America's Nuclear Renaissance
As American energy demands soar and the effects of climate change intensify, the search for a reliable, clean energy source has never been more critical. Once a proud symbol of engineering accomplishment, nuclear energy in the United States has faced a 30-year decline due to public fears and regulatory challenges. Today, however, new advancements in reactor technology are sparking renewed interest in nuclear power. This article explores traditional and novel nuclear reactor designs, and the promising role nuclear will play in the future of American energy.
The Nuclear Fission Chain Reaction
To fully grasp how a nuclear reactor functions it is necessary to start small, atomically small, and work up. The following descriptions are intended to provide a background in nuclear energy for those with an expertise in any field, from social arts to engineering.
Nuclear power, as we currently know it, originates from the nuclear fission chain reaction (NFCR). The reaction begins when a neutron - a subatomic particle with no charge - collides with an atom of nuclear fuel, most commonly uranium-235 (U-235). The neutron briefly combines with the uranium atom, creating an unstable isotope, uranium-236, which almost instantly splits apart in a process known as fission. Fission releases a significant amount of heat and 2 or 3 neutrons. These newly released neutrons then collide with neighboring U-235 atoms, triggering more fission and initiating a chain reaction.
To help visualize the NFCR, imagine a pile of grenades. The first grenade is struck by a speeding bullet, causing it to explode. The resulting shrapnel strikes other grenades, beginning a chain reaction of explosions.
The power of a nuclear reactor is harnessed from the combined heat generated by these atomic-scale explosions. In a reactor core, uranium exists as pellets, each the size of a Sharpie marker cap. Hundreds of pellets are stacked into fuel rods, which are 12-14ft long. A typical reactor core contains upwards of 50,000 fuel rods, amounting to trillions upon trillions of uranium atoms poised to undergo fission. Needless to say, keeping the NFCR stable is crucial for the safe operation of the reactor.
Neutron Moderators and Absorbers
Achieving this stability depends on carefully regulating the number of neutrons that initiate subsequent fission events. This is achieved through the use of neutron moderators and absorbers
At any moment the current state of a fission chain reaction is quantified by the effective multiplication factor (k). This factor indicates the change in the number of fission events from one generation to the next:
When a reactor is in a steady state, each fission event triggers exactly one subsequent fission event, making k=1. This state is known as critical. If k<1, the reactor is subcritical, meaning the reaction will gradually die out. Conversely, if k>1, the reactor becomes supercritical, causing the reaction to grow exponentially, which can be very dangerous.
It is imperative for a reactor to maintain k=1 to ensure the reaction neither fizzles out nor becomes uncontrollable. Neutron moderators and absorbers are vital tools used to achieve equilibrium within the reactor core by slowing down, or absorbing neutrons. In traditional reactors, the two primary mechanisms for this are water and control rods.
Neutrons released during a fission event are moving incredibly fast. Somewhat counterintuitively, if these neutrons strike uranium atoms at too high of a speed they will not trigger a subsequent fission event. To sustain the NFCR, neutrons must be slowed down. This is the job of neutron moderators. In light water reactors, ordinary water (H2O) serves as the moderator. Water is an excellent moderator due to its high hydrogen (H) content, which has a mass similar to that of a neutron. Fast-moving neutrons collide with hydrogen atoms, transferring away kinetic energy and significantly reducing their speed. These slower neutrons are far more effective at inducing fission and perpetuating the chain reaction.
In addition to moderators, neutron absorbers are essential to slow down or stop the reaction, whether to reduce power output or ensure safety. Control rods, commonly made of boron or hafnium, are used to absorb neutrons. These rods can be inserted or withdrawn from the reactor core during the fission process. The deeper the rods are inserted, the more neutrons they absorb, thus slowing the reaction. In emergency situations, fully inserting the control rods will bring the reaction to a halt.
Traditional Nuclear Reactors
As of 2024, all commercial nuclear reactors in the United States are light water reactors, which use water as both a moderator and coolant. These reactors are essentially massive steam engines, powered by the heat generated from the NFCR in the core. There are two types of light water reactors: pressurized water reactors and boiling water reactors.
