Nuclear 101
We’re often asked how nuclear power plants work. There are many different fuels, reactor architectures, and approaches to track, so this is a brief refresher that can be beneficial for even the most seasoned energy expert.
Nuclear power can be broken up into three power classes:
1. Microreactors: these are the 1 MWe - 20 MWepackaged as full plants.
2. Small Modular Reactor (SMRs): 20 MWe – 1,000 MWe reactors, usually no plant.
3. Grid-Scale Reactors: 1,000 MWe and up. This is Vogtle and its ilk. Plants are bespoke.
Regardless of class, at their (literal) core, nuclear reactors are all about making heat. That heat is then transferred to a working fluid, that fluid then spins a turbine, which in turn spins an electric generator. The working fluid is cooled as it passes through the turbine from the extraction of enthalpy¹. However, because no process is perfect the remaining heat is pulled out in the condenser. The fluid then heads back toward the reactor to be reheated and the cycle begins again.
For all deployed commercial reactors in the U.S. the working fluid is water², which is converted to steam with the reactor’s heat, and then recondensed past the exit of the turbine after the enthalpy has been extracted. This is how the Applied Atomics’ system works, though other reactor types may use a high temperature gas like helium to spin the turbines during which the enthalpy is extracted from that fluid.
Elements of a Nuclear Power Plant
The Primary Loop
The primary loop moves water from the reactor, where heat is added, through the steam generator, where heat is exchanged to the secondary loop, and then pumped back to the reactor. That reactor is, in essence, a pressure vessel with fuel rods inside. Working fluid in the primary loop (in our case pressurized water) flows past these rods and convects heat. The heat produced by the fuel rods is controlled by moving control rods made of neutron absorbing materials which are interspersed throughout the fuel. Neutrons from the fuel are born fast, then slowed (or moderated) as they pass through the water on their way to a neighboring fuel rod. As these neutrons collide with the fissile uranium-235 nuclei of the fuel, they split these atoms into byproducts and release a massive amount of energy as heat along with more neutrons - which then sustain the chain reaction. Imagine a long stack of dominos that all start falling together, except that as each domino falls, it doesn’t only knock over the next one in line but instead shoots out energy to knock over many more dominos around it. That’s an approximation of a chain-reaction in progress.
All operating commercial nuclear reactors in the United States use uranium as fuel. Some work is being done to also use thorium, but since thorium is fertile rather than fissile fuel³, most of those reactors are breeder reactors⁴. Of course, plutonium is also an option for fuel but is not available to the commercial market as a source due to proliferation concerns.
A pressurizer on the loop keeps the system (you guessed it) pressurized, providing a place for the water to expand as its temperature changes during normal reactor operation. The pressurized water flows into one side of a specialized heat exchanger, called a steam generator, where it transfers heat into the secondary loop – and isolates the radiation from the primary loop. The now-cooled water then flows to a pump which circulates it back to the reactor to be reheated and begin the journey again.
Note: the primary loop is where most SMR providers stop development. Some even stop only with the reactor core itself, choosing to sell it as one-off hardware. This leads to the end users having to develop bespoke power plants with expensive “engineering, procurement, and construction” or EPC contractors. This adds massive uncertainty to the project timeline and final price of power. More on this in subsequent sections on business model.
The Secondary Loop
The secondary loop, along with the electrical, civil and structural systems, is often called “the balance of plant” and this is where electricity is actually made. The heat from the primary loop enters the steam generator where it changes the water coming in from the secondary loop from water to steam. This steam then drives a series of turbines in a stack from high-pressure to low-pressure to extract maximum enthalpy. As the turbines spin, their output shaft is coupled to a generator that spins to produce electricity. There are also elements included in the power system to deal with variable demand, which prevent having to change the reaction rate, however those are not shared here to protect intellectual property in our approach.
Many nuclear plants use cooling towers, but not Applied Atomics.
