Ammonium Nitrate Modeling in the AN Module of IMESAFR
Michael M. Swisdak, Jr.; A-P-T Research, Inc.; Huntsville, AL, USA
William B. Evans; A-P-T Research, Inc.; Huntsville, AL, USA
Keywords: Ammonium Nitrate, Modeling, Airblast, IMESAFR, Initiation Frequency, Event
Frequency
Abstract
Worldwide, there have been several major accidents involving ammonium nitrate (AN) over the
past decade. The Institute of Makers of Explosives (IME) recognized the need for explosives
companies to be able to accurately model the risk from accidental explosions in AN stores and
commissioned APT Research, Inc. (APT) to develop the models and software tool for this effort.
This paper touches on the following topics but concentrates on those that went into the
development of the blast model:
• Background
• Assumptions that went into the model development
• Development of AN waveform
• Changes to the structural models in IMESAFR because the energetic material is assumed
to be AN rather than TNT
Finally, the paper works through an example using both the current IMESAFR models as well as
the AN Module and compares and contrasts the differences.
Background
AN is the main raw material used in the commercial explosives industry, accounting for roughly
75-80% by weight of the total explosives consumption--over 90% if one excludes water from the
raw material list. As billions of pounds of explosives are used annually in the explosives industry
globally, this means that very large amounts of AN are manufactured, stored, and transported on
a regular basis. A large AN plant can manufacture more than a billion pounds per year. There are
AN stores in remote areas of the world that contain millions of pounds of material; in more
populous areas, there are stores of 1,000,000 pounds or more.
AN is classified as an oxidizer for purposes of transport; this classification has been extended in
most jurisdictions to also cover storage. However, the United Nations (UN) definition explicitly
recognizes AN and ammonium perchlorate (AP) as the two hazard division (HD) 5.1 substances
that can explode under certain conditions. For instance, UN regulations change the classification
of AN to HD 1.1 above an organic content of 0.3%. AN is an ideal raw material for explosives
because it can be manufactured in very large quantities, is low cost, and is extremely stable in all
normal conditions. It also has a very high energy density, especially when used in an oxygen-
balanced explosive formulation, e.g., Ammonium Nitrate/Fuel Oil (ANFO) or Ammonium
Nitrate Emulsion (ANE). However, “extremely stable” is not synonymous with “inert,” as there
