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4.6. ALTERNATE FAST REACTORS
4.6.1. Advanced Oxide-Fueled LMFBRs
T. J. Burns
Oak Ridge National Laboratory
One method of improving the breeding performance of the LMFBRs discussed in the
previous section is to increase the core fertile loadings. Typically, this goal is
accomplished by one of two means: redesign of the pins to accommodate larger pellet
diameters or the use of a heterogeneous design (i.e., intermixed core and blanket
assemblies). To maintain consistency with the "classical" designs considered in the
previous section, using the same fuel elements for both concepts, the latter option was
i
pursued to assess the impact of possible redesign options. Table 4.6-1 summarizes some
preliminary results from calculations for a heterogeneous reactor core model consisting
of alternating concentric fissile and fertile annuli (primed cases) and compares them
with results from calculations for corresponding homogeneous cores (unprimed cases).
As the data in Table 4.6-1 indicate, the heterogeneous conffquration results in a
significant increase in the overall breeding ratio relative to the corresponding homo- .
geneous calculation. The heterogeneous reactors also require a much greater fissile
loading for criticality due to the increase in the core fertile loading. However, the
increase in the breeding gain more than compensates for the increased fissile requirements,
resulting in an overall improvement in the fissile doubling time. On the other hand,
because of the high fissile loading requirements, it appears that a heterogeneous model for
the denatured cases with 12% enrichment (cases 6 or 7 of the previous section) is unfeasible;
therefore, an enrichment of 120% was considered as the minimum for the denatured heterogeneous
configuration.
While the denatured heterogeneous configurations result in an increase in the
overall breeding ratio, it is significant that the 233U component of the breedinq ratio
also improves. Figure 4.6-1 depicts the breeding ratio components for both the homogeneous
and heterogeneous denatured configurations.
as the upper limit.) As Fig. 4.6-1 indicates, the heterogeneous confiaurations are
clearly superior from the standpoint of 233U self-sufficiency (i.e. , requirinq less
makeup requirements). Moreover, if enrichments in the range of 304: - 40% are allowed,
it appears possible for a denatured heterogeneous reactor to produce enough 233U to
satisfy its own equilibrium cycle fuel requirements. Production reactors would therefore
be required only to supply the initial inventory plus the additional makeup consumed
before the equilibrium cycle is reached.
(Aaain , the 233U/Th LMFBR is included