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.
When a free neutron hits the nucleus of a fissile atom like uranium U , the uranium splits into two smaller atoms called fission fragments, plus more neutrons. Fission can be self-sustaining because it produces more neutrons with the speed required to cause new fissions.
This creates the chain reaction. The very first uranium bomb, Little Boy, dropped on Hiroshima in , used 64 kilograms of 80 percent enriched uranium. In fission weapons, a mass of fissile material, either enriched uranium or plutonium, is assembled into a supercritical mass—the amount of material needed to start an exponentially growing nuclear chain reaction.
The implosion method is considered more sophisticated than the gun method and only can be used if the fissile material is plutonium.
The inherent radioactivity of uranium will then release a neutron, which will bombard another atom of U to produce the unstable uranium, which undergoes fission, releases further neutrons, and continues the process. 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 more fissionable material you have, the greater the odds that such an event will occur. Critical mass is defined as the amount of material at which a neutron produced by a fission process will, on average, create another fission event. Little Boy and Fat Man utilized different elements and completely separate methods of construction in order to function as nuclear weapons.
Most uranium found naturally in the world exists as uranium, leaving only 0. When a neutron bombards U, the isotope often captures the neutron to become U, failing to fission, and thus failing to instigate a chain reaction that would detonate a bomb. The first challenge of the project was thus to determine the most efficient way to separate and purify uranium from the overly-abundant uranium - standard methods of separation could not be used due to the strong chemical similarity between the two isotopes.
In order to avoid wasting time on one new method that could later prove insufficient to produce enough U to allow the atomic bomb to reach critical mass, General Leslie Groves consulted with lead scientists of the project and agreed to investigate simultaneously four separate methods of separating and purifying the uranium gaseous diffusion, centrifuge, electromagnetic separation and liquid thermal diffusion.
Once enough U was obtained to power the bomb, Little Boy was constructed using a gun-type design that fired one amount of U at another to combine the two masses. This combination created a critical mass that set off a fission chain reaction to eventually detonate the bomb.
The two masses of U had to combine with one another quickly enough to avoid the spontaneous fission of the atoms, which would cause the bomb to fizzle, and thus fail to explode. Powered by plutonium , Fat Man could not use the same gun-type design that allowed Little Boy to explode effectively - the form of plutonium collected from the nuclear reactors at Hanford, WA for the bomb would not allow for this strategy.
The Hanford plutonium emerged from the reactors less pure than the initial plutonium extracted from Ernest O. Thus, a new design was required. For example, a carbon atom has six protons and six electrons.
It's not that simple though. An atom's properties can change considerably based on how many of each particle it has. If you change the number of protons, you wind up with a different element altogether. If you alter the number of neutrons in an atom, you wind up with an isotope. As we see with carbon, most atomic nuclei are stable, but a few aren't stable at all. These nuclei spontaneously emit particles that scientists refer to as radiation.
A nucleus that emits radiation is, of course, radioactive , and the act of emitting particles is known as radioactive decay. If you're particularly curious about radioactive decay, you'll want to peruse How Nuclear Radiation Works. For now, we'll go over the three types of radioactive decay:. Remember that fission part especially. It's going to keep coming up as we discuss the inner workings of nuclear bombs.
Nuclear bombs involve the forces, strong and weak, that hold the nucleus of an atom together, especially atoms with unstable nuclei. There are two basic ways that nuclear energy can be released from an atom. In nuclear fission pictured , scientists split the nucleus of an atom into two smaller fragments with a neutron. Nuclear fusion -- the process by which the sun produces energy -- involves bringing together two smaller atoms to form a larger one.
In either process, fission or fusion, large amounts of heat energy and radiation are given off. We can attribute the discovery of nuclear fission to the work of Italian physicist Enrico Fermi. In the s, Fermi demonstrated that elements subjected to neutron bombardment could be transformed into new elements.
This work resulted in the discovery of slow neutrons, as well as new elements not represented on the periodic table. Soon after Fermi's discovery, German scientists Otto Hahn and Fritz Strassman bombarded uranium with neutrons, which produced a radioactive barium isotope.
They concluded that the low-speed neutrons caused the uranium nucleus to fission, or break apart, into two smaller pieces. Their work sparked intense activity in research labs all over the world. They speculated that it was the uranium isotope uranium, not uranium, undergoing fission. 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.
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