1 Introduction

Climate change is one of the current major challenges faced by all mankind. In 2018, the United Nations Intergovernmental Panel on Climate Change issued the Special Report on Global Warming of 1.5℃ [1], which figures out that the global temperature rise has been observed, and the impact of temperature rise on human beings is much higher than expected. The impact of temperature rise of 2℃ on the world will be unbearable, so mankind must strive to control the temperature rise within 1.5℃.

In response to the aforementioned global challenges, the international community has formulated the United Nations Framework Convention on Climate Change and its Paris Agreement, which provide the basic legal framework for climate change governance. Countries around the world have also set the timeline for achieving carbon neutrality based on their own financial and social conditions.

On September 22, 2020, at the general debate of the 75th Session of the United Nations General Assembly, Chinese president made a solemn commitment to "peak carbon emissions and achieve carbon neutrality", and first proposed the "Dual-Carbon" strategic goal of "striving to peak carbon emissions by 2030 and achieve carbon neutrality by 2060". This is also an inevitable requirement for promoting the transformation and upgradation of China's energy structure, industrial structure, and economic structure. It is of great significance for China to achieve high-quality development and build a community with a shared future for mankind.

Carbon neutrality, is not only an energy and environmental issue, but also a global and systemic issue. As an important part of the energy system, nuclear energy has the potential to play the supportive role to achieve the "Dual-Carbon" goal through its innovative-driven and high-quality development. The opportunities for the development of nuclear energy will be presented in Chapter 2, and the problems that need to be concerned and solved in the development of nuclear energy will be analyzed in Chapter 3. Next, the related studies on the safety, economy and sustainable development of nuclear energy conducted by NuTHeL under the "Dual-Carbon" strategy will be briefly introduced in Chapter 4. In the end, Chapter 5 is the conclusion, which briefly summarizes the paper.

2 Opportunities for nuclear energy development

Nuclear energy has the advantages of non-intermittency, high energy density, low pollutants emission, small occupation and less constraint by natural conditions [2]. Under the general trend of global energy conservation and emission reduction, nuclear energy has become a promising industry that requires more vigorous development and research.

CO2 Emissions from Fuel Combustion 2020: Overview [3], issued by the International Energy Agency, states that in 2018, global CO2 emissions were 33.63 billion tons. Among them, the electricity and transportation accounted for more than 2/3 of the emissions, and the remaining 1/3 mainly caused by the industry and construction.

The global energy consumption structure in 2020 is shown in Fig. 1 [4]. Coal, oil, natural gas and other fossil energy still occupy an absolute proportion, whilst hydropower, renewable energy and nuclear energy account for a relatively small proportion. There is still a long way to go for global carbon emission reduction. In particular, coal accounts for more than 50% of China's energy consumption, much higher than the proportion of coal in the global energy consumption structure [5]. Therefore, nuclear energy will play an indispensable role in the future as China achieves its "Dual-Carbon" goal and contributes to the global realization of carbon neutrality [6].

Fig. 1
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Global and China’s energy consumption in 2020 [4, 5]

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In 2011, the Chinese Academy of Engineering carried out research on greenhouse gas emissions from different electricity generation energy chains. The main conclusions of the study are as follows: the current actual normalized greenhouse gas emission of the front part of the nuclear fuel cycle in China (including uranium mining and smelting, uranium conversion, uranium enrichment, component manufacturing and utilization in nuclear power plant) are 6.2 g CO2/kWh, and the total normalized greenhouse gas emission of the whole nuclear fuel cycle (including spent fuel reprocessing and waste disposal) is 11.9 g CO2/kWh [7]. Whilst for the coal power industry chain, including coal production, transportation and construction, operation and decommissioning of coal-fired power stations, the normalized greenhouse gas emission of the whole life cycle is 1072.4 g CO2/kWh. Hence, the greenhouse gas emissions of the nuclear power chain are only 1% of the coal power chain. In addition, since nuclear power plants do not emit air pollutants such as SO2, nitrogen oxide and dust particles, so nuclear power is a clean energy with minimal impact on the environment [8]. It is fair to say that nuclear power is the low-carbon energy sources with the most obvious carbon emission reduction effect. Besides, the radioactivity discharge from nuclear power plants to the surrounding population is generally much lower than the local natural radiation background level. Hence, the nuclear energy can be utilized in a large scale.

