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| Fig. 1: The most studied molten salt reactor concepts. (Image Source: S. Sahel-Schackis, after the Gen IV International Forum. [7]) |
In 2019, the International Energy Agency (IEA), an autonomous intergovernmental organization, published a report emphasizing the importance of nuclear power in decarbonizing the world's energy sector, with the goal of mitigating climate change. [1] First, in the context of climate change, the report states:
“The failure to expand low-carbon electricity generation is the single most important reason the world is falling short on key sustainable energy goals.”
The role of nuclear in this transition is underscored as follows:
“For advanced economies, nuclear has been the biggest low-carbon source of electricity for more than 30 years, and it has played an important role in the security of energy supplies in several countries.”
Nuclear fission energy is today a competitive and mature low-carbon technology, operating at very high levels of safety. As an example, the European Union (EU) boasted an installed nuclear electricity capacity of 122 GWe in 2013. This capacity accounted for one-third of the EUs generated electricity, and two-thirds of the low-carbon electricity. [2] Most of the current designs are Light Water Reactors (LWR) of the second generation, capable of providing base-load electricity with a mean average capacity factor of over 80% during the reactors whole lifetime. [3]
While current Generation II and III nuclear power plant designs provide a secure and low-cost electricity supply in many markets, further advances in nuclear energy system design hold the potential to expand the applications of nuclear energy. [4] Generation IV reactors are nuclear fission reactors believed to represent “the future shape of nuclear energy”, although no precise definition exists. [5] The development of Gen IV technologies is a collaborative international effort coordinated by the Generation IV International Forum (GIF). The Forum has identified six reactor designs as the most promising: the super-critical water reactor (SCWR), the very high-temperature reactor (VHTR), the molten salt reactor (MSR), the gas-cooled fast reactor (GFR), the lead-cooled fast reactor (LFR) and the sodium-cooled fast reactor (SFR).
A thermal reactor uses slow or thermal neutrons (equal to or lower than 0.025 eV) to sustain the fission chain reaction. A neutron moderator is used to slow the neutrons emitted by the fission reaction to make them more likely to be captured by the fuel. Gen II and III reactors are of this type.
The SCWR is a high-temperature, high-pressure, water-cooled reactor that operates above the critical point of water, where distinct liquid and gas phases do not exist, but below the pressure required to compress it into a solid. It is more accurately termed an epithermal reactor than a thermal reactor, as the average fission-causing neutrons possess an energy between 0.025 eV and 0.4 eV. The working principle is similar to the light water reactor (LWR), the most commonly deployed reactor today.
The main advantage of the SCWR is improved economics due to high thermodynamic efficiency (45% or more compared to about 35% for current LWRs) and the potential for plant simplification. [6] Improvements in the areas of safety, sustainability and proliferation resistance are also possible, and are being pursued by considering several design options using thermal and fast spectra, including the use of advanced fuel cycles. [7]
The Canadian CANDU and Russian VVER pressurized water reactors (PWR) design series, belonging to the class of LWRs, have been widely deployed worldwide. The latter is reported to be developing a 1700 MWe SCWR design. [8]
High-temperature gas-cooled reactors (HTRs) are helium-cooled graphite-moderated reactors with a once-through uranium fuel cycle. HTRs are characterized by inherent safety features, excellent fission product retention in the fuel, and high-temperature operation suitable for generation of industrial process heat, in particular for hydrogen production. Typical coolant outlet temperatures range between 700°C and 950°C, thus enabling power conversion efficiencies of up to 48%. The VHTR is understood to be a longer term evolution of the HTR, targeting even greater efficiency and more versatile use by further increasing the helium outlet temperature to 1,000°C or higher. Above 1,000°C, however, VHTRs will require suitable structural materials, especially for the intermediate heat exchanger. [7]
The first commercial demonstration of a VHTR was conducted by China, which falls under the category of a pebble-bed reactor (PBR), a subtype of VHTR. This 200 MWt reactor was integrated into the power grid, commencing power production in 2021 and achieving its initial full power capacity a year later. [7]
In Japan, the High-Temperature Test Reactor (HTTR) resumed operation in 2021, following a decade-long shutdown that began after the Fukushima accident in 2011. The HTTR initially attained its full design power of 30 MWt in 1999. [9]
Several private companies in the US are actively engaged in developing HTR demonstrations as part of the Department of Energy's (DOE) Advanced Gas Reactor Fuel Development and Qualification Program. [10]
An MSR is any reactor where a molten salt has a prominent role in the reactor core (i.e. fuel, coolant and/or moderator). Liquid-fuel MSRs are a type of nuclear fission reactor in which a molten salt serves as the nuclear fuel and may also serve as the coolant. In solid-fuel MSRs, the molten salt serves as the coolant for solid phase nuclear fuel. MSRs were originally conceived in the 1940s. [7]
