Atoms, Isotopes, and Where the Energy Comes From

Every power plant on Earth is a machine for boiling water or spinning a shaft, and the differences between them come down to one question: where does the heat come from? Coal and gas plants get it from chemical reactions — rearranging the electrons that bind molecules together. A nuclear plant gets it from rearranging the nucleus itself, and that difference in scale is the entire story of nuclear power.

An atom's nucleus is a cluster of protons and neutrons bound by the strong nuclear force. The number of protons defines the element — every uranium nucleus has 92. The number of neutrons can vary, and atoms of the same element with different neutron counts are called isotopes. Natural uranium is almost entirely two isotopes: uranium-238 (about 99.3%) and uranium-235 (about 0.7%). That 0.7% is the fuel of nearly every power reactor on the planet.

What makes U-235 special is that it is fissile: if a slow-moving neutron wanders into a U-235 nucleus, the nucleus absorbs it, deforms, and splits — usually into two unequal fragments plus two or three fresh neutrons. The fragments fly apart at enormous speed, and as they slam to a stop in the surrounding fuel, their kinetic energy becomes heat. A single fission releases about 200 MeV (million electron-volts) of energy. A typical chemical reaction — burning one carbon atom — releases about 4 eV. That ratio, roughly fifty million to one per atom, is why a pellet of uranium fuel the size of a fingertip contains about as much energy as a ton of coal, and why a large reactor runs for 18 to 24 months on one batch of fuel while a coal plant of the same size consumes trainloads every week.

U-238, the abundant isotope, mostly doesn't fission in a conventional reactor — but it isn't inert. When U-238 absorbs a neutron it transmutes, through two quick radioactive decays, into plutonium-239, which is itself fissile. In an operating reactor, a meaningful fraction of the power (roughly a third, late in a fuel cycle) actually comes from fissioning plutonium that the reactor bred from U-238 during operation. This is also why spent fuel is a more complicated material than fresh fuel: it contains fission fragments, plutonium, and other heavy elements the reactor created.

The fission fragments are the source of two of nuclear power's defining characteristics. First, they are intensely radioactive — most are unstable isotopes that decay toward stability over timescales from seconds to centuries, and their decay continues releasing heat even after the chain reaction stops. This decay heat — about 6–7% of full power at the instant of shutdown, falling to under 1% within hours — is why a reactor must be cooled even when it is "off," and it is the central engineering challenge in every major reactor accident. Second, the fragments and the transmuted heavy elements are what make spent fuel hazardous and define the waste problem, covered honestly in the waste page.

One more nuance worth carrying through the rest of these pages: neutrons born in fission are fast — moving at a few percent of the speed of light — but U-235 fissions far more readily when struck by slow ("thermal") neutrons. Almost every operating power reactor therefore deliberately slows its neutrons down with a moderator. The choice of moderator, and what happens if it disappears, turns out to shape reactor design, safety behavior, and even the differences between the world's reactor accidents. That's the next page.