Explosive materials are categorized by the speed with which
they expand. Materials which detonate (which explode faster
than the speed of sound) are called high explosives and
materials which deflagrate are called low explosives.
Explosives can also be categorized according to their
sensitivity. Sensitive materials which can be initiated
by a relatively small amount of heat or pressure are
primary explosives and materials which are relatively
insensitive are secondary explosives.
Primary explosives detonate when they are subjected to heat or
shock. They are subjected to a very rapid transition from burning
to detonation,and are able to transmit the detonation to less sensitive
secondary explosives. Generally secondary explosives are more powerful
than primary explosives.
Examples of primary explosives include Lead Azide, Lead Styphnate and Mercury Fulminate.
Secondary explosives include TNT, Octogen, Hexogen, PETN and TATB.
The high explosives which are employed in nuclear weapons do not in reality burn like
gunpowder and other ordinary explosives; instead, a shock wave passes through the
material and detonates the explosive. The velocity of detonation depends on the type of
explosive and ranges from about 5,000 to 10,000 meters per second. The explosives which
are used in nuclear weapons are specially designed plastic explosives which have carefully
controlled velocity of detonation.
The choice of explosives in an implosion system is defined by the desire for high
performance, safety, ease of fabrication, or sometimes by special properties like the slow
detonation velocity which is needed in explosive lenses.
Insensitive high explosives are necessary in order to avoid accidental detonation of the
nuclear device. Although these explosive materials are insensitive to accidental initiation,
they still operate very well when they are appropriately initiated.
The desire for high performance leads to the selection of very energetic explosives which
have very high detonation velocities and pressures. The highest performance explosive
which is commonly known is Octogen (HMX).
The use of Octogen as the main explosive provides the greatest compression of the fissile
core. Octogen was widely used in American weapons from the late years of the 1950s
through the 1970s. It was often used in a formulation called PBX-9404. Octogen is also the
main explosive in many Soviet weapon designs. The chemically related Hexogen is also very
powerful. It was the main explosive used in most early American designs as a castable
mixture called "Composition B".
In recent years nuclear weapon states have become more interested in nuclear weapon
safety, as a result of some well-known accidents in which HE detonation caused wide
plutonium contamination and fatal explosions during weapon fabrication. Many of the high
explosives which were used, such as Hexogen and Octogen, are quite sensitive to shock and
heat. This has led to the use of explosives which are not sensitive to shock. These explosives
are all based on TATB. They have very unusual properties that make them extremely
insensitive to shock, impact, or heat. TATB is only a little less powerful than Composition B. A
composition known as PBX-9504 has been developed which adds 15% Octogen to a TATB
mixture. This creates a compromize between added power and added sensitivity.
Another very strong explosive which is called PETN has not been used a lot as a main
explosive in nuclear weapons as a result of its sensitivity, although it is used in detonators.
CL-20 is a quite new explosive developed by the Americans which contains nitramine groups.
It is of interest for high powered weapons systems because it is more powerful than Octogen
(which is currently the most powerful military explosive which is used in service). It can give
approximately 20% more mechanical work in terms of metal accelerating than the same
volume of Octogen explosive. One of the reasons for the extra power is the high crystal
density (2.04 g cm-3) of CL-20.
CL-20 has not yet been used in any production weapons systems, most likely because it is a
very sensitive explosive and therefore has a significant risk of accidental ignition.
It is still a research material, but it is currently undergoing testing with regard to stability,
production capabilities, and other weapons characteristics. Currently, it is probably too
sensitive to be used in nuclear weapons.
It is likely that the extra energy of CL-20 over Octogen would be quite useful to anyone who
has an interest in miniaturizing or improving an implosion system, especially if the hazard
properties can be reduced. This could create more compression of the fissile core.
This table shows the basic properties of explosives which are commonly used in nuclear
|Explosive ||Detonation Velocity |
/ m s-1
|Detonation Pressure / |
|Density / g |
|Comp B 63/36||7920||295||1.72||Low|
Scientists developed PBXs to reduce the sensitivity of high explosive crystals by means of
embedding the crystals in a rubber-like polymeric binder matrix. This reduces the shock and
impact sensitivity of the high explosive, and also permits it to be molded and shaped. The
addition of a plasticizer can also improve the mechanical properties and processing of the
The first PBX composition was developed at Los Alamos in the USA in 1952. The composition
consisted of hexogen high explosive crystals which are embedded in plasticized polystyrene.
PBXs based on hexogen/PETN which have also been developed are known as Semtex.
PBXs have replaced melt castable explosives in all nuclear weapon states.