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Nuclear Weapons


           I am, as are most people, fascinated and repelled by nuclear weapons. They are horrifying; but at the same time, they are what has kept the peace for decades, and what makes the United States a superpower. People often think it odd that the military strategy of the fifties, sixties and seventies seemed to be that we would destroy civilization, in order to save it. Such misinterpretations spawned the old “Better dead than red” types of slogans. It was not the destruction, but the threat of destruction, which was used to preserve western civilization. What is also often not made note of, is the fact that the strategy worked. The United States is now the world superpower, while the Soviet Union no longer exists.

            Though this was all classified, during the Cold War, it is now publicly known that the United States had a peak total of over 40,000 nuclear warheads. This peak occurred around 1968. The Soviets had a peak total of something like 45,000 warheads. This number has been greatly reduced, since the signing of numerous treaties between the two powers. We presently have something like 10,000 warheads; but the nature of the treaties, which generated these reductions, makes this a bit less meaningful than most people might think.


Nuclear explosives

      The main obstacle to creating nuclear weapons, and to becoming a nuclear state, is the procurement of fissionables. Only a few fissionables are suitable for use as nuclear explosives. Of these few, only Uranium 235, and plutonium 239 have been made available in quantity. Even to developed nations, like the United States, the production of U235 is a difficult, time consuming, and very energy intensive process. Plutonium, though more expensive, is more easily made in quantity, particularly in reactors optimized for it's production. This is the main reason that plutonium tends to be used more often than U235 in bomb making. Plutonium is quite a bit easier to get, as it is a waste product of nuclear reactors, and it's production can also be increased by lining the walls of a reactor with the far more abundant U238, thus transmuting it eventually to plutonium. The plutonium can then easily be chemically separated from the rest of the materials. A quick and dirty rule for plutonium production is that a 1000 MW reactor produces 1000 grams of plutonium per day and could produce roughly enough plutonium for one weapon every 6 days. This is a pretty big reactor, and it would have to be set up specially for producing plutonium, in order to get the full benefit of the yield. Reactors designed for plutonium production tend to use metallic fuel, and are unpressurized. Those optimized for power production, tend to be pressurized, and use oxide fuel pellets. Both types of design are capable of producing both, power and plutonium.

            In total, the U.S. had over 100 tons of plutonium, as well as significant amounts of uranium (as much as 1000 tons of U235), and thorium, the last time a number was given. Today, the amounts are probably higher. Though we stopped operating reactors for plutonium production in 1992, and have shut down our gaseous diffusion plants, these numbers continue to increase, as plutonium is produced as a waste product of nuclear power reactors. Though it was once highly sought after, plutonium is now considered to be a nuisance, and a proliferation danger, as the United States now has considerable amounts of excess plutonium from disassembled weapons. As of this writing, the United States has around 10,000 warheads stockpiled, and Russia has a similar number. China has several hundred, and the various European nations have, by comparison, insignificant numbers. When arms reduction agreements are signed, many people interpret these as requiring the destruction of nuclear devices; but this is not entirely the case. To understand why, a brief outline of the mechanics of nuclear weaponry is needed.


Mechanics of Nuclear Warheads

      Most people have the mistaken impression that all you have to do is put a sufficient amount of nuclear materials together, get a chain reaction going, and --- poof --- you have a big mushroom cloud. This is pretty far from the truth. Nuclear weapons are probably more difficult to make work than nuclear power plants. For a nuclear weapon to work, the core must be made to go supercritical, and then be held together, against it's natural tendency to want to blow itself apart, long enough for at least 56 generations of reactions to occur. It must also be made to go critical very quickly, without the nuclear version of pre-detonation.

       The first nuclear reactors were graphite moderated, because a graphite moderated reactor is capable of burning regular un-enriched uranium.  It was soon understood, however, that this was not a suitable material for the making of atom bombs. Two approaches were considered, and each was pursued. The first was to try and concentrate the tiny amount of U235, existing in regular uranium. Since chemical extraction was impossible, the concentration would have to be increased by taking advantage of the physical differences between U235, and U238. This equates to a difference in weight of 1.5%. A large plant was set up at Oak Ridge Tennessee, using a large scale gaseous diffusion process to perform this concentration. The code name of uranium 235 was "Oralloy", because it was created at Oak Ridge (Oak Ridge ALLOY). Natural uranium was called "Tuballoy". A gaseous diffusion plant is huge, so that the material may be processed hundreds or thousands of times, and is also a prodigious consumer of electricity.

        There was also the possibility that one of the nuclear byproducts, of uranium reactions, would be a suitable nuclear explosive.  This element was produced when U238, rather than U235 absorbed a neutron. This converted it into U239, which quickly beta decays into neptunium 239. This in turn, beta decays into the then mysterious element 94, which was eventually to be named plutonium. This new element, even before any was actually seen or produced, was calculated to hold the potential to be an excellent nuclear explosive. It also had the great advantage of lending itself to standard chemical separation techniques. The Hanford facility was set up, in Washington state, to produce and purify plutonium. The advantage of plutonium was that it could be produced in a graphite moderated reactor, and then chemically separated, making quantity production easier, and faster. Plutonium was code named "Copper", real copper was "Honest to God Copper".

     There are two types of nuclear fission warheads, which have been built. The gun type, as demonstrated by the old Little Boy bomb, is pretty much considered to be obsolete. It is quite a simple device. The other, more complex device uses explosive lenses, and was first built as the Fat Man bomb. For all practical purposes, the elements used in all nuclear weapons are either uranium 235, or plutonium 239. Though it is possible, in theory, to use any radioactive substance with a critical mass, all of the other known radioactive elements are either too unstable, too expensive, in too short a supply, or a combination of these factors. Though it is presently the favored material for the construction of the core of a nuclear weapon, plutonium has it's own share of disadvantages, when compared to uranium.

