Long Term Sustainability of Nuclear Fuel Resources

1 Long Term Sustainability of Nuclear Fuel Resources Dubravko Pevec, Vladimir Knapp and Krešimir Trontl University of Zagreb, Faculty of Electrical Engineering and Computing Croatia 1. Introduction The basic issue in considering the contribution of nuclear power to solving the world’s energy problem in the future is the availability of uranium resources and its adequacy in meeting the future needs of nuclear capacity. Increased interest in nuclear energy is evident, and a new look into nuclear fuel resources is relevant. Sufficiency of nuclear fuel for the long-term use and expansion of nuclear power has been discussed by individual analysts and by institutions, with wide spectrum of answers corresponding to variety of initial assumptions on uranium resources, reactor technologies and energy strategies (Fetter, 2009; Nifenecker, 2003; Pevec et al., 2008). With a suitable choice of assumptions arguments were occasionally constructed for the claim that nuclear power has no long-term future due to inadequate fuel resources. Oppositely, again with appropriate choice of assumptions, reassuringly long times of nuclear fuel availability were obtained, even with inefficient once-through open nuclear fuel cycle. Typical such scenarios assume extension of present slow growth of nuclear power or assume a constant share of nuclear power in the total world energy production, now slightly above 6%. With once-through fuel cycle resources then may last well over a hundred years, as will be shown below, and, argument goes on, by that time we will have nuclear fusion, so there is no reason for concern about nuclear fuel. At present state of world affairs we cannot afford the comfort of such reasoning as it neglects the outstanding potential of nuclear energy to contribute to the solution of the probably crucial problem facing humanity; how to stop climate changes threatening our civilisation, and how to do it urgently. Unlike various alternative CO2 non-emitting energy sources, fission energy is technically developed and available now, as witnessed by close to 430 reactors in operation (Knapp et al., 2010). The nuclear energy has some characteristics different from fossil fuel energy which are very important when considering the long term sustainability of nuclear fuel resources. First, unlike in the case of fossil fuels, the amount of energy obtainable from the resources per unit mass of nuclear fuel is far from being a fixed figure. Energy content of a kg of the standard coal is 29. 3 MJ. It is usable with high percentage of this figure for heating and with 30-40% if converted to electricity. Energy that can be obtained from a kg of natural uranium is highly dependent on the reactor type and on the nuclear fuel cycle. Presently dominant are so called thermal reactors. Their physically most important feature is that they fission practically only uranium isotope U235 which is present in natural uranium in only 0.7%. By 2 Advances in Nuclear Fuel presence of moderator in the core of these reactors fission neutrons are thermalised and thereby fission probability of U235 is increased by a large factor. Due to their even-even proton-neutron structure U238 nuclei can be fissioned only by fast neutrons. However, they can absorb slow neutrons and through two decays then U239 transform into a fissionable isotope of plutonium, Pu239, with properties similar to those of U235. As U235, it is fissionable by thermal neutrons and energy release per fission only slightly higher, some 2%. Consequently, in thermal reactors by neutron absorption a small amount of U238 is converted in plutonium, mainly Pu239. Plutonium is partly burnt in parallel with U235 and partly remaining in spent fuel. The thermal energy obtained per unit mass of the fuel in present thermal reactors is given in Table 1. Much the largest part of dominant isotope U238 remains unused. If plutonium is extracted from the spent fuel it can be added to the fresh fuel thereby increasing the amount of energy obtained from the unit weight of natural uranium. As the content of U238 in uranium is 99.3%, clearly a dramatically larger quantity of energy would be obtained if the energy of this isotope could be released (Bodansky, 2004). Fuel Nat. uranium Enriched U Plutonium Enrichment 0.711% 3.5% 100% * from fission of U235 only 584 GJ/kg 2870 GJ/kg 82100 GJ/kg Energy per Unit mass 6.8 MWd/kg * 7.3 MWd/kg ** 33.3 MWd/kg * 36-40 MWd/kg ** 950 MWd/kg ** in reactor, with contribution from plutonium Table 1. The thermal energy obtained per unit mass in present thermal reactors Second, contribution of uranium cost to the cost of nuclear-generated electricity is low (24%) compared to contribution of fossil fuel cost to the cost of electricity of fossil power plant (25% for coal and 65% for gas). It follows that, even for conservative approach of 4% uranium cost contribution to electricity cost, five-fold increase in uranium cost would increase the cost of electricity by 16%, and ten-fold increase in uranium cost would have modest effect on the cost of electricity by increase of 36%. It will be shown that these large increases in uranium price would produce much larger increases in available uranium resources. These uranium resources will be sufficient to support an inefficient once-through fuel cycle till the end of the century and beyond, even in the case of rapid nuclear capacity growth. Third, the operational lifetime of nuclear power plants is considerably longer than fossil power plant operational lifetime. The operational lifetime of current nuclear power plants is 40-60 years, and for Generation III nuclear power plants it will be 60-80 years. Therefore, the changes in nuclear fuel utilization will slowly change for long time periods. In this chapter we address the issue of nuclear fuel resources long term sustainability in relation to the expected and projected high limit of growth of the world nuclear power. Three main aspects have to be analyzed in order to estimate how long the world’s nuclear fuel supplies will last: nuclear fuel resources (uranium and thorium), technologies for nuclear fuel utilization, and energy requirements growth scenarios including different scenarios for nuclear share growth. In the second section of this chapter conventional and unconventional uranium and thorium resources were presented and discussed. Figures given are valid for particular moment of Long Term Sustainability of Nuclear Fuel Resources 3 time, with the rate of change of estimates dependent on the intensity of research and exploration. Detailed analysis of potential technologies for improved nuclear fuel utilization is required in order to assess long term sustainability of nuclear fuel resources. Nowadays, thermal converter reactor technology with once-through nuclear fuel cycle is dominant. The effectiveness of the technology can be improved in the area of enrichment process as well as by introducing reprocessing of the spent fuel on larger scale. Other technologies are also on the development stage that allows their implementation in short or medium period of time. These include: thermal and fast breeder reactors of different kind, thorium based fuel cycle, and conversion of uranium or thorium by particle accelerators or fusion devices. The potential technologies for improved nuclear fuel utilization are analyzed in the third section. Very important aspect of long term sustainability of nuclear fuel resources are scenarios for energy requirements growth, and scenarios for growth of nuclear share in electricity production resulting in overall nuclear capacity growth. The low growth scenario, the high growth scenarios with exponential and linear increases, and the scenario based on a compromise between low and high growth assumptions are presented in the fourth section. The long term sustainability of nuclear fuel resources is discussed in fifth section, and the conclusions are given in the sixth section. 2. Nuclear fuel resources Uranium, as well as thorium, can be used as a nuclear fuel. Uranium is relatively abundant element in the upper earth’s crust with the average content of 3 ppm. Uranium is a significant constituent of about hundred different minerals, but most minable ores belong to a dozen minerals (e.g. uraninite, davidite, uranothorite, carnotite, torbernite, autunite, etc.). Usually, uranium deposits are classified into four types: vein-type deposits, uranium in sandstones, uranium in conglomerates, and other deposits (pegmatites, phosphates). The existing nuclear power reactors use uranium as a fuel. Uranium is natural element composed mainly of two isotopes U238 (99.27%) and U235 (0.72%). As the existing nuclear power reactors are thermal reactors, the bulk of the produced energy is obtained by fission of U235 isotope. Thorium is three times more abundant element than uranium in the upper earth’s crust with the average content of 6 - 10 ppm. Thorium is widely distributed in rocks and minerals, usually associated with uranium, elements of the rare-earth group and niobium and tantalum in oxides, silicates and phosphates. Thorium is natural element composed of only one isotope Th232 (100%). Although the Th232 isotope is not fissile, it can be converted to fissile isotope U233 by slow neutron absorption. 