Tech Briefs

Verifying the Safety of a Radioactive Waste Container With Simulation Software

Simulating drop tests helped ensure structural integrity of a contaminated glovebox.

As part of decommissioning the U.S. Department of Energy (DOE) Hanford Site — an inactive nuclear facility in southeastern Washington — 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 on-site 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, a member of the cleanup team and a 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. In lieu of performing a physical test, the container was tested analytically with the simulation software. If an actual drop test had been performed — including buying the material, fabricating prototypes, performing preliminary tests, and contracting out the final testing — it would have been time-consuming and expensive. Mechanical Event Simulation (MES) software was used to demonstrate compliance with Federal regulations.

FEA was used for design verification including nonlinear simulations such as drop tests; the MES software was used to 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 design of this container, buckling and linear static stress analyses were performed on different parts of the box to verify structural adequacy. A stacking test, which is a Federal requirement for IP-2 certified containers, also was simulated. 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.

The objective of the analysis was to determine if the container maintained containment after a two-foot drop in its weakest orientation. The container was dropped 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".

Using a CAD solid model of the container, MES was used to simulate the drop test. Small, complex features were removed from the model, and only the features necessary for the simulation were kept. The finite element mesh was generated using 8-node brick elements. Mesh refinement tools were used to make the mesh finer in the corner area of impact.

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). The event duration (0.0409 second) and capture rate (0.0001 second) were defined with standard gravity as the only loading on the model.

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 container (27,000 pounds). An initial analysis indicated the need to adjust settings to achieve faster convergence. 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 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.

This work was performed by Jeff Scott, P.E., a mechanical engineer with Duratek, Inc. using finite element analysis and simulation software from ALGOR, Inc. For Free Info Visit http://info.ims.ca/5214-123