Both solid- and liquid-fueled MSRs have seen a resurgence in interest over the past two decades. Proposed designs, with molten salt fluorides and chlorides salt mixtures, include both thermal and fast spectrum systems as well as designs with time and spatially varying spectra. Nearly every form of fertile and fissile material is being considered for its potential in an MSR fuel cycle. MSRs can be grouped into three classes and six families according to their technical characteristics, as shown in Fig. 1. [7]
MSRs have a number of advantageous characteristics, ranging from high-temperature operation (and consequent increased thermodynamic efficiency) to low-pressure operation reducing the driving force for radionuclide dispersal in the event of an accident. Moreover, refueling, processing, and fission product removal can be performed online, potentially yielding high availability. Their operation can be tailored for the efficient incineration of plutonium and minor actinides, the primary contributors to the toxicity and heat generation in long-lived nuclear waste. This adaptability potentially enables MSRs to utilize waste from other reactors. However, it is essential to recognize that spent nuclear fuel reprocessing is necessary, which entails inherent proliferation risks and substantial expenses. [5]
On the other hand, the extended distribution of radionuclides by liquid fuels can necessitate fully remote maintenance. Molten salt can also become highly corrosive if exposed to oxidative impurities. Overall, MSRs have substantial technology differences from both existing light water reactors (LWRs) and other advanced reactor concepts, necessitating different approaches to safety assessment, safeguards and operations. [7]
A fast reactor is a nuclear reactor in which the fission chain reaction is sustained by fast neutrons (carrying energies greater than 1 MeV on average). There is no need for a neutron moderator, but it generally requires fuel that is relatively rich in fissile material compared to slow thermal-neutron reactors. Fast reactors can be configured to fission all actinides (the 15 metallic chemical elements with atomic numbers from 89 to 103), thus drastically reducing the amount of spent nuclear fuel (nuclear waste).
The GFR system features a high-temperature helium-cooled fast-neutron spectrum reactor and a closed fuel cycle. The main advantages of GFRs besides allowing adoption of the closed fuel cycle arise from the high operating temperature, which enables enhanced thermal efficiency and the generation of high-temperature heat for industrial applications. Another noteworthy feature is the use of helium as the coolant, known for its chemical inertness, transparency, and non-corrosive properties.
Historically, pilot and demonstration projects in Germany, the UK and the US in the 1960s and 70s, as well as in Japan and China in the 2000s, utilized thermal designs with graphite moderators. Consequently, no true gas-cooled fast reactor design has ever reached criticality. [11]
In Europe, a GFR demonstrator named ALLEGRO is presently under development through the collaborative efforts of the Czech Republic, France, Hungary, Slovakia, and Poland. The primary objectives of ALLEGRO include demonstrating the feasibility of specific GFR technologies such as fuel, fuel elements, helium-related systems, and designated safety measures. [12]
The LFR system features a fast-neutron spectrum and a lead or lead-bismuth eutectic (LBE) coolant. The main advantages lie in the burning of long-term nuclear waste and the possibility for inherent proliferation resistance. [13] Three reference systems are currently under consideration by the Gen IV Forum:
A large-scale system, with a capacity of 600 MWe, is being developed in Europe for central station power generation. Known as MYRRHA, this project combines a nuclear reactor with a proton accelerator, constituting a so-called accelerator-driven system (ADS). An ADS distinguishes itself from a standard LFR, yet it is included here due to the similarity in considered coolant and neutron energies. Construction is scheduled for Belgium, with expectations of completion by 2036. The first phase, featuring a 100 MeV linear accelerator, is projected to be finished by 2026. [14] A reduced-power prototype named Guinevere became operational in 2012.
A 300 MWe intermediate-sized system is in development in Russia, referred to as BREST-OD 300. This system's use of heat-conducting nitride fuel in combination with the lead coolant allows for complete plutonium breeding within the core, meaning there are no excess neutrons present, enhancing the reactor's inherent safety. Construction commenced in 2021. [15]
Small, transportable systems of 10-100 MWe size are being developed in various countries, primarily led by private companies. While some have construction plans in place, there are currently no ongoing construction projects. [7,16]
The SFR system employs liquid sodium as its reactor coolant. SFRs have been operated in multiple countries since the 1970s. In the US, the 400 MWt Fast Flux Test Facility (FFTF) was operational for a decade, while the 20 MWe Experimental Breeder Reactor II (EBR II) remained in operation for over 40 years before their respective shutdowns in 1992 and 1994. The largest SFR ever operated was the French Superphenix reactor, with a capacity of over 1200 MWe. Despite initial cost overruns and delays, it ultimately achieved an impressive 95% availability. However, the reactor was forced to close due to public protests and political considerations. [17]
Currently, two commercial SFRs, BN-600 and BN-800, are in operation in Russia, while test reactors are running in China, India, and Japan. [15] Limited published data is available regarding the efficiency of these reactors.
