In This Issue
Radioactive Waste Disposal
September 1, 2003 Volume 33 Issue 3

Licensing, Design, and Construction of the Yucca Mountain Repository

Wednesday, December 3, 2008

Author: Margaret S.Y. Chu and J. Russell Dyer

Investigations of the natural processes at Yucca Mountain indicate that public health and the environment can be protected.

The deep geologic disposal program in the United States began more than 20 years ago, in 1982, with the passage of the Nuclear Waste Policy Act (NWPA), which set forth processes for characterizing, recommending, selecting, and licensing sites for permanent geologic disposal of commercial spent nuclear fuel (resulting from electricity generation) and high-level radioactive waste (resulting from atomic energy defense activities). In 1987, NWPA was amended (P.L. 100-203) to limit characterization to one site, Yucca Mountain, Nevada. The mission of the U.S. Department of Energy (DOE) Office of Civilian Radioactive Waste Management (OCRWM) is to "manage and dispose of high-level radioactive waste and spent nuclear fuel in a manner that protects public health, safety, and the environment; enhances national and energy security; and merits public confidence."

The consolidation of spent nuclear fuel and high-level waste from 131 sites in 39 states and safe disposal at Yucca Mountain are vital to U.S. national interests. Disposal in a geologic repository is necessary to maintain energy options and national security, to advance the cleanup of weapons-production sites, to continue the operation of nuclear-powered ships and submarines, and to advance international nonproliferation goals.

In 2002, we completed nearly 20 years of site investigations of the natural processes that could affect the isolation of radionuclides from spent nuclear fuel and high-level radioactive waste. These investigations show that a repository at Yucca Mountain can provide the reasonable expectation required by the U.S. Nuclear Regulatory Commission (USNRC) that public health and safety and the environment will be protected. The underlying basis for these investigations and engineering designs has withstood many independent scientific peer reviews and thorough examination by national and international organizations.

In February 2002, the secretary of energy recommended the site to the president, and on July 9, 2002, Congress passed a joint resolution approving Yucca Mountain as a suitable site for repository development. The president signed the bill approving the site on July 23, 2002 (P.L. 107-200) thus completing the site characterization phase. Near-term efforts are now focused on seeking a license from the USNRC to construct a repository and develop a transportation system for shipping waste to the proposed repository.

Receiving Waste in 2010
To meet our objective of receiving waste at Yucca Mountain beginning in 2010, we must (1) seek and secure authorization to construct the repository, (2) begin constructing the repository, (3) receive a license to operate the repository, and (4) develop a system to transport waste from civilian and defense storage sites.

We will need construction authorization from the USNRC no later than 2007, which means we must submit a high-quality, defensible License Application (LA) no later than 2004, because the USNRC will require at least three years to consider the application. Because past funding constraints forced us to defer critical work on the transportation system, we must now accelerate its development. Meeting the 2010 objective will also require far greater resources than have thus far been appropriated. We estimate, for example, that it will cost about $8 billion - more than 80 percent of the budget required to meet the 2010 objective - to construct the repository and develop the transportation system.

Developing the Yucca Mountain
License Application

The LA must present a defensible position that the repository can be constructed, operated, and closed without unreasonable risk to the health and safety of the public. The USNRC has issued a site-specific licensing regulation, 10 CFR Part 63, which is risk-informed and performance-based. DOE must demonstrate that the repository will meet the specified performance objectives during operations and that after closure, the health and safety of the public will be protected for 10,000 years.

Developing a Transportation System
Even though specific routes are not expected to be identified until four years before waste transport begins, a number of critical steps are ongoing. By the end of 2003, a national transportation strategic plan will be issued that addresses policies; plans for interactions with states, local, and Native American tribal governments through whose jurisdictions waste could pass; identifies necessary activities; and describes the approach to having an operational transportation system in place by 2010.

Initial procurement of the cask fleet and orders for long lead-time transportation cask systems and equipment will be placed as soon as possible, focusing first on transportation cask designs that have not been previously developed by industry and already certified by USNRC. We will also prepare for the acquisition of transportation and logistics services, determine the approach for performing cask maintenance, develop initial site-specific service plans in consultation with nuclear utilities, and develop facility and equipment needs assessments for waste acceptance at DOE defense waste sites.

