The Waste Isolation Pilot Plant: A Potential Solution for the Disposal of Transuranic Waste (1996)

absorbed dose

—energy imparted to matter in a volume element by ionizing radiation, divided by the mass of irradiated material in that volume element. The SI derived unit of absorbed dose is the gray (Gy); 1 Gy = 100 rad = 1 J per kilogram.

accessible environment

—region outside a rectangular block of rock 5 km long on each side placed symmetrically around the Waste Isolation Pilot Plant repository, with vertical sides extending upward to the land surface and indefinitely downward into the earth.

actinide

—element with atomic number 90 (thorium) or greater.

actinide source term

—cumulative amounts, concentrations in solution, and chemical nature of all radioactive materials to be disposed of at WIPP, which could be moved in solution or suspension (or retarded by sorption on surrounding rock units) from the WIPP repository into the environment.

advection

—solute-transport process in which chemical species are carried through an aquifer by the flowing ground-water in which they are dissolved.

anhydrite

—anhydrous calcium sulfate.

aquifer

—formation, group of formations, or part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to wells and springs.

Becquerel

—radiation unit equal to one disintegration per second.

Castile Formation

—oldest of the Ochoan sequence of rocks, consisting of alternating layers of anhydrite and thin layers of limestone, with several thick layers of halite. At depth, highly pressurized pockets of brine have been found in this formation.

clastic

—term describing rocks or sediments made up of fragments of other rocks. For example, sandstone and shale are clastic rocks.

colloid

—particles of size ranging from 10^-3 to 1 micron (10^-9 to 10^-6 m), finer than clay size, and held in suspension in a medium such as water. Because of their fine size, colloids have very large surface areas.

committed effective dose

—a measure of human exposure to radioactivity. An ingestion of, inhalation of, or direct contact with radionuclides deposits energy from radioactive decays into bodily tissues. This absorbed dose is multiplied by a weighting factor dependent on the type of radiation to calculate an equivalent dose. The weighted sum of equivalent doses to all bodily tissues and summed over the time of exposure is the committed effective dose, measured in rem. The SI unit, the Sievert (Sv), is equivalent to 100 rem.

compartmentation

—filling of tunnels and drifts around rooms and panels, so that the waste is isolated in compartments.

complex

—stable association of a metal ion with one or more bulky groups (ligands), resulting in a profound change in the properties of the ion.

corrensite

—clay mineral with a structure that represents interstratification of vermiculite and chlorite structures.

creep

—slow movement over time of salt as shear stresses cause movement within or between individual crystals.

Culebra Dolomite

—second-oldest member of the Rustler Formation ranging from approximately 7-8 m thick at the WIPP site. The Culebra consists of dolomite with some clay minerals. Because it is a water-bearing unit, the Culebra is important to the ground-water flow model for the WIPP site.

curie

—measure of the quantity of radioactive material in a sample, equal to 3.7 × 10¹⁰ disintegrations per second.

Darcy's law

—empirical hydraulic principle stating that the flow rate of a fluid through a porous material is proportional to the hydraulic gradient and the hydraulic conductivity of the material.

Darcy velocity

—volumetric flow rate per unit surface area of sample, on the assumption that flow is laminar and that inertia can be neglected. This is more properly called the ''specific discharge."

Delaware Basin

—sedimentary basin in which the WIPP site is located. The Delaware Basin formed in a Permian sea and was gradually filled with thick, extensive layers of sediments and evaporite deposits.

Dewey Lake Red Beds

—youngest of the Ochoan series of rocks, consisting of thin beds of clay, silt, and sandy sediments, red in color.

diffusion length

—distance over which chemical species diffuse, or spread out and thoroughly mix, over a period of time, approximated as the square root of the product of diffusivity and time.

distribution coefficient

—ratio of the amount of a substance sorbed to the amount in solution when chemical equilibrium exists (also see isotherm); ratio of the concentrations of a substance in two phases at equilibrium.

disturbed repository performance

—radionuclide releases from the repository as a result of reasonably foreseeable natural processes as well as inadvertent human intrusion.

disturbed rock zone (DRZ)

—localized region of permeability consisting of microfractures in the salt. The DRZ is generated by stress concentrations at a new surface and spreads at a slow and decreasing rate from that surface.

dolomite

—a sedimentary rock consisting mostly of the mineral dolomite, calcium magnesium carbonate.

effective porosity

—fraction of interconnected void space per unit volume of porous material. It is less than the total porosity because it includes only pores that comprise active flow paths through the medium and excludes voids in isolated or dead-end pores.

Eh

—measure of the oxidation-reduction state of a solution.

electrical resistivity

—characteristic of a solution depending on the concentration of ions in the solution and measured as the resistance of a centimeter cube of a substance to the passage of a current perpendicular to two parallel faces of the cube.