2.1 Uranium resources Uranium resources are broadly classified as either conventional or unconventional. Conventional resources are those that have an established history of production where uranium is a primary product, co-product or an important by-product. Very low grade 4 Advances in Nuclear Fuel resources or those from which uranium is only recoverable as a minor by-product are considered unconventional resources. Resource estimates are divided into separate categories according to different confidence level of occurrence, as well as on the cost of production. 2.1.1 Conventional uranium resources The Red Book published in 2010 (Organization for Economic Co-operation and Development Nuclear Energy Agency [OECD/NEA] & International Atomic Energy Agency [IAEA], 2010) categorizes conventional uranium resources into Identified resources (corresponding to previously "Known conventional resources") and Undiscovered resources. Identified resources consist of reasonably assured resources and inferred resources. Reasonably Assured Resources (RAR) refers to uranium that occurs in known mineral deposits of delineated size, grade, and configuration such that the quantities which could be recovered within the given production cost ranges, with currently proven mining and processing technology can be specified. RAR have a high assurance of existence and they are expressed in terms of quantities of uranium recoverable from minable ore. Inferred Resources (IR) refers to uranium, in addition to RAR, that is inferred to occur based on direct geological evidence, in extension of well-explored deposits, or in deposits in which geological continuity has been established but where specific data, including measurements of the deposits, and knowledge of the deposit's characteristics, are considered to be inadequate to classify the resource as RAR. The estimates in this category are less reliable than those in RAR. IR is corresponding to Estimated Additional Resources Category I (EARI) used up to the year 2003. IR is expressed in terms of quantities of uranium recoverable from minable ore. Undiscovered resources include Prognosticated resources and Speculative resources. Prognosticated Resources (PR) refers to uranium, in addition to Inferred Resources, that is expected to occur in deposits for which the evidence is mainly indirect and which are believed to exist in well-defined geological trends or areas of mineralisation with known deposits. Estimates of tonnage, grade and cost of discovery, delineation and recovery are based primarily on the knowledge of deposit characteristics in known deposits within the respective trends of areas and on such sampling, geological, geophysical or geochemical evidence as may be available. The estimates in this category are less reliable than those in IR. PR is corresponding to Estimated Additional Resources Category II (EAR-II) used up to the year 2003. PR is expressed in terms of uranium contained in minable ore, i.e., in situ quantities. Speculative Resources (SR) refers to uranium, in addition to Prognosticated Resources, that is thought to exist, mostly on the basis of indirect evidence and geological extrapolations, in deposits discoverable with existing exploration techniques. The location of deposits envisaged in this category could generally be specified only as being somewhere within a given region or geological trend. Existence and size of such resources are speculative. SR is expressed in terms of uranium contained in minable ore, i.e., in situ quantities. The Identified resources amount to 6.306 million tonnes (4.004 million tonnes of RAR and 2.302 million tonnes of Inferred resources). The Undiscovered resources amount to 10.401 5 Long Term Sustainability of Nuclear Fuel Resources million tonnes (2.905 million tonnes of Prognosticated resources and 7.496 million tonnes of Speculative resources). These estimates refer to uranium recoverable at cost of less than 260 USD/kg. Total conventional resources amount to 16.707 million tonnes according to Red Book as of January 2009. The Identified conventional resources for different cost ranges are given in Table 2. The Undiscovered conventional resources for different cost ranges are given in Table 3. Cost ranges Resource category < 40 USD/kgU < 80 USD/kgU < 130 USD/kgU < 260 USD/kgU 796 3742 5404 6306 570 2516 3525 4004 226 1226 1873 2302 Identified Resources (Total) Reasonably Assured Resources (RAR) Inferred Resources (IR) Table 2. Identified conventional resources for different cost ranges in the year 2009 (1000 tU) Resource category Undiscovered Resources (Total) Prognosticated Resources (PR) Speculative Resources (SR) Cost ranges < 130 USD/kgU < 260USD/kgU 6553 10401 2815 2905 3738 7496 Table 3. Undiscovered conventional resources for different cost ranges in year 2009 (1000 tU) Countries with major uranium resources are Australia, Kazakhstan, Russian Federation, Canada, Niger, South Africa, USA, Namibia, and Brazil. 2.1.2 Unconventional uranium resources Unconventional uranium resources (Barthel, 2007) are found in low grade deposits, or are recoverable as a by-product. Low grade uranium deposits in black shales, lignites, carbonatites and granites were expected to be potential sources in the past. However, developing a cost effective, environmentally acceptable means of uranium extraction from this potential source remains a challenge. By-product resources are of interest in the case that conventional resources are insufficient. In by-product recovery, the greatest portion of the costs is borne by the main products. The most important unconventional uranium resources reported in Red Book 2010 (OECD/NEA & IAEA, 2010) are phosphate deposits and seawater. 2.1.2.1 Phosphate deposits At higher cost, uranium can be extracted from phosphate deposits. Uranium contained in phosphate deposits is estimated at 22 million tonnes, although annual production is limited 6 Advances in Nuclear Fuel by annual phosphoric acid production. The upper limit is below 10 000 t/year, so even if all the phosphoric acid production over time were considered, the total addition would not exceed one million tonnes. The historical operating costs for uranium recovery from phosphoric acid range from 60 to 140 USD/kgU (World Information Service of Energy [WISE], 2010). Recently, a new process (PhosEnergy) is being developed by Uranium Equities Limited, offering uranium recovery costs in the range from 65 to 80 USD/kgU. Design and construction of the demonstration plant is complete. It is expected to be in operation from late 2011 (World Nuclear Association [WNA], June 2011b). However, should uranium extraction, decoupled from phosphoric acid production, cost less than 200 USD/kgU an abundant addition to conventional resources would become available. We do not assume that this will happen much sooner than 2060 and, thus, base our consideration on estimated conventional resources. 2.1.2.2 Uranium from the seawater The uranium concentration in seawater is only 0.003 ppm, yet it can be extracted. It would require the processing of huge volumes of seawater (about 350 000 t water for 1 kg U) and use large amounts of absorber. The cost of extraction from seawater can be regarded as the upper limit of the cost of uranium. The quantity of uranium in the sea is about 4 billion tonnes, exceeding any possible needs for thousands of years. Research on uranium recovery from seawater has been going in Germany, Italy, Japan, the United Kingdom and the United States of America in 1970s and 1980s, but is now only known to be continuing in Japan. Recent Japanese research showed that uranium extraction from the seawater is technically possible. It has been developed on a laboratory scale by using either resins or other specific adsorbent. An extraction cost as low as 250 USD/kg U has been estimated, which is more than twice as high as the present spot market price (Tamada et al., 2006). Although this price appears high, and certainly is, it could be acceptable for fast breeders with a closed fuel cycle. 