Molten salt reactors might be the second generation of reactors in an alternative history with nuclear-by-startups. They require a higher fissile material concentration than found in natural uranium. To start operating a MSR you need some reactors fueled with natural uranium and moderated by graphite or heavy water in order to breed fissile plutonium from uranium, or uranium 233 from thorium, to get concentrated fissile material for starting molten salt reactors.

Or you could start a MSR with U-235 enriched from natural uranium, but that would require developing complex and expensive enrichment technology before you get your first watt of nuclear power generation.