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Fission, Fusion, and Destruction There is no doubt that the existence of nuclear weapons has brought about a significant change in human consciousness. We could well number our years as “before the bomb” and “after the bomb.” As the years have passed and we have reached the year 61 AB, we are ever more aware that we have in our hands the capacity to destroy life as we know it on our planet. In comparison with the thermonuclear weapons of today, the atom bomb which was dropped on Hiroshima on August 6, 1946, was puny, a mere 20 kilotons or so (having the destructive power of 20,000 tons of conventional explosive). Today’s “strategic” weapons are about a thousand times more powerful, at about 20 megatons. Atom bombs depend on nuclear fission from a chain reaction in a critical mass of uranium or plutonium. Nuclear fission is the process in which a highly unstable nucleus splits into two parts, each of the resulting parts being more stable, and releasing large amounts of energy, mostly as heat. The unstable nuclei are then produced by bombarding the fissionable material with neutrons; when a neutron is captured the unstable nucleus quickly releases its excess energy through fission. For an atom bomb, it is necessary to go farther: the fission must induce a chain reaction (one that is self-sustaining). When uranium 235 undergoes fission, for example, it typically produces not only two smaller nuclei, usually one about twice the size of the other, but also two or three neutrons. In other words, the captured neutron is replaced by one or two emitted neutrons, each of which would be capable of inducing another fission event, rapidly escalating the process in what is referred to as a “chain reaction.” Not all neutrons are captured, however; some escape from the surface of the mass of uranium. The proportion of neutrons escaping may be reduced by increasing the mass of uranium, so that the neutrons on average will need to travel farther before reaching the surface. The minimum mass to sustain a chain reaction is known as the “critical mass.” The key to an effective explosion is to achieve critical mass quickly, almost instantaneously. In the Hiroshima bomb a conventional explosive was used to fire a sub-critical “shell” of uranium down a cannon barrel into a sub-critical target, the combination of the two exceeding the critical mass. The Nagasaki bomb used plutonium instead of uranium and achieved criticality by inducing an implosion of the mass rather than firing a projectile, but the result is the same. Naturally-found uranium exists predominantly as the 238 isotope (that is, containing 92 protons and 145 neutrons for a total of 238), which, however, does not undergo fission. The fissionable isotope, 235 (containing 92 protons and 143 neutrons), constitutes less than one percent of the uranium found in the ore. In order to be rendered usable, the uranium must be enriched to about three percent for power and more for a bomb. Since both isotopes have exactly the same chemical properties, differing only in their mass, and that by just a little over one percent, separation requires extremely sophisticated physical (not chemical) processes. The enriched uranium used in the Hiroshima bomb was produced at Oak Ridge, Tenn., through the process of gaseous diffusion. Uranium metal was converted to its gaseous hexafluoride, and the two isotopic forms were separated by being allowed to diffuse through a series of thin membranes over miles of low-pressure tubes. While the gaseous diffusion method remained the only practical method of separation, non-proliferation could be enforced by requiring non-nuclear nations to procure fuel for power applications from the U.S. and not releasing any of the more highly enriched fuel required for the bomb. It would have been impossible for any nation to set up its own diffusion operation clandestinely, since the huge apparatus would be obviously visible from the air. With time an alternative method of separation was developed, a much more compact one, electromagnetic separation, depending on the slightly different trajectory of the isotopes as they travel through strong magnetic fields. Such an operation could easily be concealed in a small building and its implementation made enforcement of non-proliferation much more difficult. It must be noted that the immediate post-war period was marked by a naive optimism of the “Atoms for Peace” program about the role foreseen for nuclear power. It was suggested that the U.S. would disseminate nuclear power technology through the whole world, retaining, of course, a monopoly in the production and sale of fuel. It was said that electric power produced from nuclear reactors would be so cheap that it would not even be metered. With abundant cheap energy, it was hoped, there would be universal prosperity and no incentive for war. There would emerge a true Pax Americana. Thermonuclear, or fusion, bombs depend not on splitting heavy atoms, but rather on forcing light atoms, various isotopes of hydrogen, to combine into helium. The energy payoff in fusion is many times greater than for fission, and the process is essentially much simpler. All that is required is an extremely high temperature, about twenty million degrees C. Such outrageously high temperatures are not unknown in the universe; in fact, fusion is the normal reaction which produces the enormous heat of the stars, and it therefore is the source of the solar energy from our sun. On earth, however, they have been completely inaccessible─until the development of the atom bomb. A thermonuclear weapon, or hydrogen bomb, consists of a fusion reaction triggered by a fission reaction. In principle, since fusion is not limited by a critical mass, a hydrogen bomb could be made as large as desired, and its destructive power could be increased indefinitely. In actual practice, the largest thermonuclear weapons, Soviet ones, have been about 100 megatons. These days, however, the emphasis has been on more accurate “delivery systems” and on miniaturization. There is even consideration of “tactical weapons,” small enough to be used on the scene by ground troops. In the six decades since the war, many realists have felt that a nuclear exchange is certain to take place, that it is impossible that such powerful weapons would not be used. More hopeful observers have believed that the balance of terror would ensure that the weapons would not be used. A faithful Christian might perhaps be justified in trusting that the nuclear stalemate will buy us enough time to evolve the higher consciousness necessary to ensure lasting peace. At least, so we may hope. Dom Roberti |