Ammonium nitrate-fuel oil
Ammonium nitrate-fuel oil (ANFO) is a general term for a class of high explosives that are prepared at or near the point of use, by mixing ammonium nitrate granules called "prills" with a hydrocarbon such as diesel oil. It has legitimate industrial applications, especially in earthmoving and quarrying, because, in a semisolid slurry form, it can be pumped into a hole drilled into rock or earth. It is also widely used in improvised explosive devices, although optimal mixing, detonation, and effects are more complex than many amateur bomb-makers realize.
Ammonium nitrate is widely used as fertilizer. In the United States and other countries, ammonium nitrate made for the agricultural market may have a granular type less suitable for explosive making, and also may have taggants to identify the source.
While a high explosive, its detonation velocity (VOD) is slower than, for example, TNT. This is actually advantageous for earthmoving, as it tends to push rather than shatter the ground, giving it very substantial explosive power Due to its use in mining and construction, more ANFO is used in the United States than any other explosive.
The rate can be increased with additives, such as nitromethane or aluminium powder, although these can make the mixture more unstable. The aluminium-boosted form, when used in weapons, is sometimes called ammonal. VOD also varies with the prill size and the adequacy of mixing.
ANFO is relatively insensitive to shock, considerably less so than TNT, which rarely would detonate when hit by a rifle bullet but will go off with a suitable blasting cap. In general, ANFO is detonated indirectly, with a blasting cap encircled by a more sensitive booster, such as tetryl.
It is bulkier than other explosives, but, in large quantities as in a truck bomb, can be devastating. The Oklahoma City bombing used approximately 4000 pounds/1800 kg of ANFO, and wrecked a large office building with an explosion in front of it. Precision-made ANFO variants are also used in very large gravity bombs such as the BLU-82 or GBU-43 Massive Ordnance Air Blast.