Glossary

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GLOSSARY

abstracted model: Model that reproduces or bounds the essential elements of a more detailed process model and captures uncertainty and variability in what is often, but not always, a simplified or idealized form. See abstraction.

abstraction: The essential components of a process model that are extracted for use in a total system performance assessment. The abstraction retains the basic intrinsic form of the process model but does not usually require its original complexity.

accessible environment: Any point outside of the controlled area, including (1) the atmosphere (including the atmosphere above the surface area of the controlled area), (2) land surfaces, (3) surface waters, (4) oceans, and (5) the lithosphere.

actinide: Any element of the actinide series, a series of chemically similar, mostly synthetic, radioactive elements with atomic numbers from 89 (actinium) through 103 (lawrencium).

adsorption: The attachment of atoms, ions, or molecules of a gas, liquid, or dissolved substance to a surface. The term sorption is sometimes used interchangeably with this term.

advection: The process in which solutes are transported by groundwater movement.

ALARA (as low as is reasonably achievable): A process that applies a graded approach to reducing dose levels to workers and the public and reducing releases of radioactive materials to the environment. The goal of this process, often referred to as ALARA, is not merely to reduce doses, but to reduce them to levels that are as low as reasonably achievable.

alcove: A small excavation (room) off a main drift. Used for scientific study or for installing equipment.

alkaline: (1) Of, relating to, containing, or having the basic chemical properties of an alkali or alkali metal. (2) Having a pH of more than 7. See pH.

Alloy 22: A corrosion-resistant, high-nickel alloy (ASTM B 575 N06022) used for the outer barrier of the waste package.

alluvium: Sedimentary material (clay, mud, sand, silt, gravel) deposited by a stream or running water.

alpha particle: A positively charged particle ejected spontaneously from the nuclei of some radioactive elements. It is identical to a helium nucleus and has a mass number of 4 and an electrostatic charge of +2. It has low penetrating power and a short range (a few centimeters in air).

ambient: Undisturbed natural conditions such as ambient temperature caused by climate or natural subsurface thermal gradients.

analogue: Natural or man-made systems that include processes similar to those that could occur in a repository system at Yucca Mountain for which information can be obtained to evaluate long-term (e.g., millennia) or large-scale (e.g., kilometers) behavior.

anion: An atom or group of atoms having a negative charge; a negatively charged ion.

anisotropy: The condition in which physical properties vary when measured in different directions or along different axes. For example, in a layered rock section the permeability is often anisotropic in the vertical direction (from layer to layer) but is isotropic in the horizontal direction (within a layer). See isotropy.

annealing: Alternately heating and cooling a metal, alloy, or glass to relieve internal stresses in the material.

annual committed effective dose equivalent: A radiation protection term for the effective dose equivalent received by an individual in one year from radiation sources external to the individual plus committed effective dose equivalent.

aquifer: A subsurface formation or group of formations that is saturated and sufficiently permeable to transmit quantities of water to wells or springs.

areal mass loading: Used in thermal loading calculations, the amount of heavy metal (usually expressed in metric tons of uranium or equivalent) emplaced per unit area in the repository.

arid: Of a climate: very dry; a region in which annual precipitation is less than approximately 250 mm (10 in.). On average, Yucca Mountain receives about 190 mm (7.5 in.) of rain and snow annually.

ash: Fine or very fine pyroclastic particles, less than 4 mm (0.15 in.) in diameter, that are blown out from a volcanic explosion.

ash-fall tuff: Highly porous volcanic rock that is a result of magma thrown high into the atmosphere, where it cooled and fell back to earth as a blanket of ash.

ash-flow tuff: Dense, nonporous volcanic rock that is a result of magma flowing at ground level where it remained at a high temperature for a long time, welding magma and ash together.

ASHPLUME: A computer software code used to simulate the ash plume that results from an extrusive volcanic event.

assembly transfer system: System in the Waste Handling Building to transfer uncanistered spent nuclear fuel assemblies from the shipping casks to disposal containers.

autocatalytic criticality: A transient criticality in which the usual mechanisms that tend to shut down a criticality are delayed until a high fission rate is achieved.

backfill: The general fill that is placed in the excavated areas of the underground facility. If used, the backfill for the repository may be tuff or other material.

background radiation: Radiation arising from natural radioactive material always present in the environment, including solar and cosmic radiation, radon gas, soil and rocks, food, and the human body.

barrier: Any material, structure, or feature that prevents or substantially reduces the rate of movement of water or radionuclides.

base case: The sequence of anticipated conditions expected to occur in and around the potential repository, without the inclusion of unlikely or unanticipated features, events, or processes. The components that contribute to the base case model are intended to encompass this probable behavior of the repository, based on the range of uncertainty for the various parameters and conceptual models used in constructing the base case.

bioaccumulation: Means by which a living organism could ingest, inhale, or otherwise internally accumulate a foreign substance such as a radioactive particle.

biosphere: The ecosystem of the earth and the living organisms inhabiting it.

biosphere dose conversion factor: A multiplier used in converting a radionuclide concentration at the geosphere/biosphere interface into a dose that a human would experience for all pathways considered, with units expressed in terms of annual dose (i.e., the total effective dose equivalent) per unit concentration.

block-bounding fault: A normal fault that divides the crust into structural or fault blocks of different elevations and orientations; typical of the Basin and Range province.

boiling-water reactor: A nuclear power reactor in which water passing as coolant through the core is turned to steam by direct use of fission heat from the uranium oxide fuel; steam for driving the turbogenerator is formed within the reactor vessel itself rather than in an external heat exchanger and, after being condensed, returns as feedwater to the reactor vessel.

bomb-pulse: A detectable increase in radionuclide concentration related to global fallout from above-ground thermonuclear tests.

borehole: A hole drilled for purposes of collecting site characterization data or for supplying water. Sometimes referred to as a drillhole or well bore.

borosilicate glass: A material used to vitrify high-level radioactive waste in which boron takes the place of the lime used in ordinary glass mixtures.

boundary condition: For a model, the establishment of a set condition (set value), often at the geometric edge of the model, for a given variable; for example, using a specified groundwater flux from infiltration as a boundary condition for a flow model.

bounding: For a mathematical model, relating to a set condition (set value) that is considered to reflect the reasonable extreme for that value in the real-world condition being modeled. See boundary condition, bounding value.

bounding value: Specific data point that defines the reasonable extreme limit of a variable in an experiment or model.

breakthrough: The time at which the concentration of a substance, usually in groundwater, arrives at a particular point of interest.

breakthrough curve: The curve describing the rate of arrival of radionuclides transmitted through a medium. The breakthrough curve calculation includes the effects of all flow modes, flow in rock matrix, flow in fractures, and retardation and determines the expected proportion of the radionuclide mass transmitted through the medium as a function of time.

bridge crane: A large overhead crane used for material handling that spans across rails on either side of a structure.

Brownian motion: Random movement of small particles suspended in a fluid. Caused by pressure fluctuations over the surfaces of the particles as they interact with the fluid's molecules.

burnup: A measure of nuclear-reactor fuel consumption expressed either as the percentage of fuel atoms that have undergone fission or as the amount of energy produced per initial unit weight of fuel.

caldera: A large basin-shaped volcanic depression, more or less circular, resulting from the collapse of the ground surface following a rapid volcanic eruption. A caldera may be more than 15 km (9 mi) in diameter and more than 1,000 m (3,300 ft) deep.

calibration: (1) In modeling, the process of comparing the conditions, processes, and parameter values used in a model against actual data points or interpolations from measurements at or close to the site to ensure that the model is compatible with "reality" to the extent feasible. (2) For tools used for field or lab measurements, the process of taking instrument readings on standards known to produce a certain response to check the accuracy and precision of the instrument.

canister: The structure surrounding some forms of waste (e.g., high-level radioactive waste immobilized in glass logs or ceramic disks within cans) that facilitates handling, storage, transportation, and/or disposal.

capillary barrier: A contact in the unsaturated zone between a geologic unit containing relatively small-diameter openings and a unit containing relatively large-diameter openings. Water does not flow from the former to the latter due to capillary forces.

capillary force: A phenomenon that results from the force of molecular attraction (adhesion) between a fluid and different solid materials; this force causes water to rise in small diameter tubes and, in combination with the effects of gravity, is a means of water movement in the unsaturated zone.

capillary pressure: The pressure due to capillary forces; i.e., the pressure of fluids under the influence of surface tension and adhesion.

