A Study of the Advantages of Thorium Reactors for Nuclear Power

A Study of the Advantages of Thorium Reactors for Nuclear Power By Sarah Louise Penny A dissertation submitted to the Department of Physics, University of Surrey, in partial fulfilment of the degree of Master of Science in Radiation and Environmental Protection Supervisors: Prof. P. H. Regan, Dr. W. B. Gilboy Department of Physics Faculty of Engineering and Physical Sciences University of Surrey August 2010 Sarah Louise Penny 2010 1

ABSTRACT A Study of the Advantages of Thorium Reactors for Nuclear Power This report considers the advantages and disadvantages of using thorium as a fuel for Generation IV and future reactors. Nuclear power in general is compared with the current fossil fuels and the possibility of renewable energy sources. However, the main body of the report concentrates on comparisons between thorium and uranium fuelled reactors. It is shown that, in comparison to a uranium fuel cycle, a thorium cycle is safer, has a higher burn-up, produces less waste and reduces proliferation. The major disadvantage is the requirement for remote processing and fuel fabrication. Fuel reserves are also much more extensive than those of uranium. Overall, the advantages of the thorium fuel cycle outweigh the disadvantages, although the fuel cycle is less developed and there is less practical knowledge. This report concludes that, despite the initial funding requirements, thorium fuelled reactors are the best possible choice in the foreseeable future for the U.K. s main source of energy. 2

TABLE OF CONTENTS Page List of Tables....5 List of Figures....6 Glossary of Symbols and abbreviations....7 Acknowledgements....8 CHAPTERS Chapter 1 Introduction....................................9 1.1 Background...........................9 1.2 The history and development of nuclear reactors...11 Chapter 2 Characteristics of thorium Reactors........13 2.1 Thorium.....13 2.2 Thorium fuel cycle...16 Chapter 3 Fuel availability................20 3.1 Fuel reserves................20 3.2 Extraction........22 3.3 Economics........24 Chapter 4 Statistics of current nuclear reactors.................... 27 3

Chapter 5 Nuclear reactor products.........29 5.1 Non Proliferation...........29 5.2 Radioactive wastes...........31 Chapter 6 Summary of results...........36 Chapter 7 Conclusions and Recommendations.............38 References.......39 Appendices......43 4

LIST OF TABLES Pages Table 1 Annual consumption of different forms of energy..........9 Table 2 Capture and fission cross-sections..... 15 Table 3 Reserves of thorium..... 21 Table 4 Data on various thorium fuelled reactors.......27 Table 5 Weights of major actinides produced in a closed cycle.......33 Table 6 Weights of major actinides produced in a closed cycle...........33 Appendix Table 7 Relative costs of different methods of energy production.........43 Table 8 Probability Relative costs of different methods of energy production.....43 Table 9 The costs of different methods of uranium enrichment...... 44 Table 10 Fuel composition and element structure for different reactor types... 45 5

LIST OF FIGURES Pages Figure 1 Timeline of nuclear reactor development..........12 Figure 2 The naturally occurring decay chain of 232 Th.... 13 Figure 3 Graphs of the capture and fission cross-sections of 232 Th and 239 Pu... 14 Figure 4 Graph of neutron production per collision....16 Figure 5 The chain of reactions to produce fissile 233 U from fertile 232 Th...17 Figure 6 Fuel cycles for uranium and thorium.........18 Figure 7 Methods of production of 232 U........18 Figure 8 Extraction of monazite........23 Figure 9 Comparison of the amounts of plutonium burnt and produced with MOX and thorium fuels..........30 Figure 10 Timescale of different waste disposal methods........32 Figure 11 Radio-toxicity of actinides......34 Appendix Figure 12 Thorium and uranium decay chains...42 6

GLOSSARY OF SYMBOLS AND ABREVIATIONS ppm k η AGR BWR CANDU FNR HEU HTGR IAEA LFTR LMFBR LWBR LWR MA MOX MSBR MSR MWe MWt (N)NPT OECD PHWR PWR Toe UK USA WPu parts per million neutron multiplication factor number of neutrons produced by a single fission Advanced Gas Reactor Boiling Water Reactor CANadian Deuterium Uranium reactor Fast Nuclear Reactor Highly Enriched Uranium High Temperature Gas cooled Reactor International Atomic Energy Agency Liquid Fluoride Thorium Reactor Liquid Metal Cooled Fast Breeder Reactor Light Water Breeder Reactor Light Water Reactor Minor Actinides Mixed Oxide Fuel Molten Salt Breeder Reactor Molten Salt Reactor Mega Watts Electrical power Mega Watts Thermal power (Nuclear) Non-Proliferation Treaty Organisation for Economic Co-operation and Development Pressurized Heavy Water Reactor Pressurized Water Reactor Tonne of Oil Equivalent (42GJ, 12MWh) United Kingdom of Great Britain and Northern Ireland United States of America Weapons-grade Plutonium 7

ACKNOWLEDGEMENTS I would like to thank my project supervisor, Dr Walter Gilboy, and my Course Director, Prof. Paddy Regan, for their advice and assistance. I would also like to thank my Dad for his support and his assistance with the proof reading. 8

