Nuclear Reactor

• Thermal (slow) reactors are composed of fuel (fissionable material), moderating materials to slow neutrons to low velocities (to prevent capture by U238), heavy-walled pressure vessels to house reactor components, shielding to protect personnel, systems to conduct heat away from the reactor, and instrumentation for monitoring and controlling the reactor's systems. Most nuclear reactors used for electric power generation are of this type. The first plutonium production reactors were thermal reactors using graphite as the moderator.
• Fast reactors require highly enriched fuel (sometimes Weapons Grade), but no moderating material (the enrichment process removes most of the U238 that captures fast neutrons). This type of reactor is used in mobile applications, where space constraints are a major concern, as well as for the production of plutonium (see fast breeder).

Designs for fast power reactors to date have all been cooled by liquid metal. They have also been of two types, called pool and loop reactors.

To provide the power for a dynamo-electric machine, or electric generator, nuclear power plants rely on the process of nuclear fission. In this process, the nucleus of a heavy fuel element such as uranium absorbs a slow-moving free neutron, becomes unstable, and then splits into two smaller atoms. The fission process for uranium atoms yields two smaller atoms, one to three fast-moving free neutrons, plus an amount of energy. Because more free neutrons are released from a uranium fission event than are required to initiate the event, the reaction can become self sustaining--a chain reaction --under controlled conditions, thus producing a tremendous amount of energy. The newly released fast neutrons must be slowed down (moderated) before they can be absorbed by the next fuel atom. This slowing down process is caused by collisions of the neutrons with atoms of an introduced substance called a moderator.

In the vast majority of the world's nuclear power plants, heat energy generated by burning uranium fuel is collected in ordinary water and is carried away from the reactor's core either as steam in boiling water reactors or as superheated water in pressurized-water reactors. In a pressurized-water reactor, the superheated water in the primary cooling loop is used to transfer heat energy to a secondary loop for the creation of steam. In either a boiling-water or pressurized-water installation, steam under high pressure is the medium used to transfer the nuclear reactor's heat energy to a turbine that mechanically turns a dynamo- electric machine, or electric generator. Boiling-water and pressurized-water reactors are called light-water reactors, because they utilize ordinary water as the moderator. In all light-water reactors to date this water is also used to transfer the heat energy from reactor to turbine in the electricity generation process. In other reactor designs the heat energy may be transferred by light water, pressurized heavy water, gas, or another cooling substance.

The amount of energy in the reservoir of nuclear fuel is frequently expressed in terms of "full-power days," which is the number of 24-hour periods (days) a reactor is scheduled for operation at full power output for the generation of heat energy. The number of full power days in a reactor's operating cycle (between refueling outage times) is related to the amount of fissile 235U contained in the fuel assemblies at the beginning of the cycle. A higher percentage of 235U in the core at the beginning of a cycle will permit the reactor to be run for a greater number of full power days.

At the end of the operating cycle, the fuel in some of the assemblies is "spent," and it is discharged and replaced with new (fresh) fuel assemblies. The fraction of the reactor's fuel core replaced during refueling is typically one-fourth for a boiling-water reactor and one-third for a pressurized-water reactor.

The amount of energy extracted from nuclear fuel is called its "burn up," which is expressed in terms of the heat energy produced per initial unit of fuel weight. Burn up is commonly expressed as megawatt days thermal per metric ton of initial heavy metal.

• Pressurized water reactor (PWR)
• Boiling water reactor (BWR)
• Pressurised Heavy Water Reactor (PHWR or CANDU)
• Advanced gas-cooled Reactor (AGR)
• Light water cooled graphite moderated reactor (RBMK)
• D2G reactor

Advanced reactors

More than a dozen advanced reactor designs are in various stages of development. Some are evolutionary from the PWR, BWR and CANDU designs above, some are more radical departures. The former include the Advanced Boiling Water Reactor, two of which are now operating with others under construction.

The best-known radical new design is the Pebble Bed Modular Reactor, discussed below.

More could be added about advanced reactor designs the PBMR has a web page for example.

• Pebble Bed Reactor

Nuclear fuel cycle

See nuclear fuel cycle.

The first nuclear reactors were used to generate plutonium for nuclear weapons. Additional reactors were used in the navy (United States Naval reactor ) In the mid-1950s, both the Soviet Union and western countries were expanding their nuclear research to include non-military uses of the atom. However, as with the military program, much of the non-military work was done in secret. On June 27, 1954, the world's first nuclear power plant generated electricity but no headlines--at least, not in the West. According to the Uranium Institute (London, England), the first reactor to generate electricity for commercial use was at Obninsk, Russia. The Shippingport reactor (in Pennsylvania) was the first commercial nuclear generator to become operational in the United States. The Shippingport reactor was ordered in 1953 and began commercial operation in 1957.

Lots of construction in 60s and 70s (oil crisis influenced) - need some numbers here

In the aftermath of the 1979 Three Mile Island accident, the U.S. nuclear market was the first to deteriorate. No new nuclear plants have been ordered in the USA since then.

Negative influence of Chernobyl increasing regulations increased costs.

need dates, declining construction numbers, reference to legislation in US

In 1997, a total of 78 reactors were either under construction, planned, or indefinitely deferred. These units have a combined capacity of 67,484 MWe, approximately 25 percent of the total capacity already in existence. However, only 45 reactors were under construction worldwide. The remaining 33 units are either being planned or indefinitely deferred. Three U.S. units are not projected to come on-line. Some experts have predicted that Watts Bar 1, which came on-line in 1997, will be the last U.S. commercial nuclear reactor to go on-line. Other experts, however, predict that electricity shortages will revamp the demand for nuclear power plants.

need more recent figures

As of 2003, the immediate future of the industry in many countries still appeared uncertain, the most notable exceptions being Japan, China and India, all actively developing both fast and thermal technology, South Korea, developing thermal technology only, and South Africa, developing the Pebble Bed Modular Reactor (PBMR).

Statistics

In 2000, there were 438 commercial nuclear generating units throughout the world, with a total capacity of about 351 gigawatts.

In 2001, there were 104 (69 pressurized water reactors, 35 boiling water reactors) commercial nuclear generating units that are licensed to operate in the United States, producing 32,300 net megawatts (electric), which is approximately 20 percent of the nation's total electric energy consumption. The United States is the world's largest supplier of commercial nuclear power.

In France, 80% of all electric power comes from nuclear reactors.

• MinAtom (Russia)

References and Links: