Science Terms Definitions - R

Radiation - The emission of particles (i.e., alpha, beta, or gamma particles) or rays (i.e., alpha, beta, gamma, or x-rays) by the nucleus of an atom.

Radiation Shielding - Reduction of radiation by interposing a shield of absorbing material between any radioactive source and a person, work area, or radiation-sensitive device.

Radiation Sickness (syndrome) - The complex of symptoms characterizing the disease known as radiation injury, resulting from excessive exposure (greater than 200 rads or 2 gray) of the whole body (or large part) to ionizing radiation. The earliest of these symptoms are nausea, fatigue, vomiting, and diarrhea, which may be followed by loss of hair (epilation), hemorrhage, inflammation of the mouth and throat, and general loss of energy. In severe cases, where the radiation exposure has been approximately 1,000 rad (10 gray) or more, death may occur within two to four weeks. Those who survive 6 weeks after the receipt of a single large dose of radiation to the whole body may generally be expected to recover.

Radioactive (Decay) - Property of undergoing spontaneous nuclear transformation in which nuclear particles or electromagnetic energy are emitted.

Radioactivity - The spontaneous discharge of radiation from atomic nuclei. This is usually in the form of beta or alpha radiation, together with gamma radiation. Beta or alpha emission results in transformation of the atom into a different element, changing the atomic number by +1 or -2 respectively.

Radioisotope - A radioactive isotope. An unstable isotope of an element that decays or disintegrates spontaneously, emitting radiation. More that 1300 natural and artificial radioisotopes have been identified.

Radionuclides - Radioactive elements. These may be subdivided into natural radionuclides such as radium or uranium which are normally present in the earth, or artificial radionuclides which are not normally present (or normally present in very small amounts) and are produced by nuclear fission.

Radium (Ra) - A radioactive metallic element with atomic number 88. As found in nature, the most common isotope has a mass number of 226. It occurs in minute quantities associated with uranium in pitchblende, camotite, and other minerals. Radon (Rn): A radioactive element that is one of the heaviest gases known. Its atomic number is 86. It is a daughter of radium.

RAUs - Risk analysis units.

RCRA - Resource Conservation and Recovery Act. RCRA gave EPA authority to control hazardous waste from " cradle-to-grave." This includes the minimization, generation, transportation, treatment, storage, and disposal of hazardous waste. RCRA also set forth a framework for the management of non-hazardous solid wastes. RCRA focuses only on active and future facilities and does not address abandoned or historical sites.

Reasonable Maximum Exposure (RME) - The maximum exposure reasonably expected to occur in a population.

Reasonably Available Control Technology (RACT) - Control technology that is both reasonably available, and both technologically and economically feasible. Usually applied to existing sources in no attainment areas; in most cases is less stringent than new source performance standards.

Receiving Waters - bodies of water that receive runoff or wastewater discharges, such as rivers, streams, lakes, estuaries, and groundwater.

Recharge - The process by which water is added to a zone of saturation, usually by percolation from the soil surface, e.g., the recharge of an aquifer.

Recharge Area - A land area in which water reaches the zone of saturation from surface infiltration, e.g., where rainwater soaks through the earth to reach an aquifer.

Recharge Rate - The quantity of water per unit of time that replenishes or refills an aquifer.

Recommended Maximum Contaminant Level (RMCL) - The maximum level of a contaminant in drinking water at which no known or anticipated adverse affect on human health would occur, and that includes an adequate margin of safety. Recommended levels are non-enforceable health goals.

Record of Decision (ROD) - A public document that explains which cleanup alternative(s) will be used at National Priorities List sites where, under CERCLA, Trust Funds pay for the cleanup.

Reference Concentration (RfC) - An estimate (with uncertainty spanning perhaps an order of magnitude) of a continuous inhalation exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious noncancer effects during a lifetime.

Reference Dose - Toxicity value for evaluating noncarcinogenic (systemic) effects of daily exposure to contaminant levels without appreciable deleterious effects during a lifetime. See our toxicity values.

Regional Deposited Dose (RDD) - The deposited dose of particles calculated for the region of interest as related to the observed effect. For respiratory effects of particles, the deposited dose is adjusted for ventilatory volumes and the surface area of the respiratory region effected (mg/min-sq.cm). For extra respiratory effects of particles, the deposited dose in the total respiratory system is adjusted for ventilatory volumes and body weight (mg/min-kg).

Relative Biological Effectiveness (RBE) - A factor that can be determined for different types of ionizing radiation, representing the relative amount of biological change caused by 1 rad. It depends upon the density of ionization along the tracks of the ionizing particles, being highest for the heavy particles: alpha rays and neutrons.

Relative Risk - The ratio of the rate of the disease (usually incidence or mortality) among those exposed to the rate among those not exposed.

Rem  - A unit of equivalent absorbed dose of radiation, taking account of the relative biological effectiveness of the particular radiation. The dose in rems is the dose in rads multiplied by the RBE.