China is constructing two CFR-600 units, which are demonstration SFR plants each generating 600 MWe. [7] In India, the Prototype Fast Breeder Reactor (PFBR), a 500 MWe SFR, is scheduled for completion by 2024, following multiple delays spanning two decades of construction. [16] The ASTRID project was a SFR planned to be built in France until France's nuclear agency CEA announced its cancellation in 2019, because "in the current energy market situation, the perspective of industrial development of fourth-generation reactors is not planned before the second half of this century," after an estimated €738 million had been invested. [18]
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| Table 1: Comparison of Gen IV nuclear reactors. [7] (†High Pressure = 7-15 MPa) |
The potential applications of Gen IV technologies, encompassing middle/high temperature co-generation, electricity production, actinides management, and energy supply for isolated grids, surpass those of current Gen III plants. The economics of Gen IV nuclear power plants still broadly align with current Light Water Reactors (LWRs), but the "carbon cost" for fossil-fired power plants would increase their relative valuation. Nevertheless, some of these technologies still require substantial research and development efforts. The high capital costs of nuclear energy, coupled with uncertain long-term conditions, pose financial risks for utilities and investors. [19]
In practice, government policies have thus far undervalued the low-carbon and energy security aspects of nuclear power, posing challenges for the continued operation of existing plants. New projects have been marred by cost overruns and delays, leading to expectations of a decline in nuclear power in Europe and the US. Notably, Germany, Belgium, and the UK have either announced or initiated the closure of existing nuclear power plants. In the US, nuclear generation is on a downward trend, although several states have considered incentives to extend the lifespan of nuclear plants, recognizing their role in ensuring the reliability of the power grid. Recent blackouts due to extreme weather have underscored the importance of a reliable power supply and a diverse energy mix.
In Europe and the US, public acceptance remains a key issue. Although favorable opinions of nuclear power are lacking in some countries, there are signs of changing attitudes. Another significant challenge for nuclear fission is the shortage of qualified engineers and scientists due to inadequate investment in nuclear careers and a reduced availability of specialized courses at universities. The preservation of nuclear knowledge is a pressing concern, particularly as many current generations of nuclear experts are approaching retirement. [4]
Nonetheless, the United States says it is committed to trying to accelerate the deployment of nuclear energy. [20] The revised European Strategic Energy Technology Plan of 2023 actually foresees an increase in nuclear power production in the coming decades, envisioning "at least 30-45 new large reactors and small modular reactors." [21]
In the US, nuclear currently contributes nearly one-fifth of all electricity generated, making it the largest single source of power that does not directly produce carbon emissions. [22] The federal government has initiated subsidies for older nuclear plants to prevent further closures, with a $6 billion fund established in the 2021 Infrastructure Investment and Jobs Act. Over half of all US states include nuclear power in their plans to reduce carbon emissions from electricity generation. [23]
In contrast, nuclear power is set to play a pivotal role in China's decarbonization plans, with an expected nuclear capacity of 105 GW by 2035, surpassing the US (92 GW by 2030) and the EU (76 GW by 2030). [24]
India also aims to expand its nuclear capacity, albeit from a smaller base. Faced with challenges similar to its European counterparts, including technological issues, cost escalations, and substantial upfront investments, nuclear power has become less attractive than cheaper solar and wind power. In 2021, the Indian government adjusted its nuclear power capacity goals to 22.5 GW by 2031, down from the initial target of 60 GW by 2032 set 12 years earlier. [25]
Looking ahead, nuclear energy is expected to continue its stable development trajectory in China, given its essential, low-cost role in decarbonizing the economy. In Europe and the US, the consideration of future development projects will likely require substantial government support in the form of financing and a robust regulatory framework. Nuclear energy's unique ability to provide a stable base-load electricity supply can complement intermittent renewable energies. Additionally, nuclear energy can play a significant role in the low-carbon transport sector, with high-temperature applications providing synthetic fuel and hydrogen, while the generated electricity can supply a significant portion of the energy needed for electric vehicles.
© Samuel Sahel-Schackis. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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