|Stephen Blair (left), John Kelly, and Patricia Berge (rear) at work on a study of the effects of high temperature and intense compression on the tuff from the Yucca Mountain, Nevada, area where a potential nuclear waste repository may be located.|
HIGH SCHOOL teachers and college students, working side-by-side with Laboratory geophysicists and geochemists, have had the rare opportunity to contribute to the field of geomechanics and to the study of how rocks fracture.|
Obscure findings for esoteric studies of interest only to geologists, mining engineers, and rock hounds? Hardly.
The results are critical to understanding the behavior of a proposed underground repository for high-level radioactive wastes. The Yucca Mountain Repository in Nevada could become a permanent storage site for as much as 70,000 metric tons of nuclear waste, nearly 90% of it spent fuel from commercial nuclear power plants.1 Given the rigid federal, health, and safety regulations such a repository must meet, it is essential to understand how the surrounding rock behaves over time when exposed to heat and radiation generated by the nuclear waste.
At Lawrence Livermore, geomechanics expert Stephen Blair is a principal investigator conducting fundamental studies of the rock that would form the Yucca Mountain repository--Topopah Spring tuff. (Tuff is formed of compacted volcanic fragments welded together.) In his quest for answers, Blair has enlisted the skills and talents of eight high school teachers and college students, most of them recruited through the Laboratory's various education programs.
Questions that they are examining include: What happens when this rock is exposed to radioactivity over tens of thousands of years? As the temperatures increase and water in the pore spaces of the rock evaporates, how does that water move and what happens to the fractures and the rock itself? Blair and others have been conducting tests to better understand the structure of tuff and to develop and fine-tune computer models that will be used to determine the performance of the entire repository for hundreds of centuries.
"We began with small rock samples, about the size and shape of a roll of quarters, looking at what happens at the pore level, basically the size of a grain of sand," said Blair. Three studies involving students and teachers focused on the behavior and structure of rock at this level. A fourth study examines how blocks of tuff a half-meter on a side behave under increasing temperatures and pressures.
Grains and PoresThe properties and behavior of the tuff depend on its grain-scale structure and characteristics. Chris Pena, a graduate student in environmental engineering at San Jose State University, and Brian Johnson, a high school teacher now at Susanville, California, helped analyze the tuff microstructure. They used an image processing method--developed by LLNL's Blair, James Berryman and Patricia Berge--in which the microstructure of rocks is measured statistically. Under Blair's guidance, Pena used image-processing software to examine images of cross-sections taken from tuff core samples from Yucca Mountain and to determine the rock's porosity, isotropy (directional dependence of material properties), and general structure. Among her findings, which she presented at the American Geophysical Union meeting in December 1995, was that the tuff material was dominated by small pores with cross-sectioned areas of less than 10 square micrometers.2
Knowing the rock structure in such detail is important, Blair noted, because it is on this microlevel that cracks and fractures begin.
Cracks and FracturesThe grains of material that make up a piece of rock come in different sizes, shapes, minerals, strengths, and distributions. Even the most uniform rock is diverse at the grain scale, and this diversity affects the processes of rock fracturing under stress. Under heat or pressure, tiny cracks form and merge to form larger fractures. These fracture processes are not well understood and are the subject of an ongoing study.
"One of the things that might happen at elevated temperatures and stresses in an underground nuclear repository is that cracks may form in the rock and the rock's properties might change," Blair explained.
Using a two-dimensional statistical computer model developed by Blair in 1994, Diablo Valley College student Austin Woffington and San Lorenzo high school teacher John Kelly simulated what happens when rock is compressed. In the model, the rock is represented by a lattice of grain centers that can be either "strong" (breaking only under high compression) or "weak" (breaking easily under moderate compression).
This model is being used to estimate the amount of cracking that will occur over time and at the high temperatures expected in the proposed repository. The results will aid in predicting the long-term integrity of the repository tunnels.
Radiation and Rock StrengthThe tuff forming the repository must endure centuries of exposure to radioactive waste. What effect, if any, might this have on the rock? Will the rock weaken? Will it fracture more easily?
"We want to be sure radionuclides will stay in the repository," said Blair. "We need to better understand the effect radiation has on tuff and whether exposure to radiation will alter the mechanical strength or other geomechanical properties of the rock near the waste. Until now, there have been no data describing the effect of radiation on tuff from the potential repository."
A controlled study was performed to examine the effects of radiation on the strength of tuff. For this project, Blair enlisted the help of several high school teachers, including Kelly.
"We applied up to 160 megapascals--about 10 tons of force-- to rocks the size of a roll of quarters, some of which had been subjected to gamma radiation," Kelly said. "The results were impressive to watch. Samples with pre-existing cracks just crumbled. With others, nothing happened until they failed catastrophically at high stresses."
