I'm not a specialist. If I understand correctly "breeder mode" depends upon fuel composition (which actinide/isotope, in which proportion...). Non 'fast-neutron' mode is dubbed 'thermal' and seems less interesting as it obtains less energy (however some fuel, such as thorium, may enhance its performance), however it has benefits (less difficult to design/build/exploit(?)).

https://en.wikipedia.org/wiki/Fast-neutron_reactor https://en.wikipedia.org/wiki/Thermal-neutron_reactor