Types

Type of nuclear power Problems if scaled up
Fuel scarcity Weapons proliferation Nuclear waste
Conventional nuclear power Problem Low risk Problem
Conventional small nuclear reactors Problem High risk Problem
Uranium-238 breeder reactors Additional benefit: Uranium-238 reactors would make use of existing nuclear waste left over from decades of conventional nuclear power. Abundant High risk Almost none
Thorium-232 breeder reactors Abundant Low risk Almost none
Hydrogen fusion (not viable yet) Abundant Low risk Almost none

So far, it seems that thorium is the only nuclear power that could really scale up enough to replace all fossil fuels.