AT THERMAL ENERGIES a is moderate, fission cross section high. 
Thermal-reoctor neutron 
<—s. spectrum 
PASER ae 
og es Me 
Thus breeding ratio is not quite 1.0 but critical mass is low 
Fast Breeder Power Reactors— 
Aside from their long-term role as plutonium breeders 
fast reactors could become competitive as power producers. 
At the moment four major difficulties stand in the way 
Because the fast-breeder power re- 
actor is the only reactor type that 
can breed Pu from U?%8 it will cer- 
tainly find a place in any future large- 
scale atomic energy economy. How 
large this role will be, however, will 
depend not so much on the ability of 
the fast reactor as a breeder but more 
on its ability to compete successfully 
with other reactor concepts as a pro- 
ducer of commercial power. At pres- 
ent four basic difficulties tend to 
make the fast-reactor concept less 
attractive for large-scale power plants: 
1. Need for operation at a high 
core-power density. 
2. Problems of sodium technology. 
3. High fuel costs because of large 
inventory and frequent reprocessing. 
4. Criticisms have been made con- 
cerning fast-reactor safety. 
High Power Density 
High power density is an intrinsic 
feature of the fast power reactor and 
calls for heat-transfer performance 
not usually encountered in industry. 
Because the active core of a fast 
reactor is inherently smaller than the 
core of a thermal reactor for the same 
total power, the power density in the 
fast-reactor core must be an order of 
magnitude greater than the power 
density in a typical thermal reactor. 
To accommodate this high power den- 
124 
sity, high heat-transfer rates must be 
provided for in the core design. 
In addition to using liquid metal 
as a coolant, the designer must use 
finely divided fuel to provide enough 
surface area for heat transfer. Thus, 
fuel-element thicknesses and coolant- 
channel widths are small in a fast 
power reactor. In the Enrico Fermi 
Fast Breeder Reactor, for instance, a 
fuel-element assembly consists of 144 
fuel pins, 0.158 in. in diameter, 30 in. 
long, spaced 0.021 in. apart. 
In addition to being thin the fuel 
pins must be made and assembled 
accurately. 
Since the power density is never 
uniform over the core and since dimen- 
sions and physical properties of in- 
dividual elements will vary to some 
extent from the precise design values, 
the operating temperatures of some 
elements in the core will always be 
significantly above the average value. 
The average fuel operating tempera- 
ture (and thereby the thermal eff- 
ciency) is then limited by the tem- 
peratures of these “hot elements.” 
Thermal efficiency can be improved 
by making the fuel-element tempera- 
ture distribution as uniform as possi- 
ble. Fast-reactor designers minimize 
the effect of spatial power variation by 
rearranging core materials and differ- 
entially orificing coolant flow channels. 
The effect of variations in the 
properties and dimensions of indi- 
vidual fuel elements can be offset only 
by specifying strict design _ toler- 
ances for the fuel-element components. 
Thus the basic parts of a fast-reactor 
core are not only smaller but must be 
fabricated and positioned with greater 
precision than would be required in 
thermal reactors having the same 
thermal efficiency. The additional ex- 
penses for constructing and maintain- 
ing the more finely divided core 
represent an economic penalty for 
the fast-reactor concept. 
Sodium Technology 
Liquid-metal cooling is another nec- 
essary feature of fast-power-reactor 
designs. A fast reactor needs a coolant 
that does not moderate neutrons 
and has good heat-transfer properties. 
Liquid metals in general fulfill both 
these requirements. Sodium in par- 
ticular has outstanding heat-transfer 
properties and does not attack con- 
tainers made of widely used structural 
materials like low-carbon iron, stain- 
less steel and Inconel. In addition 
the melting point of sodium is low 
enough to be used with ordinary struc- 
tural materials, and its boiling point is 
high enough to allow a high thermal 
efficiency. For these reasons all fast 
power reactor designs use sodium (or 
