Explosives come in various forms, most commonly referred to as Hazard Divisions (HD). What categories of explosives exist and how are munitions assigned into the various groups?
This article provides an introduction into explosives hazard classification and associated testing. This material is presented from a US DoD perspective. Joint Technical Bulletin TB 700-2, “Department of Defense Explosives Hazard Classification Procedures” is the primary source for US DoD hazard classification procedures for substances and specific Ammunition & Explosives (AE). The Department of Defense Explosives Safety Board (DDESB) serves as the final technical review authority.
Types of Explosives Safety Hazards
There are many types of hazardous substances, with explosive materials being just one of these categories. A numbering system is used to classify items (e.g., 1.1, 1.2.1, etc.). The first number is the Hazard Class of the item. The second number is the HD, and subsequent numbers are subdivisions where applicable.
The DoD, United Nations (UN) and Department of Transportation (DoT) hazard classification systems define explosives as Class 1. Other classes are defined by their predominant hazard. These other classes include:
Class 2 Gases
Class 3 Flammable liquids
Class 4 Flammable solids
Class 5 Oxidizing substances, organic peroxides
Class 6 Toxic and infectious substances
Class 7 Radioactive material
Class 8 Corrosive substances
Class 9 Miscellaneous dangerous substances and articles
Within Class 1, there are currently six divisions (HDs). HDs 1.5 and 1.6 are rare and are not commonly included in explosives safety planning.
1.1 - Mass Explosion
1.2 - Non-Mass Explosion, Fragment Producing (Subdivisions 1.2.1, 1.2.2 and 1.2.3)
1.3 - Mass fire, minor blast or fragment
1.4 - Moderate fire, no blast or fragment
1.5 - Explosive substance, very insensitive (with mass explosion hazard)
1.6 - Explosive article, extremely insensitive (no mass explosion hazard)
Qualified hazard classification authorities, within the DoD or in the private sector, can produce Interim Hazard Classifications (IHC) and Final Hazard Classifications (FHC) based on testing or analogy to similar items. The Joint Hazard Classification System (JHCS) is the authoritative record for DoD classifications for transportation or storage of ammunition and related items.
Explosives Safety Hazard Classification Testing
So how are materials and AE classified? If a similar item does not exist and if the worst-case default classification (HD 1.1) is not acceptable, physical testing must be accomplished. This process follows a logical, results-based testing process to identify whether specific hazards are generated in the event of an initiation of an item. The testing process follows UN test series procedures.
UN Series 1 - Detonability of New Substances
UN Series 2 - Insensitivity Tests for New Substances
UN Series 3 - Hazard Tests for Handling and Transporting New Substances
UN Series 4 - Hazard Tests for Handling and Transporting New Articles, Packaged Articles & Substances
UN Series 5 - Hazard Division 1.5 Tests
UN Series 6 - Hazard Classification Tests (1.1, 1.2, 1.3, 1.4)
UN Series 7 - Hazard Division 1.6 Tests
UN Series 1 through 4 serve to determine if a substance or article is a Class 1 explosive. These tests include various shock, impact, friction and thermal stability tests. If a material or item can be shown to be other than Class 1, this can greatly impact how the item is shipped, stored and handled. Some of these tests (Series 1 and 2) are optional, and some are required (Series 3). Series 4 tests are used to determine if the final packaged configuration (substance or article) is safe to transport over public roads. This test series includes thermal stability testing and drop impact testing. An item could be an effective explosive article but not be safe enough to be practical for transport and storage.
UN Series 6 Testing – Assigning an HD
The focus of this article is UN Series 6 testing, to determine the HD of a Class 1 material. (HD 1.5 and 1.6 materials require unique testing (UN Series 5 or 7) and won’t be discussed here.)
The UN Series 6 testing subjects an item to a sequence of input stimuli and intentional initiation scenarios to see if various explosive hazards (e.g., overpressure, debris) are generated and to what extent. If an item fails an initial test (meaning that it generates the hazards) it is classified as HD 1.1. Items known or expected to be HD 1.1 can be classified as such without the need for testing, if desired.
If an item can pass the sequence of tests (meaning that some hazards are not generated or generated to an acceptably low level) the item can be considered for lesser HDs. The testing sequence proceeds until a classification is made.
The first test is a Single Package Test (UN Test 6(a)). This test ignites or detonates a single item configured in its transportation or storage configuration. This test seeks to determine if effects of the test are contained within the package. This test is conducted in unconfined (open air) and confined (typically buried) configurations.
If hazards or effects can be seen outside of the package (ranging from damage to surrounding items up to full destruction of the item and package) the sequence proceeds to a Sympathetic Reaction (SR) Test (UN Test 6(b)). This test uses a specifically configured stacked group of items to determine if one initiated item will cause adjacent rounds to also initiate or detonate. One donor item is used and is surrounded by multiple acceptor items. The test is repeated for unconfined and confined configurations. The item passes the test if less than 50% of the acceptor items simultaneously detonate. If more than 50% detonate, the item fails the test and is classified as HD 1.1.
