Thorium Resources Research Articles

Investigation on the effect of operational parameters in a microreactor system on the morphology and size distribution of thorium oxalate

Thorium oxide has recently garnered attention as a nuclear fuel due to the scarcity of uranium resources and the abundance of thorium resources, as well as its favorable thermal and neutronic properties. One of the most desirable characteristics of thorium nuclear fuels is their thermal properties, which are affected by the size distribution and morphology of thorium oxide particles. Thorium nitrate is the most common method for producing thorium oxide. Since no purification processes occur in the production of thorium oxide from oxalates, particle size and shape control are crucial. In this paper, the synthesis of thorium oxalate particles is carried out in a microreactor system to control the parameters of the precipitates. The main parameters that influence the efficiency, size distribution, and morphology of thorium oxalate are the concentration ratio of oxalic acid to thorium nitrate solution, as well as the flow rate ratio of these feed materials. Results show that at lower flow rate ratios of thorium nitrate to oxalic acid solution and higher concentration ratios of acid to thorium nitrate solution, a uniform particle size distribution and smaller particles are obtained, which are suitable for further calcination to prepare high-density and small grain size pellets.

TRU utilization and MA transmutation in thorium-based fluorinated molten salt fast reactor

In this paper, a thorium-based fluorinated molten salt fast neutron reactor is taken as the research object, and the utilization of thorium resources, reuse of transuranic waste (TRU), and transmutation of minor actinide (MA) in fast reactor are the research purpose. The fast reactor adopts a multi-layer core structure, in which the fuel is divided into an inner fission fuel zone and an outer breeding fuel layer, respectively loaded with fluorinated molten salts LiF+ThF4+XF4 (where X is 233U, TRU, or 233U+MA) and LiF+ThF4. In addition to serving as nuclear fuel, these two molten salts also act as coolants in the primary circuits.Through the cross sections of nuclear reactions, transmutation chain of actinide nuclides, and the particle population density formula of actinide nuclides in nuclear reaction equilibrium, the possible nuclear reactions of actinide nuclides in thorium-based fast reactors are analyzed. The calculated results show that the thorium-based fast reactor has the following characteristics: fast neutron energy spectrum, negative temperature reactivity coefficient, effective utilization of thorium resources, the ability to breed fissle fuel such as 233U, production of high-purity radioisotope 238Pu material, and the ability to incinerate Pu and MAs in the TRU spent fuel.In the 233U fuel scheme, the 233U fuel is breeded, breeding ratio of fissile fuels is 1.21, the reaction products contain very little Np, and almost no Pu, Am, and Cm long-lived radionuclides. In the TRU fuel scheme, the annual capacity to incinerate MAs and Pu is 1072.6 kg, while producing a large amount of 233U and 233Pa fuel (together 1162.5 kg/year).In the 233U+2.0 wt% TRU-MAs fuel scheme, 127.35 kg of MA waste can be incinerated annually, and their transmutation rate is 15.95%, which is equivalent to the annual production of MA nuclear waste from 4.9 conventional 1000 MWe light water reactors. Here, TRU-MAs is the mixtured MA fuels after separation of Pu from the TRU fuel. At the same time, radioactive fuel 238Pu, which can be used for space nuclear power, is produced with an annual yield of 87.89 kg and an isotopic abundance of 87.57%. If the added TRU-MAs fuel is replaced with 2.0 wt% 237Np fluorinated salt, the annual yield and isotope abundance of 238Pu are further increased to 130.24 kg and 95.4%.

X-ray diffraction and Raman spectroscopic studies on ThO2−UO2 solid solutions

In view of the vast resources of thorium in India, thorium-based fuel cycle has been envisaged as one of the most promising options to meet sustainable energy requirements. In this direction, ThO2−233UO2 and ThO2−UO2 (low-enriched) mixed oxide fuels with < 30% UO2 have been proposed for the Advanced Heavy Water Reactor. In the present work, phase relations in the Th1-xUxO2+δ (x ≤ 0.3) system under reducing and oxidizing conditions have been investigated by means of X-ray diffraction and Raman spectroscopic studies. Diffraction studies confirmed the formation of single-phase fluorite-type solid solutions; however, the samples synthesized in air exhibited slightly lower lattice parameters than those of their reduced counterparts, attributed to oxidation of U4+. Raman spectroscopic studies indicate the formation of various defects upon uranium substitution in parent ThO2, depending on synthesis conditions. It also highlights the existence of more disorder in the air-synthesized samples as compared to their reduced counterparts, plausibly due to the presence of a variety of oxidation states of uranium and associated oxygen anions interstitials. Thermal annealing of the solid solutions under air at T ≥ 1873 K results in heavy uranium evaporation loss, which has been established by XRD and XRF studies.

