In nuclear power

Nuclear power

Nuclear power is the use of sustained nuclear fission to generate heat and electricity. Nuclear power plants provide about 6% of the world's energy and 13–14% of the world's electricity, with the U.S., France, and Japan together accounting for about 50% of nuclear generated electricity...

 technology, online refuelling is a technique for changing the fuel of a nuclear reactor

Nuclear reactor

A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

 while the pile is critical

Critical mass

A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The...

.

Online refuelling has been provided for three main reasons:

• To allow low-burnup
  Burnup
  In nuclear power technology, burnup is a measure of how much energy is extracted from a primary nuclear fuel source...
  
   fuel to be extracted for the production of plutonium
  Plutonium
  Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
  
   suitable for nuclear weapons.

• To allow frequent rearrangement of fuel within the core for the purposes of balancing the thermal load and allowing higher fuel burnup, reducing both the fuel requirements and the amount of high-level nuclear waste for disposal.

• To improve availability and economy by reducing the downtime required for refuelling and other maintenance.

Power reactors provided with online refuelling facilities include:

• The Magnox
  Magnox
  Magnox is a now obsolete type of nuclear power reactor which was designed and is still in use in the United Kingdom, and was exported to other countries, both as a power plant, and, when operated accordingly, as a producer of plutonium for nuclear weapons...
  
   and UNGG generation 1 Gas Cooled Reactor
  Gas Cooled Reactor
  A gas-cooled reactor is a nuclear reactor that uses graphite as a neutron moderator and carbon dioxide as coolant...
  
  s.

• The AGR
  Advanced gas-cooled reactor
  An advanced gas-cooled reactor is a type of nuclear reactor. These are the second generation of British gas-cooled reactors, using graphite as the neutron moderator and carbon dioxide as coolant...
  
  . However, problems with the mechanism led to the discontinuation of full load refuelling of this class.

• The CANDU.

• The RBMK
  RBMK
  RBMK is an initialism for the Russian reaktor bolshoy moshchnosti kanalniy which means "High Power Channel-type Reactor", and describes a class of graphite-moderated nuclear power reactor which was built in the Soviet Union. The RBMK reactor was the type involved in the Chernobyl disaster...
  
  .

• Most pebble bed reactor
  Pebble bed reactor
  The pebble bed reactor is a graphite-moderated, gas-cooled, nuclear reactor. It is a type of very high temperature reactor , one of the six classes of nuclear reactors in the Generation IV initiative...
  
  s.

Suitable types

Reactors provided with online refuelling have to date been one of two types:

• Reactors cooled by gas such as the Magnox.

• Reactors cooled by water in pressurised channels, rather than in a pressure vessel. Channel-type reactors include the CANDU and RBMK.

Unsuitable types

Refuelling a water-cooled, pressure-vessel type reactor has, to date, always involved a major shutdown in which the coolant is depressurised to allow for dissassembly of the pressure vessel. These types of reactors include the popular PWR

Pressurized water reactor

Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...

 and BWR types and their generation III

Generation III reactor

A generation III reactor is a development of any of the generation II nuclear reactor designs incorporating evolutionary improvements in design developed during the lifetime of the generation II reactor designs...

descendents.