Introduction The end of the second world war, with the nuclear bombing of Hiroshima and Nagasaki, marked the entry of the world into the era of nuclear weapons and its terrifying implications. Indeed, the underlying principle of Mutually Assured Destruction (which conveniently abbreviates to MAD), is that all major players on the world stage have the ability to annihilate each other. Through different delivery methods, it is impossible to stop an incoming attack, but at the same time impossible to destroy all means of retaliation from the enemy. This means that no first strike from any player will be able to fully ensure that there would be no retaliation from the attacked nation.Nuclear weapons and their delivery systems have evolved in technology and doctrine since “Little Boy” and “Fat Man”, they have become more destructive and at the same time impossible to intercept, and were at the heart of the cold War.

Even though weapons of mass destruction, and in particular nuclear weapons, are often attacked by politicians and NGOs who want a more peaceful world, one could argue that it is the threat of nuclear annihilation that has prevented the Soviet Union and the United States from attacking each other. Nuclear weapons, because of their incredible power, have kept Europe at peace since the end of WW2, and have now lead to the most peaceful era mankind has ever seen.We will cover the functioning of the main different types of atomic weapons as well as their main delivery system, the InterContinental Ballistic Missile (ICBM), in the optic of demonstrating why it would be impossible to stop an atomic attack or a retaliatory strike.     I/ Functioning of Atomic weapons There are two types of chain reaction: fission and fusion. The first one implies splitting a heavy atom by bombarding it with neutrons, releasing two product atoms, neutrons, and vast amounts of energy. If there are enough neutrons produced and a high enough of fissile atoms, the reaction goes out of hand into a chain reaction. This is precisely what is desired in a nuclear bomb, which is the opposite of what is searched for in a nuclear powerplant where the reaction is kept stable. Fusion happens under very high temperature and pressure conditions, in a plasma, and two hydrogen atoms combine to form a helium atom and a neutron that will then slit, releasing energy.

Fusion bombs (also called thermonuclear bombs), are much more powerful and complex than their fission counterparts. The first fission bomb to be detonated was Trinity on the 16th of July 1945 in the New Mexico desert. The first fusion bomb was Ivy Mike on the 1st of November 1951 in the Pacific Ocean. We will cover how these weapons function and how they differ and have evolved.Explosives are usually measured in their TNT weight equivalent. However, nuclear weapons being extremely powerful, they are usually expressed in kiltons or megatons (KT and MT) of TNT. This means that Tsar Bomba, the most powerful nuclear bomb ever detonated, with its 57 MT was equal to 57 million tons of TNT A)    Fission BombsThe fission bomb relies on the principle of critical mass. The critical mass is the mass of a certain material where it will start a nuclear chain reaction.

It is dependent on the element of the element used.Figure 1: Critical masses of different Elements (ref 1) In fission bombs (also known as A-bombs) the fissile material is kept under-critical until the bombed is set to detonate, at which time the fissile material is then brought to an over-critical state, as over critical as possible to have a more powerful explosion. This can be done in two different ways, either by an insertion assembling or an implosion assembling.In the insertion configuration, two under-critical masses are kept separate while the bomb is being carried around and launched.

Once the bomb is released, before it hits the ground, a chemical explosion inserts a mobile filled in cylinder shape of uranium into an open cylinder of uranium, the combination of the two masses make the material be over its critical mass and start a fission reaction. This is achieved using a chemical explosion that shoots one of the two masses into the other. It is important, to optimize the energy of the explosion, that the two masses are merged quickly. This is the mechanism employed in the Hiroshima bomb, “Little Boy”.The other possible assembly is by “implosion”. In this configuration, the fuel is spread out in a hollow sphere in a sub-critical mass. To start the fission reaction, a large amount of small chemical explosions on the outside of the sphere compress it into a smaller sphere, and the impact and pressure exerted makes the radioactive isotope used (Uranium 235 or Plutonium) go into an over-critical mass.

