nuclear 101 · how nuclear works

energy in small places

A uranium fuel pellet is about the size of a fingertip. Put it in a reactor and, over several years, it gives off roughly as much heat as a ton of coal. No flame and no smoke. The energy comes from the nucleus, the tiny core of an atom, where a sliver of mass turns into a great deal of energy, just as Einstein’s E = mc² says it can.

This lesson starts with the people who found that energy, often by accident, and the moments that turned a laboratory curiosity into nearly a fifth of America’s electricity. Then it walks through how the machine works, with numbers you can check, and ends with why nuclear power matters again: AI, small reactors, and the Moon.

No physics degree needed. A calculator helps, and curiosity helps more.

the story · from a fogged plate to a power plant

In Paris in 1896, Henri Becquerel tucked some uranium salts into a drawer beside a wrapped photographic plate while waiting for sunshine. When he developed the plate anyway, it was fogged. The uranium was giving off rays on its own. Marie Curie took up the mystery, and in 1898 she and her husband Pierre found two new, far more active elements, polonium and radium. Marie named what they were seeing: radioactivity. The three shared the 1903 Nobel Prize in Physics.

Ernest Rutherford gave the atom its shape. In 1911 he showed that nearly all of an atom’s mass sits in a tiny, dense center, the nucleus. Enormous energy was clearly locked inside the nucleus. Nobody yet knew how to let it out.

The answer arrived at Christmas 1938. In Berlin, Otto Hahn and Fritz Strassmann had fired neutrons at uranium and found barium, a much lighter element, in the debris. Hahn wrote to his longtime colleague Lise Meitner, who had fled Nazi Germany for Sweden that summer. On a winter walk in the snow with her nephew Otto Frisch, the two worked it out: the uranium nucleus had wobbled like a drop of water and split in two, releasing about 200 million electron volts, matching E = mc² for the tiny bit of mass that had gone missing. Frisch borrowed a word from biology, fission. Their explanation appeared in Nature in February 1939.

Enrico Fermi, newly arrived in the United States from Italy, led the next step for the wartime Manhattan Project. In a squash court under the west stands of Stagg Field, the University of Chicago’s unused football stadium, his team stacked graphite blocks and uranium into a pile 57 layers high. On December 2, 1942, as George Weil slowly withdrew the last control rod, the clicking neutron counters climbed and kept climbing. At 3:25 that afternoon Fermi announced that the chain reaction was sustaining itself. Chicago Pile-1 ran for about four and a half minutes at about half a watt. Arthur Compton phoned the news in code: “The Italian navigator has landed in the New World.” Less than three years later, atomic bombs destroyed Hiroshima and Nagasaki.

Peaceful power followed. On December 2, 1957, exactly fifteen years after Chicago Pile-1, the reactor at Shippingport, Pennsylvania, went critical, and within weeks it was sending electricity into the Pittsburgh grid, the first full-scale nuclear power plant in the United States.

The record since has been mostly quiet and sometimes terrible. In 1979 a partial meltdown at Three Mile Island released a small amount of radiation; studies found no detectable health effects, and public trust took a lasting hit. In 1986 a flawed reactor and a botched test at Chernobyl in Soviet Ukraine caused an explosion and fire that killed 30 people within months and spread contamination across Europe; thousands of thyroid cancers followed among people exposed as children. In 2011 a tsunami that killed more than 15,000 people in Japan knocked out cooling at Fukushima Daiichi, causing three meltdowns and a long evacuation that itself cost many elderly lives. Each one changed how reactors are built and run.

inside the atom

Every atom has a nucleus, a core of protons and neutrons, wrapped in a cloud of electrons. The nucleus is astonishingly small. If an atom were the size of a football stadium, its nucleus would be about the size of a pea at the center, and that pea would hold more than 99.9% of the atom’s mass.

Protons repel each other, since they all carry positive charge. A much stronger force that only works at very short range, the strong nuclear force, holds the nucleus together anyway. In a very heavy nucleus like uranium, with 92 protons crowded together, that balance is delicate. Think of an overfilled water balloon: a small nudge can set it wobbling apart.

Uranium comes in versions with different numbers of neutrons, called isotopes. Uranium-235 has 92 protons and 143 neutrons, and it splits readily when a slow neutron hits it. Uranium-238 has three more neutrons and mostly does not. Natural uranium is only about 0.7% uranium-235, so most reactor fuel is enriched to roughly 3 to 5%.

splitting an atom

When a slow neutron strikes a uranium-235 nucleus, the nucleus absorbs it, wobbles, and splits into two lighter atoms, such as barium and krypton. It also throws off two or three fresh neutrons. Nearly all of the energy comes out as speed: the two fragments fly apart, crash into the surrounding material, and that crashing becomes heat.

chain reactions

Each split frees two or three neutrons, and each of those can split another atom. Physicists track the average number of neutrons from one fission that go on to cause another, and call it k. When k is exactly 1, the reaction sustains itself at a steady rate. That condition is called critical, which in reactor language simply means steady.