      Unlike the chain reactions of nuclear power generators, those suitable to the creation of a nuclear bomb can only be caused by fast neutrons. In order to have any chance at all of causing a nuclear explosion, the reactions must take place in a time frame of about 10 microseconds, rather than the 40 microsecond generations of the slow reactors.  The only way that this is even remotely possible, is by using fuel of very high purity. Weapons grade uranium, or plutonium, have purities of 90% - 95%, of the reactive isotope. There is no such thing as a moderated nuclear bomb. What is often overlooked is that a nuclear bomb is a precision engineered device. Nuclear explosions do not occur spontaneously, and nuclear explosives can only be detonated by initiating a series of very precisely orchestrated events, at very exact times.

      These bombs work, as was mentioned above, by initiating a chain reaction, and then holding the nuclear core (also called the pit) together for at least 56

 generations of fissioning to occur - the longer the better. The simplest type of atom bomb uses two masses of uranium, at opposite ends of an artillery barrel. They each consist of about three quarters of a critical mass, and are shaped to fit into each other. An explosive charge shoots one piece into the other, and upon impact, they instantly go supercritical. The great strength of the artillery barrel is enough to keep them together for the required 56 generations of reactions (for a 20kt explosion). The gun type weapon can only use uranium 235. Plutonium does not work in this kind of bomb, because it is too reactive, and will begin low scale detonation, before the two pieces even come into contact. This creates a fizzle, and makes it impossible to hold the unit together long enough for complete nuclear detonation to occur. This is similar to pre-detonation, or backfiring, in a car engine Such a fizzle begins to blow the bomb apart, before the nuclear explosion really gets going. This was the design of the Little Boy bomb, and was also used for some nuclear artillery shells. There are presently no weapons of this type in the U.S. arsenal.
      The most common contemporary design, uses plutonium, and a series of explosive lenses to make a sphere go supercritical. This is the most common design because it can be used with uranium 235, or the more easily produced plutonium. It works by taking a hollow sphere of fissionable material, and compressing it with explosive charges, which are so carefully arranged and calculated that they are called explosive lenses. The sphere reaches criticality through compression. It is actually possible to use a sphere of less than critical mass, and make it go critical, by increasing its density through compression. Recall from the previous section, that critical mass occurs, when the amount of material is sufficient to make it unlikely that a neutron will escape without a collision and fission reaction. Compression has the same effect, by crowding the atoms closer together. Still, the 20 - 40kt yields of the first nuclear bombs were grossly inefficient (the efficiency of the Fat Man bomb was 17%, while that of the Little Boy was about 1.4%). Much of the nuclear material was left unburned. This is the Fat Man type of bomb, and is the ancestor of virtually the entire world nuclear arsenal. Still, though the bomb was produced, and tested, there was much work for nuclear engineers. These men were well aware of the inefficiencies of the bomb, as were their leaders. The original Fat Man weighed over ten thousand pounds, and had a yield of 20 kt. Its modern descendants can weigh as little as a few hundred pounds and still have yields of hundreds of kilotons. Ways also needed to be found to streamline the manufacturing process. The original nuclear bombs were put together by teams of scientists, and engineers. By the end of the war, The United States had produced four nuclear bombs, out of which three had already been detonated (Trinity, Hiroshima, and Nagasaki). More would be needed for the projected new role of the U.S. as a world power, and better ways would need to be found to produce them. The theoretical limit for a fission bomb, before the core expands, or blows itself apart, is around 25 - 50%, depending upon the size of the bomb.

Increasing yield

       Figuring out the potential yield, and the time required, of a particular pit, is easy. The splitting of 1.45x10^23 atoms will yield 1 kt, so the splitting of 2.9x10^24 atoms will yield 20 kt, roughly the amount of the first nuclear bombs. The number of generations that will be required to split this many atoms may be figured exponentially by calculating: 2.9x10^24 = 2^(n-1), which  implies n = (log2 (2.9x10^24)) + 1 = 81.7 generations. So figure roughly 82 generations. Unfortunately, this does not reflect the fact that, during the chain reaction, the rate of change is also increasing, thus we need calculus to get an accurate number.

        Both the number of neutrons present in the assembly (and thus the instantaneous rate of the fission reaction), and the number of fissions that have occurred since the reaction began, increase at a rate proportional to e^((k-1)*(t/g)), where e is the natural log base (2.712...), g is the average generation time (time from neutron emission to fission capture), and t is the elapsed time. If k=2, then a single neutron will multiply to 2.9x10^24 neutrons (and splitting the same number of atoms) in roughly 56 shakes (560 nanoseconds), yielding 20 kilotons of energy. This is one-third less time than the previous approximate calculation. Due to the exponential rate of increase, at any point in the chain reaction 99% of the energy will have been released in the last 4.6 generations. It is a reasonable approximation to think of the first 53 generations as a latency period leading up to the actual explosion, which only takes 3-4 generations. Taking this a step further them, a tamper, which can hold the core together for another 46 nanoseconds, can increase yield by 99 times. The entire yield of the bomb is a direct result of the number of generations of reaction that can be sustained.

       Anything that will increase the time that the fissionable core stays together, or decrease the time of a generation of fissions, will cause an increase in bomb yield. There are essentially two known ways to increase the efficiency of a nuclear bomb. Ways can be found to hold the pit together longer, so that there will be time for more generations of reactions to occur, or ways can be found to make the reactions go more quickly. Of the two, the easiest, and most effective is to make the reactions go faster. This, in turn, may be accomplished in two ways. The first is to compress the core, so that the atoms are closer together. This happens as a matter of course in an implosion type bomb (Fat Man), and is the main reason that this type is so much more efficient than the gun type. If compression forces the atoms to half of their ordinary distance, then the generation time will be cut in half, and twice as many generations will occur in the same amount of time.

