Yucca Mountain – Looking ten thousand years into the future

LA-UR-00-4100
Roger Eckhard
July 1, 2005

It's a dry, brown, nondescript ridge near the Amargosa Desert, just north of Death Valley and west of the Nevada Test Site. The landscape is a drab mix of desert grasses, cacti, shrubs—like needleleaf rabbitbrush and Cooper goldenbush—and, of course, the occasional Yucca plant. Nothing moves but scrawny blacktailed jackrabbits and desert collared lizards. No man’s land.

Yet for the past twenty years, the ridge—called Yucca Mountain but looking more like a geologic speed bump—has been scrambled over by geologists, hydrologists, volcanologists, engineers, and environmental scientists. It has been photographed from every angle and cored at hundreds of drill sites to characterize its geology and hydrology. Its plant and animal life has been cataloged, its topography charted, and its underlying volcanic tuff captured in three-dimensional computer grids Such intense scrutiny has resulted from the mountain’s selection as a potential burial site for high-level radioactive waste.

Much of this waste comes from the nation’s nuclear power plants: spent fuel rods laden with highly radioactive fission products, unfissioned uranium, and plutonium. There are about a hundred commercial reactors in the United States, many operating since the sixties and seventies, and their spent fuel—some 39,000 tons— has been accumulating in cooling pools and dry casks with nowhere to go. By 2035, this tonnage could more than double if all power plants complete their full licensing cycles. Waste from research reactors and the Navy’s nuclear fleet plus plutonium from dismantled nuclear weapons will add another 2500 tons.

Because Congress has banned reprocessing spent fuel, all this waste must be safely stored—for eons. A critical storage issue is the lingering radioactivity of plutonium, neptunium, and other actinides in the spent fuel. The half-lives of these elements are so long that the waste must be stored for more than 10,000 years without significant leakage to the environment. No matter how clever we are in engineering containment barriers— designing storage canisters and tunnels to isolate the waste—eventually, water will seep through the repository, corrode the canisters, dissolve waste radionuclides, and carry them off. When that happens, nature itself—the natural geologic barriers—will have to lend a hand in containing the waste.

We have focused our efforts on two barriers, the unsaturated zone below the repository and above the water table, and the saturated beneath the water table. Studying and modeling the effectiveness of nature’s barriers have been the focus of work at Los Alamos National Laboratory since the early 1980s. In our studies, we have sought answers to a number of questions: Which radionuclides are most apt to dissolve and be carried off? How fast will they move through the rock? What radiation doses might citizens living in Amargosa Valley, the nearest community, receive if radionuclides reach the groundwater that feeds their wells?

Reliable answers require amassing scientific data about the site’s geochemistry and hydrology—for example, its groundwater chemistry, the sorption characteristics of site minerals, and potential groundwater flow paths—as well as about the radionuclides themselves. These data must then be combined to assess the performance of a complex physicochemical system over many millennia. Because no experiment can come close to analyzing radionuclide migration for 10,000 years or more, we have developed computer models to simulate the migration. What follows are some examples of work at Los Alamos—of how we are developing and using computer simulations to look 10,000 years into the future, and far beyond.

Modeling the Mountain

Work at Los Alamos in developing a radionuclide transport model for Yucca Mountain began by incorporating the mountain’s stratigraphy into computational grids. Using techniques that ranged from x-ray diffraction and fluorescence to microautoradiography and potassium/argon dating, we analyzed hundreds of borehole samples taken from the mountain. The picture that emerged from these and other analyses is that the mountain is composed of alternating layers of welded and nonwelded tuff—volcanic ash— that are tilted, fractured, faulted, and locally altered to zeolites and clay minerals.

Our transport model incorporates these various stratigraphic layers, including their mineral compositions, porosities, fault locations, and fracture densities. Finally, the model incorporates radionuclide transport properties derived from extensive laboratory experiments, such as radionuclide solubilities in Yucca Mountain groundwater, sorption coefficients for the various tuffs, and diffusion coefficients for the movement of dissolved radionuclides in the rock matrix.

