Recommendation 2-1. Blue Grass Chemical Agent-Destruction Pilot Plant program staff should expedite the process required to provide the hazard classification of the separated rocket motors.
The aging and degradation of the M28 propellant could cause it to have increased sensitivity to impact, shock, and thermal conditions. There were four pressure-pulse events when the motors of M55 rockets were cut at the Umatilla Chemical Agent Disposal Facility. There have also been over 20 fires when rocket motors were cut at incineration-based chemical agent disposal facilities (CDC, 2006). Although the separated rocket motors at the Blue Grass Army Depot will not be cut, those incidents indicate some sensitivity of the propellant, which could be a factor in disposing of the separated rocket motors. Because of the potential severity of incidents arising from propellant sensitivities, the Department of Defense (DoD, 2008) and the Department of Transportation (49 CFR 173.56) have instituted policies for handling these types of materials.
Nitrate esters, such as the nitrocellulose in the M28 propellant, degrade slowly and liberate nitrogen dioxide (NO2).4 One mechanism for that is the breaking of the carbon monoxide–nitrogen dioxide (CO–NO2) bond in the nitrocellulose, which is thermally labile and can be broken under storage-temperature conditions. If liberated NO2 does not react with the nitrate ester (the propellant), it can react with water in air to form acids, which will degrade nitrate esters further. For instance, NO2 is a strong oxidizer and can react with the nitrocellulose or abstract hydrogen from the nitrocellulose to produce nitrous acid (HONO). The CO–NO2 bond may also be hydrolyzed to form nitric acid (HNO3). And the degradation of the propellant can be catalyzed by the presence of bases and metals. Finally, the overall chemical reaction is exothermic (it generates heat), and can catalyze degradation further. In other words, the degradation of the nitrate esters in the M28 propellant is accelerated by its own degradation product (NO2). If the degradation reaction rate becomes high enough, the nitrate ester will self-initiate, and this can lead to ignition, deflagration, or detonation.
Standard practice is to avoid the undesirable consequences of the runaway reaction by adding an NO2 scavenger, commonly referred to as a stabilizer. The stabilizer does not contribute substantially to the energy delivered by the propellant when used for its intended purpose, so quantities of stabilizer used in propellants are limited. Over time, the stabilizer becomes depleted, and the undesirable reactions can become dominant. Surveillance programs are instituted to ensure that sufficient stabilizer remains in propellants to minimize the risk of autoignition. Such a program consists of accelerated-aging estimations of stabilizer content combined with occasional monitoring of the rocket motor inventory. Conventionally, both evaluations require the extraction of a piece of propellant, followed by chemical analysis of that piece. In 2002, the Army determined that the M28 propellant inside an intact M55 rocket assembly, in its current configuration, could be handled with minimal risk (U.S. Army, 2002). In the 10 years that have elapsed since the 2002 assessment, the propellant has degraded further. If the
4Stephanie E. Leach and Bruce P. Thomas, Naval Air Warfare Center Weapons Division, China Lake, California, “Assessment of Alternative Strategies to Determine Solid Rocket Motor Stability,” meeting poster presented at the 2012 Pittsburgh Conference, March 16, 2012, Orlando, Florida.