arpadveress 
I have tried a little to get a look at what happened from a “scientific” point of view and came to the following:
Ammonium nitrate (CAS No: 6484-52-2)
Source: link
+ crystalline solid
+ does not react with oxygen, mixed with the specified air in any particle size, no combustion occurs at any energy input
+ feeds the existing combustion violently
Above +300 C it decomposes into N2O and H2O, and at even higher temperatures into N2, O2 and H2O gases. The decomposition reaction is exothermic and the heat generated is greater than the energy required for maintenance (run-off)
+ when mixed with water, it decomposes into a mixture of ammonium ion (NH4 +) and nitrate ion (NO3-) with strong heat removal (hence the name). The former, if mixed with even more water, may form oxonium ion and ammonia gas
+ reacts exothermically with almost any metal or other oxidizable organic or inorganic material
+ mixed with already oxidized material (eg metal oxides, rust, sulfur dioxide) can displace oxygen (typically in an exothermic reaction), it can further oxidize quasi-already oxidized material
+ (this is more for information only, does not belong closely here) when mixed with certain substances (eg phosphorus) it is also prone to spontaneous combustion, it can react exothermically without the presence any of the 13 ignition sources.
If these are true, I think the following outcome we may have:
1. No hazardous / explosive atmosphere is created around ammonium nitrate in accordance with Article 2 of 1999/92/EC.
2. Neither passive storage nor its processing (unless the chemical reaction during the processing results in an explosive substance being intentionally or as a by-product) is directly covered by the ATEX Directives (1999/92 / EC and 2014/34 / EU).
3. Consequently, there would be no requirement in the EU to create a hazardous area classification or EPD (explosion protection documentation) for this storage technology.
Ammonium nitrate decomposes to several gases at high temperatures, if you look at the decomposition reaction equation at high temperatures, it turns out that 2 moles of material very suddenly becomes 7 moles of gases:
2NH4NO3 → 2N2 + O2 + 4H2O
In practice, this is the explosion itself, and it does not really require the presence of air, it is enough to provide ignition (E.g: high temperature in a fire or an initiating explosion in an igniter).
In 2016, a fertilizer factory exploded in the U.S., killing 15 (12 firefighters) in that explosion. The CSB investigated the case, here is a link to the investigation material and a demonstration video:
https://www.csb.gov/west-fertilizer-explosion-and-fire-/
Comment to Ex:
On the other side Ex safe operation is a possible way forward to define safe operation.
I speak here about EU regulations, bec there are related regulations. In all other locations we do speak about standards and local law. There should not be any difference in terms of safety of mankind and operation.
Keep up good work!
Arpad