        Unlike regular high explosive, adding more uranium, or plutonium, to the bomb, will not necessarily make a fission explosion larger. It is the amount of material actually  fissioned, which produces the yield of the bomb. This is, in turn, determined by how quickly each generation can be made to occur, how many different starting points there are, and how long the pit can be held together in super criticality. In a core that has been compressed to quadruple density, which is standard for a modern implosive type bomb, thus crowding the atoms closer together. Though the pit quickly begins to expand, and then blow itself apart, during the period of maximum compression, a shake takes 2.5 nanoseconds, rather than the more normal 10.  Adding more material, in a pure fission device, will only increase yield, if the integrity of the core can be maintained for enough generations to make the amount of fissile material the limiting factor.

      The compression created by the explosive lenses in a Fat Man type of bomb, occurs very rapidly, typically taking 1000 to 4000 nanoseconds. The period of maximum compression lasts less than 1000 nanoseconds. Recall that the cascade of fissions for a 20kt bomb needs 56 generations, which would take 560ns. This does not leave much margin for error, and leaves little room for chance. Because of this, nuclear weapons do not rely entirely on the normal spontaneous release of neutrons, from the pit. Instead, at the split second at, or just before, maximum compression, neutrons are injected into the pit.

       Today, a typical plutonium pit will be 80 mm in diameter (about the size of a baseball, or grapefruit), weigh 2.5 kg, though some may weigh as much as twice that amount, and have a dual core, or levitated pit design. The levitated pit was first used in the X-Ray test device, in 1948. This type of core has a hollow sphere, with a sold or hollow sphere in the middle of the hollow space. The smaller sphere is suspended within the hollow sphere using pins or small four pronged holders which look a bit like a set of children's jacks. In some cases a single pedestal is used. The idea behind the levitated pit is that you do not drive a nail, by resting a hammer on top of it. Instead, you raise the hammer up, and allow it to come down onto the nail. Levitated pit designs permit the use of as much as 25% less material, or are capable of doubling the yield. The outer sphere is generally surrounded by a shell of beryllium, and possibly U238. Outside of this is the shell of explosive lenses, which will contain between 20 and 50 different explosive lenses, and will usually weigh several times more than the pit itself. These use combinations of fast and slow explosives, and are set off by precision triggering circuits so that a compressive shock wave will implode the pit. Though I mentioned above that for calculation purposes, it could be considered that the core is compressed to four times it's usual density, in actuality, the pit of a modern warhead will be compressed anywhere from 10 - 21 times normal density, at maximum compression. At the same time, the hollow space in the core will be injected with boosting gas, and a shower of neutrons will be generated by a special neutron gun, to insure a positive start to the chain reaction.

      The reactions may also be made to go faster, by introducing more neutrons. This approach is known as boosting. Boosting generally works by having a material which injects extra neutrons into the core, during the explosion, or reflects neutrons back, which would otherwise have escaped. With more neutrons in play, more reactions occur, at any given time. The most favored boosting materials are tritium, and possibly a mixture of polonium, and beryllium. These can be used to greatly boost yields, and are the secret to the adjustable yield bomb. Many warheads can have their yields changed in the field, by adjusting the type or amount of boosting material injected,

       Early boosting of nuclear weapons was done with a combination of beryllium and polonium. Polonium is an extremely potent source of alpha radiation, while beryllium gives off neutrons when struck by alpha rays. The combination of the two metals acts as a very powerful neutron source. Such a combination has been used as a trigger, and as a boosting source. Serrated spheres were used, plated with gold to keep the metals separate from each other, until the reaction was initiated. At the moment of detonation, the spheres were shattered, and the two metals mixed, to give off a shower of neutrons. This technique is now considered to be obsolete, though it is still a viable means of boosting or triggering a nuclear device.

       The main drawback to using polonium is it's short half life of 138 days. Due to this, polonium quickly looses it's radioactive punch. The technique presently used, is to inject a combination of tritium, and deuterium into the hollow central portion of the pit. Once detonation takes place, the shock wave causes these two isotopes of hydrogen to fuse. During the fusion, huge numbers of neutrons are given off, as well as a bit of heat and pressure. Only about four grams of materials are injected, and the fusion reaction is not powerful enough to add significantly to the power of the bomb. The neutrons given off are enough to significantly increase the speed of the chain reaction, and to fission a considerably larger amount of the fissile pit, thus greatly increasing the efficiency of the explosion. The magnitude of the effect can be seen by noting the yields of the adjustable yield warheads. Boosting is reputed to be able to confer a 2 to 25 fold increase in yield over the same pit and weapon design, not employing boosting materials. The first tritium boosted weapon was the Item device, detonated in 1951 as part of the Greenhouse series. The boosting gas is generally pumped into the center of the pit through the cone which levitates the inner portion of the pit. It may also be forced into the space between the inner and outer spheres.

      Yields can also be increased by reflecting neutrons back into the core, or by constructing a bomb casing which will hold the core together for just a few more

 microseconds. A good tamper can accomplish both. If you figure that every generation of fission can more than double the yield of the bomb, and each generation takes only nanoseconds, it becomes obvious that a casing that will hold together for just a few fractions of a second can make a great difference in yield. Often, inner casings are made of beryllium, or uranium 238, both of which are hard, heavy, and reflect neutrons. In the case of U238, there is an additional bonus. During a thermonuclear explosion (discussed in more detail below), depleted uranium (U238) can be transmuted into various nuclear explosives, and actually add to the yield of the bomb, even to the extent that it can contribute a major portion of the ultimate yield.
       It has been said that, a fission bomb is in a race with itself: to successfully fission most of the material in the pit before it blows itself apart. The longer the pit can be held together, the more powerful will be the explosion. As soon as the pit heat expands sufficiently to loose critical mass, or blows itself apart, the reactions stop, and no further energy is released. With all other factors being the same, the pit that can be held together longer, will generate a more powerful explosion. The pit is held together in a couple of different ways. The first is by a tamper, usually of beryllium, or uranium, sometimes of tungsten, all very hard, heavy, and tough metals. This will generally be in contact with the pit itself, and will also reflect stray neutrons back into the pit to help speed up the reaction. The second, is the shock wave traveling through the pit, which may still be compressing it, even as the reaction has begun. Third, is the inertia of the pit itself. In addition, the case itself may be constructed of a heavy materials which will act as a sort of a second tamper. A generation of fission ordinarily takes about 10 nanoseconds, a very short period of time. For convenience, nuclear engineers call this a shake. For an implosion type bomb, this time be cut in half, or even more.

