<P> Nuclear reactors usually rely on uranium to fuel the chain reaction . Uranium is a very heavy metal that is abundant on Earth and is found in sea water as well as most rocks . Naturally occurring uranium is found in two different isotopes: uranium - 238 (U-238), accounting for 99.3% and uranium - 235 (U-235) accounting for about 0.7% . Isotopes are atoms of the same element with a different number of neutrons . Thus, U-238 has 146 neutrons and U-235 has 143 neutrons . Different isotopes have different behaviors . For instance, U-235 is fissile which means that it is easily split and gives off a lot of energy making it ideal for nuclear energy . On the other hand, U-238 does not have that property despite it being the same element . Different isotopes also have different half - lives . A half - life is the amount of time it takes for half of a sample of a radioactive element to decay . U-238 has a longer half - life than U-235, so it takes longer to decay over time . This also means that U-238 is less radioactive than U-235 . </P> <P> Since nuclear fission creates radioactivity, the reactor core is surrounded by a protective shield . This containment absorbs radiation and prevents radioactive material from being released into the environment . In addition, many reactors are equipped with a dome of concrete to protect the reactor against both internal casualties and external impacts . </P> <P> The purpose of the steam turbine is to convert the heat contained in steam into mechanical energy . The engine house with the steam turbine is usually structurally separated from the main reactor building . It is so aligned to prevent debris from the destruction of a turbine in operation from flying towards the reactor . </P> <P> In the case of a pressurized water reactor, the steam turbine is separated from the nuclear system . To detect a leak in the steam generator and thus the passage of radioactive water at an early stage, an activity meter is mounted to track the outlet steam of the steam generator . In contrast, boiling water reactors pass radioactive water through the steam turbine, so the turbine is kept as part of the radiologically controlled area of the nuclear power station . </P>

Where do nuclear power plants have to be located