Gas-cooled reactor

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A gas-cooled reactor (GCR) is a nuclear reactor that uses graphite as a neutron moderator and a gas (carbon dioxide or helium in extant designs) as coolant.^[1] Although there are many other types of reactor cooled by gas, the terms GCR and to a lesser extent gas cooled reactor are particularly used to refer to this type of reactor.

The GCR was able to use

reactor, using heavy water as a moderator, was designed with the same goal of using natural uranium fuel for similar reasons.

Design considerations

Historically

to be viable power reactors.

Advantages

• No
  void coefficient of reactivity
  as the coolant is a gas at room temperature and remains gaseous at operating temperature
• Able to use natural (
  neutron absorption
  cross-section than light water
• High coolant outlet temperature can be achieved, increasing
  Carnot efficiency
• Lower pressure than in a Pressurized water reactor
• Magnox reactors were designed to be
  weapons grade plutonium
  later designs instead bred reactor-grade plutonium
• Lower danger of
  hydrogen explosion
  as no water is present
• High coolant outlet temperature allows better use for process heat if desired
• Adding normal (light) water - e.g. as emergency coolant - scrams the reaction allowing better safety in dealing with unforeseen accidents

Disadvantages

• Bulky due to lower energy density of natural uranium compared to enriched fuel and the lower moderating effect of carbon compared to water
• Magnox fuel cladding cannot be stored for long times in a spent fuel pool making nuclear reprocessing mandatory
• Boudouard reaction between graphite moderator and CO₂ coolant can produce explosive and poisonous carbon monoxide
• A
  loss of coolant accident, unlike in a water-moderated reactor, does not by itself cause a scram
• Graphite is flammable and is exposed to high temperatures in operation - a graphite fire is a possible accident scenario
• nuclear graphite is more expensive than light water but less expensive than heavy water

Generation I GCR

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. Both types were mainly constructed in their countries of origin, with a few export sales: two Magnox plants to

.

Both types used fuel cladding materials that were unsuitable for medium term storage under water, making

.

Generation II GCR

In the UK, the Magnox was replaced by the

(PWR).

Types

Gas-cooled reactor types include:

• Gas-cooled reactor (graphite moderated, CO₂ cooled)
  • Magnox (British design, 28 built, 1956-2015)
  • UNGG reactor (French design, 10 built, 1956-1994)
  • Advanced Gas-cooled Reactor
    (Magnox successor, 15 built, 1962-today)
• Heavy water gas cooled reactor (heavy water moderated, CO₂ cooled)
  • Brennilis Nuclear Power Plant (1967-1985)
  • KS 150 (1972-1979)
  • Niederaichbach Nuclear Power Plant (1973-1974)
• High & Very-high temperature reactor
  (graphite moderated, Helium cooled)
  • Prismatic block reactor
    • Dragon reactor (1964-1975)
    • Peach Bottom Atomic Power Station (1967-1974)
    • Fort Saint Vrain Generating Station
      (1979-1989)
    • High-temperature engineering test reactor (1999-today)
    • Gas Turbine Modular Helium Reactor (General Atomics design)
    • Steam Cycle High-Temperature Gas-Cooled Reactor (Areva SMR design)
  • Pebble bed reactor
    • AVR reactor (1966-1988)
    • THTR-300 (1983-1989)
    • HTR-10 (2003-today)
    • HTR-PM (under construction)
    • Pebble bed modular reactor (design)
• Gas-cooled fast reactor (No moderator, Helium cooled)
  • Energy Multiplier Module (General Atomics design)

See also

• UHTREX

References