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While fission of U-235 can yield substantial energy, the world's supply of U-235 is limited. However, it is possible to use the neutrons given out by fission reaction to breed fuel from non-fissionable isotopes like U-238. As U-238 is more plentiful than U-235, it is an attractive option. U-238 can absorb a fast neutron given out by a fission reaction to yield fissionable plutonium. A typical breeding reaction is as follows : |
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U-238 absorbs a fast neutron and undergoes two successive beta decay to give rise to plutonium. The core of a fast breeder reactor consists of an inner part which contains the fission fuel consisting of enriched  UO  and  PuO and an outer part which consists of U-238, depleted uranium and  Th in some combination. The fast neutrons that are produced during fission of the inner part is captured by the outer layer to breed fissionable plutonium and U-233. Thus the reactor breeds more fuel than it consumes, which accounts for the name. No moderators are used in the reactor as the neutrons should not be slowed down. This is also the reason why the primary coolant used is not water which could slow down neutrons. The usual coolant used in the primary loop is liquid sodium. |
