The Nuclear Imperative
Plutonium breeder reactors can be designed to have a breeding margin of up to 40%, that is they will produce 40% more plutonium than they consume. As a practical matter they are usually designed to operate with a breeding margin of 20%. Thorium has a theoretical breeding margin of 8% but is only likely to be able to wash its face in a practical design and thus might need help from excess plutonium to get it over the line in terms of neutron economy. This is important because there is four times as much thorium that can be mined than uranium. A truly high-level civilisation would start working towards that end now.