In 1932, physicists Sir John Douglas Cockcroft and Ernest Walton split atoms of lithium into helium nuclei, followed two years later by Enrico Fermi achieving nuclear fission. In 1945 the first atomic bomb was made; in 1954, the first nuclear-powered submarine, the Nautilus, was launched. Splitting the atom has led to both peaceful and military applications; here's what it takes.
Obtain a sufficient quantity of fissionable material. Any element above iron on the periodic table is a candidate for fissionable material, because it will release more energy upon fissioning than it takes to cause the fission process. Ideally, the material chosen should be "fissile," that is, capable of a sustained fission reaction; both uranium-235 and plutonium-239 are excellent fissile materials.
Enrich the fissionable material by increasing its proportion of highly radioactive to less-radioactive isotopes. For uranium, this means increasing the proportion of uranium-235 to uranium-238.
Place the fissionable material in a modulating material designed to sustain the fission reaction without absorbing neutrons. Early reactors used graphite purified of boron contaminants or heavy water, while later reactors also use helium or beryllium.
Incorporate a neutron-absorbing control material that can be inserted and withdrawn as necessary to control the fission process. Most nuclear reactors use rods of cadmium to absorb stray neutrons.
Surround the fissionable material with several feet of concrete or other material to absorb alpha, beta and gamma radiation created by the material itself and during the fission process.
Withdraw the control material.
Fire a beam of neutrons at the fissionable material. A neutron impacting the nucleus of an atom will cause it to split into two nuclei of lighter elements and release additional neutrons. If these neutrons impact other nuclei and split them, a sustained fission reaction, or chain reaction is created. Using the heat generated from this reaction to power a steam turbine is how most nuclear reactors produce power. The reaction can be dampened by reinserting the control material.