Of the 94 reactors in the United States, 63 are pressurized water reactors (PWR). In PWRs, water in the reactor pressure vessel is heated by the NFCR in the reactor core. The reactor water is kept under extremely high pressure, preventing it from boiling. This heated water then travels through pipes to a steam generator, where it transfers heat to a second, separate, water source. After cooling by way of this heat transfer, the reactor water is recycled back to the core to repeat the process.
The second water source, heated within steam generator, evaporates into steam. This steam flows through the steam line into a turbine, rotating it to power an electric generator and produce electricity for the grid. After passing through the turbine, the steam is cooled by a condenser, which contains cold water.
The condenser works much like a cold beer on a humid summer day, condensing the steam back into liquid water for recycling. However, as the steam condenses, its heat is transferred to the condenser water, causing the water to become very hot. The heated condenser water is then pumped to the cooling tower, where it is sprayed over a cold water basin. During this process, about 2% of the condenser water evaporates, creating the iconic plumes of steam that can be seen rising from the cooling tower.
The remaining 31 American reactors are boiling water reactors (BWR). BWRs operate similarly to PWRs but produce steam directly from the reactor core. Water is pumped into the core, where it is heated by the NFCR, creating steam. This steam is fed directly through pipes to a turbine to generate electricity. After passing through the turbine, the steam is cooled by a condenser and recycled back to the core.
Meltdown
A nuclear reactor meltdown occurs when the cooling cycle for the reactor core is disrupted. Even with the control rods fully inserted, halting the NFCR, the fuel rods emit an immense amount of heat. If the coolant water continues to circulate this is not a problem. However, when circulation is interrupted, the hot reactor core will boil off the surrounding water until there is no coolant left. In PWR the heat can create enough pressure to overcome the pressure tank, causing an explosion.
Without adequate coolant, the fuel rods and surrounding materials begin to melt, forming a hot radioactive sludge at the bottom of the reactor containment vessel. In catastrophic situations, this sludge can melt through the containment vessel, exposing radioactive material to the outside world.
The worst nuclear disaster in history occurred in 1986 at the Chernobyl nuclear power plant, located in modern day Ukraine. Ironically, the accident happened during a test of the emergency coolant supply system. During the test, a rupture in the reactor components led to a loss of coolant, causing a meltdown, explosions, and a fire that spread radioactive ash across Europe. The catastrophe is considered one of the most costly in human history, with an estimated 4,000 deaths and damages equivalent to $700 billion USD.
Unfortunately, Chernobyl is not the only disastrous nuclear incident. In 2011, a tsunami disabled the cooling and power supply systems at the Fukushima nuclear power plant in Japan, leading to meltdowns in three reactor cores, explosions, and radiation exposure. Earlier, in 1979, the Three Mile Island Nuclear Generating Station in Pennsylvania experienced a coolant malfunction that caused a partial meltdown and the release of radioactive gas.
Outside of these events, nuclear power has actually maintained a strong safety record across hundreds of reactors. However, the severe consequences of these few catastrophic meltdowns have long influenced public opinion against the expansion of nuclear power. Nonetheless, recent advancements in reactor design may soon make any chance of meltdowns a thing of the past.
Pebble Bed Reactors
The idea of pebble bed reactors (PBR) has been around for a long time, but only recently have they gained momentum in commercial development. PBR boast two large differences from traditional reactors that make them both safer, and more efficient. They use spherical fuel assemblies, and gas cooling.
Unlike the conventional fuel rod design, PBRs are powered by tennis ball-sized spheres known as pebbles. Each pebble consists of uranium and graphite, encased in a layer of tristructural-isotropic (TRISO) particles. Graphite serves as the primary neutron moderator, while the TRISO coating provides an airtight seal that enhances containment and safety.
When enough pebbles are grouped together, they initiate an NFCR, making the reactor critical. Similar to a gumball machine, fresh pebbles are loaded in from the top daily, while spent pebbles are dispensed from the bottom of the core. Pebbles can circulate through the reactor multiple times before being fully consumed.
The mechanics of a PBR mimic those of a PWR, but with a crucial difference: PBRs are cooled by helium gas instead of water. This choice of coolant increases the reactor's efficiency by enabling it to operate at much higher temperatures. Even under high pressure, which prevents liquid water from boiling, water can only reach around 370°C before its properties begin to change. Helium gas, on the other hand, remains stable at extreme temperatures, allowing reactor operation beyond 1000°C. Simply explained, high temperatures make the reactor more efficient because they can generate more steam, and therefore more electricity.