Sustainability of Nuclear Fuel Resources in Indonesia with Open and Closed Fuel Cycle

In the wake of climate change and global warming, various alternatives are being considered as a potential replacement for fossil fuels. Despite often being overlooked, nuclear power offers many benefits as a low-carbon energy source. Being a thermal power plant, nuclear power can generate energy reliably without relying on weather without emitting greenhouse gases during its operation. Serialised construction can reduce the capital cost, which often touted as expensive. Due to the commitment to the Paris Protocol, Indonesia is obliged to achieve carbon neutrality in its energy generation, and nuclear power is a plausible option to replace fossil fuel generation. One of the questions regarding nuclear power deployment in Indonesia is the sustainability of the nuclear fuel, especially considering its domestic resources both uranium and thorium. This study estimates how long uranium and thorium resources in Indonesia will last when used to power the nuclear power plants with open and closed fuel cycles. Several reactor designs were considered. The calculation result shows that domestic nuclear fuel resources in Indonesia can be sustainable enough, provided that closed nuclear fuel cycle is deployed.

A sulfonated ligand-aqueous two-phase system for selective extraction of thorium from mining wastewater: Process optimization, structural characterization and mechanism exploration

As a potential nuclear fuel, the recovery of thorium with high efficiency, high selectivity and high extraction capacity from thorium-containing wastewater is considered to be one of the effective ways to reduce pollution and solve the shortage of thorium resources. However, the problem of efficient and selective separation of thorium from the complex environment has not been completely solved. Herein, a functional ligand called arsenazo (ASA), polyethylene glycol, and sodium sulphate were used to construct an aqueous two-phase system (ATPs) for the selectivity extraction and separation of thorium from mining wastewater. Furthermore, the complex agent, pH value, temperature, equilibrium time, coexisting substances, and other conditions for the extraction and separation of thorium were optimized, indicating that 98.7 % of thorium was efficiently and precisely separated from the acid-leaching solution to the pH of 2–3 or 5–6 with 0.08 g/L of ASA within 10 min, and the interference with multiple coexisting ions and organic substances was avoided. Importantly, the analysis of water quality parameters, thermogravimetry, infrared spectroscopy, and ultraviolet–visible spectroscopy revealed the coordination relationship between thorium ions and complex agents, indicating that the extraction of thorium in the aqueous two-phase system was a cation exchange mechanism. More importantly, SEM, TEM, EDX, and XRD verified that thorium was efficiently and selectively separated by a sulfone acid ligand-aqueous two-phase system, providing quantitative information on thorium-containing product distribution of relatively high accuracy and avoiding the interference with coexisting substance. Therefore, this work provided theoretical guidance on treating and reusing thorium in wastewater and enriched the content of functional ligand-aqueous two-phase extraction systems in green production technology.

Three-stage fuel option for VVER-1200 reactor

Possibility of three-stage fuel option is studied to enhance VVER-1200 reactor to use thorium resources for proliferation resistance without modifying the conventional fuel assembly. As stage-1, conventional UO2 is used as a fuel and Pu isotopes are retrieved from the spent fuel after burning 60 GWd/t and 10 years cooling. Retrieved Pu isotopes are used along with Th as fuel of stage-2. After 10 years cooling, 233U and 232Th are retrieved from the spent fuel of stage-2 and used as a fuel for stage-3. Stage-3 fuel can solely be prepared from the discharged fuel of stage-2. Stage-1 and 3 show a close tendency for K-inf, power peaking factor, fuel temperature, void coefficient, and neutron spectrum than stage-2. Spent fuel of stage-3 contains the least amount of plutonium, minor actinides (Am, Cm, Np), and long lived fission products, which will play an important role for proliferation resistance and spent fuel handling.

