fission to fusion

The first nuclear bomb “Little Boy” was dropped on Hiroshima on the 6th of August 1945. How far have we come in terms of nuclear warfare since then? and how are nuclear bombs actually designed?

We have actually recessed in the advancement of nuclear weapons. For a good reason, they’re incredibly destructive and devastating to our environment. In 1917, Ernest Rutherford split the atom. When an unstable atom is split into two or more nuclei it releases unstable energy as well as smaller nuclei that can then split other atoms. This process is known as nuclear fission.

how does nuclear fission work?

Within an atom, there are different forces acting on the particles, holding it together. The stability of a nucleus depends on the forces holding it together. Uranium comes in three different isotopes that have varying levels of stability u-235, u-236, and u-238.

The forces holding together a nucleus consist of protons which are positivley charged and neutrons that have no charge but a slightly greater mass. Like forces repel each other and opposite forces attract each other. Some heavy nuclei have a proton neutron imbalance and not enough binding energy to hold the nucleus together, these are called unstable atoms and will lose protons and neutrons as they attempt to become more stable.

U-235 has an odd number of neutrons (143) which makes it more susceptible to nuclear fission. When you inject a U-235 nucleus with a neutron it creates a really unstable nucleus that can easily split. It is also good at absorbing slow moving thermal neutrons which creates the unstable nucleus that can trigger a fission reaction. U-238 in comparison is too stable and requires too much thermal energy to be able to induce nuclear fission.

uranium isotopes

U-238 is the most abundant isotope of uranium, which makes up about 99.28% of naturally occurring uranium. Unfortunately U-238 cannot sustain a fission reaction in a thermal nuclear reactor. One thing to note as well, only heavy atoms can sustain a fission reaction in the first place, such as Uranium and Plutonium.

Fission begins with a U-235 atom being injected with a neutron. Essentially the neutron collides with the U-235 nucleus and is absorbed, increasing it’s mass and energy. For a brief moment, U-235 becomes U-236. This results in Nuclear excitation, where the nucleus starts distorting and vibrating. It splits into two fragments which releases a ton of kinetic energy which gets converted into heat (this is what’s used in nuclear power plants to heat water to generate electricity). It also releases other neutrons that hit other atoms and continue the chain reaction.

how does nuclear fusion work?

Why is Fusion critical for an explosive weapon? The primary mechanisms, chemical explosives and nuclear fission are easy to discuss in depth. If we want to use a more advanced form of nuclear explosions, using fusion as a secondary explosion is the way to go.

Fusion is critical for hydrogen bombs or thermonuclear warheads. It often goes hand in hand with nuclear fission which is used for the primary explosion while nuclear fusion is used for the secondary explosion.

Fusion is the same reaction that also powers stars and the sun. It works by having two light nuclei like Tritium and detrium fuse together. The total mass of the heavier nucleus which is the result of fusion is less than the combined mass of the two nuclei, meaning, there is energy produced from the fusion.

We need to remember, atomic nuclei are positively charged. One of the fundamental principles of physics is that opposite charges attract and that like charges repel. How do we overcome this to produce nuclear fusion? To overcome electrostatic repulsion, we need to meet several conditions, first that the nuclei have to be brought incredibly close together, meet very high temperatures to form a state of plasma. Plasma is a superheated matter where atoms have their electrons stripped away, which forms an ionized gas.

Once the atoms are close enough together, the nuclear force takes over and they fuse together. This releases mass amounts of energy, according the Einsteins equation E=mc^2 This energy is released as heat, light, and particles.

Deuterium and Tritium are two isotopes of hydrogen and the most efficient isotopes we can use to create a nuclear fusion reaction. All forms of hydrogen, only have one proton but the number of neutrons varies based on the isotope. This results in a lower coloumb barrier, which is also known as the electrostatic barrier. Deuterium occurs naturally in nature and is pretty common with 1 out of every 6500 hydrogen atoms. Tritium on the other hand is a radioactive, unstable atom that deteriorates quickly.

fissile material and the nuclear fuel cycle

Material than can sustain a fission reaction has to be heavy and unstable. We know that. Uranium is also abundant within the earth’s crust, U-238 at least. Because the percentage of fissile material is quite low within naturally occurring uranium, it needs to be enriched to a percentage of at least 4%-5% U-235 to sustain a fission reaction.