        Once the chain reaction begins, escaping neutrons will be reflected back into the core by the tamper. After about 1% of the pit has fissioned, the tritium/deuterium boosting gas will begin to fuse, creating a huge amount of neutrons, and adding a significant amount of heat and pressure, to the already very energetic early reaction environment. At this time, the core will still be compressing, even though nuclear reactions have already begun; but soon enough the heat and explosive force will begin to retard compression, and begin a process of expansion. When the core expands to a point, where it's density will no longer support a chain reaction, no more energy is released. In order to extend the energy release, and increase bomb yield, the tamper is designed to hold the core together as long as possible. Though the perceived explosion may go on for many seconds, this is simply the bomb transferring the already released energy to the environment. Once the pit density is no longer sufficient to support a chain reaction, energy release stops.
       All of these tricks are designed to get the most out of the scarce plutonium, or uranium 235. A bomb weighing hundreds, or even thousands of pounds will contain a pit weighing, perhaps twenty pounds (some weigh as little as 13 pounds). Most of the rest of the weight is from tamper, and casing. This is because Uranium and plutonium are very expensive, and their supply is the limiting factor in bomb production. It is also pointless, to add more material, unless the pit can be held together long enough for the fusion of these additional amounts. Uranium is a fairly common metal, and uranium 238 (depleted uranium) is quite common, and cheap, cheap enough that it is used as a counterweight material. Depleted uranium has been available for as little as a few dollars a pound. Spot price of natural uranium has varied between a low of $7 a pound in 2003, to over a hundred dollars a pound. As of this writing (June, 2008) natural uranium metal is selling for $59 a pound, due to an oil price inspired, anticipated resurgence in nuclear power generation. Uranium 235 is a different matter (pardon the pun). Pure U235 is not sold as such. Instead it is sold as a fractional portion of natural uranium, commonly called enriched uranium. Enriched uranium is sold at a cost determined by the amount of enrichment. Presently, on the DOE website, 3% enriched uranium (reactor grade) is selling for $1000 per gram, while 97% enriched (weapons grade) is selling for $3000 per gram. The current cost of plutonium is about $4000 per gram. A modern nuke has about 2.5 kg of plutonium in the pit, for a raw material cost of around ten million dollars. A uranium based weapon would need about four times that amount, costing around 30 million dollars. In some warheads, as much a twice the amount of fissionable material is used. Even so, this is just the beginning.

       Uranium, and plutonium are exceptionally difficult metals with which to work. Both are hard, chemically volatile, radioactive, and highly toxic. In addition, they must be carefully monitored, and surrounded by as tight a security system as is possible to devise. Once the pit has been painstakingly machined, then the rest of the bomb must be produced, around it. The rest of the bomb consists of the precision explosive lenses, the very advanced triggering and timing circuits, as well as the tampers, reflectors, neutron sources, and boosting devices. As there are very few pure fission bombs in the arsenal, there will also be a second stage, with a fusion device, as described below. Then there will be the safety devices, the security components, and the various coded arming systems. Because of all of these factors, a single nuclear warhead will cost tens of millions of dollars. Only a wealthy state can afford to be a nuclear power.


Fusion Weapons

      The most effective way to increase the yield of a nuclear weapon, and the most cost effective, is to initiate a fusion reaction.  This can be done in many ways, and was initially done using liquid hydrogen. Today's fusion warheads, also known as thermonuclear devices, use solids, such a lithium, or lithium deuteride, to provide hydrogen fuel through transmutation. These are set off, through the use of a standard fission bomb, which acts as a trigger. Today, such warheads are being made very small, and very powerful. Rather than breaking up large nuclei, fusion weapons work by merging small nuclei, in the case of the modern thermonuclear warhead, hydrogen is fused into helium.

        For it’s weight, a fission explosive produces 12.5 million times the power of a chemical explosive. A fusion explosive produces another 8 times this total, or approximately 100 million times that which could be produced by chemical explosive actions. It should also be noted that nuclear materials (uranium, plutonium) are exceedingly dense.

        Soon after the successful testing, and production of uranium/plutonium atom bombs, it was thought desirable to pursue the development of hydrogen bombs. There were a number of reasons for this. In theory, a hydrogen bomb could be made far more powerful. Though the power of fission bombs was being increased (up to a high of 500kt), there were limits to how long an expanding core could be contained, and thus to the number of chain reaction generations, and the ultimate yield of the bomb. Such limits were much higher on a fusion bomb. Indeed, if the Sun is considered to be an immense hydrogen bomb, note might be taken that it features no containment at all (well, technically, it's very high gravity is acting as a tamper), and is still burning, after billions of years. There was also the matter of the high cost of fissionables. It was thought that a fusion bomb could be made more powerful, and for considerably less money, than a bomb of similar yield, but more conventional design.

        Nuclear fusion is what powers the stars. It is the conversion of hydrogen in to helium. In really large, old or violent stars, it can convert helium into even heavier atoms, including uranium. It gives off huge amounts of energy, as the evidence of our own sun indicates; but the reaction can only occur under conditions of great heat and pressure. From the beginning, scientists, and bomb designers concurred, that the only way to make such a device work, was by providing the necessary initial energy through the use of a fission bomb. The fission bomb would be known as the primary, while the fusion stage would be called the secondary. The primary would be used as a trigger to set off the secondary, just as the conventional explosive lenses, and neutron generators are used to set of the primary. Still, quite a task was laid before them. Hydrogen can be an exceptionally difficult gas with which to work. It is quite explosive, very light, and is difficult to contain. In addition, it's density is very low. It seemed that the only way to get sufficient amounts, and sufficient density, would be to go with liquid hydrogen. Such a device was exploded in 1952; but hardly qualified as a bomb.

