What Is Nuclear Energy?

Nuclear energy is the energy released when the nuclei of atoms are split or combined, and, as an energy resource, it refers to the use of that release to produce heat and electricity from nuclear fuels. Every commercial nuclear plant in operation today uses fission, in which a heavy nucleus absorbs a neutron and breaks into lighter fragments, converting a small fraction of its mass into kinetic energy according to the mass-energy equivalence Albert Einstein described in 1905. The energy density involved is what distinguishes the resource: fissioning one kilogram of uranium-235 releases roughly the same energy as burning two to three thousand tonnes of coal.

The resource base rests on a small number of fissile and fertile nuclides. Uranium-235 is the only fissile isotope found in nature in usable quantity, making up about 0.7 percent of natural uranium, and most reactors run on fuel enriched to between 3 and 5 percent. Plutonium-239 and uranium-233 are bred from uranium-238 and thorium-232 respectively, which is the basis for fuel cycles that would extend the resource by orders of magnitude.

Fission and Reactor Systems

A reactor sustains a controlled chain reaction: each fission releases two or three neutrons, and the geometry, moderator, and neutron-absorbing control rods are arranged so that on average exactly one of those neutrons goes on to cause another fission. As the International Atomic Energy Agency explains in its account of the science of nuclear power, the resulting heat warms a coolant that raises steam to drive a turbine generator. Light water reactors, split between pressurized and boiling designs, dominate the installed fleet. The U.S. Department of Energy's primer on how a nuclear reactor works describes the fuel form used in these plants: ceramic uranium dioxide pellets stacked inside sealed metal tubes, bundled by the hundreds into assemblies. Heavy water reactors, gas-cooled reactors, and fast reactors cooled by liquid sodium or lead occupy smaller niches, and small modular reactor designs aim to move fabrication into factories.

The Nuclear Fuel Cycle

The fuel cycle spans mining and milling of uranium ore, conversion to uranium hexafluoride, isotopic enrichment by gas centrifuge, fabrication into fuel assemblies, irradiation in a reactor for three to six years, and management of the discharged fuel. Spent fuel remains intensely radioactive and thermally hot, so it cools in water pools for several years before transfer to dry cask storage. Some countries reprocess spent fuel to recover plutonium and residual uranium for mixed oxide fuel, while others treat it as waste destined for a deep geological repository. Safeguards and material accounting run through the whole chain, because the same enrichment and separation technologies that produce fuel can also produce weapons-usable material.

Fusion and Radioisotope Sources

Two other routes fall under the same heading. Fusion joins light nuclei, typically deuterium and tritium, and releases energy without long-lived fission products, but confining a plasma at the required conditions long enough for net energy gain remains an engineering problem under active work at ITER and at inertial confinement facilities. Radioisotope thermoelectric generators convert the decay heat of plutonium-238 directly into electricity and have powered deep space missions for decades. Both fall outside the electricity market that the U.S. Energy Information Administration tracks for nuclear power plants, which reports the fission fleet supplying close to a fifth of American generation.

Applications

Nuclear energy has applications across a range of sectors, including:

• Baseload electricity generation on national grids
• Naval propulsion for submarines and aircraft carriers
• Process heat for desalination and industrial applications
• Hydrogen production through high-temperature electrolysis
• Space power and propulsion using radioisotope and reactor sources
• Medical isotope production for diagnosis and therapy
• Materials research and neutron science at test reactors