Two more pieces make a reactor controllable. Neutrons fly out of a split very fast, and uranium-235 catches slow ones far more readily. So the fuel sits in a moderator, usually ordinary water, which slows neutrons as they bounce off hydrogen atoms. Control rods made of neutron-hungry materials such as boron or cadmium soak up extra neutrons: slide them in and k drops below 1, pull them out and it rises.

In water-cooled reactors, the water does double duty. If it boils away, neutrons stop slowing down and the reaction tends to fade on its own. And reactor fuel at 3 to 5% uranium-235 cannot explode like a bomb, which needs uranium enriched to around 90%.

one pellet versus a ton of coal

Reactor fuel is enriched uranium dioxide powder pressed and baked into ceramic pellets about a centimeter long. Hundreds of pellets stack inside a metal tube about 4 meters (13 feet) long, called a fuel rod, and rods are bundled into assemblies. A large reactor holds tens of thousands of rods, and each pellet spends several years in the core.

That matches the widely quoted figure from the Nuclear Energy Institute: one uranium fuel pellet creates as much energy as one ton of coal, 149 gallons of oil, or 17,000 cubic feet of natural gas (NEI, nuclear fuel)¹. Both sides of the comparison are heat; either kind of power plant turns roughly a third of that heat into electricity.

from heat to the wall socket

A nuclear plant is a steam plant with an unusual fire. The most common design in the world, the pressurized water reactor, uses three loops of water that never mix:

• loop 1: water flows through the core and heats to about 315 °C (600 °F). It is kept at roughly 150 times normal air pressure, so it cannot boil.
• loop 2: in the steam generator, that hot water boils a separate loop of water into steam. The steam spins a turbine, and the turbine turns a generator.
• loop 3: after the turbine, a condenser cools the steam back into water using a third loop from a river, lake, sea, or cooling tower. The white plume above a cooling tower is tiny water droplets, condensed from warm vapor.

Used fuel stays hot and radioactive for a long time. It cools for years in deep pools of water, then moves into sealed steel and concrete casks. The volume is small: the U.S. Department of Energy notes that all the used fuel the country has produced since the 1950s would fit on a single football field, stacked less than 10 yards deep. Choosing a permanent home for it is still an open question in the United States.

why it matters now

AI runs in datacenters, and datacenters run on electricity, all day and all night. The International Energy Agency estimates that datacenters used about 415 terawatt-hours (TWh) in 2024, around 1.5% of the world’s electricity, and projects roughly 945 TWh by 2030, slightly more than all of Japan uses today. Nuclear plants suit that kind of steady demand unusually well. They run around the clock, in any weather, with no carbon emissions, on a small patch of land.

Datacenter figures: IEA, Energy and AI (2025).

Technology companies have noticed. In 2024 Microsoft signed a 20-year agreement to buy the power from Three Mile Island Unit 1, the undamaged reactor next to the one that failed in 1979. It closed in 2019 for economic reasons, and Constellation is restarting it as the Crane Clean Energy Center, aiming for 2027³. That same year Google and Amazon signed agreements to support new small reactors from Kairos Power and X-energy.

small modular reactors

The International Atomic Energy Agency counts a reactor as small if it makes up to about 300 megawatts of electricity, roughly a third of a large plant. The idea is to build them in factories, ship them in pieces, and add units as demand grows, the way you might add servers to a rack. In Ontario, Canada, construction is underway on a GE Vernova Hitachi BWRX-300 at the Darlington site, the first of four planned units, aiming to reach the grid by the end of 2030⁴. The open question is cost. The first units have to show they can be built on time and on budget.

a teaser · fission beyond earth

On the Moon, night lasts about 354 hours, nearly fifteen Earth days. Solar panels and batteries struggle through that. A reactor does not care whether the sun is up. In 2018 NASA and the Department of Energy tested a small space reactor called KRUSTY in Nevada. NASA now plans to fly a 20 kilowatt reactor in space in late 2028 (SR-1 Freedom)⁵ and follow it with a lunar surface reactor aimed at launch around 2030⁶. On Mars, where dust storms can dim the sun for weeks, the same idea could keep a base warm and lit. That story deserves a lesson of its own.

closing rules of thumb

• Radioactivity was found by accident in 1896, fission was explained in 1938 and 1939, and a reactor ran in 1942.
• Splitting one uranium atom releases about fifty million times the energy of burning one carbon atom.
• A reactor holds a chain reaction at k = 1. The moderator slows neutrons; control rods catch them.
• One fingertip-sized pellet holds about as much energy as a ton of coal.
• A nuclear plant is a steam plant. About a third of the heat becomes electricity.
• The accidents were real, and each one reshaped how reactors are designed and run.
• AI demand, restarts, small reactors, and the Moon are giving fission a second look.
• All of it rests on a sliver of missing mass, and that is the wonder of it.