       The Ivy Mike device was constructed on Eniwetok Atoll, taken from Japan after the Second World War. It's detonation obliterated an entire island. The Mike device was derisively referred to as a thermonuclear installation. It was essentially a factory building, designed to produce a thermonuclear explosion. The photo to the right gives an indication of the size of the structure. Note the man sitting, to the lower right of the photo. The structure weighed in at 82 tons, and included a large thermos bottle, to the left of the photo, in which the liquid hydrogen was contained. Even had the size been more manageable, the use of a cryogenically liquefied gas was impractical for any type of deployed weapon. The 10.4 mt explosion created a 3 mile wide fireball. This was over twenty times the power of the most powerful practical fission warhead, and was roughly 1000 times the power of the bombs dropped on Japan. Still, you can't airlift a building, and drop it on a target, and it seemed unlikely that the Soviets would let a construction crew come in and build one at a targeted location. The Ivy Mike shot was a proof of concept test, rather than a weapons test.

        The Mike device not only demonstrated the viability of a fusion weapon; but also the possibility of significant cost savings, and yield increases through the use of U238 as a tamper, and mantle. This isotope had formerly been considered as a waste product; but it was now discovered that it could be made to release nuclear energy, during a fusion explosion. The five ton U238 mantle produced 77% of the device's yield. So many neutrons were given off, during the fusion reaction, that nearly any amount of U238 could be set in place, as the mantle, and be fissioned to release energy. This created the potential to produce a bomb of any size desired, using the relatively cheap U238 as fuel. This would be an exceptionally dirty bomb, as there would be considerable fallout from the leftover mantle; but in war, such things are rarely considerations. Still, a way had to be found to scale the size and weight down, and make the bomb more reliable, and storable.

         The transmutation of uranium 238, during the thermonuclear reaction of the Ivy Mike device, and it's subsequent availability to add energy to the explosion set nuclear weapons designers to thinking. It was proposed that certain isotopes of lithium might be transmuted into the required hydrogen isotopes, during a fission explosion, just as the uranium 238 had been transmuted during the fusion reaction of Mike.. The compound lithium deuteride was a solid, in which the lithium might be transmuted into hydrogen, to fuse with the deuterium. As the two substances were joined into a metallic compound, they would be at a density unachievable through any form of cryogenic storage of their gaseous components. Such a compound would make possible small, light solid fuel warheads, which would be storable, and deployable.

       Such a device, called a "dry" H-bomb was first tested in 1954. The predicted, and hoped for yield, was 6mt, and it was called the Shrimp, probably due to a size comparison with the gigantic Mike device. The Shrimp weighed 23,500 pounds, and is shown in the photo to the left.  Shrimp that it may have been, the device was still fifteen feet long, and over four feet wide (actual dimensions were 179.5" x 53.9"), in addition to weighing over ten tons. The device used lithium deuteride, as the fuel for the secondary, and had a mantle of uranium 238. The lithium had been enriched to around 40% lithium-6, as this was determined to be more amenable to fissioning into hydrogen than the more common lithium-7. This may very well have been correct; but as it turned out, it was irrelevant. Designers would have preferred a much higher proportion of lithium-6; but separation plants for isotopes of lithium had just been constructed, and more highly enriched supplies were not yet available. There was another test device, called the "Runt", which used an even even lower quality lithium, enriched to only 7.5%. The Runt test was almost cancelled, due to very grave doubts, as to the possibility of fusion occurring at such  a low level of enrichment. The Runt was predicted to have a yield of 4 mt, if it worked at all. Like the Shrimp, the Runt had a heavy mantle of U238.

        Instead of a 6mt yield, detonation of the Shrimp produced an 15mt explosion, using it's only partially enriched lithium. This was, unintentionally, the largest United States atmospheric test. The 4mt Runt, which used lithium that was hardly enriched at all, and was half expected not to work, detonated with a force of 11mt. It was eventually determined that lithium-7 was more amenable to fission than had first been though. In addition, the heavier lithium-7 nuclei tend to split into the far more energetic tritium. Though the lithium-6 nuclei were easier to split, and were the first to react, they acted as a trigger to set off the lithium-7. You may recall from the above section, tritium is a formidable boosting agent. Once produced, by fission of lithium-7, the tritium would fuse with the deuterium, and produce a far more energetic reaction, as well as many extra neutrons to fission the uranium 238 mantle. Of the 15 mt yield, 10 mt was from fission of the uranium mantle - far more than had been expected. A photo of the Shrimp shot is shown to the right.

        Though the discoveries made by the Shrimp, and Runt shots were a great boon to weapon designers, they came as a pretty unpleasant surprise to the men working the shots. The increased yield also came as a pretty unpleasant surprise to the inhabitants of a number of Pacific islands, which suddenly needed to be evacuated. dangerous levels of radiation traveled as far as 300 miles away. The shrimp was the largest nuclear warhead every tested by the U.S., though this was not by design. Today, pretty much every weapon in the U.S. arsenal is based upon this design, though there has been considerable effort made to lighten and increase the efficiency of the basic design. A typical modern warhead is diagramed in the image below. It is a gravity bomb warhead; but the designs of SLBMs and ICBMs follow along the same line, with some differences in detail.

        A contemporary nuclear warhead will have a very high yield to weight ratio, though the ultimate yield and weight will depend upon the mission. Because of the increased accuracy of modern delivery systems, today's warheads tend to have lower yields than those of the past. Gone are the gigantic, multi megaton warheads of the fifties, and sixties, to be replaced by newer, lighter, more efficient designs. Today's most powerful nuclear bomb is the 1.2 megaton B-83. The entire assembly weighs around 2400 pounds; but the warhead itself is probably only about a third of the total weight. ICBM, and SLBM warheads, generally have yields around 300kt - 475kt, and weigh 400 - 600 pounds. The warheads for the cruise missile weigh less than 300 pounds, and have yields of 100kt - 150kt. Though details may vary, and much of the design work is classified, the diagram to the left is widely regarded as being a fairly good representation of the internals of most of our current generation of nuclear gravity bombs. For those who must know, the most powerful American nuke ever deployed was the 25 MT B-41, which weighed 10,670 pounds, and was deployed from 1960 until 1976.

