(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.