Preliminary results indicated that whereas radiation had little or no effect on initially unfractured samples, it did affect samples with pre-existing open fractures. These irradiated samples failed at stresses only half those applied to the non-irradiated samples.3
"One explanation is that radiation weakened the cementing material in the cracks," said Blair. "We need to do additional studies to say for certain. However, if this is a real phenomenon, it has significant implications. The radiation is expected to penetrate only a few centimeters into the rock. But this rock will also experience high temperatures, stresses, and humidity. If the fracture-filling materials are weakened, more pieces might break off over time. In addition, changes in fracture properties--such as fracture shear strength, compressibility, and permeability--could also occur. The rock mass may be affected in unanticipated ways, including movement of rock blocks along fractures."
Next Step UpBlair's next step is to take a block of tuff a half-meter on a side (basically the size of a large computer monitor), subject it to increasing pressures while varying the temperature, and measure the deformation.
One block has been tested so far. San Jose State student Owen Pine did the data reduction and analysis on the first block of the series. Those results indicated that almost all the deformation in the block occurred across fractures and voids. Reference 4 Additional tests were completed recently.
"The results have significant implications for the flow and transport properties of the rock," said Blair. "For instance, it appears that cracks, fractures, and other open spaces perpendicular to the maximum principal stress will close over time. That means the rock will become less permeable in this direction. In addition, the tests show that pre-existing hairline cracks parallel to this stress may open over time. That will increase the permeability of the rock in that direction."
In the next test, Blair and his colleagues will add water to examine how water flow through the blocks of rock varies with pressure, temperature, and time.
Blair is one of many Laboratory principal investigators who occasionally employ students, teachers, and faculty through the Laboratory's Education Program.
The Benefits of Collaboration
"I've found that for certain projects--particularly ones that are limited in time, effort, and money and that have general tasks--students and teachers can fill a niche. The radiation tests are an example. Teachers and undergraduate-level college students have the broad skill sets we need, and I believe they get something from the experience too," he said.
High school teacher John Kelly and college students Chris Pena and Austin Woffington agree.
During his Science and Engineering Research Semester at the Laboratory, Woffington increased his knowledge of geology, worked with a variety of software programs, and learned to give effective technical presentations.
"The networking with other scientists and students was extremely valuable to me," said Woffington. After getting to know scientists in the Environmental Programs Directorate, he developed an interest in groundwater modeling.
Chris Pena, now a graduate student at San Jose State University in environmental engineering, was introduced to the Laboratory through one of her SJSU undergraduate professors.
With Blair as a mentor, she honed her analytical and research skills and gained valuable experience writing technical papers and giving technical presentations. "I couldn't have gotten that experience elsewhere," she said.
John Kelly teaches mathematics at Arroyo High School in San Lorenzo. His summers at the Laboratory as part of the Summer Research Internship Program for teachers helped him become one of two technical "mentor teachers" for the San Lorenzo School District. "As a mentor teacher, I conduct technical workshops for other teachers and work on the district's Educational Technology Committee," he said.
Blair also sees a benefit in direct outreach to the public. "This is a way to get grassroots support for science and research," he explained. "Students and teachers talk about their Laboratory experiences and pass on what they have learned. Those who come to us are the ones who go the extra mile, who are ambitious and curious. They may be future leaders, and we have a golden opportunity to introduce them to the value of research and broaden their experience. Who knows? Some may work in our fields someday and be future collaborators as well."
Key Words: geomechanics, nuclear waste repository, tuff, Yucca Mountain Project.
1. For more information on the Yucca Mountain Project and the Laboratory's role, see "The Safe Disposal of Nuclear Waste," Science & Technology Review, UCRL-52000-96-3 (March 1996), pp. 6-16.
2. C. Pena, S. C. Blair, P. A. Berge, Image Analysis of Tuff from the Yucca Mountain Project (Abstract). 1995 Fall Meeting, American Geophysical Union, December 1995.
3. S. C. Blair, J. M. Kelly, O. Pine, R. Pletcher, P. A. Berge, Effect of Radiation on the Mechanical Properties of Topopah Spring Tuff, Lawrence Livermore National Laboratory, Livermore, CA, UCRL-ID-122899 (January 1996).
4. O. Pine, S. C. Blair, P. A. Berge, Mechanical Behavior of a 0.5 m Block of Topopah Spring Tuff under Uniaxial Compression. 1995 Fall Meeting, American Geophysical Union, December 1995.
For further information about the rock mechanics projects contact Stephen Blair (510) 422-6467 (firstname.lastname@example.org).
For further information about the Laboratory's Education Program contact Eileen Vergino (510) 422-3907 (email@example.com).