Chapter 1 Introduction 1.1 Background Over the last 60 years U.K. energy statistics show a trend of increasing divergence between the energy demand and the traditional primary fuels available to meet that demand. With ever decreasing supplies of fossil fuels, a major source of energy must be developed to satisfy the increasing deficit. Natural energy sources, such as geothermal, tidal and wind cannot be used to produce a significant amount of energy. As nuclear fusion reactors are only in very early developmental stages, nuclear fission is the only major source of energy available. So far, as can be seen from Table 1, very few countries, France being one of the exceptions, are exploiting nuclear power at anywhere near maximum capacity. Country Coal Oil Gas Nuclear Total Toe per inhabitant Germany 96.7 136.9 59.6 36.9 332.8 4.29 China 581.1 100.7 13.4-695.2 0.67 France 14 92 29.2 93 228.2 3.6 U.K. 50.7 84.1 60.5 19.3 214.6 3.52 Japan 76.2 222.2 39.2 43.4 381 3.26 Russia 374.4 332 587.2 69.3 1362.9 4.88 U.S. 520.5 864.1 586.4 166.9 2137.9 7.96 Table 1. A table showing the yearly consumptions of different forms of energy in MToe. [1] The majority of nuclear power reactors are based on the uranium fuel cycle. However, there are limited supplies of Uranium reserves; less than 1% of the world s natural supply of uranium is in the fissile form of 235 U. [2] In 1975, a joint report by the OECD Nuclear Energy Agency and the IAEA estimated that, in the 10% of the Earth s surface explored at that time, 9

there was 3.5 million tonnes of commercially exploitable uranium. [1] Whilst extracting plutonium from spent fuel and feeding it back into the reactor can reduce the uranium required to fuel the reactor by 20%, Fast Breeder Reactors can, using the same amount of fuel, produce 50 times the power output expected from a uranium thermal reactor. [3] However, two of the major issues with the use nuclear power are both public and international concerns for safety and the environmental consequences of the large amount of radioactive waste produced. These are some of the many aspects that must be taken into account when developing the next generation of nuclear reactors. The use of thorium is a possible alternative to uranium fuel cycle. The thorium fuel cycle has a significantly higher burn-up than that of uranium, thus reducing the amount of radioactive waste that needs to be safely contained. During the cycle, much smaller amounts of the longlived Minor Actinides (MA), such as Np, Am and Cm are produced, causing a decreasing level of radioactivity in the spent fuel. Consequently, the time needed for the radioactivity of the spent fuel to reach an acceptable level to be safely released from containment is correspondingly reduced. However, at the end of the cycle, other radionuclides that do not appear in the uranium cycle are produced; the full effects of which are as yet unknown. The thorium cycle also has very low proliferation, as it does not lead to the production Weapons Grade plutonium, WPu, for non peaceful purposes. It also allows the incineration of all grades of plutonium in a single cycle breeding proliferation resistant 232 U. Whilst 232 U is proliferation resistant, 212 Bi and 208 Tl are produced in large amounts during the cycle. The resultant build up of radioactivity and decay heat are too great to allow manual handling and therefore require expensive remote reprocessing. As highlighted above, there are many factors relating to the type and design of nuclear reactors that must be considered when selecting the most appropriate reactor solution. Thorium reactors have both advantages and disadvantages in comparison to uranium thermal reactors. A number of criteria must be taken into account. These include; international safety standards, proliferation, economics, sustainability, environment, when considering thorium as a fuel for the next generation of reactors. 10

1.2 The history and development of nuclear reactors In the U.S., nuclear reactors were originally designed and built in the 1940s to breed plutonium for the production of atomic weapons. After World War II, scientists turned to the design of nuclear reactors for commercial power production. The first working reactor in the U.K. was built in Calder Hall and generated 50MWe. The 1 st generation of reactors were designed to be cheap to build and operate, with a relatively cheap and secure supply of fuel. Design choices and usable materials were limited as the U.K. did not have the capacity to enrich fuel at the time. This lack of enrichment capacity prevented water from being used as a moderator, as it absorbed too many of the neutrons produced by the natural uranium to sustain a chain reaction. Other reactors were designed as a means of propulsion for submarines, especially in the U.S. Whilst the overall designs were similar, reactors in submarines often used very highly enriched uranium, up to 30% 235 U. This was because a nuclear submarine cannot be refuelled, so its life span depends solely on the initial amount of fissile fuel carried. Most commercial reactors are enriched only to 3% 5% due to the increasing difficulty and expense of further enrichment. By the 1960s, there was a much larger range of choice for Generation II reactors in the U.K. as by then, enrichment had become a possibility. The AGR (U.K. design), PWR and BWR (U.S. designs) were the most viable choices, due to the expense of the heavy water for the coolant and moderator needed for the CANDU reactor (Canadian). Industry favoured the water reactors but, due to political considerations, the AGR design was chosen and seven power stations built over 14 years. The current Generation III reactors are mostly improvements on the designs of the Generation II reactors, which had been developed during their lifetime, to improve thermal efficiency, fuel handling and safety, resulting in a longer operating life. 11

Generation IV reactors are currently being developed. The design main goals are: sustainability, costs that are comparable to non-nuclear power production, enhanced safety and reliability, minimisation of waste and low proliferation. Figure 1. Timeline showing the generations of nuclear reactors, including design or specification. [4] It can be seen from the history of reactor design that the choice of reactor depends on many factors such as: the available technology, the cost of fuel and possible enrichment and fabrication, sustainability, current regulations, international politics, finance, possible environmental impact and even public opinion. No single reactor design can possibly fit all the sometimes conflicting requirements and be the most appropriate in each category. Each reactor requirement must be closely examined in order to determine if a thorium reactor is the best possible option in a particular scenario. This report will consider the advantages and disadvantages of all aspects of the thorium fuel cycle; from front end such as fuel availability, to the back end challenges of waste disposal and proliferation. There will be some comparison of nuclear power with conventional fossil fuels, but the majority of the report will concentrate on comparing details of the thorium fuel cycle with those of the uranium cycle. 12

