Better yet, stand back a few miles, then start the reaction Generation, or degeneration? Second-generation: "Hydrogen" fusion bomb, the workhorses of U.
Uses a fission bomb to start the fusion reaction. Third-generation: "X-ray laser" directed-energy and "neutron" enhanced-radiation weapons see "Third-Generation A bust. The laser didn't work, and the neutron bomb found no military use. At about the same time, other scientists discovered that the fission process resulted in even more neutrons being produced. This led Bohr and Wheeler to ask a momentous question: Could the free neutrons created in fission start a chain reaction that would release an enormous amount of energy?
If so, it might be possible to build a weapon of unimagined power. In March , a team of scientists working at Columbia University in New York City confirmed the hypothesis put forth by Bohr and Wheeler -- the isotope uranium , or U , was responsible for nuclear fission.
The Columbia team tried to initiate a chain reaction using U in the fall of , but failed. All work then moved to the University of Chicago, where, on a squash court situated beneath the university's Stagg Field, Enrico Fermi finally achieved the world's first controlled nuclear chain reaction. Development of a nuclear bomb, using U as the fuel, proceeded quickly. Because of its importance in the design of a nuclear bomb, let's look at U more closely.
U is one of the few materials that can undergo induced fission. Instead of waiting more than million years for uranium to naturally decay, the element can be broken down much faster if a neutron runs into its nucleus. The nucleus will absorb the neutron without hesitation, become unstable and split immediately. As soon as the nucleus captures the neutron, it splits into two lighter atoms and throws off two or three new neutrons the number of ejected neutrons depends on how the U atom happens to split.
The two lighter atoms then emit gamma radiation as they settle into their new states. There are a few things about this induced fission process that make it interesting:. In , scientists at the University of California at Berkeley discovered another element -- element 94 -- that might offer potential as a nuclear fuel. They named the element plutonium , and during the following year, they made enough for experiments. Eventually, they established plutonium's fission characteristics and identified a second possible fuel for nuclear weapons.
In a fission bomb, the fuel must be kept in separate subcritical masses, which will not support fission, to prevent premature detonation. Critical mass is the minimum mass of fissionable material required to sustain a nuclear fission reaction.
Think about the marble analogy again. If the circle of marbles are spread too far apart -- subcritical mass -- a smaller chain reaction will occur when the "neutron marble" hits the center. If the marbles are placed closer together in the circle -- critical mass -- there is a higher chance a big chain reaction will take place. Keeping the fuel in separate subcritical masses leads to design challenges that must be solved for a fission bomb to function properly.
The first challenge, of course, is bringing the subcritical masses together to form a supercritical mass, which will provide more than enough neutrons to sustain a fission reaction at the time of detonation. Bomb designers came up with two solutions, which we'll cover in the next section. Next, free neutrons must be introduced into the supercritical mass to start the fission. Neutrons are introduced by making a neutron generator.
This generator is a small pellet of polonium and beryllium, separated by foil within the fissionable fuel core. In this generator:. Finally, the design must allow as much of the material as possible to be fissioned before the bomb explodes.
This is accomplished by confining the fission reaction within a dense material called a tamper , which is usually made of uranium The tamper gets heated and expanded by the fission core.
This expansion of the tamper exerts pressure back on the fission core and slows the core's expansion. The tamper also reflects neutrons back into the fission core, increasing the efficiency of the fission reaction. The simplest way to bring the subcritical masses together is to make a gun that fires one mass into the other.
A sphere of U is made around the neutron generator and a small bullet of U is removed. The bullet is placed at the one end of a long tube with explosives behind it, while the sphere is placed at the other end. A barometric-pressure sensor determines the appropriate altitude for detonation and triggers the following sequence of events:.
Little Boy , the bomb dropped on Hiroshima, was this type of bomb and had a That is, 1. The second way to create a supercritical mass requires compressing the subcritical masses together into a sphere by implosion. Fat Man , the bomb dropped on Nagasaki, was one of these so-called implosion-triggered bombs. It wasn't easy to build. Early bomb designers faced several problems, particularly how to control and direct the shock wave uniformly across the sphere.
Their solution was to create an implosion device consisting of a sphere of U to act as the tamper and a plutonium core surrounded by high explosives. When the bomb was detonated, it had a kiloton yield with an efficiency of 17 percent. This is what happened:.
Designers were able to improve the basic implosion-triggered design. In , American physicist Edward Teller invented the concept of boosting. Boosting refers to a process whereby fusion reactions are used to create neutrons, which are then used to induce fission reactions at a higher rate.
It took another eight years before the first test confirmed the validity of boosting, but once the proof came, it became a popular design. In the years that followed, almost 90 percent of nuclear bombs built in America used the boost design. Of course, fusion reactions can be used as the primary source of energy in a nuclear weapon, too. In the next section, we'll look at the inner workings of fusion bombs. Fission bombs worked, but they weren't very efficient.
It didn't take scientists long to wonder if the opposite nuclear process -- fusion -- might work better. Fusion occurs when the nuclei of two atoms combine to form a single heavier atom. At extremely high temperatures, the nuclei of hydrogen isotopes deuterium and tritium can readily fuse, releasing enormous amounts of energy in the process. Weapons that take advantage of this process are known as fusion bombs , thermonuclear bombs or hydrogen bombs.
Fusion bombs have higher kiloton yields and greater efficiencies than fission bombs, but they present some problems that must be solved:. Scientists overcome the first problem by using lithium-deuterate, a solid compound that doesn't undergo radioactive decay at normal temperature, as the principal thermonuclear material.
The uranium atom can split any one of dozens of different ways, as long as the atomic weights add up to uranium plus the extra neutron. The following equation shows one possible split, namely into strontium 95 Sr , xenon Xe , and two neutrons n , plus energy:. The immediate energy release per atom is about million electron volts Me.
Of the energy produced, 93 percent is the kinetic energy of the charged fission fragments flying away from each other, mutually repelled by the positive charge of their protons. This initial kinetic energy imparts an initial speed of about 12, kilometers per second. Here, their motion is converted into X-ray heat, a process which takes about a millionth of a second. By this time, the material in the core and tamper of the bomb is several meters in diameter and has been converted to plasma at a temperature of tens of millions of degrees.
This X-ray energy produces the blast and fire which are normally the purpose of a nuclear explosion. When a single free neutron strikes the nucleus of an atom of radioactive material like uranium or plutonium, it knocks two or three more neutrons free.
Energy is released when those neutrons split off from the nucleus, and the newly released neutrons strike other uranium or plutonium nuclei, splitting them in the same way, releasing more energy and more neutrons.
This chain reaction spreads almost instantaneously. The material used was uranium It is believed that the fission of slightly less than one kilogram of uranium released energy equivalent to approximately 15, tons of TNT. Compared to the one used on Hiroshima, the Nagasaki bomb was rounder and fatter. The material used was plutonium The fission of slightly more than one kilogram of plutonium is thought to have released destructive energy equivalent to about 21, tons of TNT.
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