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Nuclear Power Stations

 

CANDU Reactor

CANDU stands for "CANada Deuterium Uranium".

It's a Canadian-designed power reactor of PHWR type (Pressurized Heavy Water Reactor) that uses heavy water (deuterium oxide) for moderator and coolant, and natural uranium for fuel.

 

CANDU-specific features and advantages

 

Use of natural uranium as a fuel

  • CANDU is the most efficient of all reactors in using uranium: it uses about 15% less uranium than a pressurized water reactor for each megawatt of electricity produced

  • Use of natural uranium widens the source of supply and makes fuel fabrication easier. Most countries can manufacture the relatively inexpensive fuel

  • There is no need for uranium enrichment facility

  • Fuel reprocessing is not needed, so costs, facilities and waste disposal associated with reprocessing are avoided

  • CANDU reactors can be fuelled with a number of other low-fissile content fuels, including spent fuel from light water reactors. This reduces dependency on uranium in the event of future supply shortages and price increases

Use of heavy water as a moderator

  • Heavy water (deuterium oxide) is highly efficient because of its low neutron absorption and affords the highest neutron economy of all commercial reactor systems. As a result chain reaction in the reactor is possible with natural uranium fuel

  • Heavy water used in CANDU reactors is readily available. It can be produced locally, using proven technology. Heavy water lasts beyond the life of the plant and can be re-used

CANDU reactor core design

  • Reactor core comprising small diameter fuel channels rather that one large pressure vessel

  • Allows on-power refueling - extremely high capability factors are possible

  • The moveable fuel bundles in the pressure tubes allow maximum burn-up of all the fuel in the reactor core

  • Extends life expectancy of the reactor because major core components like fuel channels are accessible for repairs when needed

 

Pressurized Water Reactor

One type of nuclear power plant uses a pressurized water reactor. In the plant's containment structure, water under pressure to prevent boiling flows through the reactor.

There, the nuclear chain reaction in the fuel rods heats the water to approximately 600 degrees Fahrenheit (315.5 Celsius). This hot water gets pumped to the steam generator, or heat exchanger, where the cooler water flowing from the condenser becomes steam. The steam drives a conventional steam turbine.

The condenser converts used steam back into water after it passes over the blades of the turbine, and the water recirculates on the outside of the steam generator tubes. The steam turbine turns the electric generator to produce electricity.

Boiling Water Reactor

Boiling Water Reactor uses nuclear fission to produce heat. Uranium, a naturally occurring element, is the primary fuel source. Fission occurs when a subatomic particle, called a neutron, strikes and is absorbed into the nucleus of a uranium atom. This makes the nucleus unstable and causes it to split, producing heat and additional neutrons. These additional neutrons then cause other uranium atoms to fission, resulting in a self-sustaining chain reaction. Each fission of a uranium atom releases a relatively small amount of heat; in a nuclear reactor, though, there are billions of atoms fissioning every second which produces the large amount of heat needed to boil water. 

The heat generated in the fuel core turns water into high-pressure steam. The steam is then piped to the turbine-generator, where it flows through fan blades and causes the turbines to spin at high speed (1800 RPM). This in turn causes an electric generator to spin, generating electricity.

After it flows through the turbines, the steam goes through a condenser where it is cooled and changed back into liquid water. The water is pumped back to the reactor to be reheated and turned back into steam. The heat from the condenser is released into the air through cooling towers outside of the plant.

 


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