How were nuclear reactor technologies developed?

Nuclear industry developed and improved technologies of reactors during seven decades throughout the following main generations:

Gen I reactors, which started for testing and demonstration, it was commercially developed in the 1950s, 1960s.

Gen II reactors, most of them are currently operating.

Gen III and Gen III+ reactors, which were developed 1990s and most of them include nuclear reactor under construction and recently completed

Gen IV reactors that offer new alternative fuel and coolants but are not expected to be commercially available before 2030-2040.

Some of nuclear technology vendors are also developing Small and Medium Reactors (SMRs) aiming to improve economic and flexibility of nuclear energy use with expanding access chances to new segments of energy market. Such reactors attract countries where large capacity plants are unsuitable due to electrical grid limitation or site characteristics (cooling water availability) in addition to simple design which reduces manufacturing cost and use of prefabricated technology instead of on-site construction. Thus SMRs are expected to solve the biggest obstacle facing large nuclear plants which is the long constructing period and the high capital cost.