Background
The need to cut carbon emissions has become a global priority to mitigate climate change effects. Unstable global oil prices, unreliable supply due to political instability in oil-exporting countries, and more importantly, the growing fossil fuel-related carbon dioxide (CO2) footprint are the factors driving the shift to renewable energy (Said and Omri 2). As a result, concerted efforts to promote innovation and technology development in low-carbon emission systems (wind, solar, and nuclear power) have increased to meet global carbon targets. For example, OECD nations in Europe have committed to cut carbon emissions to 20% and 80-95% by 2020 and 2050, respectively (Said and Omri 2). The goal is to create a resilient energy future that will help escape adverse climate change effects. For this reason, nations are advised to lower CO2 emissions to 9.5 gigatons by 2050 to reverse the greenhouse effect (Jin and Kim 468). To attain this de-carbonization level, renewable energy, including nuclear solutions, should be adopted in electricity, transportation, and industrial systems.
Nuclear energy can help address the climate change effects because its carbon emission is almost negligible. Its net CO2 emission over a lifecycle is about 15g per kilowatt-hour (Said and Omri 3). Thus, it is low-emission renewable energy for combating the effects of climate change. Nuclear power is the main source of low-emission electricity in developed countries, accounting for 18% of all energy produced (Jin and Kim 470). However, its share of the electricity supply worldwide has been decreasing since the 1970s due to aging nuclear reactors, high capital costs, radiation accidents, and limited capacity. Better technologies have been developed to address these challenges and meet nonelectric energy needs. This report discusses advanced nuclear energy options and their implementation in existing systems. A proposal based on the review is provided for the public or policymakers in countries adopting low-cost and safe non-emitting energy sources considering moral or ethical concerns.
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The Fast Reactor Concept
Most advanced nuclear energy solutions are classified as fast-neutron reactors or FNRs. They differ from the conventional light-water reactors in many ways that confer their specific advantages, though they have some drawbacks. Thermal nuclear reactors (TNRs) that are in use in various sites globally utilize a moderator to decelerate neutrons during a nuclear chain reaction (Howarth 174). TNRs require enriched Uranium-25 as the fuel to sustain a fission reaction. An enrichment level of 5% and moderating materials, such as light or heavy water and graphite, are needed (Ford et al. 195). In contrast, the FNR technology does not utilize moderators but requires highly concentrated U-35 or fissile plutonium. FNRs lack a neutron-moderating effect and diverse coolants, including liquefied metals, are used to achieve this objective.
FNRs can be designed as breeders or burners that generate less fissile material than the fuel added. Breeder FNRs have a high neutron-capture efficiency, which makes them more effective than TNRs. Additionally, they can utilize diverse isotopes; thus, FNRs can theoretically operate using spent fuel indefinitely (Ford et al. 195). The designs involve a closed fuel cycle, not an open one, hypothesized to increase FNRs’ lifespan. A major challenge is that a closed-fuel system separates plutonium from nuclear waste, a raw material for developing nuclear weapons.
Advanced Nuclear Reactors
Diverse nuclear energy technologies are under development, with commercial operation expected in the next decade. Several American companies operate programs expected to deliver power to the national grid in a few years. The advanced reactors under development differ in design and technology type used. They all aim to improve on existing commercial plants in “cost, safety, security, waste management, and versatility” (Morgan et al. 7186). As a result, they include advanced technical designs that enhance their performance and flexibility. Some of the additional features incorporated into these reactors are safety components, modular designs, improved physical-chemical stability, and a fast-neutron spectrum to increase yield (Ford et al. 198). Advanced nuclear energy technologies fall into three main classes: water-cooled, non-water-cooled, and fusion reactors.
Advanced Water-cooled Reactors
These technologies improve boiling water reactors (BWRs) in critical areas, including simplified design, reduced size, and greater efficiency. They include the small modular light water (SMLWR) and supercritical water-cooled (SCWR) reactors. SMLWRs have a maximum electricity-generating capacity of 300 megawatts (MW), lower than the 1,000 MW output by conventional plants (Hokenson 242). They are designed based on light water reactor (LWR) technology, but the components are miniaturized to fit in one pressure vessel. SMLWRs can be built in the factory before being shipped to a plant for installation, resulting in lower capital costs and shorter payback periods than larger LWRs (Hokenson 243). Additionally, mass production of SMLWRs is possible, which leads to economies of scale advantages.
SMLWRs under development are designed to cater to the specific energy needs of a plant. An American company, NuScale Power, has developed a 60-megawatt module reactor that can be assembled in a factory before being transported to a site for installation (Morgan et al. 7182). A public-private partnership was the funding mechanism used in developing this technology. Other American companies currently developing SMLWRs include Holtec and GE.