Chapter 2 Characteristics of Thorium Reactors 2.1 Thorium Thorium is a naturally occurring radioactive element, from the actinide group of the periodic table. Thorium is commonly found in soils, mostly on beaches with monazite sands; on average 6 ppm. The most common source of thorium is in the mineral Monazite, which contains on average 6% thorium up to a maximum of 12%. [5] There are 27 known isotopes of thorium, with half-lives ranging from milliseconds, to that of 232 Th, which is the most stable with a half-life of 14.05 billion years. [6] 232 Th is also the only naturally occurring isotope and comprises almost 100% of all thorium found in the Earth s crust. 232 Th decays naturally by alpha decay, starting off the decay chain that ends with stable 208 Pb as shown below in Figure 2. The full decay chains of both thorium and uranium are shown in Figure 12 in the Appendix. 13

Figure 2. The naturally occurring decay chain of 232 Th. A significant advantage of using thorium is that it has an absorption cross-section for thermal, 0.025eV, neutrons, of 7.4 barns, which is nearly three times the thermal cross-section of 238 U of 2.7 barns. This gives a higher conversion ratio for 232 Th to 233 U than for 238 U to 239 Pu, meaning that thorium is a superior fertile material when used in thermal reactors. However, as can be seen from the graphs below in Figure 3, of cross-sections against neutron energy, that depleted uranium is the more fertile material in fast reactors. 14

Figure 3. Graphs showing the capture and fission cross-sections of isotopes, 232 Th and 239 Pu. It is also important to consider the capture and fission cross-section of the three most common fissile nuclei present: 233 U produced by the thorium, 235 U present in original fuel and 239 Pu produced due to the capture of a neutron by 238 U. It is the difference in orders of magnitude between the capture and fission cross-section for each of the nuclei that is the principal determining factors such as quantities of waste and recycling of the fuel. It is important when maintaining a chain reaction in the nuclear reactor that the neutron multiplication factor, k, is equal to one. If k is less than one, the number of neutrons would decrease, leading to less fissions and the reactor eventually shutting down. The capture of neutrons by fissile material leads to the removal of both propagating neutrons and fuel from the reaction cycle, producing heavier isotopes that only hinder the reaction. It is vital that the capture cross-section be low enough and the fission cross-section high enough to ensure that suffucuent neutrons are produced to keep the reactor above critical. Cross-section 233 U 235 U 239 Pu Capture 46 101 271 Fission 525 577 742 Table 2. The capture and fission cross-sections for thermal neutrons in barns [7]. From Table 2, it can be seen that the fission cross-sections for each of the nuclei are of the same order of magnitude, but the capture cross-sections are very different. The large capture cross-section of 239 Pu means that more than a quarter of the neutrons interacting will lead to the production of 240 Pu. This can lead to concerns about proliferation; however the plutonium produced would be reactor-grade rather than weapons-grade unless the reactor is used for a short period of time. [8] The capture cross-section of 233 U is more than an order of magnitude less than the fission cross-section, causing greater efficiency for the 232 Th 233 U fuel cycle, so recycling is less of a problem than for the standard uranium cycles. 15

The graph below, Figure 4, shows that the number of neutrons produced by a single fission, η, remains fairly constant throughout the entire range of energies expected of a neutron in a reactor for 233 U. Figure 4. Neutrons produced per collision over the full range of neutron energies. [9] Unlike 235 U and 239 Pu, there are only small resonances, maintaining a higher value of η over much of the energy range. This higher value gives greater scope over the other factors that influence the neutron multiplication factor, such as the moderator, which, depending on material can absorb a large number of neutrons. Further data, showing both total and delayed neutron yield can be found in Table 8 in the Appendix. 16

2.2 Thorium fuel cycle Apart from natural uranium, thorium is the only other naturally occurring fertile fuel. 232 Th can absorb a thermal neutron to produce 233 Th, which decays as shown in Figure 5 to produce fissile 233 U. [10] Th 232 233 ( n, ) Th 12 Th 233 233 ( t 22.3min) Pa 233 233 ( t 27.0 days) 12 Pa U 233 U ( n, f ) fission Figure 5. The chain of reactions required to produce fissile 233 U from fertile 232 Th. Unfortunately, thorium fuel only cannot start off a chain reaction as there are no fissile isotopes present in natural thorium. While a chain reaction can be sustained, another fissile material, such as 235 U or 239 Pu, must be added to the fuel to drive a breeder in order to start up the reactor and reach criticality. A more efficient method would be to add 238 U and plutonium in a standard thermal reactor. The combination of fuels needed, in different proportions in the blanket and seed of the reactor add both complexity and cost to the design. Thorium fuel is often in the form ThO 2, which has a higher chemical stability and does not oxidise further like the uranium fuel UO 2. It also has a higher thermal conductivity and a lower coefficient of thermal expansion, thus increasing the efficiency. However, the higher melting point, at 3300 C, does complicate the fabrication. [11] Another feasible arrangement is to dissolve the ThF 4 fuel in a molten salt cooler and moderator. This removed the need to fabricate solid fuel rods. The diagram over the page shows examples of both uranium and thorium fuel cycles. 17