Carrier Preparation Building: Surface facility where waste transportation casks and their carriers are prepared before they enter the Waste Handling Building.

carrier/cask handling system: System in the Waste Handling Building that unloads the casks from the carriers and transfers the casks to either the assembly transfer system or the canister transfer system. The system also receives empty casks from the assembly and canister transfer systems and nondisposable canister overpacks from the assembly transfer system and loads them onto carriers for transfer to the Carrier Preparation Building.

cask: A large, shielded container for shipping or storing spent nuclear fuel and/or high-level radioactive waste that meets all applicable regulatory requirements.

cation: An atom or group of atoms having a positive charge; a positively charged ion.

cinder cone: A conical hill formed by the accumulation of cinders and other pyroclasts around a volcanic vent.

cladding: The metallic outer sheath of a fuel rod element generally made of a zirconium alloy. It is intended to isolate the fuel from the external environment.

clay: A rock or mineral fragment of any composition that is smaller than very fine silt grains, having a diameter less than 0.00016 in. (1/256 mm). A clay mineral is one of a complex and loosely defined group of finely crystalline hydrous silicates formed mainly by weathering or alteration of primary silicate minerals. They are characterized by small particle size and their ability to adsorb large amounts of water or ions on the surface of the particles.

climate: Weather conditions, including temperature, wind velocity, precipitation, and other factors, that prevail in a region averaged over some period of time.

climate state: Representation of climate conditions. Different climate states are used in performance assessment models to represent changes in climate over the time periods of interest.

code (computer): The set of commands used to solve a mathematical model on a computer.

codisposal: A packaging method for disposal of radioactive waste in which more than one type of waste, such as DOE spent nuclear fuel and high-level radioactive waste, are combined in disposal containers. Codisposal takes advantage of otherwise unused space in disposal containers and is more cost-effective than other methods of limiting the reactivity of individual waste packages.

cohesion: (1) In geology, the portion of the shear strength of a rock not related to inter-particle friction. (2) In physics, the attraction between molecules of a liquid that allows formation of drops or films of that liquid.

colloid: A large molecule or small particle that has at least one dimension with the size range of 10^-9 to 10^-6 m, suspended in a liquid such as groundwater. Some radionuclides can bind with colloids (either reversibly or irreversibly, depending on the type of colloid) and travel great distances in groundwater. Colloids may form directly from insoluble radioactive material (intrinsic colloids), may result from degraded spent nuclear fuel or glass waste forms (waste form colloids), or may result from other natural or man-made materials with which radionuclides can bind (pseudocolloids).

colluvium: Any loose, heterogeneous sediment deposited by rainwash, sheetwash, or slow continuous downslope creep, usually at the base of a cliff or slope.

committed effective dose equivalent: A radiation protection term for the effective dose equivalent received over a period of time (as determined by the U.S. Nuclear Regulatory Commission) by an individual from radionuclides internal to the individual following one year intake of these radionuclides.

committed dose equivalent: A radiation protection term for the dose equivalent that is committed to specific organs or tissues that will be received from an intake of radioactive material by an individual during the 50 years following the intake.

complexation capability: The ability of a chemical element to unite with other elements to form a complex compound.

component model: The analysis models that are run separately and then combined into process models for running in the TSPA computer model.

conceptual model: A set of qualitative assumptions used to describe a system or subsystem for a given purpose.

conduction: The flow of thermal energy through a material. Conduction is affected by the amount of heat energy present, the nature of the heat carrier in the material (its thermal conductivity), and the amount of dissipation.

confidence: (1) In statistics, a measure of how close the estimated value of a random variable is to its true value. (2) Degree of assurance that an argument, such as a safety case, is correct.

confidence interval: An interval that is believed, with a preassigned degree of confidence, to include the particular value of the random variable that is estimated.

conservative assumption: (1) An assumption that results in a calculated release of radionuclides exceeding actual or expected releases. (2) An assumption that uses uncertain inputs and does not attempt to include any potentially beneficial effects.

containment: (1) The confinement of radioactive waste within a designated boundary. (2) The use of design features to contain or reduce radioactive releases or radiation doses.

continuum model: A model that represents fluid flow through numerous individual fractures and matrix blocks by approximating them as continuous flow fields.

controlled area: The surface area, identified by passive institutional controls, that encompasses no more than 300 square kilometers. It must not extend farther south than 36º 40' 13.6661" North latitude, in the predominant direction of groundwater flow and must not extend more than five kilometers from the repository footprint in any other direction. It also includes the subsurface underlying the surface area.

convection: (1) The transfer of heat by the circulation of a fluid (at Yucca Mountain, either water or air). (2) The bulk motion of a flowing fluid (gas or liquid) caused by temperature differences that, in turn, cause different areas of the fluid to have different densities (e.g., warmer is less dense).

corrosion resistant material: A material that develops a protective film on its surface, creating a high resistance to corrosion. Such a material, the nickel-based alloy, Alloy 22, would be used as the outer barrier of the two-layer waste package.

creep: A phenomenon in which strain in a solid increases with time when stress producing the strain is held fixed; it may be associated with temperature or mechanical stress.

critical group: The hypothetical group of individuals reasonably expected to receive the greatest exposure to radioactive materials (proposed 10 CFR Part 63). The critical group has been replaced by the reasonably maximally exposed individual in the final NRC regulations (10 CFR 63.2).

criticality: The condition in which nuclear fuel sustains a chain reaction. It occurs when the effective neutron multiplication factor (the number of fissions in one generation divided by the number of fissions in the preceding generation) of a system equals one.

Cross-Drift, Enhanced Characterization of the Repository Block (ECRB) Cross-Drift: An excavation above and across the block of the potential repository excavated in a general northeast–southwest direction.

cumulative distribution: For grouped data, a distribution that shows how many of the values are less than or more than specified values. For random variables, this term is synonymous with distribution function.

cumulative probability: The probability that a random variable will have a value equal to or less than some specified value.

curie: A unit of radioactivity equal to 37 billion disintegrations per second, abbreviated Ci.

decommission: The process of removing from service a facility in which nuclear materials are handled. This usually involves decontaminating the facility so that it may be dismantled or dedicated to other purposes.

DECOVALEX: An international software code testing program.

defense in depth: (1) A design strategy based on a system of multiple, independent, and redundant barriers, designed to ensure that failure in any one barrier does not result in failure of the entire system. (2) A system of multiple barriers that mitigate uncertainties in conditions, processes, and events.

deliquescence: The absorption of water vapor from the air by a crystalline solid, leading to dissolution of the solid.

design bases: Information that identifies the specific functions to be performed by a structure, system, or component of a facility and the specific values or ranges of values chosen for controlling parameters as reference bounds for design. These values may be constraints derived from generally accepted state-of-the-art practices for achieving functional goals or requirements derived from analysis (based on calculation or experiments) of the effects of a postulated event under which a structure, system, or component must meet its functional goals. The values for controlling parameters for external events include (1) estimates of severe natural events to be used for deriving design bases that will be based on consideration of historical data on the associated parameters, physical data, or analysis of upper limits of the physical processes involved; and (2) estimates of severe external human-induced events, to be used for deriving design bases, that will be based on analysis of human activity in the region taking into account the site characteristics and the risks associated with the event.

desorption: A physical or chemical process by which a substance that has been adsorbed or absorbed by a liquid or solid material is removed from the material.

devitrification: The conversion of a glassy substance to a crystalline substance; for example, the alteration of glass in vitrified tuff into zeolites.

