DISPOSAL AT WIPP
Dr. David Snow
PhD, Engineering Science
University of California-Berkeley
Dr. Richard H. Phillips
PhD, Karst Geomorphology
University of Oregon
In several places in his paper, Dr. Snow states that a near-surface or above-ground, centralized, monitored retrievable storage facility, possibly at WIPP, is now the only alternative to disposal at WIPP. However, it is CARD's position that the WIPP waste should remain in monitored retrievable storage at the generator sites while research is pursued to find a truly safe method for final disposition of this waste. The Department of Energy (DOE) has stated that the WIPP waste could stay safely at the generator sites for at least another 50 years. CARD believes that the risks from transporting 35,000 shipments of waste around the country to WIPP or another centralized facility are great and outweigh the risks from properly storing the waste where it is generated or currently stored
The full technical version of this paper is also available in the Technical Section.
At WIPP, radioactive waste is being disposed of permanently in drums and boxes placed in rooms excavated in the Salado salt beds. Like all other excavations below the water table, the repository will saturate, and dissolved radioactivity could ultimately escape via boreholes, shafts or fractures to the overlying Rustler evaporites. The most evident aquifer in the Rustler, the Culebra dolomite, is claimed by DOE to provide such slow transport that the Rustler can be considered an adequate barrier to waste migration. But performance assessment modeling, based on insufficient exploration data, unsupportable deductions and faulty assumptions, led to that claim. This paper asserts that the Rustler formation overlying and down-gradient of the WIPP repository will not provide the claimed geologic containment because karst conduits are present that will facilitate rapid, ephemeral flow. If disposal is not halted and the waste removed, escaping radioactivity may reach Nash Draw within a thousand years, contaminating the Pecos River and Rio Grande. Until a suitable disposal site or method is engineered, a monitored retrievable storage facility may offer the only alternative.
In 1998, the Department of Energy (DOE) obtained certification from the Environmental Protection Agency (EPA) to dispose of transuranic (TRU) waste from nuclear weapons production, transporting it from bomb-making plants and test facilities around the country and placing it irretrievably in deep salt beds 2150 ft. beneath the surface at the Waste Isolation Pilot Plant (WIPP), near Carlsbad, New Mexico. DOE convinced EPA that plutonium and other radionuclides will not be carried by groundwater from WIPP to the accessible environment during the 10,000-year period of isolation required by federal law (40 CFR 191).
In nature, humans are largely shielded from radiation by rocks overlying the uranium ore. But sixty years of nuclear weapons production has extracted uranium and transuranic elements (heavier than uranium) and concentrated them in numerous accessible places. It seems logical to bury such radioactive waste deep in the earth from whence it came. But the reliability of geologic disposal is so difficult to prove that every nuclear nation remains burdened with dangerous stockpiles of spent fuel, obsolete weapons, and contaminated materials. WIPP is the only certified permanent disposal facility in the world, but many in the scientific community remain unconvinced of its safety. Until better technology develops, it would be more prudent to establish one or more temporary facilities at which the waste is monitored and retrievable.
Various rocks have been studied as potential hosts for the permanent disposal of nuclear waste. Crystalline rocks, including granite, basalt and metamorphics, were disqualified because it is not possible to prove the absence of preferred pathways, such as fault zones, that would shorten groundwater travel times to sources of drinking water. Since 1957, rock salt has been considered favorable because it is self-sealing (due to a process known as creep closure) and thought to be nearly impermeable. However, abandoned salt mines near Lyons, Kansas were disqualified in 1972 because the overlying rocks are perforated by many old drill holes. Salt domes of the Gulf Coast region were disqualified due to the probability that fluids in a cavern subject to creep closure would be ejected, ultimately reaching overlying aquifers. WIPP is similar in these regards. Salt beds near Hereford, Texas were eliminated by decree of Congress, as was a site in basalt at Hanford, Washington. That left only a site in welded tuff at Yucca Mountain, Nevada for nuclear power plant wastes, and bedded salt at WIPP for military wastes. Though Yucca Mountain provides an unsaturated environment, uncertainties about future climate conditions, faulting, volcanism and seismicity (Hill, et. al., 1995; Hill and Dublyansky, 1999) cast doubts on the reliability of waste isolation there. The WIPP site was selected in 1975 (U.S. DOE, 1997, p. 2-11). Early geologic investigations were clothed in military secrecy. At the onset of controversy concerning karst features over the WIPP site in 1983, the consulting role of the United States Geological Survey (USGS) was discontinued, making Sandia National Laboratories the sole consultant to DOE. Technical oversight improved in 1987 when the No-Migration Petition to the EPA disclosed project weaknesses. Since then, DOE and Sandia Labs have been single-mindedly devoted to getting WIPP certified and opened. Consequently, serious flaws in the investigations were perpetuated from the early 1980's through the 1998 certification.