Figure 6. Showing the fuel cycle for recycled uranium (left) and the recycled denatured uranium-thorium cycle (right). [12] One of the disadvantages of the thorium fuel cycle is the production of the 233 Pa nuclei, an intermediate product in the decay chain leading to production of 233 U as shown above in Figure 6. The 233 Pa nuclei is both a neutron absorber and has a relatively long half-life of 27 days in comparison to the half-life of 239 Np, 2.4 days, which is the longest-lived nuclei in the production of 239 Pu from 238 U. [13] There is always a significant amount of 233 Pa present, removing neutrons that could have gone on to produce further fissions. However, by absorbing two neutrons 233 Pa can go on to produce fissile 235 U, and so does not contribute significantly to the waste produced. It does, however, cause the reactor to require at least an additional year of cooling after shut-down because of the longer half-life. A second disadvantage is the contamination of 232 U, which requires remote handling due to the high energy gamma radiation emitted by one of its decay products. The production of 232 U can occur in a number of ways as shown below in Figure 7. n Th Th Pa U n U n 232 233 233 233 232 90 90 91 92 92 2 n Th Th Pa n Pa 2n U 232 233 233 232 232 90 90 91 91 92 n Th Th 2n Pa n Pa U 232 233 231 232 232 90 90 91 91 92 Figure 7. A number of reactions by which 232 U can be produced. 18

Despite the increased cost of the remote handling, the fuel is naturally proliferation resistant. 232 U cannot be separated from 233 U, which aids the detection of the spent fuel by the high energy gamma radiation. This will be discussed more in Chapter 5.1 19

Chapter 3 Fuel availability 3.1 Fuel reserves Current estimates suggest that there are currently in the order of five million tonnes of natural uranium reserves present in the Earth s crust. As of December 2009, there were 436 operating reactors around the world. [14] Given the current consumption of an average power plant operating at normal capacity, on average 40,000 tonnes of uranium is used per year at the current demand for nuclear power. At the present rate of usage, it is estimated that fuel reserves will last approximately 125 years. Given the rapid decrease in obtainable fossil fuel reserves, demand for nuclear power will increase causing the supply of uranium to last not much longer than the period currently predicted for oil. [15] Thorium is naturally three times more abundant than uranium in the surface of the Earth, with almost 100% of it in useful isotope form. The main source of thorium is in monazite sands, weathered from the parent rocks and separated from lighter minerals due to different physical properties. 20

The Table below shows both the Reasonably Assured Reserves (RAR) and Estimated Additional Reserves (EAR). [16] Country RAR EAR Australia 19,000 - Brazil 606,000 70,0000 Canada 45,000 128,000 Greenland 54,000 32,000 Egypt 15,000 309,000 India 319,000 - Norway 132,000 132,000 South Africa 18,000 - Turkey 380,000 500,000 United States 137,000 295,000 Table 3. Known and estimated reserves of thorium metal in tonnes. Despite the larger reserves of thorium, there is still a finite source, which would be lost in a few centuries. To be sustainable, the fuel must be artificially produced. In a breeder reactor, with good management of both thorium and uranium fuel and reprocessing, the current reserves should be sufficient for over a thousand years. On average, thorium reactors produce 3.6 billion kwh of heat per ton of thorium which, assuming a 40% efficiency, yields an estimated 6 tonnes of thorium to keep a 1GW reactor running for one year. Using the entire current estimates of thorium reserves over a one thousand year period, shared over six billion people, the power generated is 4 kwh/d per person. [18] The current energy consumption ranges from 4 kwh/d to 30 kwh/d per person in the U.K and U.S. When averaging the power supplied by the potential thorium reactors over the World population, the calculations show that using just thorium fuel would not be sufficient. However, when taking into account the extremely varied demand from different countries, the fact that many countries with high populations do not have the desire or capacity for nuclear power, and that the U.K. and especially the U.S. have some of the highest demands, the current estimated reserves should suffice. 21

The other main feasible solution would be to extract uranium fuel from sea water. The concentration of uranium in sea water measures at on average 2.3 parts per billion, with estimations of sea water contains three billion tonnes of natural uranium. However, the cost of extracting the uranium from the sea water combined with the enrichment process would cause the cost of power production to increase to ten times the current amount. [15] 3.2 Extraction The method of extracting thorium, most often from sand, shown in Figure 8, is very different to the extraction of uranium from its ores. The process requires fewer controls; while thoron ( 220 Rn, a decay product of thorium) has a high inhalation dose factor there is no need to control ventilation for control the occupational exposure, as thorium mining occurs in an open pit. Ventilation is necessary in underground uranium mines due to the high radon ( 222 Rn, from the uranium decay chain) concentration. The tailings from thorium extraction also require much simpler management than uranium tailings to control public dose, due to the shorter half-life of thoron than radon. When removed from the thorium mines, the monazite must be separated from the other minerals present by a series of chemical and physical processes, as shown on the next page in Figure 8. The resultant product contains 98% monazite, from which the thorium must be extracted. 22

Figure 8. The usual method used by most countries to separate monazite from the minerals present in the sand. [19] The two most common methods used to extract the thorium from the monazite concentrate use either an acid or an alkali. The sulphuric acid is used in a string of processes, which ends with the thorium precipitating out of solution as a phosphate, with lanthanide sulphate residues, generating a large amount of waste. 23

