Energy from Matter

Welcoming the nuclear renaissance

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In Progress’s First Principles, we discussed how matter and energy, informed by knowledge, are used to counter entropy and how these counter-entropic endeavors are the core of human progress. I was careful to note, however, that matter and energy are not truly distinct concepts. As we came to understand in the 20th century, matter is a kind of “bound energy” and with this knowledge, we unlocked atomic energy. It didn’t take long for nuclear fears, misplaced or otherwise, to regulate the nuclear energy industry to an early (near) death in the West. This was a missed opportunity. Nonetheless, innovation continued, making nuclear power more efficient and safer. With the rise of “Generation IV” nuclear technology, we may be on the cusp of a nuclear renaissance.

A Brief Introduction to Nuclear Power

Nuclear fission is relatively straightforward. A neutron is fired from a neutron gun at fissil material, most often pellets of Uranium-235. When the neutron impacts a Uranium atom, it becomes unstable and splits; releasing a huge amount of the “bound energy” as heat. It also releases more neutrons from the host material, starting a chain reaction that generates more energy. This heat energy is captured by a cooling medium, often water, turning it into high-pressure steam, which is then directed into a steam turbine connected to generators to produce usable electricity. The reaction is controlled via “control rods” that absorb neutrons; the rods are inserted deeper into the reactor's core to slow the reaction, or withdrawn to accelerate it.

In a prior essay, we tracked the history of thermal efficiency from coal through natural gas-powered engines. While nuclear power cannot (yet) achieve the same level of efficiency as natural gas, the energy density of nuclear fuel is simply unparalleled. In fact, the density disparity is so extreme we cannot represent it on a chart without using a log scale. Uranium has over 70,000 times the energy density of natural gas per kg! Further, nuclear power is emission-free. It produces no harmful pollutants or greenhouse gases that contribute to climate change. This, of course, comes with the caveat that it does produce some radioactive byproducts, but these are manageable.

Nuclear power is also extremely safe. As discussed by

Michael Woudenberg

, air pollution from fossil fuels kills millions annually. Therefore, every pollution-free atomic power plant built, on balance, saves human lives by eliminating this pollution. This is born out in the data. Per terawatt-hour of energy produced, nuclear energy causes about 0.03 deaths. Compare this with coal energy, which causes about 24.6 deaths, oil some 18.3 deaths, and natural gas at 2.8 deaths. In other words, all considered, nuclear is 350 times safer than coal and about as safe as wind or solar. This is why efforts to denuclearize Germany and Japan, for instance, have led to thousands of additional deaths and serve as another reminder of the “invisible graveyard” effect. Our fear of nuclear energy is mostly irrational, borne of a “precautionary principle” run amok from the mid-1970s.

Nuclear reactors are divided into four technology “generations,” each improving upon the next. These generations are for reference only; they overlap and are not well-defined. The period from 1950-1965 saw the Generation I reactors put into service. These early reactors were born of Cold War weapons programs and often utilized natural Uranium. These early reactors were used primarily for plutonium production for use in weaponry. They were comparatively unsafe, lacking robust containment features, for example. All Generation I facilities are now decommissioned.

By 1965, however, nuclear technology had matured enough for commercial use. These so-called “Generation II” reactors are the most common today (2025). These newer designs were slightly more efficient, more reliable, and safer than their predecessors. They included multiple redundant safety systems and reinforced concrete containment structures that prevent the release of radioactive material in the event of an accident. Most Generation II designs are Light Water Reactors (LWR,) using regular water as both a coolant and a neutron modulator with a peak thermal efficiency of about 33 percent.

Despite these improvements, in the mid-1970s, nuclear energy fell out of favor in the Western world. New orders dried up, largely stifled by the rising costs of building new reactors, itself a function of a growing regulatory thicket promoted by the Nuclear Regulator Commission. Innovation didn’t stop but development shifted East. The first “Generation III” reactor was introduced in Japan in 1995. These reactors are evolutionary, featuring fully passive emergency systems that require no external energy or human interaction for emergency shutdown and stronger containment structures. Some even include a “core catcher” underneath the plant that can safely “catch” the molten core if a reaction spins out of control. Generation III reactors are also up to 17 percent more efficient, approaching a thermal efficiency of 37 percent.