diffusion: (1) A process in which substances move from regions of higher concentrations to regions of lower concentrations. (2) The gradual mixing of the molecules of two or more substances due to random thermal motion.

diffusion coefficient: A material's weight, in grams, as it diffuses in 1 second through 1 square centimeter of a medium of a determined concentration gradient (i.e., a medium having a known variability in the concentration of the substance in solution as it travels over distance).

diffusive transport: Movement of molecules or particles due to their concentration gradient. Occurs when dissolved or suspended radionuclides move from regions of higher or lower concentration.

dike: A tabular body of igneous rock that cuts across the structure of adjacent rocks or cuts massive rocks. Most dikes are caused by the intrusion of magma. Some dikes occur as columnar structures.

dilution: The reduction of a dissolved substance's concentration in a solution, caused by an increase in the solvent's proportion of the solution. The solvent frequently is water.

dispersion (hydrodynamic dispersion): (1) The tendency of a solute (substance dissolved in groundwater) to spread out from the path it is expected to follow if only the bulk motion of the flowing fluid moved it. (2) The macroscopic outcome of the actual movement of individual solute particles through a porous medium. Dispersion causes dilution of solutes, including radionuclides, in groundwater and is usually an important mechanism for spreading contaminants in low flow velocity situations.

dispersivity: The degree to which small particles of a solid are distributed throughout either a liquid or another solid.

disposal container: The vessel consisting of the barrier materials and internal components in which the canistered or uncanistered waste form is placed. The disposal container includes the container barriers or shells, spacing structures or baskets, shielding integral to the container, packing contained within the container, and other absorbent materials designed to be placed internal to the container or immediately surrounding the disposal container (i.e., attached to the outer surface of the container). The filled, sealed, and tested disposal container is referred to as the waste package, which is emplaced underground.

disposal container handling system: System in the Waste Handling Building that prepares empty disposal containers for loading, receives full disposal containers from the assembly and canister transfer systems, welds and inspects the containers, and transfers them to the waste emplacement system. The system also receives and handles retrieved waste packages from the subsurface and disposal containers that are defective and routes them to the waste package remediation system.

disruptive processes and events: Unlikely processes or events that could affect the performance of the repository, including, for example, human intrusion, volcanic activity, and seismic activity.

disruptive scenario: A well-defined sequence of events and processes that could adversely affect repository performance initiated by a disruptive process or event.

dissolution: The dissolving of a solid or gas in a liquid.

distribution: The overall scatter of values for a set of observed data. A term used synonymously with frequency distribution. Distributions have probability structures that are the probability that a given value occurs in the set.

distribution function: A function whose values are the probabilities that a random variable assumes a value less than or equal to a specified value. Synonymous with cumulative distribution.

DOE spent nuclear fuel: Radioactive waste created by defense activities. The major contributor to this waste form is the N Reactor fuel currently stored at the Hanford Site. This waste form also includes naval spent nuclear fuel.

dose: The amount of radioactive energy taken into (absorbed by) living tissues.

dose equivalent: The product of the absorbed dose in tissue, quality factor, and all other necessary modifying factors at the location of interest. See effective dose equivalent and total effective dose equivalent.

downgradient: The direction that water will tend to flow as the result of a difference in pressure, as indicated by the elevation change in the potentiometric surface. Based on current understanding of the hydraulic gradient below Yucca Mountain, downgradient is toward the south to southeast of the potential repository location.

drift: From mining terminology, a horizontal underground passage. Includes excavations for emplacement (emplacement drifts) and access (main drifts).

drip shield: A corrosion-resistant engineered barrier that is placed above the waste package to prevent seepage water from directly contacting the waste package for thousands of years. The drip shield also offers protection to the waste package from rockfall.

dual permeability conceptual model: A conceptual model of groundwater flow in which fractures and rock matrix are represented as separate, interacting continua, with no assumption of pressure equilibrium between fractures and rock matrix. This concept allows modeling groundwater flow as occurring mostly in the fractures, with less flow in the rock matrix, depending on the degree of connection between the rock matrix and fractures and the capillary pressure gradient. The dual permeability model is one of the conceptual models for groundwater and heat flow for fractured, porous media.

effective dose equivalent: The sum of the products of the dose equivalent received by specified tissues and the appropriate weighting factors applicable to each tissue.

effective porosity: The fraction of a given medium's porosity available for fluid flow and/or solute storage.

Eh: A measure of the state of oxidation of a system. Also known as redox potential or oxidation–reduction potential.

electrolyte: A substance (e.g., an acid, base, or salt) that, when dissolved in a suitable solvent (e.g., water) or when fused, conducts electric current by the movement of ions instead of electrons.

emplacement: The placement and positioning of waste packages in the repository emplacement drifts.

emplacement horizon: The area within the repository block where emplacement drifts would be excavated.

engineered barrier: Any component of the engineered barrier system, such as the drip shield, waste package, or invert. See engineered barrier system.

engineered barrier system: The waste packages and the underground facility, including engineered components and systems other than the waste package (e.g., drip shields).

Environmental Impact Statement: A detailed written statement to support a decision to proceed with major Federal actions affecting the quality of the human environment. This is required by the National Environmental Policy Act of 1969. Preparation of an environmental impact statement requires a public process that includes public meetings, reviews, and comments, as well as agency responses to the public comments.

EQ3/6: A computer software code used to estimate equilibrium mineral phases based on thermodynamic equilibrium, thermodynamic disequilibrium, and reaction kinetics.

equivalent continuum model: A conceptual model of groundwater and heat flow, in which the fractures and matrix can be treated as a single composite material, and the hydraulic properties are a combined effect of both fracture and matrix properties.

evapotranspiration: The combined processes of evaporation and plant transpiration that remove water from the soil and return it to the air.

event: (1) An occurrence that has a specific starting time and, usually, a duration shorter than the time being simulated in a model. (2) Uncertain occurrences that take place within a short time relative to the time frame of the model.

event sequence: A series of actions and/or occurrences within the natural and engineered components of a geologic repository operations area that could potentially lead to exposure of individuals to radiation. An event sequence includes one or more initiating events and associated combinations of repository system component failures, including those produced by the action or inaction of operating personnel. Those event sequences that are expected to occur one or more times before permanent closure of the geologic repository operations area are referred to as Category 1 event sequences. Other event sequences that have at least one chance in 10,000 of occurring before permanent closure are referred to as Category 2 event sequences.

event tree: A structurally tree-like diagram that is useful in representing sequences of events and their possible outcomes. Each node, or branching point, represents an event, and each branch from that node represents one of its possible outcomes. Each possible pathway along the tree, from beginning to end of a given line of branching, represents a specific scenario.

expected behavior: (1) The nominal behavior of the repository system and the geologic barrier in the absence of disruptive events. (2) The mean value of the probability distribution describing that behavior.

expected value: A variable's mean, or average, outcome. The weighted average of the number of possible outcomes, with each outcome being weighted by its probability of occurrence. The mean of a probability distribution of a random variable that one would expect to find in a very large, random sample. The sum of the possible values, each weighted by its probability—the center of the random variable's histogram (frequency distribution).

Exploratory Studies Facility: An underground laboratory at Yucca Mountain used for performing site characterization studies. The facility includes a 7.9-km (4.9-mi) main loop (tunnel), the 2.8-km (1.7-mi) Enhanced Characterization of the Repository Block (ECRB) Cross-Drift, and a number of alcoves used for site characterization tests such as the Drift Scale Test.

exposure pathway: The course a chemical or physical agent takes from the source to the exposed organism; describes a unique mechanism by which an individual or population can become exposed to chemical or physical agents at or originating from a release site.

extrusive event: An igneous (volcanic) event occurring at the surface, i.e., a volcanic eruption. In repository performance analyses, molten material is assumed to intersect waste packages in the repository and cause a release of radionuclides. Compare intrusive event.

fault: (1) A fracture in rock along which movement of one side relative to the other has occurred. (2) A fracture or a fracture zone in crustal rocks along which there has been movement of the fracture's two sides relative to one another, so that what were once parts of one continuous rock stratum or vein are now separated.

fault displacement: Movement along two sides of a fault relative to one another.

