The nuclear reactor is the heart of a nuclear power plant. It is composed of a pressure vessel or just a delimited space housing the reactor core loaded with nuclear fuel. Controlled fission takes place inside the nuclear fuel releasing thermal energy that is removed by a coolant and used to generate electricity. Nuclear reactors can be classified based on what kind of coolant and moderator they use. The most common type of reactor is the pressurized water reactor (PWR) using ordinary water both as its coolant and moderator. The future of fission reactors is heading towards a simpler and cheaper design, easier control, higher efficiency, longer service life, and the implementation of inherent safety principles.