An assessment of the relative merits of heliumsodiumand steam-cooled fast breeders was completed in Sweden in 1969. Breeders have the potential of reducing overall system power costs provided no more than 40 million tons of uranium can be mined below $15/lb U3O8. The breeders—particularly those cooled by Na or Heset reasonable upper limits to mining and enrichment capacity requirements. Fuel development is the outstanding problem for these breeders, and especially difficult for the steam-cooled type. Steam cooling shows the lowest capital costs, followed by helium cooling, and is relatively promising with low-cost uranium. Sodium cooling involves the most extensive research and development (R&D) program; the helium- and steam-cooled breeders are largely based on thermal-reactor technology and offer real potential as advanced next-generation plants. However, the uncertainties, even of the relative assessments, still overlap the estimated power cost differences of 0.4 mill/kWh.