FEHM: A finite element heat and mass transport computer software code that is used for saturated zone flow and transport and unsaturated zone transport calculations.

fissile material: Material capable of undergoing fission with neutrons of any energy, including thermal, or slow, neutrons. The three primary materials in this category are uranium-233, uranium-235, and plutonium-239.

fission: The splitting of a nucleus into at least two other nuclei resulting in the release of two or three neutrons and a relatively large amount of energy.

fission product: Any nuclide, either radioactive or stable, that arises from fission, including both the primary fission fragments and their radioactive decay products. Also daughter product, decay product.

FLAC: A specialized computer code developed to solve soil and rock mechanics problems. It is used in conjunction with TOUGH to form a thermal-hydrologic-mechanical code to analyze those coupled effects.

flow field: A fluid's distribution in and through an area, including its velocity and density, as a function of position and time.

flux: The rate of transfer of fluid, particles, or energy passing through a unit area per unit time.

fracture: A break in rock caused by mechanical stresses. A fracture along which there has been displacement of the sides relative to one another is called a fault. A fracture along which no appreciable movement has occurred is called a joint.

fracture continuum: A continuum that represents fluid flow and transport through numerous individual fractures by approximating them as continuous flow and transport fields.

fracture permeability: The capacity of a rock to transmit fluid through fractures in the rock.

frequency distribution: Data grouped into classes (or ranges of values within the overall set of values, such as 1 to 5, 5 to 10, 10 to 20, etc.), with the classes listed in a table (or other format) showing the number of data points that occur in each class.

fuel assembly: A number of fuel rods held together by plates and separated by spacers, used in a reactor. This assembly is sometimes called a fuel bundle.

fuel blending: The process of loading low heat output waste with high heat output waste in a waste package to balance its total heat output. This process applies only to commercial spent nuclear fuel.

fuel blending inventory: The reserve of commercial spent nuclear fuel that will be inventoried in pools in the Waste Handling Building Annex. The spent nuclear fuel will be inventoried in the pools until selected, according to heat output, for fuel blending.

fuel matrix: The physical form and composition of the substance that holds the fissile material.

fugacity: A parameter that measures the chemical potential of a real gas in the same way that partial pressure measures the free energy of an ideal gas.

galvanic corrosion: Electrochemical corrosion caused by the flow of electricity that occurs when two dissimilar metals with differing electrical potentials are near each other in the presence of a conductor such as water with solutes in it.

gamma radiation: Electromagnetic radiation emitted during the radioactive decay process. The gamma ray is the most penetrating wave of radiant nuclear energy. It does not contain particles and can be stopped by dense materials such as concrete or lead.

gantry: A hoisting machine that slides along a fixed platform or track, either raised or at ground level.

GENII-S: A quasi-stochastic computer software code used to evaluate the dose from the migration of radionuclides in the biosphere that may affect humans through ingestion, inhalation, and direct radiation. It is used to develop biosphere dose conversion factors.

geologic repository: A system for the disposal of radioactive waste in excavated geologic media. A geologic repository includes the engineered barrier system and the portion of the geologic setting that provides isolation of the radioactive waste.

geologic repository operations area: A high-level radioactive waste facility that is part of a geologic repository, including both surface and subsurface areas, where waste handling activities are conducted.

glacial transition: One of the climate states of the future climate model for Yucca Mountain; others examples include modern and monsoon climates.

GoldSim: A software code used as a probabilistic shell for the TSPA component models that, in combination, simulate potential long-term behavior of the repository.

gross alpha activity: The total radioactivity due to alpha particle emissions from radionuclides estimated in the plume of contamination.

ground control: Support of rock in the subsurface of the repository (e.g., rock bolts and steel sets).

ground support: The system (rock bolt with wire mesh, steel cast, etc.) used to line the main and emplacement drifts to minimize rock or earth falling into the drifts.

groundwater: Water contained in pores or fractures in either the unsaturated or saturated zones below ground level.

groundwater flux: The volumetric rate of groundwater flow through a unit area of an aquifer.

half-life: The time in which half the atoms of a radioactive substance decay to another nuclear form. Half-lives range from millionths of a second to millions of years, depending on the stability of the nuclei.

heat pipe: A zone characterized by a continuous process of boiling, vapor transport, condensation, and migration of water back to the heat source.

heterogeneous: Being composed of parts or elements of different kinds, such as a mixture of liquid-vapor or liquid-vapor-solid. A condition in which the value of a parameter, such as porosity of rock, varies over space and time.

high-level radioactive waste: The highly radioactive material resulting from the reprocessing of spent nuclear fuel, including liquid waste produced directly in reprocessing; and any solid material derived from such liquid waste that contains fission products in sufficient concentrations; and other highly radioactive material that the U.S. Nuclear Regulatory Commission determines by rule requires permanent isolation.

high-level radioactive waste glass: The waste form of high-level radioactive waste in which the radioactive waste is mixed with borosilicate glass.

host rock: (1) The rock unit in which the potential repository would be located. For a repository at Yucca Mountain, the host rock would be the middle portion of the of the Topopah Spring Tuff of the Paintbrush Group. (2) The geologic medium in which the waste is emplaced.

human intrusion: Breaching of any portion of the Yucca Mountain disposal system within the repository footprint by any human activity.

human intrusion scenario: A disruptive event assessed in a separate TSPA according to specific characteristics defined by the U.S. Nuclear Regulatory Commission in 10 CFR 63.113(d). According to the U.S. Nuclear Regulatory Commission rule at 10 CFR 63.322, the human intrusion scenario assumes that a drill penetrates the repository and a waste package during exploratory drilling for groundwater resources. The U.S. Nuclear Regulatory Commission rule requires a determination of the earliest time after disposal that the waste package would degrade sufficiently that a human intrusion could occur without recognition by the drillers (10 CFR 63.321).

hydraulic conductivity: A measure of the ability to transmit water through a permeable medium. A number that describes the rate at which water can move through a permeable medium. The hydraulic conductivity depends on the size and arrangement of water-transmitting openings such as pores and fractures, the dynamic characteristics of the water such as density and viscosity, and the driving force.

hydraulic gradient: The difference in the height of water levels with respect to the distance between two locations.

hydraulic head: The pressure in the liquid expressed as equivalent height (head) of the water column.

hydrogen-induced cracking: Occurs when atomic hydrogen generated at the surface of a metal migrates into the metal and forms hydride phases with the metal components, causing the metal to be more brittle. Hydride phases cause the metal to be more susceptible to cracking and, thus, to localized corrosion. Also hydride cracking.

hydrogeologic nomenclature: A stratigraphic nomenclature system used for the classification of rock at Yucca Mountain based on the hydrogeologic properties that govern the rock's capacity to hold, transmit, and deliver water. Compare lithostratigraphic nomenclature and thermomechanical nomenclature.

hydrologic properties: Those properties of a rock that govern the capacity to hold, transmit, and deliver water, such as porosity, effective porosity, specific retention, permeability, and the directions of maximum and minimum permeabilities.

igneous: (1) A type of rock that has formed from molten material. (2) A type of activity related to the formation and movement of molten rock either in subsurface (plutonic) or on the surface (volcanic).

