The Pennsylvania State University

The Graduate School

Department of Nuclear Engineering

ADVANTAGES AND CHALLENGES RELATED TO THORIUM FUEL CYCLES

A Thesis in

Nuclear Engineering

by

Ryan Bradley Nielson

2013 Ryan Bradley Nielson

Submitted in Partial Fulfillment

of the Requirements

for the Degree of Nuclear Engineering

Master’s of Engineering

December 2013

We approve the paper of Ryan Bradley Nielson as fulfillment of the paper requirement of a

Master of Engineering degree in Nuclear Engineering:

Dr. Kostadin Ivanov

Distinguished Professor of Nuclear Engineering

Thesis Advisor

Chair of Committee

Date

Dr. Maria N. Avramova

Assistant Professor of Nuclear Engineering

Second Reader

Date

Dr. Arthur Motta

Professor of Nuclear Engineering and Materials Science and

Engineering

Chair of Nuclear Engineering Program

Date

iii

ABSTRACT

The nuclear power industry finds its roots in the Manhattan Project and the creation of a

nuclear bomb. Following World War II, it was not long before the first uranium powered power

plant was created. However, challenges with reactor safety, resource availability, waste

management, and proliferation have sparked innovation and the pursuit of alternate cycles such as

Thorium. Thorium presents several advantages including larger reserves and increased

proliferation resistance as compared to uranium. The chief drawbacks to a thorium fuel cycle are

economic due to the lack of commercial development up to the present.

The demand for energy is growing rapidly with the economic development of non-OECD

countries. In parallel, there is increasing interest in the minimization of greenhouse gases which

are associated with traditional fossil fuel energy sources. Nuclear energy has seen a steadily

increasing market share on account of being a clean and abundant energy source. There is

enough known uranium to last up to 267 years based on current usage rates. The amount of

thorium is estimated to be up to five times this amount (International Atomic Energy Agency).

Reprocessing methods have also been developed which extend the number of years of fuel

available.

One of the greatest concerns with embracing nuclear technology is the possibility for

nuclear weapons proliferation and diversion by disreputable states and terrorist organizations.

The technology used to create power plant fuel is essentially the same as the technology required

for creation of bomb grade material. The thorium fuel cycle presents the advantage of producing

lower quantities of weapons usable material and higher radiation levels in spent fuel, which

makes the material more difficult to handle and less appealing to would be diverters.

iv

The nuclear power industry continues to show promise, though it has many obstacles in

its future. The thorium fuel cycle does not solve all of the nuclear industry's problems, but may

help alleviate resource availability and proliferation concerns.

v

TABLE OF CONTENTS

List of Figures .......................................................................................................................... v

List of Tables ........................................................................................................................... vi

Acknowledgements .................................................................................................................. vii

Chapter 1 Introduction to Uranium and Thorium Fuel Cycles ................................................ 1

Challenges Facing the Nuclear Industry .......................................................................... 1 Early Development of the Thorium Fuel Cycle ............................................................... 2 Thorium as a Fuel ............................................................................................................ 3

Potential Advantages ................................................................................................ 3 Challenges ................................................................................................................ 7

Chapter 2 Sustainable Energy .................................................................................................. 9

Energy Demand ................................................................................................................ 9 Carbon Dioxide ........................................................................................................ 11

Abundance of Uranium .................................................................................................... 14 Uranium Demand and Cost ...................................................................................... 16

Abundance of Thorium .................................................................................................... 19

Chapter 3 Nuclear Fuel Reprocessing ...................................................................................... 21

Reprocessing Techniques ......................................................................................... 23 Thorium Reprocessing ............................................................................................. 28

Waste Reduction .............................................................................................................. 34

Chapter 4 Proliferation ............................................................................................................. 36

United States Ford-Carter Agreement ...................................................................... 37 Nuclear Weapons and Reactor Fuel ......................................................................... 38 Increasing Proliferation Resistance .......................................................................... 44 Thorium and Proliferation Resistance ...................................................................... 48

Chapter 5 Conclusions ............................................................................................................. 55

Available Resources ......................................................................................................... 55 Nuclear Fuel Reprocessing .............................................................................................. 55 Proliferation ..................................................................................................................... 56 Future Work ..................................................................................................................... 56

Chapter 6 Bibliography ............................................................................................................ 58

Appendix A ...................................................................................................................... 63 World Oil Reserves .................................................................................................. 63

Appendix B ...................................................................................................................... 64

vi

Identified Uranium Reserves by Country (Nuclear Energy Agency)....................... 64 Undiscovered Uranium Reserves by Country (Nuclear Energy Agency) ................ 65 Total thorium resources (Nuclear Energy Agency).................................................. 66

Appendix C ...................................................................................................................... 67 Isotope production chain by neutron absorption and beta decay (Kang and

Hippel) .............................................................................................................. 67

vii

LIST OF FIGURES

Figure 1-1: Early prototype thorium reactors (Unak) .............................................................. 2

Figure 1-2: Comparison of Th-232 and U-238 neutron capture products (Lung and

Gremm) ............................................................................................................................ 4

Figure 1-3: Neutron yield based on incident energy for nuclear fuels (Lung and Gremm) ..... 5

Figure 1-4: Fission cross section comparison based on fuel type (International Atomic

Energy Agency) ............................................................................................................... 6

Figure 1-5: Capture cross section comparison based on fuel type (International Atomic

Energy Agency) ............................................................................................................... 6

Figure 1-6: Comparison of fission products for different fuel cycles (Lung and Gremm) ...... 7

Figure 2-1: Comparison of energy demand (quadrillion Btu) by OECD and non-OECD

countries through 2014 (U.S. Energy Information Administration) ................................ 9

Figure 2-2: Projected demand for various energy sources (trillion kilowatt-hours) (U.S.

Energy Information Administration) ................................................................................ 10

Figure 2-3: Nuclear capacity projection comparison between different areas (Nuclear

Energy Agency) ............................................................................................................... 11

Figure 2-4: Comparsion of CO2 emissions by OECD and non-OECD countries through

2040 (billion metric tons) (U.S. Energy Information Administration) ............................ 13

Figure 2-5: Carbon intensity comparison between OECD and non-OECD countries

through 2014 (tons CO2 per million GDP) (U.S. Energy Information

Administration) ................................................................................................................ 13

Figure 2-6: CO2 emissions by fuel type with time (billion metric tons) (U.S. Energy

Information Administration) ............................................................................................ 14

Figure 2-7: Global identified uranium resources (<130 USD/kg) (Nuclear Energy

Agency) ............................................................................................................................ 15

Figure 2-8: Projected uranium consumption (Nuclear Energy Agency).................................. 16

Figure 2-9: Supply, demand, and cost comparison (Nuclear Energy Agency) ........................ 18

Figure 2-10: Trend in exploration effort (Nuclear Energy Agency) ........................................ 19

Figure 3-1: Mixer-settler extraction (Simpson and Law) ........................................................ 24

Figure 3-2: Pulse-column extraction (Simpson and Law) ....................................................... 25

Figure 3-3: Centrifugal extraction (Simpson and Law) ........................................................... 26

viii

Figure 3-4: Interim-23 process (Brooksbank, McDuffee and Rainey) .................................... 29

Figure 3-5: Thorex process (Brooksbank, McDuffee and Rainey) .......................................... 30

Figure 3-6: Pyrometallurgical reprocessing method (Bates, Jardine and Krumpelt) ............... 31

Figure 3-7: Amount of thorium dissolved in magnesium based on cadmium content

(Bates, Jardine and Krumpelt) .......................................................................................... 32

Figure 3-8: Fuel fabrication cost with radiation level (Brooksbank, McDuffee and

Rainey) ............................................................................................................................. 34

Figure 4-1: World Bank governance characteristic scores (Miller and Sagan) ....................... 37

Figure 4-2: Gun-type and implosion-type nuclear weapons (Federation of American

Scientists) ......................................................................................................................... 40

