Friday, February 13, 2015

Nuclear Weapon Design and Operation

In the previous three articles we focused on the science behind chemical and nuclear bombs. The Science of Bombs and Chemical Versus Nuclear Weapons examined how chemical and nuclear explosions work. Then, Nuclear Weapons focused further on how nuclear weapons, utilizing fission and often fusion as well, tap into the incredible power of nuclear binding energy. Now that we understand the science behind a nuclear weapon, we can explore how that information is utilized to create a nuclear bomb.

There are two basic types of nuclear weapons based on the type(s) of nuclear reaction(s) involved. Their names can be confusing, much of the confusion having to do with the highly classified environment in which these weapons were developed. All manufactured nuclear devices utilize an explosively rapid fission reaction. Some bombs, particularly those developed in the past are pure fission bombs. These bombs are often called atomic bombs, atom bombs or A-bombs, which is misleading because the energy for these weapons comes from the nucleus inside the atom, as it does in fusion bombs as well. Fusion bombs never consist of just a nuclear fusion reaction in isolation. In these bombs, a fission reaction must also be used in order to trigger the fusion reaction, which then releases far more energy per kilogram than fission does. Fusion bombs are often called thermonuclear, hydrogen or H-bombs. Again, "thermonuclear" can be misleading, as both fission and fusion are nuclear reactions that release enormous heat energy.

The "thermo" in thermonuclear weapon is derived from the fact that a very high temperature is required to initiate the fusion reaction, ignoring the equally important requirement of high pressure. The fission reaction is required to heat the fusible atoms to a plasma state and to heat that plasma to its ignition temperature while keeping it all together long enough to initiate the fusion reaction. Here, a large input of energy from fission is required in order to release a far larger output of energy from fusion. The name "hydrogen" bomb comes from the fact that isotopes of hydrogen - deuterium (H-2) and tritium (H-3) - fuse together in the reaction.

Fission Weapons

These were the first nuclear weapons built and so far they are the only type of nuclear bomb ever used in warfare. All pure fission weapons involve some way of getting a subcritical mass of fissile material to supercritical mass, where as much of the fissile material as possible detonates in a very rapid fission chain reaction.

Two fissile elemental isotopes are commonly used in fission weapons - uranium-235 (used as an example in the previous article) and plutonium-239. The critical mass of a sphere of uranium-235 is 52 kg and for plutonium-239, it is 10 kg. In reality, these two metals are never found as pure isotopes. Instead, they exist as part of an isotopic mixture where other isotopes present are fissionable but not fissile (the difference is explored in the previous article). Enriched uranium, for example, means a uranium mixture in which the percentage of uranium-235 is high enough to sustain a chain reaction. The level of enrichment is an important factor in determining critical mass, along with various other factors including shape and pressure (or density). These factors are explored in the previous article.

There are two main types of fission bombs - gun-type and implosion-type. Little Boy, the fission bomb dropped on Hiroshima, was a gun-type uranium bomb, while Fat Man, dropped on Nagasaki, was an implosion-type plutonium bomb.

Gun-Type Fission Weapon: Little Boy

In a gun-type bomb, one subcritical mass is shot into another subcritical mass in order to reach supercritical mass and detonation. Little Boy was a test of this design. It was the first uranium-based detonation in history. The Trinity Test in July 1945 was the first nuclear test explosion; it tested the implosion-type Fat Man design and it used plutonium. A mock-up of Little Boy is shown below. This design was declassified in 1960.

Little Boy was about 300 cm long and 71 cm wide, weighing 4400 kg. It contained just 64 kg of enriched (about 80% urnanium-235 on average) uranium. It wasn't an efficient design, despite its horrific damage.

Below is a scale map of the extent of blast and fire damage to Hiroshima following Little Boy's detonation at about 600 m altitude, provided by the U.S. Strategic Bombing Survey. Both types of damage zones were roughly 3.2 km in diameter, where a surface temperature of 6000°C was reached. Countless people, along with all records of their existence, were gone. The lethal radius of intense neutron and gamma radiation was 1.3 km.

The legend is difficult to read. The largest version of the map is slightly clearer.

