AGEX I: the Explosives Regime of Weapons Physics

AGEX I the explosives regime of weapons physics Timothy R. Neal he history of explosives re- hitherto not contemplated. The such as triaminotrinitrobenzene search and above-ground ex- achievement of those goals left Los (TATB)—can be dropped from great Tperimentation for nuclear Alamos, at the end of World War II, heights and will shatter but not ex- weapons began with the Manhattan uniquely in possession of the most plode. If exposed to fire in an acci- Project. During the hectic, almost advanced explosive-fabrication tech- dent, TATB will burn, but it is ex- frantic, war days at Los Alamos, it nology on earth and a mission to tremely unlikely to undergo a transi- became clear that, if possible, the fis- make nuclear weapons safer and tion from burning to deflagration or sionable material in the weapon more efficient—a mission that has detonation. Even when exposed to should be plutonium. It was equally continued into the present. high temperature, extreme pressures, apparent that the critical mass of plu- For a long period of time, the or shocks, these materials resist ex- tonium needed to produce a nuclear work on weapons implosions has plosion. Thus, they can be handled explosion would have to be assem- utilized conventional plastic-bonded quite safely with simple precautions. bled in the weapon through a spheri- high explosives, which could be pre- In addition to safety, the stability cal implosion driven by powerful ex- cisely machined. Improvements and reliability of nuclear weapons in plosives (Figure 1). Thus from the were continually made to increase the nation’s stockpile have been on- beginning the development of nuclear the accident resistance of these ma- going concerns. Scientists and engi- weapons was intimately connected terials. The emphasis on safety in neers have continued to study the with and dependent on developing nuclear weapon research led to the compatibility of materials contained fabrication, quality-control, and in- development of insensitive high ex- in weapons during long-term storage spection technology for high explo- plosive (IHE) at Los Alamos. Dur- and to develop new materials for sives (explosives with energies ing the 1970s the Laboratory pio- weapons components. The develop- greater than that of TNT). Initial ex- neered the use of IHE in nuclear ment of new materials has even led periments in the spring and summer weapons designs, which dramatical- to applications in the commercial of 1943 revealed, among other ly decreased the possibility that the sector. For example, a high explo- things, that for the weapon to work explosives would detonate during sive developed in the weapons pro- the design of the explosive charges accidental insults. Most modern gram, nitrotriazolone (NTO), is and the timing of their detonation weapons are designed to incorporate under consideration for use as a gas would have to achieve a precision insensitive explosives. An IHE— producer in automobile air bags. 54 Los Alamos Science Number 21 1993 AGEX I AGEX I fig1 3/25/93 Chemical Subcritical Compressed reaction propagates by compressing explosive mass supercritical the material ahead of it and reaches mass 90-percent completion within a few millionths of a second. Such rapid reactions produce strong shock waves. The detonation of a high explosive is typically initiated by a small shock wave that strongly compresses the explosive at a point, causing it to heat up and burn. The exothermic Figure 1. Explosive-driven Implosion chemical reaction happens so rapidly Explosion of a fission weapon is initiated by the implosive force generated by the det- that the pressure of the reaction prod- onation of a layer of high explosive surrounding the fissile fuel. The detonating high ucts compresses the fuel around it explosive compresses a subcritical mass of fissile material to form a supercritical causing that fuel, in turn, to heat up mass that then rapidly releases nuclear energy through an uncontrolled fission chain and react, and so the detonation pro- reaction. ceeds to spread out from the point of initiation just like a spherical wave. Research on Safety ered to be more tolerable. Now that This compression-driven reaction and Performance of the Soviet threat is retracted and our travels at supersonic velocities and is High Explosives current intent is to dismantle or called a detonation wave. The lead- store needed nuclear arms rather ing edge of the detonation wave is a The end of the Cold War has led than brandish them, the public de- shock front; that is, there is a discon- to increased emphasis on safety. An serves even greater assurances about tinuity in pressure, temperature, and overriding worry is that an accident safety. Accident analyses have density across the front. The pres- might cause the explosive in a nu- therefore been extended to address sures built up in the gaseous reaction clear weapon to release its energy, extremely low-probability accidents. products behind the shock front are thus causing the assembly of a criti- Complex, multiple-accident scenar- typically on the order of a few hun- cal mass and the production of some ios now being considered include dred thousand atmospheres, and the sort of nuclear yield. Even if a nu- the possibility that after a bomber temperatures are typically between clear yield is totally averted through loaded with nuclear weapons catches 2000 and 4000 kelvins. inherent design features, the explo- on fire, another large plane crashes Most accidental insults to a nu- sive-energy release might still dis- into it. Can the new “wooden” in- clear weapon would not produce perse radioactive plutonium across sensitive high explosives withstand shock waves that could initiate the the countryside. Nuclear weapons both the high temperature and the detonation of high explosives. have long been designed to avoid or severe impact that would be in- However, exposure to fire along drastically reduce such threats. For volved in such an accident? with the impact of a crash might ini- example, all weapons in the stock- In order to predict the response of tiate a deflagration, a burn front that pile are inherently “one-point” safe; explosives in various accident scenar- propagates by heat conduction rather that is, the initiation of the explo- ios, research has been under way to than compression and therefore pro- sive at some random point will not further understand the detonation ceeds about a million times more produce a nuclear yield. Weapons process in high explosives. Unlike slowly than a detonation. A defla- have also been tested against the gasoline, which must be mixed with gration in explosives and propellants raging inferno of a jet-fuel burn to the oxygen in the air in order to burn might, however, build up into a full- assure their safe response should, completely and rapidly, high explo- scale detonation. for example, a bomber loaded with sives contain enough oxygen to under- The deflagration-to-detonation nuclear weapons catch on fire. How- go extremely rapid and complete transition is a significant safety con- ever, during the Cold War, as we exothermic (heat-producing) chemical sideration in all industrial, military, stood eyeball to eyeball with the So- reactions. The high explosive is said and nuclear weapon applications of viets, certain low risks were consid- to undergo detonation if the chemical high explosives and propellants. A 1993 Number 21 Los Alamos Science 55 AGEX I comprehensive study of this problem high explosives are slower and seem sible to use basic models to simu- involving a consortium of university to depend on their location inside late the behavior of explosives even and government laboratory partici- the explosive charge. Thus the mod- in complex geometries. Thus the pants is under way, and the results eling of detonations in IHE has been wave of the future emphasizes care- of the study are being incorporated a far more difficult problem. fully selected benchmark experi- into engineering codes for predictive Through a very strong experimental ments to characterize explosive be- design and safety assessment of nu- program scientists have been able to havior followed by the linking and clear weapons. When the deflagra- confirm theoretical predictions con- extension of those results through tion-to-detonation process is proper- cerning the behavior of insensitive numerical simulations on super- ly understood, we can effect safety high explosives, in particular, that computers. measures to guard against even a reaction rates are strongly accelerat- Los Alamos scientists are extend- low-risk accident. ed by increases in temperature and ing their historic mission in high ex- The most important thrust of cur- pressure. The results of these exper- plosives research to discover at the rent explosives research is to devel- iments on reaction rates have been molecular level what an explosive is op better models of deflagration and used to develop more precise models and how it works. This fundamental detonation through a combination of of the initiation and detonation of research enlists sophisticated spec- experimental and theoretical work. insensitive high explosives and to troscopic experimental techniques to Many advances were achieved in better understand the effects of reac- learn what holds the explosive mole- modeling the detonation of conven- tion rates on the sensitivity of the cules together, how they come apart tional high explosives. The fact that explosive to heat and impact. during initiation and detonation, and chemical

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