As the energy market shifts, many overlook the potential of nuclear energy to meet the need for clean and affordable energy solutions in the future. In fact, nuclear energy remains the greatest clean power contributor in the United States, producing 800 billion kilowatt hours of electricity each year. Nuclear energy can fill the intermittency gaps of clean energy sources like wind and solar and create new high paying jobs while increasing US energy independence. Over the past 50 years, research and development have made huge strides toward a nuclear energy future that is both safe and cost-effective while maintaining low environmental impact.
NUCLEAR REACTOR BASICS
From a broad perspective, nuclear reactors function in a similar way to familiar energy plants; the heat released from a fuel source is used to produce steam or passively spin a turbine to generate electricity. But instead of burning coal or oil, nuclear plants incite a chemical reaction, nuclear fission, that produces vast amounts of energy in the form of heat. Nuclear fission happens when compressed radioactive atoms are split using neutrons. This process releases energy and initiates a chain reaction within the tightly packed atoms.
Three main fuel types help to regulate and control nuclear reactions: solid fuel rods, pebbles, and liquid nuclear solutions. Each of these different nuclear fuels houses radioactive atoms that are compressed or concentrated to proliferate the nuclear chain reaction, often uranium-233, uranium-235, or plutonium-239. Each fuel source uses different layers and coating to help control or moderate the reaction. The concentration and fuel type determine the speed of the neutrons in the nuclear reaction.
Solid fuel rods are thin metal cylinders that hold fissile pellets. The fuel rods are grouped in fuel assemblies that are placed in the reactor core. Along with the coolant, control rods can be placed in between the rods to regulate the rate of nuclear fission. Solid fuels rods are typically used in boiling water or pressurized reactors.
Nuclear pebbles, on the other hand, are tennis ball-sized spheres used as the core in high temperature nuclear reactors. The pebbles or TRISO fuel are made up of layers of moderating isotropic materials that act as buffers and contain the fissile fuel in the center. Thousands of pebbles make up the core of the reactors with new pebbles added to replace the spent fuel.
Finally, liquid nuclear fuel contains dissolved fissile particles in molten salt, aqueous salt solutions, or liquid metal. The liquid fuel acts as both the moderator and venue of reaction. This fuel type offers several safety benefits, increases fuel efficiency, and decreases nuclear waste products.
Each reactor also uses a coolant to control the temperature of the reactor and transfer the heat to generate electricity. The three main coolant categories include water, molten salt or metal, and gas. The coolant directly corresponds to the fuel type, and together they determine the rate of the reaction.
VARIOUS REACTOR TECHNOLOGIES
The following nuclear reactors are just a few of the technological developments in nuclear energy that have the potential to transform the current energy market.
High Temperature Gas Reactors
High or Very High Temperature Gas Reactors (VHTR) are nuclear systems specifically designed to support the cogeneration of electricity and hydrogen. High Temperature Reactors use TRISO coated pebbles as the fuel that runs through the graphite core structure. Helium gas acts as the coolant and heat transfer material. Uniquely, VHTR are designed with a dedicated core layout and lower power density that allows for passive heat removal. VHTR design enhances reactor safety, limiting risks traditionally associated with nuclear meltdowns. Electricity is generated using either a helium gas turbine in a direct cycle or a conventional steam generator. Also, the heat from the thermal reactors enables the efficient production of hydrogen from water through thermochemical processes with minimal byproducts.
The high operating temperatures of VHT Reactors have a variety of thermal applications that are desirable in chemical, oil, and iron industries. This versatility is particularly beneficial as essential industries seek alternative production methods to meet strict emission standards. In the current market, the middle temperature VHTR range appears to be the most advantageous given technological and material constraints. High Temperature Gas Reactors have undergone significant research and testing and will soon be ready for market implementation in many countries.
Sodium-Cooled Fast Reactors
Sodium-Cooled Fast Reactors (SFR) differ from thermal reactors in that they operate at a low temperature and pressure. The use of liquid sodium as the reactor coolant allows these reactors to operate at comparatively high-power density. Furthermore, the fast neutron spectrum of the reactors expands the compatible uranium resources, which are quite limited for thermal reactors. However, because sodium is reactive to both water and air, the reactor must function in a sealed coolant system.
To produce electricity, a working fluid of water, supercritical carbon dioxide, or nitrogen is used to transfer the heat and spin turbines. While SFRs do not offer the versatility of thermal reactors, the closed fuel cycle enables the fast reactor to utilize radioactive resources more effectively and efficiently. Consequently, radiotoxicity and heat load are reduced. SFRs achieve a high level of safety through passive heat distribution and reduce radioactive exposure in a closed system. They also use many of the basic technologies and materials of former fast reactors thereby reducing the time and investment associated with the research, development, and approval of new technology, and making them more competitive in the electricity market.