Less than 1 kg of the uranium actually underwent fission. Almost all of the entire output of three giant uranium enrichment plants in Oakridge, Tennessee blew apart before it could fission. The design was highly inefficient, allowing the fissioning uranium to rapidly expand and become subcritical before it could all fission, at which point it was no longer dense enough to sustain the chain reaction.

This is how Little Boy worked: inside the bomb a chemical explosive sends a 39 kg hollow uranium "bullet" down a barrel onto a cylinder of uranium, shown below.

Dake; Papa Lima Whiskey; Mfield; Wikipedia
The bullet, being hollow, is subcritical until it is driven onto the 26 kg uranium cylinder, powered by a cordite charge. Cordite is a smokeless low explosive that replaced gunpowder in the late 1800's. The combination of bullet and target becomes supercritical when the bullet is still 25 cm away. There are enough neutrons being spontaneously emitted from the 20% uranium-238 present to sustain the reaction when the pieces get close enough. One generation of the chain reaction takes less than a microsecond, during which time the bullet travels just 0.3 mm. This introduces a certain percentage risk of a pre-detonation, where the bomb blows apart before most of the fissile material has a chance to fission. A schematic diagram of pre-detonation is shown below right.

At the top of the diagram (right), one fissile mass is accelerated toward another fissile mass. If it is not accelerated fast enough, stray neutrons emitted from spontaneous fission will cause the two surfaces to start a chain reaction before the full critical mass is formed (before they are in contact). A much smaller explosion occurs (bottom image, right) blowing the containment apart and preventing the rest of the fissile material from fission. Two design elements prevent this. First, the cross section of the bullet is kept low enough to control the number of free neutrons being emitted (cross section is explored in the previous article). This is done by manipulating the isotopic makeup of the bullet. Second the bullet must be traveling sufficiently fast down the barrel. Speed can be increased by making a longer thicker barrel (more time to accelerate).

When Little Boy was designed there was only enough U-235 to create one bomb so it could not be tested beforehand. Little Boy detonated properly (though inefficiently) but there was a chance that it could have pre-detonated instead.

The creation of the Little Boy gun-type design was part of the top-secret Manhattan Project in Oakridge. Originally, researchers focused on using plutonium-239 as the fissile material. However, in 1944, the scientists discovered that reactor-bred Pu-239 was contaminated with Pu-240. P-240 had the effect of increasing the material's spontaneous neutron emission rate. This would have made pre-detonation inevitable. Additionally the barrel could not be made long enough to accelerate the bullet to high enough speed in order to prevent predetonation, at least not one that could fit onto an aircraft. Thereafter, they knew that gun-type bombs could only be made with enriched uranium.

Although still inefficient, the gun-type design was later refined and used to make earth-penetrating Mark 8 and Mark 11 bombs, commonly called nuclear bunker-busters. Researchers were confident that this type of bomb would reliably detonate even after being blasted into the ground. After the Mark bombs, a family of nuclear artillery shells also utilized this basic design. All gun-type weapons have since been retired and dismantled as of the early 1960's in favour of more efficient implosion-type weapons, now the only type of fission weapon in use worldwide.

Uranium-235 and plutonium-239 can be used separately or in combination in implosion-type nuclear devices, so the International Atomic Energy Agency attempts to monitor and control worldwide enriched uranium and plutonium supplies. Highly enriched uranium-235 is currently used in nuclear submarines (up to 96% enrichment) and in research reactors (up to 93%), in which maximal neutron production is desired. This high level of enrichment increases the reactor's power density and extends the life of the fuel but it is more expensive and is a greater risk to nuclear weapon proliferation. Uranium enriched to over 90% is called weapons-grade uranium. Supergrade plutonium (over 95% Pu-239) is used in U.S. Navy nuclear weapons while the U.S. Air Force uses conventional plutonium. Highly enriched supergrade plutonium, while more expensive, contains a very low amount of Pu-240, which is a high spontaneous fission isotope. Pu-240 is also a gamma radiation emitter, so submarine crewmembers would be exposed to too much gamma radiation from nuclear weapons stored onboard if conventional plutonium was used.

