Neutron Temperature - Fast Reactor and Thermal Reactor Compared

Fast Reactor and Thermal Reactor Compared

Most fission reactors are thermal reactors that use a neutron moderator to slow down ("thermalize") the neutrons produced by nuclear fission. Moderation substantially increases the fission cross section for fissile nuclei such as uranium-235 or plutonium-239. In addition, uranium-238 has a much lower capture cross section for thermal neutrons, allowing more neutrons to cause fission of fissile nuclei and propagate the chain reaction, rather than being captured by 238U. The combination of these effects allows light water reactors to use low-enriched uranium. Heavy water reactors and graphite-moderated reactors can even use natural uranium as these moderators have much lower neutron capture cross sections than light water.

An increase in fuel temperature also raises U-238's thermal neutron absorption by Doppler broadening, providing negative feedback to help control the reactor. Also, when the moderator is also a circulating coolant (light water or heavy water), boiling of the coolant will reduce the moderator density and provide negative feedback (a negative void coefficient).

Intermediate-energy neutrons have poorer fission/capture ratios than either fast or thermal neutrons for most fuels. An exception is the uranium-233 of the thorium cycle, which has a good fission/capture ratio at all neutron energies.

Fast reactors use unmoderated fast neutrons to sustain the reaction and require the fuel to contain a higher concentration of fissile material relative to fertile material U-238. However, fast neutrons have a better fission/capture ratio for many nuclides, and each fast fission releases a larger number of neutrons, so a fast breeder reactor can potentially "breed" more fissile fuel than it consumes.

Fast reactor control cannot depend solely on Doppler broadening or on negative void coefficient from a moderator. However, thermal expansion of the fuel itself can provide quick negative feedback. Perennially expected to be the wave of the future, fast reactor development has been nearly dormant with only a handful of reactors built in the decades since the Chernobyl accident due to low prices in the uranium market, although there is now a revival with several Asian countries planning to complete larger prototype fast reactors in the next few years.