Submunition fills in weapons such as BLU-97/B bomblets pose safety and/or environmental hazards if they end up as unexploded ordnance (UXO) when the munition fails to detonate. Incomplete (low-order) detonation of fills in larger munitions that employ nitramine-based charges could also lead to environmental and safety problems as UXO. A typical submunition fill employing cyclotrimethylenetrinitramine (RDX) as the main explosive charge could result in significant deleterious environmental effects on water supplies. RDX also has high to moderate mobility in soil, and its natural biodegradation is very slow.
The main objective of this Limited Scope project was to ascertain the suitability of explosive cyclic dinitrourea derivatives as “self-remediating” munition or submunition fills in weapons such as BLU-97/B bomblets employed in CBU-87/B Combined Effects Munitions and potentially in many other munition systems that employ conventional nitramines (cyclotetramethylene-tetranitramine (HMX) or RDX) as the main explosive charge. This suitability as a self-remediating munition fill would be demonstrated by quantitative measurements of the stability of specific proposed ingredients under likely environmental conditions under which such munitions might be deployed. The objective would be for such ingredients present in possible UXO to undergo adventitious degradation from environmental exposure—while remaining stable under controlled storage prior to deployment—but alternatively could be intentionally remotely and harmlessly degraded in a simply but properly designed munition configuration.
A potential solution to persistent environmental problems posed by energetic ingredients in UXO is a class of explosive energetic ingredients that have received some development over the last couple of decades: cyclic N,N′-dinitrourea derivatives. Specific examples investigated included tetranitroglycoluril (TNGU) and hexanitrohexaazatricyclododecanedione (HHTDD). A property these compounds have in common that has kept them from widespread adoption by most weapons systems is a property that makes them attractive in specific systems that are susceptible to causing environmental problems as UXO. These materials are significantly more hydrolytically reactive than simple alicyclic nitramines such as RDX. With processing problems technologically solved, these ingredients would offer attractive performance advantages, particularly in the case of HHTDD, which is recognized to be the most powerful easily accessible explosive compound, surpassing even CL-20 in explosive performance while TNGU is comparable in performance to HMX.
The main objective of this SEED project was successfully met by demonstrating the suitability of specific alternative ordnance ingredients—certain explosive cyclic dinitrourea derivatives, HHTDD and TNGU—as “self-remediating” munition or submunition fills in weapons that would be susceptible to UXO formation. These ingredients’ susceptibility to environmental hydrolysis were quantified in order to demonstrate that one or more shows acceptable stability for processing into munitions but also suitable reactivity with environmental elements that they would undergo sufficiently rapid “spontaneous” degradation as to constitute “self-remediation.” The kinetics of ingredient degradation were determined under several environmental conditions (all at ambient 23°C): humidity-controlled laboratory air (~28% RH), artificially humid air (85% RH), moist topsoil (Horizon A with 20 wt% water), and dry (ambient) topsoil. Sample analysis was performed by nuclear magnetic resonance (NMR) spectrometry following an extraction involving ultrasonic dissolution of energetics into acetonitrile-d3, allowing determination of environmental lifetimes of the ingredients under various test conditions.
TNGU and HHTDD were seen to be stable enough in humidity-controlled air to produce and process, with no significant change occurring over a few days. In humid air, however, they degrade over a very few days, and kinetic analysis shows the hydrolysis to be autocatalytic in mechanism. At 85% RH, the time required to degrade by 50% (t50) is 0.95 day for HHTDD and 3.67 days for TNGU. In moist topsoil, HHTDD is fully hydrolyzed in fewer than 19 hours, and TNGU is 99.6% hydrolyzed in 26 hours. Long-term analyses of hydrolysis over several months in dry (ambient laboratory) air as well as in intimate contact with “dry” (ambient) topsoil show extrapolations to 50% degradation (t50) on the order of 6–9 months for both ingredients. Thus, the efficiency of these ingredients’ “self-remediation” is highly dependent on ambient humidity or access to any source of water. It is concluded, therefore, that this class of ingredients, particularly HHTDD and TNGU, does offer suitable replacements for conventional nitramines in venues that are susceptible to UXO formation.
This SEED project constituted a feasibility demonstration that certain examples of cyclic dinitroureas are potentially attractive “self-remediating” munitions ingredients. Further process development would be needed in order to develop them as replacements for current problematic formulations.
This elucidation of degradation kinetics for explosive compounds may lead to a requirement for design and incorporation into munitions hardware of a mechanism whereby the explosive fills can be alternatively degraded in an environmentally friendly manner. Such a mechanism could be as simple as exposing the fill to a reservoir of water (or suitable aqueous solution) on nondetonating impact.