Implosion-Type Fission Weapon: Fat Man

The Trinity test device called the Gadget and the Fat Man bomb that dropped on Nagasaki were nearly identical designs. Both used plutonium-239 as the fissile material. Fat Man used only 6.2 kg of Pu-239, just 41% of spherical critical mass. Early research indicated that while an implosion-type mechanism would be far more efficient, a gun-type mechanism had less uncertainty associated with it. Therefore, a plutonium gun-type bomb called Thin Man was designed first. Soon after, researchers realized that Pu-239 could not work in a gun-type design so an implosion-type bomb called Fat Man was designed. A replica is shown below.

Weighing 4670 kg, it was 3.3 m long and 1.5 m wide.

High explosives in shaped charges could be used to implode a sphere of fissile plutonium. This meant a very rapid increase in plutonium density, making it all supercritical almost at once, and it had the additional advantage of requiring far less material. This mechanism created a much more efficient bomb. For an effective implosion, shaped high-explosive charges were designed into a three-dimensional explosive lens. Much like how an optical lens works, the spherical lens focuses several spherical diverging shock waves into a much more powerful singe converging shock wave. The following flash X-ray image sequence shows the converging compression shock wave formed during a high explosive lens test.

Wikipedia describes the interesting story of how this bomb was developed as part of the Manhattan Project. The troubleshooting processes involved are especially interesting to read. For this design to work, the plutonium in the centre needs to be cast into a sphere. During tests, the density of the sphere always came up as a different value, at first leading researchers to worry about contamination, but the problem turned out to be that plutonium comes in various allotropes, all of which vary in density, from 16 g/cm3 to about 20 g/cm3. This means that plutonium has a very complex and poorly understood phase diagram. Much like how water transitions from ice to liquid water to water vapour gas, plutonium transitions from a dense brittle alpha phase at room temperature to a much more malleable plastic beta phase at a slightly higher temperature and then to an even more malleable and even less dense gamma phase and so on, as shown in the graph below left, where increasing atomic volume means decreasing density.

This problem was solved by creating an alloy of plutonium by adding a small amount of gallium, which stabilized the density of the material and greatly reduced its thermal expansion as well.

As in the case of all nuclear weapons, the timing of the start of the chain fission reaction is critically important. If the chain reaction starts too soon, the bomb will pre-detonate, resulting in a reduced yield. For this reason the fissile material must have a low spontaneous neutron emission rate (low cross section). If the reaction starts too late, the fissile core will already be in the process of expanding into a less dense state in which the yield may be reduced or the reaction might not happen at all if the core is no longer at critical mass. To precisely control reaction initiation, the Fat Man was designed with a modulated neutron initiator. Located in the center of the plutonium pit, it kick-starts the fission reaction at just the right time. Activated by the converging shockwave from the synchronized high explosive detonations, it produces a sudden burst of free neutrons. The initiator is a combination of beryllium-9 and polonium-210. The two materials are held apart until the shockwave pushes them together. Polonium-210 is a radioactive alpha particle emitter. Much of its basic physics was classified until after WW II.  A silvery grey metal, it glows light blue because the alpha particles it emits excite the air around it. Beryllium-9 is a stable neutron-heavy isotope, with 5 neutrons. It releases neutrons (n, below) under bombardment by alpha particles, as shown below.

In the center of the plutonium pit, a beryllium pellet and a beryllium shell are separated from each other by a layer of polonium. Layers of gold and nickel over the beryllium shield it from alpha particles. The entire assembly is very small and weighs just 7 grams. The shock wave crushes the shell, mixing polonium with beryllium. The alpha particles from the polonium bombard the beryllium and cause it to emit a burst of neutrons. The neutrons start the fission chain reaction in the now compressed (to twice its normal density) and now supercritical plutonium core surrounding the initiator.

A simplified diagram of the fat Man assembly is shown left. The tamper (navy blue ring), composed of uranium-239, delays the outward expansion of the fissioning material. It holds the whole thing together for an additional few hundred nanoseconds. Because of its inertia, it increases the bomb's efficiency. U-239 also reflects neutrons very well, so it increases the bomb's yield. U-239 is not fissile but it is fissionable, so it undergoes fission by fast neutrons, contributing about 20% to the bomb's yield in this way as well. The animation below shows how the explosive lenses compress the fissile core and trigger the initiator.

