Digression: Fast Breeder Nuclear Fission Reactors

This technology promises to expand the amount of fissionable fuel available from natural uranium by a factor of about 60, by “breeding” more fuel than it consumes. This doesn’t change the non-renewable nature of the total uranium resource, but it does promise to make it so abundant as to encourage a casual attitude towards energy conservation.

“Fast Breeding” consists in using the stream of neutrons released by the fission of readily fissile uranium 235 to “breed” fissile plutonium from the relatively stable, only slightly radioactive uranium 238, which actually makes up 99.29 percent of natural uranium and cannot itself be used as nuclear fuel.

An atom of uranium 238 captures a neutron in a resonance capture. In doing so it becomes an atom of uranium 239. After this it can go two ways; it can lose the neutron to become again uranium 238, or it can lose merely an electron to become neptunium 239 which, in turn, over a longer period loses an electron from its nucleus to become plutonium 239, the fissile, fuel-useable isotope of plutonium.

The much more likely event on capture of a neutron by uranium 238 is the rapid loss of that neutron without any further change of the uranium 238 into higher elements.

If the layer of uranium 238 around the neutron-emitting nuclear fuel is made thicker, then each uranium 238 atom can have more “goes” at capturing a neutron and each neutron can have more “goes” at attaching itself to a uranium 238 atom. This will increase the chances of each uranium 238 atom becoming a plutonium 239 atom, thus increasing the “breeding fraction” that is the fraction of non-fissile uranium 238 converted to fissile plutonium 239.

In practice a breeding ratio of 1.2 to 1.4 should be obtained, meaning that 1.2 to 1.4 kg of plutonium 239 is obtainable for each kg of uranium 235 consumed. Evidence is lacking that a breeding ratio greater than 1 has actually been consistently obtained over a useful period with the fast breeder reactors currently operating.

Butler, Raymond and Watson-Munro in “Uranium on Trial”, 1976, give figures indicating that up to that time a breeding ratio of only 0.01 to 0.02 had been achieved – far too low to make any difference.

Assuming that breeding ratios are now, or ever can be, greater than 1, and allowing for losses, up to 60 percent of the world’s uranium could become useable as fuel in fission reactors instead of the 1 percent useable by conventional burner reactors. This 60 percent could eventually be made available even if the breeding ratio were infinitesimally greater than 1, but the time needed to get it all would be longer than if the breeding ratio were 1.1, 1.2 or 1.4. The resource’s quantity isn’t affected but its availability, the rate at which it can be got hold of, falls as the breeding ratio falls towards 1.

If the consistent long-term breeding ratio (CLTBR) were always below 1, then the proportion of the world’s uranium that could ever be obtainable as fissile fuel would never be as great as the maximum 60 percent quoted above. It would fall ever further short of 60 percent according as the CLTBR fell ever further short of 1.

If the best the world could achieve was a CLTBR of 0.75, which would be pretty good, the proportion of the world’s uranium that could ever be made available as fissile fuel would rise from 1 percent to only 4 percent – a mere fourfold increase. If the CLTBR were only 0.5, still respectable and way more than has ever been achieved, the increase would be only twofold, from 1 percent to two percent. These figures can be checked using a sort of negative compound interest – 1 kg breeds 0.5 kg, which breeds 0.25, and so on.

Fast breeder nuclear reactors cannot be seen as the path to a laid back paradise of unlimited energy consumption and no conservation effort.

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