What Is Ammonium Nitrate?

A joint EPA, OSHA and ATF chemical advisory describes two commercial forms of solid ammonium nitrate prills, technical grade used in the manufacture of blasting agents and fertilizer grade used as a fertilizer or in fertilizer blends, and says the only difference between them is the density of the prills.

The advisory says pure ammonium nitrate is stable at ambient temperature and pressure and does not itself burn, but supports and accelerates the combustion of other material and may explode when exposed to a strong shock or sustained high temperatures in confinement. It says ammonium nitrate melts and begins to decompose at 337°F, creating heat and toxic gases containing ammonia and nitrogen oxides, which can support combustion even without other sources of oxygen. It advises storing ammonium nitrate isolated from possible contaminants, and separating it from incompatible substances by separate buildings, 1-hour fire resistant walls or at least 30 feet.

The U.S. Chemical Safety Board says that on April 17, 2013, a fire and explosion at the West Fertilizer Company in West, Texas fatally injured 12 emergency responders and three members of the public. The board concluded that the construction of the bins and other building materials, along with the lack of an automatic sprinkler system, plausibly contributed to the detonation.

The federal security rule is still at the proposal stage. CISA says its Ammonium Nitrate Security Program proposed rule, published in August 2011, seeks to reduce the likelihood of a terrorist attack involving ammonium nitrate by creating a registration program for purchasers and sellers and regulating sales and transfers at the point of sale. The federal Unified Agenda lists the rulemaking as a long-term action and says CISA is planning to issue a supplemental proposed rule.