Like Little Boy, Fat Man's detonation was not perfect, despite its more sophisticated design and increased efficiency. Only an estimated 20% of the plutonium underwent fission. The rest of it, about 5 kg, was scattered.

Since then, several improvements have been made to the implosion design, all in very short order, a matter of a few years. The thick layer of uranium-239 as well as the explosives made Fat Man fat and heavy. The levitated pit implosion design, creating a hammer-in-nail kind of impact to initiate the fission reaction, more than doubled the basic Fat Man design's yield but it was still fat and heavy. The two-point linear implosion design, though inefficient compared to the levitated pit design, effectively reduced the bomb's size, making small low yield battlefield missiles possible. A two-point hollow pit implosion design followed, increasing the efficiency of small tactical nuclear weapons.

A major improvement in design, which could used to make even smaller more powerful weapons, came with the boosted fission weapon design, first tested in 1951. Though not technically thermonuclear devices, these bombs utilize a small amount of fusion fuel to further increase efficiency and cut down on size and weight. The basic idea behind using fusion in addition to fission is to speed up the rate of fission in the fissile pit so that the required inertial confinement time can be reduced. Fusion is possible because once compressed, the very center of the pit is hot and under enough pressure to trigger this reaction. The newer hollow pit designs meant there is a perfect space to insert a tiny 50/50 fusion mixture of tritium/deuterium gas into the center of the pit.

Fusion-Boosted Fission Weapons

With this design, a thick heavy uranium tamper is no longer needed to contain the reaction. Instead, a lightweight beryllium shell can be used. The beryllium reflects neutrons back into the pit, an additional advantage. The mass of the fissile pit itself can be cut in half and smaller explosive charges can also be used. These weapons are called variable-yield weapons because the yield is highly dependent of the amount of fusible material inside the pit. At any time before detonation, the amount of tritium inserted into the pit can be reduced, reducing the weapon's yield. The first device to use this technology was the Swan device, a two-point, hollow-pit, fusion boosted implosion weapon. It was just 29.5 cm in diameter and 58 cm long. Developed from this design is the Mk-54 warhead, which could be launched from a Davy Crockett gun-type launcher, similar to the one shown below right. This was deployed by the U.S. during the Cold War. Just 11 inches in diameter and weighing 23 kg, it was the smallest nuclear device ever built It could deliver the equivalent of 10 to 20 tonnes of TNT, while also delivering lethal radiation within 150 m of its target, and a likely lethal dose up to a quarter mile away. Below, fat Man and a W54 warhead, the same size as a Mk-54 but with a larger 250 t yield, are shown to the same scale with their diameters in inches shown in red.

Nuclear bombs like the Mk-54 and the W54 warheads are sometimes called suitcase nukes. Developed both in Russia and the U.S. and possibly Israel as well, they are small and light enough to smuggle across borders in a suitcase. Whether these weapons have been or are currently deployed is classified.

As strategically useful as they are, this is not the best technology for making really large bombs. Even boosted with fusion, these weapons would require a great deal of expensive fissile material and tritium to produce yields of 100 or more kilotonnes. While many scientists, who knew of the then-recent devastation caused by Fat Man and Little Boy, were morally opposed to such efforts, work was quickly underway to create a far more efficient method to deliver even more devastating power. If you click the link just above, you will find in the third paragraph a very concise account of the controversy and reasoning used by those involved in the Manhattan Project at the then-secret Los Alamos National Laboratory in order to justify the development of the thermonuclear bomb. The most successful plan was to add on a second stage (called a secondary) to the boosted fission device (now called the primary). This is the general plan of the thermonuclear device or H-bomb.

Thermonuclear Weapons: The Teller-Ulam Design

As expected, detailed information about nuclear weapons, at least those that are in current use, is classified by the nation that possesses it. The basic designs described above have been declassified. Public knowledge such as that which I am relying on, mostly Wikipedia but other sources as well, comes from at least some speculation, some reverse engineering from known facts and comparison with other fields in nuclear physics by experts in the field. One design in particular, the Teller-Ulam design is the basic technical concept behind all modern nuclear weapons. While its details remain military secrets, this is the general blueprint for a thermonuclear bomb.

