Nuclear Weapons

Depending on what type of nuclear bomb is being used:

"Atomic" bombs use the principle of nuclear fission. Fission requires a heavy element (one with a high atomic weight) atom to be split into two or more lighter (atomic weight) element atoms. This split process produces several extra neutrons and some energy as a by-product. The excess neutrons go on to cause fission in nearby heavy element atoms, resulting in a chain reaction where an enormous amount of energy is released quickly. In order to start this chain reaction (and explode the weapon), a sufficient amount of fissile material must be created. This amount of material is called the "supercritical" amount. Any amount less than this will simply not explode.

A typical atomic bomb uses Uranium-235 as heavy element. It is also possible to use Plutonium-239. Theoretically, it may be possible to use other trans-uranic elements, but so far, no one has succeeded in such a design.

Atomic bombs are generally of two designs: a "gun-type" and implosion weapon. In the Gun design, two lumps of fissile material (each about half the supercritical amount) are held at the end of a tube several feet long. An explosive charge is placed behind one of the lumps. When it explodes, the lump of fissile material is forced down the tube (in a manner identical to shooting a bullet from a gun) to smash into the other lump of fissile material. Together, they exceed the supercritical amount, and the atomic bomb goes off. In the Implosion design, a hollow sphere of sub-critical fissile material is surrounded by another sphere of chemical explosives. Detonating the explosives creates a shock wave of force, which compresses the fissile material to a point where it exceeds the density needed to achieve criticality. The atomic explosion then occurs. U-235 is generally used in a Gun design, while P-239 is the fuel for an Implosion device.

There are limits to the amount of energy released by an atomic bomb, for mechanical reasons. Thus, the largest (in terms of yield) pure atomic bomb is roughly 100kT.

"Thermonuclear" bombs (often called Hydrogen bombs) work on the principle of nuclear fusion, where two lightweight atoms are pushed together to form a heavier element (generally releasing a neutron and lots of energy). Nuclear fusion requires extremely high temperatures and pressures to occur. Currently, the only way to start a fusion explosion is to use an Atomic bomb.

Isotopes of Hydrogen (H-2 (deuterium) and H-3 (tritium)) are currently the fuel of choice for a thermonuclear weapon, with the resulting create element being standard Helium. A complex composition of exotic compounds containing H2 and H3 is placed next to a fully-working Atomic bomb. When the atomic bomb (the "trigger") is detonated, a shockwave of massive force is created. In the microseconds before this shockwave destroys the H2/H3 compound, it compresses this compound enough that nuclear fusion takes place. Even in this tiny fraction of a second, an enormous amount of energy is release, dwarfing even the atomic trigger's amount.

There is no upper limit to the size of a thermonuclear bomb. In practice, bombs bigger than 1MT are impractical. The largest H-bomb ever detonated was the USSR's "Tsara Bomb" at 50MT.

"Boosted" atomic bombs are those which use the Implosion design of the standard atomic bomb, but fill the hollow center of the P-239 sphere with gas of H2 or H3 just before exploding the weapon. This allows for a limited amount of fusion to occur at the center of the weapon, where the weapon has compressed the most. This boosts the yield of the weapon to up to 500kt, with only a little additional weight and complexity added.

"Hybrid" thermonuclear bombs (often called fission-fusion-fission weapons) are a standard thermonuclear bomb which is entirely wrapped in a spent-nuclear-fuel (i.e. mostly U-238) case. After explosion, the excess neutrons from the fusion reaction cause fission in the U-238 case. However, this particular type of weapon is very "dirty", causing a very large amount of radioactive by-products to be produced (mostly from the U-238 case, which is only partially fissioned). It will, however, increase the yield of a H-bomb by 3-5 times, with the only added issue of the weight of the U-238 case.

The above descriptions are of course conceptual. Actually building a nuclear weapon is very difficult, even if you have all the proper materials available - and, some of those materials are extremely difficult to obtain. There is a great deal of very high-tolerance machine tooling required (that is, many of the parts must be exactly a certain size, with no more than a few thousandths of an inch deviation), and there is very complex mathematics to be worked out to get the design right so that it will operate properly. Even on the very simple "gun-type" design, there is a significant chance that failure to properly calculate the right sizes, amounts, and shapes of the components will result in a dud weapon - one which, while the chemical explosive may fire, will fail to achieve a nuclear (fission and/or fusion) effect.

