Nuclear Energy Fundamentals - Quia 1236 – Nuclear Energy Fundamentals Module 2: Nuclear Fuel 1. Introduction As explained in module 1 uranium is the most commonly used fuel in nuclear

Nuclear Energy Fundamentals

Module 2: Nuclear Fuel

PREPARED BY

Academic Services

April 2012

© Institute of Applied Technology, 2012

ATM 1236 – Nuclear Energy Fundamentals



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



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. Country Tons Approximate % of world 1 Australia 1143000 24% 2 Kazakhstan 816000 17% 3 Canada 444000 9% 4 USA 342000 7% 5 South Africa 341000 7% 6 Namibia 282000 6% 7 Brazil 279000 6% 8 Niger 225000 5% 9 Russia 172000 4% 10 Uzbekistan 116000 2% 11 Ukraine 90000 2% 12 Jordan 79000 2% 13 India 67000 1% 14 China 60000 1% 15 Others 287000 7%



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



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.



5. Uranium Milling

At uranium mills, usually located

near the mines, uranium ores are

crushed and ground, and the

uranium oxide is 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”.

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.



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

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

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.2.8: Gaseous diffusion.

Fig. 2.9: Gas centrifuge cylinders.

Fig. 2.10: Gas centrifuge process.



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 U-

235. 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).



Fig. 2.11 shows the different grades of uranium.

(a) Slightly enriched uranium 0.9% - 2%U-235

(b) Low-enriched uranium (Reactor grade) 3-5% U-235

(c) Highly enriched uranium (Weapon grade) >85%-90% U-235

Fig. 2.11: Uranium grades.

9. 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.



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.

48 m3 of natural gas 3 barrels of oil. (159 Liters)

1000 kg of coal 2500 kg of firewood

One uranium pellet has the same energy available in:

Fig. 2.14: The energy equivalence of 1 uranium fuel pellet compared to

other types of fuel.



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.



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.

Fig. 2.17: Dry cask storage.

11. 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.



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.



13. Activities

13.1 Uranium crossword.

Workout the uranium crossword below. Some answers are available in

module 1.



Across 1. One method of mining uranium is called _____mining. 4. ____radiation occurs in outer space. 6. A process to separate the metal from the ____ is performed after uranium is mined. 9. Another method of mining uranium is called__ situ leaching. 10.Uranium is a _____ metal. 11 .______particles are heavy and cannot travel very far. 12. Gamma _____ are high energy and can travel through thick concrete. 19. Open ____ mining is the name given to mining on the surface.

20. ____ particles are small and can travel quite far. 21. Uranium can also be found in the ____. Down 2. Uranium is transformed into electricity in a _____ reactor. 5. Uranium is e____ before it is pressed into small pellets and made into fuel rods. 7. When an atom gives up an ____ it becomes ionized. 8. After being mined, uranium is ____. 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

Documents

Nuclear Energy Fundamentals - Quia 1236 – Nuclear Energy Fundamentals Module 2: Nuclear Fuel 1. Introduction As explained in module 1 uranium is the most commonly used fuel in nuclear