Assessing the undiscovered resources of uranium and thorium in Mamuju, West Sulawesi

The exploration of nuclear minerals in Mamuju, West Sulawesi has been started in 2013 by the Center for Nuclear Minerals Technology (CMNT) – Badan Tenaga Nuklir Nasional (BATAN). There is a strong indication of uranium (U) and thorium (Th) minerals occurrences in the location. To follow the exploration result up, a resources assessment is needed. Due to the limited early exploration data, the concept of undiscovered resources can be used. Of all resource assessment methods suggested by International Atomic Energy Agency (IAEA), Energy Research and Development Administration (ERDA) method is the most appropriate method to be applied in the Mamuju case. In this research, Pocos de Caldas, Brazil (Osamu Utsumi Mine for U reference and Morro do Ferro Area for Th reference) was selected as control area due to its similarity on the geological condition and its more-advanced exploration stage compared to Mamuju. All the parameters for the assessment can be determined with the ERDA method based on the available information of the control area.

Highly effective and selective adsorption of thorium(Ⅳ) from aqueous solution using mesoporous graphite carbon nitride prepared by sol–gel template method

Selective separation of thorium resources from aqueous solutions is an ideal strategy for recovering or removing radioactive ions, as well as reducing potential human health and environmental risks. In this study, an effective adsorbent, mesoporous graphite carbon nitride (mpg-C3N4), was synthesized by a simple thermal condensation using a sacrificial LUDOX HS-40 colloidal silica as template and applied to the selective adsorption of thorium ions from the multi-ion solutions. The results of competitive experiments illustrated that mpg-C3N4/r exhibited higher selectivity and efficiency for Th(IV) over other 14 co-existing cations, with the maximum selective adsorption capacities of Th(IV) reaching to 108.18 mg g−1 for mpg-C3N4/r=1.0 which is more than g-C3N4 (36.64 mg g−1) at pH 4.0. The interaction of Th(IV) with mpg-C3N4/r=1.0 and g-C3N4 were performed using batch processing, the adsorption isotherm was well conformed to the Langmuir adsorption model, and the maximum adsorption capacity of thorium at pH 4.0 and 298 K was calculated to be 196.08 mg g−1 which is more than three times of g-C3N4 (60.60 mg g−1). Also, the results of adsorption experiments on monazite leachate showed that mpg-C3N4/r=1.0 can be successfully used as an attractive adsorbent with the high adsorption efficiency (93.53%) of thorium. This work could broaden insights into the tailored design and synthesis of mesoporous adsorbents for the selective adsorption of thorium in multi-ion solutions.

The Use of Thorium in a Nuclear Power System with Thermal and Fast Neutron Reactors

The concept of a fast neutron reactor with a sodium coolant with metallic U–Pu fuel in the core and metallic uranium and thorium in the blankets is considered. The achievement of the required system characteristics for the initial loading and surplus fuel breeding is demonstrated. Several scenarios of the development of a two-component nuclear power system in Russia based on the VVER-type thermal reactors and fast reactors with metallic fuel involving the thorium resources are considered.

Neutronics analysis of the stellarator-type fusion-fission hybrid reactor based on the CAD optimization method

The stellarator plasmas devices have some advantages in steady state operation with less magneto hydrodynamic activities, and the development of stellarator-type FFHR (Fusion-Fission Hybird Reactor) can provide viable energy in the foreseeable future with less requirements to the plasmas technology compared with the pure fusion system. However, the neutronics design of the stellarator-type FFHR has some challenges and difficulties, since the construction of helical structures with large number of high order spline surfaces are the extremely complicated process, and the physical parameters of the complex model have to be changed frequently in the primary design stage, which make the corresponding manual input simulation work into the time consuming and error prone tasks. In this context, the CAD optimization method has been used in converting the dense mesh oriented CAD structure to the low poly style, and the CAD-PSFO code has been employed for the modeling transformation process in Monte Carlo calculation. The thorium resource has been selected as the breeding material for the stellarator-type FFHR, and the polonium and natural abundance of uranium have been used as the seed fuel for the initial loading in order to get the high energy multiplication factor. The OMCB (OpenMC basedBurnupcode) has been introduced to study the burnup features, and stellarator-type FFHR has been designed as the multi-purposes facility that can keep tritium self-sufficiency and get the high breeding ratio with steady energy output.

