Atomic Bombs (Fission Bombs): These devices rely on nuclear fission, the process of splitting heavy atomic nuclei (such as uranium-235 or plutonium-239) into smaller, lighter nuclei, while releasing a massive amount of energy.
First thing you need to do is refine some sort of fissile material. The two go to fissile materials are Plutonium-239 and Uranium-235. Both are rare isotopes that are hard to get in large quantities. Since Fission is all about reaching critical density your gonna need a lot of it in a fairly pure sample. You can find fissile material in many ores around Earth. The White Mesa Mill, 7 miles (11 km) miles south of Blanding, Utah, is the only operating conventional uranium (and vanadium) mill in the United States.
In order to get either U-235 or PU-238 we are going to need a lot of Uranium-238. The majority of Uranium is Uranium-238 with a U-235 making up less than 1%. You need to find a way to separate the U-235 from the U-238. This is called enrichment and is highly complicated. It utilizes highly precise and specialized gas centrifuges aptly named nuclear centrifuges. If set up correctly it will separate the U-235 from the U-238 and the U-235 can later be forced into a solid block that can be used as fuel.
PU-238 can be created by exposing U-238 to neutron radiation. Usually created inside reactors during the fission of U-235. Since 100% enrichment is unlikely some amount of any fuel rod will be U-238 and some amount of that will experience decay when exposed to neutron radiation eventually becoming PU-238. This will also need to be separated from the rest of the sample and enriched into a more dense block if it is to be used for fuel.
Now that we have a fissile material we need to achieve critical density. You essentially need to crush the material into hyper dense state for it to begin a runaway reaction. This can be achieved by highly precise primer explosions. You essentially want to detonate small bombs around material. Those small explosions crush the material from all directions achieving criticality. At this point the bomb should begin to release energy and explode.
All of this is highly precise and the exact specific will depend on the sample of the materials.
