Until now, the nuclear energy field has mainly depended upon uranium resources. Six key factors project the future needs for alternative nuclear fuel, these are: global energy resources, global electricity situation, global non-accessibility of electricity, lifecycle of greenhouse gas emissions, global non-renewable energy resources, and nuclear reactor technology condition (tied to estimations for future needs) (Warner and Heath, 2012). Thorium (²³²Th) is a fertile radioactive element, i.e., can capture a neutron to become ²³³Th, which through double beta decay becomes fissile ²³³U, a nuclear fuel (Banerjee et al, 2011). It is available on Earth’s crust at 3 to 4 times more than uranium (Schaffer, 2013), and is thus an important resource for future energy (Salehuddin et al, 2019). The global reserves of uranium and thorium are presented in Figure 4 (World Nuclear Association, 2020b; World Nuclear Association, 2022b). The top five nations for uranium reserves are Australia, Kazakhstan, Canada, Russia, and Namibia (accounting for 67% of world reserves, i.e., two-thirds), whereas the top five countries for thorium reserves are India, Brazil, Australia, USA, and Egypt (accounting for 47% of world reserves) (Figure 4). Notably, thorium reserves are more widespread than those for uranium, which geopolitically can be beneficial but can also signify a lower number of large concentrated reserves and hence challenges for economic exploitation. Some factors put thorium in a privileged position: its contribution to the burning of excess plutonium, less generation of long-lived waste, higher burn-ups and temperatures capability, and sustainability, considering uranium reserves [Lung and Gremm, 1998].

Global nuclear energy consumption has been estimated in terms of million tonnes oil equivalent for seven regions of the world: North America, South/Central America, the Commonwealth of Independent States (CIS) in Eurasia, Middle East, Africa, and Asia Pacific (Figure 5) (British Petroleum, 2019). It is clear that nuclear power has been concentrated in regions that have large population densities and high-income countries in combination with reduced access to fossil and renewable energy. This makes for a clear contrast between North America and Europe versus South/Central America, Middle East, and Africa. The CIS and Asia Pacific regions have complex factors that place them between these two extremes. Overall, worldwide nuclear power generation has slightly increased, with the construction of new power plants in China. In lieu, there has been reactors shutdown in Europe. A critical factor that will determine how this balance shifts in coming decades is the availability of uranium reserves, which are expected to be depleted leading to a shift toward thorium resources for future generations. Another factor is pricing pressures on electricity supply. Electricity costs for various energy sources are presented in Figure 6. With the need for carbon capture and sequestration factored in, it has been estimated that the emissions abatement cost for nuclear power is between USD 10.4 and USD 15.7 per tonne CO_2(eq) lower than for coal-fired and gas-fired power plants, respectively (Mendoza España and Bromley, 2019). Uranium, however less satisfactory regarding energy production, is, as a general rule, cheaper. Thorium-uranium fuel is expensive, having a break-even cost with uranium that would only be reached if the natural uranium price more than doubled (Serfontein, 2014). It has to be noted, however, that there are strategic cases, like India and other countries in which thorium is plentiful compared to uranium. [Lung and Gremm, 1998] In addition, the revenue generated from electricity sales combined with the reactor downtime refueling cost shows it can be advantageous - a 14.8% savings if compared in equivalent time frames (three thorium-based x four uranium cycles) (Du Toit, 2014). (Ashley 2014) presented normalized fuel cycle costs of 0.77¢/kWh for a Uranium-fueled European Pressurized Reactor (EPR) as a baseline comparison against Thorium-Uranium fueled reactors: another EPR (0.90/kWh), an Advanced Heavy Water Reactor (AHWR, 0.99¢/kWh), and a General Atomics’ Gas-Turbine Modular Helium Reactor (GT-MHR, 2.37¢/kWh).

There are several advantages of using thorium as a nuclear fuel compared to uranium. Thorium oxide has a higher melting point (3300ᴼC) compared to uranium oxide (2865ᴼC), and thorium oxide has better thermal conductivity, a lower rate of fission gas release (despite higher fission gas production per fission event), good radiation resistance, and dimensional stability up to 38 MWd/kg (Banerjee et al, 2011; György and Czifrus, 2017). UO₂ easily oxidizes to U₃O₈ and UO₃, but ThO₂ is relatively inert, hence, thorium oxide is much more stable than uranium oxide, and ThO₂-based fuels are expected to have better in-pile performance than UO₂ and UO₂-based mixed oxide (International Atomic Energy Agency, 2005; Bell et al, 2019); (Hania, 2012) also mentions that considering the focus on the burning of Pu in LWRs or HWRs, further irradiation testing of (Th,Pu)O₂ in the 6%–10% Pu concentration range to high plutonium burnups is warranted. Fabrication procedures for thorium mixed oxides are analogous to that of UO₂ and (U,Pu)O₂, with the main technical challenges lying in the reprocessing steps, and subsequent (shielded) refabrication. After reprocessing, Th²²⁸ (t_1/2 = 1.9 years) is found, making the output radioactive for decades, so blending the reprocessed uranium oxide with fresh ThO₂ and considering the spent thoria as waste is suggested.

