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Reports are emerging that yesterday’s tragedy in Beirut was the result of the explosion of a store of 2,750 tons of ammonium nitrate (AN). How did we end up with kiloton-sized depots of potential high explosives being stored in cities? A long thread follows.
Tens of millions of tons of AN is produced worldwide every year, predominately as fertilizer and is a basis of our agricultural food chain; it is not going away. AN is often not even classified as an explosive, in fact, it is not even considered flammable:
AN’s usual classification is as an oxidizer. To be used as an explosive, solid AN must be prepared with a specific porosity and other additives, which I’m not going to discuss here to assuage my conscience, but are well-known nonetheless.
Other commercial blasting uses of AN are as aqueous solutions and emulsions. Again, because AN is “as cheap as dirt” since it is produced for fertilizer, its low cost makes it attractive for breaking rock (i.e., commercial blasting).
For every country with significant natural resource extraction (e.g., US, Canada, Australia, etc.), several kilograms *per* *person* of AN-based explosive are detonated every year.
I’m not aware that pure AN has ever been detonated under controlled circumstances. Even when prepared as recommended for commercial blasting applications, porous AN-based explosives require that the explosive charges be at least a foot (30 cm) in diameter
in order to effectively detonate as a high explosive (hat tip to @cbkiyanda for the measurements of this). For charges much smaller than this, the detonation simply stops and won’t propagate.
For pure AN, the dimension of charge necessary to support a detonation is likely much, much larger but has never been measured.
This brings us to the crux of the issue: How can we store thousands of tons of potentially explosive material in industrial areas of cities, even near residential areas?
Conversely, how can a material that in its fertilizer form cannot be used as an explosive undergo an obvious high-order event as we have seen in Beirut (and previously Tianjin, PRC in 2015 and West, Texas in 2013, etc.)?
The difficult answer is that there is no simple switch in behavior from explosive to non-explosive. Many materials have the potential to react exothermally, even in the absence of oxygen.
Simple household sugar (glucose) has a large, positive enthalpy of formation, meaning it can release energy just by decomposing. Chemically, sugar is very similar to known high explosives.
In fact, U.S. DOE national labs even use sugar as inert explosive simulants in some tests due to its similarity. There are even apocryphal reports of sugar detonating, most famously by Percy Bridgman at Harvard in 1935 in experiments
wherein he simultaneously applied shear and pressure to sugar. However, we don’t lose sleep over the sugar mill next door, nor should we.
For energetic materials, a key issue is size: Small quantities, even when caught in a fire, will simply decompose (or “burn”) without undergoing violent reaction.
But as the size gets larger, the potential to release heat—that in turn accelerates reactions, that in turn releases more heat, leading to an exponential runaway—becomes more likely.
The entire history of AN and other energetic material disasters is a painful lesson in learning that large quantities of materials previously thought safe can turn into explosions. Two well-known examples are Kriewald and the BASF plant in Oppau, both in Germany in 1921:
The backstory is that AN has a tendency to absorb water and form a congealed mass. In Kriewald and Oppau, workers were using other mining explosives to try to break-up large mounds of ammonium nitrate that had become a solidified mass,
which was not known to be explosive at the time, when the mound exploded. The lesson learned was that a material not known to be explosive was found—in a sufficiently large quantity—to be explosive.
The Russian physicists Nikolay Semenov and David Frank-Kamenetskii developed an entire mathematical framework in the 1930s for understanding how heat generation and feedback occurring inside an energetic material competes with heat losses from the periphery of the material.
The competition between heat generation by reaction and heat loss to surroundings leads to a criticality of thermal runaway of the reaction. This is a highly idealized model and typically cannot be used to make quantitative predictions as to the critical size;
the main utility of Semenov–Frank-Kamenetskii theory is as a guide in understanding the qualitative nature of the problem, essentially pointing out that questions like “At what temperature will it explode?” or “How much of an energetic material is needed to form a critical mass?”
do *not* have answers that are independent of the boundary conditions: You need to specify the size, geometry, surrounding environment, etc., to even begin to have meaningful answers to these questions.
Another Russian physicist, Yulii Khariton, stated a related concept as a simple principle: *Any* exothermically reacting material has the ability to detonate if the chunk of material is made large enough; this is now called “Khariton’s principle.”
(Historical footnote: Frank-Kamenetskii and Khariton would later go on to play a significant role in the Soviet nuclear weapons program.)
