On How To Make Your Very Own Nuclear Bomb (like, seriously).

Let's make a nuclear bomb!

The Motivation

As it happens, having a nuclear bomb is a necessity for a modern life, here we go into the physics of how they work and follow that up with some practical designs for your next project. Have fun! ??

The Basic Physics

Now if you are really interested in the nitty-gritty, one needs to go and start with the quantum nature of the particles, as elements that are under consideration, namely the dynamics involved with neutron and protons that are sub-atomic particles, their size is small enough that one cannot simply imagine them as a bunch of marbles hitting each other, their behavior is more akin to waves.

So we should start by diving into the quantum mechanics, nuclear decay, radioactivity etc etc. But we are not going to, we are here to simply make things go boom!

This section hence will focus on introducing a mental model good enough to understand the presented nuclear bomb designs.

Nuclear Energy: From day dream to reality ??

Atoms have nuclei that are made up of neutrons and protons, neutrons were discovered later compared to protons, as they are charge less particles and were a bit tricky to detect, before their discovery scientists were aware of the possibility of energy stored in nucleus of atoms, but had no idea on how to access it.

In fact the sheer possibility of such a thing was considered absurd, as many scientists believe being able to tap into nuclear energy could lead to a utopian future of virtually infinite energy for humanities usage following are few of the quotes by the noble laureates before the discovery of neutron.

"There is no likelihood man can ever tap the power of the atom. The glib supposition of utilizing atomic energy when our coal has run out is a completely unscientific Utopian dream" - Robert A. Millikan (1928)

"There is not a slightest indication that nuclear energy will ever be obtainable. It would mean that atom would have to be shattered at will" - Albert Einstein (1933)

"Anyone who expects a source of power from the transformation of these atoms is talking moonshine." - Ernest Rutherford (1933)

Well, apparently moonshine drinkers were right!

After Chadwick's discovery of neutron nuclear physics developed insanely rapidly from mid 1930s to 1945, we went from nuclear energy being a day dream to a terrifying reality, with trinities explosion on July 16th 1945.

The history of nuclear physics is interesting enough to fill books rather than articles, but today we will keep things really simple and concise (at-least I will try).

On the Genesis of Atoms and you.

You see protons really don't want to be close to each other, having the same charge there is coulomb force between them, that wants to keep them away from each other.

Hydrogen has only one proton and one electron, if you remove the electron, you just a proton, if you somehow added a neutron to the nucleus you would get deuterium, an isotope. To put is succinctly

If number of protons in nucleus change, we are going from one element to some other element, e.g Hydrogen (1 proton) -> Helium (2 proton, 2 neutron) -> Lithium (3 proton, 4 neutron) and so on.
If the number of protons is the same but the number of neutron change, we are observing isotopes of the same element, e.g Hydrogen (1 proton) -> Deuterium (1 proton, 1 neutron) -> Tritium (1 proton, 2 neutron) and so on.

In the beginning of our universe things were a bit hot (that is an understatement of universal scale), after things cooled down a bit, it was almost hydrogen only, 1 proton chilling with 1 electron, and things were simple, there were no starts, or black holes, or planets, or taxes. Good times overall.

Once large enough Hydrogen cluster were there, the birth places for stars were born, which are still around, we call them nebulae.

In a nebula enough Hydrogen gets together, that the most fundamental engine for energy in our universe can start, the reaction of fusion, and hence stars are born!

In Fusion, two lighter atoms collide to give birth to a heavier one, and that is one hard reaction to do, it takes celestial bodies of the size of our sun to perform it.

Colliding two protons together to create a Helium atom is mindbogglingly hard, in fact it's so hard that even at the core of our sun, where the main energy source is actually the collision of hydrogen nuclei to create helium nucleus is neigh impossible.

It is due to Quantum effects, that add spiciness of uncertainty that give you a good enough chance to create a Helium atom using two Hydrogen atoms. The dynamics need Quantum Mechanics to make sense.

The following figure illustrates how smaller atoms combine to form larger atoms via the process of fusion.

Side Story: In college when it came to electives we had choice between Quantum Mechanics and Nuclear Astrophysics, majority of my batch chose Quantum Mechanics, except a few of us, but in the end due to scheduling issues, that course got pushed to a later semester and every one got Nuclear Astrophysics, it was in this class that Prof. Anil Gourishetty , asked us to calculate the probability of hydrogen nuclei to fuse together and form Helium atoms in the core of our sun, the probability was so low, that basically none of the heavier atoms should have existed, it was only when you include quantum mechanics into the picture that probability of formation of atoms actually becomes decent, if it wasn't for this fact, the starts just simply wouldn't exist, let alone produce enough matter for our planet, you and me to exist !

