Table of Contents


The actinides are heavy metallic elements with atomic numbers from 89 to 103. They are produced when uranium-based fuel is irradiated in a nuclear reactor, are radioactive but generally long lived, and are considered part of the waste stream. Plutonium and uranium are called the major actinides; the minor actinides consist of all other actinides, including neptunium, americium, and curium.

Base load

The base load is the minimum level of power that a power grid must supply over long periods of time, often conceived as the power output that is required 24/7. Base load contrasts to peak load, which is the maximum level of power a grid must supply for a short period of time.

Breeding ratio

The breeding ratio is the ratio of fissile material obtained to fissile material used after the irradiation of fuel in a reactor.


Burnup refers to the amount of energy that is provided by an amount of nuclear fuel. For power reactors, a common measure of burnup is megawatt-days (or gigawatt-days) per metric ton of heavy metal. Burnup is one measure of the fuel efficiency of a power reactor.

Fast neutron reactor

A fast neutron reactor, or fast reactor, uses fast neutrons to generate a fission chain reaction. Thermal reactors, by contrast, contain a moderator (such as water or graphite) to slow the neutrons.

Fission products

Fission products are light elements, including isotopes of iodine, cesium, strontium, and xenon, produced when uranium-based fuel is irradiated in a nuclear power reactor. They are radioactive and are considered part of the waste stream.


A gigawatt (GW), equivalent to one billion watts, is a standard unit used for measuring the electric power rating of industrial power plants. A gigawatt is enough power to illuminate 10 million light bulbs, each rated at 100 watts.

Heavy metal

Heavy metal refers to the actinides present in nuclear fuel. The quantity of bulk amounts of nuclear fuel is commonly measured as metric tons of heavy metal (MTHM).


Lanthanides are fifteen so-called “rare earth” metals grouped on the periodic table of the elements as having atomic numbers 57 through 71. Some of these are generated as fission products in spent nuclear fuel.

Light water reactor

The light water reactor (LWR) uses low-enriched uranium dioxide fuel, and light water (i.e., normal water, not heavy water containing more of the deuterium hydrogen isotope) serves as both the core coolant and the moderator of the nuclear chain reactions. It is the most common type of nuclear power reactor.

Mixed oxide fuel

Mixed oxide (MOX) fuel contains both uranium dioxide (UO2) and plutonium dioxide (PuO2). It can be used in both light-water reactors and fast neutron reactors; the plutonium content in MOX fuel for fast reactors is generally higher than for light-water reactors.


A moderator is a substance that slows down fast neutrons generated in nuclear fission reactions to permit them to be used in thermal reactors. The most common moderator in commercial power reactors is light water.

Neutron capture

Neutron capture happens when a neutron collides with a heavy nucleus and is absorbed by it. In a nuclear reactor, capture of neutrons by uranium leads to the transmutation of uranium into plutonium.  

Pressurized water reactor

A pressurized water reactor (PWR) is a type of LWR in which water under high-pressure absorbs heat before being fed into a steam generator. It contrasts to a boiling water reactor, in which steam is produced directly in the reactor core.


Radiotoxicity is the measurement of the adverse human health effects of a radionuclide due to its radioactivity, taking into account the type of radiation, its tendency to be absorbed by human tissue, and residence time in the body.

Solvent extraction

Solvent extraction is a technique used in reprocessing to separate and purify plutonium and uranium from other substances in spent fuel.

Steam generator

Steam generators, which are used in PWRs, remove heat from high-pressure water and use it to boil low-pressure water, producing steam.  The steam is then fed into a turbine used to generate electricity.

Supercritical water-cooled reactor

A supercritical water-cooled reactor is a concept reactor that would use water at such a high temperature and pressure that the liquid and gas phases are no longer distinct.

Thermal reactor

A thermal reactor is a reactor that uses slow neutrons.

Transuranic elements

Transuranic elements are heavy elements with atomic numbers higher than uranium (92).