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of energy-production efficiency, 233U is clearly superior for fast systems as well as for
thermal systems.
the overriding consideration is the neutron-production efficiency of the system, and for
neutron production 239Pu is superior.
Fig. 4.0-1.
pecially at the higher neutron energies, owing to the fact that 239Pu produces more neutrons
per fission than the other isotopes; that is, it has a higher v value, and that value is es-
sentially energy-independent. As a result, more neutrons are available for absorption in
fertile materials and 239Pu was originally chosen as the fissile fuel for fast breeder
reactors.
However, with the historical emphasis on fissile production in fast systems,
This can be deduced from the values for q given in
The rl value for 239Pu is much higher than that for the other nuclides, es-
The fission properties of the fertile nuclides are also important since fissions in
the fertile elements increase both the energy production and the excess neutron production
and thereby reduce fuel demands. At higher energies, fertile fissions contribute significdntly,
the degree of the contribution depending greatly on the nuclide being used. As
shown in Fig. 4.0-1, the fission cross section for 232Th i s significantly lower (by a factor
of approximately 4) than the fission cross section of 238U. In a fast reactor, this means
that while 15 to 20% of the fissions in the system would occur in 238U, only 4 to 5% would
occur in 232Th.
Thus the paired use of 233U and 232Th i n a fast system would incur a double
penalty with respect to its breeding performance. It should be noted, however, that since
denatured 233U fuel would also contain 238U (and eventually 239Pu), the penalty would be
somewhat mitigated as compared with a system operating on a nondenatured 233U/232Th fuel.
In a thermal system, the fast fission effect is less significant due to the smaller fraction
of neutrons above the fertile fast fission threshold.
In considering the impact of the fertile nuclides on reactor performance, it is also
necessary to compare their nuclide production chains.
are very similar in structure.
.corresponding to 238U and 240Pu i n t'he uranium chain, while the fissile components 233U and
235U are paired with 239Pu and 241Pu, and finally, the parasitic nuclides 236U and 242Pu
complete the respective chains. A significant difference in the two chains lies in the
nuclear characteristics of the intermediate nuclides 233Pa and 237Np. Because 233Pa has
a longer half-life (i .e. , a smaller decay constant), intermediate-nuclide captures are more
probable in the thorium cycle. Such captures are doubly significant since they not only
utilize a neutron that could be used for breeding, but in addition eliminate a potential
fissile atom. A further consideration associated with the different intermediate nuclides
is the reactivity addition associated with their decay to fissile isotopes following reactor
shutdown. Owing to the longer half-life (and correspondingly higher equilibrium isotopic
concentration) of 233Pa, the reactivity addition following reactor shutdown is higher for
thorium-based fuels.
reactivity control and shutdown systems. The actual effect of all these factors, of course,
depends on the neutron energy spectrum of the particular reactor type and must be addressed
on an individual reactor basis.
Figure 4.0-2 shows that the chains
The fertile species 232Th and 234U in the thorium chain
Proper consideration of this effect is required in the design of the
Significant differences also exist in the fission-product
yields of 23311 versus 235U, and these, too, must be addressed on an individual reactor basis.
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