Uranium and Thorium Resources of Estonia

We provide a compilation of geology of uranium and thorium potential resources in the Ordovician black shale (graptolite argillite), Cambrian–Ordovician shelly phosphorite and in the secondary resources (tailings) of Estonia. Historical and new geological, XRF and ICP-MS geochemical data and ArcGIS modeling results of elemental distribution and tonnages are presented. The Estonian black shale contains 5.666 million tons of U, 16.533 Mt Zn, 12.762 Mt Mo, 47.754 Mt V and 0.213–0.254 Mt of Th. The Estonian phosphate resources, altogether about 3 billion metric tons of phosphate ore, contain about 147,000 to 175,000 tons of U. Rare earth element concentrations in the phosphorite ore average at 1200–1500 ppm of ΣREE. Thorium can also be a possible co-product. The mining waste dump at the Maardu contains at least 3650 tons of U and 730 tons of Th. The Sillamäe radioactive waste depository contains about 1200 tons of U and 800 tons of Th. Due to the neighboring geological positions, as well as environmental constraints and mining technologies, the black shale and phosphorite can be treated as a complex multi-resource, possibly at the continental scale, which needs to be extracted together.

Frontier looking of rare-earth processed residue as sustainable thorium resources: An Insight into chemical composition and separation of thorium

Rare-earth (RE) extraction activities in Malaysia produce an average of 75000 tonnes of Water Leach Purification (WLP) residue containing an estimated 106 tonnes of thorium that leads to negative perception and concern among the public especially related to the environmental and radiological aspects. The main challenge to separate thorium from the WLP residues is the formation of insoluble thorium pyrophosphate (ThP2O7) due to the treatment process during rare-earth extraction. Therefore, the objectives of this study are to verify the thorium composition in WLP residue as well as to investigate the possibility of separating thorium from WLP. The characterizations using FTIR, SEM-EDS, XRD, and XPS verified the presence of ThP2O7 along with other thorium phosphate compounds (Th(PO3)4) and (Th3(PO4)4) in WLP residue. The digestion study using sulphuric acid (H2SO4) at various molarity ranging from 3 to 18 mol/L showed that thorium in WLP can be dissolved in acid. The dissolution of thorium up to 50% was achieved at 6 mol/L H2SO4 and further increase to 90% with the increase of acid molarity at 150 °C within an hour of the leaching time. This research showed promising findings on the separating thorium from WLP residue through the acid digestion process that might open up the possibility of recycling rare-earth residue as sustainable thorium resources.

Structural and thermal expansion studies in ternary ThO2−CeO2−NdO1.5 system: Mimicking actinide substituted ThO2

Utilization of vast resources of thorium in India by means of nuclear reactor and accelerator driven sub critical system (ADSS) is required to ensure the long term sustainability of nuclear power. ADSS also offers a solution to reduce long term radio-toxicity of minor actinides by their transmutation in suitable matrix. Consequently, the phase relations in ThO2–CeO2–NdO1.5 system have been investigated on slow cooled samples from 1873 K in context of incineration of plutonium (Pu) and minor actinides (MAs) in ThO2 as matrix component. About 24 compositions in Th0.8Ce0.2O2−NdO1.5, Th0.8Nd0.2O1.9−CeO2 and ThO2−Ce0.5Nd0.5O1.75 series, prepared by solid state route, were characterized by X-ray diffraction (XRD) and Raman spectroscopy. CeO2 and Nd2O3 have been employed as a non-radioactive surrogate for plutonia and minor actinides, respectively, to determine solubility of Pu and MAs in ThO2. Investigation shows appearance of fluorite-type phases in thoria and ceria rich region while biphasic regime was observed for neodymia-rich region. The lattice parameters of the phases are the result of interplay of ionic radii of cations and presence of oxygen vacancies. The lattice thermal expansion behaviour of few single-phase compositions in this pseudo ternary system, as investigated by high temperature XRD, is also reported in this manuscript.

Large-scale energy production with thorium in a typical heavy-water reactor using high-grade plutonium as driver fuel