This is the mechanism used in the Nagasaki bomb, “Fat Man”.The implosion mechanism is preferred over the insertion because there is no risk of mechanical failure and accidentally having the bomb prematurely explode, which is possible in insertion configuration if the mechanism fails and the two components slide into each other. Also, the implosion configuration optimizes the energy released in respect with the amount of radioactive material the bomb has, since all the material is in closer contact.   B)    Fusion bombs Fusion bombs, also called H-bombs or thermonuclear weapons, uses the nuclear fusion reaction to release more energy and therefore be an even more powerful weapon of mass destruction. Nuclear fusion is the reaction that powers the stars, and happens when two light atoms merge, releasing large amounts of energy in the process. However, to start a fusion reaction, the atoms must be in a plasma (a state of matter where electrons are dissociated from their atoms and ‘roam around’ freely).

However, the difficulty is that to obtain a plasma the temperature conditions need to be extremely high (minimum 2 000 °C).               To achieve that plasma, the bomb is composed of two stages: a first stage which is a fission bomb and a second stage that will be where the fusion takes place. When the first stage is detonated, it is going to created large amounts of energy in the form of heat and electromagnetic radiation. The X-Rays emitted turn the polystyrene that is surrounded the fusion fuel (Lithium deuterium 6LiD) into plasma. At the same time, the neutrons produced by the fission reaction make the plutonium arranged around the 6LiD start undergoing a fission reaction as well. The energy liberated from the plutonium and the plasma as well as the initial fission reaction highly compresses and heats the 6LiD to extremely high temperature and pressure. In these conditions it starts a fusion reaction, which liberates gigantic amounts of energy. The fusion also liberates a considerable amount of neutrons (property used in neutron bombs which we will not cover as they are not used anymore and had more of a tactical use rather than assuring mutual destructing ability), which is then going to filter out and activate U238 that is used as a buffer material between the polystyrene and Lithium, adding an extra fission reaction.

     II/ Intercontinental Ballistic Missile                 The idea of ICBMs was first developed in Germany during World War 2, to be able to attack and threaten the US from the other side of the Atlantic. During the war, the V1 and V2 missiles were able to deliver a payload of up to 980kg of explosives to a range of 320km. These rockets were used against British cities at the end of the conflict and were launched from the north of Belgium and France. At the end of the conflict, in the context of “Operation Paperclip” (exfiltration and recruiting of German scientists by the Americans at the end of the war), the US military began developing its own missiles. The project was put aside until the Soviet Union launched R-7 Semiorka to a range of 6 000 km (this missile was then used as the launcher for Sputnik). The arms race then took over and the US Air force and the Soviet Union developed missiles with longer range, enhanced load capacity and additional precision.

The name ICBM indicates that the range of the missile is extremely big (being able to strike the enemy wherever he is, the longest-range ICBMs can target at is 12 000 km, twice the London – New-York distance), and that the missile mainly depends on gravity (ballistic). ICBMs work on the same basis than satellite launchers: the first few stages are extremely similar to booster stages (it is the same company that builds Arianne 5 boosters and the French ICBMs) and will push the missile into space at an altitude of varying between 150 and 200 km. The missile has at this point lost all its booster stages. It then enters the intermediary phase, during which the missile, now mainly comprised only of the warheads, will reach an apogee of around 1200 km of altitude.

Once it reaches an altitude of around 100km, the missile starts the re-entry procedure and separates the warheads as well as a large number of decoys. That way, a single ICBM can hit multiple targets, while making it impossible to intercept all them, making protection against a retaliatory strike, be it just a few missiles fired from a rogue submarine, impossible. The precision of each individual warhead to its target is estimated to be of around 50 to 100 meters precise, which however doesn’t matter much in the case of nuclear weapons.The reasons the orbit is extremely elliptical and in way long (~4 minutes of boosting, ~25 minutes of intermediate phase and then around 2 minutes for reentry).

This choice is made because this path is the lowest energy path, meaning that for the same amount of propellant a larger payload can be carried. Otherwise, a depressed trajectory, which has a much lower apogee and is less energy efficient, would however be much faster and would therefore reduce the chance of the missile be tracked down and intercepted.  Figure 4: ICBM trajectory Conclusion:     References: Figure 1:Monitoring nuclear weapons and nuclear-explosive materials, by the National Research Council, Policy and Global Affairs, Committee on International Security and Arms Control, 2005 Figure 2 and Figure 3: 4: sources:éfense et Sécurité internationale no 35, mars 2008   


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