The currently preferred method uses sodium hydroxide to extract the thorium in the form of ThO 2. This allows the recovery of both the valuable phosphates and the lanthanides. Research is also ongoing, especially in India, into the use of alkyl amines. They allow high purity separation of thorium, uranium and any rare earth metals present. By changing the size and structure of the amine, the solution can be tailed to the situation as required. Primary amines are best suited to extracted thorium, while secondary amines are preferred when extracting uranium. Research has given results of thorium 99.8% purity and uranium concentrate 99.4% purity. In comparison, newly mined low grade uranium ore contains 0.01% 0.25% uranium oxide, while high grade ore contains on average 23% uranium oxide. [20] As with the monazite concentrate, the ore is leached with either acid or alkali, followed by several processes such as ion exchange and solvent precipitation, to produce yellow cake, which contains 75% uranium oxide U 3 O 8. It is this yellow cake that is sold commercially. However, unlike thorium, the lower purity requires further refinement, by electrolysis or reduction. The extraction of uranium also does not retain any of the other useful compounds present in the ore, unlike many thorium extraction techniques. Another benefit is that the preparation of thorium fuel does not require isotopic separation or enrichment as uranium fuel does. This is a useful economic advantage that will be discussed in the next section. 3.3 Economics When considering economic factors in the design and operation of a nuclear reactor, total life cycle costs must be considered. Aspects include capital costs such as building, operation and decommissioning to fuel costs, level and experience of staffing required and cost and likelihood of further development lowering production costs. It is also important to compare with other competitive sources of energy. 24

In contrast to fossil fuels, a small amount of thorium fuel can produce a large amount of energy. A single tonne of thorium can provide enough energy, 1GW/year when used in a LFTR, to power an entire city. The fact that less fuel is required for a given output drastically lowers the cost, for example for an LFTR, thorium would cost $0.00004/kWh, compared to the $0.03/kWh for coal. [21] While lower weights of thorium are needed to fuel a reactor, thorium is relatively expensive, at $5,000/kg due to lack of demand. Currently, uranium fuel is $20/kg unprocessed, directly from the ore. However, unless the uranium is to be used in a CANDU reactor, it must be enriched, which adds greatly to the cost of the fuel. Enriched fuel in the form of uranium oxide ready to be used in a reactor is priced at $1633/kg. [18, 22] Table 9 in the Appendix shows the costs of different methods of uranium enrichment. This is still lower than the cost of thorium in terms of weight/$. When looking towards the future, cost of uranium will increase dramatically as sources in the Earth s crust are depleted, while the cost of thorium is expected to decrease to a possible $10/kg if mined as extensively as uranium currently is. However, once uranium reserves in the Earth s crust have been depleted, uranium would have to be extracted from sea water, which is a much more expensive process. Research in Japan has found a technique for extracting uranium from sea water at costs of $100 300/kg. [18] Even now with thorium being more expensive by weight, the lower consumption ensures that fuelling a power plant will actually be cheaper. Using a 1GW plant as an example, a uranium plant would cost $30 million/year to fuel while a thorium plant would only cost $1 million/year. Other operating costs for a comparable thorium reactor are on average $50 million/year, again less expensive than a uranium reactor. Capital costs such as building and staffing are also cheaper for thorium reactors. Again, using a 1GW power plant in both cases, a uranium reactor would cost about $1.1 billion to construct, while building a similar thorium reactor would cost only about $250 million. This substantial difference is because thorium reactors do not require a containment shield to protect from meltdowns. They will also be low maintenance, reducing staffing costs from $50 million/year for a uranium reactor to a possible $5 million/year in the future. [22] 25

The only process in which the thorium cycle is more expensive is the closed cycle with reprocessing. Due to the high levels of radioactivity from the daughter products of 232 U and small amounts of 228 Th and 234 Th, remote handling is necessary. Reprocessing is used in order to collect the remaining 232 U for reuse. Because 232 U is a product of the decay of 232 Pa, which has a relatively long half-life of 27 days, the spent fuel must be stored on site for at least 9 months. This is to ensure that the decay heat decreases to a safe level and so that little 232 U is lost from the decay of 232 Pa which has been separated and disposed of. This storage of highly reactive material that requires heavy shielding on-site and the remote handling causes reprocessing for a thorium closed cycle to be more expensive than the U Pu closed cycle. However, uranium reprocessing has other problems such as proliferation, which will be discussed later in Chapter 5.1. 26

Chapter 4 Statistics of current nuclear reactors This chapter will look at the statistics and operation of reactors from different countries, designed for a range of purposes and with a range of power outputs. The Table below details various statistics that will form the basis of the discussion. There is a further Table 10 in the Appendix which gives details of common fuel composition and element design for each type of reactor in. Table 4. Statistics of various reactors which use a thorium fuel cycle. 27

Thorium has been used for commercial power in a few Generation II reactors since the 1970s, although they are all decommissioned or in the process of being so. Many of the currently operational reactors have much smaller power outputs as they are being used in research or, in the case of the LMFBR in India, in the burning of weapons grade plutonium. Originally, thorium fuel was developed purposely for the HTGRs, but lately research has been ongoing on the use of thorium in Generation IV reactors. Commercial reactors have been successfully been used in the U.S., U.K. and India. The Dragon, one of the earlier thorium reactors in the U.K. successfully ended its rather short lifetime, however the cost of decommissioning was underestimated, leaving the power-plant only partially decommissioned in 2005 due to lack of money. It was expense and the political climate of the time that prevented the construction of further commercial reactors after the experimental Dragon. The commercial reactor HTGR in Germany was not as successful. The testing of the experimental reactor AVR worked well and safely, but the follow on commercial reactor was shut down early after only four years in operation. The reason cited was increased cost; however an obstruction in the fuel which caused a leak of radioactive dust into the environment a few months after the Chernobyl accident most likely had a role in the decision. Uranium fuelled reactors built for commercial power typically have outputs ranging from 500MWe 1GWe. While greater than those shown in the Table on the previous page, most of those statistics were from the 1970s 1980s. Much development has occurred since in the area of thorium reactors, so they now have similar power outputs of 1GW. 28

