Appendix D

Glossary

Note: The committee adopted the definitions used in the previous National Academies report (NRC, 2008) on the same topic.

A/D ratio: Ratio of the activity of a radiation source to the activity determined to define a threshold level of danger (D) in the International Atomic Energy Agency’s radiation source categorization system.

Absorbed dose: Quantity of ionizing radiation deposited into a material, including an organ or tissue, expressed in terms of the energy absorbed per unit mass of material. The basic unit of absorbed dose is the rad or its SI equivalent, the gray (Gy).

Accelerator: A device that accelerates charged subatomic particles, also called a particle accelerator. In the context of this report, these devices are used to generate energetic beams of electrons that can be directed at an object that one wants to irradiate or at a tungsten, tantalum, or gold target, which converts the electron energy into x-rays that irradiate an object.

Activity: Rate of decay of a radionuclide. More formally, the number of decays per unit of time. Its SI unit is the becquerel (Bq) corresponding to one radioactive decay (disintegration) per second; its old unit, the curie (Ci), was originally defined as the activity of 1 gram of radium-226 or 3.7 × 10¹⁰ disintegrations per second.

Acute effect: Effects in organisms that manifest themselves soon after exposure to radiation and are characterized by inflammation, edema, denudation and depletion of epithelial and haemopoietic tissue, and hemorrhage.

Aggregated sources: Defined in 10 CFR Part 37 as “accessible by the breach of a single physical barrier that would allow access to radioactive material in any form, including any devices that contain the radioactive material, when the total activity equals or exceeds a Category 2 quantity of radioactive material.”

Agreement State: States that have assumed authority under Section 274b of the Atomic Energy Act to license and regulate byproduct materials (radioisotopes), source materials (uranium and thorium), and certain quantities of special nuclear materials. This agreement gives the state regulatory authority over radioactive materials used within that state.

Becquerel (Bq): A unit of measure for activity. One becquerel is 1 disintegration (radioactive decay) per second. A gigabecquerel (GBq) is 10⁹ Bq (1 billion becquerels) and a terabecquerel (TBq) is 10¹² Bq (1 million million becquerels).

Bremsstrahlung: Radiation emitted by the slowing down of light charged particles, such as the x-rays produced when electrons from an accelerator are stopped in a metal target.

Byproduct material: Defined by the Atomic Energy Act as radioactive material (except special nuclear material) yielded in or made radioactive by exposure to the radiation incident to the process of producing or using special nuclear material; and tailings or wastes produced by the extraction or concentration of uranium or thorium from any ore processed primarily for its source material content.

Category 1 source: A radiation source that, if not managed safely or securely, could lead to the death or permanent injury of individuals in a short period of time.

Category 2 source: A radiation source that, if not managed safely or securely, could lead to the death or permanent injury of individuals who may be in close proximity to the radioactive source for a longer period of time than for Category 1 sources.

Category 3 source: A radiation source that, if not managed safely or securely, could lead to the permanent injury of individuals who may be in close proximity to the source for a longer period of time than Category 2 sources. Sources in Category 3 could, but are unlikely to, lead to fatalities.

Curie (Ci): A unit of measure for activity equal to 3.7 × 10¹⁰ (37 billion) disintegrations (radioactive decays) per second.

Deterministic health effect: A tissue reaction that increases in severity with increasing dose, usually above a threshold dose, in affected individuals (organ dysfunction, fibrosis, lens opacification, blood changes, and decrease in sperm count). These are events caused by damage to populations of cells, hence the presence of a threshold dose.

Effective dose: Sum over the absorbed doses to different organs from different radiation types multiplied by organ weighting factors and radiation weighting factors, as defined by the International Commission for Radiation Protection. Unit: 1 Sv = 1 J/kg = 100 rem.

External cost: Cost from an action or economic transaction that is not included in the monetary cost of the activity or transaction and therefore is borne by parties not directly involved in the transaction.

Gamma ray: High-energy electromagnetic radiation. In this report, radiation emitted by decay of a radionuclide is always referred to as gamma radiation to distinguish it from radiation from an x-ray generator.

Greater-than-Class-C waste: Radioactive waste that contains concentrations of certain radionuclides above the Class C limits in 10 CFR § 61.55.

Half-life: The time during which one-half of a given quantity of a radionuclide undergoes radioactive decay.

Increased controls: A set of security measures required by the U.S. Nuclear Regulatory Commission of Category 1 and 2 radiation source or device licensees.

Ionizing radiation: Radiation that is sufficiently energetic to ionize the matter (i.e., remove electrons from the atoms) through which it moves.

Irradiation: Exposure to radiation.

Nondestructive testing (NDT): Testing that does not destroy the object under examination.