Thorium can be introduced into the energy vector in combination with high-grade plutonium (HG-Pu). Excellent neutron economy of heavy-water moderator allows use of mixed ThO2/HG-PuO2 fuel in heavy-water reactors with high efficiency, leading to the exploitation of large world thorium reserves and extending the availability of the nuclear energy by two orders of magnitude. In the present work, the criticality calculations have been performed with the code MCNPX 3-D geometrical modeling of a typical heavy-water reactor, where the structure of all fuel rods and bundles is represented individually. In the course of time calculations, nuclear transformation and radioactive decay of all actinide elements as well as fission products are considered. Five different fuel compositions have been selected for investigations: (1) 97% thoria (ThO2) + 3% PuO2; (2) 96% ThO2 + 4% PuO2; (3) 95% ThO2 + 5% PuO2; (4) 94% ThO2 + 6% PuO2; and (5) 92% ThO2 + 8% PuO2. The behavior of the criticality k∞ and the burn-up values of the reactor have been pursued by full power operation at 640 MWel (2180 MWth) for approximately 7 years. Time calculations have been conducted with MCNPX and CINDER codes under consideration of all nuclear transformation and radioactive decay processes on the actinide isotopes and fission fragments. As the reactor allows fuel recharging at on-power operation mode, the reactor criticality has been followed down to keff,end = ~1.05. The corresponding burn-up values and operation periods for the investigated modes are (1) 18 GWd/MT and 780 days; (2) 27 GWd/MT and 1200 days; (3) 35 GWd/MT and 1560 days; (4) 44 GWd/MT and 1940 days; and (5) 60 GWd/MT and 2640 days. Among the investigated four modes, 94% ThO2 + 6% PuO2 seems a reasonable choice under consideration of the high price of the HG-Pu as driver fuel. The mixed fuel has the potential of an extensive exploitation of thorium resources. Reactor will run with the same fuel charge for approximately 5 years and allow a fuel burn-up approximately 44 GWd/MT, comparable with conventional light-water reactors (LWRs). Plutonium component of the mixed fuel will become nonprolific after few months of plant operation through the accumulation of even isotopes. Addition of few percent natural uranium to the initial mixed fuel charge will keep the 233U component below 22% and hence at nonprolific level over the entire plant operation period. Replacement of 4% ThO2 with 4% nat-UO2 will practically not change main technical parameters of the reactor.

Thorium Resources and their Energy Potential in Georgian Republic, the Caucasus

Because of its unique properties thorium is considered as the main energy resource in the 3rd millennium. In Georgia are detected four Thorium ore occurrences: 1 - In the Sothern slope of the Greater Caucasus (Th con. 51 g/t - 3882 g/t), 2 - In the Dzirula massif (Th con. 117 g/t - 266 g/t), 3 - In Vakijvari ore field (Th con.185 g/t - 1600 g/t), 4 - In the Black Sea magnetite sands (Th con. 200 g/t - 450 g/t). Based on these data Georgian thorium ore occurrences should be treated as potential resources.

Closed fuel cycle with increased fuel burn-up and economy applying of thorium resources

The possible role of existing thorium reserves in the Russian Federation on engaging thorium in being currently closed (U-Pu)-fuel cycle of nuclear power of the country is considered. The application efficiency of thermonuclear neutron sources with thorium blanket for the economical use of existing thorium reserves is demonstrated. The aim of the work is to find solutions of such major tasks as the reduction of both front-end and back-end of nuclear fuel cycle and an enhancing its protection against the uncontrolled proliferation of fissile materials by means of the smallest changes in the fuel cycle. During implementation of the work we analyzed the results obtained earlier by the authors, brought new information on the number of thorium available in the Russian Federation and made further assessments. On the basis of proposal on the inclusion of hybrid reactors with Th-blanket into the future nuclear power for the production of light uranium fraction 232+233+234U, and 231Pa, we obtained the following results: 1. The fuel cycle will shift from fissile 235U to 233U which is more attractive for thermal power reactors. 2. The light uranium fraction is the most "protected" in the uranium component of fuel and mixed with regenerated uranium will in addition become a low enriched uranium fuel, that will weaken the problem of uncontrolled proliferation of fissile materials. 3. 231Pa doping into the fuel stabilizes its multiplying properties that will allow us to implement long-term fuel residence time and eventually to increase the export potential of all nuclear power technologies. 4. The thorium reserves being near city Krasnoufimsk (Russia) are large enough for operation of large-scale nuclear power of the Russian Federation of 70 GWe capacity during more than a quarter century under assumption that thorium is loaded into blankets of hybrid TNS only. The general conclusion: the inclusion of a small number of hybrid reactors with Th-blanket into the future nuclear power will allow us substantially to solve its problems, as well as to increase its export potential.

