Storage or Mixing
of Incompatible Chemicals
The U.S. Chemical Safety and Hazard Investigation Board
estimates that about 36% of hazmat incidents in U.S. industry are caused by
chemical incompatibility, due to improper storage and/or mixing of different chemicals. We don’t have a percentage internationally,
but we will look at an incident which occurred in France and see what the PEAC
tool can tell us.
Ammonium Nitrate Fertilizer Explosion in France
On 21 September 2001, at 10:18 AM, a massive explosion
occurred at the Azote de France (AZF) agricultural chemical factory near
Toulouse, France. At least 31 people
died and 650 people were hospitalized.
The total number of people injured was said to be 2442, mostly from
lacerations from flying glass and other fragments. The explosion occurred in a warehouse where between 200 and 300
tonnes of granular ammonium nitrate fertilizer was stored flat, separated by
The force of the explosion resulted in a crater more than 10
meters deep and 50 meters in diameter.
Windows were blown out in the center of Toulouse 3 km away, and window
breakage occurred up to 5 km away. An
electrical goods store 300 meters from the AZF plant collapsed. More than 500 homes became uninhabitable. Several thousand buildings were severely
damaged. The Institute for Geophysics
at Strasbourg registered the blast at 3.4 on the Richter scale.
The AZF plant was originally opened in 1924 in what then was
countryside, but urban sprawl allowed homes and businesses to be built right up
to plant boundaries. The population of
Toulouse is close to 600,000 today. The
AZF facility was completely destroyed and not rebuilt.
Initially, the possibility of terrorism was suspected as the
event occurred shortly after 9-11, and one of the plant workers killed by the
explosion was known to the police for possible Islamic fundamentalist
sympathies. Police found the worker’s
apartment completely stripped when they searched it several days later, and the
worker before the explosion had interrupted a French peaceful gathering of
respect (displaying the American flag) in honor of the 9-11 victims. There was a fair amount of confusion and
inconsistent stories during the early stages of the investigation as to the
role of possible terrorism and other causes.
The cause of the explosion was eventually concluded to be
Employee interviews led investigators to conclude that the
cause of the explosion was a chlorine-based pool maintenance chemical and
off-spec ammonium nitrate fertilizer stored in the same hanger. The two chemicals mixed, and the reaction
caused the explosion. Specifically, a
mislabeled 500 kg bin of sodium dichloroisocyanate (the chlorine-based pool
maintenance chemical) was mistakenly thought to be ammonium nitrate and placed
in the same warehouse where the fertilizer was stored. Add to this summer heat and humid
conditions. Exactly what happened next
is not clear, but the most reasonable explanation is that the two chemicals
reacted to form the highly unstable nitrogen trichloride. The nitrogen trichloride became the
detonating agent for the ammonium nitrate.
Dry, uncontaminated ammonium nitrate is stable by itself and
is safe to handle. A detonator is
of Figure: European Environmental Agency, Report No. 10, Copenhagen, 2003
blast location: 43.572o
E longitude from Earth Tools
What Can the PEAC Tool Tell Us?
We can look up information about the individual chemicals,
ammonium nitrate fertilizer and sodium dichloroisocyanate. When sodium dichloroisocyanate is pulled up,
the PEAC tool routes the information to “sodium dichloro-S-triazinetrione”. Sodium dichloro-S-triazinetrione and Sodium
dichloroisocyanate are two different
names for the same chemical. Many
chemicals are known by different names but are the same thing. The PEAC tool
attempts to list the different synonyms and spellings of this and other
chemicals in the drop-down list.
We read that the chemical is subject to violent chemical
change, and should not be stored with water.
In fact, if the chemical is wetted or stored in a very moist environment
it gives off heat and some chlorine gas.
The heat is enough that it could ignite nearby combustibles. However, the chemical by itself is not
Under ammonium nitrate, we read in the PEAC tool that heat
or shock may cause the material to detonate.
Like sodium dichloroisocyanate, ammonium nitrate is also an oxidizer. However, there are many different grades
and variations of ammonium nitrate.
This may be confusing to the PEAC tool user who wishes to select a
particular ammonium nitrate entry from a drop down list. The different grades have different explosive
characteristics. The screen for
ammonium nitrate fertilizer (low carbon content) in the PEAC tool is shown
below. Ammonium nitrate fertilizer is
stable by itself but under conditions of heat and shock it can detonate.
