Environmental Engineering FOURTH EDITION

Radioactive Waste 327

Uranium mines U 0 conversion U-235 enrichment UF, conversion

and mills 8toUF, fl / toU02and

fabrication of

fuel assemblies

Ore and ore concentrate

(U308) Recovered Plutonium

Permanent

storage

?

wastes

4

Reprocessing plant

Temporary storage - solid High waste level

in federal

repositories

Figure 16-4. The nuclear fuel cycle.

(any uranium that contains less than 0.711% U-235 by weight), an exceedingly
dense and pyrophoric material used in radiation shielding and in armor-piercing
weapons.
Nuclear fuel is inserted into a reactor core, where a controlled fission reaction
produces heat, which in turn produces pressurized steam for electric power generation.
The steam system, turbines, and generators in a nuclear power plant are essentially
the same as those in any thermal (fossil fuel burning) electric generating plant. The
difference between nuclear and fossil fuel electric generation is in the evolution of the
heat that drives the plant.
Figure 16-5 is a diagram of a typical pressurized water nuclear reactor. Commercial
reactors in the United States are either pressurized water reactors, in which the
water that removes heat from the nuclear reactor core (the “primary coolant”) is
under pressure and does not boil, or boiling water reactors, in which the primary
coolant is permitted to boiL7 The primary coolant transfers heat from the core to the
steam system (the “secondary coolant”) by a heat exchange system that ensures com-
plete physical isolation of the primary from the secondary coolant. A third cooling
system provides water from external sources to condense the spent steam in the steam
system.
All thermal electric power generation produces large quantities of waste heat.
Fossil fuel electric generating plants have, typically, a thermal efficiency of about
42%; that is, 42% of the heat generated by combustion of the fuel is converted to
electricity, and 58% is dissipated in the environment. By comparison, nuclear plants
are about 33% thermally efficient.
The nuclear reactor is perhaps the key radioactive waste producer in the nuclear
fuel cycle. The production of HLW is a direct consequence of fission reactions in

7The Fort St. Vrain Plant reactor in Colorado, now being decommissioned, used helium gas as the
primary coolant.