To enrich Uranium, the process begins with actually mining it first. Most uranium is mined via in-situ leaching. Water that has been injected with oxygen or and oxidising solution such as an alkaline or acid is put through uranium ore to dissolve uranium, which is then pumped to the surface. The uranium solution is turned into ‘yellow cake’ or uranium oxide via filtering and drying it out.

enrichment stages of uranium

The uranium oxide is converted into a gaseous form, uranium hexafluoride (UF6) through conversion. There are two main methods for enriching uranium, the less common one is diffusion where the UF6 is fed through porous barriers, because of the mass differences between the molecules, U-235 moves through the barriers slightly faster than U-238. The form of uranium enrichment, centrifugation, involves spinning UF6 through centrifuges as the heavier atoms will spread to the outside of the cylinder while the lighter U-235 atoms accumulate in the centre of the centrifuge.

The image above describes centrifugal force where the walls of the cylinder keep the UF6 gas inside. The centrifugal force is generated by the cylinder rapidly spinning. As we can see, the meavier atoms such as U-238 are pushed to the outside, against the cylinder’s walls and the U-235 are concentrated towards the centre. Often UF6 is fed through multiple centrifuges in a cascade, to achieve a higher concentration of fissile material. An important thing to note is the higher speed of centrifugation, the higher the difference in concentration.

In the context of nuclear weapons. Uranium has to have an extremely high concentration of U-235 compared to fuel. Nuclear power plants only require a concentration of 10% U-235 while a nuclear weapon requires a concentration of 90% U-235.

anatomy of an atomic bomb

Once uranium has been through the enrichment process, it can be condensed to a core. This pit/core can be lined with chemical explosives. During the manhattan project, the first potential design for the atomic bomb was called the “gun-type”, Little boy, the first bomb that was dropped on Hiroshima also used the gun type design.

The gun type is essentially two small pieces of a critical mass which cannot sustain a fission reaction on its own. To detonate the bomb, a gun fires a small bullet of fissile material into the target, which assembles the two pieces of critical mass and triggers nuclear fission. Around the fissile mass is a tamper, the job of a tamper is to reflect back neutrons into the fissile material.

This style of atomic bomb only really works for uranium, all attempts that used plutonium resulted in the fission of the material before the bomb was actually assembled. Thats why “fat man” used implosion instead of a gun-type bomb.

The other commonly used bomb was the implosion-type bomb. Implosion-type bombs are a lot more common and rely on chemical explosives being organized around the subcritical mass. Implosion is vital if the subcritical mass is made from plutonium instead of uranium. “Fat Man” which was dropped on Nagasaki on August 9th 1945, used the implosion-type bomb. One of the main differences between gun type and implosion-type is the shape of the core. Gun-type subcritical mass is often hollow so it can be injected with a critical mass bullet while implosion-type is a compressed solid sphere.

The process of detonating a bomb, obviously begins with chemical explosives being detonated around the critical mass. The purpose of the chemical explosives is to condense the core even further, before an atom is injected with a neutron, to create an explosion.

When the chemical explosives are detonated to trigger nuclear fission, there needs to be some kind of material to contain the chemical explosives to push the critical mass towards itself. This is where a tamper comes into play. It delays the thermal expansion of critical mass, while reflecting neutrons, which allows the mass to stay super-critical for longer. For example the tamper used for “little boy” was a Tungsten Carbide tamper, which has a high density and a low neutron absorbtion. Fat man used a uranium tamper almost as subcritical mass to maximise explosive yield.

anatomy of a thermonuclear warehead (hydrogen bomb)

Two different isotopes of hydrogen can be used for a thermonuclear warhead Deuterium or Tritium. It also utilises nuclear fusion instead of nuclear fission to create a chain reaction.

Thermonuclear warheads are comprised of similar components to an atomic bomb. The primary stage is almost identical to its first cousin, the implosion type atomic bomb. We have a fissile material, either U-235 or plutonium-239 surrounded by chemical explosives and a tamper as well as an explosive lens to compress the explosive material.

The secondary explosive used for nuclear fusion, will have fusion fuel in the form of lithium deuteride which is a combination of both Deuterium and tritium, surrounded by a radiation case/hohleraum to push X-rays from the primary stage to the secondary stage. As well as a spark plug wich is a small amount of fissile material to set off the secondary stage.

In the primary stage, the fission bomb is detonated which releases mass amounts of energy. Most of the energy that is release from the fissile material is in the form of X-rays. These X-rays are commpressed downwards or inwards towards the fusion fuel. This is achieved via the hohlraum or radiation case which absorbs the X-rays and redirects them back towards the fusion case, the extreme heat is what helps trigger nuclear fusion.

A ‘spark plug’ made of uranium fissions which releases outwards energy, This causes the fusion fuel (lithium deuteride) to react which releases tritium. The tritium then fuses with deuterium which forms helium and releases free neutrons. These free neutrons then cause additional fission reactions which also causes more pressure on the Lithium Deuteride, which then causes more fusion reactions. Kind of like a feedback loop or nuclear cycle, these reactions cycle until an explosion occurs.


























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