INFIL: A numerical software code used to analyze infiltration of precipitation.

in situ: In its natural position or place. The phrase distinguishes in-place experiments, conducted in the field or underground facility.

infiltration: The process of water entering the soil at the ground surface and the ensuing movement downward. Infiltration becomes percolation when water has moved below the depth at which it can be removed (to return to the atmosphere) by evaporation or evapotranspiration.

initiating event: A natural or human-induced event that causes an event sequence.

intrablock faults: Faults (i.e., rock fractures having experienced movement along their plane) found within a block of rock; in this case, within the repository block, which is located within the Topopah Spring Tuff formation.

intrusive event: An igneous event that does not disrupt the surface. In repository performance analyses, molten material is assumed to intersect waste packages in the repository and cause a release of radionuclides. Compare extrusive event.

invert: (1) The floor of a drift. (2) The structure constructed in a drift to provide the floor of that drift. In an emplacement drift, ballast in the invert serves as a barrier to migration of radionuclides that might escape from breached waste packages.

ionic strength: A measure of the level of electrical force in an electrolytic solution.

ionizing radiation: Any radiation capable of displacing electrons from an atom or molecule, thereby producing ions.

irreversible colloid: A pseudocolloid with permanently attached radionuclides.

isotope: (1) Atoms of a chemical element with the same atomic number. (2) One of two or more atomic nuclei with the same number of protons (i.e., the same atomic number) but with a different number of neutrons (i.e., a different atomic weight). For example, uranium-235 and uranium-238 are both isotopes of uranium.

isotropy: The condition wherein all significant physical properties are equal when measured in any direction or along any axes. See anisotropy.

J-13 water: Groundwater taken from Well J-13 or water made in a laboratory that has the same chemical composition. The chemical composition of this water is used as the standard for Yucca Mountain ambient groundwater composition for modeling and testing purposes.

joint: A fracture in rock, usually more or less vertical to bedding, along which no appreciable movement has occurred.

joint set: In a rock mass, a group of parallel joints (fractures without displacement).

Large Block Test: A prototype test of thermal-mechanical processes at Yucca Mountain in the middle nonlithophysal zone of the Topopah Spring unit. The test was to develop testing approaches for thermal-hydrologic and other coupled processes.

Latin hypercube sampling: A sampling technique that divides the cumulative distribution function into intervals of equal probability and then samples from each interval.

license application: An application to the U.S. Nuclear Regulatory Commission to construct a geologic repository operations area for the disposal of spent nuclear fuel and high-level radioactive waste. The application would be considered by the U.S. Nuclear Regulatory Commission in any decision whether to grant the U.S. Department of Energy authorization to begin constructing a geologic repository operations area.

line loading: Placing the waste packages at very close distances end-to-end to achieve a more uniform thermal profile along the length of the emplacement drifts. See thermal loading.

lithologic: Pertaining to rock features such as color, texture, mineral content, and weathering characteristics.

lithophysae: Small, bubble-like holes in the rock caused by volcanic gases trapped in the rock matrix as the ash-flow tuff cooled.

lithophysal: Pertaining to tuff units with lithophysae, voids having concentric shells of finely crystalline alkali feldspar, quartz, and other materials that were formed by entrapped gas that later escaped.

lithostratigraphic nomenclature: A stratigraphic nomenclature system used for the classification of rock at Yucca Mountain based on primary geologic processes (e.g., the depositional character and assemblage of the rock) and secondary geologic processes (e.g., the degree of welding, devitrification, and vapor-phase crystallization). Compare hydrogeologic nomenclature and thermo-mechanical nomenclature.

localized corrosion: A type of corrosion induced by local variations in electrochemical potential on a microscale over small regions. Variations in electrochemical potential may be caused by localized irregularities in the structure and composition of usually protective passive films on metal surfaces and in the electrolyte composition of the solution that contacts the metal.

log normal distribution: A distribution of a random variable x such that the natural logarithm of x is normally distributed.

logic tree methodology: An analytical approach that involves sequencing the studies in an analysis and then addressing certain attributes in each study.

longitudinal dispersion: Dispersion of a solute moving in groundwater in the same direction as the groundwater flow path.

lookup table: A multidimensional table containing columns of data representing relationships between parameters in the table. A lookup table is a convenient way to represent and implement functional relationships between parameters considered in the model.

low-level radioactive waste: Radioactive waste producing small quantities of ionizing radiation and that is not classified as high-level radioactive waste, transuranic waste, or byproduct tailings containing uranium or thorium from processed ore. Usually generated by hospitals, research laboratories, and certain industries.

main drift: (1) One of four main access tunnels in the potential repository design. Also access main. (2) The main north–south trending drift segment in the current Exploratory Studies Facility.

mass spectrometry: A technique for identifying chemical structures in a material. The technique involves sending a beam of ions through a combination of electric and magnetic fields, which deflects ions according to their masses, indicating the atoms and molecules.

matrix: Rock mass between explicitly considered fractures.

matrix diffusion: The process by which molecular or ionic solutes, such as radionuclides in groundwater, move from areas of higher concentration to areas of lower concentration. This movement is through the pore spaces of the rock material as opposed to movement through the fractures.

matrix permeability: The capacity of the matrix to transmit fluid.

matrix porosity: In the solid, but porous, portion of rock, the ratio of openings, or voids, to the total volume of the matrix expressed as a decimal fraction or as a percentage.

mean: For a statistical data set, the sum of the values divided by the number of items in the set. The arithmetic average.

median: A value such that half of the observations are less than that value and half are greater than the value.

metric ton heavy metal (MTHM): A metric ton is a unit of mass equal to 1,000 kg (2,205 lb). Heavy metals are those with atomic masses greater than 230. Examples include thorium, uranium, plutonium, and neptunium. The term usually pertains to heavy metals in spent nuclear fuel and high-level radioactive waste. In this report, MTHM is equal to MTU (metric tons of uranium).

microbially influenced corrosion: Corrosion of the waste package that is induced by the activity of microbes.

microsphere: Spheres of carboxylate-modified latex of varied diameters between 280 to 640 nm (0.000011 to 0.000025 in.), used as colloid tracers in the C-Wells testing.

millirem (mrem): A millirem is one one-thousandth of a rem, which is the unit of equivalent dose. Equivalent dose is a measure of the effect that radiation has on humans. The equivalent dose takes into account the type of radiation and the absorbed dose. See rem.

Miocene Epoch: The fourth of the five geologic epochs of the Tertiary Period, extending from the end of the Oligocene Epoch to the beginning of the Pliocene Epoch.

mitigation: (1) Avoiding an impact by not taking a certain action or parts of an action. (2) Minimizing impacts by limiting the degree or magnitude of the action and its implementation. (3) Rectifying an impact by repairing, rehabilitating, or restoring the affected environment. (4) Reducing or eliminating an impact over time by preservation and maintenance operations during the life of the action. (5) Compensating for an impact by replacing or providing substitute resources or environments.

mixed waste: Waste containing both radioactive hazardous substances and nonradioactive hazardous substances, regardless of whether these types of substances are combined chemically or mixed together.

model: (1) A conceptual description and the associated mathematical representation of a system, subsystem, component, or condition that is used to predict changes from a baseline state as a function of internal and/or external stimuli and as a function of time and space. (2) A depiction of a system, phenomenon, or process including any hypotheses required to describe the system or explain the phenomenon or process.

model validation: A process used to establish confidence that a conceptual model represented in a mathematical model by software or by other analytical means adequately represents the phenomenon, process, or system under consideration.

moderator: Material that contains nuclei that cause energetic neutrons to slow down. In general, the lighter the element, the better it works as a moderator. Hydrogen, the lightest element, is a very efficient moderator; since water contains hydrogen nuclei, it, too, is a very effective moderator.

modern climate: One of the climate states of the future climate model at Yucca Mountain; others include monsoon and glacial transition. Consists of two active components, the tropical and polar air masses, and a more passive component, the westerlies.

molecular diffusion: The transfer of mass in a fluid by random molecular motion.

monitored geologic repository: A system, requiring licensing by U.S. Nuclear Regulatory Commission, intended or used for the permanent underground disposal of spent nuclear fuel and high-level radioactive waste. A geologic repository includes (a) the geologic repository operations area and (b) the geologic setting within the controlled area that provides isolation of the radioactive waste.

monsoon climate: One of the climate states of the future climate model at Yucca Mountain; others include modern and glacial transition.

