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ments.
4-51
The calculations for LMFBRs operating on denatured 233U fuel cover a range of enrich-
Cases 5, 6, and 7 assume an %12% enrichment, Case 8 a 20% enrichment, and Case 9
a 40% enrichment. All these reactors are, of course, subject to the breeding ratio penalty
inherent in replacing plutonium with 233U as the fuel material.
(8 and 9) also reflect the effect of thorium in the LMFBR core spectrum. (These higher
enrichment cases were calculated in an attempt to parameterize the effect of varying the
amount of denaturing.) A further point which must be addressed regarding the denatured
reactors is their self-sufficiency in terms of the fuel material 233U.
LMFBRs typically contain both 232Th and 238U as potential fissile materials, both 233U and
239Pu are produced via neutron capture. Thus in evaluating the self-sufficiency of a fast
breeder reactor, the 233U component of the overall breeding ratio is of primary importance
since the bred plutonium cannot be recycled back into the denatured system. As illustrated
schematically by Fig. 4.5-1, the 233U component of the breeding ratio increases as the
allowable denatured enrichment is increased (which allows the amount of thorium in the fuel
material to be increased). More importantly, the magnitude of the 233U component of the
\
breeding ratio is very sensitive to the allowable degree of denaturing at the lower enrich-
ments (i.e., between 12% and 20%).
enrichment is raised, but a concomitant and significant decrease in the required 233U makeup
presents a strong incentive from a performance viewpoint to set the enrichment as high as
is permitted by nonproliferation constraints.
Table 4.5-1, the lowest enrichment limit feasible for the conventional LMFBR type systems
anaJyzed,lies in the'll-14% (inner-outer core) range.
fuel and would require significant amounts of 233U as makeup.
the 233U/Th system is not denatured.
an upper bound on the 233U enrichment.)
The less denatured cases
Since the denatured
The overall breeding ratio decreases as the allowable
In fact, based on the data summarized in
Such a system would utilize all U02
(It should be noted that
It is included in Fig. 4.5-1 because it represents
Since all denatured reactors require an initial inventory of 233U, as well as varying
amounts of 233U as makeup material, a second class of reactors must be considered when
evaluating the denatured fuel cycle. The purpose of these systems would be to produce the
233U required by the denatured reactors. Possible LMFBR candidates for this role are the
Pu/~~*U reactor with thorium blankets (Cases 2 and 3), a Pu/Th reactor with thorium blankets
(Case 41, and a 233U/Th breeder (Case lo).+ In the reduced-proliferation risk scenario, all
three of these systems, since they are not denatured, would be subject to rigorous safe-
guards and operated only in nuclear 'weapon states or in internationally controlled energy
centers. Performance parameters for these three types of systems are included in Table
4.5-1, and the isotopic fissile production (or destruction) obtained from the ORNL calculations
is schematically depicted by Fig. 4.5-2. Clearly, each system has its own unique
properties. From the standpoint of 233U production capability, the hybrid Pu/Th system is
*See discussion on "transmuters" on p.4-10.