The U.S. rail system has been used for the last 25 years to ship radioactive waste safely across the country. To link the national rail system and the Yucca Mountain site would cost an estimated $300 million to $1 billion, depending on the corridor and alignment. The final Environmental Impact Statement (EIS) for Yucca Mountain examined five potential rail corridors in the state of Nevada that could be used as transportation routes to the repository (DOE, 2002). If a decision is made to use rail transportation, then we must analyze the environmental impacts of constructing a rail line within the chosen corridor.

Yucca Mountain
Yucca Mountain is located on land controlled by the U.S. government in a remote area of Nye County in the southern part of the state of Nevada, approximately 100 miles northwest of the Las Vegas urban area. Southern Nevada, one of the most arid regions of the country, has annual precipitation of about 7.5 inches, more than 95 percent of which either runs off or is lost to evaporation or transpiration, thereby limiting the amount of water that could seep into the repository. Measurements of the water level in boreholes at Yucca Mountain indicate that the water table is approximately 1,600 to 2,600 feet below the ground surface.

Yucca Mountain consists of a series of north-south trending ridges extending approximately 25 miles. The elevation at the crest of the ridges varies from approximately 3,000 to 5,900 feet above sea level. At the proposed repository site, the crest of Yucca Mountain is 4,600 to 4,900 feet above sea level. The mountain slopes gently to the east and is incised by a series of east-to-southeast trending stream channels. The elevation at the base of the eastern slope is approximately 1,100 to 1,500 feet below the ridge crest. To the west of the crest is a steep slope that drops approximately 1,000 feet into Solitario Canyon.

Yucca Mountain consists of layers of volcanic rock, approximately 11.5 to 14 million years old, formed by eruptions of volcanic ash from calderas to the north of the mountain. Most of these volcanic rocks are ash-flow tuffs of two types (welded and nonwelded) that formed when hot volcanic gas and ash erupted violently and flowed quickly over the landscape. As the ash settled, it was subjected to varying degrees of compaction and fusion, depending on temperature and pressure. At higher temperatures, ash was compressed and fused to form a welded tuff - a hard, brick-like rock with low porosity (i.e., very little open pore space in the rock matrix). At lower temperatures, ash was compacted and consolidated between the welded layers. These nonwelded tuffs are less dense, brittle, and have higher porosity (i.e., more open pore space in the rock matrix). The resulting layers have very different hydrologic behavior.

Exploratory Studies Facility
During the site characterization phase, we conducted many studies from the surface that involved excavating approximately 200 pits and trenches, drilling more than 450 boreholes, and instrumenting more than 25 wells. To get scientists underground where they could see and test the rock near the repository horizon, the exploratory studies facility (ESF), a U-shaped tunnel (approximately 5 miles long and 25 feet in diameter) about 1,000 feet below the crest of Yucca Mountain was excavated. Additional areas were subsequently excavated to enable direct observation of geologic and hydrologic conditions, the engineering properties of the rock, and the response to construction activities. The ESF, along with a smaller cross drift (16.5 feet in diameter and 1.6 miles long), excavated in 1998, have been used extensively to conduct tests in 13 alcoves and niches. The cross drift crosses over the ESF main drift and provides access to the deeper rock units of the proposed repository. Since the start of active testing in the ESF in 1996, more than 20 major experiments have been completed or are in progress. The remainder of this paper summarizes current ambient testing and ongoing and completed thermal tests.

Ambient Testing
On the most fundamental level, the climate and the hydrologic properties of the rock units are the important factors affecting performance of the Yucca Mountain unsaturated zone as a natural barrier to radionuclide release. Estimates of parameters for percolation flux1 at the repository horizon and potential seepage 2 into waste emplacement drifts are derived from these two basic components. These, in turn, are central to development of the unsaturated zone flow and transport process model, which is part of the total system performance assessment model that will be used in the LA.


Alcove 8/Niche 3
Tests in Alcove 8/Niche 3 started in 1999 to help determine how water flows through the repository horizon and investigate flow and transport within a fault zone. Alcove 8, located off the cross drift, overlies Niche 3, which is situated on the ESF main drift. The detailed objectives are to quantify flow and seepage processes at the scale of tens of yards and to evaluate matrix diffusion mechanisms in long-term flow and transport tests across a lithophysal-nonlithophysal interface. Water containing tracers is released in Alcove 8 (lithophysal rock), and any resultant seepage is collected in Niche 3 (nonlithophysal rock), located approximately 66 feet below. Results to date include determination of seepage threshold (i.e., the value of applied percolation flux below which no seepage is observed) under high-humidity conditions (behind the hydrologic bulkhead to isolate the test from effects of tunnel ventilation) and measurements of tracer diffusion within a fault zone.

