Nuclear reactor designs have evolved over several generations. Below is a brief description of each of these generations:

Generation I reactors date back to the 1950s and 1960. Most generation I reactors tended to be “one of a kind” prototype designs, such as the Fermi I. This generation of reactor was less economical, less resilient and produced more waste than subsequent generations of reactors. The Magnox reactors still operating in the UK are the only remaining generation I reactors and quickly are approaching the end of their operating lives.

Generation II reactors date from the 1960s through the early 1990s and were the first to be used for large, commercial-scale power generation applications. Most of the reactors in service today are generation II reactors and include light water reactors, the advanced gas cooled reactors and the CANDU, among others. Generation II reactors, while generally designed from scratch, did all tend to be designed and built according to a set of design principles gleaned from lessons learned with the generation I reactors.

Generation III – The first generation III reactor came on line in the late 1990s in Japan. Generation III reactors tend to have standardized designs and mass produced parts, resulting in reduced certification/licensing, design and installation costs. Compared to generation II reactors, generation III reactors are more efficient, benefit from enhanced passive safety systems and produce less waste. The advanced boiling water reactor (ABWR) is a well-known generation III reactor.

Generation IV reactors are a set of nuclear reactor designs currently being researched and developed. They are expected to provide a range of benefits including improved safety, improved proliferation protection, reduced waste and reduced construction and operation costs. The DBI Thorium reactor could potentially be the generation IV reactor to be operational in the marketplace.

Source: http://www.dauvergne.com/technology/4th-generation-nuclear/