To ensure the steam fully converts back to water, a condenser is used. In many plants this is achieved by using cooling towers, which give nuclear power its signature look, as shown in Figure 2. However, Applied Atomics uses commercially available fin fan coolers⁵ which means that there are no steam emissions and no fresh water source is required.
Though this costs some plant efficiency versus using cooling towers, the payoff is that, other than refueling⁶, the plant requires no inputs and produces no outputs other than electricity. This means no local water supply input, or dumping cooling water to a river. These plants are suitable for use anywhere on earth with almost no modification. No other reactor or plant beyond TRL 3 can match this functionality at this size class and with this level of utility. Removing the cooling towers has the added benefit of lowering our plants visible impact, easing CapeEx, and removing a kneejerk trigger of public concern.
Finally, our secondary loop operates in the same manner as all legacy heat-engine power plants (e.g. coal & natural gas), so we are able to leverage a mature and resilient supply chain.
Nuclear Fission Advantages
Nuclear fission has many advantages over other power sources. This graphic compares these in brief, but each is discussed in more detail below.
Nuclear Power is Clean Power
Nuclear energy produces no greenhouse gas emissions during operation, and minimal amounts during plant build-out, making it a vital solution for combating climate change. It also produces less mining impact than either solar or wind power. This makes nuclear power an excellent choice for hard-to-decarbonize industries that need to meet increasing stakeholder pressure and comply with government regulations. There are also second order effects. For instance, using concrete made with nuclear power to build more nuclear power plants would cut the emissions of the materials used to make more nuclear reactors nearly in half.
Nuclear Power is Firm Power
Unlike solar and wind, which depend on local weather conditions, nuclear power provides consistent and reliable energy generation no matter where it is located⁷. It is fully dispatchable with a high capacity factor, ensuring stability for our commercial consumers. Though solar and wind are competitive in cost, their direct applications are limited in industry and their true cost increases significantly when storage buffering solutions are added to increase their suitability.
Efficient Land Use
Nuclear power requires far less land than wind farms or solar installations to produce the same amount of energy. It also compares favorably to large natural gas plants, which must bunker fuel and additional support equipment such as gasifiers and flare stacks. Nuclear power’s land efficiency allows plants to integrate into areas where other power sources cannot, preserving space for housing, agriculture, and other uses (including conservation) even as demand grows⁸.
Low Transmission Build Out
Because nuclear power is clean, firm, and energy-dense, plants can be built near the areas they serve. Unlike hydro or geothermal energy, which rely on specific geological conditions, or solar and wind that depend on favorable climates, nuclear does not have geographic constraints. This ability to locate in direct proximity to the demand, in Applied Atomics’ need for extensive transmission infrastructure. Applied Atomics’ co-location model further lowers transmission costs and complexity through co-location with our large power consumers.
Local Economic Benefits
The above advantages help attract businesses and industries seeking affordable energy to locations where Applied Atomics is deployed. This leads to the creation of energy campuses where secondary power purchasers co-locate to produce their energy-intensive products. For a campus built to primarily power e-fuel production or hyperscalers, there is excess power and heat for secondary consumers such as vertical agriculture or crypto mining. This may lead to the development of new economic hubs of mixed power-intensive product producers, providing job opportunities and fostering growth in the surrounding communities⁹.
Direct Heat Applications
Nuclear power is also an efficient provider of heat for industrial and commercial processes. Though LWRs run at lower temperatures than high-temperature gas reactors (HTGRs)the produced heat is still useful. Applications include cooling and heating for data centers and energy campuses, as well as supporting industries like e-fuel production, desalination, steel manufacturing, concrete processing, and hydrogen generation. These heat-intensive processes benefit from the reliability and scalability of nuclear energy.
Common Reactor Architectures
Here are some common types of nuclear reactors, their architectures, and the fuels they use:
Light Water Reactors (LWRs)
The most common design worldwide and the one that Applied Atomics utilizes. It includes:
Pressurized Water Reactor (PWR): Water under high pressure is heated without boiling in the primary loop and transfers heat to a secondary loop where steam is generated.
