We got extremely lucky that it's super hard to separate the isotopes of uranium and plutonium that can be used to make an A-bomb from the minerals that can be mined on Earth surface. If it was easier to do every dictatorship and every terrorist group like ISIS would build a nuclear arsenal and the biggest remaining city on Earth today would be probably 10,000 people.
We got lucky with nuclear chain reaction, but who knows what weapons of mass destruction will become possible with further advances in physics, biology, material science, AI. A discovery that will make it possible for a small group of extremists to destroy the human civilization may be the Great Filter.
Plutonium is the element with the highest atomic number to occur in nature. Neutrons are in abundance in supernova, so heavy elements like plutonium can be created in copious quantities. Trace quantities arise in natural uranium-238 deposits when uranium-238 captures neutrons emitted by decay of other uranium-238 atoms. Plutonium is separated from uranium and other transuranium elements by subjecting an aqueous solution in dilute nitric acid of the elements to be separated to electrolytic extraction by counter-current or cross-current ion migration in the presence of a complexing agent, preferably acetic acid or a mixture of hydrofluoric acid with an alkali metal fluoride, until plutonium and the other elements present accumulate in separate portions of the solution which are thereafter separately withdrawn.[1]
[1] This is all copypasta from Wikipedia and firsts of Google search results.
We got lucky with nuclear chain reaction, but who knows what weapons of mass destruction will become possible with further advances in physics, biology, material science, AI. A discovery that will make it possible for a small group of extremists to destroy the human civilization may be the Great Filter.