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Science Terms Definitions - U

Uncertainty Analysis - A detailed examination of the systematic and random errors of a measurement or estimate; an analytical process to provide information regarding the uncertainty.

Uncertainty Factor - One of several, generally 10-fold factors, used in operationally deriving the Reference Dose (RfD) from experimental data. UFs are intended to account for (1) the variation in sensitivity among the members of the human population; (2) the uncertainty in extrapolating animal data to the case of humans; (3) the uncertainty in extrapolating from data obtained in a study that is of less-than-lifetime exposure; and (4) the uncertainty in using LOAEL data rather than NOAEL data.

Unconfined Aquifer - An aquifer containing water that is not under pressure; the water level in a well is the same as the water table outside the well.

Uncontrolled Liquid Waste - Waste that meets the definition of a liquid waste and is not bounded by container or piping, e.g., runoff, natural ponds, streams.

Underground Injection Control (UIC) - The program under the Safe Drinking Water Act that regulates the use of wells to pump fluids into the ground.

Underground Storage Tank - A tank located at least partially underground and designed to hold gasoline or other petroleum products or chemicals.

Unit Risk - The unit risk factors (URFs) provide estimates of the risks due to a unit inventory of contaminant (i.e., risk/gram or risk/curie). URFs can be calculated for water, soil, air, and radiation. URFs can be used to calculate risk for quantities greater than unity only if the relationship is linear.

Unreasonable Risk - Under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), "unreasonable adverse effects" means any unreasonable risk to man or the environment, taking into account the medical, economic, social, and environmental costs and benefits of any pesticide.

Uranium - A radioactive element with the atomic number 92 and, as found in natural ores, an atomic weight of approximately 238. The two principal natural isotopes are uranium-235 (0.7 percent of natural uranium), which is fissile, and uranium-238 (99.3 percent of natural uranium), which is fissionable by fast neutrons and is fertile. Natural uranium also includes a minute amount of uranium-234.

Uranium Chips Waste - Machine turnings which were cut from uranium parts that consist of <1% U235 (loose chips or wool bundle).

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