Los Alamos team develops ‘game-changing’ approach to explosives testing

An interdisciplinary team at Los Alamos National Laboratory developed a new method for testing explosives that provides more precise data, helping researchers determine whether an explosive is safe enough to handle. The findings were recently published in the Journal of the American Chemical Society.

Called the High Explosive Initiation Time (HEIT) experiment, the interdisciplinary team at Los Alamos used a high-voltage source to very rapidly heat small explosive samples confined within narrow hypodermic needles. This enabled the team to learn exactly what temperatures were applied to the sample and how long it took for the sample to explode after the application of the voltage pulse. By applying different voltages, the team measured temperature's impact on the time to explosion.

"This method gives us reaction kinetics, which is something that researchers use to understand the safety characteristics of explosives," explains Virginia Manner, an explosives scientist at Los Alamos and leader of the project.

Prior to this experiment, the most widely used approach to testing new explosives was the so-called "drop weight impact test" - dropping a weight onto an explosive material and measuring the reaction. While effective, this method is limited, Manner says.

"With a drop weight impact test, we can analyze the sound levels produced by the impact - the loudness of the 'bang' - to determine that a reaction has taken place with the explosive material. These sound levels provide a picture, but not a complete one," she says.

For example, this standard test tells researchers how to rank explosives in terms of handling safety, but it can't tell them what is physically happening during the explosion, what temperatures were achieved, or whether the explosive sample was fully or partially consumed. The Los Alamos team sought to fill in these holes with a more modern approach to explosive testing.

High-speed imaging of an explosive sample bursting a needle during a high voltage pulse.

Los Alamos chemists, physicists, engineers, and theoreticians closely collaborated for three years to develop a test to provide greater fidelity for explosives testing than sound levels alone.

"The onset of thermal explosion in the test agrees surprisingly well with theoretical calculations of the reaction rates," said Marc Cawkwell, a materials scientist and co-leader of the project. "This effort transitions explosive safety testing from the crude, WW2-era drop weight impact test to the 21st century."

"This work is a game changer," adds Douglas Tasker, a physicist on the project who led the development of the experiment. "It links quantum molecular dynamic calculations of explosive reactions on a femtosecond timescale, to real-world explosive accident scenarios that occur in milliseconds - a trillion times slower. We can now understand explosive accidents at a fundamental level."

Paper: "An Integrated Experimental and Modeling Approach for Assessing High-Temperature Decomposition Kinetics of Explosives" The Journal of the American Chemical Society.

Funding and Support: This research was funded by the Laboratory Directed Research and Development program at Los Alamos.

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