2.2 Thorium resources The principal sources of thorium are deposits of the placer type (concentrations of heavy minerals in coastal sands), from which monazite and other thorium bearing minerals are recovered. Thorium often occurs in minerals that are mined for another commodity and thorium being recovered as a by-product. Thorium is present in seawater with a concentration of only about 0.00005 ppm, due primarily to the insoluble nature of its only oxide, ThO2. Thus the recovery of thorium from seawater is not a realistic option. Estimates of thorium resources have been given in Red Books since 1965. Classification of thorium resources is similar to uranium, e.g. Reasonably Assured Resources (RAR) and Estimated Additional Resources (EAR). The EAR is separated into EAR-Category I and EAR-Category II according to different confidence level of occurrence. Identified resources consist of RAR and EAR-I. Prognosticated thorium resources are EAR-II. Thorium resources were also classified according to cost of recovery (OECD/NEA & IAEA, 2010). The total world thorium resources, irrespective of economic availability, are at present estimated at about 6 million tonnes. The thorium resources recoverable at a cost lower than 7 Long Term Sustainability of Nuclear Fuel Resources 80 USD/kg are estimated at 4.5 million tonnes. The identified thorium resources amount to 2 million tonnes and the prognosticated thorium resources amount to 2.5 million tonnes. Countries with major thorium resources are Commonwealth of Independent States (former Soviet Union countries), Brazil, Turkey, USA, Australia, and India. Due to the fact that thorium is roughly three times more abundant than uranium in the earth’s crust and that exploration of thorium resources is poor, it is to be expected that ultimately recoverable thorium resources will be much higher than uranium resources. 2.3 Long term perspectives of nuclear fuel resources The nuclear fuel resources given in preceding sections are the today’s resource estimates published in the Red Book, compendium of data on uranium and thorium resources from around the world (OECD/NEA & IAEA, 2010). It is interesting to compare resource estimates over time (OECD/NEA, 2006). The evolution of Identified Resources, RAR, and EAR-I/IR over time (1973 – 2009) recoverable at cost of less than 130 USD/kg is shown in Fig. 1. 6000 1000 tU 5000 4000 3000 2000 1000 19 73 19 76 19 77 19 79 19 82 19 83 19 86 19 88 19 89 19 91 19 93 19 95 19 97 19 99 20 01 20 03 20 05 20 07 20 09 0 Year RAR EAR-I/IR Identified Resources Fig. 1. Changes in Identified Resources, RAR, and EAR-I/IR over time (1973 – 2009) The Identified Resources (including its components RAR and EAR-I/IR) mainly increased during a given time period except for a drop in year 1983. This drop could be explained by the facts that in year 1983 EAR have been subdivided into Category I and Category II and since 1983 RAR and EAR-I are given as recoverable resources(mining and milling losses deducted). The Identified Resources increased by around 60% in a time period of almost 40 years although for many years investment in exploration for uranium resources has been low. The evolution of Undiscovered Resources, EAR-II/PR, Speculative Resources (< 130 USD/kgU), and Speculative Resources (regardless of the price) over time (1985 – 2009) is shown in Fig. 2. 8 Advances in Nuclear Fuel The EAR-II/PR curve shows very gradual increase for the initial and final part of the given time period and for the rest of time period it remains nearly unchanged. That nearly unchanged part of the curve could be explained at least in part by the fact that countries tend to not re-evaluate their EAR-II/PR estimates on a regular bases. In contrast with the EAR-II/PR trends, both categories of Speculative Resources show considerably more volatility. 14000 12000 1000 tU 10000 8000 6000 4000 2000 0 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 2007 2009 Year EAR-II/PR Speculative regardless of the price Speculative <130 US$/kg Undiscovered Resources Fig. 2. Changes in Undiscovered Resources, EAR-II/PR, Speculative Resources (< 130 USD/kgU), and Speculative Resources (regardless of the price) over time (1985 – 2009) The Red Book nuclear fuel resources estimates are obtained simply by collecting data on conventional resources from IAEA countries. Many countries are lightly explored in uranium and many countries do not report resources in all categories so there are almost certainly large quantities of uranium that are not yet included in Red Book. Therefore, the Red Book estimates of uranium resources should be considered a today’s lower bound on the amount of uranium likely to be recoverable. For analysis of uranium resources long term sustainability it is necessary to estimate the amount of uranium that will ultimately prove to be economically recoverable. This amount is defined as “total recoverable uranium resources”. It depends on geologic parameters, as well as on development in technologies of exploration, extraction, and use. The total recoverable uranium resources could be determined from first principles by summarizing estimates of the abundance of uranium in the crust of the earth as a function of concentration and accessibility. Geologic data indicate that the total amount of uranium increases exponentially with decreasing ore grade. Synthesizing the power law for total amount of uranium and assumption that the cost of extracting a unit mass of uranium varies linearly with the inverse of the ore grade, one obtains a simple crustal model (Schneider & Sailor, 2008), Q P =  Q0  P0  ε 9 Long Term Sustainability of Nuclear Fuel Resources where Q = quantity (MtU) of uranium available at the price level P (USD/kg U) Q0 = quantity of uranium available at some reference price P0  = long term elasticity of uranium supply. This model must be calibrated through the selection of a reference point (P0, Q0) and the estimation of . The Red Book data (OECD/NEA & IAEA, 2010) could be used as a reasonable point of departure for extrapolation of total recoverable uranium resources estimates. Therefore, the uranium resources quantity of 0,796 MtU available at price 40 USD/kgU (Table 2) has been selected as the reference point. The long term elasticity of uranium supply, , is estimated by different groups and its values range from 2.35 to 3.5. The World Nuclear Association (WNA, September 2011) concludes that a doubling in price from present levels could be expected to create about a tenfold increase in measured resources over time. It implies the long term elasticity of uranium supply, ε, to be equal 3.32. Another serious attempt to estimate how much uranium is likely to be available worldwide, based on Deffeyes and MacGregor (Deffeyes & MacGregor, 1980) distribution of uranium in the earth’s crust, concluded that a ten-fold reduction in ore concentration is associated with a 300-fold increase in available resources. Using the assumption that costs are inversely proportional to ore grade the ε value of 2.48 is obtained. The U.S. Department of Energy Generation IV Fuel Cycle Crosscut Group (FCCG) study (United States Department of Energy [USDOE], 2002), basing itself on the amounts of uranium recently estimated to be available in the United States at 30 USD/kgU and 50 USD/kgU, predicted that that the ε might be as low as 2.35. Using the selected reference point and the obtained ε values, total recoverable uranium resources are calculated by simple crustal model for different cost ranges. The calculated values and the Red Book values given in MtU are shown in Table 4. These values range from 4 MtU for cost category of 80 USD/kgU to almost 400 MtU for cost category of 260 USD/kgU. All of these estimates suggest that the total amount of uranium recoverable at prices 130 USD/kgU and 260 USD/kgU is likely to be substantially larger than the amount reported in the Red Book. Source of estimate WNA Deffeyes and MacGregor Generation IV-FCCG Red Book ε 3.32 2.48 2.35 < 40 USD/kgU 0.796 0.796 0.796 0.796 Cost ranges < 80 < 130 USD/kgU USD/kgU 7.96 39.84 4.44 14.8 4.06 12.70 3.742 5.402 < 260 USD/kgU 397.91 82.59 64.75 6.306 Table 4. Total recoverable uranium resources estimated by simple crustal model for different cost ranges (MtU) In our further analysis it was assumed that conventional uranium resources according to Red Book as of January 2009 in amount of 16.7 million tonnes will be recovered until year 2065. Based on estimates obtained by simple crustal model we assumed that the total amount of uranium recoverable until the end of this century at still tolerable price of 180 USD/kgU is 10 Advances in Nuclear Fuel 50 Mt. This figure is supported by Update of the MIT 2003 Future of Nuclear Power study (Massachusetts Institute of Technology [MIT], 2009) in which “an order of magnitude larger resources are estimated at a tolerable doubling of prices”. 