I said Thorium.
Thorium to uranium, not plutonium to uranium; the plutonium is in there BECAUSE it's more advanced. Greg seems to like the idea of different nuclear cells having a loss of energy when you convert between them, and I especially agree with that for this recipe, since you're taking Thorium, which is renewable with a blaze farm, and converting it into normal uranium... Which can then be converted into plutonium in turn, so that may be somewhat difficult to balance around, depending on Greg's overall opinion on the idea.
What i read was saying to combine thorium and plutonium to make depleted uranium(which would also be a waste, but since *Plutonium makes more Eu than Uranium* it would be a waste. Btw, Greg, i saw on etho's channel that still lava makes a tungsten cell. Fixed?