In a fast reactor neutrons are a few thousand times faster than in a slow one. They are more likely to zip by a nucleus than to interact with it, a few hundred

In a fast reactor neutrons are a few thousand times faster than in a slow one. They are more likely to zip by a nucleus than to interact with it, a few hundred times more likely. But when they do interact, they are very likely to just trigger fission, rather than be absorbed. For example a U-235 has a chance of about 82% to be fissioned by an incoming slow neutron and 18% to become U-236, which is a very nasty long-live waste product. Pu-239 has about a 30% chance to become Pu-240. This itself has a 99.9% to absorb a thermal neutron rather than undergo fission [1]. When the isotope 240Pu captures a neutron, it is about 4500 times more likely to become plutonium-241 than to fission. In general, isotopes of odd mass numbers are more likely to absorb a neutron, and can undergo fission upon neutron absorption more easily than isotopes of even mass number. Thus, even mass isotopes tend to accumulate, especially in a thermal reactor. Bottom line, in a thermal reactor a lot of neutrons get absorbed and transuranic elements get produced, which have a tendency to have very long half lives. Thorium, being lighter than Uranium, will produce less of those, because it takes more successive neutron absorptions to become a transuranic isotope. So, yes, Thorium has an advantage over slow Uranium reactors. But fast Uranium reactors produce much fewer transuranic isotopes to begin with, plus they can burn U-238, which is plentiful. Slow Uranium reactors use Uranium that only contains about 5% U-235, and they only burn a part of it. Fast reactors can burn a copious amount of the U-238 they have, and so they produce much more energy from the same amount of fuel. Which is a different way to say, they produce much less waste for the same amount of delivered energy. [1] https://en.wikipedia.org/wiki/Plutonium-240#Nuclear_properties https://en.wikipedia.org/wiki/Plutonium-240#Nuclear_properti...