The formation of U²³² via (n,2n) reactions with ²³²Th, ²³³Pa, and ²³³U, makes a great proliferation-resistance impact (International Atomic Energy Agency, 2012). The half-life of U²³² is 68.9 +-0.4 years [Tuli, 2011] and the daughter products have a very short half-life, with some products, e.g., ²¹²Bi and ²⁰⁸Tl, emitting strong gamma radiations. From these considerations, it can be said that Th-based fuels can be used as an intrinsic proliferation-resistant barrier to highly enriched uranium (HEU) and weapons-grade plutonium (WPu) (International Atomic Energy Agency, 2005); (Kazimi, 2003) estimated that Th-based fuels could cost ±10% when compared to conventional U-based fuels, with the biggest uncertainty being the cost of ²³⁵U-rich seed, the cost of the fuel assemblies (which need to be designed to withstand greater exposure to heat and radiation in the core of the reactor), and the potential future savings due to a reduction in spent fuel amounts needed to be stored. HWR has a more flexible neutron economy compared to LWR, as a result of lower neutron absorption by Deuterium, in addition to presenting the necessity for online refueling, while also having a thermal neutron spectrum, which is an attractive characteristic for the thorium fuel cycle (Ault et al, 2018).

Thorium has some advantageous physical and chemical properties over uranium and that can make a reason to use thorium as a ‘future nuclear fuel’ (International Atomic Energy Agency, 2005), which has been discussed previously. Thorium is considerably more abundant than uranium in the Earth’s crust, so the use of thorium in power reactors has been considered since the start of nuclear energy in the 1950s (Kazimi, 2003). Thorium is said to be ‘fertile’ rather than ‘fissile’, as a quantity of Th²³² transforms into U²³³ by absorbing neutrons within a reactor (Alexander et al, 2020). The breeding of U²³³ from thorium is much more efficient than the breeding of Pu²³⁹ from U²³⁸ because of less production of non-fissile isotopes, which plays an important role in assessing the breeding capability of a nuclear reactor design, as a measurement of the potential for converting non-fissile material into fissile material through the use of neutron irradiation. The International Atomic Energy Agency (2005) mentions the absorption cross-section for thermal neutrons of Th²³² (7.4 barns) is nearly three times that of U²³⁸ (2.7 barns), resulting in a higher conversion (to U²³³) with Th²³² than with U²³⁸ (to Pu²³⁹). The high neutron yield per absorption ratio can be identified by looking at the fission-to-capture ratio of about 10 for U²³³, which is only about 2.5 for Pu²³⁹ (Hania, 2012). However, it is important to point out that the theoretical physics breeding is different from the actual engineering breeding–which includes reprocessing losses and large uncertainties in nuclear data of thorium fuel cycle -, meaning the effective spectrum averaged cross sections data would depend on the type of reactor design and the associated neutron energy spectrum. Ganesan (2016) mentions under-prediction for the production of U²³², given although it is possible to apply nuclear data for the cross-section of isotopes of the thorium fuel cycle to traces of thorium in natural uranium rods, it is not for the irradiation of bulk thorium rods used for power flattening in their PHWRs. There are programs, such as the IAEA CRP, that took place between the years of 2010–2014, on improving nuclear data of prompt fission neutron spectra. Gunsing (2016) references the EXFOR database, with experimental nuclear reaction data fed from the world by the International Network of Nuclear Reaction Data Centers (NRDC), as well as the contributions from the Neutron Time-of-Flight Facility (n_TOF) at CERN, measuring and recording records data, and developing instrumental, detection and beam imaging techniques, having measured fission cross sections of Th²³², U²³³, U²³⁸, among others, through different fission detectors. Moreover, thorium compounds, such as silicon-doped cubic ternary thorium phosphides (ThSi_xP_1-x, where x = 0, 0.25, 0.5, 0.75, 1), from Generation-IV nuclear breeder—theoretical—systems, have unique chemical and physical properties such as high melting point, density thermal conductivity, and oxidation resistance, being prospective fuel materials for cleaner and safer nuclear energy (Bell et al, 2019; Siddique et al, 2021). A reactor is defined as a breeder if it produces more fissile material than it consumes - i.e., even with expected neutron losses, more nuclear fuel can be made from the additional neutrons than fissioned. In theory, mastering nuclear fission thorium breeder cycle reactors could increase the amount of fissile material by over 100x (Dittmar, 2018).