Figure 4-3: Gaseous diffusion and centrifugal enrichment methods (United States Nuclear

Regulatory Commission) ................................................................................................. 41

Figure 4-4: Plutonium isotopic buildup with burnup using natural uranium in a heavy-

water reactor (Kang and Hippel) ...................................................................................... 43

Figure 4-5: Effort required to enrich uranium (World Nuclear Association) .......................... 44

Figure 4-6: Reflected critical mass by U-233 or U-235 content (Kang and Hippel) ............... 46

Figure 4-7: Plutonium production versus burnup based on fuel and reactor type (Kang

and Hippel) ....................................................................................................................... 50

Figure 4-8: Uranium-233 production versus burnup based on fuel and reactor type (Kang

and Hippel) ....................................................................................................................... 51

Figure 4-9: Dose rate in recovered U-233 fuel based on U-232 content (Brooksbank,

McDuffee and Rainey) ..................................................................................................... 53

ix

LIST OF TABLES

Table 1-1: Critical mass comparison of various isotopes (European Nuclear Society) ........... 8

Table 3-1: Typical composition of spent fuel from a light water reactor (World Nuclear

Association) ..................................................................................................................... 21

Table 3-2: Reprocessing capacity by country (World Nuclear Association) ........................... 22

Table 4-1: Incidents of terrorism in current and aspiring nuclear power states (Miller and

Sagan) .............................................................................................................................. 37

Table 4-2: Potential spikant nuclides (Selle) ........................................................................... 48

Table 4-3: Comparison of unshielded dose rates produced by a 5 kg sphere at a distance

of 0.5 meters one year after separation (Kang and Hippel) ............................................. 52

Table 4-4: Acute Radiation Syndromes (Center for Disease Control and Prevention)............ 54

x

ACKNOWLEDGEMENTS

I would like to express my gratitude to my advisor Dr. Kostadin Ivanov for his help in

reviewing and preparing this thesis and also for the instruction he has provided me while at the

school. I have had Dr. Ivanov as a professor for several years and attribute a large part of my

understanding of nuclear engineering to him.

I am also thankful to the rest of my thesis committee, including Dr. Maria Avramova and

Dr. Arthur Motta, for their time and valuable input regarding my paper.

I would like to thank my supervisor at Naval Reactors, Joseph Probst for the daily

instruction he has given me with regard to Naval nuclear systems that has provided a foundation

for understanding nuclear reactors. My professors from the Bettis Reactor Engineering School

expanded upon that foundation and prepared me for further learning with Pennsylvania State

University.

Lastly, I would like to thank my family. My wife, Susan Nielson, has had to put up with

late nights and lost weekends on account of this thesis, but she has been supportive throughout.

My parents, Catherine and Gregory Nielson, and my mother and father in-law, Linda and Alden

Hilton, have also been a great support to me and have encouraged me to complete my graduate

degree and thesis paper.

Chapter 1

Introduction to Uranium and Thorium Fuel Cycles

Challenges Facing the Nuclear Industry

The onset of the nuclear power industry is associated with World War II and the creation

of a nuclear bomb. After the conclusion of World War II, interest in nuclear power shifted

towards electricity generation based on a uranium fuel cycle. Between 1950 and 1980, uranium

supplied power plants saw rapid development. However, over the last 40 years, several events

have slowed the development of nuclear energy:

(1) The Three Mile Island, Chernobyl, and Fukushima power plant accidents

demonstrate the need for passive protection, reliability, adequate training, and constant

questioning of worst case assumptions. Reactor plants continue to improve their safety posture,

but some countries, including Germany (in response to the Fukushima accident), have decided

not to pursue nuclear energy further due to the consequences of an accident.

(2) Scarcity of uranium resources has led to the development of breeder and

reprocessing technologies and further uranium exploration effort in order to meet future demand.

(3) The cold war, terrorist activity over the last decade, and the illegal pursuit of

nuclear arms by countries such as North Korea and Iran, has also emphasized the need for

proliferation protection as nuclear technology is developed.

(3) There is not yet an operational permanent nuclear waste disposal facility in the

world. Nuclear waste currently resides at commercial facility water pools and dry storage

facilities or at temporary storage locations. Reprocessing and breeding helps to reduce the

amount of waste, but these technologies do not eliminate the need for a final disposal site. Most

2

countries have set deadlines for establishment of a geological repository between 2015 and 2035

(World Nuclear Association).

These concerns are leading to the development of alternative reactor technologies. The

thorium based fuel cycle offers a partial solution to at least two of the challenges faced by nuclear

power, namely increased fuel resource availability and proliferation resistance.

Early Development of the Thorium Fuel Cycle

Research with the thorium fuel cycle was pursued parallel to the uranium fuel cycle. In

the early 1950s, the United States produced 55 kg of U-233 from thorium for potential use in

nuclear weapons (Unak). Between 1960 and 1980, another 1.5 tons of U-233 was produced in

the United States and 2 tons in France for use in prototype power plants (Unak). Figure 1-1 lists

some of the early thorium based prototype plants built in Western countries (Unak). The thorium

fuel cycle has yet to be commercially adopted.

Figure 1-1: Early prototype thorium reactors (Unak)

3

Thorium as a Fuel

Naturally occurring Th-232 can be used to breed U-233, which is suitable for fission.

This is done by neutron capture and subsequent beta decay (both half lives ~ 23 min (Unak)) as

shown in Equation 1 below :

Equation 1

The following advantages and disadvantages are adopted from the lists provided by Unak, Lung,

and Gremm.

Potential Advantages

Th-232 has a thermal (less than 0.25 eV) capture cross section of 7.35 barns as opposed to

2.68 for U-238 (Mughabghab). This will help to drive up the breeding ratio when thorium is

used as a blanket in a thermal breeder reactor.

Th-232 breeding will primarily result in the production of U-233. It would take seven

neutron absorptions before producing Pu-239. As shown in Figure 1-2, this event is highly

unlikely since U-233 and U-235 have a high probability of fission as compared to capture

(Lung and Gremm). By contrast, U-238 capture results in the production of Pu-239. Since

proliferation resistance is largely tied to the isolation of plutonium from other actinides, the

thorium breeding has increased proliferation resistance as compared to uranium breeding

(Unak).

4

Figure 1-2: Comparison of Th-232 and U-238 neutron capture products (Lung and Gremm)

Thorium breeding also results in the production of highly radioactive U-232, which increases

U-233 proliferation resistance (Lung and Gremm).

Some thorium reactor types can be used for burning highly enriched uranium (HEU),

weapons grade plutonium, or reactor produced plutonium, thus decreasing the current

stockpile of high proliferation risk material (Unak).

Some thorium reactors can be operated without the need for reprocessing (Unak).

Thorium is more abundant than uranium (Unak).

The use of thorium could reduce fuel cycle cost (Unak).

Figure 1-3 shows that U-233 has a higher neutron yield than U-235 and Pu-239 at most

neutron energies (Lung and Gremm).

5

Figure 1-3: Neutron yield based on incident energy for nuclear fuels (Lung and Gremm)

U-233 has a comparable fission cross section to U-235 and Pu-239 (Unak), but a reduced

capture cross section as seen in Figure 1-4 and Figure 1-5 (International Atomic Energy

Agency). This will increase the neutron economy. This is especially pronounced in the

thermal range.

6

Figure 1-4: Fission cross section comparison based on fuel type (International Atomic Energy

Agency)

Figure 1-5: Capture cross section comparison based on fuel type (International Atomic Energy

Agency)

7

Thorium oxide has a melting point around 3300˚C as compared to 2865˚C for uranium oxide

and 2390˚C for plutonium oxide (World Nuclear Association). This allows for the use of

thorium in high temperature reactor designs (Lung and Gremm).