In view of these facts, nuclear energy will play a greater role in electricity generation, heat supply and comprehensive utilization.

2.1 Electricity generation

At present, electricity generation is the main way of peaceful use of nuclear energy. By January 2022, according to statistics from the International Atomic Energy Agency (IAEA), the current number of nuclear power reactors in operation reaches 439 with a total electrical capacity of 390.6 million kilowatts. Besides, 52 nuclear power reactors are under construction, with a total electrical capacity 55.1 million kilowatts. In 2020, the global nuclear electricity generation accounted for about 10% of the total electricity generation and about 27.8% of the non-fossil energy electricity generation [4]. Among the most high-ranking developed countries in terms of economic aggregates, the proportion of nuclear energy in primary energy consumption has been maintained at 4% to 9%. By 2020, nuclear electricity generation in these countries accounts for 34%-52% of non-fossil energy generation [9].

Compared with the proportion of global nuclear power, China's nuclear power has great potentialities and vast development prospects. According to the 2021 nuclear power production data released by the China Nuclear Energy Industry Association, by March 31, 2020, there were 54 nuclear power reactors in operation in the mainland with a rated installed capacity of 55.80 million kilowatts [10]. From January to December 2021 the cumulative generating capacity of nuclear power reactors in operation was 407.14 billion kilowatt hours, accounting for 5.02% of the country's total generating capacity. Compared with coal-fired electricity generation, nuclear electricity generation is equivalent to reducing standard coal by 115.58 million tons, CO2 emission by 302.82 million tons, SO2 emission by 982,400 tons and nitrogen oxide emission by 855,300 tons [11].

More significantly, China's nuclear power industry has boomed in the past few years. On January 30, 2021, the world’s first unit of "Hualong One" nuclear power reactor (HPR1000)–the fifth unit of Fuqing Nuclear Power Plant in Fujian Province of China, has been run through the continuous full-power operation assessment and put into commercial operation; in January 2022, the sixth unit of Fuqing Nuclear Power Plant has been connected to the grid for electricity generation; in 2020, the model development and engineering design of the large-scale advanced pressurized water-cooled reactor, “Guohe One” (CAP1400), has been completed, and the demonstration reactor construction is currently being promoted as planned. The development and commissioning of the above-mentioned advanced pressurized water-cooled nuclear reactors will greatly contribute to the realization of the "Dual-Carbon" goal. Specifically, on December 30, 2021, China's Experimental Advanced Superconducting Tokamak (EAST) has set the latest long-pulse high-parameter plasma operating time record, speeding up the early service of clean energy for fusion reactors.

2.2 Heat supply

As a typical clean energy, nuclear energy requires less fuel and emits lower carbon emissions. Compared with traditional coal-fired boiler heating, nuclear energy heating has significant advantages. A nuclear reactor with thermal power of 300 MW requires only about 3 tons of nuclear fuel per year. After one year of operation, about 2.5 tons of spent fuel will be discharged, and the amount of waste after reprocessing will be reduced to about 1 ton. Whilst a coal-fired boiler plant with the same power needs to burn about 0.2 million tons of coal every year, releasing about 0.7 million tons of CO2, 1,700 tons of SO2, and 60 tons of NOx [12]. In 1954, the Former Soviet Union began to apply the experimental heating reactor AST-500 with thermal power of 10 MW to develop nuclear heating technology [13]. In the following decades, a number of European countries successively used operational commercial nuclear reactors for heating [14].

More than 60% of China's regions (involving more than 50% of the population) needs heat supply in winter, and the heating source mainly produced by large coal-fired power plants or small coal-fired heating boilers, with an annual coal consumption of about 500 million tons. The emission of harmful substances produced by coal combustion is relatively huge, which is one of the main causes of serious ambient air pollution and haze weather [14]. With the improvement of public requirements for air quality, the demand for clean heating is increasingly urgent. In 1989, Tsinghua University successfully developed the low-temperature heating test reactor NHR-5 with a thermal power of 5 MW; in 2021, China's Haiyang Nuclear Power Station has launched a demonstration of comprehensive nuclear energy utilization and has already realized nuclear heating for the city. Haiyang became the first zero-carbon residential heating city in China; at present, the "Yanlong" pool heating reactor and other types of reactors have been in development by the China National Nuclear Corporation. The radioactivity produced from the pool heating reactor is attenuated by multiple shielding, and the surrounding radioactivity is about 2% of that of the coal-burning devices of the same scale (please note that it varies according to the type and the producing area of coal). Therefore, the utilization of nuclear energy for heat supply will have great economic, ecological and social benefits.