Chapter 5 Nuclear reactor products 5.1 Non-proliferation One of the nuclear industry s main aims for the generation IV reactors is to increase the proliferation resistance of the commercial nuclear fuel cycle. This requires a reduction in the amount of fissile material that is either weapons-grade or could be processed to into weaponsgrade. The main concerns are misuse of materials and facilities and also the spread of skills and technology needed to produce nuclear weapons. The two main concerns stated in the Nuclear Non-Proliferation Treaty, NPT, are 239 Pu and enriched uranium. The uranium used in reactors is not a great cause for concern, as the enrichment necessary to make the size of a nuclear bomb feasible is greater than 60%; much greater than the 3% - 5% enrichment typically used in a nuclear reactor. Most plutonium produced by reprocessing in a uranium reactor is not weapons grade due to a significant concentration of 240 Pu, produced by electron capture of 239 Pu, and small amounts of other plutonium isotopes. These isotopes with an even mass number can spontaneously fission, causing the weapon to denature. It is possible, however, to produce WPu through short fuelling cycles, which would be easily noticed by the international community due to frequent shutdowns. The purest form of 239 Pu is produced in fast breeder reactors, which produces more plutonium than it consumes. However, 233 U itself can be used as a nuclear weapon material. Depending on the weapon, a minimum of only 5 kg is necessary. Using a sphere for example 233 U has a critical mass of 16kg, in comparison 239 Pu; 10 kg and 235 U; 48kg. [22] Thorium fuel cycles have two main advantages over the uranium main U Pu cycles: a decrease in the amount of plutonium in spent fuels and an increase in the burn-up of plutonium already in existence. There are other intrinsic properties which help in preventing 29

the spread of other potential weapons material which are only significant in the spent fuel from a thorium reactor, so as not to offset these two advantages. Almost all thorium reactors have the advantage of producing less plutonium than most uranium reactors. Usually, a small amount of plutonium is present in the spent fuel, but it is possible, using pure 233 U with thorium, that no plutonium is produced. However, pure 233 U Th fuel is rarely used as it does not provide the best results in terms of burn-up and efficiency. One of the better reactors in terms of both performance and non-proliferation is the Thermal TMSR. As is shown in Table 5 in the Section 5.1, the plutonium production is a 100 th of what it would be in a PWR. [23] Even then more than 60% of the plutonium produced in any MSR is the 138 Pu isotope, so MSRs are some of the best reactors to impede the proliferation of nuclear weapons from the spent fuel they produce. Plutonium can also be used in greater amounts in each cycle in thorium reactors to remove it from circulation. Un-needed WPu can be burned in thermal reactors, but much more efficiently in high temperature reactors. Plutonium Disposition 1200 Kilograms Per Year 1000 800 600 400 Plutonium Discharged at Spent-Fuel Standard Plutonium Burned 200 0 MOX Fuel Type Thorium Figure 9. A graph comparing the amounts of plutonium burnt and produced when used in two different types of fuel. 30

Due to a combination of increased burn-up and less waste produced, the total amount of plutonium released is reduced to a seventh of what would be expected from a reactor using MOX fuel. As a result of the higher burn-up of thorium fuels, the amount of fissile material present in the spent fuel is decreased by a factor of 2 4. It is also very easy to denature the 235 U in the spent fuel by mixing it with 238 U, thereby making it proliferation resistant. The disadvantage of this approach is the increase in radioactivity in the denatured fuel when it is recycled. 233 U separated from spent fuel is always contaminated with small amounts of 232 U, which has a half-life of 69 years. Alpha decay produces 208 Tl, which is a strong gamma emitter. It has a half-life of three minutes, producing a penetrating and therefore easily detected and distinguishable gamma rays of 2.6 MeV. Because 232 U cannot be separated by chemical means from 233 U, the passive detection of 233 U is simple in comparison to the detection of spent fuel containing only uranium and plutonium, as the uranium decay chains do not contain any strong and easily detectable emitters. 5.2 Waste Radioactive waste is divided into three main categories: High Level Waste, HLW, Intermediate Level Waste, ILW, and Low Level waste, LLW. The categories are assigned by the level of radioactivity for a given weight or volume. The amounts of each type of waste produced are very important, as the method and cost of disposal, and necessary space for the disposal vary considerably, as can be seen from Figure 10. ILW, such as piping from the reactor and fuel cladding, and LLW, tools and equipment such as overalls, are similar for each type of fuel cycle, as the overall design of reactor and power plant does not vary significantly. 31

Figure 10. A diagram showing the timescale of the planning and disposal of different levels of radioactive waste. The main and most important difference between thorium and uranium cycles is the High Level Waste. It is the most expensive to dispose of due to the treatment necessary and there are currently limited resources available to store the waste. HLW comprises of mostly spent fuel. Despite having a different composition, the method of disposal after reprocessing is very similar for both thorium fuel and UO 2 fuel. As thorium in the ThO 2 form is in its highest oxidation state, temporary storage above ground is not such an issue as it is with UO 2, which can be oxidised. It is also chemically inert and will not dissolve in water, preventing leaching into the groundwater if stored underground. This together makes thorium a safer fuel for geological disposal than uranium. Spent fuel also contains radioactive fission products with half-lives ranging from less than 100 years to above 200,000 years. The major products in spent fuel with the highest radioactivity are the actinides. As shown in Tables 5 and 6, a thorium fuel cycle produces much less of the minor actinides in both open and closed cycles. 32