EPA unit

—isotopic-specific unit of measure, used by Sandia National Laboratories, that is equal to the number of curies represented by the release limit of the EPA requirements in 40 CFR part 191.

evaporite

—natural deposit of water-soluble salts formed by the evaporation of a body of water.

flocculation

—aggregation or clumping together of particles suspended in solution so that they precipitate.

flow path

—pathway a particle of ground-water would travel, starting from a specified point.

halite

—common rock salt, with chemical formula NaCl.

half-life

—time required for 50 percent of any amount of a radioactive element to decay.

high-level waste

—radioactive waste resulting from the reprocessing of spent fuel rods from nuclear reactors, or of other radioactive materials used for defense purposes.

human intrusion

—inadvertent creation of a radiation pathway by human actions.

hydraulic conductivity

—capacity for a porous medium to conduct water; defined formally as the volume of water at the existing kinematic viscosity that will move in unit time under a unit hydraulic gradient through a unit area measured at right angles to the direction of flow through a porous material. This coefficient incorporates the properties of both the porous medium and the liquid.

hydraulic diffusivity

—K/S_s, where K is hydraulic conductivity and S_s is specific storage. It represents the hydraulic conductivity (K) of a saturated porous medium when the unit volume of water diffusing is that involved in changing the head of a unit amount in a unit volume of medium. The time involved for a given head change to occur at a particular point in response to a greater change in head at another point is inversely proportional to the hydraulic diffusivity.

hydraulic gradient

—change in static (or hydraulic) head per unit of distance in a given direction. If not specified, the direction generally is understood to be that of the maximum rate of decrease in head.

hydraulic head

—height above a standard datum of the surface of a column of water that can be supported by the static pressure at a given point. The hydraulic head is the sum of the elevation head and the pressure head. Under conditions to which Darcy's law may be applied, the velocity of ground-water flow is so low that the velocity head is negligible. Stated more simply, hydraulic head is the height of free surface of water above a given subsurface point.

hydrostatic pressure

—pressure exerted in all directions by a body of water at rest.

hydrostratigraphic unit

—formation, part of a formation, or group of formations of considerable lateral extent that compose the geologic framework for a reasonably distinct hydrologic system.

ionic strength

—half the sum of the products of molality and square of charge for each ion in a solution.

ionization

—any process by which an atom, molecule, or ion gains or loses electrons.

ionizing radiation

—radiation consisting of directly ionizing particles.

isotherm

—line or curve describing the relation between the amount of a substance sorbed and the amount in solution when the chemical system is in equilibrium. The relation is typically very sensitive to temperature changes, so data used to construct such a curve are normally based on experiments conducted at a constant temperature—hence, the name isotherm. The slope of a linear isotherm is often called a distribution coefficient.

isotropic

—equal in all directions.

kinetics

—general term for rates of reaction.

langbeinite

—potash mineral with chemical formula K₂Mg₂(SO₄)₃ of economic importance as a source of K₂SO₄.

ligand

—compound or molecule in solution that can combine with the ion or molecule of an actinide to form a complex.

lithostatic pressure

—pressure exerted by a column of overlying rock at a point in the earth's crust.

Magenta Dolomite Member

—second-youngest member of the Rustler Formation, 7-8.5 m thick, consisting of dolomite with gypsum and bearing ground-water.

marker bed

—horizontally extensive nonhalite interbed in the Salado, numbered from the top of the Salado to the bottom and used to keep the repository at the same level within the Salado.

matrix diffusion

—transfer of solute mass by diffusion, driven by a concentration gradient, between an actively flowing part of a ground–water system and low-permeability (often stagnant) parts of the saturated medium. An example is diffusion from a high-permeability fracture into dead-end pores of the adjacent rock matrix.

McNutt Member

—a 120-m-thick series of beds midway between the top and bottom of the Salado Formation, which lies stratigraphically above the WIPP repository. The McNutt member contains economically significant potash deposits that have been mined in the area for many years.

molality

—number of moles of solute per 1,000 g of solvent in a solution.

Nash Draw

—broad shallow valley that developed as a stream channel at the end of the Pleistocene (1.6 million to 10,000 years ago) glaciation. Rocks of the Rustler Formation are exposed in the walls of the Nash Draw.

numerical dispersion

—artificial smearing of calculated gradients in a dependent variable (such as concentration) that results from discretization in a numerical solution to the governing partial differential equations (such as the solute-transport equation).

Ochoan

—Late Permian stratigraphic series consisting of, from lowermost to uppermost, the Castile, Salado, Rustler, and Dewey Lake Formations.

Ostwald's step rule

—rule stating that an unstable phase passes through successive steps of increasing stability as it becomes a final stable phase.

parameter

—an algebraic symbol representative of a well defined physical quantity with a numerical value. An adjustable parameter is envisioned to assume any value within its range (between the maximum and minimum numerical bounds). Any particular choice of a value renders a parameter a numerical constant.

Pennsylvanian Period

—period of time from 320 million to 286 million years ago.

permeability

—capacity of a material to transmit fluids. A measure of the relative ease with which a porous medium can transmit a liquid under a potential gradient. Permeability depends on the size, shape, and degree of interconnectedness of pores and is generally measured in millidarcies. It is a property of the medium alone and independent of the nature of the liquid.

performance assessment

—risk-based assessment of the safety performance of a nuclear waste facility.

