Nuclear fission splits heavy atoms to release energy, as in nuclear power plants, while fusion combines light atoms, as in the Sun.
Fission
Splitting a heavy nucleus, such as uranium 235, into smaller ones. Used in nuclear power plants and bombs. It leaves long lived radioactive waste.
Fusion
Joining light nuclei, such as hydrogen isotopes, into heavier ones. Powers the Sun. It needs temperatures of about 100 million degrees Celsius and produces little long lived waste.
Energy
Fusion releases about 3 to 4 times more energy per kilogram of fuel than fission.
Status
Fission is in commercial use. Fusion is still experimental, with a net gain first reported at NIF in December 2022.
Reaction type
Fission splits a heavy nucleus, such as uranium-235 or plutonium-239, into lighter nuclei. Fusion joins light nuclei, such as isotopes of hydrogen (deuterium and tritium), into heavier ones.
Energy released
Fission of 1 kilogram of uranium-235 releases about 80 terajoules, similar to burning about 3 million kilograms of coal. Fusion releases even more energy per kilogram of fuel, about 4 times more than fission.
Fuel
Fission uses uranium or plutonium. Fusion uses deuterium, which can be extracted from seawater (about 1 in 6,500 hydrogen atoms), and tritium, which can be bred from lithium.
Conditions
Fusion needs temperatures of over 100 million degrees C to overcome the electric repulsion between nuclei. The Sun fuses hydrogen into helium at about 15 million degrees C in its core, helped by immense gravity.
Waste
Fission creates radioactive waste that stays hazardous for thousands of years. Fusion produces no long lived high level waste, although reactor materials become radioactive for decades to a century.
Safety
Fission reactors need constant cooling and control to avoid meltdown. Fusion stops immediately if the conditions are disturbed, and cannot cause a runaway chain reaction.
Current status
Fission supplies about 10% of global electricity. Fusion is experimental. In December 2022, the National Ignition Facility in California achieved ignition, producing 3.15 megajoules of energy from 2.05 megajoules of laser energy delivered to the target, a first in fusion research. The ITER project in France is under construction.
Weapons
Fission powers atomic bombs, and fusion powers thermonuclear weapons (hydrogen bombs), which use a fission trigger.
Timeline
Commercial fusion power is not expected before the 2030s to 2050s according to most projects.
Energy and conditions
Nuclear fission splits a heavy nucleus, such as uranium-235 or plutonium-239, into smaller nuclei, releasing energy and neutrons that can sustain a chain reaction. Fusion joins light nuclei, such as deuterium and tritium, into helium, releasing a neutron and energy. Fusion needs temperatures above 100 million degrees Celsius to overcome repulsion between nuclei. Per kilogram of fuel, fusion releases about four times more energy than fission. The Sun is powered by fusion of hydrogen, converting about 4 million tonnes of mass to energy every second.
Status and waste
Fission powers about 440 reactors worldwide and produces long lived radioactive waste. Fusion produces no long lived high level waste, though reactor materials become activated, and it cannot melt down like a fission reactor because the reaction stops if conditions fail. In December 2022, the National Ignition Facility in California achieved fusion ignition for the first time, with 3.15 megajoules of output from 2.05 megajoules of laser energy delivered to the target, though overall system energy was far higher. ITER in France is a large international tokamak under construction, and commercial fusion power is not expected for decades.
Based on data from the US Department of Energy and ITER