Thermal Epithermal TMSR TMSR Fast TMSR FNR PWR NP 29 110 150 70 91.5 Pu 38 260 270 12,550 3,850 Am 3.1 7.1 14.8 528 248 Cm 14.1 18 2.4 135 124 Table 5. A comparison of the relative weights of the major actinides produced by various reactors in kg in a closed cycle. Thermal Epithermal TMSR TMSR Fast TMSR FNR PWR NP 7 25 9.7 23 102 Pu 1.9 2.8 0.6 12,250 1,420 Am 4.00E-04 5.00E-04 7.00E-07 192 86 Cm 1.00E-03 1.00E-04 2.00E-08 15 14 Table 6. A comparison of the relative weights of the major actinides produced in kg in an open cycle. As well as comparing the amount of radioactive waste produced, it is also important to compare the radio toxicity of the waste. The graphs below show how the radio toxicity of both major and total actinides waste from a thorium cycle decreases over the years in comparison to different uranium based cycles. The major actinides are uranium and plutonium. The important minor actinides in spent fuel are neptunium, americium, curium and californium. Overall, fewer transuranic elements, with atomic number greater than 92, none of which are stable are produced from the thorium cycle. From uranium fuel, a single neutron capture is able to produce a transuranic element. This is a very favourable interaction in comparison to the fission cross section at thermal energies. Thorium, on the other hand, would require six consecutive captures, passing through 233 U and 235 U, which both have high fission crosssections at both thermal and fast energies. Therefore it is much more likely that the nuclei 33

would fission rather than capture a neutron, resulting in far fewer long-lived transuranic elements. A single neutron capture in 238 U is sufficient to produce transuranic elements, whereas six captures are generally necessary to do so from 232 Th. 98 99% of thorium-cycle fuel nuclei would fission at either 233 U or 235 U, so fewer long-lived transuranics are produced. Because of this, thorium is a potentially attractive alternative to uranium in mixed oxide (MOX) fuels to minimize the generation of transuranics and maximize the destruction of plutonium. Figure 11. Graphs showing haw the radio-toxicity of all actinides (above) and the major actinides (below) varies with time. [23] 34

By comparing the levels of radio-toxicity on both graphs, it can be seen that for the thorium cycle, a significant amount of the radioactivity is due to the major actinides. The graph shows that, for all actinides that the radio-toxicity for thorium cycles is always less than that of an open cycle for natural uranium and radioactivity levels decrease much faster. The radioactivity of actinide waste from thorium is also less than from an enriched uranium cycle, apart from a period between 5x10 4 and 10 6 years. It is an advantage of both the thorium and uranium closed cycle over the natural uranium open cycle that the radioactivity decreases faster to an acceptable level to be retrieved from storage as there is limited space available for the disposal of radioactive waste. While thorium-based fuel cycles in general produce much less plutonium minor actinides than a uranium fuel cycle, they do produce other radionuclides which also have long halflives, such 231 Pa, 229 Th and 230 U, which have significant impact on the radioactivity of the waste. 35

Chapter 6 Summary of results With the energy demand increasing, new options are necessary to supply the deficit left by traditional fossil fuels. Due to increased concerns for safety and the environment, a clean, significant and sustainable source of energy must be developed. There are few options available; a thorium fuelled nuclear reactor seems to be one of the better options. Nuclear power in general has many advantages over other possible sources of energy and the thorium fuel cycle has itself many advantages over the currently more common uranium cycles. The thorium fuel cycle is safer than the uranium cycle. Meltdown is not a danger so a containment shielding is not necessary. The thorium fuel, ThO 2, itself is stable, cannot be oxidised and has a higher melting point than UO 2 fuel. The uranium is most often used in the form of UO 2, however, even at room temperature it will gradually convert into U 3 O 8, so it preferable that fabricated uranium fuel not remain in storage for too long. Uranium fuel will also oxidise, especially if it comes into contact with water, while thorium is relatively unaffected. Thorium nuclear fuel is a good choice for future use. Extensive reserves ensure that this option will be available for thousands of years, making the cost of any development a long term investment with big returns. Developments for the uranium fuel cycle will allow the production of energy for over a century, but then reserves would run out and another new solution, with more development costs would be necessary. Whilst practical knowledge and experience of the thorium cycle is not yet at a level with uranium, a decade or so of research and experience will put thorium at a level or above that of uranium. Thorium also has the advantage over uranium in capital costs. Building and running a thorium power plant is cheaper than a uranium plant. Currently the extraction of thorium fuel is more expensive, due to little demand, but should demand come to equal that of uranium prices will drop to below that of current uranium prices, and much less than what would be expected for uranium extracted from sea water. The only cost aspect in which uranium is less 36

expensive than thorium is the reprocessing as remote handling is necessary. Overall, thorium fuel in the future has the advantage over uranium in terms of costs. The concerns over the environmental impact of nuclear waste, along with the proliferation, are the major aspects of both public and international scrutiny. However, despite the high levels of radioactivity present in some waste, there is little other effect on the environment, unlike the fossil fuels whose combustion sends large amount of greenhouse gasses and ash into the environment. With careful treatment and disposal, the nuclear waste is of no danger, but there is limited space available for the storage of HLW. Thorium fuel holds the advantage over uranium as, with a higher burn-up and less fuel needed to run a cycle, less waste is produced per MWe of energy. Although proliferation is a significant problem for uranium fuel cycles, thorium fuel cycles can be used to reduce the amount of both Weapons and Reactor grade plutonium. Both uranium and thorium fuel cycles can be used to burn plutonium; the thorium fuel cycles on average burn three times the amount. The fissile waste that is produced is easily detectable due to the gamma radiation from the 232 U. This provides a significant barrier in the physical distribution of fissile material from the thorium cycle that is not present in the spent fuel of a uranium cycle. While the 232 U does provide an inherent proliferation resistance, the same radioactivity increases both the cost and complexity of fuel fabrication and reprocessing by the necessity of remote handling. The thorium fuel also contains 233 Pa. The long-lived isotope has a longer half-life than its parent, so it builds up in the reactor. Because it decays into a fissile isotope, the spent fuel must be stored on average nine months longer than uranium spent fuel until the decay heat is reduced to safe levels. 37

