Secondary explosives, initiation

There are three phases in a dust explosion (see Figure 3.6) initiation, primary explosion, and secondary explosion. Initiation occurs when an ignition source contacts a combnstible dust that has been suspended in air. This causes the primary explosion, which shakes more dust loose from the confined area and suspends it in air. The secondary explosion then occurs, which is usually the larger of the two explosions because there is more fuel present. 

That this is not always the case should be expected. In fact, if it was not for heterogeneous localization of some flow phenomena, it would be very diflicult to initiate secondary explosives, or to effect shock-induced chemical reactions in solids. Heterogeneous shear deformation in metals has also been invoked as an explanation for a reduction in shear strength in shock compression as compared to quasi-isentropic loading. We present here a brief discussion of some aspects of heterogeneous deformation in shock-loaded solids. 

The fact that hot spots are required for explosive initiation can be seen by calculating for the bulk temperature, say 350 K, and the anticipated hot-spot temperature, say 700 K. We take typical values of Arrhenius constants for secondary explosives QjCp 2500 K, //c = 25,(X)0 K, and V = 10 s V Hence... 

High explosives which detonate to produce shock waves. Materials which are easily detonated by mechanical or electrical stimuli are termed primary explosives . Those requiring an impinging shock wave to initiate them are secondary explosives . 

Detonator. A metal tube containing a primary explosive used for initiating a secondary explosive. 

If a dust or powder is to be used for the secondary explosion, the explosive/incendiary initiator may be enclosed in any sort of a packet and placed within or under the material to be dispersed and ignited. See Figure 62. If the initiator is to be used in gasoline or fuel it will have to be enclosed... 

Figure 14. A secondary explosion of the accumulated dust in passage ways and galleries by a minor initial explosion can propagate throughout a plant with devastating results.

An initial dust explosion can cause secondary explosions. The primary explosion sends a shock wave through the plant, stirring up additional dust, possibly resulting in a secondary explosion. In this fashion the explosion leapfrogs its way through a plant. Many times the secondary explosions are more damaging than the primary explosion. 

Primary explosives differ from secondary explosives in that they undergo a rapid transition from burning to detonation and have the ability to transmit the detonation to less sensitive (but more powerful) secondary explosives. Primary explosives have high degrees of sensitivity to initiation through shock, friction, electric spark, or high temperature, and explode whether confined or unconfined. Some widely used primary explosives include lead azide, silver azide, tetrazene, lead styphnate, mercury fulminate, and diazodinitrophenol. Nuclear weapon applications normally limit the use of primary explosives to lead azide and lead styphnate. 

Lead azide (PbN6) is a colorless to white crystalline explosive. It is widely used in detonators because of its high capacity for initiating secondary explosives to detonation. However, since lead azide is not particularly susceptible to initiation by impact, it is not used alone in initiator components. It is used in combination with lead styphnate and aluminum for military detonators, and is used often in a mixture with tetrazene. It is compatible with most explosives and priming mixture ingredients. Contact with copper must be avoided because it leads to formation of extremely sensitive copper azide. 

Secondary explosives (also known as high explosives) are different from primary explosives in that they cannot be detonated readily by heat or shock and are generally more powerful. Secondary explosives can be initiated to detonation only by a shock produced by the explosion of a primary explosive. Widely used secondary explosives include trinitrotoluene (TNT), tetryl, picric acid, nitrocellulose, nitroglycerine, nitroguanidine, cyclotrimethylenetrinitramine (RDX), cyclotetramethylenetetranit-... 

Pentaerythritol tetranitrate (PETN) is a colorless crystalline solid that is very sensitive to initiation by a primary explosive. It is a powerful secondary explosive that has a great shattering effect. It is used in commercial blasting caps, detonation cords, and boosters. PETN is not used in its pure form because it is too sensitive to friction and impact. It is usually mixed with plasticized nitrocellulose or with synthetic rubbers to form PBXs. The most common form of explosive composition containing PETN is Pentolite, a mixture of 20 to 50% PETN and TNT. PETN can be incorporated into gelatinous industrial explosives. The military has in most cases replaced PETN with RDX because RDX is more thermally stable and has a longer shelf life. PETN is insoluble in water, sparingly soluble in alcohol, ether, and benzene, and soluble in acetone and methyl acetate. 

Explosives are classed as primary or secondary. Typically, a small quantity of a primary explosive would be used in a detonator (known colloquially as a cap ), whereas larger quantities of secondary explosives are used in the booster and the main charge of a device. This collection of explosives is known as an explosive train in which a signal (mechanical, thermal, or electrical) from the control system is converted first into a small explosive shock from the detonator, which in turn initiates a more powerful explosion in the booster, which amplifies the shock into the main charge. 

The requirement for an explosive train, that is, a primary explosive to initiate the secondary explosive, is a safety feature. In the past, people wishing to illegally use explosives usually had to steal the detonators (e.g., Timothy McVey). Consequently, the effective control of access to detonators has been widely regarded as a key pubhc safety measure by many governments and law enforcement agencies. However, recently, triacetone triperoxide (TATP) has been used as the primary explosive (e.g., Richard Reid s shoe bomb) and TATP is readily, although hazardously, synthesized from acetone, hydrogen peroxide, and acid. 

