Types of Nuclear Reactors
Most nuclear reactors in the United States and in Europe use fuel composed of natural uranium that is enriched with uranium 235, and ordinary water as a coolant. These reactors are known as light-water reactors. There are two basic types: the pressurized water reactor and the boiling water reactor.
The pressurized water reactor is the most common type of nuclear reactor used for the generation of electricity. It uses ordinary water as both the moderator (to slow neutrons) and the coolant (to transfer heat). It has two separate cooling circuits: one which flows through the core of the reactor (the primary), and one which is used to drive the turbine (the secondary).
The boiling water reactor is similar in some ways to the more common pressurized water reactor. This design also uses ordinary water as both the moderator (to slow neutrons) and the coolant (to transfer heat). In the boiling water reactor, however, a single cooling circuit is used and the cooling water boils inside the reactor.
A third reactor design, CANDU (CANadian Deuterium Uranium), is also used to generate power. Developed by Canada, this reactor uses only natural uranium as a fuel, but is moderated and cooled using heavy water. Since the complex enrichment process can be skipped, this type is very popular in developing nations. It is also known as a pressurized heavy water reactor.
The Soviet designed RBMK is a pressurized water reactor with individual fuel channels, using ordinary water as its coolant and graphite as its moderator. It is very different from most other power reactor designs as it was intended and used for both plutonium and power production. The combination of graphite moderator and water coolant is found in no other power reactors. The Chernobyl accident showed that the RBMK's design characteristics caused instability when operating at low power. A number of significant design changes have now been made to address these problems.