Thorium based fuel cycles tend to produce a lower concentration of long-lived minor

actinides from fission as shown in Figure 1-6 (Lung and Gremm).

Figure 1-6: Comparison of fission products for different fuel cycles (Lung and Gremm)

Challenges

Thorium has no use by itself, but requires the presence of another fissile material from

which it can be bred (Unak).

Exploration of thorium reserves and development of extraction methods is limited.

Dissolution of thorium oxide for reprocessing is less developed than for uranium and

plutonium oxides (Unak).

8

As shown in Figure 1-2, Th-232 becomes Pa-233 after absorbing a neutron, which has a

half life of about 27 days. This will result in a reactivity spike long after shut down

(Lung and Gremm).

The critical mass of U-233 lies between U-235 and Pu-239 as shown in Table 1-1

(European Nuclear Society) indicating an equivalent need for criticality safety

precautions during production, reprocessing, transportation, and disposal (Lung and

Gremm).

Table 1-1: Critical mass comparison of various isotopes (European Nuclear Society)

U-233 can potentially be used for construction of a nuclear weapon (Lung and Gremm).

The presence of U-232 in conjunction with the U-233 necessitates advanced remote fuel

handling techniques to avoid exposure during reprocessing and manufacturing (Lung and

Gremm).

9

Chapter 2

Sustainable Energy

Energy Demand

The world's energy demand over the next thirty years is expected to be driven by

economic growth (U.S. Energy Information Administration). As shown in Figure 2-1, most of

the energy demand is expected to occur in countries outside of the Organization for Economic

Cooperation and Development (OECD) (U.S. Energy Information Administration). The non-

OECD countries include the fastest growing economies, including China and India, which grew

at 10.4 and 6.4 percent per year between 1990 and 2010 (U.S. Energy Information

Administration). The 2008 recession slowed economic progress and has left uncertainty in some

of the current OECD members, but Asia has continued to maintain above average growth rates

(U.S. Energy Information Administration). It is the rapid industrialization, development, and

population growth of non-OECD members that is forcing the energy demand upward by more

than 100% by 2040.

Figure 2-1: Comparison of energy demand (quadrillion Btu) by OECD and non-OECD countries

through 2014 (U.S. Energy Information Administration)

10

Electricity demand follows the same trend as general energy demand, with rapid growth

among clean fuel alternatives. Figure 2-2 shows that demand for all energy sources will rise

(U.S. Energy Information Administration). Renewable sources will experience the fastest

growth, followed by natural gas and nuclear power (U.S. Energy Information Administration).

Coal will continue to be the largest supplier of electricity, though its market share will reduce

(U.S. Energy Information Administration). This trend is partially the result of policy favoring

low greenhouse gas sources and the desire for energy independence.

Figure 2-2: Projected demand for various energy sources (trillion kilowatt-hours) (U.S. Energy

Information Administration)

Nuclear energy growth will be rapid, though it is subject to large amounts of uncertainty

as demonstrated in Figure 2-3 (U.S. Energy Information Administration). East Asia and non-

OECD countries lead growth in the nuclear industry. Each projection in Figure 2-3 contains an

upper and lower bound; the difference represents up an approximate 40% uncertainty

(International Atomic Energy Agency). Uncertainty in projections is attributed to the potential

for plant usage increases, changes in cycle length and discharge burn-up, and political events

(Nuclear Energy Agency). For example, the Fukushima disaster resulted in the suspension of

11

licensing for new plants in Russia and China, of operation of existing plants in Japan, and the

accelerated shutdown of all plants in Germany and Switzerland (U.S. Energy Information

Administration). Other technology changes could also impact the demand for nuclear power

including, but not limited to, the use of electric vehicles instead of liquid fuel powered vehicles

and use of nuclear power for desalination and heating (Nuclear Energy Agency). Despite the

large amount of uncertainty for the future of nuclear power, it is expected to play an increasing

role in meeting future energy demand.

Figure 2-3: Nuclear capacity projection comparison between different areas (Nuclear Energy

Agency)

Carbon Dioxide

Carbon dioxide emissions are expected to be driven by non-OECD countries, since power

demand will be driven by those same countries. In 2040, non-OECD countries are expected to

generate over 30 billion metric tons of CO2 as compared to half that amount by OECD countries

(Figure 2-4) (U.S. Energy Information Administration). However, a direct emissions comparison

12

is not fair since non-OECD countries represent a larger fraction of the world and currently do not

enjoy the same standard of living as OECD countries. Therefore, as economic conditions

improve in non-OECD areas, it should be expected they surpass OECD emissions.

The Kaya Identity provides a more accurate method for interpreting emission

comparisons. By dividing the emissions by gross domestic product (GDP), the Kaya component

carbon intensity value is obtained (U.S. Energy Information Administration). Carbon intensity

provides a gage for emission improvement, normalized by economic development. The key

assumption to this metric is that excess CO2 emissions can be justified by increased wealth.

Whether this is true or not, carbon intensity at least provides a more accurate comparison of

emission performance between countries of varying size and economic development. Figure 2-5

shows that all countries are expected to reduce their carbon intensity, with non-OECD countries

converging toward the same intensity as OECD countries (U.S. Energy Information

Administration). This demonstrates that as non-OECD countries grow wealthier in the future,

they will also become equally efficient and dedicated to the reduction of CO2 emissions.

13

Figure 2-4: Comparsion of CO2 emissions by

OECD and non-OECD countries through 2040

(billion metric tons) (U.S. Energy Information

Administration)

Figure 2-5: Carbon intensity comparison

between OECD and non-OECD countries

through 2014 (tons CO2 per million GDP)

(U.S. Energy Information Administration)

As energy demand increases, so does carbon dioxide emission as a result of spent fossil

fuels (Figure 2-6) (U.S. Energy Information Administration). The increased use of liquid fuels is

associated with transportation methods, whereas natural gas and coal use is tied to electricity

generation. Coal is the most "carbon-intensive fossil fuel" and represents an increasing fraction

of total world emissions, even though it is not the fastest growing energy producer (U.S. Energy

Information Administration).

14

Figure 2-6: CO2 emissions by fuel type with time (billion metric tons) (U.S. Energy Information

Administration)

The appeal of nuclear power is due largely to its zero greenhouse gas emissions. The

previously cited large emission increases are anticipated to result in a global temperature increase

of 3.5˚C, which would increase droughts, heat waves, melting of polar ice caps, and irregular

weather patterns (Nuclear Energy Agency). In order to obtain a 50% chance of limiting global

warming to 2˚C, CO2 levels should be kept below 450 ppm in the atmosphere (Nuclear Energy

Agency). The link between CO2 emissions and global warming is a topic of debate, but is also a

policy driver. Rapid expansion of nuclear power will be required by any policy hoping to match

recommended emissions standards.

Abundance of Uranium

The wide distribution of known uranium reserves is part of what makes nuclear power an

attractive energy source. Figure 2-7 demonstrates the availability of low cost uranium worldwide

(International Atomic Energy Agency). When higher cost uranium is also included (<250

USD/kg), the distribution becomes slightly flatter (Appendix A) (International Atomic Energy

Agency). From an energy security perspective, this helps to eliminate the dependence of energy

15

supply on a small collection of countries, as exists with oil (Appendix A) (International Atomic

Energy Agency).

Figure 2-7: Global identified uranium resources (<130 USD/kg) (Nuclear Energy Agency)

There is also a large amount of uranium reserves. Including all cost categories, there is

approximately 7,097 thousand tons of identified uranium available worldwide with another 7,595

thousand tons estimated to exist, but yet undiscovered (Appendix B) (International Atomic

Energy Agency). Identified reserves are typically high confidence and suitable for mining

decision-making or feasibility studies (Nuclear Energy Agency). At the 2010 production rate of

approximately 55 thousand tons per year (Nuclear Energy Agency), there is enough uranium to

last for 267 years. Even though the amount of uranium is abundant and there is no immediate

concern for exhaustion, the resource is scarce and a long-term energy solution will need to exist

in order to ensure continued energy availability for future generations.

