Chapter 62: The Waste Isolation Pilot Plant

1.       THE WASTE ISOLATION PILOT PLANT (WIPP):

1.1        Background:

WIPP is the world’s only operating deep geological repository for long-lived nuclear waste.  It is located in an ancient 2000-foot deep salt bed, 26 miles southeast of Carlsbad in Eddy County, New Mexico. WIPP is a DOE facility and accepts only defense TRU waste—that is, nuclear waste from past weapons programs that is not considered high-level waste, but that contains long-lived radioactive transuranic elements such as plutonium.

The Atomic Energy Commission first began looking at salt beds in southeastern New Mexico for the disposal of defense wastes in the early 1970s.  The current WIPP site was selected for exploratory work in 1974 after local officials expressed interest in being considered; five years later Congress authorized an R&D facility at the site. By this time, tensions had begun to emerge between the federal government and New Mexico, which was concerned about the inclusion of high-level waste and commercial spent nuclear fuel in some of the early plans for WIPP. Authorizing legislation adopted by Congress in 1979 stipulated that WIPP could not be used for the permanent disposal of spent fuel and high-level waste but it also heightened tensions by denying the state veto power and removing the project from the licensing authority of the NRC. Two years later, when DOE attempted to move forward with construction, New Mexico filed suit against both DOE and the U.S. Department of the Interior (which had jurisdiction over the land at the site).Slide1That suit was eventually settled out of court, but over the next decade difficulties arose in a number of areas, from problems with the design of transport casks to concerns about funding for road improvements, controversies over health and environmental standards, and plans for an early test phase during which waste could be stored at the facility without meeting final disposal standards. In 1987, DOE began withdrawing land around WIPP from general use and announced that the facility would open in 1988. This proved unrealistic, as efforts to complete the land withdrawal failed over the next few years.  In 1991, the state again filed suit—this time to prevent the transfer of land from public uses to use for a WIPP testing phase.  In response, the courts issued an injunction against proceeding with the facility according to DOE’s plans.

Progress on WIPP resumed when Congress passed the Land Withdrawal Act in 1992. This legislation required the Environment Protection Agency (EPA), not DOE, to certify that WIPP met applicable standards and gave the state authority to regulate mixed waste at WIPP under the Resource Conservation and Recovery Act (RCRA), including issuing a hazardous waste permits for the facility. Other provisions prohibited high-level waste at WIPP, even for experiments; provided additional funding for highways and emergency preparedness; and directed DOE to prepare plans for retrievability and eventual decommissioning. DOE later announced that it would move radioactive waste experiments out of WIPP and into the national laboratories.Slide2In 1998, EPA certified that WIPP met all applicable federal regulations for the disposal of TRU waste. Soon after, the 1992 court injunction was lifted and in 1999 WIPP received its first shipment of waste. As of mid-November

2011, WIPP had received 10,181 shipments for a total waste volume of approximate 68,200 cubic meters. DOE currently estimates that work to begin closing WIPP could commence as early as 2030.  In contrast to the years of controversy and delay that surrounded the development of the facility, WIPP now enjoys considerable support at the state and local level.

1.2       Site Characteristics:

Based on recommendations by NAS, DOE decided that deep underground disposal in a suitable rock formation would be the safest, most practical, and most cost-effective means of permanently disposing of transuranic wastes. For a rock formation to be suitable, it should be highly stable, contain no circulating groundwater, be in an area where severe earthquakes or volcanic eruptions are highly unlikely, and be deep enough to allow for buffers of the same rock above and below the storage area.

NAS also recommended salt deposits as one of the disposal media for radioactive waste. Salt, according to NAS, offers several advantages: most salt deposits are in stable geological areas; the presence of salt demonstrates the absence of flowing fresh water (which would have dissolved the salt beds); salt is relatively easy to mine; and salt formations will eventually “creep” and fill in mined areas and seal the radioactive waste from the environment.

