Statement of
J. Edmund Hay
U.S. Bureau of Mines
Research Physicist
Pittsburgh Research Center
before the
Committee on the Judiciary
U.S. House of Representatives
June 13, 1995
Mr. Chairman, members of the Committee, my name is Ed
Hay. I am a Research Physicist with the Fires, Explosions,
and Explosives Group of the Pittsburgh Research Center of
the U.S. Bureau of Mines. I am also chairman of the National
Fire Protection Association's Technical Committee on
Explosives and a member of Committee F-12 (Security Systems)
of the American Society for Testing and Materials.
Since its inception in 1910, the Bureau has been
charged with the responsibility of performing research to
improve safety in the mineral industries, particularly with
regard to accidental explosions. In pursuit of this mission,
the Bureau has a long history of research, testing, and test
development related to the explosive behavior of chemicals
that are either explosive ingredients or that have similar
properties. This especially includes ammonium nitrate, which
is not only a major ingredient of virtually all nonmilitary
explosives, but which has a long history of involvement in
accidental explosions.
I would like to offer for you today some observations
relative to two areas of the proposed legislation of H.R.
1710, namely:
o The proposal to render common chemicals used in
manufacturing inert as explosive ingredients and
o The subject of explosive taggants.
Ammonium nitrate is the least expensive and most widely
available used ingredient in commercial explosives. It is
available in two basic forms:
o One intended for use as a blasting agent ingredient, and
o One intended for use as fertilizer.
The principal difference is the porosity of the
granules, or "prills." It is probable that any technical
modification of the explosive-grade material to diminish its
usefulness to terrorists would have equally deleterious
effects on its legitimate use. The only practical solution
to the control of explosive-grade ammonium nitrate,
therefore, appears to be outright legislative control of its
availability. Thus, I prefer to confine my remarks to
agricultural-grade ammonium nitrate.
In this regard, there are two important considerations. The
proposed modification of the product should:
o Render it useless to the potential terrorist, and
o Cause no severe hardships for the legitimate user.
It is well established that dilution of explosives
diminishes their detonability. This fact has been, and
continues to be, exploited. There is little question that it
can be applied to ammonium nitrate and its mixtures. I use
the word "diminish" rather than Eliminate," because the
detonability of a substance depends both on the quantity of
the substance under consideration and the intensity of the
stimulus to which it is subjected, and it is thus not a
uniquely determinate property. The object is to determine a
diluent and a level of dilution that minimizes its
usefulness as an explosive while retaining its benefit as a
fertilizer. This must be done with great care, as evidenced
by an attempt in Germany in the 1920's to add 45% ammonium
sulfate to ammonium nitrate to make it inert. This reduced
the nitrogen content by 18%, yet this combination still
produced the greatest single historical disaster involving
ammonium nitrate in which the German town of Oppau was level
led. There have been several proposals to incorporate
various ingredients to reduce detonability. Although I do
not claim to know all of them, I have noticed that, in some
instances, the data supposedly substantiating their
effectiveness are based on tests with rather small charge
sizes and initiating stimuli. Such attempts can be
misleading if not validated by appropriate testing.
The quantity of product and number of legitimate users
of agricultural ammonium nitrate are very large. Many of the
users may be presumed to be vulnerable to the economic and
other impacts of degradation of an essential commodity.
Bearing this in mind, the Bureau of Mines has been
considering the potential dilution of ammonium nitrate by a
chemical that is itself a high nitrogen fertilizer, namely
urea. Urea contains 46% nitrogen, while ammonium nitrate
contains 34%. Our data indicate that, at sufficient levels
of dilution, the detonability of ammonium nitrate is greatly
reduced. We look forward to other studies on the potential
of urea and other diluents.
One might ask why the simple replacement of ammonium
nitrate by urea in this application is not completely
satisfactory. The answer is that, in certain widely
prevalent agronomic applications and conditions, urea loses
a significant fraction of its nitrogen before it is absorbed
into the soil and is thus somewhat less cost-effective.
Further assessment of the agricultural aspects of this
issue, coupled with a review of the detonation aspects, is
warranted by appropriate experts.
Another important point is that ammonium nitrate, or other
potential explosive ingredient, should not be readily
separated from the diluent. Urea is normally sold in a form
in which it is physically practically indistinguishable from
ammonium nitrate. It also has very similar solubility in
water, so that neither the physical separation process nor
methods based on differential solubility are practical for
separating the two ingredients. A careful review of all
possible
separation schemes needs to be conducted before such an
approach would be considered for final implementation. I
should note here that we are considering a simple mechanical
mixture of the individual prills of the two substances, not
a mixture in which the prills themselves would be composed
of a more intimate mixture.
The proposal to use urea may seem paradoxical to those
who are aware that, in concentrations below 20%, the
addition of urea actually increases the detonability of
ammonium nitrate, but at higher concentrations this effect
is reversed. Once an explosive mixture reaches the condition
at which the fuel content exceeds that which the available
oxygen can oxidize, further additions of fuel diminish the
detonability. In ammonium nitrate/urea, this occurs at 20%
urea. Thus, a nondetonable mixture could not be made
detonable simply by addition of fuel, as in the case of pure
ammonium nitrate.
Existing data on the effect of urea pertain only to
small-diameter charges. These data need to be extended to
much larger charge sizes and much larger initiating stimuli
to establish a level of dilution at which detonation would
require such a large quantity of material and/or such a
large stimulus as to be beyond any practical limit. This
applies to any attempted diluent, not just urea.
It may be objected that ammonium nitrate could still,
with patient effort, be extracted from the mixture and used.
I would respond that this is true. It is equally true that
with patient effort explosives can be made from chemicals
that are so common that restricting their availability is
beyond the bounds of reason. In other words, there is no
absolute solution to the problem. We can only strike a
balance between making life more difficult for the terrorist
and making life too difficult for ourselves.
It has been said that urea seems to be a strange choice
because it was used in the World Trade Center Bombing.
Although true, the urea in this case was combined with
nitric acid, and if one postulates the availability of
nitric acid, the opportunities for making explosives are so
great that I would not attempt to catalog them.
A key point here is that the level of effort required
for the potential terrorist should increase with the
magnitude of the intended explosion. At present, disaster
comes ready-made, since ammonium nitrate is detonable in its
own right without any added fuel. The fact that the
explosive energy release of pure ammonium nitrate is less
than one-half that of the normal mixture with fuel oil is of
little comfort. All that is required to detonate a storage
pile, truckload, train load, or shipload is the insertion of
a sufficiently large explosive booster. This scenario at
least would be impossible if the ammonium nitrate were
diluted at the point of manufacture.
As I indicated before, at this point the precise level
of dilution required is unknown. There is considerable
evidence that the detonability of a substance is strongly
correlated with the energy that it would release if
detonated, a value that is easily calculated. There is no
known instance of a substance detonating that has an energy
of less than about 200 calories per gram. An adequate
margin of safety would probably be about 150 calories per
gram. The level of dilution corresponding to this is about
58% urea. This, however, should be verified experimentally.
The Bureau of Mines has previously been involved in
programs of technical evaluation of the feasibility of
explosive tagging, and our experience in this area is at
your disposal. And, of course, we are willing to produce any
assistance that we can to the Department of Justice, the
Bureau of Alcohol, Tobacco and Firearms, and other
Government agencies in studying taggants and the feasibility
of rendering common chemicals inert.
Thank you, Mr. Chairman, for the opportunity to testify
before you and for your attention. The Bureau of Mines
remains ready to assist you with those technical matters in
which we have expertise. I am prepared to respond to any
questions that you may have.
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