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Strategy to render bulk AN inert
In principle, desensitization or inerting can be done in three general ways:
(1) by changing a material's physical form to make mixing less efficient to or make the material less sensitive to initiation, for example, separating the fuel from the oxidizer so that the concentrations within the reaction zone are not balanced chemically to support detonation;
(2) by diluting the explosive material with an inert additive that will take energy from the chemical reaction, possibly leading to failure of a detonation or to a lower explosive yield; and
(3) by combining the material with an active additive that will catalytically interfere with the detonation process, much as fire retardants are commonly added to textiles and polymers to reduce their potential to burn. There is a great attraction to the search for an additive that could, when added in small concentrations, render energetic chemicals inert to detonation.
An extensive British research effort over the last decade has focused primarily on inerting AN by diluting it with similar, but inert, fertilizer ingredients. However, no practical system for inerting bulk AN has yet been found.
Beginning in 1972, regulations in Northern Ireland limited the availability of sodium chlorate and nitrobenzene, and restricted the manufacture, sale, and purchase of fertilizer to formulations containing no more than 79 percent AN. Following these actions most of the AN fertilizers used in agriculture contained dolomitic limestone or chalk as additives (in amounts of 21 percent). This fertilizer mixture, known as calcium ammonium nitrate (CAN), was soon adopted by terrorists for use in making bombs, several of which have been set off with devastating effects in Northern Ireland and in London. Although the 1972 regulations do not prevent AN bombings, they do make the construction of AN bombs somewhat more difficult.
Regulations in South Africa classify porous prilled AN as an explosive, raising its cost and effectively eliminating its use as a fertilizer. As a result, the agricultural and commercial mining industries in South Africa use lime ammonium nitrate, which is not regulated (Rorke et al., 1995). Like CAN, lime AN contains about 20 percent calcium carbonate (limestone) intimately mixed with the AN and manufactured to have little porosity. Lime AN can be combined with roughly equal weights of undiluted prilled AN and fuel oil and used as a material similar to regular ammonium nitrate/fuel oil for blasting. Although this combination is chosen for reasons of economy, its use suggests that the desensitizing additives do not materially degrade the performance of the explosive under all circumstances.
Changing Ammonium Nitrate's Physical Form
The physical form of an AN prill can be altered by changing properties such as particle size, density, crystal structure, or porosity. A hard, dense prill (or a prill with a nonporous outer shell) is more difficult to detonate than a low-density porous prill. Thus, decreasing AN particle porosity, perhaps through adjustments made in the prill manufacturing process, can desensitize the material (Hopler, 1995). Although a change in morphology may make detonation more difficult, it does not necessarily make it impossible. Nonporous fertilizer-grade AN prills may still be detonable in large charges. In addition, terrorists can make dense AN prills more easily detonable by simple (if tedious) means.
Diluting Ammonium Nitrate
The problems with diluting AN can be understood quite easily by looking at extreme cases. A slight dilution (for example, adding 1 part diluent per 99 parts AN) would likely have an insignificant effect on AN's function as a fertilizer. However, such a small dilution similarly would have very little effect in reducing the detonability of an AN/fuel mixture. Clearly, slight dilution of AN is not effective in inerting.
On the other hand, a drastic dilution (for example, a mixture of 99 parts inert diluent to 1 part AN) would certainly not be detonable, since the active components (the AN molecules) would be too widely separated in the mixture to sustain a detonation. Of course, this highly dilute mixture would also be virtually useless as an AN fertilizer, and so drastic dilution clearly is not practical.
Between the extremes of slight and drastic dilution, both of which present problems, there may exist a mixture that under most circumstances is nondetonable but still is useful as an agricultural fertilizer. To the committee's knowledge, no such mixture exists that has a diluent concentration of less than 20 percent. It is likely that mixtures exist with a diluent concentration of 50 percent or more that are nondetonable under most circumstances. However, unless the diluent were an equivalent agricultural fertilizer, up to two times as much product would have to be used to yield the same agricultural benefit to farmers.
Limitations of the Porter Patent
Samuel J. Porter's 1968 patent claims to render fertilizer-grade ammonium nitrate resistant to flame and insensitive to detonation by the addition of specific amounts of ammonium phosphates and small amounts of potassium chloride or ammonium sulfate. It is interesting to note that Porter's intention seems to have been to reduce accidental detonation of fertilizer-grade AN, primarily as a result of initiation by fire, rather than to prevent intentional detonation.