Monte Carlo simulation: An analytical method that uses random sampling of parameter values available for input into numerical models as a means of approximating the uncertainty in the process being modeled. A Monte Carlo simulation comprises many individual runs of the complete calculation using different values for the parameters of interest as sampled from a probability distribution. A different final outcome for each individual calculation and each individual run of the calculation is called a realization. Each realization is equally likely to occur in the Monte Carlo process.

mountain scale: (1) Similar to far-field for processes that are related to the area of the geosphere and biosphere far enough away from the repository that, when numerically modeled, show that releases from the repository are represented as a homogeneous, single source term. The effects of individual, small-scale components such as individual waste packages are not modeled because they are considerably smaller than the scale of the model. (2) A scale of hundreds of meters, or even kilometers, as opposed to tens of meters.

MULTIFLO: A computer software code used by the U.S. Nuclear Regulatory Commission to simulate the flow of groundwater and heat in unsaturated porous and fractured media. The code MULTIFLO is similar to TOUGH2.

natural barriers: The physical components of the geologic environment that individually and collectively act to limit the movement of water or radionuclides.

near field: The area and conditions within the repository including the drifts and waste packages and the rock immediately surrounding the drifts. The region around the repository where the natural hydrogeologic system has been significantly impacted by the excavation of the repository and the emplacement of waste.

net infiltration: The water that has infiltrated down from the soil zone or exposed rock surface to a depth below which it cannot be removed by evapotranspiration. Net infiltration is the total infiltration at the surface minus water lost to evaporation and plant transpiration.

neutron absorber: A material (such as boron or gadolinium) that absorbs neutrons. Used in nuclear reactors, transportation casks, and waste packages to control neutron activity.

neutron logging: The analysis of the water content of soil and rocks in a borehole by means of neutron bombardment and the measurement of the reflected radiation.

nominal scenario or nominal case: The performance assessment case, or conceptual model, representing the expected conditions of the disposal system as perturbed only by the presence of the repository, in the absence of disruptive events.

NUFT: A nonisothermal unsaturated zone flow and transport software code used for simulation of three-dimensional flow of groundwater, heat, and contaminant transport. It is used for drift-scale thermal-hydrologic calculations.

numerical model: An approximate representation of a mathematical model that is constructed using a numerical description method, such as finite volumes, finite differences, or finite elements. A numerical model is typically represented by a series of program statements that are executed on a computer.

occupational dose: The radiation dose received by an individual in the course of employment in which the individual's assigned duties involve exposure to radiation or to radioactive material. Occupational dose does not include, for example, dose received from background radiation, from any medical administration the individual has received, from voluntary participation in medical research programs, or as a member of the public.

one-off sensitivity analysis: A method used to examine the effects of each component model or parameter on overall system performance. These analyses are conducted by fixing one important parameter of a particular component model at either its expected value, the median (or 50^th percentile value), or at a specified extreme value (the 5^th or 95^th percentile value). These analyses are used to display the effect of the change on other measures of system or subsystem performance, as well as the effect on the variance of the projected dose history. By fixing a particular parameter that significantly affected the spread of the overall system performance results, it was possible to directly examine the significance of that parameter on the performance outcome.

order of magnitude: A range of numbers extending from some value to 10 times that value.

oxidation: (1) A chemical reaction, such as the rusting of iron, that increases the oxygen content of a substance. (2) A reaction in which the valence of an element or compound is increased as a result of losing electrons.

paleoclimate: The climate of a past interval of geologic time.

Paleozoic: (1) A geologic era extending from the end of the Precambrian to the beginning of the Mesozoic, dating from about 600 to 230 million years ago. (2) The rock strata formed during this era.

patch: For corrosion modeling, one of two geometries for an opening in a waste package layer created by corrosion (the other geometry is a pit). A patch is generally wider than it is deep.

perched water: Groundwater of limited lateral extent separated from an underlying body of groundwater by an unsaturated zone.

percolating water: Water passing through a porous substance. In rock or soil it is the movement of water through the interstices and pores under hydrostatic pressure and the influence of gravity. The downward or lateral flow of water that becomes net infiltration in the unsaturated zone.

percolation: The downward or lateral flow of water that becomes net infiltration in the unsaturated zone.

percolation flux: (1) Volumetric percolation rate per unit area. The flux anywhere below the root zone of plants and is no longer susceptible to removal back into the atmosphere by evapotranspiration. (2) Volume of water moving downward or laterally through the unsaturated zone in a given period.

performance assessment: An analysis that forecasts the behavior of a system or system component under a given set of constant and/or transient conditions. Performance assessment includes estimates of the effects of uncertainties in data and modeling. See total system performance assessment.

permeability: In general terms, the capacity of a medium (like rock, sediment, or soil) to transmit liquid or gas.

phase stability: A measure of the ability of matter to remain in a given phase.

picocurie: One trillionth of a curie (3.7 × 10^-2 disintegrations per second or 2.22 disintegrations per minute), abbreviated pCi. See curie.

pit: For corrosion modeling, one of two geometries for an opening in a waste package layer created by corrosion (the other geometry is a patch). A pit is generally deeper than it is wide.

playa: A nearly level area at the bottom of an undrained desert basin, sometimes temporarily covered with water.

Pliocene: The last of the five geologic epochs of the Tertiary Period, extending from the end of the Miocene to the beginning of the Pleistocene, and the rocks formed during that time.

plume: A measurable discharge of a contaminant, such as radionuclides, from a point of origin. The contaminants are usually moving in groundwater, and the plume may be defined by concentration gradients.

plume of contamination: That volume of groundwater in the predominant direction of groundwater flow that contains radioactive contamination from releases from the Yucca Mountain repository. It does not include releases from any other potential sources on or near the Nevada Test Site.

pore water: The water and any material it is carrying that exist in the pore spaces of the rock matrix. Also pore fluid.

porosity: The ratio of openings, or voids, to the total volume of soil or rock, expressed as a decimal fraction or as a percentage.

postclosure: The period of time after closure of the geologic repository.

potentiometric: Pertaining to the distribution of groundwater level.

precipitate: A solid particle that has separated from a liquid as a result of physical or chemical changes.

precipitation: (1) The process of substance coming out of solution by the action of gravity or by a chemical reaction. (2) Any form of water particles, such as frozen water in snow or ice crystals, or liquid water in raindrops or drizzle, that falls from clouds in the atmosphere and reaches the earth's surface. (3) An amount of water that has fallen at a given point over a specified period of time, measured by a rain gauge.

preclosure: The period of time before and during closure of the Yucca Mountain disposal system.

preclosure safety evaluation: A preliminary assessment of the adequacy of repository support facilities to prevent or mitigate the effects of postulated initiating event sequences and their consequences and the site, structures, systems, components, equipment, and operator actions that would be relied on for safety.

pressurized water reactor: A type of nuclear power reactor that uses uranium fuel elements cooled and moderated by water under high pressure to keep the water from boiling. The water boiled to generate steam is in an external heat exchanger rather than in the reactor vessel body.

probabilistic analyses: Analyses in which uncertainty in processes and events is represented through probability distributions for the parameters of those processes and events.

probabilistic risk assessment: (1) A systematic process of identifying and quantifying the consequences of scenarios that could cause a release of radioactive materials to the environment. (2) Using predictable behavior to define the performance of natural, geologic, human, and engineered systems for thousands of years into the future using probability distributions.

process model: A depiction or representation of a process along with any hypotheses required to describe or to explain the process.

pseudocolloid: A colloid from natural or man-made materials, as distinguished from a colloid from insoluble radioactive materials (intrinsic colloid) or from altered fragments of spent nuclear fuel or glass waste forms (waste-form colloid). See colloid.

pyroclastic: Of or relating to clastic rock material of any size that is formed by volcanic explosion or ejected from a volcanic vent.