The nexus for all these data is FEHM—a finite element heat- and mass-transport code. FEHM solves the equations of heat and mass transport in porous and fractured media in two or three dimensions. The code also offers a comprehensive set of models for simulating the transport of dissolved species in either the gas or liquid phase. It combines the capability of simulating transport using either finite element or particle-tracking solutions with a dual permeability capability that captures the effect of fractures on flow and transport.

To model the transport of waste radionuclides through Yucca Mountain, we must generate a grid, or mesh, on which our FEHM calculations can be run. Our primary tool for generating, optimizing, and maintaining computational meshes is LaGriT (for Los Alamos Grid Toolbox), a general-purpose software package.A mesh consists of nodes (points) at specific locations in space that are connected to form elements. These elements can be triangles or quadrilaterals in 2-D models and tetrahedra, hexahedra, prisms, or pyramids in 3-D models. The elements fit together like the pieces of a puzzle to represent physical systems such as the rock layers in Yucca Mountain.

Developing flow and transport models for Yucca Mountain has pushed the limits of mesh generation technology. The models’ size requires us to keep the number of elements as low as possible, their complex physics requires us to accurately represent the geology of the repository site, and the need for timely results requires us to automate mesh generation whenever possible. These often conflicting demands have been met by a collaborative effort in enhancing mesh generation capabilities to meet the challenges of modeling Yucca Mountain.

Our scientists are developing and applying new mesh generation and modeling tools to advance the scientific understanding of Yucca Mountain.

Neptunium Chemistry

Of the several hundred radionuclides present in spent fuel, only six are long lived, soluble, mobile, copious, and hazardous enough to contribute significantly to calculated radiation exposures should the nuclides reach well water in Amargosa Valley. Four of them (technetium- 99, iodine-129, uranium-234, and neptunium-237) could be transported by groundwater because of their high solubility and weak adsorption to minerals. The other two (plutonium-239 and plutonium-242) tend to adsorb to minerals (because of their IV oxidation state) but could be transported on or as colloids. Although technetium-99 and iodine- 129, both abundant in spent fuel, would be the dominant radionuclides to reach the valley in the first 10,000 years, radiation doses from them are not expected to exceed EPA limits. After 10,000 years, neptunium-237 starts to become the radionuclide of concern. While neptunium concentrations in spent fuel are small (only 0.03 percent), they will increase over time through the decay of americium- 241, which has a half-life of 432.7 years. Because of its radiotoxicity, long half life (2.14 x 106 years), high solubility, and relatively low sorption on Yucca Mountain tuffs, neptunium has been the radionuclide of prime concern in our transport calculations.

Thus we have conducted extensive laboratory tests to determine its solubility, speciation, sorption, and transport. Our scientists are leading the effort to understand the properties of neptunium as it is transported in water beneath the Yucca Mountain repository.

Matrix vs Fracture Flow

Another key issue for radionuclide transport is groundwater flow—how quickly will water move through the mountain and what pathways will it follow? Early theories ranged from a “tin roof” scenario in which the upper thin layer of nonwelded tuffs acts as a relatively impervious barrier, diverting water laterally and drastically reducing percolation through the repository, to a scenario of rapid flow along major faults and fractures that allows water to infiltrate the repository within only a few decades. Our studies have shown the actual flow mechanisms to be a bit more complex than either of these two extremes. Through a process that weighs the various surface, chloride, and chlorine- 36 data and that reexamines the hydrologic properties of the various strata (i.e., their porosity, matrix permeability, and fracture density), we are moving closer to establishing a valid flux rate for assessing repository performance. Laboratory transport tests indicate that fracture coatings also affect flow rates. These tests involved columns of Yucca Mountain tuff containing both natural and induced fractures. The natural fractures were coated with minerals that had been deposited over the eons; the induced fractures had no mineral coatings. By using a variety of tracers, we were able to sort out the relative effects of flow through fractures, of matrix diffusion into rock micropores, and of sorption by fracture minerals. We found that because of sorption, neptunium arrived at the bottom of a column long after a nonsorbing tracer. Thus, minerals that appear to contribute insignificantly to sorption when they are present in trace quantities in the bulk rock may quite effectively retard transport when they are concentrated on fracture surfaces.

