("Thermal" does not mean hot in an absolute sense, but means in thermal equilibrium with the medium it is interacting with, the reactor's fuel, moderator and structure, which is much lower energy than the fast neutrons initially produced by fission.)
A fast-neutron reactor, on the other hand, operates using high-energy neutrons that are not slowed by a moderator.
These reactors can efficiently use a broader range of fuels, including plutonium and other heavy atoms, and have the capability to breed more fissile material, such as uranium-238 into plutonium-239, which is not possible in thermal reactor.
These reactors are capable of recycling nuclear waste and breeding new fuel, which enhances sustainability.
Additionally, IFRs incorporate passive safety features that allow them to safely shut down without external power or human intervention Most nuclear power plant reactors are thermal reactors and use a neutron moderator to slow neutrons until they approach the average kinetic energy of the surrounding particles, that is, to reduce the speed of the neutrons to low-velocity, thermal neutrons.