2.3 Hydrogen production

Hydrogen energy is generally recognized as an ideal energy source in the 21st century. It has a wide application range in transportation, chemical industry, construction, etc., and developing hydrogen energy has been elevated to a strategic level by many countries. Generally, gray, blue or green hydrogen is classified according to the source of energy used and the method of production [15]. Green hydrogen refers to hydrogen production from renewable energy and nuclear energy, which is the mainstream of future hydrogen production. Nuclear energy has natural advantages in hydrogen production. For example, the outlet temperature of the high-temperature gas-cooled reactor can reach 800 ~ 950℃, it can be used to produce hydrogen through traditional water electrolysis and high-temperature steam electrolysis with less energy consumption and higher efficiency (higher than 40%) [16].

In 2020, China produced about 25 million tons of hydrogen. It is estimated that hydrogen demand in 2030 and 2050 will reach 35 million tons and 60 million tons, respectively [17]. In December 2021, the world's first industrial-scale demonstration plant of a high-temperature gas-cooled reactor at Shidaowan in Shandong Province has been connected to the grid for electricity generation. This reactor has an installed capacity of 200,000 kilowatts, which can meet the energy requirements of 600,000 tons of steel production capacity for hydrogen, electricity and part of oxygen [18]. It can be seen that under the "Dual-Carbon" goal, nuclear energy will play a greater role in cooperation with the hydrogen energy development strategy.

2.4 Seawater desalination

Water resources are indispensable for human survival and development. Although most of the earth's surface is covered with water, 96.5% of it is seawater and cannot be used directly. Only 0.26% of freshwater resources such as groundwater, lake water, and river water can be directly utilized by people. According to data released by the World Bank, by 2025, more than 1 billion people will live in water-scarce areas, and as many as 3.5 billion people may face water shortages [13]. Therefore, seawater desalination is the only way to alleviate water shortage.

Seawater desalination through nuclear energy can use existing nuclear power plants in operation to build seawater desalination facilities, and operate in cogeneration mode; alternatively, multipurpose advanced nuclear energy systems for desalination and other fields could be developed. Advanced nuclear energy systems have broad application prospects in the field of seawater desalination due to the comprehensive advantages such as compact size, strong flexibility, large power ratio, great adaptability etc. According to the report of IAEA, the cost of nuclear energy desalination on an industrial scale is about 0.47$/m3 ~ 0.5$/m3, while the corresponding cost of seawater desalination using fossil energy is about 0.77$/m3 ~0.8$/m3 [19]. Therefore, nuclear energy desalination has significant economic advantages as well as clean and pollution-free environmental merits.

3 Concerns of nuclear energy development

To achieve the "Dual-Carbon" goal, China must strive to establish a "near zero carbon emissions" energy system dominated by new and renewable energy by 2050. The proportion of non-fossil energy will further increase in the total energy system. Nuclear energy, as a member of non-fossil energy, is expected to be vigorously developed in the future. However, the high-quality development of nuclear energy still faces many uncertainties and challenges. The following issues should be concerned and properly handled in the development.

3.1 Safety

Safety is the prerequisite and lifeline for the development of the nuclear energy. The Three Mile Island nuclear accident in 1979 and Chernobyl nuclear accident in 1986 not only brought harm to local people's health and ecological environment, but also led to the development of the global nuclear industry into a low ebb. Since the Fukushima nuclear accident in 2011, Switzerland, Germany and Italy have proposed permanent nuclear abandonment. Meanwhile, the construction and approval of nuclear power projects or industrial parks at Pengze in Jiangxi Province, Beihai in Guangxi Province and Jiangmen in Guangdong Province were shelved or cancelled after being opposed by the public. In recent years, countries around the world have learned lessons and actively adopted measures to improve nuclear energy safety and rebuild confidence in development [20]. Only when safety issues are properly addressed can nuclear energy gain greater public acceptability. Consequently, the nuclear energy would have larger possibility for further development. In order to achieve the goal of "Dual-Carbon", the nuclear energy industry should make great efforts in the areas that may affect safety and public acceptability, laying the foundation for sustainable development.

