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between these two extremes, and thus the proposed system's power ratio will be a crucial
evaluation parameter.
The symbiotic system depicted by Fig. 2.2-1 can also be characterized by the type
of reactors utilized inside and outside the center. In general, systems consisting of
thermal (converter) reactors only, systems consisting of both thermal converters and fast
breeder reactors, and systems consisting solely of fast breeder reactors can be envisioned.*
One important characteristic of each system is the extent to which it must rely
on an external fuel supply to meet the demand for nuclear-based generating capacity. The
thermal-thermal system would be the most resource-dependent. The breeder-thermal system
could be fuel-self-sufficient for a given power level and possibly also provide for moderate
nuclear capacity growth. The breeder-breeder scenario, if economically competitive with
a1 ternative energy sources, would permit the maximum resource-independent nuclear contribu-
tion to energy production.
While such considerations serve to categorize the symbiotic systems themselves, the
transition from the current once-through LEU cycles to the symbiotic systems is of more
immediate concern. A1 though a1 l-breeder systems would be resource-independent, commercial
deployment of such systems is uncertain. The transition to the denatured cycle could be
initiated relatively soon, however, by using moderately enriched 235U/2 38U mixed with
thorium (sometimes referred to as the "denatured 23sU fuel cycle") in existing and projected
thermal systems. The addition of thorium (and the corresponding reduction of 238U
over the LEU cycle) would serve a dual purpose: the quantity of plutonium generated would
be significantly reduced, and an initial stockpile of 23% would be produced. It should
be noted that this rationale holds even if commercial fuel reprocessing is deferred for
some time. Use of denatured 23% fuel would reduce the amount of plutonium contained in
the stored spent fuel. In addition, the spent fuel would represent a readily accessible
source of denatured 233U should the need to shift from 23% arise. However, substituting
232Th for some of the 23*U in the LEU cycle would require higher fissile loadings and thus more
235U would be comnitted in a shorter time frame than would be necessary with the LEU cycle.
An alternative would be to utilize energy-center Pu-burning transmuters to provide the initial
source of 233U for dispersed 233U-based reactors. From these starting points, various scenarios
which employ thermal or fast energy-center reactors coupled with denatured thermal or fast
dispersed reactors can be developed.
On the basis of the above, eight general scenarios have been postulated for this study,
with two sets of constraints on Pu utilization considered: either plutonium wiZZ no& be allowed
as a recycle fuel but recycle of denatured 233U will be permitted; or plutonium wiZZ
be allowed within secure energy centers with only denatured fuels being acceptable for use at
dispersed site reactors. The eight scenarios can be summarized as follows:
* See Section 4.0 for discussion of reactor terminology as applied in this study.