The first generation of nuclear power plants were the experimental plants ot the 1950s and early 60s, which were also used to power nuclear submarines. The second generation were the commercial plants from the later 1960s to the 1990s.
After the Three-mile Island and Chernobyl accidents, a third generation of nuclear plants was developed. These emphasise improved fuel technology, superior thermal efficiency, passive safety systems and standardized design for reduced maintenance and capital costs and longer life (60 years compared to 40 years for Generation 11 reactors). However, the technology is basically the same as in older reactors.
Most of these are "light water reactors" meaning that they use ordinary water for cooling and to transfer the heat from the nuclear reation to the turbines that generate electricity. Most do this by making steam; most of the rest use pressured water.
A different type of reactor, sometimes called Generation 111+, is the gas-cooled pebble-bed reactor. In these, the uranium fuel is encased in spheres of graphite, rather than the long rods used in conventional reactors, and helium is used as the coolant rather than water. Their advantages are higher effeciency and the fact that they shut down automatically if the coolant is shut off. These reactors are being developed in South Africa and China. The first commerical plants could be in operation by 2016.
The successor to the pebble-bed reactor is the Very High Temperature Reactor. This uses a similar technology but operates, as the name suggests, at very high temperatures – between 550°C and 1000°C. This greatly improves the efficiency of electricity production and opens up the possibility of also producing hydrogen or using the heat for water desalination or industrial processes. The United States and Japan are both working on this type of reactor with the American Idaho National Laboratory aiming to have a demonstration plant, linked to hydrogen production, in operation by 2015. Commercial plants could be in operation by about 2021.
Although the Very High Temperature Reactor is classified as Generation IV, it is unlike most planned Geneation IV reactors which are "fast" reactors. They are called "fast" because they don’t have a moderating material (like graphite) to slow down the neutrons. This means that they oparate at very high temperatures and consume almost all of the nuclear fuel.
Generation 11 and 111 reactors "burn" uranium 235 which makes up only 0.7% of natural uranium and produces highly radioactive, and very long-lived, waste. Fast reactors not only use all of the uranium, but they can also "burn" most of the waste materials from Generation 11 and 111 reactors. It has been estimated that the world already has enough nuclear waste to supply its electricity needs using fast reactors for 500 years. The small amount waste produced by fast reactors has a half-life of around 300 years.
The difficulties in building these reactors include developing methods and materials for containing the high temperatures and radiation and the technology for separating the various highly radioactive materials in nuclear waste for use as fuel.
Several countries, including the United States, Western Europe, Russia and Japan are working on developing fast Generation IV reactors but commercial electricity production is not expected before 2030.
(Based on sources including the Netherlands Research Platform for Sustainable Energy Supply, the American Institute of Physics and Wikipedia)