Systematic Hydrologic Characterization
Tests in the cross drift systematically characterize the hydrologic properties of the Topopah Spring welded (TSw) lower lithophysal unit. Testing started in 1999 in a section of the cross drift approximately 1,640 feet long. A series (nine planned) of inclined boreholes, 98 feet long, were drilled into the crown of the cross drift, and water was released from packer-isolated sections of the individual boreholes. Additional boreholes were also drilled perpendicular to the drift in the horizontal plane. Water moving through the fracture system of the rock is being collected and analyzed to determine the seepage threshold and matrix-diffusion properties of the TSw lower lithophysal medium. Because of the considerable size of the test bed, this study will also provide data to assess scaling issues concerning hydrologic-property spatial variability within this repository unit.

Tests in the cross drift also help evaluate the effects of ventilation on moisture. Observations designed to quantify the effects of dry-out (from ESF ventilation) and rewetting (in areas isolated behind hydrologic bulkheads) began in 1999. A large section of the cross drift (approximately 2,900 feet) is presently being monitored to see if seepage under ambient conditions can be observed. This portion of the cross drift underlies an area that receives relatively high surface infiltration; if seepage were to occur under ambient conditions, this part of the tunnel system should be the most conducive to development and observation of active seeps. Instrumentation within this isolated section includes heat-dissipation probes, temperature probes, relative-humidity sensors, pressure sensors, chemically treated drip cloths, remote television cameras, and sample-collection bottles attached to rock bolts and other potential points of water accumulation. Accumulations of water within the drift have been observed. However, chemical analyses of collected samples suggest the most probable mechanism of condensation is driven by temperature gradients in heat generated from mining and data collection instruments. To date, no seepage has been observed.

Niche 5
Tests in Niche 5 help determine the hydrologic properties and seepage threshold of the TSw lower lithophysal unit. Similar to tests completed in the other niches devoted to seepage studies (ESF Niches 1, 2, 3, and 4), Niche 5 was excavated in 2000 to investigate potential seepage through controlled releases of water. To obtain more realistic estimates of seepage potential than were investigated in Niches 1 through 4, however, the rates of water release were substantially slower, and the time period of injection was longer.

An additional objective of testing in Niche 5 is to see if the effects of the lateral diversion of flow due to the capillary barrier imposed by the excavation of the drift itself can be observed and quantified (account for mass balance). This is being pursued through the observation of water released from boreholes located above the niche to see if it migrates into a collection slot cut into the side of the niche.

Alcove 7
Tests in Alcove 7 help determine whether seepage can be observed in the vicinity of faults. Alcove 7 was excavated in 1997 to provide access to the southern portion of the Ghost Dance Fault. Tests in this alcove concentrate on moisture monitoring to see if any ambient seepage can be detected in the vicinity of the Ghost Dance Fault. Isolated behind multiple hydrologic bulkheads, this sealed alcove has been monitored for about four years. No ambient seepage has been observed to date despite penetration of the alcove by a through-going structural feature that provides a potential flow path from the surface.

Chlorine-36 Validation Study
The objective of the chlorine-36 (Cl-36) validation study is to evaluate whether the Sundance Fault and Drillhole Wash Fault zones are "fast" (50 years or less) flow pathways. Cl-36 is a radioactive isotope produced in the atmosphere and carried underground with percolating water. High concentrations of this isotope were added to meteoric water during a period of global fallout from atmospheric testing of nuclear devices during the 1950s and 1960s. This "bomb-pulse" signal has been used to test for the presence of fast transport paths in the unsaturated zone at Yucca Mountain. Because of the important implications of the occurrence of "bomb-pulse" Cl-36 to the site-scale unsaturated zone flow and transport model, a study is ongoing to confirm the apparent Cl-36 signal detected in earlier studies.

The elevated Cl-36 signature appears to be confined to the immediate vicinity of faults (i.e., where structural features provide continuous flow paths from surface to depth). Fifty boreholes, 13 feet long, have been drilled in areas adjacent to the two faults, and the core obtained is presently being analyzed for Cl-36 concentrations. Corroboration of these results would demonstrate the existence of fast flow paths from the surface to repository depths. Final results of this study are expected during 2003.

