Addessio joined the Laboratory in 1978 to work on modeling loss-of coolant accidents related to nuclear reactor designs. Following the Three Mile Island incident, which altered the face of that industry, he transferred into the Theoretical Division. Today he’s working at the single-crystal and polycrystal length scales, zeroing in on the properties of individual materials, to help scientists understand how the materials that make up weapons behave under normal—and unexpected—conditions.
What happens if a weapon is dropped during transport? Or if there’s heat from a fire in the storage facility, or a sympathetic shock from a nearby explosion? Scientists at LANL examine safety scenarios, as well as design performance under rigorous and demanding operating conditions. Addessio points out that nuclear weapons are composed of relatively exotic materials. “Some of these materials decay radioactively, emitting decay elements and generating heat. That is, like most things, they age!” To explore conditions of interest, scientists use a combination of experimentation and computational modeling.
Addessio draws a simplified analogy to testing the design of an automobile frame made of titanium and steel. “You could build the frame,” he says, “then run it into a wall and observe what happens. If it didn’t work, you would change the design and run it into a wall again, over and over.” He points out that this process can get expensive, and, in the assessment of nuclear weapons, there are safety and environmental considerations, as well as the constraint of the nation’s “zero yield” policy.
“To understand how the materials—in our example, titanium and steel—are going to deform when they hit the wall,” he says, “we perform‘simple,’ small-scale experiments—and I put simple in quotes—to develop stress-strain models. Using finite element analysis, we can model the entire frame, and the impact of that structure against the wall. Now we can run thousands of simulations on computers that are much cheaper and quicker.”
The synergy between lab research and the power of the Advanced Simulation and Computing program is the basis of SBSS. It takes a lot of back-and-forth to refine the models. Ideally, theory and practice agree— but, he says, even when the models aren’t perfect “the hope is that they put us in the ball park, so maybe instead of doing 25 experiments we only have to do 10.”
Addessio has closely studied the properties of titanium crystals and composites. A current focus is a plastic-bonded explosive, or PBX, which contains organic energetic grains held together by polymer binders.
“Because of the atomic structure, the grains of the energetic component are subject to deformations in their orientation,” he notes. “As with wood, if you cut it with the grain or against it, it’s going to react differently, depending on direction. In some of our simulations, we consider the deformation of a collection of atoms to observe how the crystalline structure changes (e.g., phase transformations), and the impact those changes have on the stress and strain response. That determines the response of the engineering structure—just as in the earlier analogy of the automobile frame.”