Why no Thorium? Because of the Uranium-Plutonium Fuel Cycle

According to the "Energy from Thorium" website, "We get somewhat of an insight into the thinking of the Atomic Energy Commission with regards to breeder reactors. If they were to use uranium-plutonium, then plutonium supplies were crucial due to the fact that each fast breeder needed 10 to 15 times as much fissile material to generate a unit of power as a thermal reactor did. The light-water reactors at the time were producing plutonium as a byproduct. The fast-breeder needed and wanted that plutonium. Reactors like LFTR needed a tiny fraction of the fissile inventory as the fast breeder did and could be started on HEU.

Here’s an image of how the AEC envisioned light-water reactors running on enriched uranium and producing plutonium, and fast-breeder reactors needing that plutonium, working together.

The picture begins to become clearer, especially when we consider how Weinberg described what the AEC did with this report, establishing a “Fast Breeder” office but no “Thermal Breeder” office."

Uranium and plutonium went hand-in-hand. There was no need for thorium under this model.