Offsite Source Recovery Project: An effort by the National Nuclear Security Administration (NNSA) to recover and secure radiation sources that may pose a danger to public health, safety, and security. The project is run by the Los Alamos National Laboratory.

Panoramic irradiator: An irradiation device that does not have shielding built into the device. In such devices, the sources must be housed in thick, shielded structures.

Radiation dose: The quantity of radiation energy deposited in an object or medium divided by the mass of the object or medium. The radiation dose of interest in this report is ionizing radiation. Ionizing radiation doses can be expressed as an absorbed dose, equivalent dose, or effective dose. Its SI unit, gray (Gy), is defined as 1 joule (J) of energy absorbed per kilogram (kg) of absorbing medium; its old unit is the rad defined as 100 erg of energy absorbed per gram (g) of absorbing medium.

Radiation dose rate: The quantity of ionizing radiation absorbed by a medium per unit mass of the medium per unit time.

Radiation exposure: The act of being exposed to radiation. Also referred to as irradiation. Formally in radiation detection and measurement, radiation exposure is related to the ability of photons to ionize air.

Radioactive: Elements that are unstable and transform spontaneously (i.e., decay) through the emission of ionizing radiation, a process known as radioactive decay.

Radioactive source: Radioactive material packaged to use the radiation it emits.

Radiography: Use of radiation to create images of a subject, especially the internal features of a subject. Medical radiography is familiar from routine dental examinations. Industrial radiography is a form of nondestructive testing for aircraft wings, pipes, turbines, reinforced concrete construction, and other applications.

Radiological dispersal device (RDD): Device used to spread radioactive material for malevolent purposes. An RDD or “dirty bomb” involves use of conventional explosives such as dynamite to disperse radioactive material but does not use that material to produce a nuclear chain reaction that leads to an explosion, as is the case with a nuclear weapon. The objective of such a device might be to cause social disruption (panic, evacuation), acute physical harm, the potential for physical harm from chronic exposure, and/or economic damage. An area denial RDD is one intended to cause contamination that prevents occupation of the contaminated area for an extended period of time.

Radiological exposure device (RED): Device used to cause direct radiation exposure for malevolent purposes.

Radionuclide: Atom with an unstable nucleus, which undergoes radioactive decay.

Radiosurgery: Focal irradiation techniques that use multiple, non-coplanar radiation beams to deliver a prescribed dose of radiation to a lesion, primarily in the brain.

Radiotherapy: Treatment of disease with ionizing radiation.

Risk: As used in this report, the potential for an adverse effect from the accidental or intentional misuse of a radiation source. This potential can be estimated quantitatively if answers to the following three questions can be obtained: (1) What can go wrong? (2) How likely is it that something will go wrong? And (3) What are the consequences? Risk can be expressed in absolute terms or in comparison to other types of risks.

Safety: In the context of this report, concerning prevention of failure, damage, human error, and other inadvertent acts involving radiation sources that could result in accidental radiation exposures.

Safety risks: In the context of this report, risks that arise from exposures of people to radiation as a direct result of accidents involving radiation sources.

Security: In the context of this report, concerning protection against theft, sabotage, and other malevolent acts involving radiation sources.

Self-contained irradiator: “An irradiator in which the sealed source(s) is completely contained in a dry container constructed of solid materials, the sealed source(s) is shielded at all times, and human access to the sealed source(s) and the volume(s) undergoing irradiation is not physically possible in its designed configuration,” according to the American National Standards Institute (ANSI) Standard N433.1, “Safe Design and Use of Self-Contained, Dry Source Storage Gamma Irradiators (Category I).” Also called a self-shielded irradiator.

Social cost: Costs to society, including direct and indirect costs paid as money and undesirable effects that are not readily monetized.

Societal risk: All risks that affect society, including the health and safety risks and social risks discussed in this report.

Stochastic effect: One in which the probability of occurrence increases with increasing dose but the severity in affected individuals does not depend on the dose (radiation carcinogenesis and genetic effects). There is no threshold dose for effects that are truly stochastic, because these effects arise in single cells, and it is assumed that there is always some small probability of the effect occurring no matter how small the radiation dose.

Transuranic waste: Radioactive waste containing long-lived radioactive transuranic elements (elements with atomic numbers greater than 92, such as plutonium) in concentrations greater than 100 nanocuries per gram.

Ultrasonics: Use of high-intensity acoustic energy for materials examination.

Vitrification: Process for immobilizing radioactive material in a glass matrix.

Well logging: Practice of measuring the properties of the geological strata through which a well has been drilled and recording the results as a function of depth.

X-ray: High energy electromagnetic radiation. In this report, radiation emitted by a machine such as an x-ray tube or an electron accelerator with a high-Z target is always referred to as x-ray radiation to distinguish it from radiation from decay of a radionuclide.