Inside a PBR, helium gas is pumped down through the gaps between the fuel pebbles, cooling the nuclear fuel while absorbing heat. The heated helium then rises to a steam generator, where it transfers its heat to an external water source, creating steam. This steam drives a turbine, producing electricity. After passing through the steam generator and releasing its heat, the cooled helium is recycled back into the system to repeat the process.
In the event of a power failure, like the one that occurred at Fukushima, the helium gas’s heating and cooling cycle will maintain a convection current, continuing to cool the reactor core. This passive cooling system ensures that a power loss won’t result in a meltdown.
The remarkable safety of PBRs was recently demonstrated at China’s Shidaowan nuclear power plant. In a controlled test simulating a meltdown, the plant successfully relied on passive cooling after the power supply was deliberately cut off. This marked the first time passive cooling has been proven on a commercial scale.
In the U.S., X-energy is developing its own PBR, the Xe-100. In 2021, the company received $1.1 billion from Congress through the Department of Energy's Advanced Reactor Demonstration Program. The reactor, currently under construction in Washington state, is expected to be operational by 2030.
Small Modular Reactors
What’s unique about the Xe-100, compared to Shidaowan, is that it is a small modular reactor (SMR). SMRs are becoming the focal point of nuclear development in the U.S. because of their convenient design and lower construction cost.
Conventional large reactors require massive upfront investments and are frequently plagued by delays, as each reactor has a unique and complex design. For example, Vogtle-3 and Vogtle-4, the first U.S. reactors completed in 30 years, were delayed by seven years and ended up costing over $34 billion—more than double the original estimate. Such delays and cost overruns are common and have deterred new nuclear projects for decades. SMRs offer a solution with their modular design.
An SMR is composed of one or multiple uniform modules, each of which can function independently or be combined to scale up power production. The standardization of these modules allows them to be mass-produced, with most of the assembly happening offsite in factories. This streamlines the installation process, minimizes potential delays, and, most importantly, drastically reduces costs compared to traditional nuclear power plants.
Additionally, the smaller size of SMRs enables them to be deployed in locations where traditional reactors wouldn't fit, such as small electrical markets, remote regions, or areas with limited water resources and acreage.
X-energy is not alone in the SMR space. Multiple startups are working to revolutionize nuclear power in the U.S. Leading contenders include TerraPower, backed by Bill Gates, which recently broke ground on an SMR project in Wyoming, and California-based Radiant, which is developing even smaller microreactors that can be transported by truck for rapid energy deployment.
America's Nuclear Renaissance
From 1970 to 1990, the construction of nuclear reactors in America surged, with over 100 reactors built, peaking at 111 operational reactors in 1990. However, the industry soon stagnated and has since declined to just 94 functioning reactors today. After a 30-year hiatus, the nuclear energy landscape is starting to shift. Advances in reactor technology, the green energy movement, and a new wave of energy demand have positioned the U.S. for a nuclear renaissance.
In the early 1970s, nuclear development was booming. But as the number of reactors grew, so did regulatory oversight. The turning point came with the 1979 Three Mile Island incident, which sparked widespread public fear of nuclear energy - a fear that was only heightened by the catastrophic Chernobyl disaster in 1986. This led to a sharp decline in public support and the implementation of heavy regulations, which slowed the industry’s progress.
These new regulations significantly increased the time and cost to build nuclear reactors. The chart below from Andressean Horowitz illustrates how overnight construction costs (the cost to build excluding interest rates) and time to completion both dramatically rose following Three Mile Island.
The enormous upfront investment required for nuclear power projects has long deterred new development. However, SMRs offer a promising, low-cost alternative. Their modularity and mass manufacturing capabilities are expected to dramatically reduce construction costs. Additionally, their standardized design should simplify regulatory approval, further lowering expenses.
Moving forward, thoughtful regulation and legislation will be key to the success of the nuclear industry. While some oversight is necessary to prevent accidents, the enhanced safety of new technologies like PBRs could enable a reduction in costly regulations.
Nuclear energy is also one of the rare bipartisan issues in American politics, providing hope that lawmakers will work across the aisle to uncuff the industry. A Pew Research Center study found that Republicans and Democrats are less divided on nuclear energy than on any other energy source, with public support steadily increasing over the past decade.