In the event that the Single Package Test showed no external effects, or the Sympathetic Reaction Test showed that less than 50% of the acceptor items detonated, the testing proceeds to the Liquid Fuel/External Fire (LF/EF) Test (UN Test 6(c)). This test places at least three test articles above a large basin of fuel. A fire is started below and around the acceptor items and is maintained for a prescribed duration at a minimum temperature. An example of this type of test is shown below.
This test is surrounded by witness plates, blast pressure gauges and thermal gauges to measure the effects of an initiation. Data is gathered and reviewed by the test experts to determine if any initiation event could be considered a detonation. In general, if more than 50% of the AE is shown to detonate, the item is classified as HD 1.1. If less than 50% of the AE detonates, the item is not considered HD 1.1 and proceeds to further analysis.
Testing for HD 1.2, 1.3 and 1.4
If an item passes all UN Series 6 tests and is confirmed to not be classified as an HD 1.1 item, the results of previous tests are analyzed to determine the final HD. If any previous test generated “projections” (i.e., debris) exceeding a set energy threshold (20 joules - based on fragment weight and throw distance) and any type of partial or full detonation occurred, the item is classified as HD 1.2. Within this HD, the size of the individual item and the number of items detonated in various tests determine the subdivision and corresponding Maximum Credible Event (MCE) characteristics where applicable.
If it can be shown that either projections (greater than 20 joules) are not generated or that true detonation (a blast wave exceeding the speed of sound) did not occur, the item is then considered for HD 1.3 or 1.4 classification.
The classification as either HD 1.3 or 1.4 depends on the level of thermal effects, including fireball or flame jets and fiery projections. If these effects do not exceed established thresholds the item is classified as HD 1.4. If testing shows that the thermal effects are NOT apparent outside of the package the item can be further downgraded to HD 1.4S requiring no Quantity-Distance (QD).
Importance of Understanding Hazard Classification Testing
Understanding how hazard classification testing works can help site planners gain a better appreciation for what the various HDs mean, and what they do not mean. Typically, when we learn explosives safety site planning by studying the site planning criteria manuals (e.g., DoD Explosives Safety Regulation – DESR 6055.09) we can be given the impression that the HDs are very distinct groups. While these categorizations have significant differences from a criteria standpoint and required standoff distances, the difference between two items in different HDs can be very small in terms of actual physical reactions and explosives hazards.
A few examples can illustrate how explosives hazards of various HDs can be over or underestimated.
The difference between HD 1.1 and HD 1.2 is primarily determined by the amount of material that is found to sympathetically detonate in either a Sympathetic Reaction Test or a Liquid Fuel/External Fire Test. An AE item that is shown to have slightly more than 50% of the items detonate is classified as a full HD 1.1 item. From that point forward, from a siting criteria standpoint all adjacent items must be assumed to completely detonate in the event of an accident, and all items throw hazardous fragments to full extent of the HD 1.1 criteria set. However, if during testing slightly less than 50% of the items sympathetic detonate, the item is allowed to be classified as HD 1.2 (and then subclassified further), with not all items detonating, and often with reduced fragment distances. This has significant impacts in the required standoff distances.
HD 1.3 items have been shown through testing to not sympathetically detonate, though they can propagate through burning reactions. However, the difference between a violent, non-supersonic overpressure event (considered a deflagration) and a low-level supersonic overpressure event (considered a detonation – HD 1.1 or 1.2) can be very small. HD 1.3 events can be very violent, though the siting criteria only require minimal distances (e.g., 75 ft Inhabited Building Distance for up to 1,000 lbs) and no direct consideration of fragmentation distances. The siting does, however, require HD 1.3 to be treated as HD 1.1 in extremely confined conditions. HD 1.3 material is also required to be treated as HD 1.1 if stored in the same location (i.e., it is assumed to detonate if in an event with other detonating material).
HD 1.4 is considered a standalone explosives group in terms of explosives mixing rules. This means that if HD 1.4 is stored with other HDs, the weight of 1.4 does not have to be aggregated with the weight of other, higher HDs. This implies that the 1.4 material would not contribute to a larger detonation event, even if in a confined environment. However, the hazard classification testing does not directly test for this behavior. HD 1.3 is assumed to contribute to a detonation, whereas HD 1.4 is not. But the main difference between classifying HD 1.3 and 1.4 is the analysis of various thermal effects from burning tests.
In summary, understanding the actual characteristics tested in the UN test series can help a site planner better appreciate the actual potential effects of AE material, beyond the basic HD classifications found in the criteria manuals.
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