The site consists of a thick layer of rock salt deposited about 225 million years ago. The low rainfall in the desert environment limits the amount of water that will move through the ground in the vicinity of the WIPP. Fewer than 30 people live within 10 miles of the WIPP site.

Although the WIPP site is under the control of DOE and is in a sparsely populated area, there is oil drilling, gas drilling, and potash mining in the vicinity. Because transuranic waste remains radioactive and must be kept isolated for thousands of years, some have expressed concern that future drilling or mining could disturb the site centuries from now, when government controls over the repository may have deteriorated.

WIPP excavation began in 1981 and continued throughout the 1980s. Four vertical shafts provide access and ventilation to the underground portion of the WIPP, where transuranic wastes will be deposited if the facility opens. This underground portion, which is 2,150 feet below ground level, is to consist of 56 large rooms–each about 300 feet long, 33 feet wide, and 13 feet high. By 1988, seven of these rooms had been constructed. Construction of additional rooms will resume when the need arises. Upon completion, the WIPP as currently designed could hold more than 6 million cubic feet of transuranic wastes or about 850,000 55-gallon drums.Slide3The above-ground portion of the WIPP facility includes the Waste-Handling Building, where containers of transuranic wastes are to be unloaded and their contents inventoried, inspected, and prepared for disposal underground; a health physics laboratory; an exhaust filter building; emergency electric generators; and staff offices. The WIPP site also has its own fire department, ambulance service, and mine rescue capability.

1.3        Natural Setting and Resources:

The natural setting of the Waste Isolation Pilot Plant (WIPP) was instrumental in meeting the selection criteria for a radioactive waste repository.  The natural setting consists of the geology, hydrology, geochemistry, climate, and natural resources of the region and local area immediately surrounding WIPP.

For a large part of the early geologic history of the North American continent, the land now occupied by southeastern New Mexico was part of an ancient ocean south of the continent.  For hundreds of millions of years, this region experienced almost uninterrupted deposition of marine shoreline sediments, primarily beach sands, and shallow water lime and clay muds.  In the Pennsylvanian Period and later, coarser continental shelf and slope deposits occasionally slid into deeper basins.  The relatively warm, calm seas promoted an abundance of marine life, which, upon death, accumulated in the muds and sands, altered to simple hydrocarbons, and eventually became the oil and gas found in some of these rock layers in recent years.

In pre-Permian period, the collision of tectonic plates of the earth’s mobile crust caused mountains to form to the southeast, north, and west of the WIPP site.  The southeastern corner of New Mexico and western Texas remained relatively stable through the Permian Period of earth history (286 million to 245 million years ago), although some instability caused the earth’s crust beneath this region to rise in some places and sink in others, in a belated response to the collision pressures. These fluctuations resulted in shallow areas on the sea floor (“Shelves” or “Platforms”) and deeper areas (“Basins”) at the beginning of the Permian Period in southeastern New Mexico and western Texas.

It was in the sediments accumulating in these deep basins that oil was generated, which moved into adjacent shelf environments to form the richest oil pools. One of these deep basins, the Delaware Basin, underlies WIPP and part of southeastern New Mexico. Most of these older rocks beneath WIPP are far too deep to bear on the performance of the repository. However, the presence of oil is considered a possible reason for inadvertent, or even deliberate, human intrusion into the repository in the future.Slide4Late in the Permian, the warm and shallow seas encouraged the formation of reefs, similar to contemporary coral reefs in shallow tropical marine environments.  As they grew, the reefs blocked off parts of the sea from time to time. With circulation of seawater restricted by the reef, the seawater began to evaporate, resulting in highly concentrated brines. As the brines continued to evaporate because of the warm temperature and restricted inflow of seawater, crystalline salts began to precipitate and accumulate on the bottom of the restricted basin. The salts varied in chemical composition, depending on the amount of water that had evaporated and on the concentrations of calcium (Ca), magnesium (Mg), sodium (Na), potassium (K), and other components of the common “Evaporite” deposits.

