Environmental Engineering Reference
In-Depth Information
Containment vessel
Steam
Load
Turbine
C
ooling
Condenser
Secondary
heat
exchanger
(steam
generator)
Primary
heat
exchanger
Reactor
Pump
Feedwater
heaters
Pump
Pump
Figure 6.5
Schematic of a liquid metal fast breeder reactor (LMFBR) power plant.
6.5
NUCLEAR FUEL CYCLE
The nuclear fuel cycle starts from mining uranium (or thorium) ore, through extraction of the useful
uranium concentrate, gasification to UF
6
, enrichment of
235
U, conversion to metallic uranium or
oxide of uranium, fuel rod fabrication, loading of a reactor, retrieval of spent fuel, reprocessing of
spent fuel, and finally fuel waste disposal. A block diagram of the nuclear fuel cycle is presented
in Figure 6.6.
6.5.1
Mining and Refining
Uranium ores containing variable concentrations of uranium are found in many parts of the world.
Rich ores may contain up to 2% uranium, medium-grade ores 0.5-1%, and low-grade less than
0.5%. In the United States, ores are found in Wyoming, Texas, Colorado, New Mexico, and Utah.
Large deposits are found in Australia, Kazakhstan, Canada, South Africa, Namibia, Brazil, and
Russia.
The most economic way of extracting the ore is from open surface mines. Because uranium
deposits are always associated with decay products (daughters) of uranium, such as radium and
radon, these ores can be radioactive, and workers' protection must start at the mining phase. Open
pit mines are well-ventilated, so most radiation, especially that associated with radon, which is a
gas, escapes into the atmosphere. However, masks must be worn to prevent inhalation of mining
dust which can contain radioactive elements.
The ore is crushed and ground. The ground ore is leached with sulfuric acid. Uranium, together
with some other metals, dissolves. Uranium oxide with the approximate composition U
3
O
8
, called
yellow cake, is precipitated, dried, and packed into 200-liter drums for shipment. The radiation
from these drums is negligible. However, the solids remaining after leaching with acid may contain
