Chain reactions explosions

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). 

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

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. ... 

There is a third explosion limit indicated in Figure 4.1 at still higher pressures. This limit is a thermal limit. At these pressures the reaction rate becomes so fast that conditions can no longer remain isothermal. At these pressures the energy liberated by the exothermic chain reaction cannot be transferred to the surroundings at a sufficiently fast rate, so the reaction mixture heats up. This increases the rate of the process and the rate at which energy is liberated so one has a snowballing effect until an explosion occurs. 

Afonso, M. Dos Santos et al., Chem.Abs., 1987, 107, 175302 The thermal homogeneous chain reaction to give mainly octafluoropropane and hexafluoropropylene oxide becomes explosive above a minimum oxygen pressure of 26 mbar. 

Oxidation of benzene to phenol. This was attempted in the former U.S.S.R. and Japan on a pilot-plant scale. High yields were reported, but full-scale operation apparently was discontinued because of destruction of product by irradiation and the possibility of explosion in the reaction vessel. The latter danger can be controlled in the oxidation of halo-genated hydrocarbons such as trichloro- or tetrachloroethylenes, where a chain reaction leads to the formation of dichloro- or trichloro-acetic acid chlorides through the respective oxides. 

In some reactions involving gases, the rate of reaction estimated by the simple collision theory in terms of the usually infened species is much lower than observed. Examples of these reactions are the oxidation of H2 and of hydrocarbons, and the formation of HC1 and of HBr. These are examples of chain reactions in which very reactive species (chain carriers) are initially produced, either thermally (i.e., by collision) or photochemically (by absorption of incident radiation), and regenerated by subsequent steps, so that reaction can occur in chain-fashion relatively rapidly. In extreme cases these become explosions, but not all chain reactions are so rapid as to be termed explosions. The chain... 

Bmnched-Chain Mechanisms Runaway Reactions (Explosions)... 

The explosion at Chernobyl started a chain reaction. It s forever. You cannot stop it. The half-lives of various radioactive elements are different. The half-life of plutonium is twenty-four or twenty-five thousand years. We don t know how all of the radioactive elements affect our health, and there is mutation of different radioactive elements that have not been studied. To study this you need a lot of money. 

Some reactions proceed explosively. The explosion are of two types (i) thermal explosion and (ii) explosion depends on chain reaction. The basic reason for a thermal explosion is the exponential dependence of reaction rate on the temperature. In an exothermic reaction, if the evolved energy cannot escape, the temperature of the reaction system increases and this accelerates the rate of reaction. The increase in reaction rate produces heat at an even greater rate. As the heat cannot escape, hence the reaction is even faster. This process continues and an explosion occurs. 

The other basic type of explosion depends on a chain reaction. In some chain reactions, each chain carrier produces more than one free radical in propagation steps resulting in a rapid increase in the concentration of active species with time with a consequent rapid increase in the reaction rate. This in turn would further increase the production of free radicals. The reaction thus occurs instantaneously and an explosion takes place. The chain is called to branch when an active species or chain carrier produces more than one free radical or carrier, e.g. 

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