Nuclear Energy Fundamentals
Module 2: Nuclear Fuel
PREPARED BY
Academic Services
April 2012
© Institute of Applied Technology, 2012
ATM 1236 – Nuclear Energy Fundamentals
Module 2: Nuclear Fuel
Module Objectives
Upon successful completion of this module, students will be able to:

Identify the different processess comprising uranium fuel cycle.

Describe the three different methods used in mining uranium.

Explain the milling operation of uranium.

Describe the conversion operation of uranium.

Explain the three methods used for uranium enrichment.

Distiguish between the different uranium grades and their uses.

Describe the fuel fabrication process of uranium.

Identify and explain the two method of the spent fuel storage.

Explain the reprocessing operation of uranium.
Module Contents:
Topic
Page No.
1.
Introduction
3
2.
Nuclear Fuel
3
3.
Uranium Fuel Cycle
4
4.
Uranium Mining
4
5.
Uranium Milling
6
6.
Uranium Conversion
6
7.
Uranium Enrichment
7
8.
Uranium Grades
8
9.
Fuel Fabrication
9
10.
Spent Fuel Storage
11
11.
Reprocessing
12
12.
Summary of Fuel Cycle
12
13.
Activities
14
14.
References
14
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Module 2: Nuclear Fuel
ATM 1236 – Nuclear Energy Fundamentals
1.
Introduction
As explained in module 1 uranium is the most commonly used fuel in
nuclear power plants (NPP). In this module we are going to focus on the
fuel cycle of uranium which comprises many processes to convert uranium
from the ore stage to the fuel stage.
2.
Nuclear Fuel
In the last 50 years, uranium (Fig. 2.1) has become one of the world’s
most important energy minerals. Traces of it occur almost every where,
including oceans. There are uranium mines in about 20 countries, although
more than two third of the world production comes from just 10 mines.
Today there are strict controls on the buying and selling of uranium. It is
only sold to countries that have signed the Nuclear Non-Proliferation
Treaty. This allows international inspectors to check that it is used only for
peaceful purposes.
Table 2.1 shows which countries have
the largest uranium resources and the
percentage of uranium they have in
relation to that of the whole world.
Fig. 2.1: Uranium ore.
Table 2.1: World resources of uranium.
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Country
Australia
Kazakhstan
Canada
USA
South Africa
Namibia
Brazil
Niger
Russia
Uzbekistan
Ukraine
Jordan
India
China
Others
Tons
1143000
816000
444000
342000
341000
282000
279000
225000
172000
116000
90000
79000
67000
60000
287000
Approximate % of world
24%
17%
9%
7%
7%
6%
6%
5%
4%
2%
2%
2%
1%
1%
7%
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ATM 1236 – Nuclear Energy Fundamentals
3.
Uranium Fuel Cycle
The first step before the uranium fuel cycle starts is the exploration of ore.
The ore bodies containing uranium are first located by drilling and through
other geological techniques. The preparation of uranium to change to a
usable fuel involves five main processes. These are mining, milling,
conversion, enrichment and fuel fabrication. After being used in the power
plant the spent fuel has to be stored for several months to several years in
order to reduce the radiation levels. In a reprocessing facility the used fuel
is separated into different components to produce fresh fuel and to reduce
the amount of waste. Fig. 2.2 shows the nuclear fuel cycle.
High Level Waste
Fig. 2.2: Uranium fuel cycle.
4.
Uranium Mining
Uranium is mined from underground and the ore is crushed and extracted.
The natural uranium is composed of 99.28% U-238 and 0.72% U-235.
Uranium ore is removed from the ground in one of three ways depending
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Module 2: Nuclear Fuel
ATM 1236 – Nuclear Energy Fundamentals
on the characteristics of the deposit. Uranium deposits close to the surface
can be recovered using the open pit mining method (Fig. 2.3), and
underground mining methods (Fig. 2.4) are used for deep deposits. In
some circumstances the ore may be mined by in-situ recovery (Fig. 2.5),
a process that dissolves the uranium while still underground and then
pumps a uranium-bearing solution to the surface.
Fig. 2.3: Uranium open pit mining.
2.4: Uranium underground
mining.
Fig. 2.5: Uranium in-situ recovery.
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ATM 1236 – Nuclear Energy Fundamentals
5.
Uranium Milling
At uranium mills, usually located
near the mines, uranium ores are
crushed
and
uranium
ground,
oxide
is
and
the
chemically
extracted. The mill product, called
uranium
concentrates
or
“yellowcake” (Fig. 2.6), is then
marketed and sold as pounds of
U3O8
or
kilograms
of
uranium Fig. 2.6: Uranium “Yellowcake”.
content.
6. Uranium Conversion
After the yellowcake is produced at the mill, the next step is conversion
into
pure
uranium
hexafluoride
(UF6)
gas
suitable
for
use
in
enrichment operations. During this conversion, impurities are removed and
the uranium is combined with fluorine to create the UF6 gas. The UF6 is
then pressurized and cooled to a liquid. In its liquid state it is drained into
14-ton cylinders where it solidifies after cooling for approximately five
days. The UF6 cylinder, in the solid form, is then shipped to an enrichment
plant (Fig. 2.7). UF6 is the only uranium compound that exists as a gas at
a suitable temperature.
Fig. 2.7: The cylinders of uranium hexafluoride are transported to the
enrichment facility.
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ATM 1236 – Nuclear Energy Fundamentals
7.
Uranium Enrichment
Throughout
the
global
nuclear
industry, uranium is enriched by one
of two methods: gaseous diffusion
or gas centrifuge. Many methods
like laser enrichment and others
have been proposed for use in the
recent years.
Gaseous diffusion
Fig.2.8: Gaseous diffusion.
Combining uranium with fluorine is
followed
by
gaseous
diffusion
to
increase the percentage of uranium
fissionable isotope U-235. The UF6
output
from
gaseous
diffusion
is
divided in two streams (Fig. 2.8). One
is
increased,
or
enriched,
in
its
percentage of U-235, and the other is
reduced,
or
depleted,
in
its
Fig. 2.9: Gas centrifuge cylinders.
percentage of U-235. The depleted
uranium
hexafluoride
product
is
referred to as "depleted UF6." After
gaseous
diffusion,
the
enriched
uranium hexafluoride is subjected to
further processing, while the depleted
UF6 is generally stored.
Gaseous diffusion is based on the
separation effect caused by the flow
of gas through small holes.
Fig. 2.10: Gas centrifuge process.
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ATM 1236 – Nuclear Energy Fundamentals
Gas centrifuge process
The gas centrifuge process uses a large number of rotating cylinders in
series and parallel formations (Fig. 2.9). Each cylinder's rotation creates a
strong centrifugal force so that the heavier gas molecules containing U-238
move toward the outside of the cylinder and the lighter gas molecules rich
in U-235 collect closer to the center (Fig. 2.10). It requires much less
energy to achieve the same separation than the older gaseous diffusion
process, which it has largely replaced and so is the current method of
choice and is termed second generation.
8.
Uranium Grades
The enrichment can produce several grades. Each grade has it is own use.
The following explains each grade and where it is used:

Slightly enriched uranium: has a U-235 concentration of 0.9% to
2% and is used in some heavy water reactors. Reprocessed
uranium is a product of nuclear fuel cycles involving nuclear
reprocessing of spent fuel recovered from light water reactor spent
fuel typically contains slightly more U-235 than natural uranium, and
therefore could be used to fuel reactors that use natural uranium as
fuel.

Low enriched uranium: has a lower than 20% concentration of U235. For use in commercial light water reactors, in the most common
power reactors in the world, uranium is enriched to 3 to 5% U-235.
In research reactors the enrichment level usually reaches 12% to
19.75% U-235.

Highly enriched uranium: has a greater than 20% concentration
of U-235. Uranium in nuclear weapons usually contains 85% or more
of U-235 known as weapons-grade, though for a crude, inefficient
weapon 20% is sufficient (called weapons-usable).
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Module 2: Nuclear Fuel
ATM 1236 – Nuclear Energy Fundamentals
Fig. 2.11 shows the different grades of uranium.
(a) Slightly enriched uranium (b) Low-enriched uranium
(Reactor grade)
3-5% U-235
0.9% - 2%U-235
Fig. 2.11: Uranium grades.
99.2% U-238
0.72% U-235
9.
(c) Highly enriched uranium
(Weapon grade)
>85%-90% U-235
99.2% U-238
0.72% U-235
Fuel Fabrication
Fuel fabrication for light water (regular) power reactors typically begins
with
receipt
of
low-enriched
uranium
hexafluoride
(UF6)
from
an
enrichment plant (Fig. 2.12). The UF6, in solid form in containers, is
heated to gaseous form, and the UF6 gas is chemically processed to form
uranium dioxide (UO2) powder.
After that the powdered UO2 is pressed
into small cylindrical shapes and baked at a high temperature (1600 1700°C) to make hard ceramic pellets (Fig 2.13b). Fig. 2.14 shows the
energy equivalence of one fuel pallet compared to other types of fuels used
in power generation.
Fig. 2.12: Fuel fabrication process.
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ATM 1236 – Nuclear Energy Fundamentals
In a light water reactor, the fuel pellets are packed in thin tubes called fuel
rods. The rods are grouped together into a bundle called a fuel assembly
(Fig. 2.13a). A typical 1,100 megawatt pressurized water reactor contains
193
fuel
assemblies
composed
of
nearly
51,000
fuel
rods
and
approximately 18 million fuel pellets.
(a)
(b)
Fig. 2.13:(a) Fuel assembly, rod and pellets. (b) Fuel pellet size.
One uranium pellet has the same energy available in:
48 m3 of natural gas
1000 kg of coal
3 barrels of oil.
(159 Liters)
2500 kg of firewood
Fig. 2.14: The energy equivalence of 1 uranium fuel pellet compared to
other types of fuel.
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Module 2: Nuclear Fuel
ATM 1236 – Nuclear Energy Fundamentals
10.
Spent Fuel Storage
There are two storage methods for spent fuel after it is removed from the
reactor core, namely:

Spent fuel pools

Dry cask storage
Spent Fuel Pools
All nuclear plants have storage pools for spent fuel. These pools are
typically 13 meters or more deep. In the bottom 5 meters are storage
racks designed to hold fuel assemblies removed from the reactor (Fig.
2.15). In many countries, the fuel assemblies, after being in the reactor for
3 to 6 years, are stored underwater for 10 to 20 years. The water serves 2
purposes:

It serves as a shield to reduce the radiation levels that people
working above may be exposed to.

It cools the fuel assemblies that continue to produce heat (called
decay heat) for some time after removal.
Fig. 2.15: Spent fuel pool.
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ATM 1236 – Nuclear Energy Fundamentals
Dry cask storage
In the late 1970s and early 1980s, the need for alternative storage began
to grow when pools at many nuclear reactors began to fill up with stored
spent fuel. Utilities began looking at options such as dry cask storage for
increasing spent fuel storage capacity.
Dry cask storage allows spent fuel that has already been cooled in the
spent fuel pool for at least one year to be surrounded by inert gas inside a
container called a cask. The casks are typically steel cylinders that are
either welded or bolted to close them (Fig. 2.16). The steel cylinder
provides a leak-tight container of the spent fuel. Each cylinder is
surrounded by additional steel, concrete, or other material to provide
radiation shielding to workers and members of the public. Some of the
cask designs can be used for both storage and transportation (Fig. 2.17).
Fig. 2.16: Dry cask construction.
11.
Fig. 2.17: Dry cask storage.
Reprocessing
Nuclear reprocessing uses chemical procedures to separate the useful
components (especially the remaining uranium and the newly-created
plutonium) from the fission products and other radioactive waste in spent
nuclear fuel obtained from nuclear reactors. The reprocessed uranium can
in principle also be re-used as fuel, but that is only economic when
uranium prices are high.
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Module 2: Nuclear Fuel
ATM 1236 – Nuclear Energy Fundamentals
12.
Summary of Fuel Cycle
The following figure summarizes the fuel cycle of uranium from ore to
disposed fuel stage.
Fig. 2.17: Summary of uranium fuel cycle.
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ATM 1236 – Nuclear Energy Fundamentals
13.
Activities
13.1 Uranium crossword.
Workout the uranium crossword below. Some answers are available in
module 1.
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Module 2: Nuclear Fuel
ATM 1236 – Nuclear Energy Fundamentals
Across
1. One method of mining uranium is 20. ____ particles are small and can
called _____mining.
travel quite far.
4. ____radiation occurs in outer
space.
21. Uranium can also be found in
the ____.
6. A process to separate the metal Down
from the ____ is performed after
uranium is mined.
2. Uranium is transformed into
electricity in a _____ reactor.
9. Another method of mining
uranium is called__ situ leaching.
5. Uranium is e____ before it is
pressed into small pellets and made
10.Uranium is a _____ metal.
into fuel rods.
11 .______particles are heavy and 7. When an atom gives up an ____
cannot travel very far.
it becomes ionized.
12. Gamma _____ are high energy 8. After being mined, uranium is
and can travel through thick ____.
concrete.
19. Open ____ mining is the name
given to mining on the surface.
15. Uranium is exported as uranium
___ concentrate.
18. People are exposed to radiation
when they go out in the ____.
13.
References

Chemistry Concepts and Applications Mc Graw-Hill Glenco.

Why Science Matters, Using Nuclear Energy by John Townsend,
Heinemann.

http://www.uraniumsa.org/education/

http://www.energyquest.ca.gov/projects

http://www.nrc.gov/materials/

http://www.nfi.co.jp/e/product/prod02.html

http://www.euronuclear.org/info/encyclopedia/g/gascentrifuge.htm

http://en.wikipedia.org
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