(S-9) Nuclear Weapons

    The preceding web page on nuclear power, written two years ago, deliberately avoided the subject of weapons. The world is changing, and reluctantly this part was added, in the belief it is be better to know than to remain ignorant.

    To web-users led here by a search engine: this web page is the 3rd in a sequence discussing nuclear energy at an elementary level, part of a very large web course "From Stargazers to Starships" on astronomy, Newtonian mechanics, the Sun and spaceflight. . The two preceding it are The Energy of the Sun and Nuclear Power.

Explosive release of Fission Energy

    However, it also has a second aspect--the making of nuclear bombs. Instead of gradually "burning up" U235 or plutonium (the main nuclear fuels), producing heat for generating electricity, a bomb releases its energy very abruptly, creating an intense concentration of heat. Even a few kilograms (of which just a small part undergoes fission) can destroy a city.

    Such a sudden release is not easily achieved. Rather sophisticated technology is needed, otherwise heat released early in the fission "chain reaction" blows the fuel apart and stops the process. A nuclear reactor can never explode like a bomb: the most it can do is explode like a steam boiler without safety valves, or more likely, its fuel might melt down into expensive slag, as happened on Three Mile Island. Malfunctioning reactors can certainly be dangerous, since they hold intensely radioactive fission products (remember Chernobyl!). However, they cannot become nuclear bombs.

    A second reason also exists: the process used in power stations is not suitable for building bombs. Each neutron released by the fission of a nucleus must bounce around inert matter (e.g. water or carbon) and slow down, before it can initiate a new "thermal" fission. The extra matter interferes with the explosive process, and the slowing-down process, fast as it is, stretches out the energy release.

Fission by Fast Neutrons

    Enrichment is a difficult and expensive process, using gas centrifuges or large magnetic separators (methods used by Iraq prior to 1991), or else large gas-diffusion arrays. Plutonium, on the other hand, requires only chemical methods to separate it from the intensely radioactive fission products in spent reactor fuel. Because of the deadly radiation, such separation is always done by remote control. The possibility that nuclear bombs may be constructed from plutonium extracted from commercial power plants has been the main motivation of international efforts to control the spread of nuclear power technology.

    Even given purified fuel, it is not easy to build a nuclear bomb. A "critical mass" of fuel is needed, and it must be assembled very rapidly--indeed, compressed into a smaller volume--to allow enough of it to react before its generated heat blows everything apart. The first "atomic bomb" dropped on Japan in 1945 ("nuclear bomb" may be more appropriate, but science fiction had already written about "atomic bombs") was a modified gun barrel, in which one chunk of U²³⁵ was fired at another. Plutonium bombs need to be assembled even faster and compressed even more, so there a spherical explosive charge "implodes" upon a sphere of fuel in its middle. The technology for doing so involves the most tightly controlled "nuclear secrets," and of course a source of neutrons must be provided to initiate the chain reaction during the critical microsecond of greatest compression.

Effects of Nuclear Weapons

    It is too bad such "weapons of mass destruction" can be built and used (and they are not the only kind: see here). We cannot wish them away, but some day a new understanding among nations should bring them under control. By being aware of their peril, we might perhaps resolve to do so before bitter experience forces it upon us.

"Dirty Bombs"

    A new danger raised recently is that a "dirty bomb" exploded by terrorists. That would not be not a nuclear bomb, but an ordinary explosive device, loaded with radioactive waste such as is found in spent fuel from nuclear reactor.

    Such radioactivity can contaminate the location where the bomb explodes, to a range of perhaps a hundred feet or a few hundred feet. Coming close to such radioactivity is not likely to kill or seriously harm anyone. Much more harmful is ingesting it with food or air, so that it ends inside the body. Prompt medical treatment can remove most of it, but the main point would be to instill fear, in a visible and conspicuous way

    The contamination must be cleaned up: failing to do so would create a long term health hazard. If a dirty bomb were exploded at a national shrine or monument, or at a focus of public activity, that place would have to be closed down, at the very least temporarily, and expensive clean-up would have to start, all these very visible actions. Furthermore, the general public is unfamiliar with nuclear physics and dreads anything tied to it. It is likely to react in fear.

    Nuclear waste is usually well-guarded, since it is in the interest of any government to keep dangerous radioactivity away from its own citizens. Yet severely stressed societies exist, which lack the resources to prevent determined terrorists from breaking into depositories of nuclear waste, or from finding ways to quietly steal some of their contents. It is yet another danger we must address in this day and age.

Books about Nuclear Weapons

    The Making of the Atomic Bomb by Richard Rhodes, a thorough history, quite large and very well written. Simon and Schuster, 1987.
    Dark Sun by Richard Rhodes, a continuation of the above story--the nuclear bomb effort of the Soviet Union (including its spying on the US) and the hydrogen bomb. Simon and Schuster, 1995.
    The Effects of Nuclear Weapons, edited by Samuel Glasstone, published 1962 by the U.S. Government Printer for the U.S. Atomic Energy Commission. Detailed, full of graphs and figures.
    Atomic Energy for Military Purposes, Henry DeWolf Smyth, Princeton U., 1945. The "Smyth Report," the first report published soon after the revelation of the US effort to produce the first nuclear bomb.