The Hydrogeologic Setting of WIPP
The underground repository lies 2150 ft. below the surface and will ultimately comprise 500 acres. The WIPP site itself, also known as the Land Withdrawal Area (Figure 1), is 16 square miles, or 10,240 acres. The geology has been summarized by Powers, et al., (1978) and Brinster (1989). As Figure 2 shows, there here are about 1300 ft. of salt and anhydrite beds of the Salado Formation above the repository. Above the salt is the Rustler Formation (Figure 3), comprised of thick beds of anhydrite interspersed with salt and dolomite, dipping about 10 degrees ENE. The Rustler is 400 ft. thick at P-10 beneath the east boundary of the site, thinning to 302 ft. at P-6 beneath the west boundary. Above the Rustler lies the Dewey Lake Redbeds, 199 to 534 ft. of fine-grained, uncemented muddy sandstones and siltstones. Overlying the Dewey Lake is the Santa Rosa formation, 0 to 217 ft. of coarse sands and conglomerates interbedded with shales, mainly beneath the eastern half of the site. Windblown sand and Mescalero caliche cover most of the semi-arid surface of the Mescalero Plain, which slopes gently west to Livingston Ridge, a scarp or cliff that marks the edge of Nash Draw. (Figures 1 and 5). It is DOE's position that any contaminated brines that escape the repository through exploration boreholes will be conveyed upward through the Salado but not to the surface. Rather, contaminants are predicted to move laterally through the thin Culebra dolomite aquifer in the Rustler. On the site, an ill-defined water table is in the Dewey Lake Redbeds, sloping westerly. But because bedding dips northeasterly, nearly in the opposite direction, the water table to the west of the site is in the Rustler, leading to outcrop areas in Nash Draw, 1.1 to 4 miles beyond (Figure 1), or to Malaga Bend on the Pecos River, 13 to 15 miles southwest of the site. Nash Draw is a karst valley formed by dissolution of the underlying Salado salt, which caused fracturing, brecciation and subsidence of the Rustler (Kelly, 2000). Should contaminants escape the repository and enter the Rustler groundwater, the paths, destinations, and travel times to the accessible environment remain uncertain.
Repository performance is judged according the radioactivity accumulated offsite over a 10,000-year period. Allowable limits are prescribed by the EPA in 40 CFR 191.13 for all radionuclides in the waste expected at WIPP. Performance Assessment (PA) is a modeling exercise of prodigious complexity (Helton, et. al., 2000), designed to take into consideration all physical properties of the wastes, the geology, the climate, and human disturbance. It is a collection of models, each of which embodies a time-dependent mathematical representation, a method of numerical evaluation, and a computer code. There is a model to represent inflow and outflow of brine and gas. There is a model to calculate the changing radionuclide inventory as it decays. There is a model to describe groundwater flow in the Culebra dolomite (assumed to be the sole aquifer) above the repository, and a model to calculate transport of each radionuclide along Culebra pathways. PA incorporates several different scenarios, such as no human disturbance, or mining of potash in the overlying Salado, or drilling through the repository and into pressurized brine reservoirs in the underlying Castile Formation. Many factors in the computations are ill-defined. Ranges of values are entered into the equations at random, resulting in an even broader range of answers. The proportion of results that satisfy the EPA criteria measures the probability that the site will be in compliance. Any scientist has to admire the elegance of the computation procedure, while the manager or politician reveres that which is understandably baffling. But underlying the mathematical elegance and the comprehensive array of data are numerous unwarranted assumptions, many of which profoundly affect the results. Some of the hydrogeological assumptions run contrary to the observations of qualified critics, (such as Anderson, 1978, Ferrall and Gibbons, 1980, Barrows, 1982, Snyder, 1985, Phillips, R. H., 1987, Snow, 1998 and Hill, 1999), and arguably differ from actual field conditions. Management has influenced the scientific staff to adopt models and select studies favorable to EPA certification, thus biasing the results of PA. A critic must use DOE's own data and draw inferences from his own observations to show where investigations have gone astray. There are areas of geology, rock mechanics and hydrology that deserve reassessment in that light.
