Supercomputer Models Explosives Critical for Nuclear Weapons

By Oliver Peckham

August 6, 2022

Lawrence Livermore National Laboratory (LLNL) is one of the laboratories that operates under the auspices of the National Nuclear Security Administration (NNSA), which manages the United States’ stockpile of nuclear weapons. Amid major efforts to modernize that stockpile, LLNL has announced that researchers from its own Energetic Materials Center and Purdue University’s Materials Engineering Department have leveraged LLNL supercomputing to better understand the chemical reactions that detonate explosives that are “critical to managing the nation’s nuclear stockpile.”

The explosive materials in question are those based on triaminotrinitrobenzene (TATB), a high explosive that is insensitive to shock and which LLNL has characterized as “virtually invulnerable to significant energy release in plane crashes, fires, or explosions, or … small arms fire.” (“In fact,” they continued, “TATB is so stable that researchers had to discover how to reliably initiate an explosion of the material.”) TATB – which is expensive to produce – is almost entirely used for the detonation of nuclear weapons.

But the mechanisms of TATB’s invulnerability had remained relatively opaque – until now. Researchers did know that the explosions resulted from “hotspots” created by shockwaves hitting defects in the material and causing temperature spikes, but models struggled to reproduce much beyond that basic understanding.

To change that, the researchers turned to the Quartz supercomputer at LLNL, a Penguin Computing-supplied system powered by Intel Xeon CPUs. Quartz, which delivers 2.63 Linpack petaflops, placed 233rd on the most recent Top500 list. On Quartz, the researchers conducted atomically resolved molecular dynamics simulations in an effort to simulate the formation of these hotspots and their explosive reactions.

The Quartz supercomputer. Image courtesy of LLNL.

Even after the simulations, the researchers had some trouble deciphering the results. “Recent molecular dynamics simulations have shown that regions of intense plastic deformation, such as shear bands, can support faster reactions,” said Matthew Kroonblawd, a computational chemist at LLNL, in an interview with LLNL’s Anne Stark. “Similar accelerated rates also were observed in the first reactive molecular dynamics simulations of hotspots, but the reasons for the accelerated reactions in shear bands and hotspots were unclear.”

“These simulations generate enormous quantities of data, which can make it difficult to derive general physical insights for how atom motions govern the collective material response,” added Ale Strachan, a professor of materials engineering at Purdue.

So the researchers turned to clustering analysis to better understand the data. In doing so, they found two descriptive features of the more reactive molecules: temperature, of course — but also the “intramolecular strain energy,” or the energy resulting from deformations of the molecules.

“At ambient conditions, TATB molecules adopt a planar shape,” explained Brenden Hamilton, a researcher at Purdue University. “This shape leads to a highly resilient crystal packing that is thought to be connected to TATB’s unusual insensitivity.” The molecules that had deviated from this planar shape were quicker to react.

To learn more about this research, read the reporting from LLNL’s Anne Stark here.

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