A fission reactor contains a number of different parts:
nuclear fuel - the uranium isotopenuclei that will split when triggered by an incoming neutron. The fuel is held in rods so that the neutrons released will fly out and cause nuclear fission with uranium in nearby rods;
graphite core - graphite slows down the fast moving neutrons so that they are more likely to be absorbed by uranium nuclei in nearby fuel rods and cause further fission;
control rods - these are raised and lowered to stop neutrons from travelling between fuel rods and therefore change the speed of the chain reaction or stop it altogether. On average, only one of the fission neutrons goes on to produce further fission;
coolant - this is heated up by the energy released from the fission reactions and is used to boil water to drive turbines in the power station to generate electricity;
concrete shield - the daughter products of the fission reaction are radioactive and can be a hazard. The concrete shield stops alpha and beta particles, gamma rays and fast moving neutrons from escaping into the environment of the power station.
Many of the features of the reactor are designed to control the speed of the reaction and the temperature inside the shielding.
An uncontrolled fission reaction is the basis of an atomic bomb.
There are many political, social, environmental and ethical issues relating to using nuclear energy to generate electricity.