The term atom bomb (or atomic bomb) usually refers to a bomb that obtains its energy solely through the process of nuclear fission.

However technically the term is considered interchangeable with nuclear bomb, and can refer to any bomb obtaining its energy through either nuclear fission, nuclear fusion, or any combination of the two processes.

Simple explanation:Conventional bombs explode as a result of chemical reactions, but the atoms themselves that make up the chemicals stay unchanged by those reactions. On the other hand, the "Atomic Bomb", also called a "Nuclear Bomb", is so named because it explodes as a result of reactions which actually do change the atoms. When those atoms are changed in this way, they create explosive energy as a direct result of the changes of the atoms.

Technical Answer: To explain how a nuclear weapon (sometimes called an atomic bomb) works, we need to jump around a bit to pick up the necessary ideas that we'll knit together to build this critter. We can start by separating the nuclear weapons into two basic types: there is the fission weapon and there is the fusion weapon. We'll start with the first one and go from there. But first we need to review some physics. Buckle up. Let's take a ride.

Among those quirky elements at the upper end of the periodic table we find a couple or three that are fissile. What that means is that if they capture a neutron, they can fission; the atomic nucleus can be broken apart. They also spontaneously fission, and they do this to some extent all the time. All the elements at the upper end of the periodic table are unstable and undergo radioactive decay; they have no stable isotopes. But this is just a "breakdown" of the nucleus and the ejection of a particle or two and some energy. Fission is actually a "splitting" of the nucleus of an atom. It breaks into "chunks" we call fission fragments. A neutron or two or three is also ejected in the event. You can imagine the violence of this phenomenon on the atomic scale. It's horrendous. A lot of energy is released, and this is the key to the use of these materials in a weapon.

When we consider the fissionable materials, there is a threshold called critical mass associated with them. When it is exceeded, that is, when we "put together" enough material to exceed the critical mass, the material will spontaneously begin to fission. This is because a tiny number of spontaneous fissions occur naturally all the time, and the neutrons released in these events, which always are occurring, will start a chain reaction. (This is actually how an atomic bomb blast or the chain reaction in a nuclear reactor begins.) Enough material is around, that so-called critical mass, that a chain begins spontaneously. There is no way to stop this from happening if critical mass is reached. It will always occur. But which elements do this?

It is uranium and plutonium that we are most familiar with as nuclear weapons materials. Let's just look at them. As regards uranium, only the specific isotope U-235 will work for this application. Over 99% of the uranium in the ground is U-238, and only a tiny portion of the metal is the isotope U-235. We have to refine the uranium to separate the tiny bit of that lighter isotope out to make a weapon. And that's no mean feat! It takes a lot of equipment and energy to process the material and concentrate the preferred isotope sufficiently. We call this process enrichment, and the finished product is enriched uranium. Plutonium is created by exposing uranium to the neutron flux in an operating nuclear reactor and letting it "soak up some neutrons" and transform into plutonium. This is the most common approach to obtaining weapons materials that the nuclear powers use. So we have our nuclear material, and all we need to do now is make a bomb.

There is a thing called "geometry" that we associate with nuclear weapons. It speaks to the sizes and shapes of the sub-critical masses of the fissionable material that we are using in the bomb. It will probably also include how they will be brought together to achieve critical mass. Certainly there will be some safety features associated with the geometry, the physical design of the bomb. Remember that if we put enough fissionable material together to cause a chain reaction to begin, lots of energy will be liberated very quickly. And this energy will serve to "push apart" the material that was brought together to create the critical mass. So just "joining together" sub-critical masses of material won't work for a bomb because the immediate release of energy will force the material apart. There are a couple of basic ways to arrange the sub-critical masses and "force" them together, but that's what will have to be done. The fissionable material will have to be "driven together" somehow to make it work well. This is where conventional explosives come into the picture.

