Neutrons travelling at high velocities, or equivalently, with high energies. Fast neutrons from fission have energies in the range of 1–10 Megaelectronvolt (MeV). Fast neutrons produced by deuterium-tritium neutron generators have energies of 14 MeV.
Fast Neutron
The process of the descent to the earth's surface of particles contaminated with radioactive material from a radioactive cloud. The term is also applied in a collective sense to the contaminated particulate matter itself. The early (or local) fallout is defined, somewhat arbitrarily, as those particles which reach the earth within 24 hours after a nuclear explosion. The delayed (or worldwide) fallout consists of the smaller particles which ascend into the upper troposphere and stratosphere, to be carried by winds to all parts of the earth. The delayed fallout is brought to earth, mainly by rain and snow, over extended periods ranging from months to years.
Fat Man
The implosion bomb dropped on Nagasaki, Japan at 11:02 am on August 9, 1945. The bomb was dropped 2 miles off target. 74,000 people were killed and another 75.000 sustained serious injuries.
See; Nagasaki, Fission Bomb, Fission, Implosion Weapon, Manhattan Project.
The luminous sphere of hot gases which form a few millionths of a second after a nuclear explosion. It is the result of the absorption by the surrounding medium of the thermal X-rays emitted by the extremely hot (several tens of million degrees) weapon residues. The exterior of the fireball in air is initially sharply defined by the luminous shock front and later by the limits of the hot gases themselves.
Stationary mass fire, generally in builtup urban areas, causing strong, inrushing winds from all sides; the winds keep the fires from spreading while adding fresh oxygen to increase their intensity.
First strike
The launch of a surprise attack to considerably weaken or destroy an adversary's military installations or nuclear forces and thus severely reduce its ability to attack or retaliate.
Fissile Isotope
Isotopes whose nuclei may be split by neutrons moving at various speeds and therefore fission very easily. Uranium-233, Uranium-235, and Plutonium-239 are all fissile isotopes. Fissile isotopes typically undergo fission more easily than other fissionable isotopes.
See; Fission.
Fissile Material
Nuclear material, containing a high proportion of fissile isotopes, which is essential for making nuclear explosives. High-enriched uranium (HEU) and weapons-grade plutonium are examples of fissile material.
See; Fission.
The process whereby the nucleus of a particular heavy element splits into (generally) two nuclei of lighter elements, with the release of substantial amounts of energy. The most important fissionable materials are uranium-235 and plutonium-239; fission is caused by the absorption of neutrons.
See; Fission Bomb, Nucleus.
Fission Bomb
A nuclear bomb based on the concept of releasing energy through the fission (splitting) of heavy elements such as U 235 or Pu 239.
See; Fission.
Fission Probability
The probability that a nucleus will split (fission) when an additional neutron is added to the nucleus.
See; Fission.
Fission Products
A general term for the complex mixture of substances produced as a result of nuclear fission. A distinction should be made between these and the direct fission products or fission fragments which are formed by the actual splitting of the heavy-element nuclei. Something like 80 different fission fragments result from roughly 40 different modes of fission of a given nuclear species (e.g., uranium-235 or plutonium-239). The fission fragments, being radioactive, immediately begin to decay, forming additional (daughter) products, with the result that the complex mixture of fission products so formed contains over 300 different isotopes of 36 elements.
Fission Weapon
A nuclear warhead whose material is uranium or plutonium that is brought to a critical mass under pressure from a chemical explosive detonation to create an explosion that produces blast, thermal radiation, and nuclear radiation through fission. The complete fission of one pound of fissionable material has a yield equivalent to 8000 tons of TNT.
Fission Yield
The amount of energy released by fission as distinct from that released by fusion in a thermonuclear (fusion) explosion.
The initiation of the fission chain reaction in the fissile material of a nuclear weapon at any time before the designed or the maximum compression or degree of assembly is attained.
Caused by the reflex glance toward the intense light given off by a nuclear explosion. Its effects can range from permanent blindness to retinal burns to temporary visual loss.
Flash Burn
A burn caused by excessive exposure (of bare skin) to thermal radiation.
See; Thermal Radiation.
