Physics of Nuclear Devices
Nuclear bombs involve the forces, strong and weak, that hold the nucleus of an atom together, especially atoms with unstable nuclei (see How Nuclear Radiation Works for details). There are two basic ways that nuclear energy can be released from an atom:
Nuclear fission - You can split the nucleus of an atom into two smaller fragments with a neutron. This method usually involves isotopes of uranium (uranium-235, uranium-233) or plutonium-239.
Nuclear fusion -You can bring two smaller atoms, usually hydrogen or hydrogen isotopes (deuterium, tritium), together to form a larger one (helium or helium isotopes); this is how the sun produces energy.
In either process, fission or fusion, large amounts of heat energy and radiation are given off.