SOLAR CONSTANT: The solar radiation falling on a unit area at the outer limits of the earth’s atmosphere.

SPECTRAL-SHIFT REACTOR: A reactor in which a mixture of light water and heavy water is used as the moderator and coolant. The ratio of light to heavy water is varied to change (shift) the energy spectrum of the neutrons in the reactor core. Since the probability of neutron capture varies with neutron velocity, a measure of reactor control is thus obtained.

SYNTHESIS GAS: A fuel gas containing primarily carbon monoxide and hydrogen; it can be used after careful removal of impurities, particularly sulfur compounds, for conversion to methane (high-Btu gas), methanol, liquid hydrocarbons, and a wide variety of other organic compounds.

TAILINGS: Waste material from a separation process. Commonly the finely divided waste from a mineral separation operation.

TAILS: Contraction of “tailings” (q.v.).

TAILS (OR TAILINGS) ASSAY: The percentage of valuable material that remains unrecovered in the tailings of a separation process.

TAR SANDS: Hydrocarbon-bearing deposits distinguished from more conventional oil and gas reservoirs by the high viscosity of the hydrocarbon, which is not recoverable in its natural state through a well by ordinary production methods.

TERTIARY RECOVERY: Use of heat and methods other than air, gas, or water injection to augment oil recovery (presumably occurring after secondary recovery).

THORIUM: A radioactive element of atomic number 90; naturally occurring thorium has one main isotope—thorium-232. The absorption of a neutron by a thorium atom can result in the creation of the fissile material uranium-233.

THROWAWAY FUEL CYCLE: A fuel cycle in which the spent fuel discharged from the reactor is not reprocessed to recover residual plutonium and uranium values.

TRANSURANIC ELEMENTS: Radioactive nuclides generated as fission products from the fissioning of nuclear fuel during reactor operation and as induced activity from the capture of neutrons in fuel cladding, reactor structures, and reactor coolant.

URANIUM: A radioactive element of atomic number 92. Naturally occurring uranium is a mixture of 99.28 percent uranium-238, 0.71 percent uranium-235, and 0.0058 percent uranium-234. Uranium-235 is a fissile material and is the primary fuel of light water reactors. When bombarded with slow or fast neutrons, it will undergo fission. Uranium-238 is a fertile material that is transmuted to plutonium-239 upon the absorption of a neutron.

URANIUM HEXAFLUORIDE (UF₆): A compound of uranium, which is used in gaseous form in the enrichment of uranium isotopes.

YELLOWCAKE: A uranium concentrate that results from the milling (concentrating) of uranium ore. It typically contains 80–90 percent uranium oxide (U₃O₈).