Oppau Explosion: 450 Tons Turned a Factory Into a Crater

Estimated reading time: 12 minutes

On 21 September 1921, roughly 450 tons of ammonium nitrate mixture detonated at the BASF plant in Oppau, Germany. The blast killed 560 people, injured another 2,000, and left behind a crater big enough to make an entire chemical factory look like it had simply ceased to exist.

Before Oppau: My Own Introduction to High Explosives

Blowing up stuff is one of the few marketable skills I retain from the military. Soon after I bought my rural Mississippi farm back when dinosaurs roamed the earth, it became obvious that we were going to have to get rid of an even dozen beaver dams if we ever hoped to make the place habitable. A skilled operator can do that with a track hoe. Or you can follow the manly route and call upon energetic chemistry.

Sourcing Explosives Was Once a Very Different Proposition

Back before al Qaeda ruined absolutely everything with 9/11, acquiring certain explosive materials was a radically different proposition than it is today. No kidding, I once arranged a legal purchase over the phone and picked up commercially available explosive supplies from a dealer in a church parking lot. Suffice to say, it’s a bit more complicated than that today. I swear sometimes it seems that the primary function of modern government is to keep American males from enjoying themselves.

My kids and I spent two amazing days clearing those twelve beaver dams. I don’t know when I’ve had so much fun. We even inadvertently blew up one terribly unfortunate beaver, but that’s a story for another day.

I later experimented with commercially familiar reactive-target chemistry on my property as well. Tannerite is the best-known consumer example of the general concept. Used lawfully and according to the manufacturer’s directions, reactive targets provide the sort of immediate feedback that can put a memorable punctuation mark on a shooting session.

High explosives are indeed notoriously unforgiving of stupidity, inattention, or sloth. Normally, only folks like highly-trained professional gun writers should be allowed access to such volatile stuff. However, sometimes large volumes of explosive precursors are entrusted to irresponsible stewards with predictably exciting results.

Oppau 1921: Inside the BASF Ammonium Nitrate Plant

In September of 1921, the BASF plant in Oppau, Germany, was busy churning out ammonium nitrate fertilizer by the literal ton. This same material has a dual use application as an oxidizer in explosives. The facility began producing ammonium sulfate in 1911 to support Germany’s war effort during WW1. They later made ammonium nitrate using something called the Haber Process. Oppau was the first chemical plant in the world to do so.

When heated, ammonium nitrate produces large quantities of oxygen that can then be used to service an explosive reaction. Additionally, ammonium nitrate is fiercely hygroscopic. This means that it rapidly absorbs moisture from its surroundings.

The Dangerous Storage Problem Building Inside BASF

Let this stuff get a little moist, and it sets up into a plaster-like substance. BASF was storing their fertilizer in 20-meter-high silos. German workers had to employ pickaxes to get it out of the storage facilities. As being crushed by massive collapsing mounds of fertilizer was quite a serious concern, workers eventually adopted an explosive method for breaking apart the hardened material.

This was admittedly crazy, but it was sort of based on science. Testing conducted in 1919 demonstrated that formulations containing less than 60% nitrate could not be convinced to explode. The fertilizer BASF made was typically about 50/50. They actually took a load of this stuff out into the woods and subjected it to testing just to see what would happen. Convinced that they were good, these maniac Krauts began storing ammonium nitrate in 50,000-ton lots. Whenever hardened material needed to be loosened, workers relied on a practice that history would eventually reveal to be catastrophically overconfident. What could possibly go wrong?

In their defense, it is estimated that they had used explosive charges to break up solidified chunks of ammonium nitrate at least 20,000 times in the past. However, despite that much experience, this stuff is still the oxidizer for a very potent high explosive compound. Then, on 21 September, something very bad happened.

The Oppau Explosion Turns BASF Into an Enormous Smoking Hole

Nobody knows the details. There was nobody left to interview. Anybody even close to the point of detonation was utterly vaporized. 560 people lost their lives. A further 2,000 were seriously injured.

There were actually two explosions that occurred about half a second apart. Roughly 10% of the fertilizer in one silo detonated. 450 tons of ammonium nitrate were consumed. The blast left a crater 90 meters wide, 125 meters long, and 19 meters deep. The noise was detectable in northeastern France some 186 miles away. The cumulative effect was equivalent to nearly two kilotons of TNT. This was about one-eighth the explosive yield of the Hiroshima bomb.

Oppau Explosion Facts: 560 Dead and a 125-Meter Crater

Date	21 September 1921
Location	Oppau, Germany
Facility	BASF plant
Fatalities	560
Seriously Injured	2,000
Material Consumed	450 tons of ammonium nitrate
Crater	90 meters wide, 125 meters long, and 19 meters deep
Estimated Effect	Nearly two kilotons of TNT
Distance Heard	Some 186 miles away

The Fallout: Oppau Was Shattered for Miles

80% of the buildings in Oppau were destroyed. 6,500 local citizens were left homeless. There was not an intact window to be found for twenty miles.

