This article talks about how a nuclear reactor functions and what really went wrong in Fukushima Nuclear plant…
The uranium fuel generates heat by neutron-induced nuclear fission. Uranium atoms are split into lighter atoms (aka fission products). This process generates heat and more neutrons (one of the particles that forms an atom). When one of these neutrons hits another uranium atom, that atom can split, generating more neutrons and so on. That is called the nuclear chain reaction. During normal, full-power operation, the neutron population in a nuclear reactor core is stable (remains the same) and the reactor is in a critical state.