Boiling Water Reactor (BWR): Water boils directly in the reactor vessel to produce steam for the turbine.
Fuel: Low Enriched [< 5%] uranium dioxide pellets, encased in fuel rods.
Working Fluid: Water, which serves as both a coolant and a neutron moderator (slows neutrons for better fission).
High Temperature Gas Reactors (HTGRs)
Uses graphite as a neutron moderator and helium gas as a coolant.
Fuel: TRISO (tri-structural isotropic) HALEU (High-Assay, Low-Enriched) [>5% and < 20%] fuel particles, often uranium or plutonium encased in ceramic layers.
Working Fluid: Helium. This gas heats up significantly but doesn't change phase. This makes it suitable for high-efficiency power conversion, however helium is hard to procure in large quantities and is expensive.
Fast Reactors
Fast reactors don’t use a moderator, allowing neutrons to maintain high energy. They often use liquid metal coolants like sodium to transfer heat efficiently.
Fuel: Uranium or plutonium isotopes. Some designs allow breeding of more fuel, like thorium.
Working Fluid: Liquid metal (sodium or lead), which has excellent heat conduction properties.
Molten Salt Reactors (MSRs)
Architecture: Fission occurs within liquid fuel mixed with molten salt. The reactor operates at high temperatures and low pressures.
Fuel: Uranium or thorium dissolved in molten salt.
Working Fluid: The molten salt itself transfers heat to a secondary loop.
A Brief Note on Fusion
Fusion is the process in which atomic nuclei combine to form a heavier nucleus, releasing vast amounts of energy in the process. Fusion reactors typically fuse hydrogen isotopes like deuterium and tritium under extreme pressure and temperature, creating helium and freeing energy due to the mass difference. This is the same process that powers stars, including our Sun, and as such is a major effort to sustain and control. While there are vast benefits to fusion if it becomes commercially available, there is still a long road to navigate even after the completion of a fusion reactor itself.
The totality of work in fusion thus far has been an attempt to create a sustaining reaction that generates more power than it consumes¹⁰. After that work is completed, the fusion reactor will still need to be coupled to a power plant in much the same way we do with our fission reactor. While some forms of fusion may eventually enjoy the benefits of direct energy conversion (DEC), it is important to note that whether it uses DEC or traditional Carnot heat engines¹¹ for energy conversion to electricity, some power plant infrastructure will be required. These plants will also need to navigate an NRC process, albeit potentially simplified, just like a fission plant.
Fusion is not a shortcut past the challenging balance of plant or NRC processes, which means that it is also not a given that it will be more affordable than any other source of power. Given the massive resources committed to its development, both public and private, it is more likely that it will not be economical for a long time after its technical development is complete. In this way, its commercial viability will follow the path of fission over time.
Footnotes
¹ I recommend learning about enthalpy. If you want to be a superstar in all future energy discussions, next learn about exergy!
² Advanced reactors such as HTGRs and MSRs use different working fluids, as touched on later in this section.
³ Fissile? Fertile? Fissionable? Read more here: Distinction between Fissionable, Fissile and Fertile | nuclear-power.com
⁴ A resource on breeder reactors: How do fast breeder reactors differ from regular nuclear power plants? | Scientific American
⁵ Also known as ‘air cooled heat exchangers’ these are used all across commerce and industry. More info here.
⁶ AA will first refuel on 24-month intervals; however, this may be extended to 48 months, making these highly independent installations.
⁷ Nuclear Power is the Most Reliable Energy Source and It's Not Even Close | Department of Energy
⁸ Easy reference: 1GW of nuclear = Central Park, 1 GW solar = all of Brooklyn, 1 GW wind = all five boroughs.
⁹ More on the energy park concept in the customer information section.
¹⁰ You’ll hear teams talk about this in terms of “Q>1” where Q is the “fusion energy gain factor.”
¹¹ Look, if you followed the advice of a previous footnote and learned about enthalpy and exergy, you might as well learn about Carnot.
