Unattend Storage of Explosives in Aluminum Dayboxes Produces Lethal Fragmentation but Limited Overpressure Hazards


The Bureau of Alcohol, Tobacco, Firearms and Explosives authorized State and local law enforcement agencies to store 2.5 lb net explosive weight overnight in official response vehicles under ATF Ruling 2009-3. Further evaluation by the National Association of Bomb Squad Advisory Board has determined that an increase to 5.0 lb net explosive weight is needed to further ensure public safety and expedite response during critical incidents. In evaluating the increase from 2.5 lb to 5.0 lb net explosive weight ATF felt it was critical to understand the impacts to the public in the event of an accidental explosion. A test series was conducted to measure the lethal effects of both overpressure and fragmentation using cast TNT inside aluminum type III day boxes. The overpressures measured were consistent with the Hopkison-Cranz scaling laws and matched well with Kingery Bulmash TNT surface burst model. Hazardous fragmentation distances using the Pseudo-Trajectory-Normal (PTN) method predicted distances comparable to ATF’s American Table of Distances for unbarricaded inhabited buildings and were less conservative than the U.S. Department of Defense (DoD) guidelines for the unintentional detonation of munitions. Considering only consequence, i.e. the probability of event is ignored, these results support the established standoff distances of both ATF and DoD for the accidental explosion of these charges sizes in a type III day box.

Test Date: December 30, 2015

Report Date: June 3, 2016

Performed by Explosive Research and Development Division, National Center for Explosives Training and Research



This report summarizes a study requested by the Explosives Industry Programs Branch (EIPB), Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF), to quantify the hazards associated with detonating 2.5 lb (1.1 kg) and 5.0 lb (2.3 kg) of trinitrotoluene (TNT) in a type III magazine. ATF currently allows State and local bomb technicians to store 2.5 lb net explosive weight11Net explosive weight is considered the weight of explosive excluding packaging. It is not to be confused with the “TNT equivalent weight”. (NEW) overnight in a type III magazine in an unattended vehicle, as defined in ATF Ruling 2009-3 (ATF 2012). The National Association of Bomb Squad Commanders Advisory Board (NBSCAB) requested an increase to 5.0 lb NEW. NBSCAB has expressed concerns that the current 2.5 lb limit is insufficient to adequately respond to critical incidents. This study will allow EIPB to evaluate the impacts of the increase from 2.5 lb to 5.0 lb NEW in the event of an accidental detonation in a public space. This study did not address the probability of the event only what the consequences would be if an accidental explosion were to occur.

The typical hazards associated with an explosion are thermal, overpressure, primary fragments, secondary fragments, and building collapse. This study will only assess the hazards associated with overpressure and primary fragmentation. Both ATF and the DoD have established standards to address safe distances from an accidental explosion.

Under the current ATF standards, a 5.0 lb NEW would have an unbarricaded inhabited building distance (IBD) of 140 ft (ATF 2012). Under this standard IBD is the minimum permissible distance allowed between a quantity of explosives and any building inhabited by the public or where people are accustomed to assemble. This minimum distance provides a high degree of protection against structural damage based on blast or shock wave effects to frame or masonry buildings.

The Department of Defense Explosive Safety Board (DDESB) quantity distances (QD) for unintentional detonation of 2.5 lb and 5.0 lb are 55 ft and 69 ft respectively based on overpressure22\(D=40\sqrt[3]{W}\) were D is the quantity distance in feet and W is the weight of explosives in lb.. For fragmentation DDESB uses the hazardous fragmentation distance (HFD) or 200 ft, whichever is larger, as the minimum safe distance for an accidental explosion. The HFD is based upon a fragment having an impact energy of 58 ft-lbs. (79 Joules) or greater and a debris density less than or equal to one fragment per 600 ft2 (55.7 m2) (DDESB 2007).

For this study, the DDESB HFD was used to assess the hazards associated with fragmentation. The National Fire Protection Association (NFPA) 921 Human Injury Criteria for Direct Overpressure establishes a threshold for fatality due to direct blast effects of 14.5 psi (NFPA 2014). This was the overpressure threshold used in this study.

Only one other reference could be found that allowed for unattended storage of explosives for official response vehicles. The United Kingdom (UK) Army licensing authority allows explosive ordnance disposal (EOD) garaged vehicles to store less than 11.0 lbs (5.0 kg). The positioning of such garages is controlled to minimize risk to personnel outside the garage in the event of an accidental explosion (Goold 1994).

Overpressure Modeling


Initial calculations using the Blast Effects Calculator (BEC) were run on 2.5 lb and 5.0 lb TNT to estimate the distance from the charge to achieve 14.5 psi (DDESB 2008). Table 1 shows the results of these calculations.

Blast effects calculator results
Charge Wt (lb) Distance (ft) Scaled Distance \(\frac{ft}{\sqrt[3]{lb}}\) Time of Arrival (ms) Overpressure (psi)
2.5 11.2