Chapter 7 Conclusions and Recommendations With the decreasing reserves of fossil fuels, it is necessary to choose and develop another, secure supply of power now. Sources of natural power are not sufficient to fill the growing demand. Uranium powered reactors could be developed to produce all the power needed for at least a century, until reserves run out, placing us back in the same position we are now. While thorium fuelled nuclear reactors are not as developed as uranium nuclear reactors, the potential life-span of thorium power is much greater than that of uranium. Thorium has many advantages over uranium: it is sustainable, safer, has higher burn-up, produces a lower mass of waste with lower radio-toxicity, is cheaper to build and run the power plants, and most importantly is inherently proliferation resistant. Thorium extraction is currently more expensive than uranium extraction, but increased demand would bring the costs down to the level of uranium. The only unalterable disadvantage is that remote handling is required for reprocessing and fabrication. Because of the proliferation resistance, those countries with well developed nuclear power can form agreements to help countries wanting nuclear power. Countries with the greatest knowledge of nuclear power, especially the thorium fuel cycle can help countries that wish to develop nuclear power with technology and knowledge, if the countries agree to only use thorium fuel. This could stop some of the many proliferation worries. An example is the recent concerns resolving around the announcement that Iran is currently fuelling its first nuclear reactor and planning on enriching uranium up to 20%. Many of the countries that hold large reserves of thorium do not currently have the capacity for nuclear power, so agreements could be formed to trade knowledge or subsidising developments for mining rights. Overall, this report shows that, once fully developed, thorium fuel has many advantages over uranium, making initial investments to ensure such development are well worthwhile and the best possible choice for the future power supply. 38

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Appendix Figure 12. A diagram of the thorium and uranium decay chains. 42

U.S. Average Levelised Costs (2008 $/MWh) for Plants Entering Service in 2016 Plant Type Total Capacity Fixed Variable Transmission Capital System Factor (%) O&M O&M Investment Costs Conventional Coal 85.0 69.2 3.8 23.9 3.6 100.4 Advanced Coal 85.0 81.2 5.3 20.4 3.6 110.5 Advanced Coal with CCS 85.0 92.6 6.3 26.4 3.9 129.3 Advanced Nuclear 90.0 94.9 11.7 9.4 3.0 119.0 Wind 34.4 130.5 10.4 0.0 8.4 149.3 Wind - Offshore 39.3 159.9 23.8 0.0 7.4 191.1 Solar PV 21.7 276.8 6.4 0.0 13.0 396.1 Solar thermal 31.2 224.4 21.8 0.0 10.4 256.6 Geothermal 90.0 88.0 22.9 0.0 4.8 115.7 Biomass 83.0 73.3 9.1 24.9 3.8 111.0 Hydro 51.4 103.7 3.5 7.1 5.7 119.9 Table 7. Relative costs of different methods of energy production. Nuclear Data 232 Th 233 U 235 U 238 U 239 Pu 241 Pu Neutron Yield - 2.48 2.43-2.87 2.97 Delayed Neutron Yield - 0.0031 0.0069-0.0026 0.0050 Neutron/fission average - 2.5 2.4 - - 2.9 Table 8. A comparison of the neutron yield for different fissile isotopes. 43

Characteristics Gaseous diffusion Centrifuge Jet-nozzle Advanced vortex tube Separation factor 1.004 1.2 1.5 1.010 1.030 1.015 Specific power consumption (kwh/kgswu) 2050 2500 200 400 2400 4400 3300 Minimum economic capacity (tonnes SWU/year) 9000 1000 2500 > 5000 Specific investment costs ($/kgswu/year) 145 300 145 300 150 ~200 Construction time (years) 6 3 4 5 6 Table 9. The costs of different methods of uranium enrichment. 44

Reactor Type Composition Fuel shape Fuel element High temperature ThO 2, (Th, U)O 2, ThC 2 (Th, U)C 2 Microspheres 200-800µm diameter coated with multiple layers of Mixed with graphite and pressed into larger spheres ~60mm or fuel rods buffer and pyrolitic carbon and SiC Light water ThO 2, (Th, U)O 2, (Th, Pu)O 2 (<5% Pu, High-density Sintered Pellets Zircaloy clad Pin Cluster encapsulating Pellet-Stack 235 U or 233 U) High-density microspheres Zircaloy clad Pin Cluster encapsulating fuel microspheres PHWR ThO 2 - - AHWR (Th, U)O 2, (Th, Pu)O 2 (<5% Pu, 235 U High-density Sintered Pellets Zircaloy clad Pin Cluster encapsulating Pellet-Stack or 233 U) Fast ThO 2 blanket, (Th, U)O 2, (Th, Pu)O 2 (~25% Pu, 235 U or U) fuels High-density Sintered Pellets Stainless steel (SS) clad Pin Cluster encapsulating Pellet- Stack Th metal blanket, Injection-cast fuel rods SS clad Pin Cluster Th-U-Zr and Th-U- encapsulating Fuel Rods Pu-Zr fuels Molten salt Li 7 F, BeF 2, ThF 4 and Molten salt in liquid Circulating molten salt acting as breeder UF 4 form fuel and primary coolant Table 10. Fuel composition and element structure for different reactor types. 45