NSWCDD-PN-18-00254; Distribution Statement A:
Approved for Public Release; distribution is unlimited 1
Investigating a Proper Heating Rate for the Slow Heating Test Using Documented Incidents
David Hubble
Naval Surface Warfare Center Dahlgren Division; Dahlgren, Virginia, USA
Abstract
The slow heating test is performed on ammunition and explosives (AE) to simulate accident scenarios in which a
munition is slowly heated over an extended period of time. This can result when a fire occurs but is separated from
the munition by some barrier such as the walls of a transport container. If the AE cooks off, the results can be severe
because much of the energetic material is at an elevated temperature when the reaction occurs.
Historically, the slow heating test has been performed by heating the munition in an oven at a constant rate of
3.3°C/hr until a reaction occurs. Recently, however, the validity of this heating rate has been disputed and it has
been argued that it is too slow to represent a realistic threat scenario. While many agree that the heating rate should
be increased, there has been no real consensus on what the new rate should be. This investigation was performed to
help determine what heating rates are possible for munitions and to help select a more appropriate heating rate for
future testing. This was done by examining historical accidents and using the data to conservatively estimate the
heating rates experienced by the AE involved. In the course of this analysis, no data was found which supports a rate
as slow as 3.3°C/hr and it is concluded that a heating rate faster than 10°C/hr is more appropriate and better
represents real-world threats to munitions. It is the recommendation of the author that the heating rate used for the
slow heating test be increased to 15°C/hr.
Keywords
Slow heating, Hazard Classification Testing, Cook-off, Accident Investigation
Background
All ammunition and explosives (AE) must be hazard classified prior to being transported or stored. The procedure
for determining the hazard classification of AE is outlined in TB 700-2 [1] and apply to all DoD and DoE AE in
their storage and transportation configuration. These procedures subject the AE to a number of tests that are
designed to determine if and how the AE will react when subjected to a number of different stimuli. The slow
heating test is one of the tests that is used to characterize the hazards associated with the AE under test.
The slow heating test is performed on AE to simulate accident scenarios in which a munition is slowly heated over
an extended period of time. This can result when a fire occurs but is not in direct contact with the AE but is instead
separated from the munition by some barrier such as the walls of a magazine or shipping container. This is in
contrast to the fast heating test where the munition is in direct contact with the fire. While it might seem that the
worst case scenario would be to have the AE directly exposed to the fire, this is typically not true. In the slow
heating scenario, the heat fluxes into the item are much smaller than in the fast heating case and the resulting
temperature gradients are much lower. This means that at the time that the munition cooks-off, the average
temperature of the energetic material is higher than it would have been if it had been heated more quickly. In many
cases, this higher average temperature can cause the resulting reaction to be more severe because the elevated
temperature of the energetic fill can cause normally stable materials to become more sensitive and detonate during
slow heating. It is this phenomenon that is examined by the slow heating test.
The classification given to explosive items prior to shipment or storage consists of a hazard class followed by a
hazard division (also referred to as a sub class number) and a compatibility group. Most AE fall under class 1 for
explosives. For example, a common explosive designation is 1.4D. This classification defines the item as class 1
(explosive) division 4 (minor explosion hazard) compatibility group D (secondary explosive). This classification
then defines how the material can be shipped, where it can be stored, and with what it can be shipped and stored.