As you keep going on creating heavier Nuclei, you need neutrons in the mix to sort of act as a glue between protons, but this soup of neutrons and protons can be stable only in certain configurations.

Following is chart that shows different neutron/proton configurations for various element nuclei, the black line in the middle represent the stable atoms that most of the matter around you is made of, if the configuration is unstable it decays (breaks-down) and releases energy in form or particles of electromagnetic waves.

It is this discharge of particles and electromagnetic wave that carry with it energy that is referred to as nuclear radiation.

Now there is a whole lot of discussion that need to be had over nuclear radiation, it's types (Alpha, Beta, Gamma) and their properties, but I will keep the discussion short, here's a brief summary in the below table, and there are nuances that should be studied for anyone actually serious.

Skipping over a whole bunch of theory and a lot of ideas like 'liquid drop of nucleus', semi-empirical formula for binding energy etc, we simply introduce a mental model to think to provide an idea to understand later concepts.

Let's just say there is something called binding energy, which is a property of a nucleus, for our purpose , think of it as the "stability" per nucleon, i.e if it high the nucleus doesn't want to break, while also meaning it is hard to combine it with some other nuclei to form an even heavier nuclei.

Following is a chart of binding energy per nucleon w.r.t mass number.

If you notice Iron (Fe) is at the top, interestingly enough when a star dies Iron is formed, as it takes massive amounts of energy to do anything with it, so a lot of it gets ejected as the start implodes.

That also sheds light on why we have so much of it, it has an incredibly stable nuclei. (We had a lot of unstable U-235, 2 Billion years ago, it has naturally decayed away).

NOTE: This chart does not directly correlate with the abundance of an element in the Universe, there is a famous physics problem called "Cosmological lithium problem", google it, physics still has a lot of unanswered questions.

THE FORMULA !!

Again, contrary to popular belief, the formula coined by the Albert Einstein was not the above but the one below.

As sub-atomic particles can approach speeds comparable to the speed of light, the relativistic nature of things has to be accounted for, but as it is with many things we are simply going to ignore that as well, if the v << c, we simply just don't consider it, and that's how we get to famous equation presented first.

Putting the ideas together and reaching -> Fission and Fusion

We already saw how atoms are fundamentally created in the core of stars through Fusion, and while we did not do the math, just trust me bro, it is incredibly hard to do fusion.

In both Fission and Fusion the energy released is due to the difference between binding energy of the reactants and the products, let's understand the pathways to tap into nuclear energy via the binding energy chart shown before.

We can observe two pathways to energy, either we climb the binding energy curve towards higher stability (more stable nuclei) from the right, or from the left, meaning...

Either we combine lighter nuclei to form more stable heavier nuclei -> Fusion.
Or we use breakable heavy nuclei to rise towards stability -> Fission.

To have bit deeper understanding, let's take a general reaction, in this, some atom 'x' is coming with incredible speed (enough to overcome coulomb barrier in case of Fusion), or perhaps a neutron slowly heading towards, a stable nuclei of say something like Uranium - 235 in case of Fission.

In such a reaction we can define something called the Q-value (energy released) as follows:

This is it everyone, we are here finally! ????.

This is the energy released in a nuclear reaction, as you can see it the difference of mass between the reactants and products multiplied by the speed of light squared as given by Einsteins formula.

Note: There is a lot of discussion that has been skipped over to get to this point, my college professors may retroactively reduce my grades reading this stuff.

Last bit of theory, I swear.

While we are done with forming the mental models required, we need to understand, how we are going to initiate the reaction in a nuclear bomb, and for that we need to look at some real reactions:

Fusion Examples:

These are called deuterium-deuterium or D-D reactions, notice how we are reacting isotopes of Hydrogen, generally D-D or Deuterium-Tritium reactions are more probable than you vanilla Hydrogen-Hydrogen (proton - proton) fusion reaction.

Okay, but fusion is hard, and the condition required to make fusion go require you to almost have fusion reaction going already, but as we saw with the binding energy chart, we have another way.

Fission Example:

In the above reaction, a thermal neutron strikes a U-235 nucleus, and breaks it to produce Rubidium and Cesium with 2 extra neutrons.

It's this reaction, that changed the world towards the end of world war II. Let's examine a few ideas that we need to be aware of going forward.

