Explosive chain reactions

Figure 2.1 The lower and upper limits of an explosive chain reaction as a function of temperature and pressure.

Chain Explosions and Chain Reactions. See Vol 2, p C146-L and in this section under Detonation (and Explosion), Chain Reactions... 

The ideas from the dissertation were then further developed in a paper with Kramers whose intimate knowledge of the theories of Bohr concerning atomic structure contributed essentially to the correct representation of the theory. In this paper inter alia the possible occurrence of explosive chain reactions, i.e., chain reactions in which no steady state concentrations of the intermediates is obtainable, was mentioned. The surprising results by Moureau and Dufraisse (17) concerning antioxidants were explained as being due to autoxidation proceeding as a chain reaction, the chains being broken by the antioxidant. 

Safe separation studies were conducted to achieve increased production and cost effectiveness with improved safety. A typical ammunition production line flow diagram (in this case for the manufacture of 105 mm projectile) consists of several work areas as shown in Fig 8. ( (l) Receiving and storage, (2) Box open and inspect (3) Melt Pour (U) Cool (5) Hold (6) Funnel Pull and (T) Riser Preparation.) Explosive material is transferred by automatic conveyor between these work areas. The requirement was to establish safe separation between explosive boxes, pallets with and without funnels, buckets, and to determine critical height of continuous feed flake Comp B and TNT. The objective of these tests was to establish minimum nonpropagation distances between these items so that an explosion chain reaction will be prevented. 

Arts. Neutron-induced fission reactions of the heavy elements have as their products two or more neutrons of high energy. This multiplies the eifect of the initiating neutron, and can lead to an explosive chain reaction. 

Not long from Nazi Germany, Frisch found his argument against a violently explosive chain reaction reassuring. It was backed by the work of no less a theoretician than Niels Bohr. With satisfaction he published it. 

Sometime in February 1940 he looked again. There had always been four possible mechanisms for an explosive chain reaction in uranium ... 

If Frisch now glimpsed an opening into those depths he did so because he had looked carefully at isotope separation and had decided it could be accomplished even with so fugitive an isotope as U235. He was therefore prepared to consider the behavior of the pure substance unalloyed with U238, as Bohr, Fermi and even Szilard had not yet been. I wondered— assuming that my Clusius separation tube worked well—if one could use a number of such tubes to produce enough uranium-235 to make a truly explosive chain reaction possible, not dependent on slow neutrons. How much of the isotope would be needed ... 

These experiments gave direct evidence of an explosive chain reaction. They gave an energy production of up to twenty million watts, with a temperature rise in the hydride up to 2 C per millisecond. The strongest burst obtained produced 10 neutrons. The dragon is of historical importance. It was the first controlled nuclear reaction which was supercritical with prompt neutrons alone. 

Mixtures of chlorine and hydrogen reaa only slowly in the dark but the reaction proceeds with explosive violence in light. A suggested mechanism for the photochemical chain reaction is ... 

Bromine, like chlorine, also undergoes a photochemical chain reaction with hydrogen. The reaction with bromine, however, evolves less energy and is not explosive. 

The reaction with fluorine occurs spontaneously and explosively, even in the dark at low temperatures. This hydrogen—fluorine reaction is of interest in rocket propellant systems (99—102) (see Explosives and propellants, propellants). The reactions with chlorine and bromine are radical-chain reactions initiated by heat or radiation (103—105). The hydrogen-iodine reaction can be carried out thermally or catalyticaHy (106). 

Ha/ogenation. Heats of reaction are highly exothermic for halogens, particularly fluorine (qv), and chain reactions can result in explosions over broad concentration ranges. Halogens also present severely challenging corrosion problems (see Corrosion and corrosion control). 

Criticality Precautions. The presence of a critical mass of Pu ia a container can result ia a fission chain reaction. Lethal amounts of gamma and neutron radiation are emitted, and a large amount of heat is produced. The assembly can simmer near critical or can make repeated critical excursions. The generation of heat results eventually ia an explosion which destroys the assembly. The quantity of Pu required for a critical mass depends on several factors the form and concentration of the Pu, the geometry of the system, the presence of moderators (water, hydrogen-rich compounds such as polyethylene, cadmium, etc), the proximity of neutron reflectors, the presence of nuclear poisons, and the potential iateraction with neighboring fissile systems (188). As Httle as 509 g of Pu(N02)4 solution at a concentration Pu of 33 g/L ia a spherical container, reflected by an infinite amount of water, is a critical mass (189,190). Evaluation of criticaUty controls is available (32,190). 

Notice from the fission equations written above that two to four neutrons are produced by fission for every one consumed. Once a few atoms of uranium-235 split, the neutrons produced can bring about the fission of many more uranium-235 atoms. This creates the possibility of a chain reaction, whose rate increases exponentially with time. This is precisely what happens in the atomic bomb. The energy evolved in successive fissions escalates to give a tremendous explosion within a few seconds. 

For nuclear fission to result in a chain reaction, the sample must be large enough so that most of the neutrons are captured internally. If the sample is too small, most of the neutrons escape, breaking the chain. The critical mass of uranium-235 required to maintain a chain reaction in a bomb appears to be about 1 to 10 kg. In the bomb dropped on Hiroshima, the critical mass was achieved by using a conventional explosive to fire one piece of uranium-235 into another. 

Although the first reaction is of an ordinary sort, the next two are unusual in that one propagating intermediate is converted into two. These are branching reactions. As each occurs, the total rate speeds up. When that happens, even more branching occurs, and so on. If unchecked, the exponential growth of chain carriers leads to explosion, just as in nuclear chain reactions. 

When the Plutonium Project was established early in 1942, for the purpose of producing plutonium via the nuclear chain reaction in uranium in sufficient quantities for its use as a nuclear explosive, we were given the challenge of developing a chemical method for separating and isolating it from the uranium and fission products. We had already conceived the principle of the oxidation-reduction cycle, which became the basis for such a separations process. This principle applied to any process involving the use of a substance which carried plutonium in one of its oxidation states but not in another. By use of this... 

Chain reactions begin with the initiation of a reactive intermediate that propagates the chain and concludes with termination when radicals combine. Branching chain reactions can be explosively fast. 

Fluorine reacts explosively by a radical chain reaction as soon as the gases are mixed. A mixture of hydrogen and chlorine explodes when exposed to light. Bromine and iodine react with hydrogen much more slowly. A less hazardous laboratory source of the hydrogen halides is the action of a nonvolatile acid on a metal halide, as in... 

The recapture ratio does not have to be 2 for this effect to occur. Any recapture value larger than 1.0 results in explosive growth of the fission chain. The critical mass is called critical because any mass greater than this value sustains a chain reaction and may explode. 

The use of PbEt4 as an anti-knock agent in petrol depends in part on the ability of the ethyl radicals, generated on its thermal decomposition, to combine with radicals produced in the over-rapid combustion of petroleum hydrocarbons chain reactions which are building up to explosion (knocking) are thus terminated short of this. The complete details of how PbEt4 operates are not known, but there is some evidence that minute Pb02 particles derived from it can also act as chain-stoppers . 

Chain reactions can lead to thermal explosions when the energy liberated by the reaction cannot be transferred to the surroundings at a sufficiently fast rate. An explosion may also occur when chain branching processes cause a rapid increase in the number of chains being propagated. This section treats the branched chain reactions that can lead to nonthermal explosions and the physical phenomena that are responsible for both branched chain and thermal explosions. 

The generalized mechanism by which branched chain reactions proceed provides a basis for a semiquantitative understanding of explosions resulting from chain branching. ... 

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