Pure ammonium nitrate detonates but it needs some initiator, which can be a big enough deflagration (=slower than detonation) of ammonitrate if mixed with some fuel. So a limited amount of some fuel somewhere on ammonitrate suffices to detonate everything just with an ignition source.
https://en.wikipedia.org/wiki/Ammonium_nitrate#Explosivescheck also the MSDS (=Material Safety Data Sheet)
An other initiator class just sensitizes ammonitrate, and hypochlorite (=bleach) is a known example. Not chlorine, which is a gas. It has been cited as a possible cause at AZF. Though, tainting ammonitrate with anything, and certainly with bleach, is not what you can expect from workers at an ammonitrate plant who know their job.
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A part of the story is little known. On that same day in Toulouse, few kilometres and seconds apart,
SNPE's plant producing hydrazines for Ariane exploded too. Probabilities speak against a coincidence. Alternately, both plants can have been attacked (this was little after the Twin Towers). Or the explosion at one plant damaged the other.
http://www.usinenouvelle.com/article/proces-azf-faut-il-aller-voir-du-cote-de-l-armee.162919fun: usinenouvelle sends to a different article now, but never mind, I append the original one here under. Replace the extension txt by htm.
http://www.sudouest.fr/2012/08/29/azf-la-these-officiel-le-remise-en-cause-806412-4697.phpHydrazines are very toxic, and building such a plant within the already existing neighbourhood was a criminal negligence that has not been investigated by the Justice.
The linked newspapers tell about 10t of UDMH had leaked. This would have had catastrophic sanitary consequences for the inhabitants. One SNPE worker died in the following explosion. While UDMH ignites very easily, possibly the worker sacrificed himself to ignite the leak and avoid to poison 100k people.
4s to the explosion at AZF, that can be the flight time of a debris that hit the ammonitrate there. Other people suggest the air or ground shake ignited the ammonitrate.
Anyway, rocket operators want to get rid of hydrazines, and the production plant was at the wrong place.
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Ammonitrate has already killed more than thousand people over more than a century.
https://en.wikipedia.org/wiki/Ammonium_nitrate_disastersIt would be about time to
qualify it legally as an explosive, not as a fertilizer. Then the industry would take the appropriate steps to prevent explosions. Maybe store and transport it as a solution in water rather than a dry powder, if its use is to spread on fields anyway.
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