Combinations of thorium and highly enriched uranium have been tested in gas or water-cooled reactors, and several countries such as India, Germany, the UK, France, Japan, Russia, Canada, Brazil, and China have tested the use thorium-based fuels for their reactors (Jiyang et al, 2004; Şahin et al, 2004; Bell et al, 2019; Mendoza España and Bromley, 2019), while some countries like Türkiye are considering it (Demirbaş, 2005). However, the existence of natural resources, the cost of exploiting those resources, a country’s international trade balance position, and its economic growth policies, the profitability of the electrical power and nuclear industries are important factors that may help or prevent the adoption of thorium-based nuclear fuel technology, making technical development critical to overcoming economic challenges (Mendoza España and Bromley, 2019). For example, the excellent neutron economy in Canada deuterium uranium (CANDU) reactors potentially enables the implementation of thorium fuel cycles in these systems as a strategy to conserve uranium fuel resources and as an alternative to fast breeder reactors (FBR) (Şahin et al, 2004). An FBR typically uses uranium and/or plutonium as fuel and liquid sodium as a coolant, having the potential to produce electricity more sustainably and efficiently than conventional nuclear reactors. However, due to the large availability in its territory, India has been over essential microstructural, mechanical and. thermophysical properties of thorium-uranium alloys, showing great interest in Thorium-based metallic fuel applications to LMFBRs (Ramanna, 1986), and its excellent safety features—more specifically, (Th, Pu)O₂ containing from 20% to 80% PuO₂ (Lainetti, 2017).

We are, at present in the third decade of the 21st century, waiting to see when factors will align to see global dissemination of the thorium nuclear fuel cycle. Jordan et al, (2015), a few years ago, summarized the main uncertainties that have slowed this factor alignment: (i) competing proposals for reactor designs; (ii) fuel configurations; and (iii) reprocessing options. As a result of these uncertainties, which will be played out by social, political, and economic interests, the certainty that we can end with is that the crustal abundance of thorium is a simpler question than the complex matter of usable availability.

Thorium is a future prospective nuclear fuel based on major considerations such as resource availability globally, economic evaluations that consider it as both a primary and secondary resource associated with other valuable mining products, environmental protection in terms of its relatively low short- and long-term carbon and environmental footprints, and its association with the mining of natural resources needed for green technologies such as REE, and the safeguarding of future generations due to the lower potential for proliferation of fissile materials (Kakodkar, 2006). Carbon emissions are threatening future generations as well and are a major driver in thinking about meeting future societal needs with an alternate energy source. At the same time, with society having negative opinions on nuclear power plants, especially as a consequence of recent events in Japan (Fukushima incident) and Ukraine (military threats), it is critical that future studies also consider the lesser-known health benefits resulting from the interlink between human lives and nuclear energy such as air pollution and birth weight (Shellenberger, 2017). Other studies have considered the health of mine workers in the pursuit of thorium resource exploitation; Lindsay et al, (2022) investigated the health hazards at the Steenkampskraal mine in the Western Cape Province in South Africa, where high natural thorium concentrations lead to high thoron activity concentrations. The study concludes that while thoron concentrations cannot be fully eliminated by mine ventilation, it should be possible to significantly reduce thoron progeny so that radiation should be an avoidable hazard with adequate engineered protections. On top of this, thorium natural resources are more abundant in Earth’s crust than uranium (Figure 4), so a mindset change towards thorium exploration is all but expected.

Nuclear energy has a vast scope to fulfill human needs, but there is a clear need to design nuclear power plants that are properly planned and that utilize high standards of safety precautions. The low to negligible extent of air pollution from well-operated nuclear power plants can support a carbon-free society. After the Fukushima accident in 2011, Japan decided to close nuclear power plants and replaced them to some extent with coal power plants, jeopardizing carbon emissions reduction goals. The country’s nuclear generation dropped to zero by September 2013 for almost 2 years, which jeopardized carbon emissions reduction goals—as they were replaced mostly with coal and natural gas power plants, as shown in Figure 7. The reactors have been being resumed slowly through the years. By September 2020, Japan had 33 operable nuclear reactors with a total installed net generating capacity of about 32 GW, down from 54 reactors with 47 GW of capacity before the Fukushima accident in 2011 [Ministry of Economy, 2023, Trade and Industry–Agency for Natural Resources and Energy, 2023]. The government’s stated aim is for nuclear power to provide 20%–22% of electricity by 2030 (World Nuclear Association, 2023). Looking back in history, the carbon emissions from the top twenty nations from 1960 are compared to the same group in 2018 (Figure 8) (Global Carbon Atlas, 2021). Cumulative carbon emissions have risen from 9.104 Gt CO₂ in 1960 to 35.331 Mt CO₂ in 2018 (Figure 9) (Global Carbon Atlas, 2021), an enrichment of 3.88 times which is coupled with a global population expected to reach 9 billion in the coming years (Lam, 2013). This has called and propelled the high-tech and fourth industrial revolution era that has accompanied marked lifestyle changes in society already in the last two decades. Such changes illustrate that despite the nuclear power industry seemingly being slow in development in recent years, progress can rapidly pick up and thorium can play a leading or important role (Hargraves, 2012). Moreover, Hargraves (2012) delves into the early days of nuclear power development to exemplify how the first version of a technology (the uranium fuel cycle with LWR developed in the first half of the 20th century) can be far from the technical ideality when economic and political factors take precedence. What has changed since then is the climate emergency (Santos and Bakhshoode, 2021).

