Science Media Centre Fact Sheet

Nuclear reactors

Key components of a

- Fuel - usually -based (uranium oxide UO2 is the most common), in the form of pellets in tubes that form fuel rods, which are then arranged in the reactor core - Moderator - a material in the core which slows down the released from fission so that they cause more fission. Usually water, but may be heavy water or . - Control rods - made from -absorbing material which absorbs the neutrons produced by . Inserted into or withdrawn from the core as necessary to control or halt the reaction - - a liquid or gas that circulates through the core to transfer heat from it - Steam generator - part of the coolant loop that generates steam to power the turbine - Turbine - driven by steam to produce electricity - Containment - structure around the core to protect it from the external environment and prevent radioactive material from getting out

Pressurised Water Reactor (PWR) - Most common type - present in US, France, Japan, Russia, China - Fuel is oxide (UO2) - usually as pellets arranged in tubes to form fuel rods - A type of light water reactor, in which water is used as the coolant, circulating through the core to transfer heat away. The primary cooling circuit flows through the core and transfers heat via a steam generator to a second circuit in which the turbine is located. - Water also acts as the moderator, which slows down the reaction.

Boiling Water Reactor (BWR) (Fukushima reactor type) - Second most common reactor type - present in US, Japan, Sweden - the Fukushima plant is an early generation of this type, designed in the 1960s - though the design has significantly evolved over the past 40-50 years - Fuel is enriched uranium oxide - Also a type of light water reactor, with water as the coolant and the moderator - Different from a PWR in that there is only one cooling circuit, which runs to the generator and back to the reactor in one loop - The coolant system is currently out of action in the Fukushima plant, as are the diesel generators that provided backup - engineers are working to cool the plant and get this system back up and running.

Pressurised Heavy Water Reactor (PHWR) - Developed since the 1950s in Canada (where it is known as the CANDU)

- Uses heavy water (D2O) as the moderator and coolant - this is necessary as the reactor runs on natural instead of enriched uranium oxide, which requires a more efficient moderator in the form of heavy water

Gas-cooled Reactor (UK reactor type) - These are the type used in the UK, either (Generation I) or Advanced Gas-cooled Reactors (AGRs, Generation II) - Both types use a graphite moderator and as the coolant - AGRs run on enriched uranium oxide pellets in tubes - Magnox reactors run on in metal form - In the UK, there are currently 14 AGRs still operating (across seven sites, each with two reactors) and four Magnox reactors. - Some Generation II (AGR) reactors are nearing the end of their 40-year operating lives and many are likely to close over the next 15 years or so. Almost all Generation I (Magnox) reactors are now in the process of being decommissioned - If nuclear new build goes ahead, the new reactors will be Generation III, which will be extensions of existing designs

Light Water Graphite-moderated Reactor - Soviet design, known as RBMK - Fuel is low-enriched uranium oxide, with a graphite moderator and water coolant - Have never been built outside the Soviet Union due to limitations in the design

Sources / further information World Nuclear Association - excellent resource on various reactor types - also page on how works

Factfile from the IET - slightly more technical

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