These differ from the secondary explosives shown in Figure 1, in that AN-based explosives are generally so insensitive that in addition to a blasting cap , a strong booster is also required for initiation. 

Initiation by shock can be instantaneous, or practically so, if the shock is strong enough. In secondary explosives the essential factor appears to be a pressure increase, specifically a pressure "jump of 25-50 atm1 ... 

To achieve high-order detonation in secondary explosives, it has always been necessary to allow much longer delays in order to let the low-order process initially started "jump to high order [Compare with Detonation (and Explosionjby Influence] Note 2 In a review of 23 papers on initiation, ignition, and growth of reaction presented at the 4thONRSympDeton bv G.P. 

M.A. Cook etal, TrFaradSoc 56, 1028-38(1960 Promotion of shock initiation of detonation by metallic surfaces) 36a) Andreev Belyaev (i960), 265-68 (Starting impulse and mechanism of initiation) 268-70 (Initiation by heat) 270-73 (Initiation by flame) 273-86 (Initiation by shock or friction) 287-89 (Initiation of expln in projectiles on hitting a target) 36b) J. Favier C. Fauquignon, MP 42, 65-81(1960) (Initiation of expls. and transmission of detonation) 37) D.B. Moore J.C. Rice, Detonation of Secondary Explosives by Lead Azide , SRI (Stanford Research Institute), Poulter Laboratories, Technical Report 004-60(1960) 37a) S.J. Jacobs, AmRocket-... 

High Explosives (HE). A high explosive is a substance that undergoes extremely rapid chemical reaction, when properly initiated, to produce gaseous products at high temperature and pressure. These products are then capable of doing useful work as they expand. It has been customary to divide HE into two categories primary explosives and secondary explosives. 

This distinction is more in kind than in degree. Small quantities of primary or initiating explosives usually detonate when exposed to flames or high temperatures whiie secondary explosives usually burn or deflagrate under these conditions. However under slightly altered conditions primary explosives can be made to deflagrate and secondary explosives can be made to detonate. Examples of primary explosives are Lead Azide, Mercury Fulminate, DDNP, etc Examples of secondary explosives are PETN, RDX, HMX, Tetryl, TNT, as single HE compns and Comp B, Comp C, PBX 9404, Dynamite ANFO (Ammonium Nitrate/Fuel Oil) as HE mixtures... 

For pure primary secondary explosives (except for border-line HE such as Ammonium Nitrate or Ammonium Perchlorate) dcr decreases as p0 increases until p0 approaches very close single crystal density when dcr may increase drastically. Thus if we limit ourselves to p0 < 0.9pcrySt, increases in p0 (according to Eq 8) should result in a greater sensitivity to impact. This is quite the opposite of what is found for shock initiation and will be examined more closely later on. At a fixed density, dcr increases as p. increases (see A B, p 90). This increase is fairly pronounced at small p (<0.2 mm) but levels off becomes almost asymptotic at large p (>0.4mm, except for cast TNT or TNT with 1% paraffin oil). Thus an increase in p, as expected, leads to a decrease... 

Initiation by Primers (and Boosters) is the standard method of initiating secondary explosives. Thus hot wires (or other means) are used to initiate the primer charge (Lead Azide, Mercuric Fulminate etc) explosive whose detonation then initiates the main charge of PETN,... 

Farber (Ref 31) points out that LP rocket destruct systems (specifically Saturn V) can initiate dangerous secondary explosions in the discharged fuel/oxidant mixture... 

Secondary high explosive A less sensitive high explosive initiated by another explosive. 

Military explosives comprise explosives and explosive compositions or formulations that are used in military munitions (bombs, shells, torpedoes, grenades, missile or rocket warheads). The bulk charges (secondary explosives) in these munitions are insensitive to some extent and are, therefore, safe for handling, storage and transportation. They are set off by means of an explosive train consisting of an initiator followed by intermediates or boosters. 

A major difference between primary and secondary explosives arises from the fact that primary explosives are initiated to detonate by burning whereas secondary explosives are initiated to detonate by shock waves. Therefore, the most important property of a primary explosive is its ability to undergo a fast deflagration-to-detonation transition (DDT). Thus, fast DDT is the strength of primary explosives as well as their weakness. All other parameters being equal, the faster the DDT, the better the primary explosive. At the same time,... 

On the other hand, Dudek et al. found that the impact sensitivity (h50% ) of e CL-20 28 cm is comparable to that of [5-HMX (h50% = 30 cm) [124]. Agrawal et al. have not only given impact sensitivity of CL-20 but also the mechanism of initiation on impact leading to its detonation. This is in line with the mechanism of initiation by hot spot formation on impact followed by detonation similar to other secondary explosives [125]. 

The manufacturing process of BX4 was found to be more favorable than that of BX3. The detonation pressure and test of their ability to initiate a secondary explosive have shown that BX4 is the most powerful of these formulations and therefore, it is considered the most promising booster formulation. Another attribute of BX4 is that it could be converted into a blast formulation by the addition of aluminum powder and this modified formulation may find application as a main-charge blast formulation. 

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