Fission occurs in several heavy elements, including the two main isotopes of uranium, uranium-235 and uranium-238. In the 1930s it was found that bombarding a uranium nucleus with high energy neutrons causes it to divide into two roughly equal fragments. However, the mass of the two fragments is less than the mass of the original nucleus. This difference in mass is liberated as energy as defined by Albert Einstein's equation E=mc^2 (where E is energy, m is mass and c is the speed of light). Splitting generates more high energy neutrons, which can then split more uranium nuclei. This is called a chain reaction.
There are a number of different designs of nuclear reactor in operation worldwide; however, all are governed by two essential principles: how to control chain reactions and how to contain the heat generated by these reactions. The density of neutrons in a reactor is moderated using control rods made from cadmium, boron or graphite, which also absorb excess neutrons. A stable chain reaction is one where the number of neutrons generated approximately equals the number absorbed. Anything less and the chain reaction will slow down and stop; anything more and there is risk the reaction will grow exponentially out of control.
The issue of excess heat is addressed by the various different types of reactor. The pressurized water reactor (PWR), the most common design in the United States, France and Germany, uses highly pressurized water as a coolant. Heat is removed from the reactor core, which is then channeled to a heat exchanger to produce steam. This steam spins turbines to generate electricity. Gas-cooled reactors (GCR), the most common design used in the United Kingdom, use pressurized carbon dioxide as a coolant rather than water.
Nuclear accidents or meltdowns occur if either the cooling or moderating systems fail. In 1986 in Chernobyl, USSR (now Ukraine), a poorly planned and executed experiment led to an uncontrolled chain reaction in the reactor core. The energy generated caused a huge explosion and a release of highly radioactive material that contaminated land as far away as Sweden, Norway and Scotland.
In March 2011 an earthquake and tsunami resulted in the failure of the cooling system at Fukushima Daiichi power plant in northern Japan. The extremely hot fuel in one of the reactors could not be contained, causing some of it to literally burn through the bottom of the containment vessel.