3. Technologies for improvement of nuclear fuel utilization A fuel utilization of present power reactors is low because they mainly utilize energy of U235 nuclide. Therefore, technologies and methods have been considered, that make possible to utilize enormous energy of U238 and of Th232 as well. Some of these technologies and methods are developed and proven technically viable, while some others are well researched with developmental problems identified. In the past, characterized by relatively slow nuclear energy expansion, with low cost of uranium and high cost of reprocessing of spent fuel, the simplest once-through fuel cycle has been generally accepted. Consequently, better utilization of nuclear fuel was not interesting to a private nuclear industry. Situation is different in the countries where governmental support made long term planning possible. For our purpose two aspects have to be understood. First, from the technical point of view, what these new technologies can achieve regarding uranium resources extension. Second important technical consideration is the time for their commercial development. It also has to be evaluated whether they could be introduced by the time of exhaustion of uranium resources used by present thermal reactors operating in the open cycle regime as is practice today. The following technologies and methods for improvement of nuclear fuel utilization have been considered: a. b. c. d. e. f. Plutonium and uranium recycle with thermal reactor technology Thermal breeder reactors Fast breeder reactors Zonal fuel burning in the so called „candle reactor“ Accelerator conversion of U238 into plutonium and of Th232 into U233 Conversion of U238 and Th232 by fusion neutrons The short survey of each of the considered technologies and methods is given below. 3.1 Plutonium and uranium recycle with thermal reactor technology Technology of plutonium recycle has been developing for many years. The PUREX process for recycling uranium and plutonium from spent nuclear fuel is implemented in several countries. Plutonium is mixed with enriched uranium for fabrication of the so called MOX fuel as both components are in the chemical form of oxides. There are many years of experience with the use of MOX fuel. Plutonium recycle is also a way to use surplus military plutonium. Except for such special situation, in the past there was little general interest in recycling at current high reprocessing and low uranium prices. Recent quantitative cost assessment of plutonium recycle has been given in EPRI Report 1018575 in 2009 (Electric Power Research Institute [EPRI], 2009). According to EPRI analysis fuel costs for oncethrough fuel cycle would be lower than for plutonium recycle for uranium cost below USD 312/kg and PUREX reprocessing cost above USD 750/kgHM. The same holds for uranium recycle except for some special concepts of reactors operating in tandem. The effect of plutonium and uranium recycle in present light water reactors on resources extension would not be very high; typically 5 kg of spent fuel contains enough plutonium for one kg Long Term Sustainability of Nuclear Fuel Resources 11 of fresh fuel with plutonium replacing U235. Natural uranium resources extension is of the order of around 30%, as can be seen in a number of publications and reports (Garwin, 1998). 3.2 Thermal breeder reactors Thermal breeder reactors were investigated in the early days of nuclear technology development before selection of light or heavy water cooled thermal reactors for commercial energy production. Thermal breeding is achieved either by benefiting from larger neutron yield of U233 in thermal fission, or by better neutron economy achieved by extracting neutrons absorbing fission products from the liquid fuel. First approach was investigated in the experimental Shippingport reactor. This light water solid fuel thermal breeder prototype reactor was in operation in US from 1957-1982 using uranium and thorium fuel, but the same concept could run on thorium fuel and U233 as fissile material produced by conversion of thorium. (United States Nuclear Regulatory Commission [USNRC], 2011). Other approach was also investigated in the early years of nuclear development. Small experimental molten uranium fluoride fuelled reactor (8 MW thermal power) was operated in the years 1965-69 at Oak Ridge National Laboratory in the US (Briggs, 1967; Rosenthal et.al., 1970). Using on-line extraction of fission products from circulating molten fuel, neutron losses by absorption in fuel were reduced with the effect of increasing conversion ratio above 1. Development did not proceed at the time due to corrosion problems. Latest development of this reactor type was Japanese FUJI MSR 100-200 MWe reactor. With several attractive features, such as reduced radioactivity inventory, low pressure of primary circuit, high thermal efficiency, possibility to run on thorium fuel, this concept is again taken up in a selection for Generation IV reactors. Corrosion problems were largely resolved in the meantime. Work on the molten salts technology is in progress in EU, China, India, with long interest in thorium, and other countries (Forsberg et.al., 2007; Gen. IV International Forum, 2011b). Another concept of thermal breeder is a version of Canadian heavy water reactor CANDU using U233 as fissile material and thorium as fertile material. Commercial use of this fuel cycle, usable with little additional technical development required, depends on the costs of uranium and reprocessing of thorium for extraction of U233, and is ruled out at present uranium and reprocessing costs. 3.3 Fast breeder reactors Concept of fast breeder reactor developed in early days of nuclear energy uses the physical property of Pu239 which when fissioned by fast neutrons releases considerably more fission neutrons than U235 or U233 fissioned at low or high neutron energy. Thus in reactor with Pu239 as fissile material and U238 as fertile, and with little or no moderation to avoid degradation of high neutron energy, conversion coefficient will be increased. With additional plutonium production by neutrons escaping from the reactor core into the uranium blanket surrounding the core, conversion ratio can reach values well above 1. Since these early days several concepts of fast reactors were developed to utilize energy of U238. One concept, sodium cooled fast reactors has been developed from the first small experimental reactor EBR 1 in USA, in operation 1951, to large reactors close to commercial stage, such as Superfenix of 1200 MW in France operating from 1984 to 1998, with a number of working prototypes in between in several countries. Last construction was reactor Monju 12 Advances in Nuclear Fuel of 300 MW in Japan, in operation from 1993. List of major experimental, pilot and demonstration fast breeder reactors is given in Table 5 (Cochran et.al., 2010; WNA, August 2011). Country China France Germany India Japan USSR/Russia United Kingdom United States Name CEFR Rapsodie Phenix Superphenix KNK 2 FBTR PFBR Joyo Monju BR-5 BOR-60 BN-350 (Kazakhstan) BN-600 BN-800 Dounreay FR Protoype FR EBR-I EBR-II Fermi 1 SEFOR Fast Flux Test Facility MWe 20 MWth 40 250 1240 21 40 500 140 280 5 12 350 600 800 15 250 0.2 20 66 20 400 Operation 20101967-1983 1973-2009 1985-1998 1977-1991 19852010? 19771994-1995, 2010? 1959-2004 19691972-1999 19802014? 1959-1977 1974-1994 1951-1963 1963-1994 1963-1972 1969-1972 1980-1993 Table 5. Major experimental, pilot and demonstration fast breeder reactors Other concepts of fast reactors using lead or lead-bismuth alloys as coolant, thus avoiding safety risks associated with sodium coolant, are selected as promising new projects for Generation IV reactors (Gen. IV International Forum, 2011a). Theoretical resource extension by fast breeder technology is very large, as the energy of dominant isotope of uranium is liberated. Extension is not only by a factor of about 50 coming from conversion of U238, but also from the possibility to use uranium resources too expensive for the present light water reactors with their inefficient use of uranium. It is correct to state that fast breeder reactors present technical option which can remove the resources constraint on any conceivable future nuclear energy strategy. Their deployment depends on economic and safety considerations, such as investment and reprocessing costs and plutonium diversion safety. New concepts in development attempt to preserve attractive safety features, such as low primary pressure, but avoid the use of sodium coolant which burns in contact with water. 