Thermal Testing
Key objectives of the thermal tests in the ESF have been to obtain data necessary to understand thermally coupled processes. Because the waste in the repository will result in heating of the geologic system, we need to understand the effects of heat on hydrologic, mechanical, and chemical processes and validate the models of those thermally coupled processes.

Drift Scale Test in Alcove 5
Tests in Alcove 5 help determine how heat affects the interactions of hydrologic, mechanical, and chemical processes (i.e., thermally driven coupled processes) in the middle nonlithophysal unit of the proposed repository horizon. The ongoing drift scale test (DST) site consists of an observation drift, a connecting drift, and a heated drift (HD) that is separated from the other drifts by a thermal bulkhead door. In the thermal testing program, the DST is the largest scale test with an HD approximately 156 feet long and 16 feet in diameter. It is also the longest duration test - eight years - in the thermal testing program at Yucca Mountain. The heating stage of the test started in late 1997; the heaters were turned off in January 2002 after slightly more than four years of heating. The cool-down phase is expected to last four years, after which the test equipment will be removed, and portions of the affected rock mass will be sampled for post-test observations and characterization.

The rock was heated using a large number of resistance heaters that provided a total maximum power of approximately 280 kilowatts. The heaters were distributed in two ways. First, inside the HD, nine steel canisters (to simulate the cylindrical waste packages) contain 30 primary heating elements for a total of 7,500 watts per canister. These canister heaters also contain a duplicate set of heating elements for backup to the primary elements (although not planned to be used, these backup elements could be run concurrently for a total of approximately 135 kilowatts output from the in-drift canister heaters). Second, two resistance heaters are located in each of 25 additional boreholes on both sides of the heated drift (50 total) extending into the rock. These "wing heaters" represent an additional 144 kilowatts of heating power. These produce heat sources that were laterally offset from the heated drift, mimicking heat flow from adjacent drifts in the proposed repository. The strategy was to raise the temperature of the HD wall to about 390?F. Over the duration of the DST, the heated volume of rock was approximately 706,200 cubic feet, with more than 70,600 cubic feet of the rock mass driven above the boiling temperature for water (about 205?F at the elevation of this test).

To monitor and quantify the coupled thermal, hydrologic, mechanical, and chemical processes that occur, almost 150 boreholes were drilled to house the wing heaters and instrumentation packages. Approximately 4,000 sensors are located throughout the rock mass and within the HD to record temperature, relative humidity, gas pressure, mechanical changes in the rock, microseismic events, changes in water saturation, moisture movement, and fracture permeability. In addition, instrumentation allows the collection of water, gas, and rock samples for analyses of bulk chemistry and isotopic composition of gas and water in the test and mineral alteration. Just outside the HD bulkhead, an associated niche includes a plate loading test to determine bulk thermomechanical properties at ambient and elevated temperatures.
Beyond the studies of these natural coupled processes, the DST includes a number of tests to evaluate materials processes in the heated environment:

  • mechanical measurements on a cast-in-place concrete liner in the last 41 feet of the HD
  • sample coupons of metal alloys placed in the HD and within boreholes that will be retrieved at the end of the test and evaluated to characterize corrosion processes
  • samples of microbes retrieved at the end of the test to evaluate their survivability
Many of the data on coupled processes are used to either validate, or in a few cases calibrate, the coupled-process models that will be used in the LA (i.e., the coupled thermal-hydrologic, thermal-hydrologic-chemical, thermal-hydrologic-mechanical, and, ultimately, thermal-hydrologic-mechanical-chemical processes) to provide confidence that the models capture these processes appropriately.

The rate of temperature decrease has been rapid initially, as expected, based on the thermohydrologic models. Temperature throughout the test block fell below the boiling temperature for water after approximately one year of cooling. The rate of cooling is decreasing in a manner similar to the modeled behavior, and the system is expected to be nearly back to ambient temperature after four years of cooling.

Modeling of this test provided important insights into the hydrologic, mechanical, and chemical properties and responses of the fractured tuff. The test confirmed, at the field scale, that the dual permeability model of the rock-water system is more appropriate to describe the processes than alternate equivalent continuum models. Results show that water moves away from heat sources as vapor, condenses where it is cooled below boiling, and tends to drain downward through fractures. These model results and test observations suggest that gravity drainage through fractures would prevent water from perching above the heated region.

