Nuclear Fission and Fusion: How They Differ

A heavy nucleus that splits and two light nuclei that join are, in physics, two different routes to the same outcome: releasing energy from inside the atom. The first process is called fission, the second fusion, and they sit behind very different technologies: power plants already in operation, as the Recap on how nuclear power plants work explains, and research experiments still underway.

How nuclear fission works

Nuclear fission starts with an impact: a neutron strikes the nucleus of a heavy atom, typically uranium or plutonium, and splits it into two smaller nuclei. In the case of uranium-235, each fission releases about 200 MeV of energy and an average of 2.45 new neutrons. Not every one of those neutrons needs to hit another nucleus; one is enough to sustain the chain reaction, while the rest are lost or absorbed without causing a new fission. A faster, uncontrolled version of the same principle sits behind the atomic bombs dropped on Hiroshima and Nagasaki.

About 85% of the energy released by fission shows up as kinetic energy in the nucleus fragments, which later turns into heat.

Worked example: one kilogram of uranium-235 fuel that undergoes fission releases about 82 terajoules of energy. That heat, generated inside the reactor, is what drives the turbines of a nuclear plant.

How nuclear fusion works

Nuclear fusion starts instead from light nuclei: hydrogen isotopes such as deuterium and tritium, elements near the top of the periodic table. Fusing them on Earth requires heating them to extreme temperatures, on the order of 50 million degrees Celsius. At that temperature matter turns into plasma, a very rarefied gas of charged particles (less than a milligram per cubic meter), held together through magnetic confinement across volumes of hundreds of cubic meters.

The ring-shaped chamber used by most experimental fusion reactors is called a tokamak, first designed in 1951 by Soviet physicists Andrei Sakharov and Igor Tamm. Each fusion between a deuterium nucleus and a tritium nucleus releases 17.6 MeV, less than the 200 MeV of a single uranium-235 fission, yet fusion releases more energy per unit of fuel mass.

The ITER project, under construction at Saint Paul-lez-Durance in France, aims to heat its plasma to 150 million degrees Celsius, ten times the temperature of the Sun’s core, and to produce 500 MW of fusion power starting from 50 MW of input heating, a tenfold energy gain sustained for 400 to 600 seconds. As of October 2026 the machine’s assembly is underway, with components built by suppliers from six domestic agencies across China, the European Union, Japan, South Korea, Russia, and the United States. Once assembly finishes, ITER will hold the record for the biggest tokamak ever made, enclosing a plasma volume of 830 cubic meters, a scale meant to bring fusion research closer, for the first time, to a machine that gives back more energy than it takes in.

Two recent experiments

Two results, in different years, show how far the research has come. On December 5, 2022, the National Ignition Facility at the U.S. laboratory LLNL ran the experiment that first passed scientific energy breakeven in a fusion laboratory: lasers delivered 2.05 megajoules to the target, which produced 3.15 megajoules of fusion energy. In January 2025, China’s EAST tokamak instead sustained a high-confinement, steady-state plasma for 1,066 seconds, a world record more than double the previous 403-second mark set in 2023.

The two processes, side by side

Per single event, fission releases more energy than fusion: 200 MeV against 17.6 MeV. Look at a kilogram of fuel instead, and fusion comes out ahead, because a gram of deuterium and tritium packs far more nuclei available to fuse than a gram of fission fuel packs uranium-235 nuclei.

The more practical difference, as of October 2026, is about use. Fission runs nuclear plants operating in several countries, and the electric current that reaches homes passes through some of them. Fusion, as of October 2026, remains a research goal: a plant like ITER has not yet started operating, and the strongest results so far are laboratory records, like those from NIF and EAST, not electricity delivered to a grid.

Fission’s long-lived waste stays radioactive for that very reason: it keeps releasing energy through radioactive decay, the same spontaneous process behind any unstable nucleus.