3.4 Zonal fuel burning in so called “candle reactor” Zonal burning concept, respectively, Travelling Wave Reactor (also called “candle reactor”) (Ellis et.al., 2010) is an old idea proposed in 1958 by S. Feinberg (Feinberg, 1958). Recently it was given new attention by several investigators, especially by H. Sekimoto from the Tokyo Long Term Sustainability of Nuclear Fuel Resources 13 Institute of Technology (Sekimoto et.al., 2008). This reactor concept promises very high uranium utilization, about 40% of U238 in fuel, without the need for reprocessing. Needles to say, that would dramatically increase energy obtainable from uranium with very great advantage that reprocessing is not required. Fissile material is burnt and created in situ in the zone that moves through the reactor core. Concept is certainly very attractive, but real perspective is not yet clear. It could be a major advance in the use of nuclear fission energy, but it has not been demonstrated and is still in the early phase of development. Open problems are fuel and other core materials capable to sustain very high burn-up. Clarifications on the initiation of the burning are needed. Attempt to construct a prototype of this reactor type is supported by Bill Gates foundation. 3.5 Accelerator conversion of U238 into plutonium and of Th232 into U233 Electronuclear breeding investigation started early within the US MTA project (1949-1954), initiated by E.Lawrence (Heckrotte, 1977). Although technically successful, project was terminated when new uranium deposits large enough for US nuclear programme were discovered. Number of studies in 70ties dealt with the accelerator production of fissile materials Pu239 or U233, but low cost of uranium and proliferation consideration worked against further development. Concept was recently again taken up by C. Rubbia of CERN. In electronuclear accelerator breeding, particle accelerator is optimized in particle energy and target selection to produce thermal neutrons at minimum energy cost. Using protons in the range of 1000- 1500 MeV or deuterons with twice this value, minimum energy is lost on ionization in the large uranium or thorium target, whilst energetic ions produce neutrons first in spallation reactions and then in fast neutron reactions such as (n,2n) or (n,3n) which further increase number of neutrons of lower energy before they are thermalized and absorbed in fertile materials U238 or Th232. Project studies show that economy of plutonium production requires the proton beams of 200-300 mA corresponding to a beam power of about 200-300 MW. It is believed that extrapolation of present accelerators to such beams would not require new physical development. Accelerator target would in size and power dissipation resemble nuclear reactor core, profiting thereby from the existing reactor technology. Such an accelerator combined with the conventional thermal reactors fed by fertile nuclides produced by accelerator-breeder would present a system producing energy with an input of natural uranium or thorium fuel only. While in principle such hybrid system offers as effective use of natural uranium as a fast breeder reactor, it has an important advantage that fissile material production can be separated in time and location from the energy production. Accelerator and reprocessing installation would parallel enrichment installations, with a difference that the largest part of natural uranium input could be turned into fissile isotopes. Another advantage is that produced fissile materials could be fed into existing proven conventional reactors (Bowman et.al., 1992; Fraser et.al., 1981; Kouts & Steinberg, 1977; Lewis, 1969; Steiberg et al., 1983). 3.6 Conversion of U238 and Th232 by fusion neutrons Several studies have shown that fusion devices unable to reach positive energy balance required to operate as pure fusion power producer, could still serve as neutron source producing neutrons for conversion of uranium or thorium. With fissile materials produced by neutron irradiation fed into conventional fission reactors, hybrid system of fusion device 14 Advances in Nuclear Fuel and fission reactors can produce energy with input of natural uranium or thorium only, as accelerator breeder systems. Many general and economic considerations are similar to those for accelerator breeders, with an advantage of less complexity in case of accelerator system, where the accelerator target technology could use much of reactor core technology. At present development required for accelerator breeders appears less demanding than development of fusion breeder devices (Maniscalco et.al., 1981). 3.7 Perspectives of nuclear fuel utilization improvement At this moment it is difficult to foresee which, if any, of these ways to utilize the energy of U238 and Th232 will be developed. Molten salt thermal breeder might have the best chance, being one of the Generation IV selections. Second chance could be one of the fast breeder concepts with the coolant more acceptable than sodium. When we look at the technologies that may require more time for development, such as accelerator breeders or fusion –fission hybrids, we should note that time is not a limitation, as with effective burning of U238 nuclear fission energy is a source for the next thousands of years. At that time scale it does not matter whether they are developed in 50 or in 100 years. What is however important is to know that technologies exist which if developed and applied would make nuclear fission an energy source we cannot run out. Cost of enriched sea extracted uranium determines the upper limit on the costs of any of above concepts for utilization of U238. An essential reduction of seawater uranium extraction cost would consequently reduce the number of economically acceptable concepts out of the list of physically and technically possible concepts presented above, respectively, move them into the more distant future. 4. Projections of long term world nuclear energy demand and nuclear fuel requirements In order to assess long term sustainability of uranium resources a number of scenarios with different nuclear energy development strategies have been analysed. In the upcoming subsection we first give general assumptions and calculational methodology used in the analysis of all scenarios. We then proceed with detailed description of each particular scenario including specific assumptions and overall calculational results. 4.1 General assumptions and calculational methodology In all the development strategies, i.e., scenarios, once-through fuel technology has been used. Spent fuel was assumed to be stored in spent fuel casks on controlled sites, enabling possibility of future reprocessing. The year 2010 has been chosen as the starting year for all the scenarios. The initial parameters used are those for the year 2009 and are based on the World Energy Outlook (WEO) 2009 (IEA, 2009) reference scenario data, the joint report by OECD Nuclear Energy Agency and the International Atomic Energy Agency regarding uranium resources (OECD/NEA & IAEA, 2010), and some assumptions based on engineering judgement and experience. These parameters are as follows: • conventional uranium resources have been used in all scenarios as availability merit; these resources equal to 16.7 million tonnes (OECD/NEA & IAEA, 2010), Long Term Sustainability of Nuclear Fuel Resources • • • • 15 conversion factor addressing the amount of uranium required for production of 1 TWh of electricity equals 25.0 tU/TWh; the factor has been conservatively set based on the analyses of electricity production in nuclear power plants and corresponding uranium demand over the last decade (OECD, 2006; OECD/NEA & IAEA, 2010); the value for the conversion factor has been verified theoretically (Bodansky, 2004), conversion factor addressing the mass of plutonium in spent fuel based on energy production is 0.17 tPu/GWye (Bodansky, 2004), constant capacity factor for nuclear power plants of 0.88 has been used for the entire investigated period in all scenarios, scenarios 2, 3 and 4 are selected in order to see the adequacy of uranium resources for essential contribution to carbon emission reduction, as required by WEO 2009 450 Strategy that would keep temperature increase below 2°C (IEA, 2009). Owing to general safety consideration we assume conventional reactor technology until the end of century and postponement of reprocessing until 2065, respectively 2100. This is also the reason for using conservative parameters for evaluation of uranium consumption. Scenario 1 is a low growth scenario which would not contribute essentially to carbon emission reduction. 