Generation II nuclear reactors were the first class of reactors designed and intended for commercial use on a large a scale. They were designed to be economical and reliable with a typical lifespan of around 40 years. They began operation in the lat 1960s an make up a large majority of the 104 reactors currently operating in the United States. Most of these reactors were manufactured by one of three companies: General Electric,Westinghouse, or Framatome (now part of Areva). Typical Generation II reactors include boiling water reactors (BWR), pressurized water reactors (PWR), and Canada Deuterium Uranium Reactors (CANDU). CANDU reactors are unique because they can operate on natural uranium as fuel; eliminating the enrichment step in their fuel cycle and drastically reducingproliferation concerns.
The reactors at Fukushima Daiichi plant were GE Mark I boiling water reactors. Since all generation II reactors produce considerable amounts of spent fuel there must be measures taken to deal with this in either an open fuel cycle or closed fuel cycle.