PLENTIFUL ENERGY

core is around 150 o C. For higher temperature rises, the coolant mass flow rate can
be reduced, but this results in a higher thermal stress on cladding and a greater
difference in outlet temperatures between adjacent assemblies. This can lead to
thermal striping (a random joining of hot and cold flows, leading to cyclical stresses
that contribute to fatigue cracking) on upper internal structures.
14.4.3 Design Tradeoffs
As discussed above, the selection of linear heat rate and temperature rise involve
tradeoffs. Tradeoffs continue as the design progresses. For a given linear heat rate
and temperature rise, the mass flow rate is determined by the heat balance equation.
The coolant area of the unit lattice cell is then given by the mass flow rate divided
by the coolant velocity. Higher coolant velocities reduce the coolant area and
increase the fuel volume fraction. However, the pressure drop increases
(proportional to the square of the velocity), which in turn increases pumping power
requirements and reduces natural circulation tendency. Therefore, the coolant
velocity is restricted by the need to keep the pressure drop below a given target
value. For the design parameters presented in Table 14-3, a linear rate of 30.8
kW/m, a temperature rise of 155 o C, and a pressure drop of 15 psi were taken. For a
given pin diameter, the coolant area can be calculated and the unit lattice
configuration is fixed to arrive at the volume fractions of fuel, coolant and
structures.
A cladding thickness of 0.05 cm, hexagonal duct wall thickness of 0.3 cm, and
wirewrap spacers as illustrated in Figure 14-3 are reasonable choices, as is the
choice of 169 pins per fuel assembly. The number of pins for n-hexagonal arrays is
given by 3n(n-1)+1, and illustrated in Table 14-4. If the number of pins per
assembly is increased to 217, the structural fraction is further reduced and the fuel
volume fraction is increased, as shown in Figure 14-5. The assembly weight is also
increased, and this places additional burden on the in-vessel fuel handling machine.
Also the number of the assemblies for the entire core is reduced, which results in
less flexibility in optimizing the number and location of the control rods. In general,
a smaller reactor has a smaller number of pins per assembly, and as the reactor size
increases the number of pins per assembly increases.
In general, the higher fuel volume fraction design gives better neutron economy,
resulting in lower fissile enrichment, a higher internal conversion ratio, and a
reduced reactivity swing during burnup. By minimizing the initial excess reactivity
requirements, the control rod requirement is reduced and accidental reactivity
insertion events are more easily handled. In general, the excess reactivity
requirements for the IFR are much less than those of thermal reactors, as illustrated
in Figure 14-6. In a thermal spectrum, the reactivity change between refueling
intervals is rather large, and in addition, the buildup of fission products such as Xe
and Sm consume significant amount of reactivity. The thermal reactor core must
312