16

Uranium Demand and Cost

The rate of uranium production is expected to grow significantly as nuclear energy

demand increases. In Figure 2-8, the Nuclear Energy Agency estimates the tonnes of uranium

required per year to increase from approximately 65,000 to between 100,000 and 140,000 by

2035. Committed and existing production centers are sufficient to meet high demand estimates

through 2020. Planned and prospective production centers allow high demand estimates to be

met though 2030. By 2035 when demand begins to exceed production, 35% of the existing

identified reserves are expected to be exhausted (Nuclear Energy Agency). If power plants built

between now and 2035 have an assumed lifetime of up to 80 years, then identified resources

could become exhausted prior to the end of plant life based on high demand estimates (Nuclear

Energy Agency).

Figure 2-8: Projected uranium consumption (Nuclear Energy Agency)

17

The cost of uranium is linked to the relationship between production levels and demand

as shown by the Nuclear Energy Agency in Figure 2-9. When demand exceeds production, cost

rises and vice-versa. Supply exceeded demand from the onset of nuclear power through 1980

which created a large stockpile of uranium. Post 1980 and through the end of the cold war in

1990, production dropped sharply. The years of excess uranium were marked by declining cost.

However, as the uranium stockpile was consumed and the gap between requirements and

production persisted, there was a sharp rise in cost, reaching peak values around the year 2008.

When cost rose, exploration effort increased dramatically as seen in Figure 2-10 (Nuclear Energy

Agency). Exploration effort corresponded with greater production levels and decreased cost.

Based on anticipated production capability, it is reasonable to expect stable uranium prices

through 2030 with rising prices thereafter. Rising prices would be curbed by the discovery and

production of additional uranium, reduced demand through recycling and breeding, or use of

alternative fuels.

18

Figure 2-9: Supply, demand, and cost comparison (Nuclear Energy Agency)

19

Figure 2-10: Trend in exploration effort (Nuclear Energy Agency)

Abundance of Thorium

Adopting a thorium based fuel cycle would significantly increase available nuclear

material reserves. The "World Thorium Resources" meeting in October 2011 produced estimates

for total thorium resources found in Appendix B (Nuclear Energy Agency). Since thorium is not

currently used in commercial reactors, there is no distinction between identified reserves and

undiscovered reserves. The total amount available is up to 7.6 million tons, spread among greater

than 34 countries. Some of the countries with the highest amounts of thorium, such as Turkey,

India, and Brazil, have some of the lower amounts of uranium. The wide distribution of thorium

helps to further level nuclear resource availability worldwide and increase energy independence.

There is still a tremendous amount of thorium undiscovered. Thorium is generally

estimated to be between three and five times as abundant as uranium based on concentration in

the earth's crust (International Atomic Energy Agency). However, the total thorium identified is

only about half of the total uranium reserves in Appendix B. The small amount of identified

20

resources as compared to theoretical amounts in the earth's crust can be attributed to the lack of

thorium exploration effort.

The Nuclear Energy Agency identifies the byproduct nature of thorium as one of its

challenges with exploration and mining. Thorium has not yet been commercially adopted, so it is

uneconomical to produce as a primary product at present. Whereas countries reported that only

26% of uranium is produced as a co-product or bi-product (Nuclear Energy Agency). Thorium is

extracted entirely as a byproduct along with other rare earth elements (REE), phosphates (as in

monazite), and uranium (Nuclear Energy Agency). This makes it difficult to estimate the cost of

thorium production.

21

Chapter 3

Nuclear Fuel Reprocessing

The primary purpose of nuclear fuel reprocessing is to recover uranium and plutonium

from used fuel. The World Nuclear Association shows that spent fuel is composed mostly of

uranium with a small amount of plutonium, both of which are valuable fuel sources. Reprocessed

uranium (RepU) can be re-enriched for use in conventional reactors or used for production of

mixed oxide fuel (MOX) (Table 3-1). The amount of fissile U-235 in spent fuel is between 0.5%

and 0.9% as compared to approximately 0.7% in naturally occurring uranium (World Nuclear

Association). This means that spent fuel from light water reactors, RepU, or a blend of spent fuel

and RepU to produce a natural uranium equivalent (NUE), is also a suitable substitute for natural

uranium in pressurized heavy water reactors (PHWR) such as the CANDU reactor (World

Nuclear Association). This process is called DUPIC. Plutonium is used for primarily for

production of mixed oxide fuel (MOX). MOX is most suitable for use in fast reactors (World

Nuclear Association). The combination of RepU and MOX from reprocessing is a significant

source of fuel. It is estimated that reprocessing techniques could reduce the amount of naturally

occurring uranium required by up to 30% (World Nuclear Association). Reprocessing is clearly

advantageous from an energy sustainability perspective.

Uranium 95.6%

Stable fission products 2.9%

Plutonium 0.9%

Other long-lived fission products (caesium,

strontium, iodine, technetium, etc.)

0.5%

Minor actinides (americium, curium,

neptunium)

0.1%

Table 3-1: Typical composition of spent fuel from a light water reactor (World Nuclear

Association)

22

As new uranium reserves are discovered, the immediate need for reprocessing

diminishes, but many countries have continued developing the technology in order to ensure

sufficient fuel is available in the long term. Table 3-2 compares the reprocessing capacity for

various countries, with France and the UK having the highest capacity (World Nuclear

Association).

Most RepU is stored rather than used due to the economic advantage of purchasing new

fuel. In total, 90,000 tons out of 290,000 tons of used fuel has been reprocessed to date, with

approximately 45,000 tons currently in storage (World Nuclear Association). France has allowed

for storing reprocessed uranium for up to 250 years (World Nuclear Association). Stored RepU

will provide the same benefit that stockpiled uranium has since the 1980's: to supplement

uranium production capability.

Table 3-2: Reprocessing capacity by country (World Nuclear Association)

23

Reprocessing Techniques

Aqueous Reprocessing

Aqueous reprocessing is the most common and advanced reprocessing method and is

described as follows by Simpson and Law. The term aqueous reprocessing refers to liquid-liquid

solvent extraction methods used to separate uranium, plutonium, actinides, and fission products.

The spent fuel is first dissolved in an acidic solution. The separation takes when a second liquid

with higher affinity for the fissile elements or actinides is mixed with the solution. The second

liquid is typically an organic compound and has a different density than the original solute. The

difference in density allows for separation of the two liquids after adequate mixing has taken

place to allow for extraction. The three most common mixing and separation methods are

described by Simpson and Law as follows:

(1) Mixer-settler (Figure 3-1): The aqueous solution and organic compound are

mechanically mixed and then allowed to stratify based on density difference, with the more dense

of the two liquids settling toward the bottom of the tank. These machines typically have a large

footprint and require multiple stages in order to increase removal efficiency (Simpson and Law).

24

Figure 3-1: Mixer-settler extraction (Simpson and Law)

(2) Column (Figure 3-2): The aqueous and organic liquids are introduced at the top

and bottom of a column, with the denser fluid on top, such that a countercurrent is introduced in

the column. Within the column, perforated plates or other obstructions are used to create a

tortuous path and promote mixing. The efficiency of uranium (or plutonium or actinide) removal

is determined by the height of the column (Simpson and Law). Mechanical pulsing of the liquid

decreases the droplet size of the dispersed liquid, promoting mixing, and reducing the required

column height (Simpson and Law). The footprint for these machines are small, but the height is

typically 40-50 feet (Simpson and Law).

25

Figure 3-2: Pulse-column extraction (Simpson and Law)

(3) Centrifuge (Figure 3-3): The two liquids flow into a lower plenum where they

are mechanically mixed and then fed into the centrifuge. The centrifuge can produce force as

high as 300g, forcing the more dense liquid toward the outside of the centrifuge (Simpson and

Law). This method has very high single stage efficiency and a small footprint (Simpson and

Law).

