All nuclear weapons rely on the same fundamental science, that is fission. Modern nuclear weapons also use fusion to increase the efficiency and reduce the size of the warhead.
How they go about initiating and sustaining fission differs between different warheads.
there are 2 different ways of initiating fission. Both can be seen by looking at the nuclear weapons dropped on Hiroshima and Nagasaki.
Little boy is a “gun type” nuclear weapon.
Fat man is an implosion type nuclear weapon.
Gun type nuclear weapons are incredibly simple.
You take one sub critical mass of highly enriched uranium (plutonium doesn’t work very well unless SIGNIFICANT processing is done) and throw it into another subcritical mass of uranium. Together you get more than critical mass slamming together at the same time, resulting in a runaway fission reaction.
This design is less safe than implosion methods (only a single explosive charge has to accidentally detonate for a nuclear blast to occur) and it’s incredibly inefficient. It also has no real way to develop further. Thus nobody uses it anymore.
Fat man was an implosion type nuclear bomb.
This design takes a sub critical mass of uranium or plutonium (plutonium is better but more expensive generally) and compresses it under tremendous pressure. this makes it become more dense than normally possible, and Fission begins.
The (usually) plutonium core is surrounded by a layer of depleted uranium called the tamper, Which acts as a neutron reflector, making it so the plutonium in the core is closer to critical mass, without being nearly as large. additionally the tampers momentum holds the weapon together for fractions of a second longer, allowing more fission to take place.
Actually achieving fission using this method is significantly harder than the gun type.
The first problem is turning the shockwave of the conventional explosions from convex to concave.
Convex shockwaves would not evenly compress the fissile material, instead the material would simply deform, resulting in no, or insignificant, fission.
This was solved by by making an explosive lens that was a combination of high explosive and low explosive.
The lower detonation velocity of the low explosive slows the shockwave down in such a manner as to cause the wave to impact the entire surface of the HE charge beneath it at the same time. So, aslong as all of the detonators go off at the same time, the inner HE layer will detonate perfectly. The result is the fissile material is compressed evenly, and fission occurs. Except there’s another problem. The shockwave travels so quickly that the entire plutonium core may not be compressed at the same time.
A layer of aluminum or some other light material is added around the uranium tamper.
It’d because of these 2 layers, plus the larger explosive charge needed to compress them, that the fat man was so fat.
Later designs incorporated a series of advancements that decreased the necessary weight and volume of the weapon, while increasing the yield.
There are 4 main improvements that I’m aware of.
- 2 point implosion
- hollow pit
- boosted fission
- fusion
2 point implosion simplified the warhead considerably. Instead of several dozen explosive lenses, these warheads used 2. This means it was smaller in diameter (in one dimension Atleast) And easier to produce. It also had the result of making the plutonium pit more egg shaped than spherical, due to the way the shockwave was shaped.
I’ll try and find a good image for one later and put it here.
A hollow pit implosion device had to be crushed into a sphere by the explosions. This made it so the pit carried inward momentum itself, so the tamper could be made much thinner. It also completely got rid of any need for a pusher. The resulting decrease in mass meant less explosives were needed to initiate it.
Hollow pit implosion devices also made it incredible easy to introduce the next item on our list, which is boosted fission.
Boosted fission is simply a fission warhead with some fusion thrown in to make it more efficient.
The hollow pit offered a perfect place to put the fusion fuel, which is deuterium and tritium. When the pit collapses and begins fissioning, the heat and pressure ignites fusion, which releases energy and neutrons. The neutrons speed up the fission reaction. So it becomes more efficient, and can be made smaller.
Example of boosted two point hollow pit implosion warhead
Combined this all together and you have a small and incredibly efficient nuclear warhead.
Throw it on top of a fusion secondary, and you’ve got a thermonuclear warhead
As for emitting radiation, that does make up the vast majority of the energy released. Technically, the conventional explosives also release some energy that isn’t radiation, but it’s not really much energy compared to the nuclear material.
Much of this radiation is quickly absorbed by the atmosphere, super heating it. That’s what causes the nuclear fireball
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