Fuel Assemblies
Structures that contain about 240 fuel rods of uranium pellets. Fuel for a nuclear power plant is loaded in the reactor core in fuel assemblies.
See; Light-Water Reactor, Pressurized-Water Reactor, Reactor.
Fuel Cycle
The process of mining and refining uranium for use as reactor fuel. The process usually involves uranium enrichment and/or reprocessing.
Fuel Pellets
A small body of fuel, often cylindrical, designed to be stacked in a can to form a fuel element.
See; Light-Water Reactor, Pressurized-Water Reactor, Reactor.
Fuel Rods
A long, slender tube that holds fissionable material (fuel) for nuclear reactor use. Fuel rods are assembled into bundles called fuel elements or fuel assemblies, which are loaded individually into the reactor core.
See; Light-Water Reactor, Pressurized-Water Reactor, Reactor.
Nuclear fusion is a type of nuclear reaction in which two atomic nuclei combine to form a heavier nucleus, releasing energy. For a fusion reaction to take place, the nuclei, which are positively charged, must have enough kinetic energy to overcome their electrostatic force of repulsion. Thermonuclear fusion of deuterium and tritium will produce a helium nucleus and an energetic neutron. This is one basis of the Hydrogen Bomb, which employs a brief, uncontrolled thermonuclear fusion reaction. A great effort is now under way to harness thermonuclear fusion as a source of power.
See; Hydrogen Bomb, Thermonuclear.
Fusion Bomb
See; Fusion Weapon.
Fusion Weapon
Two-stage nuclear warhead containing fusion materials, such as deuterium and tritium, that are brought to critical density and temperature conditions by use of a primary fission reaction in order to initiate and sustain a rapid fusion process. This process in turn creates an explosion that produces blast, thermal radiation, and nuclear radiation. This type of device is commonly known as a hydrogen bomb or thermonuclear weapon.
See; Fusion, Fission, Fission Bomb.
The Gadget
The Gadget was built to test the implosion design. It was placed atop a 100 foot tower and detonated at 5:29:45 am on July 15, 1945. It had a yield of 19 kilotons 
See; Trinity Test, Manhattan Project.
Gamma Radiation
High-energy electromagnetic radiation emitted by nuclei during nuclear reactions or radioactive decay. These rays have high energy and a short wave length. Shielding against gamma radiation requires thick layers of dense materials, such as lead. Gamma rays or radiation are potentially lethal to humans, depending on the intensity of the flux.
See; Gamma Rays.
Gamma Rays
Electromagnetic radiations of high energy photons, originating in atomic nuclei and accompanying many nuclear reactions. They can penetrate deeply into body tissue and many materials. Shielding against gamma radiation requires thick layers of dense materials, such as lead. Gamma rays are potentially lethal to humans.
See; Photon.
Gas-Centrifuge Process
A method of isotope separation in which heavy, gaseous atoms or molecules are separated from light ones by centrifugal force.
Gas-Graphite Reactor
A nuclear reactor in which gas is the coolant and graphite is the moderator.
See; Reactor.
Gaseous Diffusion
A method of isotope separation based on the fact that gas atoms or molecules with different masses will diffuse through a porous barrier at different rates. The method is used to separate uranium-235 from uranium-238. It requires large plants and significant amounts of power.
See; Site X.
Ground Based Interceptor (GBI)
The missile intercept of the proposed U.S. National Missile Defense (NMD) system, the GBI will intercept incoming ballistic missile warheads outside the earth's atmosphere (exo-atmospheric) and collide with the incoming ballistic missile, thereby destroying the missile.
Ground Launched Cruise Missile (GLCM)
See; Cruise Missile.
Ground Zero
The point on the surface of land vertically below or above the center of a burst of a nuclear (or atomic) weapon; frequently abbreviated to GZ. For a burst over or under water the corresponding term is surface zero (SZ). Surface zero is also commonly used for ground surface and underground bursts.
Gun-Type Weapon
A device in which two or more pieces of fissionable material, each less than a critical mass, are brought together very rapidly so as to form a supercritical mass which can explode as the result of a rapidly expanding fission chain reaction.
See; Critical Mass, Supercritical.