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4,1.2 Converter operation with fuel processing
Because the results of the currently completed neutronic calcuba-
tions will not support any final conclusions about the breeding potential
of fully optimized DNSR cores, consideration must be given to the conse-
quences of co~~ersion ratios lower than 1,OO. The evaluations were pes-
formed for the two-zone flux-flattened core described for the 30-year fuel
cycle with the fuel processing concept for the break-even breeder added.
If this system were operated with no constraint on the enrichment sf the
uranium in the reactor and no 2 3 8 ~ addition, it would gradually develop
into an MSBR as the 23*U was consmed.
self-sustaining on thorium with a breeding ratio of about 1.03 but with a
very high enrichment of fissile uranium. Breeding ratfos as high as 1.11
could be attained by changing the thorium concentration and/or the size of
the inner core zone.
plant uranium denatured at all times, this particular reactor system would
ultimately require am additional 2% in nuclear reactivity to be indefi-
nitely operable. This reactivity deficit, if real, could be supplied in
a number of ways.
*
The system would then be fully
with the addition of enough 2"~ to keep the in-
Amoderate feed of 23% at 20% enrichment would extend the fuel syc~e
to about 308 years. At that time, the 238U loading would become exces-
sive, and the reactor could no longer be wade critical. While even 300
years may be much longer than any reasonable planning horizon, this re-
sult indicates that a fully denatured MSR could have a very long, if not
unlimited, fuel lifetime. I€ the enrichment of the feed material were
allowed to rise to 33% 235U9 reactor operation could be sustained indefi-
nitely WfthOblt fuel discard.
Because the buildup of 23% is the limiting phenomenon in the fuel
cycle of any nonbreeding DMSW, any process that would have the effect of
removing 2381~ would improve the characteristics sf the cycle.
With the
fate% feed enrichment set at 20% 235U9 the bufldup of 238U could be limited
* lt
Indefinitely operable" is arbitrarily defined here as maintaining
kefg 1. 1.0 for 600 years or bong el^. In all extended fuel cycles, the
fuel is presumed to be transferred without loss frm one reactor plant to
another as required by hardware lifetime considerations,