The neutron striking U-235 is a thermal neutron, meaning it is not moving at high speed (~2.19 Km/s), thinking in classical terms one would think that neutron needs to be really fast and impart a lot of energy to the nuclei in order to destabilize it an break it. But that is not true.
Nuclear reactions have something called nuclear cross-section, given is barns (microscopic cross-section, cm^2), let's just say the probability and rate of reaction is a function of density of the material, it's type and also the energy of the incoming particle, and this quantity helps us provide a number to that probability.
To have a mental model, think of how many neutrons are moving around, and how many U-235 nuclei are available to be struck to generate even further neutrons.
The way the reaction goes is dependent on this factor, there are different kinds of nuclear chains, some involve fast neutrons, some involve slow neutrons etc, there are multiple paths for a nuclear reaction to take, as their are multiple radioactive pathways for nuclear reaction to release energy with.

Finally we are here!

So to finally understand how the nuclear bomb works:

As the reaction starts, nuclei start to break, they release the energy alongside, 2 extra neutrons are released.
These two neutrons strike two other nuclei and then they release 4 neutrons.
And hence the number of atoms being broken increases exponentially, resulting in the material going supercritical, i.e a reaction in which once the fission has started, the rate will go ever increasing.
This due to the stated fact that every fission results in 2 or higher neutrons being released, i.e the ratio of neutrons released vs neutron absorbed is greater than 1, (k > 1).

SUPER IMPORTANT FACT.

Nuclear reactors cannot become nuclear bombs!!!!!

Nuclear reactors by design operate in critical range, i.e the number of neutrons is just enough to sustain the reaction. THEY CANNOT UNDER ANY CIRCUMSTANCE GO SUPERCRITICAL, THE PHYSICS SIMPLY DOES NOT ALLOW THAT!!
In nuclear reactors k = 1, there is not enough material around for them to go supercritical, it is simply against the dynamics and physics of everything.
So the next time you see a show like HBOs, Chernobyl (2019), where they claim the nuclear reactor became a nuclear bomb, mail them this article, and tell them go attend a basic physics class!

Alright, with that out of my system, we finally have everything we need to discuss the designs and build process for what we are here for.

Building Our Nuclear Bombs, Finally!

Getting the good stuff!

You see building a nuclear bomb is quite easy, the hard part is getting enough Nuclear Fissile material to put into the bomb.

For example 99.3 % of Uranium on Earth is U-238, the chill brother of U-235, you can throw a neutron at U-238, but it simply won't go boom!

You need to spend a ton of money on either centrifuges like shown below.

And then wait penitently, as they separate few atoms atom of U-235 from atoms of U-238.

Or you can invest in breeder reactors, these reactors have nuclear reaction chains designed to specifically gather weapon grade nuclear material, a lot of exotic nuclear bomb material like Plutonium simply doesn't exist in high enough quantities in nature, we literally have invested billions in order to make power plants that serve the dual purpose of providing electricity and generating materials for bombs on the side.

It is incredibly difficult to gather > 90% pure Uranium. That is why it was U.S.A that was able to build the first nuclear bomb, due to their industrial might, the physics involved had been figured out by almost all the parties involved, whether it be the Heisenberg in Nazi Germany, or the Japanese or the Britishers.

Fun fact, the Frisch–Peierls memorandum, was the letter that was sent to U.S.A by Churchill during WWII included the calculation for the critical mass required to make a nuclear bomb (~600 gm U-235).

Both Frish and Peierls were scientists, that were ousted from Germany as Hitler rose to power, as they weren't "Aryans".

And it was them who first did the calculation while taking refuge in Britain.

If Hitler wasn't racist, the world could've had a much different history, think about that when you go to sleep tonight.

Churchill was concerned of a German invasion, and the calculations falling into the hand of the Nazis, this calculation used to be top secret, now it is part of basic introductory nuclear physics, and you can read about it on Wikipedia.

So in reality making nuclear bombs isn't that hard, the hard part is being able to enrich enough fissile material to put into a bomb.

Nuclear supply chains are internationally controlled and monitored by all nations with utmost sincerity, to say it is hard to get your hands on U-235, does not do justice on the amount of restrictions placed on nuclear material movement.

Unless of-course, if you are Israel, and suddenly +500 pound of enriched Uranium disappears from American facilities without trace, only for you to suddenly have a nuclear bomb, funny how things turn out! - Source

So is true for other nations, who have nuclear bombs, they need technical support to build the facilities required to get the spicy nuclear fissile material, the rest is relatively easy work to do, once you know the physics.

Alright let's just say, you got lucky (and I do really mean "lucky"), and you found yourself with a pile of material belonging to the following class:

Perfect! Now we can finally begin the discussion for the various designs you can choose for you first nuclear bomb.

The design used on Hiroshima, the gun assembly.

The bomb dropped on Hiroshima was puny!

Yes, as I will discuss later, the puny 15 kiloton yield of the "Little Boy" (kiloton of TNT equivalent), is nothing compared to modern nuclear bombs, and when I say nothing I mean modern nuclear bombs have yields in Megaton, 1000x Hiroshima.

