Principles of Nuclear Weapons

This page will explain some very basic ideas about the scientific principles behind nuclear weapons. Nuclear physics is obviously a very complex and deep subject, so anyone seriously interested has to look at additional references.

>Crossroads atomic bomb test at Bikini Atoll, July 1946
Crossroads atomic bomb test at Bikini Atoll, July 1946.

In 1905 Albert Einstein published his revolutionary Special Theory of Relativity, now universally accepted as the basis of modern physics. A consequence of this theory is that the relation between mass and energy is expressed by the famous equation:

Energy equals mass times the square of the speed of light

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Turning Matter into Energy

What Einstein's equation means is that mass is equivalent to energy. The amount of energy is obtained by multiplying the mass by a huge factor, the square of the speed of light, a very large number. If you apply the equation to 1 kilogram (2.2 pounds) of matter you find it would be equivalent to the energy released by exploding 22 megatons of TNT (if 100% converted). This is quite amazing -- 2.2 pounds of material can explode like 22 million tons of TNT!! But can this equivalence be actually realized? How can the energy be released?

When fission of uranium ("splitting the atom") was discovered in 1938, precise measurements revealed a curious fact: When the uranium atom was split into two lighter elements, not all of the matter was accounted for, some disappeared during the fission. That is, the products of the fission weighed less that the uranium you started with, by a tiny amount. The difference was converted to pure energy exactly as predicted by Einstein.

This subtle fact is the basis of all so-called nuclear energy based on fission. Matter in the nucleus of an atom is converted into energy. This conversion is entirely different from the kind of chemical reaction energy released by conventional explosives like TNT or nitroglycerine. In nuclear reactions, the matter to energy conversion rate is so high (E = mc2) that it does not take much matter to produce a lot of energy.

Turning Nuclear Fission into a Bomb

It was immediately realized, in Germany, Great Britain, the U.S. and elsewhere, that the potential for a bomb of unprecedented power was present in the experiments of the physicists. In the U.S. the Manhattan Project was launched in 1942 to turn the potential into an actual atomic weapon, a long and expensive journey requiring a vast investment of scientific, engineering and industrial talent that succeeded in 1945. But even though the creation of a working bomb is very complex, the basic ideas are fairly simple.

Uranium has a very heavy nucleus which is naturally unstable. That's what it means to be radioactive -- uranium will slowly break up on its own, giving off small amounts of energy. The slow, natural decay is not useful for a bomb, but uranium fission (splitting) can be stimulated fairly easily by striking the uranium nucleus with neutrons. [If you don't know what this terminology means, see this web page.] Anytime the uranium atom captures one more neutron, it will split. The result of the fission is two lighter elements plus more neutrons! This last point is the basis of the "chain reaction" in which each atom split sends out additional neutrons to split other atoms. The cascading chain reaction will cause an energetic nuclear explosion in a few millionths of a second.

If such a chain reaction happened easily in nature, the world would have disappeared long ago. So there has to be a catch, and there is. Only certain configurations of uranium, and a few other elements, are fissionable. That is, they have to have a certain specific nuclear weight and composition (referred to as an isotope). The fissionable isotopes are naturally rare and have to be concentrated to get bomb-making material. The great industrial effort at Oak Ridge during the Manhattan Project was primarily directed at refining and enriching enough of the right kind of uranium (U-235) for "weapons grade" material, starting with the naturally relatively abundant "wrong" kind of uranium (U-238). Another element, plutonium, was found to have an isotope suitable for weapons: isotope P-239. Plutonium does not occur in nature at all, but small amounts could be created from radioactive uranium in a reactor and then purified, the object of work at the Manhattan Project Hanford plant.

Making a Fission Bomb

Very small amounts of uranium are harmless and moderate amounts will not explode, only sicken you from their radiation. But as you reach a concentrated amount called the "critical mass" that is the point where a chain reaction will start by itself and will explode. The exact nature of the explosion depends on the mass and density of material involved plus the geometry and timing of how the critical mass of material comes together.

