A nuclear reactor is a special apparatus used to perform nuclear fission. Since the nuclear fission is radioactive, the reactor is covered by a protective shield. Splitting up of nuclei of heavy atoms is called as nuclear fission, during which a huge amount of energy is released. Nuclear fission is done by bombarding slow-moving neutrons on the nuclei of heavy elements. As the nuclei break up, it releases energy as well as more neutrons which further cause fission of neighboring atoms. Hence, it is a chain reaction and it must be controlled, otherwise, it may result in an explosion. Two types of nuclear reactors that are widely used.
Pressurise Water Reactor (PWR) –This type of reactor uses regular water as a coolant. The coolant (water) is kept at very high pressure so that it does not boil. The heated water is transferred through a heat exchanger where water from the secondary coolant loop is converted into steam. Thus, the secondary loop is completely free from radioactive stuff. In a PWR, the coolant water itself acts as a moderator. Due to these advantages, pressurised water reactors are most commonly used.
Boiling Water Reactor (BWR) – In this type of reactor, only one coolant loop is present. The water is allowed to boil in the reactor. The steam is generated as it heads out of the reactor and then flows through the steam turbine. One major disadvantage of a BWR is that the coolant water comes in direct contact with fuel rods as well as the turbine. So, there is a possibility that radioactive material could be placed on the turbine.