       While the original dry deployable warheads appear to have used a cylindrical secondary, and spherical primary, many modern thermonuclear warheads use spherical secondary's, and, ellipsoid primaries. Still, the basic theory of operation remains the same. Please note that this is inferred by scientists and engineers, and that nowhere does the government publish any official documentation regarding the structure, or function of nuclear warheads.

        The first stage in the detonation of a thermonuclear bomb, is the triggering of the primary. A primary is a boosted fission device, basically a standard atom bomb, with a yield of around 5kt. Un-boosted, such a primary would have a yield of around .2 kt. In the diagram to the left, the primary is at the top; you might recognize it, by comparison to the standard fission bomb warhead, show towards the top of this section. .  The primary produces heat, radiation, and pressure, enough to start the secondary. Note that the boosted yield is 25 times the unboosted yield. The first stage is the trigger; but as most of the yield of the warhead will come from subsequent reactions, it need not be particularly powerful, nor particularly efficient. Note also, the dial a yield control, which controls the amount of boosting material, and thsu the ultimate yield of the bomb.

       In the early hydrogen bombs, the fusion portion, known as the secondary, was a large cylinder. Early on it had been thought, or at any rate hoped, that merely putting fusionables in proximity to the that and pressure of a nuclear explosion would do the trick. It turned out be a bit more complicated. In the design finally made operational, the fusion stage consists of layers of lithium 6-deuteride fusion fuel, encased within a mantle of U238. In the cylindrical design, a part of the fission energy from the trigger is channeled down an internal tube of Pu239 or U235, called a spark plug. Often, the spark plug is encased in a pusher of U238 or U235. the secondary is the long cylinder shown below the primary in the diagram above. This spark plug ignites, creating heat, pressure, and a cascade of neutrons to transmute lithium into deuterium, or tritium.

        While the spark plug is igniting on the inside of the secondary, High energy X-rays are concentrated to make a high pressure plasma of the polystyrene foam surrounding the outside of the secondary. This compresses the fusion fuel and inner spark plug tube and its pusher, both fissioning, creates conditions to convert the lithium-6 fusion fuel component into tritium which then fuses with the deuteride under the intense pressures created by the outer plasma and inner spark plug fission reactions. Once the fusion reaction begins, it sustains itself pretty well, creating enough heat and pressure to burn up a large amount of it's fuel. In addition, it produces huge numbers of neutrons. These neutrons are present in sufficient numbers, with sufficient energy, to transmute, and then fission the surrounding mantle of uranium 238, even without a tamper. So though adding more fissionable material to a standard fission bomb would not increase yield unless the bomb was sufficiently contained, adding material to the mantle of a hydrogen bomb will continue to increase yield, up to the point where the neutrons generated by the fusion reaction are no longer sufficient to completely fission all of the material. These fission-fusion-fission processes deployed in a thermonuclear warhead can be cascaded by increasing the number of blanket wraps in the fusion-fission stage. Bomb designers can calculate, from the known amount of neutrons generated by a given yield of fusion reaction, just how much U238 can be consumed in the mantle. The largest nuclear weapons ever produced are of this type. The 50 MT Tsar Bomba detonated by the Soviets was such a weapon. It was designed to have a yield as high as 100 MT.


Variable Yield, and other Advanced Feature Warheads

     As was mentioned above, the Soviet Tsar Bomba was a variable yield bomb. many modern bombs are designed to have their yields changed, either through mission determined retrofitting, or even on the spot, by the dial a yield system. Dial a yield is a system for metering the amount of boosting gas that is pumped into the pit. It can increase or decrease the primary stage yield by as much as 25 times. It is a simple system to use, quick and easy to change on the fly. The currently deployed B-83 nuclear bomb can be adjusted to have a yield from 1200 KT, down to just a few KT.

        Yields in modern warheads are adjusted by a system of boosting gas regulation, and through a system of adding and removing panels, rings, and donuts of U238 from around the mantle. Change of the removable mantle sections is not easily accomplished in the field, and is generally done ahead of time at forward bases, or before deployment.

        This system gives a considerable amount of flexibility to the  field commander for tactical use. Yields can be set according to proximity of friendly troops, or amount of damage desired. For strategic use, or retribution, it may be assumed that maximum yield is always used.

        In addition to variable yield, the advent of the ICBM, and SLBM made warhead weight an important factor. Available bombers were (and are) capable of carrying huge bomb loads, particularly with development of in-air fueling. Missiles, particularly submarine launched missiles, were different. The SLBM had to be made small enough to fit in a submarine of reasonable size. It was also desirable to have the capacity of a number of missiles aboard each boat. The original Polaris subs held 16 missiles each. These missiles, in addition to their size and weight constraints, also needed to be capable of enough range, so that a sub could hit a useful target from out in the ocean. What this meant was a rather low payload, compared to what a bomber could carry. Smaller, lighter warheads were needed. When cruise missiles were developed, an even smaller warhead was required. Suddenly, it was weight and size that mattered, rather than amount of fissionable used.

        These new warheads were made possible, in part, by the increasing amounts of plutonium, and U235 available. As it was no longer the first priority to get the most out of each gram of fissionable, and as pretty much the whole stockpile now included fusion secondary's, it was no longer necessary to used huge, heavy tampers or cases, to increase fission efficiency. Because of this change in approach, the new warheads could weigh in at as little as a few hundred pounds, and yet have yields of several hundred kilotons. Eventually, these advances made their way to gravity bombs. Part of the new compact design was the use spherical secondary's, rather than the cylindrical secondary's of the original designs. Even more recent designs have ellipsoid primaries, for which it would have been impossible to design and calculate the lenses, before modern computers.