26

Figure 3-3: Centrifugal extraction (Simpson and Law)

The elements that are extracted in each stage varies with the aqueous extraction process

used. Plutonium and Uranium Recovery by Extraction (PUREX) is the most widely used process

(Simpson and Law). Spent fuel is chopped and then dissolved in nitric acid (World Nuclear

Association). The organic compound is typically tributyl phosphate (TBP) dissolved in a

hydrocarbon diluent (Simpson and Law). The TBP extracts uranium and plutonium in the +4 and

+6 oxidation states as shown in Equation 2 and Equation 3 below (Simpson and Law). A

reducing agent is then introduced to the TBP to change the plutonium to the +3 oxidation state,

which will draw the plutonium out of the TBP (Simpson and Law). The uranium that remains in

the organic compound is finally removed from the TBP using diluted nitric acid (World Nuclear

Association). The final plutonium and uranium solutions are then evaporated and calcined to

produce PuO2 and UO2 (World Nuclear Association). The drawback to this method is the

isolation of plutonium from other actinides, which is a proliferation concern (Chapter 4).

27

Equation 2

Equation 3

There are numerous alternatives to the PUREX method, each with the goal of increasing

proliferation resistance and reducing long-lived radioactive waste (by separation of actinides from

fission products). Uranium Extraction (UREX) is a variation to PUREX with increased

proliferation resistance by extracting plutonium with other actinides (World Nuclear

Association). There are several variations to the UREX method which strive to produce a

plutonium product more similar to traditional MOX fuel, but with increased proliferation

resistance, including UREX +, UREX +1a, UREX +3, and NUEX (World Nuclear Association).

COEX allows for uranium and plutonium to be co-precipitated to yield an approximate 50/50

(Pu,U) oxide product (World Nuclear Association). There are numerous other methods pursued

to lesser degrees.

Pyroprocessing

Simpson and Law describe a second method for reprocessing called pyroprocessing,

which refers to high temperature methods for removing unused fissile elements from spent fuel.

The used fuel is typically dissolved in a molten salt electrolyte as opposed to aqueous and organic

solutions as previously discussed. The dissolved solution contains a cathode and anode, so that as

voltage is applied across the solution, positive actinide ions collect on the anode for removal. The

typical process is described by Simpson and Law as follows:

28

(1) The spent metal fuel is chopped and then dissolved in an electrolyte, typically

LiCl-KCl-UCl3 at 450-500˚C with the electrolyte melting temperature around 350˚C. If the fuel

is an oxide, it is dissolved in LiCl at 650˚C in order to convert the UO2 to Li2O and metal U.

(2) Current is passed through the electrolyte, causing oxidation of uranium at the

anode and metal deposition at the cathode. Other transuranics (TRU) are oxidized by the UCl3,

but will only collect on the cathode if the TRU to U ratio is high and a liquid cadmium cathode is

used. Molten cadmium has a high activity coefficient compared to U, which helps to overcome

the back reaction with UCl3.

(3) Both the cathode and anode are placed in a vacuum distillation furnace which

separates the metals from the liquid salt. The cathode metal is a combination of U, Pu, and other

TRU which will become the reprocessed fuel ingot. The anode metal contains fission products,

cladding, and leftover TRU that become a solid waste ingot.

Thorium Reprocessing

Thorium and U-233 reprocessing methods resemble uranium and plutonium reprocessing

methods. The two main methods are again aqueous and pyroprocessing, though the chemicals

used and some details of the processes are different. Since thorium is not commercially used,

thorium reprocessing is also in developmental stages and small amounts of spent fuel have been

reprocessed as compared to uranium fuel cycle.

29

Aqueous Reprocessing

The Interim-23 process is essentially the same as the PUREX method, as shown in Figure

3-4 and described by Brooksbank, McDuffee, and Rainey. The spent fuel cladding and thorium

are dissolved in nitric acid and then TBP is used to extract U-233. The U-233 is then stripped

from the organic compound with a low concentration nitric acid, after which it can be calcined.

Thorium is left with other actinides and fission products and discharged as waste.

Figure 3-4: Interim-23 process (Brooksbank, McDuffee and Rainey)

The Thorex method is used to co-extract thorium and U-233, as shown in Figure 3-5 and

again described by Brooksbank, McDuffee, and Rainey. This is accomplished by use of

Al(NO3)2 rather than HNO3 as the acid. The dissolved solution is then evaporated to remove

most of the acid and nitrate from the dissolver solution before using a high concentration TBP to

extract thorium and U-233. The aluminum and fission products are then disposed of as waste and

30

the reprocessed fuel is stripped from the organic compound and calcined. If stainless steel or

zirconium clad fuel is used, then fluoride-catalyzed HNO3 is used as the dissolver instead and the

process is termed Acid Thorex (Brooksbank, McDuffee and Rainey).

Figure 3-5: Thorex process (Brooksbank, McDuffee and Rainey)

Pyrometallurgical

The Argonne National Laboratory proposed a method for pyrometallurgical reprocessing

of thorium-based fuels shown in Figure 3-6 and summarized as follows (Bates, Jardine and

Krumpelt):

31

Figure 3-6: Pyrometallurgical reprocessing method (Bates, Jardine and Krumpelt)

(1) If the fuel is an oxide, then calcium salt (CaCl2 or CaF2) is used as a reducer to create

CaO and isolate the metal actinides in the fuel being reprocessed. Calcium has a free energy for

producing the desired reaction of -6.6 kcal/mole at 1000˚C. The reaction continues until the

ThO2 is completely reduced since it is the slowest reacting actinide. The salt also dissolves the

CaO, alkali and alkaline fission products, and iodine, which provides separation from the reduced

actinides.

(2) The uranium, plutonium, and thorium are then separated by dissolving the actinides

in magnesium which has a solubility of 0.002 wt %, 55 wt %, and 44 wt % of the respective

actinides at 650˚C. The low solubility of uranium and high solubility of plutonium provides two

product streams. The amount of thorium dissolved in the plutonium stream is controlled by either

limiting the volume of magnesium or introducing cadmium to the magnesium. High cadmium

content drives down the solubility of thorium in the solution. The temperature can also be

decreased in order to reduce solubility further to a limit of 5 wt % (Figure 3-7). Fission products

are soluble to varying degrees based on type and both streams will contain some amount of

fission products.

32

(3) The uranium/thorium product stream is allowed settle in order to separate the

uranium/thorium from the magnesium/cadmium. The uranium/thorium that does not come out of

solution is retorted. The plutonium/thorium is also recovered by retort. The leftover solution is

then recycled.

(4) The amount of CaO in the salt can reach 30 atom % before reduction rate decreases.

The CaO is removed from the salt by electrolysis using a carbon cathode. The Ca collects on the

cathode and the oxygen combines with the carbon to produce CO2.

Figure 3-7: Amount of thorium dissolved in magnesium based on cadmium content (Bates,

Jardine and Krumpelt)

Radioactivity Concerns During Fabrication of Fuel

As previously mentioned (Equation 1), thorium must be irradiated to produce fissionable

U-233. During irradiation, some small amount of U-232 will be created, following one of the

33

processes shown in Equation 4, Equation 5, or Equation 6 below (Appendix C) (Kang and

Hippel).

Equation 4

Equation 5

Equation 6

The U-232 that is bred in parallel with U-233 is highly radioactive. The decay chain of

U-232 includes Tl-208, as shown in Equation 7, which beta decays with a 3.1 month half life

releasing a 2.6 MeV gamma (Brooksbank, McDuffee and Rainey). This is compared to

approximately 0.5 MeV (t1/2 = 367 days) and 0.1 MeV (t1/2 = 284 days) for the fission products

Ru-106 and Ce-144 and 1.1 MeV for the activated metal Co-60 (t1/2 = 5.26 years) commonly

found in commercial uranium fuel cycle power plants (Siegel). As the content of U-232 increases

in U-233 fuel recovered from thorium irradiation, the expected dose rate increases. High dose

rates are advantageous from a proliferation standpoint (Chapter 4), but are more difficult for

manufacturing and reprocessing.

