# How is the neutron capture cross-section increased in thermal breeder reactors?

The related Wikipedia article says:

A breeder reactor is a nuclear reactor that generates more fissile material than it consumes. Breeder reactors achieve this because their neutron economy is high enough to create more fissile fuel than they use, by irradiation of a fertile material, such as uranium-238 or thorium-232 that is loaded into the reactor along with fissile fuel.

Thus, a breeder reactor somehow solves, that the thermal neutrons in it create equal or more $$^{239}\rm Pu$$ (by the neutron capture of $$^{238}\rm U$$, which decays to $$^{239}\rm Pu$$ in two steps), than the fission (of $$^{239}\rm Pu$$ or $$^{235}\rm U$$) wastes.

Now my question is, how do they reach this?

In a thermal reactor, the neutrons are thermal (i.e. their kinetical energy is roughly the same, than the medium around them). Thus, also the cross sections for neutron capture are roughly fixed. And these are far smaller, which would be required for breeding (as far I know, roughly half of it).

• Took lot of time to understand the question because of misleading content. I answered it as per my understanding. – SRD Apr 8 '19 at 2:48