Nuclear weapons

(Presentation)
Good morning, ladies and gentlemen. First, let me introduce myself. My name is Lenny and I'm student at Faculty of Manufacturing Technologies of Technical University in Kosice. I'm going to talk today about nuclear weapons. I would like to divide my presentation into two parts. Firstly, I’ll talk about the A-bomb and fission. Secondly, I’ll define the H-bomb and fusion reactions. If you don't mind, we'll leave the questions until the end.

THE BOMB

The simplest fission bomb, or A-bomb, consist of two pieces of 235U such that separately their masses are less than the critical mass, but jointly their masses add up to more than the critical mass. To detonate such a bomb, the two pieces of 235U, initially at a safe distance from one another, are suddenly brought closely together. The assembly of two subcritical masses into a single supercritical mass must be carried out very quickly; if the two are brought together slowly, a partial explosion (predetonation) will push them apart prematurely, before the chain reaction can release its full energy – the explosion fizzles. The device commonly used for the assembly of the two pieces of uranium toward the other at high speed (Figure 1); the propellant is an ordinary chemical high explosive.

Fig. 1. A fission bomb using a gun deviceFig. 2. An implosion device

A more sophisticated fission bomb consist of a (barely) subcritical mass of 239Pu; if this is suddenly compressed to a higher than normal density, it will become supercritical. The sudden compression is achieved by the preliminary explosion of a chemical high explosive such as TNT. If this explosive has been carefully arranged in a shell around a sphere of 239Pu (Figure 2), then its detonation will crush the sphere of 239Pu into itself; this implosion of the plutonium very suddenly brings its density to the supercritical value and triggers the chain reaction. The implosion technique is used with 239Pu because this isotope has a strong tendency to predetonate; if one were to use the gun technique to bring together two subcritical masses of 239Pu, the chain reaction would start while the masses were still moving toward one another; the consequent premature explosion would push the masses apart and prevent a full development of the chain reaction. The implosion technique assembles the supercritical mass much faster and therefore avoids the problem of a premature explosion.

During World War II, a scientific-military-industrial complex known as the Manhattan District produced three A-bombs: one plutonium bomb exploded at Alamogordo, New Mexico, on July 16, 1945, another plutonium bomb exploded at Nagasaki, Japan, on August 9, 1945, and one uranium bomb exploded at Hiroshima, Japan, on August 6, 1945. All of these bombs had yields of about 20 kilotons, i.e., an explosive energy equivalent to that of 20,000 tons of TNT. This is the energy released by the fission of 1 kg of uranium (or plutonium). Hence these devices were quite inefficient – only a small fraction of the total mass of fissionable material actually underwent fission; the rest was merely scattered in all directions, blown apart before it had a chance to react.

The uranium used for bombs has to be highly purified, “weapons-grade” 235U. Uranium ores contain a mixture of 99.3% of the undesirable isotope 238U and only 0.7% of the isotope 235U. Since these isotopes are chemically identical, their separation is very laborious. The separation process depends on the small difference in the masses: in a gaseous compound of uranium, such as UF6, at a given temperature, the molecules containing 235U have a slightly higher average speed than the molecules containing 238U, and they will diffuse slightly faster through a porous membrane; hence such a membrane acts as a (partial) filter the separates 235U from 238U. This is the basis of the gaseous-diffusion process which is still the main source of highly enriched 235U.

Plutonium is not found in nature, except in insignificant trace amounts. It is made artificially, by transmutation of uranium in a nuclear reactor.
Fig. 3. The bomb dropped on Hiroshima. This device was 120 in. Fig. 4. The bomb dropped on Nagasaki. This device was 128 in. long
long and weighed 7000 lb. and weighed 10,000 lb.

As can be seen from Figures 3 and 4, the bombs used in World War II were rather cumbersome. Advances in the technology of shaping chemical high explosives used for the implosion of plutonium have made it possible to construct bombs in which the lump of plutonium is only about as large as a golf ball; the overall diameter of such a bomb is only about a foot, and yet it releases an amount of energy in the kiloton range.

Much higher yields can be achieved by taking advantage of nuclear fusion. Binding energy of light nuclei is relatively low – when two such light nuclei are made to fuse together to form a heavier nucleus, energy will be released. Fusion is the opposite of fission: in the former two nuclei merge into one, in the latter one nucleus splits into two. Furthermore, the energy released in fusion is strong energy while the energy released in fission is Coulomb energy. The strong force favors fusion since, by merging, the nuclei reduce their surface area. The Coulomb repulsion between the two nuclei opposes fusion, but in the case of light nuclei the strong force overcomes this opposition.

The heat given off by the Sun is due to a fusion reaction called hydrogen burning: hydrogen nuclei fuse together to make helium nuclei. This reaction involves several intermediate steps, which were discovered by theoretical calculations by Hans Bethe. This reaction cannot by duplicated on Earth because it will only proceed at extremely high temperatures and pressures, such as are found near the centre of the Sun. However, some fusion reaction involving deuterium and tritium (the isotopes 2H and 3H) can be made to work on Earth:

2H + 2H → 3He + n(1)
2H + 2H → 3H + p(2)
2H + 3H → 4He + n(3)
5 2H → 3He + 4He + p + 2n + 24.3 MeV

The net results of the three reactions taken together is disappearance of five 2H nuclei and formation of 3He, 4He, one free proton , and two neutrons, with the release of 24.3 MeV. The amount of energy released per nucleon of reactant is 24.3 MeV per 10 nucleons = 2.43 MeV per nucleon, whereas for the fission of uranium the energy released per nucleon of reactant is 200 MeV per 235 nucleons = 0.85 MeV per nucleon. Thus the fusion of a given mass of 2H will yield about three times as much energy as the fission of an equal mass of 235U.

The reactions (1)-(3) are called thermonuclear because they will proceed only at very high temperature and pressure. The requisite temperatures and pressures are attained at the place of explosion of an A-bomb. Hence, the fusion reactions can be initiated by exploding a fission bomb next to a mass of heavy hydrogen; this results in self-sustained explosive “burning” of the hydrogen nuclei. This so-called H-bomb is really a fission-fusion device in which fission triggers fusion. What is more, the fusion reactions release a large number of energetic neutrons which can be used to further enhance the violence of the explosion: the trick is to surround the fusion bomb by a blanket of cheap, natural uranium, consisting of mainly 238U; although this isotope will not maintain a chain reaction, it will fission when exposed to the large flux of neutrons from the fusion. This kind of H-bomb is a fission-fusion-fission device; typically one-half of the total energy yield is due to fusion, one-half is due to fission. The fission of a large amount of uranium leaves behind a residue of highly radioactive fission products; hence fission-fusion-fission bombs are dirty, i.e., they generate a large volume of radioactive fallout.

The total energy yield of an H-bomb is of the order of one or several megatons – roughly a thousand times the yield of an A-bomb. Explosions of up to 60 megatons have been tried with great success, and there seems to be no limit to the suicidal madness that nature will let us get away with.

That brings me to the end of my presentation. Thank you for your attention and if you have any questions, I’ll be glad to answer them.