        Some of these advanced fusion designs include encapsulated tritium-deuterium to accelerate the lithium- dueteride reaction through a lithium hydride or beryllium moderator. Though it is impossible to make a moderated fission bomb, moderation can be used to improve the efficiency of a fusion bomb, and greatly increase the neutron production of the secondary. This permits the fission of much larger U235 mantles, and makes for a far more powerful bomb. In a single stage fusion design of this type, if the U235 mantle is omitted, the high energy fusion neutrons are directly released. The result is called a neutron bomb. Use of materials other than U235 in the mantle, is known as salting, and would produce a bomb deadly beyond imagining, though with much less destructive force. If this sounds like a contradiction, it is not. Salted bombs were also known as Doomsday Weapons, and have never been produced by any nation. A salted bomb uses a non fissionable material in the mantle. When such a material is exposed to the high energy neutron flux produced by this type of secondary, it does not add to the energy of the explosion, as a U235 mantle would. Instead, it becomes radioactive, and is spread as fallout. The radioactive isotope produced depends upon the original material used.

        The stereotypical doomsday weapon, is the cobalt bomb. This would be a thermonuclear bomb, with enhanced neutron production, as described in the paragraph above. Instead of a mantle of U238, this bomb would have a mantle of cobalt. The neutron exposure would convert the normal cobalt into the cobalt-60 isotope. This isotope has the property of being radioactive enough to be lethal, while at the same time having a long enough half life to be widely dispersed while still deadly. Most nuclear fallout is either too radioactive to remain long in the environment, or is very long lasting, but not radioactive enough to be lethal. Cobalt-60 has a half life of 5.27 years, more than enough time for wide dispersal. At the same time, it is radioactive enough to kill anything exposed to it. This is the ultimate dirty bomb. Other salting materials have been conjectured, such as zinc, or even gold; but cobalt is cheap enough, easy to machine, and will do the job. It is unlikely that a cobalt bomb, or any other type of salted bomb will ever be built. Such a device will destroy the maker as certainly as it will destroy the target against which it is deployed. There is no future in the deployment of salted bombs - literally.

        With the end of nuclear testing, it is unlikely that any new nuclear weapon designs are forthcoming any time soon.  America presently has over 100 tons of plutonium, from which to make weapon pits, as well as something like 1000 tons of weapons grade uranium. A typical pit takes from 9 to 13 pounds of material, though some can take up to several times this amount. Many new designs have been proposed, and have been proven through computer simulation; but it is only a matter of time until the tests resume.


Effects of Nuclear Weapons

        The effects of nuclear weapons are horrible beyond description, and I say this as a supporter of nuclear weaponry, and of nuclear power. The destructive effects come in several stages, according to the speed with which the energy released can be transferred across the environment. The actual energy release takes place in a fraction of a second, and is all over by the time any effects are noticed. Within 500ns - 1000ns, the reactions are over, and all of the energy has been released. It is the transference of this quickly released energy, which we observe as the effects of the explosion.

        First comes the photon induced radiation, traveling away from the core of the nuclear reaction, at the speed of light. This consists of gamma rays as well as X-rays, and an intense flash of light. The flash is so powerful that it is capable of causing blindness at 30 miles away, for a present day nuke. Watching films of atomic tests, shows how amazing this phase of the explosion can be. Even for the 20kt Trinity device, considered a very small nuke by today's standards, and not even employing a thermonuclear stage, an observer was blinded at 10 miles away. The initial flash can cause third degree burns many miles from the explosion site. Test films show paint burning off of cars, houses, and other structures. Anything that can burn, will burst into flames, including clothing. Anything that can melt will show signs of melting. If you are wearing polyester clothing, it will melt to your body, even if you are miles from the explosion.

        Then there is the blast. The effects of a nuclear blast have to be seen to be believed. This is not just the bang, of a conventional explosion, which lasts for a fraction of a second. The blast, and overpressure from a nuclear explosion is more like a howling wind, or a super hurricane, and seems never to stop. It is so powerful, that it actually creates a vacuum, as the air is blown out from around it's center. Eventually, the pressure subsides, and the air rushes back to fill the vacuum. You can see this effect on the old nuclear test films, where the blast seems to move first in one direction, and then the other.

       For certain types of weapons, the blast is preceded by a burst of neutrons. Much other radiation is given off by a nuclear detonation; but most does not travel very far from ground zero.  Alpha and beta particles do not travel very far, even when ejected from a nuclear explosion. Alpha particles generally travel at about 0.6 the speed of light, while beta particles only attain about a tenth of light speed. Still, this is a considerable amount of speed and energy. For the most part, alpha and beta particles are stopped pretty quickly; but their energy does not just go away. It is transferred to the surrounding environment, as they are neutralized by it. Neutrons travel quite a it farther, usually many miles, and will seriously affect any living thing in their path.

        Finally, there is fallout. This is simply the rain of radioactive dust, particles and debris, which are produced by the fission reaction, the stream of neutrons, and irradiation from other radiation at the site of the explosion. Part of the fallout will also consist of the fission particles from the core explosion itself, and any irradiated and not fissioned pats of the actual bomb and bomb casing. It can take as long as several hours for fallout to descend, to reach a given area, and these particles can remain lethal for days or weeks afterwards. This was the reason for the home shelters, so popular in the fifties, and early sixties. A nuclear attack is a nightmare, in every detail.