On the role of fusion neutron source with thorium blanket in forming the nuclide composition of the nuclear fuel cycle of the Russian Federation

The possible role of available thorium resources of the Russian Federation in utilization of thorium in the closed (U–Pu)-fuel cycle of nuclear power is considered. The efficiency of application of fusion neutron sources with thorium blanket for economical use of available thorium resources is demonstrated. The objective of this study is the search for a solution of such major tasks of nuclear power as reduction of the amount of front-end operations in the nuclear fuel cycle and enhancement of its protection against uncontrolled proliferation of fissile materials with the smallest possible alterations in the fuel cycle. The earlier results are analyzed, new information on the amount of thorium resources of the Russian Federation is used, and additional estimates are made. The following basic results obtained on the basis of the assumption of involving fusion reactors with Th-blanket in future nuclear power for generation of the light uranium fraction 232+233+234U and 231Pa are formulated. (1) The fuel cycle would shift from fissile 235U to 233U, which is more attractive for thermal power reactors. (2) The light uranium fraction is the most “protected” in the uranium fuel component, and being mixed with regenerated uranium, it would become reduced-enrichment uranium fuel, which would relieve the problem of nonproliferation of the fissile material. (3) The addition of 231Pa into the fuel would stabilize its neutron-multiplying properties, thus making it possible to implement a long fuel residence time and, as a consequence, increase the export potential of the whole nuclear power technology. (4) The available thorium resource in the vicinity of Krasnoufimsk is sufficient for operation of the large-scale nuclear power industry of the Russian Federation with an electric power of 70 GW for more than one quarter of a century. The general conclusion is that involvement of a small number of fusion reactors with Th-blanket in the future nuclear power industry of the Russian Federation would to a large extent solve its problems and increase its export potential.

Natural Thorium Resources and Recovery: Options and Impacts

This paper reviews the front end of the thorium fuel cycle, including the extent and variety of thorium deposits, the potential sources of thorium production, and the physical and chemical technologies required to isolate and purify thorium. Thorium is frequently found within rare earth element–bearing minerals that exist in diverse types of mineral deposits, often in conjunction with other minerals mined for their commercial value. It may be possible to recover substantial quantities of thorium as a by-product from active titanium, uranium, tin, iron, and rare earth mines. Incremental physical and chemical processing is required to obtain a purified thorium product from thorium minerals, but documented experience with these processes is extensive, and incorporating thorium recovery should not be overly challenging. The anticipated environmental impacts of by-product thorium recovery are small relative to those of uranium recovery since existing mining infrastructure utilization avoids the opening and operation of new mines and thorium recovery removes radionuclides from the mining tailings.

Перспективный топливный цикл ядерной энергетики РФ с привлечением незначительного количества тория от термоядерного источника нейтронов с Th-бланкетом

Перспективный топливный цикл ядерной энергетики РФ с привлечением незначительного количества тория от термоядерного источника нейтронов с Th-бланкетом

Utilization of nuclear waste plutonium and thorium mixed fuel in candu reactors

Summary Spent nuclear fuel out of conventional light water reactors contains significant amount of even plutonium isotopes, so called reactor grade plutonium. Excellent neutron economy of Canada deuterium uranium (CANDU) reactors can further burn reactor grade plutonium, which has been used as a booster fissile fuel material in form of mixed ThO2/PuO2 fuel in a CANDU fuel bundle in order to assure reactor criticality. The paper investigates incineration of nuclear waste and the prospects of exploitation of rich world thorium reserves in CANDU reactors. In the present work, the criticality calculations have been performed with 3-D geometrical modeling of a CANDU reactor, where the structure of all fuel rods and bundles is represented individually. In the course of time calculations, nuclear transformation and radioactive decay of all actinide elements as well as fission products are considered. Four different fuel compositions have been selected for investigations: ① 95% thoria (ThO2) + 5% PuO2, ② 90% ThO2 + 10% PuO2, ③ 85% ThO2 + 15% PuO2 and ④ 80% ThO2 + 20% PuO2. The latter is used for the purpose of denaturing the new 233U fuel with 238U. The behavior of the criticality k∞ and the burnup values of the reactor have been pursued by full power operation for ~10 years. Among the investigated four modes, 90% ThO2 + 10% PuO2 seems a reasonable choice. This mixed fuel would continue make possible extensive exploitation of thorium resources with respect to reactor criticality. Reactor will run with the same fuel charge for ~7 years and allow a fuel burnup ~55 GWd/t. Copyright © 2016 John Wiley & Sons, Ltd.