Fission

.. Nuclear fission is a process in nuclear physics where the nucleus of an atom splits into two or more smaller nuclei as fission products, and usually some by-product particles [] 

==Critical mass is the minimum mass of fissionable material required to sustain a nuclear fission reaction. == http://science.howstuffworks.com/nuclear-bomb4.htm




 * ==Nuclear fission is the splitting in half of a heavy nucleus.The halves, called "fission fragments," form smaller and lighter elements that mostly radioactive. Nuclear fusion is the combining of two very light atomic nuclei to form a heavier element. Fusion has a small amount of radioactivity, but it is usually the product of harmless gasses. The other main difference between fusion and fission is that fission occurs easily where fusion only occurs at the extremely high temperatures found at the center of stars, or in a nuclear bomb blast. == ||

http://www.nytimes.com/books/first/f/fowler-fusion.html