A major driver of this growing support is the green energy movement. Nuclear energy is a carbon-free power source, unlike fossil fuels. For comparison, a one-gigawatt nuclear reactor can power one million homes annually without emitting any direct carbon dioxide, whereas a coal plant with equivalent output would generate over 13 billion pounds of CO2.
The Biden Administration released a strategy for the United States to reach net-zero carbon emissions by 2050. The plan includes a slight increase in nuclear power generation, however it is mainly focused on wind and solar. The administration cites the low costs of wind and solar as a key reason for their implementation, so a decrease in nuclear pricing should open up an avenue for greater involvement in their plan. Despite its higher costs, nuclear power still has significant advantages.
One of the key benefits of nuclear power is its space efficiency. A nuclear power plant requires far less land than solar or wind farms to produce the same amount of electricity - about 75 times less than solar panels and 360 times less than wind turbines. Additionally, wind turbines are often criticized for their visual impact on the skyline. If the goal of green energy is to preserve the environment, then the power source which least alters the landscape, nuclear, is clearly the path to take.
Nuclear power is also far more reliable than other sources of renewable energy. Unlike wind or solar, which depend on weather conditions, nuclear plants can generate power continuously. Its capacity factor - a measure of how often a power source operates at maximum output - is 92.5%, more than twice that of wind or solar.
Another key factor driving the nuclear comeback is the rising demand for energy, particularly from artificial intelligence (AI) data centers. The rapid growth of AI has created an exponential demand for computational power, and with it, more electricity. Data centers, essential for training AI models, are composed of thousands of GPUs and consume massive amounts of energy. In the U.S., data center energy use is projected to account for 9% of total power consumption by 2030, double what it is today.
Unlike other renewables or fossil fuels, nuclear energy offers a stable, reliable, and low-carbon power source, making it an ideal solution to meet the energy needs of these data centers. Tech giants have already begun investing in nuclear options to secure their power requirements.
In a recent earnings call, Oracle founder Larry Ellison revealed the company is building an 800-megawatt data center, featuring “acres of GPU clusters” dedicated to training one of the world’s largest AI models. Ellison also announced that this facility will be powered by nuclear energy, noting that Oracle has “already got the building permits for three nuclear reactors. These are small modular nuclear reactors to power the data center.” Oracle is also planning an even larger one-gigawatt data center, presumably powered by nuclear as well.
Oracle is not alone in the nuclear pursuit. In March 2024, Amazon purchased a 960-megawatt nuclear powered data center from Talen Energy for $650 million. The data center is directly powered by the adjacent Susquehanna Steam Electric Station, which generates 2.5 gigawatts of nuclear power. Talen CEO Mark McFarland commented on the deal, noting that as power demand continues to rise, “data centers are at the heart of that growth.” This strategy of situating data centers next to nuclear power plants to draw electricity directly, bypassing the grid, is gaining traction, with similar proposals in New Jersey, Virginia, Texas, and Ohio.
AI leader Microsoft is also embracing nuclear power. Just days ago, the company announced an unprecedented agreement with Constellation Energy to restart the dormant Three Mile Island nuclear power plant. Under this deal, Microsoft would be the sole recipient of the plant's 835-megawatts of nuclear energy for 20 years. Naturally, the plan has sparked controversy due to the plant’s infamous history. However, if regulators approve the deal, Three Mile Island could be operational again by 2028. This move follows Microsoft’s 2023 partnership with nuclear fusion startup Helion, aiming to begin receiving power also by 2028.
In the wake of tech giants moving toward nuclear energy, fourteen of the world’s largest financial institutions have pledged to increase their support for the nuclear sector. Major players like Bank of America, Barclays, Citi, Morgan Stanley, and Goldman Sachs have committed to backing a global initiative aimed at tripling nuclear energy capacity by 2030.
The convergence of AI and nuclear power is setting a new standard for energy consumption in the digital age. Coupled with improved reactor safety, increased political backing, and the growing demand for clean energy, the U.S. stands on the cusp of a nuclear renaissance. As regulations evolve and costs decrease, the country has an opportunity to lead a new era of nuclear development, ensuring a sustainable and reliable energy future.