Anyone who drives along Interstate 35 in Texas between Waco and Dallas or Fort Worth will pass through West, a small rural Texas town with a Czech heritage that was known, at least until last week, mainly for the pastries that you can buy at a bakery right next to the Interstate.

It would not have surprised me to learn that a distributor of fertilizers to local farmers operated on the east edge of town, nor that one of the kinds of fertilizer sold by the dealer was ammonium nitrate. But when I learned of the tremendous explosion that killed at least 14 people, injured hundreds, and destroyed a good fraction of West’s built environment on April 17, my sadness was tinged with the knowledge that in warehousing large quantities of ammonium nitrate fertilizer, the firm was taking a chance that such a thing could happen.

Ammonium nitrate is a curious chemical. A “molecule” consists of an ammonium ion (four hydrogens arranged around a nitrogen) with a positive charge and a nitrate ion (a nitrogen atom surrounded by three oxygen atoms) with a negative charge. At room temperature, it is a solid, but its constituent elements are all gases. And the only thing holding it together are the opposite charges retained by the ammonium and nitrate ions.

When heated gently in an open container, it breaks down into nitrous oxide (laughing gas) and water. But when it is in contact with easily oxidized materials, such as the fertilizer urea or even some metals, heating can cause it to release oxygen, which greatly increases the heat of the reaction and can lead to a fire.

When confined by walls or even the pressure of a high stack of the material itself, burning ammonium nitrate can self-detonate. A detonation is an explosive shock wave that travels at very high speed through a volume of material, and differs from burning as travelling by jet aircraft differs from walking. This is apparently what happened a little after seven in the evening at the burning warehouse in West.

A back-of-the-envelope calculation of the power of the resulting blast can be done by beginning with an aerial view of the fertilizer plant before the explosion, which is still available on Google Maps. Comparison with views of the devastated explosion site indicate that the explosion was probably centered in a large, flat warehouse building that appeared to be one story high and measured about 60 feet by 110 feet.

If we assume it was packed to a height of eight feet with ammonium nitrate (not an unreasonable assumption as distributors stock up for the summer growing season), the total mass of chemical in that building could have been as much as two thousand tons. Pure ammonium nitrate has about a fourth of the energy content per pound as TNT. Still, given these rough assumptions, if the whole mass went off at once, which it appears to have done, the force of the explosion could have been as great as a thousand tons of TNT, or one kiloton.

You may have run across the word “kiloton” in reference to nuclear explosions. While there were fortunately no nuclear weapons or radioactive materials involved in the West explosion, the nuclear weapon dropped on Hiroshima at the end of World War II had a yield of only about 16 kilotons of TNT. So what happened in West was one-sixteenth of a small nuclear bomb, in terms of destructive power. No wonder it showed up on seismographs as a magnitude-2 earthquake.

If ammonium nitrate is so dangerous, why isn’t handling and use of it more regulated? That’s a good question. The Wikipedia article on ammonium nitrate notes that in 2005, Australia passed a Dangerous Goods Regulation law which requires a license for the sale or use of the material. But if even licensed users store huge quantities of the stuff in places where it can catch fire and explode, licensing would not prevent disasters such as the one that happened to West last week, or Texas City in 1947, or over twenty other occasions since 1916 listed in a separate Wikipedia article devoted to ammonium nitrate disasters.

Chemical companies that deal routinely with explosives know how to handle these materials so that when they explode, the explosions are limited to a small area that is sacrificed in order to protect the rest of the property and lives involved. You simply restrict the amount of explosive allowed in one place to a maximum amount that you can afford to blow up, and then physically isolate it from all other concentrations of explosive in a series of small bunkers.

If the fertilizer stored in the West Fertilizer Company plant had been dispersed in this way, perhaps one of the small storage areas might have blown up, but with sufficient earth-berm isolation and other precautions, the explosion would not have spread.

That is small comfort for the survivors in West.

And as a practical matter, you can handle ammonium nitrate in an ordinary way without special precautions, as it is done thousands of times each year around the world, and most of the time, nothing bad will happen. If the West firm had been required to invest in the additional storage facilities needed to treat ammonium nitrate as a true explosive, it would have gone out of business for sure. (News reports indicate the firm nearly went bankrupt a few years ago and was rescued at the last minute by the present owner.)

So we face the dilemma of either requiring a huge investment in safety facilities on the part of fertilizer manufacturers and retailers everywhere to prevent disasters like West, or we leave things as they are and wait for the next one.

A compromise solution might be the rigorous training of anyone who deals with ammonium nitrate, enforced by a licensing law similar to the one on Australia. This would include mandatory evacuations based on scientific calculations of a worst-case explosion whenever a fire occurs near large quantities of the stuff. While regulations like this would not have prevented the damage caused by the West detonation, it could have reduced the death toll.

Our thoughts and prayers are with the residents of West, whose tragic experience may lead to changes that at least mitigate the dangers involved in dealing with ammonium nitrate in the future.

Karl D. Stephan is a professor of electrical engineering at Texas State University in San Marcos, Texas. This article has been republished, with permission, from his blog, Engineering Ethics.

Sources

I referred to the Wikipedia articles “Energy density,” “Ammonium nitrate” and “Ammonium nitrate disasters” as well as Google maps of the vicinity and photographs in various publications of the disaster site, and the book The Science of High Explosives by Melvin A. Cook (Reinhold, 1958).

Karl D. Stephan received the B. S. in Engineering from the California Institute of Technology in 1976. Following a year of graduate study at Cornell, he received the Master of Engineering degree in 1977...