Sensitivity studies are under way to assess the impact such sorptive minerals could have on radionuclide transport.

Unsaturated-Zone Transport

Our modeling studies of how effectively the natural barriers at Yucca Mountain will contain migrating waste radionuclides began with modeling travel times for neptunium through rock barriers in the unsaturated zone. To help us benchmark our modeling parameters, large scale field experiments were conducted to measure the actual in situ transport properties of fractured rock. The experiments were conducted at Busted Butte, about 8 kilometers southeast of the potential Yucca Mountain repository. Overall, our studies indicate that zeolitic sorption of neptunium in the unsaturated zone results in travel times that are much longer than 10,000 years, and thus the repository’ primary regulatory goal can be met for this important limiting actinide. However, our modeling work is based on measurements from small-scale laboratory experiments that do not characterize the effects of larger geologic features such as faults and stratigraphic boundaries. To characterize the effects of these heterogeneities in the unsaturated zone, we conducted large-scale field experiments at Busted Butte.

Saturated-Zone Transport

Although our unsaturated-zone modeling indicates that neptunium travel times to the water table are long, radionuclides will eventually reach it, releasing a radioactive plume into the saturated zone. The second part of our modeling work, therefore, entailed simulating neptunium transport through the saturated zone and again benchmarking our modeling parameters with field experiments. As in the unsaturated zone, groundwater will encounter both fractured and porous media in the saturated zone. In fractured media, groundwater may move relatively quickly through the fractures, but some water will move into progressively smaller cracks and pores, where advection, matrix diffusion, and sorption will help retard radionuclide transport. In porous media, such as alluvium, groundwater travel times will lengthen because the water must diffuse through the matrix rather than flow along fractures. Large-scale dispersion, or dilution, will also lower radionuclide concentrations in such media.

To benchmark our modeling of the saturated zone, we conducted a series of field tests with tracers at a complex called the C-Wells. These wells are located about 2 kilometers southeast of the potential repository site and are drilled into fractured volcanic tuff. In terms of groundwater flow, they are directly downstream from the southern end of the repository.

In the C-Wells tests, we injected a variety of tracers into the saturated zone at one well and simultaneously pumped water out of another well about 30 meters away, establishing a recirculation loop between them. By adjusting packers in the injection well that sealed off selected intervals along its length, we were able to test transport through distinct stratigraphic layers having different hydraulic conductivities. The tracers used were lithium bromide (composed of a small cation and small anion), pentafluorobenzate (PFBA, a large anion), and polystyrene microspheres (simulated colloids with a negative surface charge). The microspheres were tagged with a fluorescent dye so that they could be detected with flow cytometry. The bromide and PFBA are nonsorbing solutes with different diffusion coefficients, and lithium is a weakly sorbing solute. We conducted separate laboratory tests to characterize the sorption of lithium to C-Wells tuffs and the matrix diffusion coefficients of all tracers.

Volcanic Hazards

Los Alamos researchers have also been involved in analyzing the probability and consequences of volcanic activity at the Yucca Mountain repository site. The possibility of such activity must be considered because a dozen small volcanoes lie within 20 kilometers of the mountain. All but one are within Crater Flat, a region of alluvium-filled basins to the southwest of Yucca Mountain. The twelfth, and youngest, volcano lies farther south at Lathrop Wells. Six of the volcanoes have erupted within the last 1 million years; the other six, within the last 4 million years.

The pioneering research conducted to assess the probability of a volcanic eruption at the Yucca Mountain site is critical to predicting the risk posed by the repository.

Summary

The characterization of Yucca Mountain has progressed for more than 20 years with Los Alamos National Laboratory scientists leading many efforts related to radionuclide transport by water away from the repository. As the Yucca Mountain project evolves toward licensing and construction of the repository, our scientists will continue to apply cutting edge science in support of this important national program.

EES Focus Areas

EES Group Pages