Nuclear power plants have only a few decades of working life before they have to be shut down – so how big is the problem? Roll over the circles to find out more about each site, and use the controls on the right to home in on what interests you.

For instance, to view reactors of similar type and vintage to those at Fukushima Daiichi, Japan, choose “boiling water” as the type and use the slider to select those that began construction in the 1960s and 1970s. If you’d like to see how China has been expanding its nuclear industry, check the “under construction” box on the east Asia map.

For more information, consult our source: the World Nuclear Association’s Reactor Database. To learn about the most common reactor types, scroll below the graphic. Note that a small number of reactors in South America and South Africa are not shown on these maps. Peter Aldhous, San Francisco bureau chief


Pressurised water reactors (PWRs)

This is the most common type worldwide. Enriched uranium oxide is formed into rods and water is used both as a coolant, flowing through the reactor core to transfer heat away, and as a moderator, slowing down neutrons released by fission so that they promote further nuclear reactions. The main cooling circuit transfers heat via a steam generator to a second circuit, which drives the electricity-generating turbine.

Boiling water reactors

This is the next most common type, used in countries including the US, Sweden and Japan – the reactors at Fukushima Daiichi are ageing examples. The fuel is enriched uranium oxide, and water is used both as a coolant and as a moderator. However, these reactors differ from PWRs in that there is only one cooling loop, flowing between turbine and reactor.

Heavy water-moderated reactors

These all use heavy water as a moderator. Most reactors of this type are known as pressurised heavy water reactors, favoured especially in Canada. PHWRs are similar to PWRs, but use raw uranium rather than enriched uranium oxide as fuel, and deploy heavy water – in which hydrogen is replaced by deuterium – as both moderator and coolant.

Gas-cooled reactors

Developed in the UK, these reactors use graphite to moderate neutrons and carbon dioxide to cool the core. Older versions, known as Magnox reactors, use uranium metal as fuel, while newer ones use enriched uranium oxide.

Fast breeder reactors

These reactors are cooled by liquid sodium, which is not an efficient moderator. In addition to driving fission reactions, “fast” neutrons are readily captured by uranium-238, which is then converted to plutonium-239. These reactors therefore “breed” plutonium, which can be used to make more fuel or nuclear weapons.

Light water-cooled graphite-moderated reactors

Fuelled by low-enriched uranium oxide, these reactors use graphite as a moderator and water to cool the core. The most common variant of this type, known as RBMK, was responsible for the 1986 Chernobyl disaster. The design is considered inherently hazardous because graphite can react explosively with metal pipes, yet some reactors of the type continue to be operated in the former Soviet bloc.