As a result of these conditions, the WIPP area is underlain by a total accumulation of hundreds to thousands of meters of reef limestone (calcium carbonate, CaCO3); dolomite [calcium magnesium carbonate—CaMg(CO 3)]; gypsum [a hydrous (water-bearing) calcium sulfate, CaSO4·2H2 O)]; halite rock salt, NaCl, the source of common table salt); smaller amounts of anhydrite (CaSO4); and potassium salts (collectively known as “potash,” a commercially useful deposit).

1.4       Hydrologic Setting of WIPP:

Because flowing ground-water is one of the most likely means by which contaminants could reach the accessible human environment, it is important to understand the hydrogeologic character of the region.

Most of the hydrologic studies of the WIPP site and surrounding region have focused on the Rustler Formation in general, and on the Culebra Dolomite Member in particular.  Because the latter is a water-bearing unit that is exposed in Nash Draw, it is a major concern in assessing the capabilities of WIPP to isolate radioactive wastes. However, the other major formations also play a role in WIPP hydrology for varying reasons.

The top of the Castile Formation lies about 200 m below the level of the Salado Formation at which the WIPP facility is located (Figure A.2). Under natural conditions, there is no flow of ground-water between the Castile and Salado Formations. However, during exploratory drilling to locate a site for WIPP, project scientists found large, highly permeable zones (pockets) at depths within the Castile, filled with brine under very high pressure (from the weight of hundreds of meters of overlying rocks). This is considered significant in evaluating the WIPP facility because brine in pressurized pockets, once penetrated by drilling, flows out to the surface rapidly and could continue to flow for hours or days, depending on the depth of the pocket, the volume, and pressure of the brine.Slide51.4.1     Hydrology of the Salado Formation:

The mechanical behavior of salt, especially massively bedded salt, provides the basis for the inference that water cannot flow continuously through salt along interconnected pathways as it does in many other types of geologic materials. As a weak solid, salt flows under low pressure after a period of time, much as glacial ice flows.

Both are solids, but under pressure such as its own weight or the weight of material above, the crystal structure of the individual minerals that make up the rock salt deforms, causing movement within the minerals (intracrystalline gliding) and movement between crystals (intercrystalline slip) over time. This type of movement, termed ”creep,” is a very slow process in human terms, in which the actual movement of the salt cannot be seen, although the results of creep are readily observable in room closure and the deformation of boreholes.

The continuous creep of rock salt causes opening and closing of pore spaces that trap droplets of water and prevent the continuous flow of ground-water through the interconnected pore spaces in the rock from one location to another. Creep also closes fractures and “heals” them as salt crystals flow into them and interconnect with other salt crystals. For this reason, the permeability of salt (the degree of interconnected pore spaces that allows flow of ground-water through the rock matrix or fractures) is generally regarded as extremely low to zero. Pressure tests done on the Salado Formation at various times, both from wells drilled at the surface and from underground tests after the facility was constructed, have indicated that the salt permeabilities are very low, close to the sensitivity limits of the instruments.

During the construction phase of the WIPP facility, brine was observed flowing or seeping into the space created whenever a new opening was made in the salt, whether in a narrow hole or in a large room. These observations led to consideration of the possibility that the disposal rooms could be flooded over the time required for isolation of the long-lived radionuclides and that the consequent chemical reaction of the brine with metal containers would produce large volumes of hydrogen gas.  These considerations assumed that brine was flowing in from the far field and had a continuous source. However, tests to monitor and measure the inflow of brine over a period of years have shown that the rate of inflow is greatest immediately following the disturbance of the salt to produce a new opening, then declines rapidly, and finally tapers to almost immeasurable amounts.Slide6These observations have led WIPP project researchers to consider that brine inflow into a newly created opening results when the salt is disturbed by drilling or mining and fracturing occurs in a zone around the new opening (disturbed rock zone, or DRZ), thus releasing brine from entrapped pore spaces. The only likely source of measurable Darcy flow in the Salado is the nonhalite interbeds, especially the anhydrite interbeds, which show varying degrees of fracturing. Fracture permeability can provide interconnected pathways for ground-water.