4.2 Scenario 1 – Low growth scenario A scenario of low nuclear capacity growth is a typical scenario showing that for a small share of nuclear energy in the total world production of energy, resources are not a limiting factor. This scenario assumes moderate growth strategy of 0.6% per year for the period 2011 – 2025, and 1.3% after the year 2025, following the 450 Policy Strategy of WEO 2009 (IEA, 2009). The scenario aims at preserving the share of nuclear energy in the total energy production. Although the present growth of total energy production and consumption is higher, we do not consider it appropriate for the longer periods in question. The investigated period is the entire 21st century, with special attention placed on the year 2065, which is later used as a milestone in scenario 2 and scenario 3. The results are given in Table 6. Cumulative uranium requirements up to the year 2065 would be approximately 5.4 million tonnes, while for the entire 21st century cumulative requirements would reach 11.3 million tonnes. By the year 2100 installed nuclear capacity would reach 1080 GWe producing more than 8,000 TWh of electricity per year. It is also interesting to notice that cumulative mass of plutonium in spent fuel by the year 2100 would be slightly below 9,000 tonnes. If the same level of nuclear capacity increase would be used beyond the year 2100, the conventional uranium resources of 16.7 million tonnes would be exhausted by the year 2123. 4.3 Scenario 2 – Exponential high growth scenario Exponential high growth scenario is determined by asking for the maximum nuclear buildup that can be reached by the year 2065, compatible with present estimate of uranium resources and their use with once-through nuclear technology, i.e. without reprocessing. Exponential growth with annual increase of 2.35% is used for the initial period 2011 – 2025. The aim of the scenario analysis is to deduce the maximum growth, i.e., the maximum nuclear build-up that can be achieved throughout the period 2026 – 2065, with the 16 Advances in Nuclear Fuel assumption that at the end of the period the current uranium resources of 16.7 million tonnes would be exhausted. The year 2026 has been chosen as the starting year for rapid nuclear build-up based on the estimate of present status of nuclear industry and the time needed to prepare such a massive undertaking. The results are given in Table 7. Nuclear Year capacity (GWe) 2010 2015 2020 2025 2030 2035 2040 2045 2050 2055 2060 2065 2070 2080 2090 2100 375 386 398 410 437 467 498 531 566 604 644 687 733 834 950 1,080 Annual electricity production [TWh] 2,890 2,978 3,068 3,161 3,372 3,597 3,837 4,093 4,366 4,658 4,968 5,300 5,653 6,433 7,320 8,329 Annual U requirements (ktU) Cumulative U requirements (ktU) 72 74 77 79 84 90 96 102 109 116 124 132 141 161 183 208 72 440 819 1,210 1,620 2,059 2,526 3,025 3,557 4,124 4,729 5,375 6,064 7,582 9,310 11,276 Annual Cumulative mass of mass of Pu in Pu in spent fuel spent (tPu) fuel (tPu) 56 56 58 342 60 636 61 939 65 1,258 70 1,598 74 1,961 79 2,348 85 2,761 90 3,201 96 3,671 103 4,172 110 4,707 125 5,886 142 7,227 162 8,753 Table 6. Scenario 1 (low growth scenario) results Under the condition of uranium resources exhaustion by the year 2065, the maximum possible annual growth rate for the period 2025 – 2065 is 5.7%. Thus, by the year 2065 installed nuclear capacity would reach 4,878 GWe producing more than 37,000 TWh of electricity in that year. Under the scenario terms, the maximum increase of nuclear capacity is observed during the last year of examined period and equals 263 GWe. It is also interesting to notice that cumulative mass of plutonium in spent fuel until the year 2065 would slightly exceed 13,000 tonnes. Very high contribution, over 50%, to the carbon emission reduction as required by WEO 2009 450 Strategy would be reached by 2065. Based on previous discussion on long-term perspective of nuclear fuel resources presented in subsection 2.3, one can assume that the current estimate of 16.7 million tonnes of conventional uranium resources is likely to increase in the next 50 years. Therefore, it would be interesting to see the uranium requirements for the entire 21st century. A number of development strategies for the period 2066-2100 could be taken into account. However, we limit our investigation on a simple one, foreseeing constant nuclear capacity that equals the one reached by the year 2065 - 4,878 GWe. The results are also given in Table 7. Cumulative uranium requirements for the period 2066-2100 would amount to approximately 33 million tonnes. If reprocessing of spent fuel and plutonium cycle (MOX 17 Long Term Sustainability of Nuclear Fuel Resources fuel) is envisioned as possible after the year 2065 (WNA, 2011a), then cumulative mass of plutonium in spent fuel up to the year 2098 would amount to slightly more than 37 thousand tonnes. The year 2098 has been taken as final for plutonium accumulation to enable reprocessing of spent fuel and MOX production. Assuming that 70% of accumulated plutonium in spent fuel is fissile (Bodansky, 2004) reduction of uranium requirements in the amount of 5.7 million tonnes could be expected. Nuclear Year capacity (GWe) Annual electricity production [TWh] Annual U requirements (ktU) Cumulative U requirements (ktU) Annual Cumulative mass of mass of Pu in Pu in spent fuel spent (tPu) fuel (tPu) 2010 375 2,890 72 72 56 56 2015 421 3,246 81 460 63 357 2020 473 3,646 91 895 71 695 2025 531 4,095 102 1,384 79 1,074 2030 701 5,403 135 1,990 105 1,545 2035 925 7,128 178 2,790 138 2,166 2040 1,220 9,405 235 3,846 183 2,985 2045 1,610 12,409 310 5,238 241 4,066 2050 2,124 16,372 409 7,076 318 5,492 2055 2,802 21,601 540 9,500 419 7,374 2060 3,697 28,500 712 12,698 553 9,857 2065 4,878 37,603 940 16,918 730 13,133 2070 4,878 37,603 940 21,618 730 16,781 2075 4,878 37,603 940 26,319 730 20,430 2080 4,878 37,603 940 31,019 730 24,079 2085 4,878 37,603 940 35,719 730 27,727 2090 4,878 37,603 940 40,420 730 31,376 2095 4,878 37,603 940 45,120 730 35,025 2100 4,878 37,603 940 49,821 730 38,673 Table 7. Scenario 2 (exponential high growth scenario) results 4.4 Scenario 3 – Linear high growth scenario As in the previous scenario, a scenario of linear high growth is determined by asking for the maximum nuclear build-up that can be reached by the year 2065 with the assumption that current conventional uranium resources would be exhausted by the same year. However opposed to scenario 2, it assumes linear growth rate. Also for the period 2011-2025 linear growth rate is envisioned similar to the WEO 2009 reference scenario (IEA, 2009) resulting in 459 GWe of installed nuclear capacity in the year 2025. Annual increase in nuclear capacity for the period 2011 – 2025 is approximately 5.6 GWe. The results of scenario 3 analysis are given in Table 8. 18 Advances in Nuclear Fuel Under the same conditions as in the previous scenario (current uranium resources exhaustion by the year 2065), the maximum possible annual increase of installed nuclear capacity for the period 2025 – 2065 is 75.5 GWe. Thus, by the year 2065 installed nuclear capacity would reach 3,479 GWe producing almost 27,000 TWh of electricity per year. Compared to previous scenario, scenario 3 results in larger penetration of new nuclear capacity at the beginning of investigated period. This is an advantage from the carbon emission reduction considerations. For example, scenario 2 projects 30 GWe of new nuclear capacity for the year 2026, as opposed to 75.5 GWe of scenario 3. Graphical representation of annual increase in nuclear capacity for scenario 2 and scenario 3 is given in Fig. 3. Cumulative mass of plutonium in spent fuel until the year 2065 would slightly exceed 13,000 tonnes just as in the case of the previous scenario. As well as for scenario 2, extension of scenario 3 up to the year 2100 has been analysed, assuming nuclear capacity of 3,479 GWe for the period 2066-2100. The results of extended scenario 3 are also given in Table 8. Nuclear Year capacity (GWe) 2010 2015 2020 2025 2030 2035 2040 2045 2050 2055 2060 2065 2070 2075 2080 2085 2090 2095 2100 375 403 431 459 836 1,214 1,591 1,969 2,346 2,724 3,101 3,479 3,479 3,479 3,479 3,479 3,479 3,479 3,479 Annual electricity production [TWh] 2,890 3,106 3,322 3,538 6,448 9,358 12,269 15,179 18,089 20,999 23,909 26,819 26,819 26,819 26,819 26,819 26,819 26,819 26,819 Annual U requirements (ktU) Cumulative U requirements (ktU) 72 78 83 88 161 234 307 379 452 525 598 670 670 670 670 670 670 670 670 72 450 854 1,286 1,946 2,971 4,359 6,110 8,226 10,705 13,548 16,755 20,108 23,460 26,812 30,165 33,517 36,869 40,222 Annual Cumulative mass of mass of Pu in Pu in spent fuel spent (tPu) fuel (tPu) 56 56 60 350 65 665 69 1,001 125 1,515 182 2,312 239 3,392 295 4,756 352 6,403 409 8,332 465 10,545 522 13,041 522 15,650 522 18,260 522 20,869 522 23,478 522 26,087 522 28,697 522 31,306 Table 8. Scenario 3 (linear high growth scenario) results Cumulative uranium requirements for the period 2066-2100 would amount to approximately 23.5 million tonnes. The cumulative mass of plutonium in spent fuel up to 19 Long Term Sustainability of Nuclear Fuel Resources Annual increase of nuclear capacity [GWe] the year 2098 would amount to 30.2 thousand tonnes. If reprocessing of spent fuel and plutonium cycle (MOX fuel) is envisioned as possible after the year 2065, and using the same assumption as in the previous scenario a reduction of uranium requirements in the amount of 4.6 million tonnes would be expected. 