Lawrence Livermore National Laboratory
A Chilling Tale of Nuclear Weaponry
Freezing and maintaining the reliability of the United States’ nuclear weapons stockpile would appear to be diametrically opposed concepts. Think of one and the notion of shivering in the cold comes to mind. The other conjures images of an inferno.
But in the world inhabited by Jim Glosli, a staff physicist at the Department of Energy’s (DOE) Lawrence Livermore National Laboratory (LLNL), and other members of the Simulations Group, freezing and nuclear weapons mesh perfectly. Their job is to conduct computer simulations of what happens when metals freeze, or change from a liquid to a solid state.
This work is part of the Advanced Simulation and Computing Program managed by DOE’s National Nuclear Security Administration. The program is designed to maintain the safety and reliability of the nation’s nuclear weapons stockpile without nuclear testing. Modeling metals is particularly important to this program because, as with virtually all metal parts, the metal components of nuclear weapons are formed by converting molten metal into a solid state.
And, in something of an understatement, Glosli notes, “When liquids freeze it is not a simple process. When a material solidifies it is not necessarily homogeneous at the atomic scale. Lots of little crystallites form. The way they arrange and organize themselves affects the material properties of that metal.”
Understanding those properties can provide useful information for improving the metallic materials used to build nuclear weapons. “Imagine a nuclear explosion,” Glosli says. “There are violent things happening. Materials are going through lots of different states and all of those details have an effect on the performance of those weapons and their reliability. By understanding those material properties the designers can do a better job of weapon design.”
A deeper knowledge of the metals that go into those weapons also can enable scientists to understand what happens to stored weapons over time. This is so because even after metal solidifies and seems stable, changes in its structure continue to take place. It is these kinds of changes at the microstructure level that lead to occurrences such as metal fatigue.
As Glosli puts it, “These structures continue to evolve over very long periods of time. The system would like to get to its lowest energy state, which would be a single crystal. But that wouldn’t happen. It would just take too long — geologic sorts of time.”
Pounding and Stretching
The art of heating, pounding and stretching metals to give them different characteristics, such as strength and brittleness, predates even the blacksmith’s shop. Though early metalworkers didn’t know it, what they were doing was manipulating the microstructures within metals. Today, it’s crucial to understand metals at the atomic level. Modern metal devices are more complex and face extreme demands — especially when it comes to safety and national defense.
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