Nuclear fusion bombs work by undergoing a process called nuclear fusion. Nuclear fusion occurs when the nuclei of two atoms are forced together to form a different and heavier element. Nuclear fusion can only occur with elements lighter than iron. In the case of a nuclear bomb hydrogen is used, thus giving it the more common name of a hydrogen bomb.
The hydrogen used in a hydrogen bomb is deuterium (a hydrogen with one proton and one neutron in its nucleus) and tritium (a hydrogen atom with one proton and two neutrons in its nucleus). During the fusion process the deuterium is forced together to form an unstable isotope of helium, which will then “throw off” a neutron to stabilize its nucleus.
Hydrogen is not the only substance used in a nuclear fusion bomb though. First a large amount of energy must be harnessed to force the hydrogen nuclei together. This is because hydrogen atoms are positively charged, and since like polars repel, the hydrogen atoms will push away from each other. So in order to harness all that energy the bomb starts out by undergoing the fission process, which is when the nuclei of an atom is split apart into two or more lighter elements.
The only fissionable substances that can be used are uranium-233, uranium-235, and plutonium-239. These are fissionable because the nucleus of the atoms are unstable and can absorb a neutron, and when the nucleus does absorb a neutron it will split apart releasing more neutrons from its own nucleus bombarding other atoms which absorb the neutron and split creating a chain reaction.
Before any of these elements can undergo the fission process they first need a neutron source. Elements such as radium, polonium, and americium are used for this because they naturally emit alpha rays. These will then be mixed with aluminum or beryllium (also used to augment a chain reaction during the fission process) to emit neutrons. Because of everything that takes place during the execution of a nuclear fusion bomb, a one kiloton bomb will release enough energy to destroy everything within a ten mile radius.