300 250 200 150 100 50 0 2000 2010 2020 2030 2040 2050 2060 2070 Year Scenario 2 annual increase [GWe] Scenario 3 annual increase [GWe] Fig. 3. Annual increase in nuclear capacity for scenario 2 and scenario 3 4.5 Scenario 4 – An intermediate scenario Scenarios 2 and 3, i.e., high growth scenarios, provide illustration on maximum growth of nuclear capacities possible under stated resources constraint. Scenario 4 illustrates a less demanding nuclear build-up strategy that would replace all coal power plants without Carbon Capture and Storage (CCS) system, with nuclear power plants during the 2026-2065 period. Unlike scenario 1 this scenario would still give important contribution to carbon emission reduction, albeit not as high as the scenarios 2 and 3. It is assumed that all new coal power plants build after the year 2025 would have CCS installations. Linear replacement dynamics starting in the year 2026 is assumed without specifying the exact dates of coal power plant replacement. As in the previous scenario linear growth rate is envisioned for the period 2011-2025, similar to the WEO 2009 reference scenario (IEA, 2009), resulting in 459 GWe of installed nuclear capacity in the year 2025. Same WEO 2009 reference scenario (IEA, 2009) states that electricity production in coal power plants would be 13,387 TWh in the year 2025. With availability factor of 0.88, installed nuclear capacity of 1,736 GWe would be required to replace coal power plants electricity production. The results of scenario 4 analysis are given in Table 9. Goal of all non-CCS coal power plants replacement throughout the period 2026-2065 would require an annual increase of nuclear capacity in the amount of 43.4 GWe. The total installed nuclear power by the year 2065 would reach 2,195 GWe with electricity production of almost 17,000 TWh. Cumulative mass of plutonium in spent fuel until the year 2065 would slightly exceed 9,000 tonnes which is rather lower than in previous two scenarios. 20 Advances in Nuclear Fuel As in the previous two scenarios, extension of scenario 4 up to the year 2100 has been analysed assuming nuclear capacity of 2195 GWe for the period 2066-2100. The results of extended scenario 4 are also given in Table 9. Intermediate nuclear growth envisioned in scenario 4 results in cumulative uranium requirements up to the year 2065 in the amount of slightly less than 12 million tonnes. The current conventional uranium resources would be exhausted by the year 2077. Cumulative uranium requirements for the period 2078-2100 would amount to approximately 9.7 million tonnes. If reprocessing of spent fuel and plutonium cycle (MOX fuel) is envisioned as possible after the year 2065, then cumulative mass of plutonium in spent fuel up to the year 2098 would amount to 19.9 thousand tonnes resulting in possible reduction of uranium requirements in the amount of 3.1 million tonnes. Nuclear Year capacity (GWe) 2010 2015 2020 2025 2030 2035 2040 2045 2050 2055 2060 2065 2070 2075 2080 2085 2090 2095 2100 375 403 431 459 676 893 1,110 1,327 1,544 1,761 1,978 2,195 2,195 2,195 2,195 2,195 2,195 2,195 2,195 Annual Annual Cumulative Annual U Cumulative U electricity mass of Pu mass of Pu in requirements requirements production in spent spent fuel (ktU) (ktU) [TWh] fuel (tPu) (tPu) 2,890 72 72 56 56 3,106 78 450 60 349 3,322 83 854 64 663 3,538 88 1,286 69 998 5,211 130 1,853 101 1,439 6,884 172 2,630 134 2,042 8,557 214 3,616 166 2,807 10,230 256 4,811 199 3,735 11,902 298 6,215 231 4,825 13,575 339 7,829 263 6,077 15,248 381 9,651 296 7,492 16,921 423 11,683 328 9,069 16,921 423 13,798 328 10,710 16,921 423 15,913 328 12,352 16,921 423 18,028 328 13,994 16,921 423 20,143 328 15,636 16,921 423 22,258 328 17,278 16,921 423 24,373 328 18,920 16,921 423 26,488 328 20,562 Table 9. Scenario 4 (intermediate growth scenario) results 5. Discussion on the long term sustainability of nuclear resources As we stated introductory, energy that can be released by nuclear fission from uranium or thorium is not determined, or not essentially determined, by the quantity of resources. This is an essential difference to note when comparing nuclear with fossil fuel resources. On the other hand physical quantities of resources are, similarly as for fossil fuels, defined by extraction costs and by accepted criteria for categorization and estimates of deposits. Energy that can be liberated from unit mass of natural uranium varies by a large factor depending Long Term Sustainability of Nuclear Fuel Resources 21 on the reactor and fuel cycle technology. Economic criteria on uranium deposits are consequently much more dependent on the energy conversion technology than in the fossil energy use. If the technology applied releases much more energy per unit mass than the present conventional reactors, then more expensive uranium or thorium deposits can be economically exploited. However our approach on the nuclear technologies to be used in this century is conservative. Therefore, our first interest is to see how far we can go with conventional, or essentially conventional nuclear technology. When considering present and future nuclear technologies which determine the requirements we must not take a narrow technical view on the possible fuel and reactor technologies. Development of nuclear safety is a slow process, reactors built in the nuclear boom in the late seventies and early eighties of the last century are still running, albeit approaching retirement. Although there are some 14 000 years of reactor experience, change of generations is a slow process, and such is the rate of change in basic reactor concepts. As the recent accidents at Fukushima show there is still a room for improvement even on the dominant line of light water reactors operating in a once-through fuel cycle. This is a reason why we estimate the uranium requirement in this century without introduction of breeder reactors. Also, we do not foresee before the end of century any major contribution of other technologies for extension of uranium or thorium utilization (Section 3). Our further basic assumption is on the role that nuclear fission should play in the critical period of about 50 years from now before wind, solar, nuclear fusion and CCS may contribute essential part of energy production. Nuclear fission energy is a proven, developed and economical source of carbon free energy. It is very difficult to see that the internationally accepted target to keep the mean global temperature increase below 2 °C could be achieved without the use of nuclear energy. Therefore in estimating the future needs of uranium we consider such deployments of nuclear power as can give an essential contribution to reduction of carbon emission. Often shown strategies with low growth, such as scenario 1 included in previous Section 4, result in assurances about the long life of resources, but are pointless for the purpose of climate control. For our purpose relevant are strategies 2, 3, and 4 of Section 4. These strategies are an extension of the strategies we investigated earlier (Knapp et al., 2010) with the aim to determine what could be maximum contribution of nuclear energy in reduction of carbon emission down from the projected WEO 2009 Reference scenario to the sustainable WEO 450 scenario limiting the temperature increase to 2°C. Strategies were constrained to the use of proven conventional reactors operating in the once-through nuclear fuel cycle, without fuel reprocessing and plutonium recycle. Maximum nuclear contribution was obtained in strategies 2 and 3 by further assumption that total conventional uranium resources estimated in 2009 Red Book be consumed by the year 2065. The point of the study was not in proposing any specific growth strategy, but rather to see whether with conventional reactor technology, without spent fuel reprocessing, nuclear energy can essentially contribute to the carbon emission reduction. An argument for selection of the year 2065 for the final year of nuclear build-up is essentially derived from the status of nuclear and renewable technologies, as well as CCS and fusion prospects and their perspective for large contributions in carbon emission reduction. Under these constraints maximum annual nuclear capacity growth for the linear growth strategy (scenario 3), between the years 2025 and 2065 was 75.5 GW, reaching installed nuclear power of 3479 GW in 2065. By that year nuclear contribution to the required GreenHouse Gasses (GHG) emission reduction comes to the value of 39.6% of the WEO 450 Strategy 22 Advances in Nuclear Fuel requirements (Knapp et al., 2010). This is a very serious contribution which still leaves large space of remaining about 60%, respectively of 38.4 GtCO2–eq reduction to be