SCWRs are a high-temperature version of the LWR technology used in nuclear reactors. Supercritical water in a critical pressure and temperature point where liquid and gaseous phases cannot be distinguished is used in SCWRs (Mignacca and Locatelli 2). The aim is to enhance the efficiency of conventional BWRs. Efficiency levels of 44% have been achieved in SCWRs, compared to 34% in BWRs (Mignacca and Locatelli 2). In its design, supercritical water from a reactor core turns into vapor, driving a turbine generator to produce electric power. SCWR designs depend on the neutron spectrum type used in the reactor.
Advanced Non-water-cooled Reactors
These modules are fission reactors that utilize different substances for cooling but not water. The materials used include molten metals, such as lead and gases, for example, helium (Mignacca and Locatelli 4). Liquefied salts are also utilized as coolants in these reactors. High-temperature gas reactors (HTGRs) use this technology to produce heat for industrial purposes and electricity generation. Various HTGR variants are under development, with some being adopted for commercial use.
Very High-Temperature Reactors
These HTGRs are helium-cooled, graphite-moderated reactors that are the most mature among nuclear energy technologies. They can reach outlet temperatures of up to 1,000°C, compared to 300°C for SMLWRs (Ford et al. 196). The heat generated supports the cogeneration of electric power and hydrogen and iron smelting industries. More research attention has turned to produce modest outlet temperatures due to the associated commercial viability in recent years. HTGRs involve two main designs that differ in the type of graphite moderator used. The first module involves a graphite-based core with detachable components that contain fuel particles (Ford et al. 196). In contrast, the second design includes small graphite balls (pebbles) with particulate fuels stacked into the core.
The fuel comprises small-sized particles enclosed in silicon carbide – a thermo-resistant coating. Thus, experimental results have shown that the reactor and the fuel can tolerate high temperatures that can cause a meltdown and harmful radioactive emissions (Mignacca and Locatelli 6). HTGRs are the most well-developed advanced nuclear energy solutions that have gained commercial adoption in several nations, including the US, UK, Japan, and China.
Gas-cooled Fast Reactor (GFRs)
GFRs are a technology that differs from HTGRs in that it operates in a fast spectrum, not in the thermal one. They are high-temperature, enclosed fuel-cycle fast reactors whose main coolant is helium (Ford et al. 197). GFRs do not need the graphite moderator used in HTGRs to decelerate the neutrons. A basic GFR design involves an enclosed U-Pu fuel cycle that recycles the fuel source when set up as a breeder (Krall and Macfarlane 327). The fuel can be plutonium or uranium, and higher temperatures (850°C) can be achieved in HTGRs than in LWRs; hence, they have potential use in industries and electricity generation.
The main drawback with HTGRs is that the coolant (helium) used has a lower heat-removal capacity than molten metal agents in case of a meltdown. Some European countries, including Hungary, Poland, and Slovakia, are constructing a GFR reactor (ALLEGRO) that uses a French design (Krall and Macfarlane 327). Further, an American firm, General Atomics, is building a GFR design based on the gas-cooled fast reactor concepts.
Lead-cooled Fast Reactors (LFRs)
LFRs comprise an enclosed fuel cycle that uses liquefied lead or lead-bismuth alloy as a proposed coolant. They have many advantages over other reactors that enhance their commercial viability. LFRs, like sodium-cooled fast reactors (SFRs), use molten metal coolant that supports low-pressure systems and passive cooling in case of a meltdown (Nguyen et al. 256). However, unlike the sodium used in SFRs, lead is mainly inactive; hence, safer. Another advantage is that lead can retain byproducts of fission, and thus, they can prevent harmful radioactive emissions from reaching the environment during an accident. Additionally, LFRs can be built to burn actinides in spent fuel, reducing their half-life significantly.
LFRs present some design challenges that must be addressed to ensure their viability. For example, liquid lead-related corrosion of the steel structure of nuclear reactors is a major problem. Thus, further technological advances are needed to develop corrosion-resistant steel. Additionally, lead is very dense and opaque, making it difficult to monitor the reactor’s core (Nguyen et al. 272). Another design challenge relates to this metal’s high melting point – maintaining it in a liquid state under lower temperatures for easy circulation is difficult. The countries developing LFR facilities are Russia, the US, and Japan.