Evidence of this drainage includes liquid collected in boreholes and changes to saturation distribution around the heat source. Coupled thermal-hydrologic-mechanical models of the DST compare well with observations of the air-permeability changes due to the combined effects of fracture saturation changes and mechanical deformation. Detailed observations of water and gas compositions and of mineral alterations in rock samples taken from the test demonstrate the validity of the coupled (thermal-hydrologic-chemical) modeling that has simulated those changes through the four years of heating.

Single Heater Test in Alcove 5
The single heater test (SHT) was conducted in Alcove 5 to evaluate coupled thermal-mechanical-hydrologic-chemical processes that could occur in a heated rock mass, as well as to improve planning for the larger DST (discussed previously). The SHT consisted of a nine-month heating period followed by a nine-month cooling period, followed by a period of postcooling characterization.

For the SHT, a 900 cubic yard block (approximately 14 yards wide by 11 yards deep by 6 yards high) of the TSw (middle nonlithophysal unit) was exposed on three sides and was heated with a single 6-yard long 4-kilowatt heater. A total of 41 boreholes parallel to, perpendicular to, and surrounding the heater hole were instrumented to monitor the thermal, hydrological, mechanical, and chemical changes. When the test was completed, core was taken from six newly drilled boreholes, and four existing boreholes were overcored to evaluate the thermally altered properties of the rocks.

Modeling of this test provided important insights into the hydrologic properties and responses of the fractured tuff. The test confirmed, at the field scale, that the dual permeability model of the rock-water system is more appropriate to describe the processes than alternate equivalent continuum models.

Condensate drainage was collected in a borehole segment intersected by a fracture drainage pathway. The apparent rapid drainage through fractures indicates that reaction between condensate and fracture-surface mineralogy is limited to relatively short times. The analyzed water compositions indicated that the gas composition plays a role in the water chemistry with carbon dioxide affecting the pH. Overall, the compositions of the collected fluids were consistent with the pore water compositions from this unit. Calcium carbonate, calcium sulfate, and silica minerals were found to have precipitated, and their form suggests that they were deposited by evaporative concentration of fluids.

These observations provided constraints on the possible magnitude and extent of heat-driven geochemical effects on water compositions and fracture mineralogy. In addition, the SHT supplied constraints on thermal properties of the rock-water system, as well as the behavior of thermal-mechanical properties before and after heating. This test was completed in 1998.

Summary
To obtain a license, construct, and operate a repository, we will rely on information gained from more than two decades of scientific investigations at the Yucca Mountain site. The proposed repository would consolidate spent nuclear fuel and high-level waste that is currently stored at 131 sites in 39 states. The repository would be isolated from large population centers, in a desert location, in a closed hydrologic basin, secured 1,000 feet below the surface, surrounded by land controlled by the U.S. government, and protected by multiple natural geologic barriers and robust engineering barriers.

As steward of the U.S. nuclear waste, not just for a few decades after the start of repository operation in 2010, but for hundreds and hundreds of years, we consider the proposed Yucca Mountain repository a key strategic resource for the United States, a critical asset that will pay immeasurable dividends for our citizens.

Reference
DOE (U.S. Department of Energy). 2002. Final Environmental Impact Statement for a Geologic Repository for the Disposal of Spent Nuclear Fuel and High-Level Radioactive Waste at Yucca Mountain, Nye County, Nevada. DOE/EIS-0250F, 4 vols. Washington, D.C.: Office of Civilian Radioactive Waste Management, U.S. Department of Energy.



Footnotes
1Percolation flux, the flow of liquid water to the repository horizon, strongly influences drift seepage and radionuclide transport. Percolation flux is a quantity derived from knowledge of various parameters, including climate and infiltration, chemical analyses (i.e., major/minor ions, total chlorides), environmental isotopes, perched water occurrences, heat flow, and analysis of fracture-fillings.
2Seepage is the flow of liquid water into an underground opening. Potential seepage into the repository drifts is an experimentally determined quantity investigated through water-release studies.
About the Author:Margaret S.Y. Chu is director, Office of Civilian Radioactive Waste Management, U.S. Department of Energy. J. Russell Dyer is assistant deputy director, Office of Repository Development, Office of Civilian Radioactive Waste Management, U.S. Department of Energy.