achieved by renewable energy sources, respectively, by energy efficiency and other ways of carbon emission reduction. If consumption of total uranium resources, as estimated in 2009, was required to achieve a serious contribution of nuclear energy to carbon emission control by 2065, should one then conclude that nuclear energy cannot continue in production of carbon free energy with the same share in total energy production? This is question certainly very relevant for judgment on sufficiency of uranium resources and we try to answer it in Section 4. To obtain a quantitative base for this we continued our scenarios 2, 3, and 4 from the year 2065 up to 2100 on the power levels reached by the year 2065, i.e. with powers of 4878 GW, 3479 GW, and 2195 GW for strategies 2, 3, and 4, respectively. In view of the expected slow growth of total energy consumption in the last decades of the century the contributions of all three strategies to carbon emission reduction will remain substantial, not much below their values in 2065. For all three strategies we have calculated cumulative uranium requirements from 2010 through to 2100 without reprocessing and with reprocessing after 2065. Assumption of study was to postpone fuel reprocessing as late as 2065 in order to give sufficient time for development of all political, institutional and technical condition for safe use of plutonium. The required quantities of uranium without reprocessing are 49.8 Mt, 40.2 Mt, and 26.5 Mt for strategies 2, 3, and 4, respectively. The required quantities of uranium with reprocessing after 2065 are 44.1 Mt, 34.3 Mt, and 23.4 Mt for strategies 2, 3, and 4, respectively. The estimated uranium requirements until 2100 are upper limits as they are obtained by conservative assumption on the efficiency of uranium use, i.e. by assuming operation of present technology conventional reactors. Even for the highest nuclear capacity growth of scenario 2 the uranium requirements are less than 50 Mt, the uranium resources estimated by simple crustal model. For scenario 3, assuming plutonium recycle after 2065, the conservative estimate, based on the use of conventional reactors and ignoring reductions by the more efficient Generation 4 reactors, ends with uranium requirements on the level of 35.6 million tonnes up to the year 2100. In other words, keeping the present proven reactor technology, with plutonium recycle postponed to 2065, one could go on with a nuclear share of about one third in the total energy production until 2100 with approximately double uranium resources as estimated in 2009. Our figure without reprocessing until 2100 is about 13% higher and it amounts 40.2 Mt. While we can expect the conditions for reprocessing to exist by 2065, we can say that even the postponement of reprocessing until 2100 for strategy 3 with a very large contribution of carbon free energy results in still acceptable requirements. This is certainly so for the intermediate Strategy 4, which still contributes with about one quarter to required emission reduction, while the uranium requirements are lower. Whether the introduction of reprocessing after 2065 will be necessary will depend on many future developments, such as the improvement of conventional nuclear technology, progress in fusion and CCS technology, rate of deployment of renewable resources, and of course, on the rate of increase of uranium resources. About this we cannot speculate. Also, we do not want to discuss in this place the wisdom or the feasibility of giving up nuclear energy in view of the enormous tasks world is facing to control the climate changes by GHG emissions. What we do want to show is that until the end of century uranium resources are not a limiting factor for a large nuclear contribution on the level of 3479 GW approximately, Long Term Sustainability of Nuclear Fuel Resources 23 i.e. on the level of one third of total energy production, without introduction of such technologies as fast breeder reactors. That should be sufficient for a reasonable assurance that a strategy such as WEO 450 could be achieved, provided, of course, that renewable source and other ways of GHG emission control contribute their large shares. After 2065 there could be a welcome contribution from CCS installation, and, less likely, from fusion. If these developments fail, our estimates show that continued share of nuclear energy could be supported by conventional reactor technologies up to the end of century. Large scale introduction of fast breeders after 2100 would make the issue of uranium or thorium resources irrelevant for future energy production. Needless to say, in that case the uranium from the seawater would open as economically acceptable and for all practical purposes inexhaustible uranium source. However, we do not want to overplay these future possibilities. It is not enough to show that nuclear energy is sustainable. This is easily done by assuming an early introduction of breeder reactors. However, in democratic societies nuclear energy must also be acceptable to most citizens. Nuclear energy must prove itself to be evidently safe, technically and politically. That is why it would be preferable to continue with proven technology till about the end of century. We show that possible from the point of resources. Many safety improvements were applied on the light water reactors after the Three Mile Island accident in 1979. There will be some lessons after Fukushima 2011 accidents. Applied, they will contribute further to the safety of present reactor line. Rather than changing basic technology too soon, it may be wiser to demonstrate several decades of safe and reliable operation of present one. That would be a good preparation for later introduction of new technologies, such as breeders. This is not a long delay, considering that with new technologies to use U238 and Th232 nuclear energy can serve humanity for thousands of years. 6. Conclusions Under the long term sustainability of nuclear resources we understand the capability to support long term large share of nuclear energy (of about one third) in total energy production and in reduction of carbon emission. We determined the uranium requirement for corresponding nuclear strategies to 2065 and to the end of century. In view of our survey of non-conventional uranium resources with potential to substantially expand conventional uranium resources, as well as expected increase of conventional resources estimates relative to their 2009 values, and looking at the results of above presented nuclear strategies 2,3 and 4, we feel justified to conclude that, after nuclear build-up in the period 2025-2065, nuclear energy share on the achieved level of about 3479 GW, respectively about one third in the total energy production, can be sustained until the end of century using only proven conventional reactor technology or with introduction of plutonium recycle after 2065. Our conservative estimate indicate, that in later case about 35.6 million tonnes of uranium would be required by 2100 in that case. Postponing the spent fuel reprocessing until the end of century would increase uranium requirement to about 40.2 million tonnes. Technologies and methods for improvement of nuclear fuel utilization have been considered. Even though some of these technologies are developed and proven technically viable, substantial implementations of these technologies are not expected in this century. While some effects on reduction of uranium requirements before the end of century may be possible, our aim for conservative estimates does not take them into account. 24 Advances in Nuclear Fuel Looking to the end of century we note that based on a geochemistry model the total amount of uranium recoverable at price of 180 USD/kg U is estimated to 50 million tonnes. On the technology side, large scale introduction of fast breeders after 2100 would make the issue of uranium or thorium resources irrelevant for future energy production. Shorter and long term sustainability potential of nuclear fuel resources is enhanced by expected extraction of uranium from phosphates and seawater. Finally, it may be concluded that nuclear fuel resources will not be a constraint for long term nuclear power development, even if the use of nuclear power is aggressively expanded. 7. References Barthel, F.H. (2007). Thorium and Unconventional Uranium Resources, Proceedings of a Technical Meeting „Fissile Materials Management Strategies for Sustainable Nuclear Energy”, ISBN 92–0–115506–9, Vienna, Austria, September 2005. Bodansky, D. (2004). Nuclear Energy Principles, Practices and Prospects, (Second Edition), Springer, ISBN 978-0387-20778-0, New York, USA Bowman, C. D., Arthur, E. D., Lisowski, P. W., Lawrence, G. P., Jensen, R. J., Anderson, J. 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