Btu (British thermal unit): The amount of energy necessary to raise the temperature of one pound of water by one degree Fahrenheit, from 39.2 to 40.2 degrees Fahrenheit.

CAPACITY FACTOR: The ratio of the amount of product (e.g., electrical energy or geothermal brine) actually produced by a given unit per unit of time to its maximum production rate over that period. Also called “load factor.”

COGENERATION: The generation of electricity with direct use of the residual heat for industrial process heat or for space heating.

COMBINED CYCLE: A combination of a steam turbine and a gas turbine in an electrical generating plant, with the gas-turbine exhaust heat used in raising steam for the steam turbine.

CONVERSION RATIO: The ratio of the number of atoms of new fissionable material produced in a converter reactor to the number of atoms of fissionable fuel consumed. See “breeding ratio.”

CONVERTER REACTOR: A reactor that produces some fissionable material, but less than it consumes. In some usages, a reactor that produces a fissionable material different from the fuel burned, regardless of the ratio. In both usages the process is known as conversion.

CURIE: A measure of intensity of the radioactivity of a substance; i.e., the number of unstable nuclei that are undergoing transformation in the process of radioactive decay. One curie equals the disintegration of 3.7×10¹⁰ nuclei per second, which is approximately the rate of decay of one gram of radium.

DEPLETION ALLOWANCE: A tax credit based on the permanent reduction in value of a depletable resource that results from removing or using some part of it.

DRY HOT ROCK (geothermal): See “hot dry rock.”

ELASTICITIES OF DEMAND: The arithmetic relations used by economists in quantifying the change in demand for a commodity in response to a change in another economic quantity. In this report, elasticities of demand in terms of price and income are especially important. These elasticities are calculated as the ratio of the percentage change in demand to the percentage change in price or income that evokes it.

FERTILE MATERIAL: A material, not itself fissionable by thermal neutrons, that can be converted into a fissile material by irradiation in a reactor. There are two basic fertile materials, uranium-238 and thorium-232. When these materials capture neutrons, they are partially converted into plutonium-239 and uranium-233, respectively.

FLASHING: The rapid change in state from a liquid to a vapor without visible boiling, resulting usually from a sudden reduction in the pressure maintained on a hot liquid.

FLUIDIZED BED: A body of finely divided particles kept separated and partially supported by gases blown through or evolved within the mass, so that the mixture flows much like a liquid.

FLY ASH: Fine solid particles of noncombustible ash entrained in the flue gases arising from the combustion of carbonaceous fuels. The particles of ash may be accompanied by combustible unburned fuel particles.