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(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.

    David P. Stern                  30 March, 2003.


Explosive release of Fission Energy

    The fission of uranium currently provides electrical power in several countries. Some day it may do so on a much larger scale, if the world can agree on ways to prevent its misuse (see below) and on the safe disposal of fission products.

    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

    Fission can, however, proceed much more rapidly by a different process, using "fast" neutrons. Freshly created from the fission process, they initiate another fission before moving very far. "Fast fission" makes bombs feasible (as well as "fast breeder" reactors, of which France has built two). Its chain reaction is damped by the U238 isotope, so that unlike the fuel in a power station, where even natural uranium (0.7% U235) can be used, the one used in a bomb must be heavily enriched in U235 ("weapons grade uranium"). Or else, the artificially produced element plutonium is used, extracted as a by-product of nuclear reactors.

    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

    The world has every reason to dread the use of nuclear weapons and to do its best to prevent it. Their sudden release of energy produces a tremendous concentration of heat, which radiates energy as an immensely intense flash of light. A small fraction of a second later, a "fireball" forms--a sphere of very hot air (or a hemisphere, if the bomb explodes near the ground), which absorbs the heat and immediately re-radiates it, so the intense radiation of heat persists for seconds.

    In the first wartime use of the bomb, on the large Japanese city of Hiroshima on 6 August 1945, thousands of exposed citizens suffered hideous burns, and many of them died. The heat also ignited the city, most of which burned down, with even more casualties. Three days later another bomb was dropped on Nagasaki, with similar effects, and Japan surrendered, ending World War II.

    The intensely heated air around the bomb also expands forcefully. This has two effects: a powerful shock wave, which adds to the destruction, and a huge bubble of hot air, whose buoyancy makes it rise rapidly--like a huge hot-air balloon--to about 60-80,000 feet. The bubble drags behind it a "stem" of dust, smoke and debris, creating the famous "mushroom cloud."

    People near the explosion who have managed to shield themselves against the heat and the blast may be killed or sickened by nuclear radiation. In addition, radioactive debris from the bomb, sucked up by the mushroom cloud, ultimately comes down again and adds contamination. Still, the most harmful effects in Japan were probably the flash and the fires, with the blast also contributing. Describing them takes more the skills of a talented writer than the training of a physicist; John Hersey did it well in his short book "Hiroshima," although, his book, which concentrates on survivors, may not have told enough.

After Hiroshima

    A much greater peril is the "H-bomb" ("hydrogen bomb"), in which the fission of uranium and plutonium is supplemented by the fusion of isotopes of hydrogen and of lithium, combining to form helium and releasing energy, a process related to the one taking place in the core of the Sun. Since the H-bomb uses a regular A-bomb as its detonator, the compression of its fuel is much faster, completely overcoming the problem of blowing apart before the reaction has proceeded very far. As a result, more fuel can be used, creating a much larger explosion. While the Hiroshima bomb was equivalent to about 13,000 tons of high explosives (later A-bombs were larger), H-bombs have been exploded which were 1000 times more powerful.

    The flash of such a bomb can ignite cities and forests 40 kilometers away and more, while the deadly radioactive debris ("fallout"), which ultimately drops back to Earth, can blanket areas many hundreds of kilometers from the explosion. The fact that so much fission-debris is released suggests that although this is labeled a "hydrogen bomb," much of its energy (perhaps most of it) may come from the fission of plutonium and uranium. In the 1960s, when both the US and Soviet Russia produced such weapons and envisioned their use, "fall-out shelters" were identified or built in the US, locations shielded by enough thickness of material (about 20-30 cm of concrete) to stop the deadly rays of fallout. They were stocked with food and water for 1-2 weeks, the time needed for the worst radioactivity to decay.

    Luckily, it was soon realized that such large bombs had no clear military use. They were only suited for wanton destruction, which would merely invite retaliation in kind. After 1970 they were quietly abandoned--let us hope, forever.

    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


    Out of the huge existing literature, here are a few samples. Be warned they my be out of print, though you might find them in libraries:

    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.

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Author and Curator:   Dr. David P. Stern
     Mail to Dr.Stern:   stargaze("at" symbol)phy6.org .

Last updated: Last updated: 9-23-2004