Equation 7 (Brooksbank, McDuffee and Rainey)

34

High dose fuel is expensive to handle since it requires special remotely operated

equipment and shielding. Brooksbank, McDuffee, and Rainey use Figure 3-8 to provide a

comparison between fuel fabrication cost increases and the content of Th-228, which is the first

decay daughter of U-232. As Th-228 content increases, curie content increases dramatically, with

a $150/Kg approximate step increase occurring at 1 Ci Th-228/Kg U. When compared to current

natural uranium retrieval costs of less than $130/kg, fabrication of U-233 is cost prohibitive.

Figure 3-8: Fuel fabrication cost with radiation level (Brooksbank, McDuffee and Rainey)

Waste Reduction

The most developed method and current plant of record for all nuclear powered countries

is to deposit spent fuel in geological repositories (World Nuclear Association). These repositories

depend on immobilizing the waste in borosilicate glass or synthetic rock, encasing the waste in a

stainless steel container, locating the container deep underground, and surrounding the containers

35

with impermeable backfill such as clay (World Nuclear Association). However, there are not

currently any permanent disposal facilities in operation and there are political and technical

challenges associated with ensuring that radioactive waste will be safe for millions of years after

disposal.

There is no way to completely eliminate radioactive waste from the nuclear fuel cycle,

but reprocessing technology can be used to decrease the amount of waste. This is accomplished

through offsetting the amount of additional fuel required and through transmutation. It is

estimated that adoption of the DUPIC process could reduce waste disposal by 70% (World

Nuclear Association). Transmutation refers to conversion of long-lived radionuclides (e.g.

neptunium, americium, curium, iodine-129, technetium-99, caesium-135, and strontium-90) into

shorter lived nuclides by neutron bombardment (World Nuclear Association). This can occur as a

result of fission by the actinides or neutron absorption and conversion into a more stable or

shorter lived nuclide as occurs with certain fission products. During the reprocessing effort, these

radionuclides can be partitioned along with the unspent fuel and then burned in a reactor (World

Nuclear Association). The process is most efficient in a fast reactor, but would be equally well

suited to a thorium based or uranium based fast reactor (World Nuclear Association).

36

Chapter 4

Proliferation

There is a concern that as nuclear technology is adopted and expanded, nuclear arms will

become more readily available to dangerous countries and organizations. Nuclear bombs have

traditionally been constructed from highly enriched U-235 or Pu-239. Similar methods used to

separate unused uranium and plutonium from commercial spent fuel can be used to collect bomb

grade material. There is risk that without proper controls such as policy and technology

restrictions, it would not be difficult for countries which possess nuclear power capabilities to

quickly obtain nuclear weapon capabilities.

Once weapon capability is obtained, there is also risk that weapons may be lost, sold, or

stolen. Figure 4-1 was prepared by the World Bank to rank existing and aspiring nuclear power

states in various categories indicative of risk for improper use or control over nuclear arms (lower

numbers designate higher risk). This figure demonstrates that aspiring nuclear power states are at

significantly higher risk. Table 4-1 shows that if aspiring nuclear power states had obtained

nuclear capability, they would have accounted for six of the top ten terrorist incidents among

nuclear powered states between 2004 and 2009 (Miller and Sagan). Therefore, there is cause for

concern that only responsible and stable states should obtain nuclear power, and that nuclear

capability should be strictly regulated to prevent inadvertent proliferation.

37

Table 4-1: Incidents of terrorism in current and aspiring nuclear power states (Miller and Sagan)

United States Ford-Carter Agreement

At the onset of nuclear power in the United States, uranium was perceived as a scarce

resource which encouraged the development of reprocessing technology. In 1956, the Atomic

Figure 4-1: World Bank governance characteristic scores (Miller and Sagan)

38

Energy Commission announced a government sponsored program to assist in reprocessing

(Andrews). This program laid the foundation for a closed fuel cycle. However, as more uranium

deposits were discovered, the need for a closed cycle became less apparent. Further, concerns for

proliferation increased with Cold War tension. In 1976, President Ford announced:

“…the reprocessing and recycling of plutonium should not proceed unless there is sound

reason to conclude that the world community can effectively overcome the associated

risks of proliferation… that the United States should no longer regard reprocessing of

used nuclear fuel to produce plutonium as a necessary and inevitable step in the nuclear

fuel cycle, and that we should pursue reprocessing and recycling in the future only if they

are found to be consistent with our international objectives” (Andrews).

One year later, President Carter announced,

“We will defer indefinitely the commercial reprocessing and recycling of plutonium

produced in the U.S. nuclear power programs” (Andrews).

President Carter further stated,

“We have concluded that a viable and economic nuclear power program can be sustained

without (plutonium) reprocessing and recycling” (Society for Science & the Public).

These statements, and a presidential veto of the Energy Research and Development

Administration proposed legislation required for building a breeder reactor and reprocessing

facility, resulted in the termination of reprocessing for commercial power in the United States

(Andrews).

Nuclear Weapons and Reactor Fuel

Reactors are designed to generate electricity based on energy output from nuclear

criticality, whereas nuclear weapons are a result of a supercritical reaction. The critical masses

39

for each isotope were previously shown in Table 1-1 (European Nuclear Society). By exceeding

the critical mass a supercritical, or runaway, reaction can be started. However, shortly after

becoming supercritical, the system will heat up and melt or explode. The reaction stops shortly

after expanding since the critical mass of a system decreases with the square of density (Bunn and

Wier).

In order to produce a high yield bomb the system must transform from sub-criticality to

super-criticality in a short period of time before the fuel expands and becomes sub-critical again.

There are basically two methods for producing this effect described by the Federation of

American Scientists and shown in Figure 4-2:

(1) Gun-devices use an explosion to propel a subcritical piece of fuel toward another

subcritical piece of fuel. They typically only successfully fission a small amount of the active

material. For example, less than 2% of the 60 kilogram bomb dropped on Hiroshima fissioned

(Bunn and Wier).

(2) Implosion-devices use a chemical explosion to generate an inward directed

implosion wave in a hollow fissionable material sphere which causes the material to rapidly

collapse upon itself. This method compresses the nuclear material much faster than the gun-type,

and therefore requires less material and can create higher yield bombs. The exact amount

depends on the timing of the explosion and other complicated design features. However, the

implosion bomb dropped on Nagasaki required only 6 kilograms of weapons grade plutonium,

which would be equivalent to 18 kilograms of highly enriched uranium (HEU) (Bunn and Wier).

40

Figure 4-2: Gun-type and implosion-type nuclear weapons (Federation of American Scientists)

Both the gun-type and implosion-type bombs are within the technical reach of a

sophisticated terrorist group or state. Hans Bethe of the Manhattan Project stated that the gun-

type design was "'well taken care of' by one scientist and two of his graduate students during a

summer study at Berkley" and there is substantial unclassified literature available on both bomb

types (Bunn and Wier). Of course, weapons delivery methods such as ballistic missiles are more

complicated, but the creation of a crude bomb, assembled on site or delivered locally is feasible

(Bunn and Wier). The main difficulty in the creation of a bomb is in the isolation of high purity

fissionable isotopes (U-233, U-235, and Pu-239) (Bunn and Wier).

Uranium must be highly enriched to be used in a bomb. Weapons grade uranium is

classified as having greater than 90% U-235 enrichment, though HEU includes anything over

20% since it is possible to construct a nuclear weapon from this material (Bunn and Wier). HEU

is suitable for either gun-type or implosion type bombs (Bunn and Wier). Nuclear reactors are

limited to the use of low enriched uranium (LEU) which is between 0.7 and 20% enriched (Bunn

and Wier). As previously discussed, natural uranium has approximately 0.7% U-235 (World

Nuclear Association).