Contaminated Brine Discharge from the Repository
The starting point for all PA calculations is the brine inflow after the repository is filled and sealed. As noted by Brinster (1989, p. II-19), the Salado is not pure salt, but contains thin beds of anhydrite, polyhalite, glauberite, and mudstone. The salt was formerly believed to be so impermeable that the rooms would remain dry, but small brine seeps appeared soon after the first research rooms were opened, showing that DOE must contend with a wet waste environment. PA recognizes that waste could be carried to the surface along with cuttings from inadvertent oil wells, but all other scenarios involve radionuclides transported in flowing groundwater. The WIPP project might have been aborted if DOE had been more respectful of the historic problems of water in salt and potash mining. At WIPP, brine will accumulate downdip, corrode the steel drums, and dissolve radionuclides. Generated gas will collect updip. The computed brine inflows depend on the measured permeabilities of fractured anhydrite beds above and below the repository horizon, but only four beds, all within 40 vertical feet of the repository, have been modeled as inflow contributors. That limitation was due to the assumed extent of the disturbed rock zone (DRZ) of fractures expected to form around the rooms as they close. The consequences of gas generation, cavity pressurization and outflows of brine and gas through those four anhydrite beds indicated (by PA) that the undisturbed scenario poses no hazard of a significant breach or accumulation of radionuclides beyond the WIPP site boundary.
The fallacy of that conclusion stems from a misconception of the behavior of the Salado Formation. The 13-ft. high by 33-ft. wide rooms will be short-lived. Large open fractures appear in the ceilings of all rooms within months of mining. Several roof-falls and floor heaves have already occurred at WIPP, so an extensive array of roof bolts has been installed to delay the failure of the remaining rooms long enough to fill them with drums. These and all future rooms will suffer collapse of major roof slabs bounded above by weak clay-bed partings. Such falls will crush the drums, and uncontained waste will enter the fractures. DOE has assumed roof fractures extending upwards only to Marker Bed #138, forty feet above the rooms, but as creep subsidence incorporates whole panels and then the repository width, horizontal slip and openings will occur on successively higher clay seams, most of them bounding stiff anhydrites. Inclined fractures will interconnect the rooms and panels with fractured anhydrite beds farther above the repository, each of which will contribute to increasing inflows of brine. Experience at potash mines in similar salt sequences (notably at K-2 Mine in Saskatchewan) indicates that such roof behavior is typical. At the Canadian mines, the fractures sometimes breach the top of salt into an aquifer, causing inflows that flood the mine (Tofani, R., 1983, Van Sambeek, 1993). After shaft leakage, such roof breaching is the next most common cause of flooding of salt and potash mines, all of which ultimately flood because they lie below the water table. Already there is leakage occurring from the Dewey Lake Redbeds into one of the WIPP shafts, and thence into the repository. In European potash mining experience, such leakage has been irreparable. The first drop of water signals the eventual flooding of the mine.
In PA, the Salado above anhydrite marker #138 up to the Rustler is assumed to be salt with very low permeability. What is significantly wrong with the model is that it assumes no fracture conduits reaching high above the waste panels. Rather, as pressures in the sealed repository rise, gases will cause the subsidence fractures to propagate unstably to higher levels where smaller rock stresses prevail, facilitating subsequent brine leakage to the Culebra and other aquifers much sooner and at higher rates than the PA model predicts. Because of the non-conservative assumption that the Salado is uniform, the rock mechanics model is unrealistic of long-term subsidence, and the conclusion from PA calculations that the undisturbed scenario is innocuous has to be wrong. Subsidence of the upper Salado above the McNutt interval at nearby potash mines of Eddy County, NM could have been studied, reported and modeled, to derive more realistic subsidence predictions for the WIPP site. Sandia rock mechanics wanted to do that at the Horizon (Amax) Mine (Crosser, 1998), but funding was denied them. It is common geotechnical experience that in a significant proportion of dams, tunnels, aqueducts or deep mines, failure will occur by reason of unexpected hydrologic effects. Therefore, in sensitive and doubtful situations, especially conservative assumptions are appropriate. Instead, WIPP modeling employed idealistic assumptions of continuous rocks, when discontinuous (fractured or karstic) properties would have been appropriate. The conservative expectation is that subsidence fractures at WIPP will propagate unstably upwards to the Rustler due to the gas pressure generated in the repository, followed by contaminated brines after the gas has dissipated and rooms become saturated. Sealed shafts and boreholes nearby will probably retain their integrity unchallenged, because fractures will provide easier egress for fluids.
Because human intrusion is a potential cause of repository breaching, any radioactive waste disposal site should be free of valuable natural resources that could stimulate future explorations. The WIPP repository is overlain by potash beds and is surrounded by oil and gas wells (Figures 2 and 4). Inadvertent interceptions of the waste rooms by future drill holes are very credible. Direct flows of contaminated brine along boreholes open to the surface have been minimized in PA by assumptions of borehole plugging by future operators. The results are unrealistic for an open borehole that intercepts a saturated repository at near- lithostatic pressures, or along fractures initiated by subsidence or oil-field water- flooding (Bredehoeft, 1997).