The first full test of this design was the test detonation of Ivy Mike (10.4 MT, 450 times more powerful than Fat Man dropped on Nagasaki) in 1952. The simple schematic diagram below shows how a thermonuclear warhead basically works. As we explored in the previous article, fusion is a spontaneous reaction but in order to happen, it requires tremendous heat under pressure. A thermonuclear device delivers those conditions. The legend that follows is taken directly from the image page on Wikipedia.

Teller-Ulam hydrogen bomb firing sequence, modified from Howard Morland, The Secret that Exploded (Random House, 1981).
A Warhead before firing; primary at top, first at top. Both components are fusion-boosted fission bombs.
B High-explosive fires in primary, compressing plutonium core into super-criticality and beginning a fission reaction.
C Fission in primary emits X-rays which channel along the inside of the casing, irradiating the polystyrene foam channel filler.
D Polystyrene foam becomes plasma, compressing secondary, and plutonium sparkplug inside the secondary begins to fission, supplying heat.
E Compressed and heated, lithium-6 deuterium fuel begins fusion reaction, neutron flux causes tamper to fission. A fireball is starting to form...

Energy is transferred from the primary to the secondary through X-rays from the fissioning primary. The X-rays compress the secondary fusion cell as well as the spark plug, which starts the fusion reaction. Exactly how this happens is secret.

Because this is the most efficient design for a high yield nuclear device, it is the only type of nuclear weapon deployed by the five nuclear weapon states under the Nuclear Non-Proliferation Treaty (at least officially): United States, Russia, United Kingdom, France and China. Other states that are not members of the treaty possess less well known numbers and types of devices, such as India, Pakistan and North Korea.

Perhaps the most advanced warhead design is the American W88 warhead, designed in the 1970's at the Los Alamos National Laboratory. The Trident submarine-launched ballistic missile can be armed with up to 12 W88 warheads (limited to 8 under the Strategic Offensive Reductions Treaty, as if this makes a difference). This device is shown below.
Dan Stober and Ian Hoffman;Wikipedia
Why Have a Nuclear Arsenal?

Just one W88 could completely destroy a city as large as London, Moscow or New York, killing millions of people. The aftermath of such devastation is unimaginable, considering too that all of the city's support infrastructures would also be obliterated. How vulnerable are these and other nuclear weapons to hacking, a military nuclear accident (sometimes called broken arrows; Wikipedia has compiled an astonishingly long list of worldwide accidents dating from the 1940's to the present), or to terrorism?

I find myself trying to imagine the general state of mind of those researchers in Los Alamos. It was the late 1940's and they were frantically working to design a better more destructive weapon, knowing that Russia at the time was also developing its nuclear program. The paranoid spectre of WW II was still fresh in everyone's mind and the extent of horror of the bombings of Nagasaki and Hiroshima could not have yet been fully realized in the west. At that time, the concept of mutual deterrence must have seemed reasonable to those in charge of the top-secret nuclear program. Today, over 60 years later, that thinking is outdated. The Cold War is over and attention is more focused on terrorist threats made by rogue militant groups not attached to any single country and the increasing threat of cyber-terrorism, rather than the threat of a full-scale nuclear exchange between the U.S. and Russia.

Yet mutual deterrence, still the only rationale, supports the world's present level of nuclear armament, one capable of destroying modern society as we know it. Two superpowers - the U.S. and Russia - have reduced their nuclear inventories, and while both the U.S. and Russia have signed the Treaty on the Non-Proliferation of Nuclear Weapons in which nuclear disarmament is one of the three "pillars' of the treaty, both countries as well as several others maintain large stockpiles of nuclear weapons. According to the Arms Control Association, an American organization the promotes public understanding and effective arms control policies, the U.S. alone currently has over 1600 deployed nuclear warheads, a number closely matched by Russia. China has 250 total warheads, France has 290 deployed warheads and the U.K. has 40 deployed warheads.