Longevity and nuclear stockpiles

     The frustrating thing about producing and particularly about maintaining nuclear warheads, is the chameleon like quality of all of the materials being used. This capricious behavior was useful in the early days of nuclear development, permitting many problems to be sidestepped, by transmutation. In particular, it permitted the transmutation of large amounts of the plentiful U238 into plutonium, greatly speeding up the production, and stockpiling of nuclear materials, when compared to the painstaking separation of U235 from natural uranium. It also permits a virtually unlimited fission warhead yield, by using casings of U238, which is transmuted during detonation. Granted, this produces an exceptionally dirty bomb; but war is war. It also greatly reduced the weight, and eased the manufacture, of fusion devices, by permitting the transmutation of lithium (actually, lithium deuturide) into hydrogen, during the nuclear fission of the first stage of the device. There is, however, a downside to this nuclear chimera.

     These warheads do not remain viable forever, particularly if they are boosted. Recall that boosting is done, largely, via the insertion of tritium. With a half life of a bit over twelve years, tritium needs to be replaced periodically. Even more critical are the polonium/beryllium packages used as neutron triggers, or initiators. Polonium has a half life of a bit over 138 days. Solutions have been found for some of these problems. Neutrons can now be generated though electronic assemblies, and are no longer necessarily dependant upon the half lives of isotopes. Even this, though, is only a partial solution. The materials of the core pit itself, undergo change, particularly of plutonium fueled warheads.

      Many people are under the impression that nuclear weapons can be stockpiled, and kept in readiness forever. This is not quite true. Most of today's nuclear warheads use plutonium as a nuclear explosive, and thus have shelf lives of between one and three decades. Because of this, they need constant attention. Plutonium gives off quite a bit of heat, and radiation, which causes the explosive lenses around the pit to degrade. These lenses must be replaced. As plutonium decays, it produces unwanted impurities, which compromise its usefulness as an explosive. From time to time, plutonium weapon pits need to be replaced, and their plutonium recycled and purified. All of these materials have a nightmarish quality for anyone who must work with or around them. They are also very demanding of other materials with which they are combined to form nuclear devices.

      The same instability which permits plutonium pits to be a quarter the weight of uranium pits, also means that the quicker decay introduces more impurities, more quickly. In particular, P240, and Americinium 241 begin to build up. These elements have the dual disadvantages of making the pit both more radioactive, and less reliable. In addition to radioactivity, plutonium pits give off quite a bit of heat. A plutonium pit has a normal temperature of around 400 degrees. This, along with the radiation, degrades the explosive lenses, which need periodic replacement. The odd numbered isotope, plutonium-241, is also a highly undesirable isotope as it decays to americium-241 which is an intense emitter of alpha particles, X and gamma rays. Plutonium-241 has a half-life of 13.2 years which means americium-241 accumulates quickly causing serious handling problems. Eventually, the pit itself will need to be removed, and reprocessed, to purify the plutonium. This is why the idea of weapon stockpiles, and the fear of old former Soviet weapons is a bit misleading. These devices need constant attention, and periodic inspections, as well as component replacement. to remain viable. Fusion fuel in the secondary's will keep for a long time, fission fuel in the primaries will not.

   Because of its larger critical mass, and the difficulty in purifying it to weapons grade levels, uranium 235 was seen as somewhat less desirable, by weapons designers, in favor of plutonium, which was initially easier to accumulate, and could be used to make smaller lighter warheads; but now the emphasis seems to be changing. With decades of constant operation, American gaseous diffusion plants have actually produced far more U235, than the amounts of plutonium produced by our reactors. In part, this is due to the difficulty of licensing for new reactor production. Because of this, we now have something like ten times the amount of U235, than our stockpile of plutonium. So we are seeing much more use of uranium primary's, or uranium/plutonium composite primary's. Plutonium has been favored because it is possible to make a plutonium pit of a quarter the size and weight of a corresponding uranium pit; but uranium has it's own set of advantages. Uranium 235 is far more stable than plutonium 239, meaning that warheads can go for much longer periods of time, without being recycled. When the Minuteman missiles were designed in the sixties, they were given thermonuclear warheads with uranium core triggers. This was because the Minuteman was designed to be lowered into its silo, and then left alone, with no maintenance crew, for years at a time. As we are no longer in a race with the Soviets, and have a pretty comfortable margin compared to any possible enemy, we are once again considering large scale use of uranium weapons. We have also, by now, accumulated a pretty good stock of U235, more than enough to maintain our present stock of nuclear weapons. When U235 decays, it transmutes into Thorium, which can be chemically separated and transmuted into U233, a potent fissionable, for possible use in reactors, or perhaps even in a new line of nuclear weapons.  

        Unlike uranium 235 and 238 (which decay into thorium - from which chemical separation is relatively easy), when plutonium 239 ages, it decays into other forms of plutonium. The even numbered isotopes (plutonium-238, 240 and 242) fission spontaneously producing high energy neutrons and a lot of heat. In fact, the neutron and gamma dose from this material is significant and the heat generated in this way would melt the high-explosive material needed to compress the critical mass prior to initiation. The neutrons can also initiate a premature chain reaction thus reducing the explosive yield, typically to a few percent of the nominal yield, sometimes called the "fizzle yield". Such physical characteristics make aging plutonium extremely difficult to manipulate and control. With a half life of roughly 24,000 years, these changes occur much more rapidly, than in uranium 235 with it's 710 million year half like, or U238 with it's 4.5 billion year half life. So in addition to becoming much more difficult with age, plutonium ages much more rapidly that uranium.

       In the photo above, I am standing next to a pair of casings for modern nuclear warheads. These devices are 15 - 20 times more powerful than those developed for WWII, and only weigh a few hundred pounds. The power to weight ration of these is around 1 kt per pound, as compared to that of the Fat man bomb, which was around 1 kt per hundred pounds. The casings are obviously only mock ups, probably made of aluminum. In real life, these casings would be lined with uranium 238, which would have turned black due to its reactivity, and rapid oxidation upon exposure to the air. The diagram above, gives details as to the internal workings of these warheads. One kilogram is equivalent to about 22 million kilowatt hours of heat energy. The complete detonation of a kilogram of plutonium produces an explosion equal to about 20,000 tons of chemical explosive.

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