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Ammonium nitrate decomposes into gases including oxygen when heated (non-explosive reaction); however, ammonium nitrate can be induced to decompose explosively by detonation. Large stockpiles of the material can be a major fire risk due to their supporting oxidation, and may also detonate, as happened in the Texas City disaster of 1947, which led to major changes in the regulations for storage and handling.
There are two major classes of incidents resulting in explosions:
In the first case, the explosion happens by the mechanism of shock to detonation transition. The initiation happens by an explosive charge going off in the mass, by the detonation of a shell thrown into the mass, or by detonation of an explosive mixture in contact with the mass. The examples are Kriewald, Morgan, Oppau, Tessenderlo and Traskwood.
In the second case, the explosion results from a fire that spreads into the ammonium nitrate itself (Texas City, Brest, Oakdale), or to a mixture of an ammonium nitrate with a combustible material during the fire (Repauno, Cherokee). The fire must be confined at least to a degree for successful transition from a fire to an explosion (a phenomenon known as "transition from a decomposition or deflagration", or DDT). Pure, compact AN is stable and very difficult to initiate. However, there are numerous cases when even impure AN did not explode in a fire.
Ammonium nitrate decomposes in temperatures above 210 °C. Pure AN is stable and will stop decomposing once the heat source is removed, but when catalysts are present (combustible materials, acids, metal ions, chlorides. ..) the reaction can become self-sustaining (known as self-sustaining decomposition, SSD). This is a well-known hazard with some types of NPK fertilisers, and is responsible for the loss of several cargo ships.
On October 4, 1918, the T. A. Gillespie Company Shell Loading Plant explosion occurred leading to many artillery shells being launched into the air, some of which landed on a neighbouring warehouse where 4000 tonnes of ammonium nitrate were stored in barrels. One of the shells caused a large explosion, but the majority of the ammonium nitrate did not detonate.
Krieweld, Germany, 1921
- On July 26, 1921 in this railway town (now in Poland) workers tried to dislodge 30 tonnes of ammonium nitrate which had aggregated in two wagons. When mining explosives were used on solid mass the wagons exploded and killed nineteen people.
Oppau, Germany, 1921
Main article: Oppau explosion
- Another attempt at disaggregation of a fertilizer mix with industrial explosives caused the death of 450 people and the destruction of 700 houses on September 21, 1921. The fertilizer was a 50:50 mixture of ammonium nitrate and ammonium sulfate and the factory had used this method of disagregation over 20,000 times without incident. It is thought that, on this occasion, poor mixing had led to certain parts of the mass to contain more ammonium nitrate than others. Only 450 tonnes exploded, out of 4500 tonnes of fertilizer stored in the warehouse.
Nixon, New Jersey, 1924 (Now Edison Township)
- A fire and several large explosions destroyed a warehouse containing ammonium nitrate on March 1, 1924. The explosivity of the product was perhaps enhanced, as it had been prepared using nitric acid which had previously been used for the production of TNT.
Muscle Shoals, Alabama, 1925
On April 4, 1925, and May 3, 1925, two carloads, each containing 220 barrels of ammonium nitrate, were dispatched from Muscle Shoals, Alabama and caught fire in transportation. The barrels had been stored in a warehouse with varying humidity for 6 years, so it is believed that they were ignited by friction with their nitrate-impregnated manilla paper lining. Other shipments were reportedly more successful.
Rouen, France, 1940
- During a bombing raid on June 5, 1940, a bomb exploded in a warehouse containing ammonium nitrate: the fertilizer was dispersed around the crater, but did not explode.
Miramas, France, 1940
- August 5, 1940: 240 tonnes of ammonium nitrate in sacks exploded after being hit by a shell from a nearby fire in a munitions train.
Tessenderlo, Belgium, 1942
- another attempt to disagregate a pile of 150 tonnes of ammonium nitrate with industrial explosives ended tragically on April 29, 1942: several hundred people were killed.
Texas City, United States, 1947
Main article: Texas City Disaster
The cargo ship Grandcamp was being loaded on April 16, 1947 when a fire was detected in the hold: at this point, 2600 tonnes of ammonium nitrate in sacks were already aboard. The captain responded by closing the hold and pumping in pressurised steam. One hour later, the ship exploded, killing several hundred people and setting fire to another vessel, the High Flyer, which was moored 250 metres away and which contained 1050 tonnes of sulfur and 960 tons of ammonium nitrate. The Grandcamp explosion also created a powerful earthshock and knocked two small planes flying at 1500 feet out of the sky. The High Flyer exploded the next day, after having burned for sixteen hours. 500 tonnes of ammonium nitrate on the quayside also burned, but without exploding, probably due to the fact that it was less tightly packed.
