REPORT ON IN NAVAL NUCLEAR JUNE, 1995

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TABLE 1
23SU Fissile Content of Various Fuels
Enrichment Level Volume Required to Get Same Fissile l3SU
Content as One Cubic Centimeter of
93%-Enriched REV
2O%-Enriched 4.7 cc
S%-Enriched 18.6 cc
Natural Uranium 129 cc
That is, the lower the enrichment, the more non-fissile uranium comes along Vroith the fissile uranium. The
non-fissile uranium in the lower enrichment fuel represents a very inefficient use of fuel volume. The nonfissile
uranium does not produce energy directly, but first has to absorb a neutron and transmute to fissile
plutonium -an inefficient process in small moderated reactors.
Naval reactor cores have evolved in compactness to the point where the maximum amount of uranium is
packed into the smallest volume, and the only way to make more volume available for uranium would be to
remove cladmng, structure or coolant. In other words, no more uranium could be packed into a modern
long-lived core Vroithout degrading the structural integrity or cooling of the fuel elements. Therefore, using
LEU Vroith its lower fissile content in place of REU wastes volume in the core, and offers only two design
choices for a given Naval reactor application:
.Using the same core volumes as in current design ships, pack in about the same amount of uranium, in
the form of LEU instead ofHEU, and as a consequence reduce the fissile loading and substantially
decrease the endurance of the core.
.Alternatively, in redesigned ships, substantially increase the volume of the core, and pack in more
uranium in the form of LEU, so that the total amount of energy which can be taken out of the core
over its life is the same.
These two choices are further developed below.
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