5. Kinds of reactors

Pressurized water reactors (PWRs) constitute the large majority of all Western nuclear power plants and are one of three types of light water reactor (LWR)In a PWR, the primary coolant (water) is pumped under high pressure to the reactor core where it is heated by the energy generated by the fission of atoms. The heated water then flows to a steam generator where it transfers its thermal energy to a secondary system where steam is generated and flows to turbines which, in turn, spin an electric generator. All LWRs use ordinary water as both coolant and neutron moderator.

PWRs were originally designed to serve as nuclear marine propulsion for nuclear submarines and were used in the original design of the second commercial power plant at Shippingport Atomic Power Station.

PWRs currently operating in the United States are considered Generation II reactors.

The boiling water reactor (BWR) is a type of light water nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor (PWR). The main difference between a BWR and PWR is that in a BWR, the reactor core heats water, which turns to steam and then drives a steam turbine. In a PWR, the reactor core heats water, which does not boil. This hot water then exchanges heat with a lower pressure water system, which turns to steam and drives the turbine. The BWR was developed by the Idaho National Laboratory and General Electric (GE) in the mid-1950s. The main present manufacturer is GE Hitachi Nuclear Energy, which specializes in the design and construction of this type of reactor.

The RBMK is a class of graphite-moderated nuclear power reactor designed and built by the Soviet Union.

The RBMK is an early Generation II reactor and the oldest commercial reactor design still in wide operation. Certain aspects of the RBMK reactor design , such as the positive void coefficient properties, the graphite-tipped control rods and instability at low power levels, contributed to the 1986 Chernobyl disaster, in which an RBMK exploded during a mishandled test, and radioactivity was released over a large portion of Europe. The CANDU (short for CANada Deuterium Uranium) reactor is a Canadian-invented, pressurized heavy water reactor used for generating electric power. The acronym refers to its deuterium-oxide (heavy water) moderator and its use of (originally, natural) uranium fuel. CANDU reactors were first developed in the late 1950s and 1960s

A gas-cooled reactor (GCR) is a nuclear reactor that uses graphite as a neutron moderator and carbon dioxide (helium can also be used) as coolant.

The GCR was able to use natural uranium as fuel, enabling the countries that developed them to fabricate their own fuel without relying on other countries for supplies of enriched uranium, which was at the time of their development only available from the United States or Soviet Union.

There were two main types of generation I GCR:

The Magnox reactors developed by the United Kingdom.

The UNGG reactors developed by France.

The main difference between these two types is in the fuel cladding material.

The very-high-temperature reactor (VHTR), or high-temperature gas-cooled reactor (HTGR), is a Generation IV reactor concept that uses a graphite-moderated nuclear reactor with a once-through uranium fuel cycle. The VHTR is a type of high-temperature reactor (HTR) that can conceptually have an outlet temperature of 1000 °C. The reactor core can be either a "prismatic block" or a "pebble-bed" core. The high temperatures enable applications such as process heat or hydrogen production via the thermochemical sulfur–iodine cycle.

A breeder reactor is a nuclear reactor capable of generating more fissile material than it consumes. These devices are able to achieve this because their neutron economy is high enough to breed more fissile fuel than they use from fertile material such as uranium-238 or thorium-232. Breeders were at first found attractive because their fuel economy was better than light water reactors, but interest declined after the 1960s as more uranium reserves were found and new methods of uranium enrichment reduced fuel costs.

The thorium fuel cycle is a nuclear fuel cycle that uses the isotope of thorium, 232Th, as the fertile material. In the reactor, 232Th is transmuted into the fissile artificial uranium isotope 233U which is the nuclear fuel.

A hybrid reactor is an anaerobic digester that combines a UASB reactor with an anaerobic filter. This combination is an advanced form enabling improved solid retention time in the treatment of waste water. This waste water can be built up in the secondary chamber and must be removed daily or an explosion is imminent to occur.

(Carlo Rubbia)