The United States Nuclear Regulatory Commission describes three methods for enriching

uranium including gaseous diffusion, centrifuges, and laser separation (Figure 4-3). Gaseous

diffusion applies high pressure gaseous UF6 to a porous membrane such that smaller particle,

U-235, is capable of passing through the membrane at a higher rate than U-238 (which is the most

41

abundant isotope in naturally occurring uranium). When gaseous UF6 is introduced to a

centrifuge, the heavier U-238 isotope will be forced toward the outside of the rotating case, while

U-235 will tend toward the center. Both the centrifugal and gaseous diffusion methods require

multiple stages in order to reach high enrichment. Laser separation uses monochromatic light to

ionize a specific isotope, which can then be chemically or physically extracted.

Figure 4-3: Gaseous diffusion and centrifugal enrichment methods (United States Nuclear

Regulatory Commission)

42

Plutonium does not require any enrichment for use in bombs since all of the plutonium

isotopes are fissile (Bunn and Wier). However, Plutonium is most likely to be used in the

implosion-type bomb since plutonium has a high neutron emission rate which may cause super-

criticality before the bomb material has time to sufficiently compress in a gun-type bomb

(Federation of American Scientists). The highest yield bombs use greater than 90% Pu-239

(weapons grade) since Pu-240 has a higher neutron emission rate and reduces bomb effectiveness

(Bunn and Wier).

Plutonium Commercial nuclear power plants typically yield a 30% Pu-240 content in

recovered plutonium, which on average will reduce bomb yield by 10 fold (Cohen). This high

Pu-240 content is due to fuel being left in the reactor for long periods of time in order to reach

high burn-up and reduced operation costs. However, the operating cycle could be reduced in

order to obtain lower Pu-240 content. Kang and Hippel use Figure 4-4 to show how the various

isotopes of plutonium buildup depending on operating cycle length (burnup). The Pu-240

content rises with burnup, whereas the Pu-239 content decreases. Weapons grade plutonium can

be obtained for burnups of approximately 1000 MWd/tHM or less, although weapons usable

material can be obtained at any burnup value (Kang and Hippel). The Soviet Union designed a

commercial reactor which was used for both electricity generation and plutonium production for

weapons based on a balance between burnup and Pu-240 content (Cohen). North Korea also used

plutonium from a commercial reactor to create a bomb (Federation of American Scientists). The

most sophisticated nuclear weapon states are judged to be capable of creating bombs from reactor

grade plutonium of comparable yield and reliability to weapons grade plutonium (Bunn and

Wier).

43

Figure 4-4: Plutonium isotopic buildup with burnup using natural uranium in a heavy-water

reactor (Kang and Hippel)

The proliferation concern for both plutonium reprocessing and uranium enrichment is

that the same technology used to create reactor grade material can be used for the production of

weapons usable material. Nuclear weapons states usually produce weapons material in dedicated

plutonium production reactors due to lower cost (Cohen). However, it is estimated that research

reactors, which cost much less to build than power plants, could produce enough plutonium for a

bomb every two years (Cohen). Any method of plutonium recovery by reprocessing could then

be applied by either a state or terrorist group to obtain material for a bomb (Chapter 3).

The effort required to start producing weapons grade uranium instead of reactor grade or

research reactor grade material is also minimal. The World Nuclear Association provides Figure

4-5 to show the energy input in separative work units (SWU) per tonne uranium feed in order to

reach a desired U-235 enrichment level. One tonne of uranium input can result in 120-130 kg of

44

reactor grade, 26 kg of research reactor grade, or 5.6 kg of weapons grade material (World

Nuclear Association). The most effort is expended in producing power reactor grade material and

relatively minor additional effort is required to move to higher enrichments. A rogue state,

unstable state, or sophisticated terrorist group may divert and weaponize reactor grade material.

They could also disguise weapons production with peaceful power plants or research reactors

unless closely supervised. This creates concern for the advancement of enrichment technology by

rogue emerging nuclear power states since they can easily transition from peaceful power plant

fuel production to weapons production.

Figure 4-5: Effort required to enrich uranium (World Nuclear Association)

Increasing Proliferation Resistance

The threat of proliferation and diversion can never be completely eliminated so long as

uranium and plutonium are used in reactors, but the threat can be significantly reduced. Broadly

45

speaking, proliferation resistance techniques can be categorized as follows (Selvaduray and

Heising-Goodman):

(1) Dilution of weapons usable material - Mixing weapons usable material with any

other material will increase the critical mass of the mixture as shown by Kang and Hippel in

Figure 4-6. When U-233 is the material of concern, it must be diluted to much lower levels than

U-235. Only 12% enrichment of U-233 is required to obtain an equivalent critical mass as 20%

enriched U-235 (LEU) (Kang and Hippel). Plutonium would require even more dilution since it

has a reduced critical mass as compared to U-233. The goal would be to keep weapons material

below the LEU limit; the point when critical mass increases dramatically (note that the term LEU

is typically used only in reference to U-235). When kept below LEU levels, diluted material

would require additional processing to enrich to weapons useable levels and requires larger

quantities of material.

46

Figure 4-6: Reflected critical mass by U-233 or U-235 content (Kang and Hippel)

Dilution is often accomplished by diluting plutonium with other actinides. Dilution is

often accomplished by co-processing, which extracts the multiple materials at once, but can also

be accomplished by mixing the diluting material with the weapons usable material after

extraction (Selvaduray and Heising-Goodman). COEX is an example of co-processing dilution.

Dilution is most effective when it becomes physical impossible to extract weapons usable

material from other isotopes (Selvaduray and Heising-Goodman). This can be accomplished by

eliminating weapons usable material from the product stream. The pyroprocessing technique

described earlier is a good example because the plutonium that is reprocessed will always exist

either with the waste stream or along with uranium and other actinides.

(2) Spiking - Introduction of high radiation spikants to weapons usable material

increases the difficulty of handling the material and increases detectability (Selvaduray and

47

Heising-Goodman). Large amounts of shielding are required for transportation, further

enrichment, and fabrication. This makes the material easier to detect. Some desirable attributes

for a spikant include: (1) gamma energy greater than 1000 KeV, (2) high activity, and (3) has a

half life between 8 months and 50 years (Selle). High gamma energy and activity ensure that the

material is dangerous to handle, whereas the half life ensures that it will maintain its lethality for

a significant amount of time. Selle uses Table 4-2 to list some spikant candidates, their half-life,

and the concentration of the material required to produce 27,000 R/h after two years. The dose

rate of 27,000 R/h is the amount necessary to cause a 50% chance of death after 1 minute of

exposure (Selle). Cobalt-60 is an especially good choice based on half life, minimum

concentration requirements for lethality, as shown in Table 4-2: Potential spikant nuclides .

Fission products also make excellent choices since they are already present in the spent fuel.

48

Table 4-2: Potential spikant nuclides (Selle)

Thorium and Proliferation Resistance

Uranium-233 is suitable for use in nuclear weapons. The critical mass for U-233 is

approximately 1/3 of that required for U-235 (Table 1-1). U-233 can be used in gun-type bombs,

49

which are easier to construct though less destructive (Kang and Hippel). These reduced

requirements for bomb construction as compared to U-235 could make U-233 a target for

diversion and proliferation. However, the thorium fuel cycle has several characteristics that can

increase its diversion and proliferation resistance.

