Uranium used in electricity generation in nuclear reactors is generally sourced
from ores with uranium oxide concentrations up to 10%. However, lower grade ores
with uranium oxide concentration of 0.2% or less are the most common and the most
mined (Ross 2015; Thomas 1981). The uranium concentrate used in nuclear reactors
is typically at 75–95% uranium oxide (U 3 O 8 ). Mining of such readily available
lower-grade uranium ore to achieve about 75–95% U 3 O 8 concentrate used in nuclear
power plants results in the release of large amounts of waste rock tailings with
significant environmental consequences due to the presence of residual uranium in
the rocks (Thomas 1981).
Uranium is sourced from rich ores with concentrations up to 10%. However, ores
with uranium oxide concentration as low as 0.2% are also mined and are the most
common (Sovacool 2008; Tudiver 2009). Uranium producers have been able to
utilize ores with uranium oxide concentration as low as 0.0004%. Uranium is
recovered from ore by comminution of the rocks followed by leaching using
alternative solutions of acid and/or alkaline chemicals. The end product from ore
milling and leaching results into a bright yellow powder called yellow cake (U 3 O 8 )
which is about 75–90% uranium oxide (Sovacool 2008). Before this uranium oxide
concentrate can be used in a reactor for generating electricity, it must first be
converted into uranium hexafluoride (UF 6 ), which is used in a gaseous diffusion
enrichment process. During the uranium enrichment process, U-235 concentration is
increased to least 3.5% for atypical commercial light-water reactor and up to 4–5%
for other modern reactors, while at the same time, the U-238 isotope is decreased
notably. Suffice to say, U-235 is the only natural occurring isotope that can sustain a
fission chain reaction by capturing neutrons and splitting into two parts yielding
large amount of energy (Soudek et al. 2006; WNA 2008). On average, the specific
radioactivity of natural uranium is 25 kBq/g, double that of U-238. During its decay
process, uranium may generate 0.1 watts/tonne which is enough to warm the earth’s
mantle (WNA 2008).
After the enrichment process, about 85% of oxide comes out as waste in the form
of depleted UF 6 , and the remaining 15% emerges as enriched uranium and is
converted into ceramic pellets of UO 2 . Fresh UO 2 which contains up to 5% of
U-235 is then packaged in zirconium alloy tubes and bundled together to form fuel
rod assembles for reactors. Thereafter, the used reactor fuel which contains up to
95% U-238, 3% fission products and transuranic isotopes, 1% plutonium, and 1%
U-235 is removed and stored to be reprocessed prior to disposal (Soudek et al. 2006;
WNA 2008). During the reprocessing stage, uranium (U-235) and plutonium
(Pu-239) are separated from the spent fuel using the PUREX method and then
reused as mixed-oxide (MOX) fuel in the reactor. This process is referred to as the
closed fuel cycle (Fig. 2.2). The majority of radioactive organic waste is produced in
the enrichment and reprocessing operations. All values in the tables are reported in
cubic meters per Gigawatt electricity-year (m
3 /GWe-year).
28
E. M. Nkhalambayausi-Chirwa et al.
from ores with uranium oxide concentrations up to 10%. However, lower grade ores
with uranium oxide concentration of 0.2% or less are the most common and the most
mined (Ross 2015; Thomas 1981). The uranium concentrate used in nuclear reactors
is typically at 75–95% uranium oxide (U 3 O 8 ). Mining of such readily available
lower-grade uranium ore to achieve about 75–95% U 3 O 8 concentrate used in nuclear
power plants results in the release of large amounts of waste rock tailings with
significant environmental consequences due to the presence of residual uranium in
the rocks (Thomas 1981).
Uranium is sourced from rich ores with concentrations up to 10%. However, ores
with uranium oxide concentration as low as 0.2% are also mined and are the most
common (Sovacool 2008; Tudiver 2009). Uranium producers have been able to
utilize ores with uranium oxide concentration as low as 0.0004%. Uranium is
recovered from ore by comminution of the rocks followed by leaching using
alternative solutions of acid and/or alkaline chemicals. The end product from ore
milling and leaching results into a bright yellow powder called yellow cake (U 3 O 8 )
which is about 75–90% uranium oxide (Sovacool 2008). Before this uranium oxide
concentrate can be used in a reactor for generating electricity, it must first be
converted into uranium hexafluoride (UF 6 ), which is used in a gaseous diffusion
enrichment process. During the uranium enrichment process, U-235 concentration is
increased to least 3.5% for atypical commercial light-water reactor and up to 4–5%
for other modern reactors, while at the same time, the U-238 isotope is decreased
notably. Suffice to say, U-235 is the only natural occurring isotope that can sustain a
fission chain reaction by capturing neutrons and splitting into two parts yielding
large amount of energy (Soudek et al. 2006; WNA 2008). On average, the specific
radioactivity of natural uranium is 25 kBq/g, double that of U-238. During its decay
process, uranium may generate 0.1 watts/tonne which is enough to warm the earth’s
mantle (WNA 2008).
After the enrichment process, about 85% of oxide comes out as waste in the form
of depleted UF 6 , and the remaining 15% emerges as enriched uranium and is
converted into ceramic pellets of UO 2 . Fresh UO 2 which contains up to 5% of
U-235 is then packaged in zirconium alloy tubes and bundled together to form fuel
rod assembles for reactors. Thereafter, the used reactor fuel which contains up to
95% U-238, 3% fission products and transuranic isotopes, 1% plutonium, and 1%
U-235 is removed and stored to be reprocessed prior to disposal (Soudek et al. 2006;
WNA 2008). During the reprocessing stage, uranium (U-235) and plutonium
(Pu-239) are separated from the spent fuel using the PUREX method and then
reused as mixed-oxide (MOX) fuel in the reactor. This process is referred to as the
closed fuel cycle (Fig. 2.2). The majority of radioactive organic waste is produced in
the enrichment and reprocessing operations. All values in the tables are reported in
cubic meters per Gigawatt electricity-year (m
3 /GWe-year).
28
E. M. Nkhalambayausi-Chirwa et al.