3.2 Economy

Although nuclear energy has received certain attention and developed as a low-carbon and clean energy, it must maintain a superior economy in order to integrate into the international and domestic economy. Solar photovoltaics (PV) has fallen 82% since 2010, followed by concentrating solar power (CSP) at 47%, onshore wind at 39% and offshore wind at 29%, according to cost data collected by the International Renewable Energy Agency (IRENA) from 17 000 projects in 2019 [21].

However, the current deal price for nuclear power is below the benchmark price. From 2016 to 2020, the trading price of nuclear power market in Qinshan and other six nuclear power plants falls within 0.26~0.37 ¥/kWh, while the online electricity price benchmark is about 0.4 ¥/kWh in the same period. Hence, under the market-oriented trading system, nuclear energy-related companies must comply with market requirements, tap potential capacity, strictly control investment and operating costs such as project costs, establish a new management system that adapts to power market reforms, and enhance their market competitiveness.

3.3 Flexibility

The flexibility of nuclear energy includes matching nuclear power with other energy sources to participate in grid peak shaving and nuclear energy's multi-energy co-generation. In the context of the competitive development of nuclear energy and other renewable energy sources, the double randomness and volatility of a high proportion of new energy sources and huge electrical loads have brought enormous pressure to the safe operation of the power grid. To realize the complementary advantages of various energy types, and then to improve the economy, stability and flexibility of electricity generation, are the prominent problems that needs to be solved urgently in the high-quality development of nuclear energy and renewable energy under the goal of “Dual-Carbon” [22]. The nuclear energy-renewable energy hybrid power generation system [23] can not only solve the disturbance problem of wind energy, solar energy and other renewable energy electricity generation to the power grid, but also improve the comprehensive utilization efficiency. At present, a variety of schemes have been designed. To realize the commercial utilization of hybrid power generation system, the overall design optimization and economic analysis of the system need to be further improved [24].

At the same time, nuclear energy can also play an important role in carbon emission reduction of non-electric application fields such as heat supply, hydrogen production and seawater desalination in addition to power supply, providing new ideas for improving the flexibility of nuclear energy. By adjusting the proportion of non-electricity utilization of nuclear power plants, the problem of limited depth of peak shaving of nuclear power grids can be compensated to a certain extent. Nuclear energy has inherent advantages in non-electric utilization. For example, the New Nuclear Watch Institute (NNWI) in the United Kingdom pointed out that the cost of hydrogen production by nuclear power is 20% less than that of wind power [25]. By 2019, 79 nuclear reactors worldwide have been used for non-electric applications, some of which are multipurpose reactors, including seawater desalination (10 reactors), residential heating (56 reactors) and industrial heating (32 reactors) [26]. It has accumulated 750 reactor years of experience in safe operation [26]. With the technology development, especially the gradual maturity and application of the fourth-generation nuclear energy system technology, nuclear energy should perform more functions in a variety of non-electric comprehensive utilization fields, and play a greater supporting role in reducing carbon emissions and ensuring energy safety (as shown in Fig. 2 [27]).

Fig. 2
figure 2

Maximum coolant outlet temperature of Generation IV reactor and process with temperature range [27]

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3.4 Sustainability

In the development of nuclear energy, efforts should be made to achieve long-term sustainable development. At present, the key problems restricting the long-term and large-scale sustainable development of global nuclear energy include the supply of fission fuel and the reprocessing of spent fuel. Nuclear fission fuel is a non-renewable mineral resource with limited reserves. Given the global uranium demand as of January 2019, the identified recoverable uranium resources, including reasonably determined and inferred resources with a mining cost of less than 260$/ kgU, are only available for human usage for about 135 years [28]. Besides, the exploration, mining and enrichment of new uranium resources will require more energy consumption. At present, the pressurized water reactor adopts a once-through fuel utilization method, and the utilization rate of uranium resources is only about 0.6%. At the same time, the utilization of nuclear energy also discharged high-level radioactive waste, resulting in environmental pressure. The high-level radioactive waste that requires geological treatment of a 1GW nuclear power unit can reach 2 m3/tU per year [29]. In addition, the pressure on global spent fuel storage is increasing and the geological disposal of high-level radioactive waste is encumbered, and many countries are facing public opposition. Therefore, it is imperative to explore the long-term sustainable development of nuclear energy.