The most straightforward desensitization scheme from the patent is the mixture of 10 percent ammonium phosphate and 90 percent ammonium nitrate. The patent claims that this AN mixture (when mixed with 5.5 percent fuel oil) is nondetonable under specific test conditions (i.e., when tested in a 4-inch-diameter by 10-inch-long cardboard container holding approximately 3 pounds of material and initiated by a No. 8 blasting cap or a blasting cap plus 24 inches of 50-grains-per foot detonating cord).
Following the 1995 bombing in Oklahoma City, additional tests were performed to evaluate the claims of the Porter patent (Eck, 1995). These tests showed that mixtures of AN with diammonium phosphate, claimed by Porter to be nondetonable, would detonate when tested in larger amounts and with greater confinement (in 6-inch-diameter steel pipes or in 80-pound quantities). The tests quoted in the patent were performed on too small a scale and with insufficient confinement to predict whether the mixtures were, in fact, detonable or not in sizes and conditions likely to be found in an illegal bombing situation.
Based on its examination of efforts abroad to render ammonium nitrate inert, the claims of the Porter patent, and its own knowledge and experience, the committee concluded that there is no established technical basis at this time to recommend a method for inerting bulk AN. To the committee's knowledge, no approach yet proposed—such as dilution of AN by 20 percent with inert additives such as limestone—achieves the desired inerting of AN, while preserving its utility as a fertilizer for use in agriculture.
Alternatives to Inerting—Limiting Access and Availability
Retail Sale of Packaged Ammonium Nitrate Fertilizers
Approximately 90 percent of all fertilizer-grade AN is shipped as prills and used as a bulk material. Of the 10 percent that is sold in packaged form, only half is bagged at the production site; the other half is bagged at subsequent points in the distribution system (IFDC, 1997), either as a mixture with other fertilizer ingredients or as pure ammonium nitrate. Much of the distribution and end-point sale of bulk AN occurs through agricultural distributors who are likely to know their customers or keep business records of the sale, thus potentially preventing untraceable large-scale sale of AN to terrorists. The small-scale retail fertilizer market, on the other hand, is a commercial source where AN can be purchased without purchaser identification or retailer record keeping. It is unlikely that records exist for purchases of AN from these sources, which include home improvement centers and discount retailers, where a potential terrorist might buy AN for the production of a large bomb.
The committee believes that obtaining pure prilled AN from these sources can be made more difficult without causing undue effects on the marketplace. Much of the fertilizer sold at the retail level is already blended and is likely nondetonable . The nondetonability of such mixtures could be established by following a suitable test protocol. Probably many fertilizer mixtures could be certified as nondetonable by analogy to similar mixtures with the same ingredients and with the same or lower concentration of AN, as is done in the Department of Transportation's classification of materials for transport (United Nations, 1995). Retail purchase of pure, packaged AN fertilizer could still be allowed, provided that purchasers provided identification and records of the sales were maintained.
Sale of Explosive-grade Ammonium Nitrate for Fertilizer
In considering the question of whether the markets for explosive-and fertilizer-grade ammonium nitrate should be kept separate, the committee observed that the prilled ammonium nitrate used by the fertilizer industry can also be used by a determined bomber. With respect to explosive performance, the basic difference between low-density, explosive-grade prills and high-density, fertilizer-grade prills—assuming the same prill particle size—when formulated as ammonium nitrate/fuel oil (ANFO) is (1) minimum charge diameter (explosive-grade has a smaller minimum diameter than fertilizer-grade), and (2) detonation velocity (fertilizer-grade AN has a lower detonation rate, at least in charges close to the minimum diameter). This implies a lower detonation pressure for fertilizer-grade AN.
It has been shown that ANFO made from modified fertilizer-grade prills can be made to have explosive characteristics comparable to those achieved with explosive-grade prills. Even unmodified, fertilizer-grade AN mixed with fuel has been demonstrated to be detonable (Hopler, 1961). Therefore, there would be little public safety benefit in excluding explosive-grade AN from the fertilizer market.
Testing for Detonability of Inerted Bulk Fertilizer Mixtures
Unfortunately, questions about the detonability of various ammonium nitrate mixtures cannot be answered easily. Because ANFO, a so-called nonideal explosive, consists of a separate fuel (usually fuel oil, but possibly other carbonaceous materials) and an oxidizer (ammonium nitrate), it releases its energy more slowly over a longer period of time than does an "ideal" explosive such as TNT. In addition, the behavior of a nonideal explosive does not scale linearly with the mass of the explosive mixture (Cook, 1958). Thus, even though tests on a smaller charge mass indicate that a mixture will not detonate, a large charge of an AN mixture can in fact detonate (Eck, 1995). This nonideal behavior of AN has been the source of much confusion concerning the applicability of the claims of the Porter patent (see "Limitations of the Porter Patent" above).
