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Appendix B: Nuclear Reactor Technology
Figure 14.5: Schematic Drawing of an Advanced Gas Cooled Reactor (AGR)
14.5 Appendix B5: Fast Breeder Reactor (FBR)
The reactors described above have in common that they are all thermal reactors. In thermal
reactors the fast neutrons released in the fission process must be “slowed down” (thermalized) by
collisions with the moderator atoms. Fast reactors, on the other hand, utilize the fast neutrons
directly for producing fissions. While fast neutrons are less likely to be absorbed by U-235 or
plutonium-239 than thermal neutrons, the highly enriched fuel used in fast breeder reactors
allows for a self-sustaining nuclear chain reaction. For this reason, no moderator is required to
thermalize the fast neutrons.
All large-scale FBRs have been Liquid Metal Fast Breeder Reactors (LMFBRs) cooled by liquid
sodium to transfer heat from the core to steam used to power the electricity generating turbines.
FBRs usually use a mixed oxide (MOX) fuel core of up to 20 % plutonium dioxide (PuO2) and at
least 80 % uranium dioxide (UO2).
The fast breeder reactor (FBR) is a fast neutron reactor designed to breed fuel by producing more
fissile material than it consumes. In many FBR designs, the reactor core is surrounded in a
blanket of tubes containing non-fissile uranium-238 (U-238) which, by capturing fast neutrons
from the reaction in the core, is partially converted to fissile plutonium (Pu-239), which can then
be reprocessed for use as nuclear fuel. Other FBR designs rely on the geometry of the fuel itself
(which also contains U-238) to attain sufficient fast neutron capture.
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