In a move that could make demolition teams breathe a bit easier, scientists at the Lawrence Livermore National Laboratory (LLNL) are studying the structure of high explosives to find out how to make them safer. Based on microscopic examination and computer modeling, they have found that by engineering the porous microstructure of explosives, their sensitivity to detonation can be controlled.
Ever since explosives were invented, chemists and others have been working on ways to make them less likely to explode when they shouldn't. This has involved everything from the first attempts to "corn" gunpowder into grains instead of loose powder, to creating complex mixtures of different explosives and inert additives that result in an end product that can level a building, yet doesn't mind being struck with a hammer.
However, even the best explosives have to be handled with respect because the factors involved in making them detonate aren't completely understood. But what the LLNL team led by research scientist Keo Springer of the LLNL's High Explosives Applications Facility found was that it wasn't just the chemistry that plays a part, but the tiny holes, pores, and defects in the explosive itself.
The new research suggests that when a high explosive is set off by means of a detonator and gain, the actual explosion is caused by the pores in the plastic substance being compressed by the detonating shock wave. This heats up the immediate area, starting the chemical reaction in the microscopic crystalline grains of explosive material.
To understand this chain reaction better, the team looked at an explosive called HMX (why it is called this, no one knows), a powerful nitroamine high explosive similar to RDX. It is difficult to manufacture and is so sensitive that it is only used by experts.
By running simulations of the LLNL supercomputers , the team found that if the pores were on the surface, the detonation was much faster. In addition, if the pores were smaller, they accelerated the reaction even more. They were also able to determine a threshold where the pores become small enough that the reaction stops.
"We found out that when pores are at the surface, they speed up the reaction," says Springer. "We also discovered that if a shockwave hits a number of surface pores at once, they bootstrap each other. It's an explosive party, and they party well together."
LLNL is now working to verify the initial simulations to better understand the physical and chemical processes, before going on to actual micro-scale experiments with real explosives.
"Validation is the tough part," says Springer. "Ideally, we would need a really good magnifying glass and the ability to stop time. We're talking about sub-micron resolution with a shutter speed on the order of nanoseconds. What's neat is that the research community is starting to work on this.
"If we can engineer initiation properties into the microstructure of explosives, it would be a game changer for industry and for the safety of the nuclear stockpile. But we have a long way to go to realize that vision. This type of research is very important, but just one of the first steps."
The research was published in Propellants, Explosives, Pyrotechnics.
Source: LLNL
