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Fast reactors (FBR's)- fuel use could be extended by 60 times, waste reduced.
While producing power from fast fission, the Fast Neutron Breeder Reactor can also produce more useable fuel, say 20% more each use than it uses by converting U238 to fissionable Pu239 - plutonium thus, after processing, extending considerably the use of uranium as a fuel, probably by 60 times (ref 145) and decreasing very considerably the long term storage waste.
Simplified diagram of fast neutron reactor :-
In contrast to the thermal fission process, for every 6 fissions of Pu239 in a fast reactor, 17 neutrons are emitted, 6 go on to maintain the chain reaction, 7 neutrons breed PU239 from U238 and four are lost. Thus more fissionable fuel can be created than is used up.
A 'fast' reactor has no 'moderator' (graphite, water, heavy water) and therefore water cannot be used as a coolant. Heat is more concentrated so a coolant such as Liquid sodium, or sodium/potassium, lead, lead-bismuth at 500 - 550 C is suitable which does not require a pressure vessel. Control rods are boron and/or cadmium.
A blanket of mainly U238 is wrapped around the core and spare neutrons convert this to PU239 to maintain the reaction which with used Uranium is reprocessed as further fuel.
Due to cost, abundance of cheap fossil fuels at the time and various handling and leakage problems, plans to build large numbers of fast reactors were abandoned or put on hold in the 1960s and 1970s but there is renewed interest today.
Of the 18 FBRs 6 are still in operation including the 250 MWe Phoenix in France since 1973; the , 40 MW FBTR India since 1985; 500MW reactors under construction at Kalpakkam; the BN 600 in Russia since 1981; and the 280 MW reactor at Monju in Japan
Long life fission products in high level waste can be fissioned in a FBR to products with a shorter life, so easing disposal. (Ref 6)
The amount of Pu239 fuel obtainable from weapons disposal could when reprocessed (as MOX) supply all our needs for several decades.
The thermal fission reaction
Natural uranium consists of a small amount of U235 which is 'fissionable' but also a much greater quantity of U238 which is not. The thermal reaction is as follows:-
U235 + n (a neutron) --> X + Y (fission products) + energy (= lost mass) + 2.5 n
The figure '2.5 n' means that out of 6 fissions 6 X 2.5 = 15 neutrons are emitted. 6 maintain the chain reaction, 5 are lost and 4 breed Pu 239 (from U238.)
U 238 is a non fissionable isotope and in much greater quantity in uranium. As U235 is fissioned, some U238 in the uranium is converted to Pu239 which is then also fissioned extending the life of the fuel :-
U238 + n --> U239 --> Np239 --> Pu 239 (fissionable).
Thorium is present in greater quantity than uranium and non fissionable but can also become fissionable if in a reactor it catches a neutron or "breeds":-
Th 232 + n --> U233 (fissionable) .