Remedial Action (RA) - The actual construction or implementation phase of a Superfund site cleanup that follows remedial design.

Remedial Design - A phase of remedial action that follows the remedial investigation/ feasibility study and includes development of engineering drawings and specifications for a site cleanup.

Remedial Investigation (RI) - An in-depth study designed to gather data needed to determine the nature and extent of contamination at a Superfund site; establish site cleanup criteria; identify preliminary alternatives for remedial action; and support technical and cost analyses of alternatives. The remedial investigation is usually done with the feasibility study. Together they are usually referred to as the "RI/FS".

Remedial Project Manager (RPM) - The EPA or state official responsible for overseeing on-site remedial action.

Remedial Response - Long-term action that stops or substantially reduces a release or threat of a release of hazardous substances that is serious but not an immediate threat to public health.

Remediation - Cleanup or other methods used to remove or contain a toxic spill or hazardous materials from a Superfund site; for the Asbestos Hazard Emergency Response program, abatement methods including evaluation, repair, enclosure, encapsulation, or removal of greater than 3 linear feet or square feet of asbestos-containing materials from a building.

Remote Handling - Techniques used for the handling of radioactive materials behind the protection of walls which will absorb the radiation. It includes the use of robotics in radioactive areas.

Removal Action - Short-term immediate actions taken to address releases of hazardous substances that require expedited response.

Reportable Quantity (RQ) - Quantity of a hazardous substance that triggers reports under CERCLA. If a substance exceeds its RQ, the release must be reported to the National Response Center, the State Emergency Response Commission, and community emergency coordinators for areas likely to be affected.

Repository - A permanent resting place for radioactive wastes, which will finally decay to natural levels of radioactivity.

Reprocessing - Chemical treatment of spent fuel from a nuclear reactor to separate unused uranium and plutonium from radioactive fission product wastes. This allows recycle of valuable fuel material and minimizes the volume of high-level waste materials.

Response Action - Generic term for actions taken in response to actual or potential health-threatening environmental events such as spills, sudden releases, and asbestos abatement/management problems; A CERCLA-authorized action involving either a short-term removal action or a long-term removal response. This may include but is not limited to: removing hazardous materials from a site to an EPA-approved hazardous waste facility for treatment, containment or treating the waste on-site, identifying and removing the sources of groundwater contamination and halting further migration of contaminants; 3. Any of the following actions taken in school buildings in response to AHERA to reduce the risk of exposure to asbestos

RH (Remote Handled) Waste - TRU waste that 1) emits an external dose rate greater than 0.2 rem/hr (200 mrem/hr) and less than or equal to 1000 rem/hr. 2) has a relatively large quantity of beta and gamma emitting radionuclides with half lives typically 30 years or less. 3) requires shielding.

Risk - The product of: impact of severity (consequence) and impact of likelihood (probability). Specifically for carcinogenic effects, risk is estimated as the incremental probability of an individual developing cancer over a lifetime as a result of exposure to a potential carcinogen. Specifically for noncarcinogenic (systemic) effects, risk is not expressed as a probability but rather is evaluated by comparing an exposure level over a period of time to a reference dose derived for a similar exposure period.

Risk Analysis - A detailed examination including risk assessment, risk evaluation, and risk management alternatives, performed to understand the nature of unwanted, negative consequences to human life, health, property, or the environment; an analytical process to provide information regarding undesirable events; the process of quantification of the probabilities and expected consequences for identified risks.

Risk Assessment - The process of establishing information regarding acceptable levels of a risk and/or levels of risk for an individual, group, society, or the environment.

Risk Characterization - This last step in the risk assessment process characterizes the potential for adverse health effects and evaluates the uncertainty involved.

Risk Communication - The exchange of information about health or environmental risks among risk assessors and managers, the general public, news media, interest groups, etc.

Risk Estimate - A description of the probability that organisms exposed to a specific dose of a chemical or other pollutant will develop an adverse response (e.g., cancer).

Risk Estimation - The scientific determination of the characteristics of risks, usually in as quantitative a way as possible. These include the magnitude, spatial scale, duration and intensity of adverse consequences and their associated probabilities as well as a description of the cause and effect links.

Risk Evaluation - A component of risk assessment in which judgments are made about the significance and acceptability of risk.

Risk Factor - Characteristic (e.g., race, sex, age, obesity) or variable (e.g., smoking, occupational exposure level) associated with increased probability of a toxic effect.

Risk Identification - Recognizing that a hazard exists and trying to define its characteristics. Often risks exist and are even measured for some time before their adverse consequences are recognized. In other cases, risk identification is a deliberate procedure to review, and it is hoped, anticipate possible hazards.

Risk Management - The process of evaluating and selecting alternative regulatory and non-regulatory responses to risk. The selection process necessarily requires the consideration of legal, economic, and behavioral factors.

Risk Specific Dose - The dose associated with a specified risk level.

Roentgen - A unit of gamma radiation measured by the amount of ionization in air. In non-bony biological tissue 1 roentgen delivers a dose approximately equal to 1 rad.