Let's see how it worked.

The bomb has two sub-critical (not going to go boom!), pieces of Uranium - 235, one towards the nose, and one towards the tail.
The cylinder at the tail side has a hollow space inside matching the shape of Uranium block on the other side.

At the time of detonation, a conventional explosive fires the tail end of Uranium block towards the front.
Due to the fact that U-235 is naturally radioactive, there are some neutrons already going about here and there, but the nuclear cross section of the reaction requires higher density to go super-critical.
As the tail block hits the front with extreme speed, it compresses, and in an instant the density of the block increases, while there is enough mass of U-235 around to go super-critical.
AND BOOM!
This is it! The random neutrons that were going about suddenly start breaking U-235 atoms, which break even more atoms, which break even greater amount of atoms.

And finally you have successfully vaporized thousands of human lives into nothingness, congratulations! YOU HAVE A NUCLEAR BOMB!

A look at the O.G!

A look at the original trinity design just for the sake of completion.

The first nuclear explosion, i.e the trinity explosion, was much different design, let's have a look,

The trinity bomb design is a little bit more involved, as it is of greater yield than the gun assembly we just saw.

You see nuclear bombs are not that efficient, the gun assembly design, the one responsible for wiping 100s of thousands, merely used Uranium with mass comparable to single U.S.D note (yeah, seriously), most of the atoms simply moved away from the neutrons so fast that they never exploded!

So in order to make our death machine a little more competent we can use other materials, in the design of trinity for example, we have high explosive covering the main core of Plutonium, not Uranium.

The Boron is a neutron reflector, so when things get spicy the neutrons can't escape to the outside but get reflected back to break even more atoms.
The small Polonium-Beryllium core is the initiator of the reaction, cutely named "Urchin", it is the initial source for neutrons.

Other than that the idea is similar to what we had before.

Use conventional explosives to get the exotic conditions required for the material to go super-critical
Witness one of the most fundamental forces of universe reek havoc on a scale incomprehensible.

Simple stuff!

The Fat Man

The design discussed above is the one that got dropped on Nagasaki, in the bomb that was code named the "Fat Man".

SIDE NOTE: Americans are particularly good at coming with acronyms and code-names.

Guess what we can make things so much more spicier !!

Fusion Enters the Chat!

Remember how I told you it is incredibly hard to achieve fusion?

Let me correct that, it is incredibly hard to do fusion in a sustained controlled manner, if the goal is to just make things go boom, we can actually do it!

Fusion bombs.

You see the energy released in fission blast is enough to cause a fusion given the proper conditions.

The above design is two-stage nuclear bomb, let's see how it works.

The primary (fission bomb) goes off first.
It results in incredibly compression and heat, which initiates a secondary FUSION explosion using lithium-6 deuteride reaction.
And boom, now we are talking megatons of yield, baby!

To give you and idea of how powerful nuclear bombs can be, following is graph showing the scale of warheads that were detonated during cold war.

You know what? Tsar Bomba, the biggest nuclear bomb ever tested had to be scaled down from a 100 megaton design, cause otherwise the pilot dropping it would simply wouldn't make it, lol!

And yes, if we wanted we can go much bigger in a nuclear bomb design.

Final Thoughts

God had given us permission to peek into his creation, and have for ourselves his most divine power, yet instead of creating a world of prosperity we have chosen to wield it for destruction.

As we live in this modern world with two nuclear powers in hot wars, with more nations like Iran ready to take hold of this gargantuan strength, just to point it towards human souls, it is of incredibly shame for the entire humanity that as our children face ever so greater threat of an Earth that cannot sustain us, we use this incredible power not save ourselves, but put a hanging sword on the neck of our existence.

The tons of material currently sitting in silos, on-top of rockets, ready to strike any part of our globe in minutes can power humanities growth for not decades, centuries but millennia.

We evolved from so called "uncivilized" animals, only to realize that we remain too disgraceful and unintelligent to command the power that has been bestowed upon us.

The people who live today, including me, can look at the numbers, study the physics, do a simulation, look at the footage, but to witness one of the most fundamental forces of our universes dance is an opportunity that very few of us ever had.

It is therefore very hard to understand the might of these weapons for normal people, I cannot not explain to you what peering into the horizon looks like, an astronaut can write a book, sing a song, capture a photo, make a movie, but to witness Earth's beauty from out there is an experience that can only be theirs.

So true is for the sheer strength that one witnesses when seeing these weapons go off, and it is therefore, that one cannot simply avoid invoking a sense spiritual nature within, it is therefore that Oppenheimer was compelled to think of Bhagvat Gita.

Even with all the Physics and Math said and done, it was too little to describe what he and his peers witnessed on July 16th.