Brest, France, 1947
- The cargo ship Ocean Liberty was loaded with 3300 tonnes of ammonium nitrate and various inflammable products when it caught fire at 12:30 July 28, 1947. The captain ordered the hold to be sealed and pressurised steam was pumped in. As this did not stop the fire, the vessel was towed out of the harbour at 14:00, and exploded at 17:00. The explosion caused 29 deaths and serious damage to the port of Brest.
Red Sea, 1954
- a fire was detected on the cargo ship Tirrenia on January 23, 1954, while it was carrying 4000 tonnes of ammonium nitrate. Attempts to extinguish the fire with steam were unsuccessful, and the ship was abandoned before it exploded later in the night.
Roseburg, Oregon, 1959
- a truck carrying dynamite and ammonium nitrate caught fire early in the morning of August 7, 1959. When it exploded it killed 14 people and injured 125 more. Several blocks of downtown Roseburg were destroyed. The accident is locally referred to as "The Blast."
Kansas City, Missouri, 1988
- on November 29, 1988, at 4:07 AM two trailers containing approximately 50,000 lbs of ammonium nitrate exploded at a construction site located near the 87th street exit of Highway 71 in Kansas City, Missouri. The explosives were to be used in the blasting of rock while constructing Highway 71. The result of the explosions were the deaths of six firemen from the Kansas City Fire Department's Pumper Companies 30 and 41. Both companies were dispatched after 911 calls indicated that a fire had been set to a pickup truck located near the trailers. The responding companies were warned that there were explosives on-site; however, they were unaware that the trailers were essentially magazines filled with explosives. At 4:07 AM one of the "magazines" caught fire and a catastrophic explosion occurred, killing all six firemen instantly — only sparing remains were found. A second blast occurred 40 minutes later, although all fire crews had been pulled back at this time. The blasts created two craters, each approximately 100 feet wide and 8 feet deep. The explosions also shattered windows within a 10-mile area and could be heard 40 miles away. It was later determined that the explosions were acts of arson, set by individuals embroiled in a labor dispute with the construction company contracted to build the highway.
Toulouse, France, 2001
- on September 21, 2001, at 10:15 AM, in the AZF (Azote de France) fertiliser factory in Toulouse, France, an explosion occurred in a warehouse where the off-specification granular AN was stored flat, separated by partitions. About 200–300 tons is said to be involved in the explosion, resulting in 31 people dead and 2,442 injured, 34 of them seriously. The blast wave shattered windows up to 3 kilometres away and the resulting crater was 10 metres deep and 50 metres wide. The exact cause remains unknown. The material damage was estimated at 2.3 billion euros. 
Cartagena, Murcia, Spain, 2003
The fertilizer storage facility of Fertiberia holded a self sustained decomposition (SSD) fire in January 2003. The fire was controlled after most of the material was removed by mechanical means.
Barracas, Spain, 2004
- a truck carrying 25 tonnes of ammonium nitrate fertilizer exploded half an hour after a traffic accident on March 9, 2004, killing two people and injuring three others. The explosion, which could be heard at a distance of 10 km (6 miles) caused a crater five metres deep.
Ryongchŏn, North Korea, 2004
Main article: Ryongchon disaster
- a freight train carrying ammonium nitrate exploded in this important railway town near the Chinese border on April 22, 2004, killing 162 people and injuring over 3000 others. The station was destroyed, as were most buildings within 500 metres, and nearly 8000 homes were destroyed or damaged. Two craters of about ten metres in depth were seen at the site of the explosion. The authorities blamed "human error" for the explosion, although rumours persist that it was in fact an attempt to assassinate the Korean leader Kim Jong-Il, who was due to be passing through the station at the time.
Estaca de Bares, Spain, 2007
The NPK fertilizer cargo of the ship Ostedijk sustained a self sustained decomposition (SSD) fire for 11 days. The fire plume reached 10 m in diameter and several hundred meters in length. Special water spears were inserted inside the cargo to extinguish the fire.
Monclova, Coahuila, Mexico, 2007
- Another case has been registered on September 10, 2007 near Monclova, Coahuila, México, when a trailer loaded with 22 tons of ammonium nitrate crashed to a truck leaving three dead on the crash, then a fire on the trailer cabin started, and approximately 40 minutes after that, a very huge explosion occurred, resulting in around 150 people injured and 37 more dead, also a crater 30 ft wide and 6 ft deep was created due to the explosion.
^ The Economist 15 September 2007 p 47 but in less detail than above