Quaternary: The second period of the Cenozoic Era, beginning about 2 million years ago at the end of the Tertiary Period and extending to the present.

radioactive decay: The process in which one radionuclide spontaneously transforms into one or more different radionuclides called decay products or daughter products.

radioactive waste: High-level radioactive waste and other radioactive materials, including spent nuclear fuel, that are received for emplacement in the geologic repository.

radioactivity: The property possessed by some elements (e.g., uranium) of spontaneously emitting alpha, beta, or gamma rays by the disintegration of atomic nuclei.

Radiologically Controlled Area: An area of the surface repository enclosed by security fences, control gates, lighting, and access detection systems. This area includes the facilities and transportation systems required to receive and ship rail and truck waste shipments, prepare shipping casks for handling, and load waste forms into disposal containers for underground emplacement. It also includes the facility and systems required to treat and package site-generated, low-level radioactive waste for offsite disposal.

radiolysis: The chemical dissociation of molecules caused by exposure to radiation. For example, under certain circumstances, radiation can cause the hydrogen and oxygen molecules in water to separate.

radionuclide: A radioactive atom with an unstable nucleus that spontaneously decays, emitting ionizing radiation in the process.

raise: From mining terminology, an upward opening, either vertical or inclined, driven in rock from one level to that above it.

reasonably maximally exposed individual: Under the U.S. Nuclear Regulatory Commission rule, a hypothetical person who meets the following criteria: (1) lives in the accessible environment above the highest concentration of radionuclides in the plume of contamination; (2) has a diet and living style representative of the people who now reside in the Town of Amargosa Valley, Nevada (DOE must use projections based upon surveys of the people residing in the Town of Amargosa Valley, Nevada, to determine their current diets and living styles and use the mean values of these factors in the assessments conducted for 10 CFR 63.311 and 63.321); (3) uses well water with average concentrations of radionuclides based on an annual water demand of 3,000 acre-ft; (4) drinks 2 L (0.53 gal) of water per day from wells drilled into the groundwater at the location specified in the regulations; and (5) is an adult with metabolic and physiological considerations consistent with present knowledge of adults.

recharge: The movement of water from an unsaturated zone to the saturated zone.

reference biosphere: The description of the environment inhabited by the reasonably maximally exposed individual. The reference biosphere comprises the set of specific biotic and abiotic characteristics of the environment, including but not necessarily limited to climate, topography, soils, flora, fauna, and human activities.

rem (roentgen equivalent man): The unit of a dose equivalent from ionizing radiation to the human body. It is used to measure the amount of radiation to which a person has been exposed.

repository block: The portion of rock in Yucca Mountain that would house the geologic repository if the site is found suitable.

repository footprint: The outline of the outermost locations of where the waste is emplaced in the Yucca Mountain geologic repository.

retardation: Slowing of radionuclide movement in groundwater by mechanisms that include sorption of radionuclides, diffusion into rock matrix pores and microfractures, and trapping of large colloidal molecules or particles in small pore spaces or dead ends of microfractures.

reversible colloid: A pseudocolloid to which radionuclides are reversibly bound.

revised supplemental TSPA model: The model used in supplemental calculations of total system performance assessment as documented in Total System Performance Assessment—Analyses for Disposal of Commercial and DOE Waste Inventories at Yucca Mountain—Input to Final Environmental Impact Statement and Site Suitability Evaluation. The model is a modification of the supplemental TSPA model that conforms to the requirements of the final U.S. Environmental Protection Agency rule at 40 CFR Part 197. This model was also used in sensitivity analyses in Total System Performance Assessment Sensitivity Analyses for Final Nuclear Regulatory Commission Regulations to address provisions of the final Nuclear Regulatory Commission rule at 10 CFR Part 63.

rhyolite: A volcanic rock type with a chemical composition similar to granite.

risk: The probability that an undesirable event will occur multiplied by the consequences of the undesirable event.

risk assessment: An evaluation of risks associated with a potential system or action. This assessment focuses on potential impacts on human health or the environment.

runoff: Water from rain and snow that flows over land to streams.

run-on: The volume or depth of the routed surface-water flow.

safety case: The set of data and analyses that, collectively, are intended to provide the reasonable assurance that a successful license application would require.

saturated zone: The region below the water table where rock pores and fractures are completely saturated with water.

scenario: A well-defined, connected sequence of features, events, and processes that can be thought of as an outline of a possible future condition of the repository system. Scenarios can be undisturbed, reflecting performance under the expected, or nominal, behavior for the system. The scenario can also be disturbed if altered by disruptive events such as human intrusion or natural phenomena such as volcanism.

seepage: (1) The inflow of groundwater moving in fractures or pore spaces of permeable rock to an open space in the rock; the amount of percolation flux that enters the drift in a given time period. (2) Flow of liquid water into an underground opening such as a waste emplacement drift or exploratory tunnel. Does not include water vapor movement into openings or condensation of water vapor within openings.

seepage threshold: A critical percolation flux below which seepage into the openings is unlikely to occur.

seismicity: A seismic event or activity such as an earthquake or vibratory motion.

semiarid: Of a climate: having precipitation only sufficient for growth of sparse vegetation; a region in which the annual precipitation is about 250 to 500 mm (10 to 20 in.).

sensitivity study: An analytic or numerical technique for examining the effects of varying specified parameters in a computer model. Shows the effects that changes in various parameters have on model outcomes and illustrates which parameters have a greater impact on the predicted behavior of the system being modeled. Also called sensitivity analysis because it shows the sensitivity of the consequences (e.g., radionuclide release) to uncertain parameters (e.g., the infiltration rate that results from precipitation).

site characterization: Activities, whether in the laboratory or in the field, undertaken to establish the geologic and hydrologic conditions and the ranges of the parameters of a candidate site relevant to the location of a repository. These activities include borings, surface excavations, subsurface excavations and borings, and in situ testing needed to evaluate the suitability of a candidate site for the location of a repository but do not include preliminary borings and geophysical testing needed to assess whether site characterization should be undertaken.

sorption: The binding, on a microscopic scale, of one substance to another. A term that includes both adsorption and absorption. The sorption of dissolved radionuclides onto aquifer solids or waste package materials by means of close-range chemical or physical forces is an important process modeled in this study. Sorption is a function of the chemistry of the radioisotopes, the fluid in which they are carried, and the mineral material they encounter along the flow path.

sorption coefficient (K_d): A factor to calculate sorption of one substance to another (e.g., sorption of a radionuclide to a colloid or sorption of a radionuclide to the rock).

source term: Types and amounts of radionuclides that are the source of a potential release of radioactivity from the repository.

spent nuclear fuel: Fuel and the associated hardware withdrawn from a nuclear reactor following irradiation, the constituent elements of which have not been separated by reprocessing. Spent fuel that has been burned (irradiated) in a reactor to the extent that it no longer makes an efficient contribution to a nuclear chain reaction. This fuel is more radioactive than it was before irradiation, and it is thermally hot.

splay: A branch of a fault or fault zone.

split: The two halves of an emplacement, performance confirmation, or reserve drift, separated by an exhaust raise.

staging area: An area in the Waste Handling Building or a part of the waste-handling process in which spent nuclear fuel or high-level radioactive waste is retained for future loading in a disposal container.

standard deviation: (1) For a set of observations or a frequency distribution, the square root of the average of the squared deviations from the mean divided by n-1 (where n is the sample size). (2) The square root of the variance.

steady-state modeling: Modeling a system under the assumption that the variables are not changing with time. For example, flow fields can be simulated at a steady state if the boundary conditions, saturations, and fluxes are not changing with time.

stochastic: Involving a variable (e.g., temperature, porosity) that may take on values of a specified set with a certain probability. Data from a stochastic process is an ordered set of observations, each of which is one item from a probability distribution.

stress corrosion cracking: Preferential corrosion initiation in response to unrelieved tensile stresses in an aggressive corrosion environment.

stress intensity: The amount of stress at a given point in a structure. Derived from combined totals of both positive (tension) stress and negative (compression) stress.

subsurface facilities: The repository's underground structures and systems. The subsurface facilities include the main drifts, exhaust mains, turnouts, emplacement drifts, ventilation shafts, mechanical and structural support systems, underground utilities, waste emplacement and retrieval equipment, and surface-based control systems.

supplemental TSPA model: The model used in supplemental calculations of total system performance assessment as documented in Volume 2 of FY01 Supplemental Science and Performance Analyses. The model is a modification of the TSPA-SR model that incorporates new component models and input parameter values for some components. The model was based on specifications of proposed U.S. Environmental Protection Agency and U.S. Nuclear Regulatory Commission regulations. See total system performance assessment. Compare TSPA-SR model and revised supplemental TSPA model.

surface facilities: All permanent facilities within the restricted area constructed in support of site characterization activities and repository construction, operation, and closure activities, including surface structures, utility lines, roads, railroads, and similar facilities, but excluding the underground facility.

system model: The analytical tool to examine the future behavior of the potential repository and its component barriers.

system performance: The complete behavior of a geologic repository system at Yucca Mountain in response to the features, events, and processes that may affect it.

tectonic: Pertaining to geologic forms or effects created by deformation of the earth's crust.

