Heterogeneous explosives

Military and ammunition sites contaminated with explosives can cover substantial areas (Gerth et al. 2005). Soil contamination in these sites is often heterogeneous. Explosives are relatively non-volatile, and have low aqueous solubility. Sampling from sites within a few decimetres of one another can result in concentration differences of up to one hundredfold (Jenkins et al. 1996). For example, the coefficients of variation across samples taken from 11 abandoned sites in the USA were 248% for TNT and 137% for Hexogen (Crockett et al. 1998). As a result, sampling error greatly exceeds measurement error. Thus to obtain representative results... 

The basic two-dimensional processes involved in the shock initiation of heterogeneous explosives have now been numerically described. 

E.E. Walker R.J. Wasley, Critical Energy for Shock Initiation of Heterogeneous Explosives , Explosivst (1), (1969), 9—13... 

Campbell R. Engelke, Diameter Effect in High-Density Heterogeneous Explosives", LA-UR-76-1115, Los Alamos Sci Lab, Los Alamos (1976) CA86, 157801 (1976) 46) R,K. 

Mist explosions are, of course, dust explosions in which the particles happen to be liquid. A study of criteria for these, with especial reference to organic heat transfer fluids, is reported [39]. Other less common types of heterogeneous explosion involving gaseous oxidants are reviewed [37] [40]. These include wick explosions, foam explosions, and surface explosions. Studies aimed at early detection and suppression of dust explosions are reported [38]. 

As previously stated, this discussion is valid for homogeneous explosives, such as the ones used in the military, since their reactions are predominantly intramolecular. Such explosives are often referred to as ideal explosives, in particular when they can be described using the steady state model of Chapman and Jouguet. In heterogeneous explosives (non-ideal), which are currently used in civil applications, intermolecular (diffusion controlled) mechanisms are predominant for the air bubbles, cavities or cracks (etc.). As a general rule detonation velocities increase proportional to the diameter. 

In the case of lead azide, Andreev [42] and Bowden and Yoffe [43] suggest that lead azide detonates immediately after being ignited and that a burning regime is absent. The theory of fracture that was subsequently developed to explain the initiation of fast reaction [44,45], and the previous observations lead to the conclusion that the shock initiation mechanism of this primary explosive is not likely to exhibit the same characteristics as those exhibited by the secondary explosives. However, examination of the shock sensitivity of dextrinated and polyvinyl lead azide to pulse durations vaiying from 0.1 to 4.0 psec shows that the initiation characteristics are indeed similar to those observed for heterogeneous explosives. 

Trott W. M., and Renlund A. M., Single-pulse Raman scattering studies of heterogeneous explosive materials, Appl. Opt., 24, 1520-1525, 1985. 

The initiation mechanism of heterogeneous explosives (e.g., mixture-type of explosives) is somev at different compared to the initiation mechanism of the homogeneous explosives. 

The initiation mechanism of heterogeneous explosives is mainly the result of the local heating of an explosive, leading to the formation of hot spots due to... 

The critical values of the shock wave pressure that may cause the initiation of heterogeneous explosives are 1-5 GPa, while the hot spot temperature is 700-800 K. 

In another recent study, Spear and Nanut (34) have reviewed the physical processes involved in initiation and build up of reaction in heterogeneous explosives. They point out that the molecular processes occurring during shock initiation are not known and that there is disagreement as to whether grain burning or physical processes such as permeability, thermal conduction, convector and diffusivity are the key parameters controlling the subsequent build-up of reaction. 

Three-Dimensional Reaction Zones of Heterogeneous Explosives... 

The heterogeneous explosive reaction zone that has been the most studied is PBX-9404 (94/3/3 HMX/Nitrocellulose/Tris-/3-Chloroethyl phosphate). A summary of the estimated reaction zone thickness and Von Neumann spike pressure is given in Table 1.4 along with the calculated reaction zone parameters using the solid Arrhenius HMX constants of... 

Three-Dimensional Reaction Zones of Heterogeneous Explosives TABLE 1.4 PBX-9404/9501 Reaction Zone... 

The metal free-surface measurements of B. G. Craig used the technique described in reference 8. The infrared radiometry measurements of W. Von Holle used the technique described in reference 12. The interferometer measurements were performed by W. Seitz. The application of this method to reaction zone measurements is described in reference 13. The bromoform measurements were made by R. McQueen and J. Fritz using the technique described in reference 14 and a sound speed and overdriven detonation method in reference 17. Sheffield studied the reaction zone of heterogeneous explosives using a foil-water system and a visar technique described in reference 18. 

Cross-sectional plots of pressure and burn fraction through the 15th cell in the X direction (1=15) are shown in Figure 1.28. A complicated time-dependent, multi-dimensional reaction region proceeds through the heterogeneous explosive. 

Thus, any experimental study of a reaction region in a heterogeneous explosive is actually measuring some mean value of an irregular, complicated multi-dimensional flow. It is not surprising that different experimental techniques may give quite different reaction zone thicknesses, Von Neumann spike pressures and profiles. 

As shown in Table 1.5 for PBX-9502 and Table 1.6 for Composition B, the measured reaction zone parameters for heterogeneous explosives vary considerably with the experimental technique. The reported reaction zone thickness for PBX-9502 (95/5 TATB/Kel F, p = 1.894) varies by a factor of 8 between the metal free-surface measurement of Craig and the foil-water measurement reported by Sheffield As shown in Table 1.6, the reaction zone thickness of Composition B (64/36 RDX/TNT, p = 1.713) varies by a factor of 4 between the bromoform measurement and the conductivity measurement of Hayes . 

The reactive region in heterogeneous explosives is complicated, time-dependent, and multi-dimensional (non-laminar). 

Theoretical analytical or numerical models that assume a unique reaction zone thickness and pressure profile throughout the reaction zone may be appropriate for certain homogeneous explosives that are stable at the C-J state, but such models can NOT describe the reaction zone of any heterogeneous explosive or propellant. 

Experimental measurements of the reaction zone of heterogeneous explosives need to be made with high resolution in space and time. The space scale required must be less than the size of the smaller density discontinuities, and the time scale must be small enough to follow the variable flow associated with the density discontinuities. 

It is important to understand that this discussion is about build-up of the effective C-J pressure of detonating 9404 initiated at pressures less than infinite-medium effective C-J pressure, but greater than that pressure required for prompt initiation ( 100 kbar for most explosives). This is not to be confused with the build-up to detonation, characteristic of the process of shock initiation of heterogeneous explosives initiated by a shock wave of a few tens of kbars described in Chapter 4. 

Build-Up TO Detonation - Characteristic of the process of shock initiation of heterogeneous explosives initiated by a shock wave of a few tens of kbars. 

The voids or density discontinuities in a heterogeneous explosive cause irregularities of the mass flow when shocked. The heterogeneous explosive is initiated by local hot spots formed in it by shock interactions with density discontinuities. The hot spot mechanism is important in the propagation and failure of the detonation wave. As shown in Chapter 1, the density discontinuities are also a dominant feature of the heterogeneous explosive reaction zone. 

Since the interaction of shocks with density discontinuities as simple as corners produces a very complicated fluid flow, it appears that detailed numerical studies are essential for an understanding of the experimental results of more complicated systems such as heterogeneous explosives. 

---