Molten Salt Reactors
Molten Salt Reactors were first proposed 50 years ago and offered a completely different approach to traditional nuclear energy generation. Molten Salt Thermal Reactors use molten or superheated salt as the coolant, which is then used in a heat exchange to generate electricity with a turbine. This technology uses solid fuel rods to house the fissile material in the reactor core. The reactors use a molten salt solution to house the nuclear fuel particles and act as the heat transfer liquid.
Unlike traditional reactors, the control rods in Molten Salt Fast Reactors act as the mediator or resistance material instead of holding the nuclear fuel. The unique design decreases radiation damage and fuel burn-up as well as the requirement to handle or fabricate solid fuel rods. Furthermore, Molten Salt Reactors extend the capacity and life of fuel resources and can use a larger range of actinides including thorium, which ultimately reduces the amount of nuclear waste. The waste that is produced can be processed in real time and the reactor can be refueled without shutting down operations.
Unfortunately, Molten Salt Reactors, both thermal and fast, are not currently feasible energy alternatives. Significant research and development regarding safety and performance are required. Also, the added layer of licensing and risk assessment for a host of new instruments, controls, and related equipment will take time and investment creating a large hurdle to the potential use of Molten Salt Reactors.
The three technologies highlighted above – High Temperature Gas, Sodium-Cooled Fast, Molten Salt Reactors – are categorized as Advanced Modular Reactors. None of these reactors are ready for the market; however, they are the furthest along in the approval process of the new designs.
Small Modular Reactors
Small Modular Reactors (SMR) are not an exclusive category of reactor designs, but rather describe the capacity of energy output. These small reactors are designed to accommodate remote communities and small utility-grade applications, generally producing anywhere between 50 to 300 MW of energy. SMR show particular promise as they can be fabricated and mass-produced off-site. For this reason, Small Modular Reactors can be produced much faster and cheaper. As small to medium-sized coal plants are decommissioned, small modular reactors can utilize the brownfield sites left behind and fill the production void.
Corresponding to research, both federal and state energy policies must unbiasedly make way for nuclear energy development. Over the past twenty years, the private sector has reinvigorated the production of nuclear power in the United States. But the federal government plays a key role through safety and environmental regulations, research and development funding, and setting national energy goals.
The Energy Policy Act of 2005 worked to promote the development of new nuclear reactors through loan guarantees and subsidies. The Department of Energy oversees the federal and civilian research and development of nuclear energy projects while the Nuclear Regulatory Commission retains the responsibility of licensing both active and theoretical nuclear designs like the reactors described above.
Recently, the commission widened the scope of certification to include novel designs outside the traditional boiling water and pressurized designs but the nuclear power industry remains the most heavily regulated in the United States. The licensing process is long and grueling, often pushing construction far past deadlines and budget. Before construction can begin, the initial approval process for a new reactor can take anywhere from 3 to 5 years. The smaller-scale modular reactors may help to cut through some red tape; the SMR design can be approved and produced offsite instead of needing to restart the whole licensing and approval process for each project. Significantly, the first small modular reactor design was approved in 2020 by the U.S Nuclear Regulatory Commission.
STATE AND LOCAL INFLUENCE
State and local governments also greatly influence the feasibility of nuclear development even though a large part of the regulation is done at the federal level. Each state maintains control of nuclear power use and capacity through state utility commissions as well as property regulations, taxes, and commercial activities. The energy deregulation movement in the 1990s opened state energy markets to a greater concentration of nuclear power to meet emission standards. However, state-led initiatives have created barriers to nuclear development, largely due to negative public perception of nuclear energy.
Three nuclear accidents in the last century and the cold war significantly tainted opinions, and negative stereotypes have been reinforced by misleading portrayals of nuclear technology in movies and the media. In California, a law enacted in 1976 still restricts nuclear development by preconditioning new construction on the approval of waste disposal methods. Also, under the Nuclear Waste Act, states maintain veto power over the location of nuclear waste repositories unless overridden by Congress.
Advanced nuclear technologies show great potential, but there are still many hurdles to address before the benefits can be fully realized. Public and private funding should be focused on a broad range of technological developments. Failing to diversify the national research and development portfolio will lead to a vulnerable energy market that relies too heavily on a limited number of energy resources. Additionally, while federal, state, and local regulations are necessary to ensure safety and reliability, regulations must not place an undue burden on the energy industry as ultimately the load falls upon consumers. All levels of government should work in tandem to streamline the licensing and approval process of nuclear designs. Furthermore, energy policies should not determine the winners and losers of the market, but rather impartially culture innovation and development.
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