Sodium-cooled Fast Reactors (SFRs)
This unconventional nuclear technology is currently at an advanced stage of development. SFRs involve a fast reactor circuit with molten sodium as the coolant (Hokenson 242). The liquid metal enables the system to operate at low pressure close to atmospheric levels. Another advantage of SFRs is that the heat-transfer character of molten sodium makes it possible to perform passive cooling. The outlet temperature is modest, ranging between 500°C and 550°C (Lou and Gandy 2834). This thermal level (lower than that achieved by HTGRs) means that they can utilize materials that cannot be used in other reactors. SFRs involve two designs that differ in the heat exchange systems implemented. The first one is the loop-type concept, where both the core and heat exchanger are embedded in a pool of liquid sodium (Lou and Gandy 2834).). The second design is the loop-type reactor, where the heat exchange component is contained in a distinct vessel.
Modular SFRs that are assembled in the factory before being shipped for installation on site are possible. However, the sodium used in SFRs is highly reactive to oxygen and water and can cause fires. Therefore, the coolant system must include an intermediary cooling component to remove this metal from vapor to avoid its release during accidents (Howarth 175). As a result, SFRs have additional costs associated with maintenance and safety. They include an enclosed fuel cycle that can utilize spent fuel but refueling may be necessary after a few decades. Like LFRs, SFRs also destroy actinides, reducing the half-life of radioactive waste. Given the safety and cost concerns, only a few SFR plants occur globally in Russia, China, and India.
Molten Salt Reactors (MSRs)
The coolant for these reactors is liquefied salts used to cool the core. The MSR design is similar to that of the HTGR but utilizes salt blocks to cool the circuit. An MSR variant is the salt-fueled MSRs in which molten fuel is combined with the salt coolant (Mignacca and Locatelli 3). They can operate in the fast or thermal spectrum, and the actinide burn-up potential of fast-MSR reactors is high; hence, useful in reducing radioactive emissions during accidents. Another advantage is that a high outlet temperature (700-1000°C) can be achieved using MSRs (Mignacca and Locatelli 7). However, attaining this thermal level is difficult, and further technological research is needed. A distinctive feature of salt-fueled MSRs is the freeze plug, a passive safety component useful during an accident. American and Chinese firms are currently developing MSR reactors.
Fusion Reactors
These advanced nuclear designs generate power by fusing light atomic nuclei. Research and development (R&D) of this concept has attracted significant investment to build a testing facility. Fusion energy generation uses light atoms (hydrogen) subjected to high temperatures to produce a plasma with free electrons (Howarth 174). Keeping the plasma intact during heating to fuse the nuclei is a design problem. A proposed experimental project in the United States plans to use a magnetic field to achieve high temperatures needed by fusion reactions. The advantage of fusion reactors is that it produces no harmful emission or radioactive waste. Additionally, their design and energy source (hydrogen) reduce the risk of a meltdown.
A Public Policy Proposal
From the discussion above, advanced nuclear technology could address safety issues, high capital cost, hazardous spent fuel, and nuclear arms development that seem to hamper its widespread adoption in electricity generation, transportation, and industries. Given these concerns, the diversity of designs with different benefits and drawbacks, and the rising demand for non-emitting sources of energy, a central regulatory agency is proposed. This publicly-funded body will coordinate R&D efforts and mitigate safety and other risks to the environment and the public using a multi-pronged approach.
Environmental groups oppose advanced reactor development based on waste management and radioactive emission to the atmosphere. Current LWRs pose spent fuel disposal challenges, but advanced reactors such as LFRs and SFRs yield less radioactive waste by destroying actinides (Krall and Macfarlane 326). The agency will evaluate the new designs to determine the extent to which waste management and air emission challenges have been minimized. This approach will help alleviate environmental and health concerns inhibiting the commercialization of advanced reactor technology. Federal support for versatile reactors through this agency will enhance industrial use. The advanced reactors, including small modular reactors (SMRs), allow for modular installation in different sites due to their small size (Hokenson 244). Funding R&D for high-temperature micro-reactors can reduce capital costs and scale up their development for industrial processes requiring heat.
A major concern with nuclear energy projects is the weapon proliferation risk. Non-state actors can use fuels (enriched uranium) to build nuclear arms, posing a threat to national and global security. However, proponents note that advanced reactors include sealed designs that make it difficult to reach the core (Howarth 176). Additionally, they use fissile materials in small quantities, and some (LFRs and SFRs) produce non-radioactive waste that may not be useful in weapon development. Thus, the agency would support fast-reactor designs that use highly enriched fuel – not useful for building nuclear arms.
The operational safety of the reactors is another public concern that this body will need to address. The chemical properties of components used in advanced reactors (cooling agents, fuels, and moderators) pose unknown risks. Therefore, testing facilities are needed to ascertain their safety before commercial use. Additionally, advocates suggest that SMRs may be safer because of the small amount of fuel needed (Hokenson 248). Siting the advanced reactors underground couple with remote monitoring can also reduce the safety risk to people and the environment.
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