Science Terms Definitions - D

Decay - The decrease in the amount of any radioactive material with the passage of time due to the spontaneous emission from the atomic nuclei of either alpha or beta particles, often accompanied by gamma radiation. Every decay process has a definite half-life.

Decay Correction - The amount by which the calculated radioactivity (for example, of a release of radioisotopes) must be reduced after a period of time, to allow for its radioactive decay during that time.

Decay Products - Degraded radioactive materials, often referred to as "daughters" or "progeny", radon decay products of most concern from a public health standpoint are polonium-214 and polonium-218.

Decommissioning - Decontamination and dismantlement of retired, contaminated facilities and removal and/or disposal of the resulting wastes.

Decontamination - Removal of harmful substances such as noxious chemicals, harmful bacteria or other organisms, or radioactive material from exposed individuals, rooms and furnishings in buildings, or the exterior environment.

Deep-Well Injection - Deposition of raw or treated, filtered hazardous waste by pumping it into deep wells, where it is contained in the pores of permeable subsurface rock.

Depleted Uranium - Uranium having a percentage of uranium-235 smaller than the 0.7 percent found in natural uranium. It is obtained from spent (used) fuel elements or as byproduct tails, or residues, from uranium isotope separation.

Department of Energy (DOE) - This Federal agency's mission is to achieve efficiency in energy use, diversity in energy sources, a more productive and competitive economy, improved environmental quality, and a secure national defense. DOE was created on October 1, 1977 out of the Energy and Research and Development Agency as well as various aspects of non-nuclear federal energy policy and programs. The DOE complex, which is located over 22 States with sites that range in size from small to very large, produced and tested nuclear weapons.

Dermal Exposure - Contact between a chemical and the skin.

Dermal Toxicity - The ability of a pesticide or toxic chemical to poison people or animals by contact with the skin.

Deterministic Effect - The health effects, the severity of which varies with the dose and for which a threshold is believed to exist. Radiation-induced cataract formation is an example of a deterministic effect (also called a non-stochastic effect)

Dewater

1. Remove or separate a portion of the water in a sludge or slurry to dry the sludge so it can be handled and disposed.

2. Remove or drain the water from a tank or trench.

Direct Discharger - A municipal or industrial facility which introduces pollution through a defined conveyance or system such as outlet pipes; a point source.

Direct Filtration - A method of treating water which consists of the addition of coagulant chemicals, flash mixing, coagulation, minimal flocculation, and filtration. Sedimentation is not used.

Direct Runoff - Water that flows over the ground surface or through the ground directly into streams, rivers, and lakes.

Discharge - Flow of surface water in a stream or canal or the outflow of groundwater from a flowing artesian well, ditch, or spring. Can also apply to discharge of liquid effluent from a facility or of chemical emissions into the air through designated venting mechanisms.

Dose - The absorbed dose, given in rads (or the international system of units, grays), that represents the energy absorbed from the radiation in a gram of any material. Furthermore, the biological dose or dose equivalent, given in rem or sieverts, is a measure of the biological damage to living tissue from the radiation exposure.

Dose-effect - The relationship between dose (usually an estimate of dose) and the gradation of the effect in a population, that is a biological change measured on a graded scale of severity, although at other times one may only be able to describe a qualitative effect that occurs within some range of exposure levels.

Dose Equivalent - The product of the absorbed dose from ionizing radiation and such factors as account for differences in biological effectiveness due to the type of radiation and its distribution in the body as specified by the International Commission on Radiological Units and Measurements (ICRU).

Dose-response - A correlation between a quantified exposure (dose) and the proportion of a population that demonstrates a specific effect (response).

Dose-response Assessment - The process of characterizing the relation between the dose of an agent administered or received and the incidence of an adverse health effect in exposed populations and estimating the incidence of the effect as a function of human exposure to the agent.

Dose-Response Relationship - The quantitative relationship between the amount of exposure to a substance and the extent of toxic injury or disease produced.

Dosimetry - The theory and application of the principles and techniques involved in the measurement and recording of ionizing radiation doses.

Downgradient - The direction that groundwater flows; similar to "downstream" for surface water.

Drawdown

1. The drop in the water table or level of water in the ground when water is being pumped from a well.

2. The amount of water used from a tank or reservoir.

3. The drop in the water level of a tank or reservoir.

DWPF - Defense Waste Processing Facility, the name of the vitrification plant for high-level radioactive wastes at the Savannah River Site.