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.

1  

 See, for example, Yancik (1960), p. 89.



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OCR for page 341
Containing the Threat from Illegal Bombings: An Integrated National Strategy for Marking, Tagging, Rendering Inert, and Licensing Explosives and Their Precursors 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. 1    See, for example, Yancik (1960), p. 89.

OCR for page 341
Containing the Threat from Illegal Bombings: An Integrated National Strategy for Marking, Tagging, Rendering Inert, and Licensing Explosives and Their Precursors MATERIALS AND RECOMMENDED TESTING ENVIRONMENT Container—schedule 40 steel pipe, 12 inches (30 cm) in diameter, 5 ft (1.5 m) in length, with a 1/4-inch (6-mm) steel plate welded to one end to serve as a witness. The witness plate should have a hole in one corner beyond the perimeter of the pipe for attachment of a cable to enable its retrieval when used in the underwater mode. The plate should also have a hole within the perimeter of the pipe, for insertion of a continuous-detonation-velocity probe. (This size pipe has the following characteristics: internal diameter, 11.938 inches [303.2 mm], weight, 54 lb/ft [80.4 kg/m], capacity of material, 39.74 lb/ft [59.1 kg/m] of material of specific gravity 0.82. A test charge as described would thus weigh approximately 470 lb [213 kg] [not including the witness plate], of which 200 lb [91 kg] would be test material.) Instrumentation—continuous-detonation velocity probe within the mass of test material. The instrumentation should have the capability of generating a printout for later reference. Booster—cast pentolite, 5-lb (2.26-kg) weight. (This is a commercially available item.) Test site—possibly underwater, to contain shrapnel. (Some sites may be able to run the test in air if suitable instrumentation shelters ["bombproofs"] are available.) Underwater sites could also determine shock and bubble energy for further refinement of the data. PROCEDURE Insert the continuous-detonation velocity probe through the hole in the witness plate, and bring it out of the top of the pipe. Mix No. 2 diesel fuel oil with the fertilizer material, in the amount of 2 percent by weight. Mixing can be carried out in a cement-mixer apparatus or in large containers using plastic or wooden shovels for agitation. The percentage of oil is arbitrary, based on the following reasoning: the oxygen balance of the various fertilizer mixtures likely to be tested may be difficult or impossible to determine; thus it may not be possible to calculate the proper amount of fuel to achieve a stoichiometric mixture. It has been determined that with particulate oxidizers, such as ammonium nitrate prills or grained ammonium nitrate, the greatest sensitivity is achieved with underoxidized mixtures. Thus, although ammonium nitrate/fuel oil has the best energy output, detonation velocity, and after-blast fumes at 5.7 percent fuel oil, it is most sensitive to initiation at lower percentages of fuel oil. It is thus thought that settling on 2 percent fuel oil for all fertilizer detonation tests is reasonable as a standard. Load the fueled fertilizer material into the pipe, tapping gently to eliminate voids. Weigh the material emplaced, and calculate the specific gravity.

OCR for page 341
Containing the Threat from Illegal Bombings: An Integrated National Strategy for Marking, Tagging, Rendering Inert, and Licensing Explosives and Their Precursors (Note: the material must not be tamped or otherwise compressed. Insensitive granular materials are easily "deadpressed," and this must be avoided. The light tapping on the side of the pipe will result in what is known as "poured" density in explosives industry testing.) Place the booster in the top of the pipe, with the initiation system inserted. Attach the continuous-detonation velocity probe to the booster. The initiation system may be of the electric or shock tube variety. For underwater testing, enclose the entire unit in a waterproof plastic wrapping. Attach a strong cable ("airplane" cable should be suitable) to the hole in the outer corner of the witness plate, and secure the other end of the cable to an anchorage on the shore of the test pond. Fire the charge, and record the detonation trace and the appearance of the witness plate, if recovered. To ensure accuracy and consistency of results, the test should be repeated three times. TEST INTERPRETATION Detonable fertilizer mixtures will be indicated by a continuous-detonation velocity probe trace that shows a sustained, steady-state or an increasing velocity from the booster area to the end of the pipe, and/or a retrieved witness plate that has a hole punched through it, or has deep cupping. Nondetonable mixtures will be indicated by a continuous-detonation velocity probe trace that shows a decreasing velocity, or velocity falling to zero, and/or a retrieved witness plate with no deformation or only slight denting. Borderline results should be interpreted as detonations, since this may indicate that a slightly larger charge would sustain detonation. SUMMARY The above test, although it may be difficult to administer and may even be impossible to run at all test sites, should determine definitively whether or not a particular fertilizer mixture poses a potential threat in the hands of a person attempting to construct a destructive device. This test should have to be run only one time (in triplicate, as noted above) on each "family" of fertilizer materials. Although some may consider that a larger mass of material might sustain a detonation even though the material was nondetonable in the test with a 12-inch-diameter pipe, the committee believes that potential explosive mixtures of this low level of sensitivity pose little threat to the public.