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4.6.3 Gas-Cooled Fast Breeder Reactors
T. J. Burns
Oak Ridge National Laboratory
In addition to the sodium-cooled fast reactors discussed above, the impact of the various
a1 ternate fissile/fertile fuel combinations on the Gas-Cooled Fast Breeder Reactor (GCFR) has
also been addressed (although not to the degree that it has for the LMFBR). A 1200-MWe Pu/U
GCFR design with four enrichment zones was selected as the reference case.7-8 The various
alternative fissile/fertile fuel combinations were then substituted for the reference fuel.
No design modifications or optimizations based on the alternate fuel properties were per-
formed.
It should also be emphasized that the results of this scoping evaluation for
alternate-fueled GCFRs are not comparable to the results given in Section 4.5 for LMFBRs
due to markedly different design assumptions for the reference cases.
The results of the preliminary calculations for the alternate-fueled GCFRs, sum-
marized in Table 4.6.3, reflect trends similar to those shown by LMFBRs; i.e., relative
to the reference case, a significant breeding ratio penalty occurs when 233U is used as
the fissile material and 232Th as the core fertile material. >loreover, the magnitude
of the penalty (ABR) is larger for the GCFR than for the LMFBR.
coolant, the characteristic spectrum of the GCFR is significantly harder than that of
a comparably sized LMFBR. In light of the relative nuclear properties of the various
fissile and fertile species discussed in Section 4.5, this increased penalty due to the
harder spectrum is not surprising.
the fissile Pu isotopes in the GCFR is significantly higher than the number produced In
the softer spectrum of an LMFBR.
tively insensitive to spectral changes. Hence, the larger penalty associated with
233U-based fuels in the GCFR is due to the better performance of the Pu reference system
rather than to any marked changes in 233U performance. A similar argument can be made
for the replacement of core fertile material. Owing to the harder spectrum, the fertile
fast-fission effect is more pronounced in the GCFR than in an LMFBR.
in the fertile fission cross section resulting from replacement of 238U by 232Th results
in a larger decrease in the breeding ratio.
case, 233U-fueled GCFRs require smaller fissile inventories than do the corresponding
Pu-fueled cases.
Owing to the helium
The number of neutrons produced per fission (v) of
The value of v for 233U, on the other hand, is rela-
Thus, the reduction
It should also be noted that as in the LMFBR
The better breeding performance of Pu in the harder spectrum of the GCFR, on the
other hand, indicates that the GCFR would be a viable candidate for the role of energy
center "transmuter," either as a Pu/Th system or as a Pu/U + Tho, radial blanket system.
It must be emphasized, however, that these conclusions are tentative as they are based
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