1.4.2    Hydrogeology of the Culebra Dolomite:

Ground-water flow in the Rustler Formation is restricted mostly to the Rustler-Salado contact zone, the Culebra Dolomite Member, and the Magenta Dolomite Member.  Of these, the Culebra is the most transmissive. Although it is relatively thin (the thickness of the Culebra is generally between 7 and 8 m), it is also extensive in area.

The Rustler Formation, especially the Culebra Dolomite Member, has been the principal focus of hydrogeologic characterization of the WIPP vicinity.  The ground-water system in the Rustler is generally characterized by relatively low permeability (or transmissivity) and relatively poor water quality (i.e., too salty to be potable). The Rustler contains a predominance of secondary permeability features, such as fractures, bedding planes, and dissolution features; these may dominate flow in parts of the system, complicate analysis, and render quantitative descriptions difficult. At the very least, these features imply that the ground-water flow system is three-dimensional in a heterogeneous framework that is a challenge to model and understand properly. Both observation and water-supply wells are relatively scarce, contributing to the difficulty and expense of characterization. Although much excellent work has been done during the past 20 years in an attempt to characterize the relatively complex hydrogeology of the area near the WIPP site, some large uncertainties remain in the understanding of the subsurface system above the Salado.

A number of water-level measurements and observations of drawdown and/or recovery during hydraulic tests have been used to measure the hydraulic heads and transmissivities in the Culebra. The changes in head over distance determine the hydraulic gradient, Transmissivity, which can vary significantly in value from point to point, is a measure of the ability of the rock to let water flow through it under a given hydraulic gradient.

However, some inconsistencies remain in the interpretation of the long-term flow history of the Culebra based on hydrologic evidence, compared to that inferred on the basis of geochemical and isotopic indicators. These inconsistencies, which reflect some degree of lack of understanding of present or past flow regimes within the Culebra, mean that there is uncertainty in the conceptual model that underlies the performance assessment (PA) model.Slide71.4.3    Ground-Water Flow Directions:

In general, the direction of ground-water flow in the Culebra Dolomite in the WIPP area is from north to south. Because the salinity of the ground-water is high and variable, the fluid density also varies significantly.  Davies (1989) analyzed variable density flow in the Culebra and concluded that, at least in areas south of WIPP, flow directions can be calculated accurately only if variations in fluid density also are evaluated. Potentiometric data in the Magenta Dolomite indicate that the flow direction is predominantly from east to west. Differences in flow directions within the Rustler Formation reflect the complexity of the hydrogeologic system and may be influenced by vertical components of flow.

A fair degree of uncertainty remains about the amounts and locations of recharge to and discharge from the Culebra. In general, Brinster notes that recharge occurs north of the WIPP site, although some recharge may also occur east of WIPP because of vertical leakage from the Magenta.  Natural discharge is south of WIPP, with some discharge to the Pecos River likely occurring at Malaga Bend.

1.4.4    Hydrogeology of the Dewey Lake Red Beds:

The Dewey Lake Red Beds are believed to be less permeable than the Culebra Dolomite.  However, very few hydraulic tests of this unit have been completed, and few observation wells are available to characterize the flow system.  It was reported that that although a continuous saturated zone has not been found, some localized zones of relatively high permeability have been identified.  Potable water has been reported in some parts of Dewey Lake, perhaps because it is sufficiently shallow to receive direct recharge from precipitation events.

Resources:

  1. The National Academics Press;
  2. The Blue Ribbon Commission on America’s Nuclear Future; and
  3. A Reporter’s Guide to WIPP – The WIPP Facility.

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