Tertiary: The first of two geologic periods of the Cenozoic Era extending from the end of the Mesozoic Era to the beginning of the Quarternary Period, covering a time span from about 65 million to about 2 million years ago.

thermal loading: (1) The areal density at which waste packages are emplaced within the repository as characterized by the areal power density and the areal mass loading. See line loading and point loading. (2) The application of heat to a system, usually measured in terms of watt density. The thermal loading for a repository is the result of heat produced by the radioactive waste in the active disposal area.

thermal stress: Stress caused by temperature changes in a material that is physically restricted and unable to expand or contract accordingly.

thermal-mechanical effects: Changes in the geomechanical properties of the repository host rock produced by heating of the rock associated with the emplacement of radioactive waste in the repository. An example might be decreased rock strength related to increased fracturing caused by heating of the rock.

thermomechanical nomenclature: A stratigraphic nomenclature system used for the classification of rock at Yucca Mountain based on the thermal and mechanical properties of the rock. Compare hydrogeologic nomenclature and lithostratigraphic nomenclature.

three-dimensional model: A three-dimensional representation of physical conditions and/or processes.

total effective dose equivalent: For purposes of assessing doses to workers, the sum of the deep-dose equivalent (for external exposures) and the committed effective dose equivalent (for internal exposures). For purposes of assessing doses to members of the public (including the reasonably maximally exposed individual), total effective dose equivalent means the sum of the effective dose equivalent (for external exposures) and the committed effective dose equivalent (for internal exposures). See annual committed effective dose equivalent.

total system performance assessment (TSPA): A risk assessment that quantitatively estimates how the proposed Yucca Mountain disposal system will perform in the future under the influence of specific features, events, and processes, incorporating uncertainty in the models and data.

TOUGH2: A computer software code used to simulate three-dimensional flow of groundwater and heat in unsaturated or saturated porous and fractured media. It is the basis for the unsaturated zone flow process model.

TOUGHREACT: Thermal-hydrologic-chemical software code used to simulate the water composition on the drip shield and waste package.

tracer: A substance or dye used in hydrologic tests to observe the movement of groundwater and sorbing and nonsorbing chemical species. Also applicable to gas injection tests using gaseous tracers to measure the breakthoughs at observation points.

transmissive fracture: A fracture in rock through which groundwater could flow.

transpiration: The process by which water absorbed by plants, usually through the roots, is evaporated into the atmosphere from the plants' surfaces. It is an important process for removal of water that has infiltrated below the zone where it could be removed by evaporation.

transport: A process in which substances carried in groundwater move through the subsurface by means of the physical mechanisms of convection, diffusion, and dispersion and the chemical mechanisms of sorption, leaching, precipitation, dissolution, and complexation. Types of transport include advective, diffusive, and colloidal transport.

transportation cask: A heavily shielded container that meets applicable regulatory requirements used to ship spent nuclear fuel or high-level radioactive waste.

transverse dispersion: The spreading of a solute in groundwater in directions perpendicular to the direction of the groundwater flow path.

TSPA-SR model: The model used in the calculations of total system performance assessment as documented in Total System Performance Assessment for the Site Recommendation. The model was based on specifications of proposed U.S. Environmental Protection Agency and U.S. Nuclear Regulatory Commission regulations. See total system performance assessment. Compare supplemental TSPA model and revised supplemental TSPA model.

tuff: Igneous rock formed from compacted volcanic fragments from pyroclastic (explosively ejected) flows with particles generally smaller than 4 mm (0.16 in.) in diameter. The most abundant type of rock at the Yucca Mountain site. Nonwelded tuff results when volcanic ash cools in the air sufficiently that it does not melt together, yet later becomes lithified through compaction. Welded tuff results when the volcanic ash is hot enough to melt together and is further compressed by the weight of overlying materials.

two-dimensional model: (1) A two-dimensional slice through an entity, such as the earth's crust, usually in the horizontal and vertical directions, on which known features are placed and are used to predict likely features that may exist between points of known data. (2) Mathematically, a model that represents physical conditions or processes; this mathematical model is composed of both horizontal rows and vertical columns of grid cells arrayed in L-shaped configurations only one grid cell thick.

uncertainty: A measure of how much a calculated or estimated value may vary from the unknown true value.

underground facility: The underground structure, backfill materials, if any; and openings that penetrate the underground structure.

unsaturated zone: The zone of soil or rock below the ground surface and above the water table.

unzipping: The splitting of the cladding on a fuel rod.

uptake: Intake by and exposure of the receptor to a contaminant.

van Genuchten's capillary-strength parameter: A parameter in a functional relationship between saturation (or water content) and potential (a measure of the suction due to capillary forces). Also α-parameter.

variability (statistical): A measure of how a quantity varies over time or space.

vitric tuff: Volcanic rock composed of glassy shards of volcanic ash.

vitrified high-level radioactive waste: A type of processed high-level radioactive waste where the waste is mixed with glass-forming chemicals and put through a melting process. The melted mixture is then put into a canister where it becomes a dry, solid "log" of waste in a glassy matrix.

WAPDEG: A computer software code used to analyze drip shield and waste package degradation.

wash: Term used in the southwest for a broad, gravelly, dry bed of an ephemeral stream, generally in the bottom of a canyon.

waste form: A generic term that refers to the different types of radioactive wastes.

Waste Handling Building: In the North Portal Area, a structure designed to support waste handling operations and the loading and staging of waste packages.

waste package: A sealed container containing waste that is ready for emplacement. The waste package includes the waste form and any containers, spacing structures or baskets, and other absorbent materials immediately surrounding an individual waste container placed internally to the container or attached to the outer surface of the disposal container.

waste package remediation system: A repair facility for disposal containers and waste packages that have failed the weld inspection processes, that are defective or abnormal, or that have been selected for retrieval from the repository for performance confirmation examinations.

waste stream: Input of waste into the repository over time.

water table: (1) The upper limit of the portion of the ground wholly saturated with water. (2) The upper surface of a zone of saturation above which the majority of pore spaces and fractures are less than 100 percent saturated with water most of the time (unsaturated zone) and below which the opposite is true (saturated zone).

welded tuff: A tuff that was deposited under conditions where the particles making up the rock were heated sufficiently to cohere. In contrast to nonwelded tuff, welded tuff is considered to be denser, less porous, and more likely to be fractured (which increases permeability).

Yucca Mountain disposal system: A combination of underground engineered and natural barriers within the controlled area that prevents or substantially reduces releases from the waste.

zeolites: A large group of hydrous aluminosilicate minerals that act as molecular "traps" because they can adsorb molecules with which they interact. At Yucca Mountain, they are secondary alteration products in tuff rocks, caused by exposure to groundwater. Zeolites could act to retard the migration of radionuclides.

Zircaloy: A family of alloys of zirconium that may have any of several compositions. These alloys are frequently used as a cladding material.

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