Information about what happens if ammonium nitrate
fertilizer is mixed with sodium dichloroisocyanate (= sodium
dichloro-S-triazinetrione) can be obtained using the PEAC tool. The first step is to click on the reactivity
, which appears in the tool bar shown below.
The next step is to make a list of chemicals that are to be
mixed together, in this case, ammonium nitrate fertilizer and sodium
dichloroisocyanate. The list appears
on the left part of the screen. The
user under “Lookup By” selects “Reactivity Chemicals”. A display, as shown below, appears. The heat generated by the chemical reaction
may initiate an explosion, which is what happened.
Could the chemicals stored at Azote de France (AZF) result
in that much explosive damage? There
was between 200 and 300 tonnes (metric tons) of ammonium nitrate (another
report said 300 tonnes) and 500 kg of
sodium dichloroisocyanate stored in the building. There was more ammonium nitrate elsewhere at the AZF factory, and
there were probably secondary explosions.
The sodium dichloroisocyanate was the source of the detonator. It was the ammonium nitrate that
exploded. We will therefore look at
the explosive potential of 200 to 300 tonnes of ammonium nitrate. One tonne equals 1000 kilograms. We will work in metric units. To use metric units, the PEAC user selects
“Options” under the “edit” toolbar, and then selects “metric” when the PEAC-WMD
Options screen appears
The explosive calculator is initiated by selecting the
icon on the tool
But there is a problem.
The explosive calculator icon does not appear on the PEAC toolbar if
ammonium nitrate fertilizer is selected.
The reason is that the explosive power of ammonium nitrate (in terms of
TNT equivalents) depends upon the form or type of ammonium nitrate,
specifically, how much the material is compressed. All we know that it is off-spec ammonium nitrate fertilizer, it
is in granular form, and was stored flat separated by partitions. This is not enough information to specify an
explosive power of ammonium nitrate in terms of TNT equivalents. The PEAC tool lists many varieties of
ammonium nitrate, most of which will not pull down the
icon because the
description is too vague. What we will
do instead is use the military (U.S. Navy) calculator for explosive ordinance
disposal (EOD). We will select
“Ammonium nitrate (EOD Explosive)” .
This will link up to a military specification of ammonium nitrate which
has a defined TNT equivalent. Ammonium
nitrate fertilizer will probably have a lower TNT mass equivalent because the
bulk material will likely be less dense.
A qualification statement then appears (below, left). The PEAC tool then asks the user to specify
a GPS Location. It is not necessary to
specify a location to do an explosive blast calculation unless we wish to
overlay the results on a map, but we will do it anyway. If 43.572 (see figure for Location of the
Toulouse accident” is entered, the calculator automatically displays as
degrees, minutes, and seconds. If the
GPS measurements are not done remotely, there is no Source Offset.
The weight (mass) of ammonium nitrate is entered. We will use 250 tonnes (= 250000 kg). An overpressure of 0.7 kPa is about the minimum pressure that window
glass breakage can occur.
The distance corresponding to an overpressure of 0.7 kPa is
5415 meters. This is very roughly consistent with the observation that
glass breakage was observed up to a distance of 5 km. The EOD military calculation may have over predicted the distance, but we don’t know the
characteristics of the off-spec ammonium nitrate fertilizer stored in the
warehouse, and we are using a military calculator that has a set TNT mass
equivalence. The TNT equivalence used
in the calculation can be examined by pulling up the chemical information
The TNT equivalent used in the calculation is 0.7, and the
ammonium nitrate is assumed to be compressed to a specific gravity of 1.72
The U.S. military defines hazardous fragments as a fragment
thrown from the blast center having an impact energy of 79 joules (58 ft-lb) or
greater. The range is based on one
hazardous fragment per 55.7 square meters (600 square feet). Hazardous fragments do not include glass
injury from shattered windows which are broken due to blast effects at some
distance from the blast center. The
distance for hazardous fragments is 794 meters
(almost 0.8 km).
The PEAC user can use the drop-down box to calculate
distances corresponding to other overpressures, which are also matched with
A table can then be constructed.
Distance (meters) for 250 tonnes ammonium nitrate
Possible breakage of windows under stress; very loud noise
Minimum safe evacuation distance, possible breakage of
Some minor damage to frame house, typically 10% window
Significant window breakage, other minor damage to frame
Minimum safe building distance for protection against
Partial demolition of houses; skin laceration from flying
Partial collapse of walls and roofs of houses; Eardrum
rupture; significant human lethality from flying glass and missiles
Probable total building destruction; many deaths; lungs
The numbers roughly approximate what actually happened.