In a bomb, sub-critical quantities of the fissionable material (uranium U-235 or plutonium P-239, for example), are initially arranged physically apart. When the bomb is triggered, the fissionable material is brought together and compressed in a precisely engineered way so that a critical mass is formed and a rapid chain reaction ensues. Two general methods were invented at the Los Alamos Laboratory during World War II to accomplish this:

  • The gun method ("Little Boy"): A critical mass of U-235 was divided into two parts, a cylinder and a rod that exactly fits the hollow center of the cylinder. In the bomb, a conventional explosive detonator shoots the rod rapidly into the center of the cylinder forming a critical mass.

  • The implosion method ("Fat Man"): Conventional explosives are arranged around a sub-critical sphere of P-239. When detonated, the explosives cause a powerful shockwave that compresses (implodes) the plutonium beyond critical mass.

The detonating events in the bomb happen in small fractions of a second after which the nuclear forces take over. If the design is correct, the chain reaction converts enough mass into energy to achieve the design yield before the materials vaporize in an expanding fireball. The actual percentage of bomb material converted to energy in the reaction is very low, a few percent, but more than enough to create a huge explosion.

The bare bones designs described do not deal with a host of other engineering issues that determine whether you get a blast or a fizzle. Refer to references if interested in more details.

What is Nuclear Fusion?

The application of Einstein's mass-energy equivalence is not limited to fission. Edward Teller took the ideas of Hans Bethe and Enrico Fermi and, with the contributions of Ulam and others, developed the theory of nuclear fusion into practical reality in 1952 when the first thermonuclear (hydrogen) bomb was tested.

Nuclear fusion, the reverse of fission, occurs when two atomic nuclei join, forming a larger nucleus. That is, two light elements (perhaps two hydrogen atoms) join to form a heavier element (e.g. helium). Physicists discovered that this process, like fission, results in excess energy and a chain reaction, although for different reasons. The nuclear fusion process in nature is the energy source for stars, generating the light and heat we get from our sun and the light we see from distant stars.

It takes a lot of energy to initiate nuclear fusion because atoms resist being pushed together. Fusion happens in stars because the tremendous gravitational energy in the mass of a star forces the nuclei together. But on the surface of the earth no such forces are available. That problem was solved by the fission bomb. The atomic bomb is the trigger, creating extremely high temperature and pressure for just long enough to ignite the fusion reaction. That is why the H-bomb came after the A-bomb; you need a solid understanding of atomic bomb design (fission) to be able to design a thermonuclear hydrogen bomb (fusion).

Making a Hydrogen Bomb

The availability of early computers in 1951 made it possible to complete the design calculations for the first practical H-bomb design. In the first tests, a mixture of deuterium and tritium (hydrogen isotopes) was used to boost fission bomb yield, adding a little fusion energy. The Ivy Mike test, detonated 31 October 1952 (GMT) on Elugelab Island in the Enewetak Atoll, was the first full-scale H-bomb, a cylinder about 20 feet high by almost 7 feet in diameter, weighing 82 tons. A version of the Mk-5 implosion fission bomb at one end was the primary or trigger stage. A stainless steel vacuum flask held several hundred liters of liquid deuterium as the secondary fusion stage, surrounded by a massive natural uranium jacket that would fission in response to the fusion reaction. A complex system of additional engineering features was incorporated to make the inner apparatus efficiently trigger the fusion reaction. This was the Teller-Ulam design and it worked, yielding 10.4 megatons, 77% of which was fission, the rest fusion.

A number of other designs, involving two or three stages, have been successfully tested. The goal is always to achieve a planned yield for a given fuel input with control of other factors such as size, weight, or radioactive fallout. As design has grown more sophisticated, specialized warheads have been developed for specific weapons such as tactical artillery, cruise missiles, or strategic ICBMs.

The book, Weapons of Mass Destruction , by Robert Hutchinson is a good, inexpensive guide to this subject with additional material on chemical and biological weapons.

Find More Information on the Internet

There are many fine websites that have additional information on this topic, too many to list here and too many to keep up with as they come and go. Use this Google web search form to get an up to date report of what's out there.

For good results, try entering this: nuclear weapons theory. Then click the Search button.

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