In a bomb, the sub-critical masses will be "blasted together" by a chemical explosive, and this blast will "hold" the material together for a split second to let the chain build to criticality and beyond to the point where the chain is actually supercritical. Proper design and construction will permit a good "burn" of the nuclear material with a large resultant release of energy. A triggering mechanism will set off the conventional charges and they will drive the sub-critical masses together. The "assembled" mass will be a bit more than critical, and the spontaneous fissions that are always occurring in the material will initiate a chain reaction. Atoms will spontaneously fission, the neutrons released will be captured by other nuclei, and they in turn will fission releasing more neutrons. The chain has begun and expands almost instantly. The periods of time associated with the initiation and buildup of the chain are ridiculously short. It all basically happens "in an instant" and the weapon detonates. This is the fission weapon, and a fission weapon is needed to set off a fusion weapon.

In nuclear fusion, protons are fused together to form helium nuclei - at least in the simplest form, which is what is used here. That is how our sun is operating now. At least mostly. Later in the sun's life, the fusion of heavier nuclei will be more predominate. Then heavier still. Anyway, it takes a lot of energy to force hydrogen nuclei (the protons) together and fuse them. This happens in an environment of extreme heat. Only the heat of a nuclear blast can create enough energy here on earth to fuse quantities of hydrogen nuclei into helium nuclei. (This though the laser pumped fusion reactor is still being experimented with.) The fusion of hydrogen, the fusing of those two protons and a pair of neutrons, is the hydrogen bomb. We effectively build a fission bomb "around" a supply of "hydrogen" and set it off to create the fusion explosion. It's really a huge blast. And that's the long and short of it.

There are some other "exotic" weapons out there. The fission-fusion-fission weapon is one that takes it a step further. It just uses the fusion device to trigger another fission device which is built into it. It's really hairy. There are neutron bombs, too, which just pack more "neutron producing material" in the package to generate more neutron radiation to kill things without increasing blast damage - which is already substantial. We've paved the road for you and given you a map to get you started. Should you wish to continue on down it, there will be some school work you'll have to master to fully appreciate the in's and out's of these notoriously quirky devices. Oh, and don't think you'll be "tickling the dragon's tail" any time soon. It takes a PhD and a security clearance above "Top Secret" to even get on the reservation, let alone get in the door.

Naturally we have some links you can follow to learn more, and those links can be found below.

An atomic bomb is any bomb which obtains its destructive energy from the excess binding energy of atomic nuclei. The term is most commonly applied to bombs that release the excess binding energy of heavy atoms by fissioning them to form lighter atoms, but can equally correctly be applied to bombs that release the excess binding energy of light atoms by fusing them to form heavier atoms (however such bombs are most commonly called hydrogen bombs).

The fine details of this are somewhat blurred as most modern nuclear weapons use some combination of both fission and fusion, regardless of what they are called (e.g. dial-a-yield tritium gas fusion boosted atomic bombs, conventionally built hydrogen fusion bombs usually get more than 90% of their yield from fast fission of their depleted uranium tamper) to optimize their characteristics.

The next energy transformation occurs when the two pieces of fissionable material rush towards each other. The atomic particles emited by each piece cause more particles to be knocked loose from both pieces, resulting in a 'chain reaction', where so many particles are emitted that the process is self-sustaining. As the number of nuetrons knocked loose increases, very high temperatures are acheived. Very quickly, all available nuetrons are separated from the atoms of the fissionable material, resulting in a brief period of temperatures of millions of degrees Celsius. This is what is called an atom bomb, or a fission bomb, and is the kind of weapon used on Hiroshima and Nagasaki during World War Two.

A thermonuclear, or hydrogen, bomb, incorporates the above steps, but continues the procss into nuclear fusion, where hydrogen atoms are hurled together at such velocities that they fuse into helium atoms, and, at the same time, release huge amounts of energy. Temperatures reach hundreds of millions of degrees Celsius for a fraction of a second, in a fireball which expands rapidly, cooling as it does so.

So, chemical compounds are burned rapidly, resulting in high temperatures and pressures, which trigger a fission reaction, where atoms of fissionable material actually desintegrate in a flash of temperatures high enough to permit hydrogen to fuse with itself, releasing even higher temperatures. Thus, there are three entirely separate and distinct energy transformations when a thermonuclear weapon is detonated.

I suggest reading Richard Rhodes books: The Making of the Atomic Bomb and Dark Sun.