Unlike a conventional war, there can be no winner in a nuclear war. Even a limited regional exchange would have dire consequences for everyone on Earth. Current computer models reveal that Earth would suffer a 20-year-long nuclear winter as well as worldwide famine following a "limited" nuclear war. The model is based on an exchange between Pakistan and India, each detonating 50 15-kt nuclear weapons. The yield of each detonation is within the range of the Little Boy and Fat Man detonations (if you scroll up with this link, you will see a comprehensive list of nuclear testing treaties, well worth a look). 5 Mt of black carbon (soot) would be released into the atmosphere, absorbing sunlight. Black carbon rain would kill millions of people. Average surface temperature would drop by 1.5°C by year 5 postwar. This sounds minor, but growing seasons worldwide would be shortened by 10-40 days, causing a significant disruption to food production. The ozone layer would be 25% thinner and possibly up to 50% thinner over populated areas, increasing deadly UV radiation and rates of skin cancer would be substantially higher. There would be a 9% reduction in average rainfall. Even after 26 years, global rainfall would be decreased by 4.5%, leading to widespread crop failure and famine.

If this is not terrifying enough, I've come up with a list of possible post nuclear war effects, based on a variety of online sources.

1. Thermal radiation. The fires at Hiroshima accounted for half of the casualties.
2. Radiation contamination from either ground level or low detonations. The area contaminated by fallout exceeding a safe level of 300 roentgens from a one MT detonation (about the yield of a typical thermonuclear bomb) is thousands of square kilometres.
3. In a war, orderly evacuation is impossible. Millions of people might evacuate one contaminated area only to find themselves in another. Deteriorating sanitary conditions and hunger will lead to more deaths as well as genetic damage to all animal and plant life must be taken into account.
4. Ecological damage is difficult to estimate but entire ecosystems will be stressed or destroyed.
5. Continuous forest fires may destroy the majority of Earth's forests. Smoke could black out sunlight for weeks and a significant amount of oxygen in the atmosphere could be consumed. There is a very real threat of nuclear winter as well, which began to be studied in detail in the early 1980's. A study presented at the American Geophysical Union in 2006 found that even a small-scale regional nuclear war (100 Hiroshima-size nuclear detonations) could disrupt the global climate for a decade or more.
6.  High-altitude detonations could destroy the ozone layer, maximal estimates of the resulting UV radiation could destroy all life.
7. Disruption of transportation and communication, with serious economic impact.
8. Disruption of food distribution, water supplies, fuel and electricity, sewage and waste disposal, medicine, clothing. No assistance for millions of people who are wounded, burned or exposed to radiation.
9. Widespread hunger, the spread of diseases such as cholera, influenza, dysentery, typhus, etc., political chaos, disintegration of rules of law and order.


In short, a nuclear exchange could hurl mankind back into the dark ages, if our species survived. The conclusion of this is that nuclear warfare has no place among humankind. The technology, as fascinating as it is, needs to be mothballed. In the video, White Light & Black Rain: The Destruction of Hiroshima & Nagasaki, survivors said that the reason they agreed to relive and share their memories of the horror and to show their disfigurements on camera was to say that this could never happen again. The link to watch it online can be found in the previous article Nuclear Weapons: Understanding Nuclear Binding Energy.

Perhaps miraculously, no nuclear weapon has detonated since the bombings of Hiroshima and Nagasaki. It is disheartening however that nuclear weapon technology ever advanced at all, following what happened then. It also seems discouraging that once mankind harnessed the awesome power of the nuclear force, it was immediately turned into weapons of mass destruction. We can make a great deal of peaceful use out of nuclear energy. Nuclear fission power plants and possibly future nuclear fusion plants turn nuclear binding energy into heat, which can be turned into electrical power with great efficiency and little waste. However, there are many reasons why nuclear power may not reach a golden age. For the layman, it is not easy to separate in one's mind the threat of nuclear weapons from the promise of nuclear power. Catastrophic accidents such as Three Mile Island, Chernobyl and the Fukushima disaster have also, and with good reason, made many people wary of nuclear power. We will explore the possibilities of nuclear energy, and how it works, next.


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