The high cost of Gen 3 nuclear power is largely due to the very large steam turbine and heat exchangers that are required if you are using water as a working fluid, coolant, and moderator.
From the 1950s to the 1990s the interest in fast reactors has been in the 60x better fuel economy and reduced waste problem. (e.g. less radioactive than the uranium ore in 1000 years) It was believed back then that a fast reactor coupled to a steam turbine would have a higher capital cost than an LWR. There also was a lot of concern that it takes a lot of uranium or plutonium to form a critical mass and that would be an expense.
Recently fast reactors and other high temperature reactor types are of interest because getting rid of the water could allow miniaturizing the whole system and get the cost competitive with natural gas not to mention solar and wind on the days that solar and wind feel like supplying power. The stockpile of plutonium in spent fuel is getting bigger and bigger every day and the public seems entirely uninterested in throwing away 98% of the energy content of the spent fuel away in a place like Yucca Mountain. Thus high-quality fissile material seems a lot less scarce than it did in the EBR I - Superphenix era.
