BREEDER REACTOR A reactor that is designed to produce more fissile material than it consumes. Most breeder reactors use fast neutrons for sustaining the nuclear chain reaction, and are therefore called "fast breeders." A fast reactor that does not produce more fissile materials than it consumes is called a "fast neutron reactor." BURN-UP The amount of energy that has been generated from a unit of nuclear fuel; usually measured in megawatt-days thermal per metric tons of initial heavy metal (MWdth/MTIHM). ELECTRON A negatively-charged elementary particle. FERTILE MATERIAL Material that is not fissile, but which can be converted into a fissile material. Uranium-238 and thorium-232 are the principal fertile materials. FISSILE MATERIAL Material whose nucleus can be fissioned when it absorbs a low-energy (ideally zero energy) neutron. Fissile materials can sustain nuclear chain reactions. FISSIONABLE MATERIAL Material that can undergo nuclear fission when bombarded by a high-energy neutron. Most fissionable materials that are not fissile cannot sustain chain reactions. HALF-LIFE The amount of time it takes half of a given quantity of a radioactive element to decay. ISOTOPE A variant of an element that has the same number of protons but a different number of neutrons in the nucleus. Isotopes of elements have the same atomic numbers, but different mass numbers. MODERATOR A material used in a nuclear reactor to slow down the fast neutrons emitted in the process of fission. NEUTRON A neutral elementary particle that occurs in the nuclei of elements (except ordinary hydrogen). Free neutrons decay into a proton, an electron and an anti-neutrino. A neutron is about 1,838 times heavier than an electron. NUCLEAR FISSION The splitting of a nucleus of a heavy element into two lighter nuclei, generally accompanied by the release of one or more neutrons and energy. PROTON An elementary particle with a positive charge equal to that of an electron, but which is about 1,836 times heavier than an electron. REACTOR CORE The core of a reactor, consisting of the fuel, moderator (in the case of thermal reactors), and coolant. REPROCESSING The separation of irradiated nuclear fuel into uranium, plutonium, and fission products. THERMAL REACTOR A reactor that uses thermal (or slow) neutrons to sustain the chain reaction. VITRIFICATION The process of making glass. In the context of plutonium and nuclear waste management, it means the mixing of a material with molten glass in order to render it immobile, safe for storage, and not easily usable for weapons.

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Last updated: February, 1997