4 Related research conducted by NuTHeL

NuTHeL was established in 1958 at Xi’an Jiaotong university in China. After more than 60 years of development, NuTHeL has conducted a series of theoretical, experimental and numerical study on nuclear thermal–hydraulic and safety characteristics analysis. In recent years, NuTHeL has also undertaken a series of efforts to improve the safety and economy of advanced nuclear reactor systems, such as the third-generation large-scale advanced PWRs "Hualong One", "Guohe One" and the design and development of SFR, these achievements have contributed to help nuclear energy play a supporting role in the realization of China's "Dual-Carbon" goal.

4.1 Thermal–hydraulic design and safety analysis of the third generation PWR

The third generation PWR has significant advantages in economy and safety, and it is the promising reactor type of commercial nuclear power plants in the future, as well as the potential reactor type of nuclear energy applied in the "Dual-Carbon" strategy. The third-generation advanced nuclear energy system has complex structures and variable operating conditions. Hence, it is indispensible to conduct operation characteristics tests of system equipment and refined coupling simulation of full scale integration, so as to provide guarantee for the design and safe operation of nuclear power system. The related work carried out by NuTHeL will be briefly introduced in the following section.

4.1.1 Experimental study

A sequence of experimental research has been conducted by NuTHeL to support the development of "Hualong One" and "Guohe One". Lots of separate test facilities were built to analyze the specific phenomenon in the key equipment of the reactor, such as Automatic Depressurization System (ADS) [30,31,32,33], Steam Generator (SG) [34], Emergency Core Cooling (ECC) [35, 36], and containment dome [37, 38], pressure-swirl nozzle [39]. The representative test facilities are shown in Fig. 3. The operation characteristics of the equipment are obtained, which provides experimental data support for the equipment R&D and application in the third generation PWR.

Fig. 3
figure 3

Test facilities for Gen III PWR. a ADS-4 test facility. b ECC test facility. c Spray test facility

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4.1.2 Development of numerical simulation software

NuTHeL has developed a series of thermal hydraulic analysis software for key equipment and phenomena in the third generation advanced PWR, including: reactor system safety analysis code [40], sub-channel analysis code [41], multi-compartment containment code [42] and steam generator thermal hydraulic analysis code [43], etc. The software can be used to perform the full-scale thermal–hydraulic simulation of complex equipment. The simulation models for system analysis and sub-channel analysis are shown in Fig. 4. These findings offer more predictive values for thermal hydraulic and safety analysis of advanced nuclear power system.

Fig. 4
figure 4

Simulation models for Gen III PWR. a Simulation model for nuclear reactor system analysis. b Simulation model for sub-channel analysis [41]

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4.1.3 Safety characteristics analysis under severe accident

NuTHeL carried out experimental research on the heat transfer characteristics of the molten pool [44,45,46,47] (Fig. 5). The natural circulation characteristics in the lower head after a severe accident of the reactor are obtained, which provided a foundation for design optimization and heat transfer enhancement. A simulation and analysis code for severe accidents in PWR has been developed [48], which includes early behavior module, core degradation module, debris bed module, molten corium in-vessel retention module and thermal hydraulic module, providing effective support for analysis and mitigation of severe accidents in PWR.

Fig. 5
figure 5

COPRA test facility

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4.2 Thermal–hydraulic design and safety analysis of SFR

To solve the problems of limited uranium resources and spent fuel reprocessing and realize the sustainable development of nuclear energy, the research and development of fast neutron reactor has great application potential. SFR has received much attention by many countries due to its advantages in proliferating nuclear fuel and transmutation of long-life radioactive waste. It is one of the most advanced reactor types in the fourth generation of nuclear energy systems and it is relatively close to satisfy the requirements of commercial nuclear power plants. NuTHeL conducted a series of mechanical experimental studies and system numerical simulation studies for the key issues in the design, construction and safe operation of SFR.

