The primary stage has the same basic design as an implosion fission weapon, described in section 1. After the primary stage is detonated, the x-rays it releases cause the pressure and temperature inside the weapon casing to reach the conditions necessary to achieve a thermonuclear reaction in the fusion fuel.
The yield of the fusion fuel is increased when the fissile rod in its center reaches a supercritical state and begins itself to fission. As the fusion fuel reacts, it releases high-energy neutrons that also fission the uranium nuclei that are in the uranium metal casing wrapped around the fusion fuel. In a typical configuration, fission and fusion each contribute about half the overall energy yield.
These are called enhanced radiation, or neutron bombs. They rely on fusion between deuterium and tritium to produce a lethal radius of neutrons and gamma rays. The goal is to produce a low yield weapon deliverable by an artillery shell, for example that inflicts prompt casualties on troops by radiation but leaves intact structures that otherwise would be destroyed by blast effects. Because fusion releases many times more neutrons than fission for a given weight of fuel, a neutron bomb can create a larger radius inside which there is a lethal dose of nuclear radiation than a small fission bomb can.
A one kiloton neutron bomb, for example, creates about the same lethal radius of nuclear radiation as a 10 kiloton fission weapon. This means that by using a neutron bomb, it is possible to achieve a given radius of lethality with only one tenth of the blast damage that would otherwise be required. These are tactical, not strategic weapons because of their small size. When detonated in the air, they have the additional advantage of producing little residual radiation fallout so it is plausible to think of them as battlefield weapons.
Both plutonium and uranium have been used as nuclear explosives in fission weapons. Plutonium is created when an atom of uranium absorbs a neutron and becomes plutonium The reactor generates the neutrons in a controlled chain reaction. For graphite to succeed as a moderator it must be exceptionally pure; impurities will halt the chain reaction. Heavy water looks and tastes like ordinary water but contains atoms of deuterium instead of atoms of hydrogen. For heavy water to succeed as a moderator, it too must be pure; it must be free of significant contamination by ordinary water, with which it is mixed in nature.
To achieve this separation, a specially shielded chemical plant is needed to chop the fuel rods into pieces, dissolve the radioactive spent fuel in acid, and then extract the plutonium in pure form. This isotope, like plutonium, is unstable and fissions when struck by a neutron. It is, however, found in natural uranium at a concentration of only 0. To be useful in a nuclear weapon, the concentration must be increased. This is accomplished by a process known as enrichment.
Because the isotopes of uranium are identical chemically, the enrichment process exploits the slight difference in their masses. Uranium enriched to greater than twenty percent uranium is called highly enriched. Nuclear weapons typically use a concentration of more than 90 percent uranium Thus, a separate chemical processing plant must be constructed to convert the uranium into gaseous form.
These typically require high-precision manufacturing, which can be accomplished only with specialized equipment or materials. Such components also require specialized testing equipment. Selected components and equipment are listed below. The energy released by a nuclear explosion comes in several forms: pressure from the blast, thermal radiation, nuclear radiation, and an electromagnetic pulse. The damage inflicted by the various effects depends upon the size and type of the explosion.
The detonation produces a drastic increase in atmospheric pressure and severe transient winds. Due to the extreme temperature and pressure created, a massive shock wave is promulgated outward from the detonation point. A standard chemical high-explosive produces only 5, degrees centigrade 9, degrees Fahrenheit. A one megaton explosion can produce third degree burns which destroy skin tissue at a distance of five miles.
The extent to which burns are inflicted depends on weather conditions. The initial radiation consists of neutrons and gamma rays, which can travel great distances, penetrate considerable thicknesses of material, and inflict fatal damage on human tissue. Initial radiation can be intense but has a limited range. For large nuclear weapons, the range of initial radiation is less than the range of lethal blast and thermal effects.
For small weapons, direct radiation may be the lethal effect with the greatest range. For a one kiloton blast, initial radiation levels of at least rem extend out 0. For a one megaton surface blast, the rem exposure radius would be about 2.
Residual radiation is often termed fallout, and it can affect both the immediate blast area and areas farther away. Fallout is caused by particles that are scooped up when the nuclear fireball touches the earth. If the nuclear burst is high in the air, fallout is minimal. The scooped-up particles can be carried some distance by the wind before falling back to earth, and their concentration in any one location depends on local weather conditions.
Fallout can cause severe contamination to soil, vegetation and groundwater. A steady northwest wind, for example, blowing across a one megaton ground burst in Detroit, could carry enough residual radiation to inflict acute radiation sickness to exposed persons in Cleveland.
The residual radiation decays over time, by a factor of ten after seven hours, a factor of after 49 hours and a factor of 1, after two weeks. Depending on the conditions of the blast, radiation levels can persist above permissible peace time levels for months or years in areas around the explosion.
This can also be estimated as a ring inside which the mean lethal overpressure is approximately five pounds per square inch. This is the amount of pressure needed to collapse a typical residence. Imagine that a nuclear weapon is detonated in Washington, D. Using the definition of lethal radius as the area inside which the mean overpressure is five pounds per square inch, the lethal radius for such an event with weapons of various yields can be calculated. Table 2 and the map figure display the five pounds per square inch radii for weapons with yields of one kiloton, 20 kiloton, kiloton and one megaton.
Table 2 - Five psi radii for various yield nuclear weapons Weapon Yield 5 psi radius km 1 kiloton 0. The radii given in Table 2 assume that the weapon detonates in air at the optimum height for creating damage.
The bombs dropped on Hiroshima and Nagasaki, Japan, for example, exploded at a height of approximately 1, feet.
Buildings in Washington, DC are limited to feet. A weapon delivered in a vehicle, such as a cargo van, would not detonate at the optimum height.
An air burst creates little fallout because the fireball does not touch the ground. In , North Korea successfully tested a nuclear weapon as powerful as the atomic bomb that destroyed Hiroshima. The underground explosion was so significant that it created an earthquake with a magnitude of 4.
While the political landscape of nuclear warfare has changed considerably over the years, the science of the weapon itself -- the atomic processes that unleash all of that fury -- have been known since Einstein. This article will review how nuclear bombs work, including how they're built and deployed. Up first is a quick review of atomic structure and radioactivity. Before we can get to the bombs, we have to start small, atomically small.
An atom , you'll remember, is made up of three subatomic particles -- protons , neutrons and electrons. The center of an atom, called the nucleus , is composed of protons and neutrons. Protons are positively charged, neutrons have no charge at all and electrons are negatively charged. The proton-to-electron ratio is always one to one, so the atom as a whole has a neutral charge. 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.
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. 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. Physicist Seth Neddermeyer at Los Alamos constructed a design for the plutonium bomb that used conventional explosives around a central plutonium mass to quickly squeeze and consolidate the plutonium, increasing the pressure and density of the substance.
An increased density allowed the plutonium to reach its critical mass, firing neutrons and allowing the fission chain reaction to proceed. To detonate the bomb, the explosives were ignited, releasing a shock wave that compressed the inner plutonium and led to its explosion.
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