The pressure pulse traveled to Mannheim just across the Rhine from Oppau, tearing the roofs off of houses fifteen miles away. A streetcar was blown off of its tracks in Heidelberg, some 19 miles distant. Rescuers recovered more than 500 bodies in the first 48 hours.

Why the Oppau Explosion Happened Despite 20,000 Earlier Blasts

It turned out that the less than 60% rule was not quite accurate. Lower percentages of nitrates are tougher to conflagrate, but it can obviously still happen. The ease with which one can coax this stuff into detonating is driven by lots of different things.

Particle size, density, and ambient humidity all played a role. Prior to the accident, chemical engineers had tweaked the production process to decrease the moisture content and lower the relative density. Both of these factors contributed to the accident. Investigators also determined that the lot in question was not homogenous, containing pockets of ammonium nitrate that were more enriched than others. Once one of these pockets touched off, the resulting shockwave sympathetically detonated the rest of it.

My own experience with reactive targets reinforced how much apparently minor physical differences can change the behavior of energetic materials. Things that seemed important in theory sometimes made little practical difference, while variables a casual observer might overlook could matter enormously. With the benefit of hindsight, that is precisely the sort of complacency the Oppau disaster warns against.

Two months prior to the Oppau tragedy, some thirty tons of ammonium nitrate fertilizer exploded in Kriewald, Germany, under comparably suspicious circumstances. That blast claimed nineteen lives. History does not record why this event did not drive a re-evaluation of the industrial processes at Oppau.

Oppau Was Not the Last Ammonium Nitrate Catastrophe

Investigators originally thought that the blast had originated in the reactor facility where the product was actually made. It was discovered only later that the blast originated from a storage silo. Not that it much mattered.

The Oppau blast was the third-largest ammonium nitrate explosion in history. This follows the legendary Beirut, Lebanon, blast in 2020 and the detonation of some 2,300 tons of ammonium nitrate in Texas City, Texas, in 1947. We may explore those simply epic events at some point in the future.

At the time of this detonation, there were no ammonia plants of this sort in the United States. American engineers rightfully believed that the state of the art simply did not support the safe mass production of ammonium nitrate fertilizer. Lots of folks make it these days, but it is easier to manage safely now that we understand the physics behind this reaction so much better.

From the Oppau Disaster Back to a Mississippi Farm

It really is great fun to spend a couple of days turning binary explosives into noise. Back on the farm, we cleared our beaver dams using the training, equipment, safety margins, and procedures I had learned through prior experience. The kids naturally considered the whole affair premium entertainment. A grand time was had by all.

Once the beaver dams were done, we contracted with a dirt guy to build us a proper dam across the little creek that runs through my backyard. The end result is seven acres of heaven that is now teeming with bass, bream, and water moccasins. However, in the immediate aftermath, the lake was just bug ugly with dead trees sprouting up everywhere.

I tried lots of different techniques for felling these trees. I can attest that running a chainsaw out of a canoe is a suboptimal solution. As soon as you touch the trunk, the reactive force begins spinning the canoe around in circles. As the mission is to cut these old derelict trees down, you can never be sure which direction they might fall. Being crushed and drowned by a tree I cut down myself would look pretty stupid on a tombstone.

I eventually experimented with energetic methods rather than balancing a running chainsaw in a canoe. Properly handled explosive force can do remarkable work, but the operative words there are properly handled. The entire Oppau story exists because familiarity and repetition can convince otherwise intelligent people that a dangerous process has somehow stopped being dangerous.

The Real Lesson of Oppau: Experience Does Not Rewrite Physics

Blowing stuff up really is fun so long as you don’t also wipe out yourself, your home town, and 560 of your closest mates in the process. Some commercially available reactive-target products can be legally transported and possessed under circumstances that differ dramatically from regulated blasting materials. Other explosive components and activities are governed by extensive federal, state, and local requirements. Anybody dealing with such materials needs to know the law, follow applicable storage and handling rules, and understand that energetic chemistry does not grade on a curve.

That is perhaps the most enduring lesson of the Oppau explosion. BASF workers had repeated their process thousands upon thousands of times. Success became routine. Routine became confidence. Confidence became complacency. Then chemistry reminded everybody in the most savage way imaginable that twenty thousand uneventful repetitions do not constitute immunity from physics.

In 1921, in Oppau, Germany, a bunch of guys got in a bit of a rush and blew themselves to smithereens. Something similar happened in 1947 in Texas and then again in 2020 in Beirut. However, with knowledge, proper controls, respect for the law, and serious attention to detail, energetic materials can be managed far more safely than they were a century ago. Oppau remains the spectacular, crater-shaped reminder of what happens when experience is mistaken for invulnerability.

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