Nuclear reactors produce energy by heating steam that turn steam turbines. The heat is generated from an atomic reaction that occurs when an atom of Uranium 235 splits, releasing a large amount of energy. Uranium 235 (U-235) is one of many isotopes of the heavy, unstable metal element Uranium and happens to be the most easily fissionable (split). This particular isotope is only .7% of all of the naturally occurring Uranium. This isotope splits and releases a large amount of energy, as well as 2 or 3 neutrons when bombarded with a neutron or other sub-atomic particle. This makes a chain reaction possible, where one neutron fissions one U-235 atom, releasing 2 or 3 more neutrons to cause the fission of more atoms of U-235. This chain reaction produces a large amount of heat energy, which is used to heat pressurized water, which is used to produce steam, which is used to produce electricity. The pressurized water also acts to slow down the uranium and increase the chance of collisions. In this way, the pressurized water is also acting as a moderator. This chain reaction would quickly grow out of control if two or three atoms of U-235 split as a result of one atom splitting. To maintain a constant rate, only one neutron must be allowed to strike an atom of U-235 for each atom that splits. The rate of reaction is thus controlled by the number of neutrons that are available to contact U-235 atoms. This is controlled through the use of cadmium and boron rods that absorb neutrons, and which can be raised and lowered into the reaction core to absorb more or less neutrons. The reactor is surrounded by a heavy concrete shield, one of the few things which blocks radiation from escaping into the environment. Modern reactors are designed in such a way that they shut themselves down if they are left to their own devices, constant intervention is needed to keep them operating. This is in contrast to the Chernobyl reactor, which required constant intervention to keep the reaction under control, making modern reactors much safer.