Wikipedia also contains good explanations.

There are a number of different types that operate by different principles:

1. Fission: splitting large unstable atoms into smaller ones with neutron chain reaction.
2. 1. Gun design, always uses Uranium, fires a bullet and target together with cordite down a cannon barrel to assemble a supercritical mass followed by a pulse of neutrons to start the chain reaction.
   2. Implosion design, can use Plutonium, Uranium, or a composite of both, shaped charge high explosives crush this core to assemble a supercritical mass followed by a pulse of neutrons to start the chain reaction.
3. Fusion: joining small atoms into larger ones at high temperature and pressure. Note that pure fusion bombs (with no fission component at all have never been built).
4. Mixed Fission-Fusion: most modern bombs use some mixture of the reactions to optimize the weapon for the mission, the delivery vehicle, and production costs.

• Fusion boosted low yield tactical fission weapon. These use a small amount of tritium gas inside the fission core to boost efficiency of a tiny low yield core with extra neutrons from the tritium fusion.
• Multi-stage high yield fusion weapons. These even raise yield by fissioning the normally nonfissionable Uranium-238 isotope with high energy fusion neutrons.
• etc.

Einstein has some credit in the development of the nuclear bomb, but never thought of it himself... I believe it was Hitler who first thought of the atomic bomb. The Germans were working on a way to harness the energy from a split atom. Einstein sent the US president a letter, telling him of the power such a weapon could have, and the Germans were working on splitting atoms. That brought about the Manhattan Project.

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The Einstein letter to FDR was actually entirely written by Leo Szilard, all Einstein did was sign it.

The writer H.G. Wells had imagined them and wrote about them in his 1914 novel "The World Set Free", but this was just SciFi.

Leo Szilard was the first to actually invent the process that makes the atomic bomb work (neutron chain reaction) in 1933 while in London and patented this on June 28, 1934.

A team of German scientists working in Germany and Sweden discovered fission in uranium-235 in 1939 and after the publication of their paper, Germany blocked further publication of the results of all research in nuclear physics.

By the time the Manhattan Project became fully active, Germany had already decided to scale back their nuclear project to just basic reactor research with the single goal of building power reactors after Germany won the war. They considered a bomb impractical in the time estimated to win the war by conventional means, even if it was possible (and Heisenberg's calculations suggested such a bomb would be too large for any available aircraft to deliver).

To Hitler the idea of an Atomic Bomb sounded like Jewish Physics, and thus nonsense. Hitler knew nothing of physics, nuclear or otherwise, and the teaching of Einstein's theories of relativity were already banned in Nazi Germany simply because Einstein was a Jew (implicitly making it Jewish Physics) regardless of any experimental evidence confirming the theories.

Although the underlying theory of the atomic bomb workings draws loosely from Einstein's ideas on mass to energy conversion (the famous equation E=Mc2), he did not himself invent the atomic bomb neither did he himself discover fission. Leo Szilard invented the neutron chain reaction that makes both bombs and reactors possible, but Einstein knew nothing of this as the British kept the patent classified from 1936 until 1949. Einstein was an absolute pacifist and refused to participate in any type of war related work.

Before WWII Szilard wrote a letter and had Einstein sign it (as if he had written it) to warn President Roosevelt that Germany was likely working toward the development of an atomic bomb. Shortly after this the US started a project to beat the Germans to the development of this weapon.

The atomic bomb was the product of cooperation of many scientists and engineers participating in the Manhattan project. Chief among the people who unleashed the power of the atom was Robert Oppenheimer, who oversaw the project from conception to completion.

Enrichment of the U-235 from U-238 was conducted at Oakridge Tennessee. Harold Urey and his team at Columbia University devised an extraction system using the principle of gaseous diffusion.

Production reactors to make Pu-239 were built and operated at Hanford Washington.

Bomb casing development was done at Wendover Utah and in southern California.

Production of different parts was contracted out to several dozen companies. Often as the companies were not given information on the things their part attached to (for secrecy) things did not fit right and had to be reworked in the field. For example wartime MK-III Fat Man bombs were virtually custom built with no interchangeable parts. Even the Plutonium core of one bomb would not fit in the pit of others.