A generation II reactor is a design classification for a nuclear reactor, and refers to the class of commercial reactors built until the end of the 1990s. Prototypical and older versions of PWR, CANDU, BWR, AGR, RBMK and VVER are among them.
These are contrasted to generation I reactors, which refer to the early prototype of power reactors, such as Shippingport, Magnox/UNGG, AMB, Fermi 1, and Dresden 1. The last commercial Gen I power reactor was located at the Wylfa Nuclear Power Station and ceased operation at the end of 2015. The nomenclature for reactor designs, describing four 'generations', was proposed by the US Department of Energy when it introduced the concept of generation IV reactors.
The designation generation II+ reactor is sometimes used for modernized generation II designs built post-2000, such as the Chinese CPR-1000, in competition with more expensive generation III reactor designs. Typically, the modernization includes improved safety systems and a 60-year design life.
Generation II reactor designs generally had an original design life of 30 or 40 years. This date was set as the period over which loans taken out for the plant would be paid off. However, many generation II reactors are being life-extended to 50 or 60 years, and a second life-extension to 80 years may also be economic in many cases. By 2013 about 75% of still operating U.S. reactors had been granted life extension licenses to 60 years.
Fukushima Daiichi's three destroyed reactors were generation II reactors; specifically Mark I Boiling water reactors (BWR) designed by General Electric. In 2016, unit 2 at the Watts Bar Nuclear Generating Station came online and is likely to be the last generation II reactor to become operational in the United States.
The only remaining commercial Gen I plant, the Wylfa Nuclear Power Station in Wales, was scheduled for closure in 2010. However, the UK Nuclear Decommissioning Authority announced in October 2010 that the Wylfa Nuclear Power Station will operate up to December 2012.