Nuclear Waste Program

Bo Bodarsson, gsbodvarsson@lbl.gov

The role of ESD's Nuclear Waste Program (NWP) is to assist the U.S. Department of Energy, the United States, and other countries in solving the problem of the safe disposal of high-level radioactive waste--by means of high-quality scientific analyses and technology development. The major portion of this program involves investigating the feasibility and potential of the Yucca Mountain site in Nevada for permanent storage of high-level nuclear waste. The NWP has also collaborated on nuclear-waste disposal issues with such countries as Japan, Switzerland, Sweden, China, Romania, and others.

The Yucca Mountain site is located about 120 km northwest of Las Vegas in a semi-arid region. The proposed repository will be located about 350 m below ground surface within a thick unsaturated zone (UZ). Subsurface rocks at Yucca Mountain consist primarily of fractured volcanic tuffs that vary in degree of welding. To date, a total of about 60 deep surface boreholes have been drilled in the area. In 1996, an 8 km long underground tunnel, the Exploratory Studies Facility (ESF), was completed at Yucca Mountain to facilitate more extensive subsurface testing.

NWP's work at Yucca Mountain consists of solving many problems related to multiphase, nonisothermal flow and transport through the UZ. Some of the key questions addressed by NWP scientists include:

• How much water percolates through the UZ to the repository at Yucca Mountain?
• What fraction of the water flows in fractures and what fraction flows through the rock matrix blocks?
• How much of this water will seep into the emplacement drifts (tunnels)?
• How will radionuclides migrate from the repository to the water table?
• How will coupled TH (thermal-hydrological), THC (thermal-hydrological-chemical) and THM (thermal-hydrological-mechanical) processes affect flow and transport?

To address these questions, the NWP is organized into the Ambient Testing, Thermal Testing, and Modeling groups, with support from geophysical studies.

Ambient Testing Group

The Ambient Testing group investigates how water flows through the mountain and how much of this water will seep into the emplacement drifts. This group has performed various tests within the ESF, including fracture-matrix interaction tests, drift-to-drift tests, the Paintbrush unit test (PTn test), and niche (short drift) testing. Fracture-matrix interaction tests are relatively small-scale tests (i.e., covering a few meters) that focus on the components of water flow in fractures and matrix blocks and on the interaction between the two continua. The drift-to-drift tests address the same issues, but on a much larger spatial scale (10-20 m). The test in the Paintbrush unit, which is an unwelded tuff unit, addresses issues of episodic flow, effects of faults and large-scale features, and lateral continuity of flow and transport. This mostly unfractured unit, directly above the potential repository, is key to dispersing fracture flow from the fractured units above it, and buffering the transient behavior of episodic flow. The niche studies address perhaps the most crucial problem of Yucca Mountain, i.e., determining the fraction of water that will flow into the emplacement drifts. The niche studies are carried out by introducing water into boreholes above the drift opening and measuring what fraction actually seeps into the opening.

Thermal Testing Group

The Thermal Testing group works in collaboration with other national laboratories to evaluate the effects of heat on thermodynamic conditions, fluid flow and transport, and permanent property changes in the fractured tuff at and near the emplacement drifts. The Yucca Mountain Project has completed the first in situ heater test, called the Single Heater Test. The project is now conducting a large-scale heater test in a 50 m long drift. This second test, called the Drift Scale Test (DST), is intended to resemble the actual conditions that would exist when the high-level radioactive waste is placed in the emplacement drifts. NWP's roles in the heater tests are to characterize the heater-test rock block (area) prior to testing; to monitor potential changes in fracture and matrix saturations through air injections, tracer testing, and ground-penetrating radar measurements; and to perform predictive TH, THC, and THM calculations.

The initial characterizations of the heater test areas were performed with air-injection tests that yield the 3D permeability structure of the fracture network. Continued air-injection testing during heating yielded changes that can be attributed to changes in fracture saturations or mechanical effects. Crosshole radar tomography has also yielded very promising results regarding change in global saturations of the system caused by heating. Laboratory scientists are also involved with measurements of the isotopic compositions of gases and condensate water collected in instrumented boreholes. Detailed 3D TH, THC, and THM calculations were used to predict the behavior of the tests.

Modeling Group

Berkeley Lab has the primary responsibility for the development of the UZ Flow and Transport Model. This is a comprehensive, 3D, dual-permeability numerical model that represents the entire UZ at and near Yucca Mountain. The model is intended to integrate, within a single computational framework, all of the relevant geological, hydrological, geochemical, and other observations that have been made at the surface, in boreholes, and in tunnels at Yucca Mountain. The model is calibrated against pneumatic moisture tension, matrix potential, temperature, geochemical, perched water, and other data from the UZ. The model is then used to predict all of these variables in new boreholes and new drifts to be drilled. The degree of agreement between model predictions and subsequent field observations indicates the reliability of the model, and provides guidance as to what additional data need to be collected and incorporated.

A very important submodel of the UZ model is the seepage model, which is on a tens-of-meters scale, versus the UZ model's hundreds-of-thousands-of-meters scale. The seepage model, similar to the UZ model, predicts the results of the niche tests, which are subsequently modified to match the actual observations. Another submodel of the UZ model is the coupled-process THC model, calibrated using the heater test data and used to estimate the chemistry of water and gas entering the drifts. All these models--the UZ model, the seepage model, and the THC model--are key to the Total System Performance Assessment of Yucca Mountain, since performance of the potential repository is only as reliable as these underlying key models.

Funding

The Nuclear Waste Program's Yucca Mountain Project research is supported by the Director, Office of Civilian Radioactive Waste Management, U.S. Department of Energy, through Memorandum Purchase Order EA9013MC5X between Bechtel SAIC Company, LLC, and Berkeley Lab. The support is provided to Berkeley Lab through U.S. Department of Energy Contract No. DE-AC03-76SF00098.