Most nations operating nuclear power plants have relied on LWR despite its several disadvantageous features: very short life span of the new fuel (1–3 years); generated plutonium waste; high pressure leading to hazard in case of failure and high risk for steam explosion (namely, the Chernobyl accident in 1986); high risk of fuel melting (namely, the Fukushima incident in 2011); high core radioactivity; and waste disposal problems (Dolan, 2017). On the other hand, Molten Salt Reactors (MSRs) have resurged in the search for constant advances in nuclear energy technologies. Its concept has been considered promising for further research and development by the Generation IV International Forum (GIF), as it offers significant improvements regarding environmental and safety concerns, by using liquid fuel, as well as economics, as shown in a model developed by Rykhlevskii (2019) in a simulation around reprocessing in an MSR. The thorium-uranium (Th-U) fuel cycle is an approach that can ensure the long-term supply of nuclear fuel, and that could be implemented in small modular MSR (Yu et al, 2019). Yu et al, (2019) found that in such applications the thorium energy contribution starts at 37.6% for the once-through mode of operation, increasing after several years to 47.4% once the batch reprocessing mode is implemented. At longer time scales of decades, the fuel transitions from low enriched uranium to ²³³U with online reprocessing mode, whereby the Th energy contribution can reach 89.1%.

In addition, International Atomic Energy Agency (2002) reports studies around more reactor systems and associated fuels, coolants/moderators. Some other reactors investigated on the thorium fuel cycles are PWR, CANDU, VVERT reactor, HTGR, WWER type reactors, FR, HWR, and ADS. Table 1 is a benchmark between other types of reactors, which shows the merit of MSR for the transmutation of minor-actinides (MA).

The lack of dedicated mines for thorium is somewhat worrying when compared to current commercial operations of uranium. However, thorium would be far from being the only natural resource from the periodic table requiring co-exploitation. Several examples can be obtained of valuable by-products of mining, simply by looking at the flowsheet of the Vale Base Metals (former Inco) operations in Ontario, Canada. While nickel is the main commodity of these operations, several additional valuable metals are co-extracted and refined, such as cobalt, platinum group metals (PGMs), selenium, and tellurium. Ault et al, (2015) thoroughly discuss the opportunities for integrated mining of thorium, which would be particularly beneficial as a means to close the resource gaps of what are currently terms ‘critical metals’ (or ‘critical minerals’). According to their study, the adoption and scale-up of the thorium nuclear fuel cycle should not result in thorium mines being constructed, because there is plenty of opportunities to recover thorium together with uranium and titanium mining (as well as smaller amounts recovered during tin, REE and iron mining), amounting to at least 100,000 t Th annually (mostly from titanium mines) from such active mines before new mines need to be brought online. Jordan et al, (2015), on the other hand, discuss that recovery costs will depend on whether thorium is a direct by-product of heavy mineral sand mining or a by-product of a by-product (i.e., tailings recovery), the former suggesting higher recovery costs. Considering the energy generation potential of this much thorium, it would take decades to require new resources. Ault et al, (2015) suggested that if all current nuclear energy generation switched from uranium to thorium, the by-product potentials would supply up to 12 times the amount of thorium required, assuming no reprocessing, and 250 times if reprocessing fuel cycle were implemented. It should be reminded, according to Ault et al, (2015), that the once-through thorium cycle still requires enriched uranium or plutonium as a seed starter, but a closed breeding fuel cycle does not, plus if uranium mining is reduced the titanium mining activities would more than sufficiently provided the needed thorium supply. Ault et al, (2016) go further in discussing the environmental benefits of not opening new mining activities and reducing the number of radionuclides ending up in mining tailings by switching from uranium to thorium fuel cycle. They also point out that while chemical processing for thorium recovery is needed, it is not a particularly challenging barrier in terms of technical and economic requirements.