PLENTIFUL ENERGY

neutron leakages from the core are a lot less than from the top and bottom (the
diameter of the core as a rule is at least double the height), the optimum radial
blanket thickness even for maximum breeding is only two or three layers. And for
self-sustaining reactivity only, perhaps one layer of external blanket is necessary; a
net burner of actinides will have no uranium blankets at all, only steel shielding.
The core layout presented in Figure 14-4 is for a 350 MWe reactor core. Size is
determined by the number of fuel assemblies; the assembly design itself probably
remains the same.
14.4.4 Reactor Size Effects
The unit lattice cell design principles described in the previous section are
applicable independent of the reactor power. Larger reactors require more unit cells
or more fuel pins and assemblies. The core size can grow in both radial and axial
directions. But when the core size grows to about three feet in height, any further
expansion is in the radial direction. Even with a three-foot active fuel column
length, the overall assembly length could be more than twelve or thirteen feet when
the gas plenum, upper and lower shields (or blankets), and inlet and outlet nozzles
are added. Since the spent fuel has to be handled under sodium when transferred
across the core, the sodium pool depth and the vessel height have to be increased as
the active core height increases. But the three-foot optimum is not set in concrete.
Both the height and the diameter can differ with the same design for electrical
output if different properties of the core are emphasized. But a typical core diameter
would be about ten feet for a 350 MWe plant and some fifteen feet or so for a 1,000
MWe, both with core heights of about three feet.
A key question in the effects of reactor size is whether the inherent safety
features demonstrated in the small EBR-II can be achieved in larger reactor sizes, or
if indeed there is a size limit for such behavior. Some inherent safety characteristics
sought in other reactor concepts, such as the radiative heat removal, do depend on
the reactor size. However, the IFR‘s inherent safety is more or less independent of
the reactor size. As explained in detail in Section 7.8, this independence is due to
the reactivity feedback mechanisms themselves being quite independent of the
reactor size. The net result is that the margins to coolant boiling during unprotected
loss-of-flow and unprotected loss-of-heat-sink events are very similar independent
of reactor size, as presented in Table 7-1.
One important effect of reactor size is its effect on the specific fissile inventory
requirement. Since the unit cell design remains the same, the total amount of heavy
metal increases directly proportional to the reactor power. However, the fissile mass
does not follow this proportionality, since the fissile enrichment (ratio of fissile
mass to heavy metal mass) is reduced due to the reduced neutron leakage of a larger
core. More neutrons are then available inside the core for in situ breeding, which
further reduces the enrichment level. The amount of fuel needed for a given power
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