A Nuclear Renaissance?

There is no definitive definition of a Generation IV reactor. Instead, this category includes a mix of efficiency, safety, and cost improvements. However, unlike prior generations, which were evolutionary, Generation IV reactors will be revolutionary. Six different approaches are being considered around the world; most envision new fuels, higher operating temperatures, and new cooling mediums like helium, nitrogen, lead, or sodium. This generation of nuclear technology promises to reignite a renaissance in energy production on a scale thus far unseen.

The efficiency of a heat engine is determined by Carnot’s Theorem; the higher the temperature, the better the efficiency. Conventional nuclear reactors are limited to an operating temperature of about 300C, constrained by the materials used and the water coolant. Some have suggested pressurizing the water coolant, enabling an operating temperature up to 374C. This “supercritical water,” however, is extremely corrosive, requiring expensive and exotic alloys and thicker vessels, which together drive up costs. To achieve a breakthrough in efficiency and cost, we need to rethink nuclear power from the reactor core outward. I will focus this essay on “pebble bed” reactors because they are perhaps the most mature of Generation IV designs.

Arguably, the first Generation IV reactor to come online is the HTR-PM which began operation in China in 2021. This reactor represents a radical departure from prior designs. Instead of uranium pellets, for instance, the uranium fuel is encased in a sphere of carbon and silicon carbide. This TRISO (tristructural-isotropic) particle, called a “pebble,” is stable to over 1600°C. Its outer layers serve as a self-containment system, preventing the release of the fissile material under all conditions. Thousands of these pebbles are placed inside the reactor core. The “pebble bed” approach can operate at temperatures of 900C, enabling higher efficiency, and is inherently safe because it is impervious to a meltdown. Chinese researchers demonstrated this in September 2023, simulating a complete cooling system failure and watching as the reactor naturally and safely cooled itself.

Due to its higher operating temperatures, the HTR-PM is cooled using Helium instead of water. Helium is often the preferred choice for Generation IV reactors because it has high heat transfer efficiency, is “transparent” to neutrons, and is chemically inert (won’t corrode the reactor core.) Together, the inherent safety brought by the TRISO particle fuel and high-temperature operation, means that the reactor can achieve higher thermal efficiencies than was previously possible, up to about 42 percent. The HTP-PM, however, doesn’t use the superheated Helium directly. Instead, it’s used to heat water for a traditional steam turbine.

This is a missed opportunity, but understandable due to the relative immaturity of Helium gas turbines. In the future, we could harness the superheated Helium directly by running it through its own turbine. This could increase the thermal efficiency of the system to nearly 50 percent. It may also open the door to employing a combined cycle system, similar to what is done with natural gas plants today. With a combined cycle, we would run the superheated Helium gas through a turbine and then capture the still-hot flue gas for another cycle, such as a Rankine cycle (steam turbine). The combined cycle efficiency could reach as high as 60 percent.

Pebble bed reactors’ inherent safety allows us to build them smaller too. Many companies are exploring Small Modular Reactors (SMRs) that, instead of being constructed piecemeal on-site, are manufactured on assembly lines. The idea is that mass production will leverage experience curve effects, lowering costs and improving safety and efficiency through scale. Some of these designs hope to be “drop-in” replacements for existing power plants; simply drop in an SMR into an existing coal power plant to improve safety and reduce pollution. Pebble-bed SMRs could even unlock Martian travel. As I discussed here, they could be used as the core of a Nuclear Thermal Rocket, shaving months off a journey to Mars or other distant locations.

Whether we use them on Earth or in space, the time has come to revisit nuclear energy. Nuclear power leads in terms of energy density, efficiency, and safety. Generation IV reactors promise to extend that lead further by making reactors safer and more efficient still. Maybe, just maybe, a nuclear renaissance will get us back onto the “Henry Adams” curve, ushering a new era of pollution-free and cheap energy, restoring our ability to grow and innovate in the world of atoms once more.

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