Permian Period

—period of time from 286 million to 245 million years ago¹.

pH

—measure of the acidity of a solution phase; negative logarithm of the hydrogen ion concentration.

Pitzer parameters

—correction factors for calculating solubilities in strong electrolyte solutions.

potash

—group of potassium salts, mined for various uses including plant fertilizers.

potentiometric elevation

—elevation that represents the hydraulic head. For an aquifer, it is defined by the level to which water will rise in a tightly cased well. The water table is a particular potentiometric surface.

radiolysis

—decomposition brought about by high-energy irradiation.

radionuclide

—radioactive nuclide.

recharge

—addition of water to the saturated zone; either natural or artificial.

redox potential

—number specifying the "oxidation state" of a solution or, alternatively stated, a measure of the ability of a natural environment to control the equilibrium of oxidation-reduction reactions. Redox potential is a synonym for oxidation potential.

retardation factor

—in solute-transport analysis, a parameter that describes the ratio of the net apparent velocity of the concentration of a particular chemical species to the velocity of a nonreactive species (or of water). It is proportional to the slope of a sorption isotherm; thus, if the isotherm is nonlinear, the retardation factor is not constant and depends on solute concentration.

risk

—probability of a selected set of consequences from the existence of a hazard (e.g., nuclear waste in a repository) or the operation of a facility.

Rustler Formation

—second youngest Ochoan formation, overlying the Salado Formation, and consisting of five sequences (members) of thin-bedded strata. The lowermost beds consist of mudstone and sandstone interbedded with evaporites. The upper part of the formation consists of alternating evaporite and dolomite beds. The Culebra Dolomite Member is the second member from the bottom of the formation. The total thickness of the Rustler Formation near the WIPP site is approximately 100 m.

safety

—protection against the adverse health effects of hazards (such as radiation and chemicals), regardless of cause.

Salado Formation

—second-oldest Ochoan geologic formation consisting of a 230-million year old deposit of rock salt (halite) in near-horizontal beds, in total 200-400 m thick. Very thin layers of clay, anhydrite, and potash minerals are interbedded with the halite beds. Lying at a depth of approximately 660 m (2,160 ft) at the WIPP site, the Salado hosts the WIPP repository.

saturated zone

—that part of the water-bearing material in which all voids are filled with water under pressure greater than atmospheric.

sorbing tracer tests

—field experiments designed to measure the amount of sorption of a substance.

sorption

—chemical reaction processes that result in the accumulation of ions and molecules at a fluid-solid interface. Sorption depends on the balance between the affinity of a substance for a surface and its affinity for an aqueous solution. The term is sometimes used in a general sense to include ion exchange, surface complexation, and surface precipitation.

spalling/spallation

—breaking off of very thin sheets from a rock surface.

specific discharge

—rate of discharge of ground-water per unit area measured at right angles to the direction of flow (see also Darcy velocity).

specific storage

—volume of water released from or taken into storage per unit volume of a porous medium per unit change in head. This property is normally considered in problems of three-dimensional

transient flow in a compressible ground–water system.

specific weight

—mass per unit volume of a substance.

storativity (or storage coefficient)

—volume of water an aquifer releases from or takes into storage per unit surface area of the aquifer per unit change in head.

sylvite

—potash mineral with chemical formula KCl, the principal ore of potassium.

tortuosity

—ratio of the length of a fluid particle's path to the length of a straight line between the beginning and ending points of the path. According to this definition, the tortuosity will always have a value greater than one. Note that tortuosity is sometimes defined on the basis of an inverse of this relation; in such cases it will always have a value less than one.

transmissivity

—rate at which water of prevailing properties is transmitted through a unit width of an aquifer under a unit hydraulic gradient. For horizontal flow in a homogeneous aquifer, it is equal to the hydraulic conductivity times the saturated thickness. Transmissivity is measured in units of length squared per time (e.g., m²/s).

transuranic waste

—radioactive waste consisting of radionuclides with atomic numbers greater than 92 in excess of agreed limits. A more precise definition, in DOE Order 5820.2A, EPA regulation 40 CFR 191, and the Land Withdrawal Act, is waste that is not high level waste that is "contaminated with alpha-emitting radionuclides of atomic number greater than 92 and half-lives greater than 20 years in concentrations greater than 100 nanocuries per gram." The regulatory definition excludes actinide elements with atomic numbers between 90 and 92 (most significantly, Th and U isotopes), which is in agreement with the literal meaning of "transuranic." However, common usage of "transuranic waste" is often understood to include all actinides.

undisturbed repository performance

—radionuclide releases from the repository as a result of reasonably foreseeable natural processes

unsaturated zone

—subsurface zone above the water table in which some of the pore spaces are occupied by air and some by water.

waste characterization

—process of identifying and classifying the chemical, physical, and radiological constituents of each drum of waste.

waterflooding

—technique used in the secondary recovery of petroleum and gas whereby water is injected into a petroleum or gas reservoir so that the pressure of the water expels the petroleum or gas.

water table

—surface in an unconfined ground–water system at which the pressure is atmospheric, in the field environment defined by the levels at which water stands in wells that penetrate the saturated zone just far enough to hold standing water.