4.2.1 Experimental investigation

Compared with traditional water-cooled reactors, liquid metal reactors have significant differences in heat transfer mechanism and design philosophy. It is necessary to reveal their flow and heat transfer mechanism and related safety characteristics through experimental investigations. Accordingly, NuTHeL carried out experiments on the thermal hydraulic characteristics of single- and two-phase sodium metal, and obtained the flow and heat transfer characteristics of liquid sodium in the rod bundle channel [49] (Fig. 6(a)). An experiment was designed to measure the flow rate between the components and sub-channels of SFR, and the resistance characteristics of the sub-channel in the bundle with wrapped wire were obtained [50] (Fig. 6(b)). In addition, based on the mechanics characteristics of SFR components, the experimental data of wall stiffness, flexural stiffness, thermal bending deformation, contact force, component spacing change and temperature distribution were obtained, which provided data support for the validation of theoretical calculation model and component deformation analysis code for SFR [51] (Fig. 6(c)).

Fig. 6
figure 6

Test facilities for SFR. a Test facility of sodium flow and heat transfer. b Test facility of sub-channel hydraulic characteristics [50]. c Test facility of assembly section stiffness and bending stiffness

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4.2.2 Development of numerical simulation software

Due to the characteristics of high thermal conductivity and low Prandtl number of liquid sodium, as well as the unique thermal hydraulic phenomenon of SFR, the transient thermal hydraulic and safety analysis codes widely used in PWR design and safety analysis are not applicable. Hence, based on the latest research worldwide, NuTHeL further considered the coupling heat transfer effect between components of SFR, flow redistribution effect and local three-dimensional effect, etc., and then developed the thermal hydraulic transient analysis code of SFR, which was validated by a large number of experimental data. This model covers primary loop, secondary loop, intermediate loop of passive heat removal system and all important components in each loop, as shown in Fig. 7. The software can be used for transient thermal hydraulic and safety analysis of different types of SFRs, with strong practicability and adaptability [52, 53]. In addition, a reactor core sub-channel analysis program [54] and a 3D numerical simulation tool [55] were also developed to meet the calculation requirements of different scales and precision.

Fig. 7
figure 7

Numerical model of the SFR [53]

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4.2.3 Safety characteristics analysis under severe accident

According to the heat and mass transfer behavior characteristics of SFR under severe accident conditions, the metal, metal oxide, and mixture of metal and metal oxide were respectively used as the replacement material of the core melt in the experiment, and liquid metal sodium was used as the coolant. The fragment characteristics of high-temperature melt in the coolant were obtained [56], and the facility is shown in Fig. 8. Additionally, a simulation code was developed for the possible sodium fire phenomenon after the accident [57], which can be used to offer more predictive values in this phenomenon, providing technical support to ensure the safety of SFR.

Fig. 8
figure 8

Test facility of melt fragmentation in sodium pool

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5 Conclusions

The current situation of energy system and the basis of nuclear energy development in China are investigated in the paper. The innovative development opportunities of nuclear energy in the fields of electricity generation, heat supply, hydrogen production and seawater desalination under the background of "Dual-Carbon" goal are analyzed. The main problems that need to be considered in the current nuclear energy development in China are discussed, including safety, economy, flexibility and sustainability, etc.

To meet the "dual carbon" goal, NuTHeL conducted a series of experimental, theoretical and numerical simulation studies on critical thermal hydraulic and safety issues in the third generation PWR and SFR. The experimental studies provide indispensable models for the thermal–hydraulic design, such as flow and heat transfer model. The code development research provides high precision software for the design and safety analysis of PWR and SFR, which helps to improve the design economy while considering the safety margin. The safety and accident research gives mitigation strategies for designed basis accident and severe accident, ensuring the reactor safety throughout its life cycle. The research of SFR will contribute to the early service of commercial reactor and realize the proliferation of nuclear fuel and the transmutation of spent fuel, so as to promote the sustainable and high-quality development of nuclear energy.

Under the "Dual-Carbon" goal, countries around the world will gradually develop a diversified and clean energy system. As a typical low-carbon energy, nuclear energy can surpass the role of electricity supplier, and achieve the complementarity of multi-energy co-generation and co-supply. Consequently, nuclear energy will continue to make contributions in promoting clean and low-carbon energy transition in the future. In order to finally achieve high-quality development of nuclear energy, it is necessary to systematically plan and deploy the supply chain, industrial chain and innovation chain. Besides, great emphasis should be placed on solving the practical and fundamental issues as well as making breakthroughs of forward-looking and disruptive technologies in nuclear energy development. Eventually, nuclear energy could play a more prominent supporting role in achieving the "Dual-Carbon" goal and realizing the long-term sustainable development of human society.