4th Generation Needs 3rd Generation's Waste, to Breed

4th Generation nuclear reactors are meant to safely achieve at least these three things:

Nuclear waste that lasts decades instead of millennia.

100-300 times more energy yield from the same amount of nuclear fuel.

The ability to consume existing nuclear waste in the production of electricity.

__GenIV Reactor Wikipedia

To get so much energy from nuclear materials, Gen IV reactors will be "breeder reactors." They will take fertile fuels such as Uranium 238 or Thorium 232, and turn them into fissile materials such as Plutonium 239 and Uranium 233. But they will need a lot of starting materials: fissile materials from weapons and waste -- lots and lots of it.

As a significant number of generation IV units start to come online over the next few decades, they will need fissile “start charges” to kick them off. A new 1 gigawatt fast spectrum reactor, for instance, needs to be fuelled with about eight tonnes* of fissile uranium 235 or plutonium (or some other mixture of fissile actinides) to get it going. After that, it can breed all the new fuel it will ever need from uranium 238.
Yet, if all of the world’s stockpile of weapons material and used nuclear fuel were reprocessed, we could still produce only enough fissile material (about 3,000 tonnes) to launch just 400 1-GWe fast reactors in the decade 2020 to 2030. After that, if nuclear power is to continue to expand at a rapid rate, we would need a reactor deployment program where we continue to build both generation III and generation IV units for the next few decades. _BraveNewClimate

Barry Brook is saying that for Generation IV reactors to be sustainable -- with all of their many advantages -- they will need a lot of Generation III reactors to keep them stocked with waste starting material.  The two types of reactors complement each other, since each produces part of the fuel for the other.   The combination could provide abundant heat and electricity to the world for many thousands of years.

 Go to the link above and read the entire argument, and follow the links to supplementary materials.

More on advanced reactors.