PLENTIFUL ENERGY

in the same proportion, since the neutron leakage is reduced in larger cores and the
fissile enrichment is reduced accordingly. The fissile enrichment can be reduced
substantially in a similar fashion, as illustrated in Figure 14-7, depending on the
reactor power.
Parenthetically, the uranium resource utilization cannot be noticeably improved
by once-through long-life cores. As a reference point, a high-burnup LWR fuel
requires 4.5% enrichment to achieve 5% burnup. About 88% of the original
uranium ends up in the tailings during the enrichment process, then called depleted
uranium. Of the 12 % loaded into the reactor, only 5% is fissioned, resulting in a
0.6% utilization of the original uranium. A large fast reactor would typically require
15% enrichment, instead of the 4.5% which would mean that 96.5% is discarded as
depleted uranium. Even a 20% burnup of the remaining 3.5% results in an overall
utilization of 0.7%—not much different than the 0.6% of the LWR‘s once-through
cycle. This example case is already on a flat side of Figure 14-7, and therefore the
best one can expect by extreme designs of long-life cores would be a little over 1%
uranium utilization.
The second design constraint for long-life cores is the fast neutron (>0.1 MeV)
fluence limit. The fast neutron fluence (the neutron flux multiplied by irradiation
time) causes damage to the cladding material. The fluence limit is in the same range
as the burnup limit for conventional design conditions, and hence normally no
particular attention is given to the fluence. However, for long-life cores, the fluence
continues to increase with time. The cladding strain due to irradiation-induced creep
will continue beyond the conventional design limit and will become the limiting
constraint for the long-life cores. Most long-life cores reach fast neutron fluences
on the order of 34 times those of conventional designs.
The third constraint involves the thermal-hydraulics. The cladding lifetime is
very sensitive to temperature, which sets a peak temperature limit. Within this limit,
the coolant outlet temperature needs to be as uniform as possible among assemblies
in order to maintain a high average temperature. In conventional core designs, this
is achieved by providing a few orificing zones of assembly inlet nozzles to match
the coolant flow to power. Because the assembly power shifts during irradiation, it
is impossible to achieve a constant power-to-flow ratio for all assemblies. For larger
long-life cores, the power shift will be more prominent and widespread throughout
the core, and it will be much more challenging to develop a workable orificing
scheme that will remain effective throughout the long core life.
In summary, the neutronic and burnup constraints can be met readily by
judicious design choices, but the fluence constraint cannot be met unless a ―magic‖
cladding material is developed. The thermal-hydraulic orificing constraint is also a
tough challenge, especially for large cores.
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