysis settings to achieve faster convergence. "By changing the convergence criteria, we were able to get our results quickly and still obtain an accurate solution," said Scott.

Interpreting the Analysis Results

Results evaluation and presentation tools including numerical results, contour displays, graphs and animated replays of the event were used to examine the stress distribution and displacement in the IP-2 glovebox container due to the corner drop. "Although the corner plastically deformed as expected, the gasket sealing surfaces did not displace enough to breach the seal," said Scott. "As long as the gasket seal was not breached, the package maintained containment of the glovebox." Using ALGOR MES, Duratek verified that the container design successfully passed the drop test.

Scott summarized, "Using ALGOR MES software to simulate the drop test was much faster and more economical than performing physical prototype tests, which saved significant time and money for our client."

Future Plans for FEA

Reflecting on the drop-test simulation, Scott commented, "I learned a lot about convergence criteria and other nonlinear analysis parameters. This experience helped with another recent project in which we performed a drop test for a 55-gallon drum using ALGOR MES. It's a useful tool for nonlinear analysis that helps us verify our product designs more economically."

Jeff Scott, P.E. is a Mechanical Engineer with Duratek, Inc. in Richland, Washington. He earned a bachelor degree in Mechanical Engineering at Gonzaga University in Spokane, Washington and is currently pursuing a master's degree in Mechanical Engineering at Washington State University. For more information about Duratek, visit www.duratekinc.com.