Dilution

The thorium fuel cycle results in the production of less plutonium than the uranium fuel

cycle. Appendix C shows that it takes seven neutron absorptions and four beta decays to

transform Th-232 into Pu-239 as compared to a single neutron absorption and two beta decays

from U-238 (Kang and Hippel). The increased number of reactions required to produce

plutonium from thorium drives down the plutonium production rate as shown by Kang and

Hippel in Figure 4-7. At the lowest burnup (which is most favorable for plutonium production), a

natural uranium fueled heavy water reactor might produce 1 gram of Pu-239 per MWd as

compared to 0.2 grams in a 70% ThO2 and 30% UO2 (19.5% enrichment) reactor. It is also

important to note the trend toward lower plutonium production with higher enrichment. The

combination of slightly higher enriched uranium (still LEU) and thorium to fuel reactors

significantly decreases plutonium production levels.

50

Figure 4-7: Plutonium production versus burnup based on fuel and reactor type (Kang and

Hippel)

The challenge in adoption of thorium based fuels is balancing Pu-239 and U-233

production (Kang and Hippel). The best designed reactors will burn U-233 as it is created, with

no excess available for recovery. Kang and Hippel use Figure 4-8 to show that as thorium levels

in the core drop and consequently uranium levels increase, production of U-233 decreases. The

heavy water reactor (HWR) with 7% thorium cuts U-233 production in half compared to cores

with 70% thorium. Still, the 70% ThO2 core would only produce approximately 0.35 grams U-

233. When combined with the 0.2 grams of plutonium produced, the total weapons usable

material produced is still half the amount of plutonium produced by a natural-uranium fueled

HWR and roughly equivalent to the plutonium produced by a 4.5% U-235 enriched pressurized

51

water reactor (PWR). It is clear that thorium has potential for reducing the quantity of weapons

usable material produced.

Figure 4-8: Uranium-233 production versus burnup based on fuel and reactor type (Kang and

Hippel)

Dilution can also be accomplished by restricting power plant operation to high burnup

levels. The average power plant reaches reactor burnups of approximately 40000 MWd/tHM

(International Atomic Energy Agency). Using the previous figures, average plant burnup levels

would result in 0.05 grams of plutonium and 0.2 grams U-233 production in a 70% ThO2 reactor:

a 50% decrease in weapons usable material as compared to low burnup. Applying minimum

burnup limits in addition to the use of thorium and enriched uranium would minimize weapons

material production.

52

The U-232 Spikant

The radioactivity of thorium based fuel marks a dramatic difference from enriched U-235

fuel and plutonium. Thorium fuel radioactivity comes from U-232 produced during irradiation of

Th-230 and Th-232 (Appendix C) (Kang and Hippel). Table 4-3, provided by Kang and Hippel,

compares the dose rate associated with weapons-grade and reactor-grade plutonium and various

concentrations of U-232 in U-233. These dose rates correspond to 0.5 meters, which is a good

approximation for the distance between a worker and the material in a glove-box (Kang and

Hippel). A 5 kg sphere is approximately the amount of plutonium required for creation of a bomb

(Table 1-1: Critical mass comparison of various isotopes (European Nuclear Society). The

average time that fuel remains in a reactor is 12-36 months, so approximating the dose rate after

one year is fair for comparison (World Nuclear Association). At high U-232 concentrations, the

dose rate is thousands of times higher than in plutonium.

Table 4-3: Comparison of unshielded dose rates produced by a 5 kg sphere at a distance of 0.5

meters one year after separation (Kang and Hippel)

The high concentration of U-232 in U-233 from power plants acts as a spikant. The

concentration of U-232 in spent fuel is between 1000 and 4000 ppm (Brooksbank, McDuffee and

Rainey). For fuel that has been in the reactor for 12 to 24 months, these concentrations yield an

approximate 1000 R/hr dose rate at 1 foot (Figure 4-9, (Brooksbank, McDuffee and Rainey)).

High dose rates applied over a short amount of time (acute exposure) can have devastating health

53

effects. Table 4-4 summarizes the potential for bone marrow, gastrointestinal, and cardiovascular

illnesses resulting from progressively higher doses (Center for Disease Control and Prevention).

Within half-an-hour of exposure to a 1000 R/hr source (mostly gamma radiation with a quality of

1), there is a 50% chance of death within 60 days. After one hour of exposure, there is a 100%

chance of death within two weeks. After five hours of exposure, death is likely to occur within

three days. A 1000 R/hr dose rate does not meet the 27,000 R/hr criteria used previously in

defining spikants, but it is still a lethal level and dramatically increases proliferation resistance.

Figure 4-9: Dose rate in recovered U-233 fuel based on U-232 content (Brooksbank, McDuffee

and Rainey)

The thorium fuel cycle's increased proliferation resistance as compared to other reactor

fuels is dependent on process and cycle length. A shorter refueling or reprocessing cycle would

result in lower concentrations of U-232. It is possible to design a reactor such that thorium is

only used in target channels that can be refueled more frequently than the uranium channels,

lowering U-233 content to only a few ppm which is non-lethal (Kang and Hippel). The thorium

fuel cycle would have to be implemented with corresponding restrictions against U-232 reducing

methods in order to maintain its diversion and proliferation advantage over the uranium fuel

cycle.

54

Table 4-4: Acute Radiation Syndromes (Center for Disease Control and Prevention)

Chapter 5

Conclusions

Available Resources

The amount of world-wide available uranium, though not in immediate danger of

expiring, is limited. Based on current and projected demand, there is enough known uranium to

last for multiple generations and perhaps centuries. There is little economic motivation for

pursuit of thorium as a primary resource at present due to the current low cost of uranium.

However, it is prudent from a global energy sustainability and individual national security

perspective to understand the limitations of the current fuel cycle and begin development of

technology that will allow the use of a thorium fuel cycle. The thorium fuel cycle should also be

of particular interest to countries with large stores of thorium as compared to uranium. Adoption

of the thorium fuel cycle would supply additional resources to meet world energy demands.

Nuclear Fuel Reprocessing

Nuclear fuel reprocessing is an important technology for both the uranium and thorium

fuel cycles in order to increase available resources while minimizing waste. The uranium and

plutonium reprocessing methods are well developed, though they deserve continued improvement

in proliferation resistance and waste disposal. The thorium reprocessing methods are very similar

to other reprocessing methods, though they are not refined due to lack of economic interest and

commercial adoption of the thorium fuel cycle. There are also technical challenges and high

56

costs associated with U-232 induced radioactivity levels in spent fuel from thorium based

reactors.

Proliferation

Nuclear fuel cycles come with a threat of weaponization of nuclear material. Within the

last decade, several countries have defied international law and sanctions in order to obtain

weapons capabilities. Though there has been no use of a nuclear weapon since World War II, the

threat is sufficient to slow the development of nuclear power technologies (i.e. reprocessing, new

reactors, breeder reactors, etc.).

Thorium offers significant improvements in proliferation resistance. The use of thorium

results in less weapons grade material production than natural and enriched uranium cores. It also

results in the highly radioactive byproduct U-232 which can deter would be diverters with the

threat of physical illness and death due to exposure. Given the current political and economic

environment, thorium's improved proliferation resistance could be a more significant driver

toward adoption of a thorium based fuel cycle than the benefit of increased resources as

previously discussed.

Future Work

There are many areas left uninvestigated by this paper that need to be addressed in order

to fully weigh the advantages and disadvantages of a thorium based fuel cycle. These include,

but are not limited to: a comparison between uranium and thorium breeder reactors in terms

proliferation resistance, the impact of breeder reactors on estimated uranium and thorium

demand, geological repository alternatives, detailed environmental impact comparison between

57

the thorium and uranium fuel cycles, a review and comparison between promising thorium and

uranium reactor designs, and the safety of thorium based reactors.

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63

Appendix A

World Oil Reserves

(U.S. Energy Information Administration)

(Organization of the Petroleum Exporting Countries)

64

Appendix B

Identified Uranium Reserves by Country (Nuclear Energy Agency)

65

Undiscovered Uranium Reserves by Country (Nuclear Energy Agency)

66

Total thorium resources (Nuclear Energy Agency)

67

Appendix C

Isotope production chain by neutron absorption and beta decay (Kang and Hippel)