Small-scale tests currently are used by industry to assess the detonability of explosive mixtures. In addition, it will be necessary to have a standard test protocol to evaluate the detonability of any proposed, inerted bulk fertilizer mixtures, whether they are based on ammonium nitrate or other ingredients, under the conditions likely to apply in large-scale bombings.
Tests to evaluate the detonability of bulk fertilizer mixtures and proposed inerting schemes should be performed at a sufficiently large scale to ensure that the conclusions will also hold true for car or truck bomb quantities (approximately 80 to 5,000 pounds). They must also employ a booster charge of sufficient size to adequately test the detonability of candidate inerted materials. A booster of several pounds would be typical for testing car-or truck-bomb quantities of candidate materials. For a nonideal explosive, the minimum, or critical, diameter of a cylindrical explosive charge that will detonate may be relatively large (e.g., 2 inches or more). It is important, in evaluating the detonability of a candidate inerted material, that the experimental tests be performed on charge sizes larger than the critical diameter. A suitable container must also be used to ensure adequate confinement.
Appendix H describes a proposed test protocol supplied for illustrative purposes. This test, or any other that is proposed, must be experimentally validated to confirm that it correctly predicts the detonability of known and candidate (inerted) formulations. Such a test should serve to uniformly indicate whether mixtures pose a potential threat in the hands of a person attempting to construct an illegal explosive device. Once a suitable test has been developed, many organizations should be capable of running it without difficulty (see Appendix I).
H Test to Evaluate Detonability
An example of a standard test protocol for evaluating the detonability and destructive capacity of bulk ammonium nitrate-based fertilizer mixtures is given below. Small-scale tests are currently used by industry to assess the detonability of explosive mixtures. However, no standard test protocol is available to test the detonability of bulk fertilizer mixtures under the conditions likely to be used in large-scale bombings.
It has been determined through years of design and testing of explosive materials such as water gels and blasting agents that some of these require emplacement in containers or boreholes of large cross-sectional area (i.e., must have a large minimum diameter) before they will sustain a detonation reaction. This is a good safety feature for commercial applications, assuming that the minimum diameter is less than the diameter of the boreholes being drilled at a mine. An attractive explosive product is one that will detonate in a borehole of a certain size but will be incapable of sustaining detonation in the smaller diameter of a pumping apparatus or the hose used to place the explosive in the borehole.
The material used by the explosives industry to simulate borehole conditions is schedule 40 steel pipe. Many tests have shown that this pipe provides the same detonation conditions in the same diameter as a competent rock borehole,1 as evidenced by the achievement of the same detonation velocity in both media. Testing in other forms of confinement such as stovepipe, cardboard tubes, or tile pipe has required far larger diameters to achieve the same detonation velocity, or indeed any detonation at all.
Research to Develop Methods of Inerting
Although no effective inerting or desensitizing methods have yet been found for use with bulk AN, research should be conducted to ensure adequate options for action in the event that terrorist bombings with AN become more frequent.
Although a number of common chemicals could be used in illegal bombings, the common explosive chemical likely to be of greatest threat is ammonium nitrate. The committee's qualitative ranking of common explosive chemicals, based on availability and accessibility, ease of bomb making, cost, and history of prior use, indicated that ammonium nitrate (AN) is by far the most obvious material for making large bombs.
Despite ongoing research in both the United States and abroad, no practical method for inerting ammonium nitrate has yet been found. No additive (such as claimed by the Porter patent) has been shown to be capable of rendering fertilizer-grade AN nondetonable under all circumstances when the additive is present in concentrations of about 20 percent or less. The present state of knowledge identifies neither the additive nor the critical levels of inertant needed to guarantee nondetonability. High concentrations of inertants may not be practicable, because of both their cost and their deleterious effect on the utility of the fertilizer.
Other inerting additives/experiments you should be familiar with is:
Coal combustion byproducts (CCBs, Fly-ash C, Fly-ash F and FGD) were evaluated for their effectiveness as blast mitigating agents when applied as a coating to CAN fertilizer. 5 kg ANFO bombs confined in steel containers were prepared coated with 10-50% of the inerting compounds. Tests showed that they had to use a 15% or more of the additive to prevent detonation. When the AN prills were crushed into powder, they had to use 20% or more of the substance to prevent detonation. Conclusion: failure to efficiently inert.
1. Fertilizer-grade AN fertilizer labeled as 34-0-0 in hardware store nomenclature is 34 percent nitrogen by weight. Most small-scale retail fertilizers intended for home and garden use typically include some fraction of phosphorus and potassium, represented by the other two digits in the fertilizer marking system.
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