Scientists look to nature for insight into how a repository would perform

Before a repository at Yucca Mountain can be built, scientists and engineers must demonstrate to the Nuclear Regulatory Commission a reasonable expectation that the repository would safely isolate highly radioactive materials for thousands of years. This is a difficult task — no country has ever buried spent nuclear fuel or high-level radioactive waste in an underground repository. How can we know it can be done safely and effectively?

Researchers have developed mathematical models that show how a repository would most likely respond to changing climatic, geologic, and hydrologic conditions over a vast expanse of time. These models encompass much of what we have learned about Yucca Mountain during more than two decades of intense study. But there is no real way to test the models directly, to see if events actually do unfold as predicted. And no model can inspire the confidence that comes from evidence people can actually touch or see.

Fortunately, the real world offers many examples of geologic, and even man-made, structures that provide or otherwise embody the kind of protective systems that a repository would present against escaping radioactive particles. Researchers call these examples analogues. These analogues offer direct, real-world analogies for some of the long-term processes that would occur in a repository.

Many of the analogues studied suggest that a repository at Yucca Mountain would protect the public from hazardous exposures to radiation for thousands of years. For example, analogues imply or reinforce the idea that a deep geologic repository at Yucca Mountain would probably be very effective:

• The analogue systems show us that dry environments can be maintained for thousands of years despite the presence of water.
• Many different kinds of things have been preserved in dry environments for very long periods.
• Similar structures and situations — caves, rock ledges, underground tunnels and tombs, and even the insides of the Great Pyramids — have preserved normally very fragile items for thousands of years in dry, oxidizing environments similar to that of Yucca Mountain.

By studying analogues, we can learn how naturally occurring prehistoric materials and human artifacts can be, and have been, preserved underground, sometimes in environments more extreme than Yucca Mountain’s. We can also learn about the behavior of water in places similar to Yucca Mountain. And we can see how naturally occurring radioactive particles have moved through similar types of rock over long periods of time.

No single analogue contains all aspects of the proposed repository at Yucca Mountain. But striking examples of the major processes that would take place in a repository do occur in nature. This fact sheet offers a brief overview of some analogue sites that share many of the conditions, features, and processes that exist at Yucca Mountain.

Project researchers study those analogues that have the greatest similarity to conditions at a possible Yucca Mountain repository. First, they study the great volume of work already done on analogue sites around the globe.

Project scientists also do their own fieldwork.

With this combined research, they have identified existing natural and man-made analogues for the following aspects of the proposed repository:

• Highly radioactive waste materials and spent nuclear fuel that would be buried at Yucca Mountain
• Materials that would be used to construct the man-made (engineered) barrier system, such as waste packages and drip shields
• Movement of water in the unsaturated rock above the proposed repository level
• Movement of radioactive particles in the rock below the proposed repository level

Analogues alone cannot inspire complete confidence in a mathematical model, of course, but they can add to the sometimes limited data available from field and laboratory studies and from site monitoring. Carefully selected analogues provide a practical and realistic way to develop and test models for, and to build understanding in, how a repository would function under different conditions.

Transport by water is the primary way radioactive particles could move out of a repository to the human environment. Mathematical models suggest, however, that little, if any, water would get into the tunnels where the waste would be disposed.

Researchers believe that caves containing prehistoric paintings, such as those found in southern France, provide excellent natural analogues for the flow of water in the unsaturated¹ zone that would host a repository at Yucca Mountain. The 30,000-year-old paintings in these caves were made with oxides of iron and small amounts of manganese, as well as clay, charcoal, and silica. None of these materials would survive long in the presence of abundant water. Yet many such paintings have survived in locations far more humid, and with more than three times the rainfall, than Yucca Mountain.

Those paintings survived because water tends to flow around caves and tunnels, not into them, in part because of the comparative size of the different openings. In unsaturated rock, what little water is available in small fractures has a tendency to remain in the fractures rather than flow into larger openings, such as caves or tunnels. Most of the caves identified as natural analogues have as large or larger a cross-sectional area than that proposed for the repository at Yucca Mountain. Seepage into repository tunnels, therefore, is expected to be minimal.

Another example of nature’s own power to preserve can be found in the many very old, highly preserved packrat middens (refuse heaps, used by packrats as their nest as well). Packrats compose their middens of twigs, debris, and their own droppings, all held together by their own dried urine. Such biologic remains quickly decompose if exposed to much water. Yet in underground burrows throughout the desert Southwest, scientists have found numerous completely intact packrat middens that are up to 50,000 years old.

The Oklo uranium mine, in Gabon, Africa, contains the only known examples of natural fission reactors. It remains perhaps the world’s best-known natural analogue.

In 1972, French scientists discovered a peculiarity in the Oklo ore. They concluded that it could only be explained by assuming that a naturally occurring nuclear chain reaction had taken place there two billion years earlier. Today, uranium must be enriched by human beings to sustain fission (atom splitting). When the Oklo chain reactions happened, though, nature created enough enriched uranium to sustain underground nuclear chain reactions that continued, intermittently, for a million years or more.

Oklo’s natural reactors are of special interest in studying possible repositories. Scientists can look at the remains of the radioactive by-products produced during fission and see where these radioactive particles ended up. Some of these fission products are the same as those that will be disposed of in a repository. By studying evidence of the by-products’ behavior over thousands of years, scientists gain a better understanding of how these same substances would behave over long periods of time if released from a geologic repository.

Geochemical observations of the Oklo site suggest that less than 10 percent of the uranium and fission products involved in the spontaneous chain reaction moved into the surrounding rock. The distance moved by radioactive particles at Oklo was small because clays and iron-bearing minerals, similar to those in Yucca Mountain, held the particles close to the site of origin. Scientists anticipate that a repository at Yucca Mountain would perform at least as well as the Oklo site in preventing radioactive particles from escaping into the environment.

The Peña Blanca uranium district is in the northern part of the Sierra Peña Blanca mountain range in Mexico. During the 1980s, geologists identified the Nopal I mine, in the northwest portion of the district, as a useful analogue for unsaturated zone¹ processes at Yucca Mountain:

• The uranium deposits at Nopal I occur in a similar environment.
• Both sites are located in semi-arid to arid regions.
• Both are situated in fractured volcanic rock.
• Both sites share a similar mineralogy.
• Both sites lie within the unsaturated zone¹ and 100 meters (328 feet) or more above their respective water tables.

Rainier Mesa lies some 32 kilometers (20 miles) northeast of Yucca Mountain. Workers excavated a number of tunnels into the mesa for the underground weapons program at the Nevada Test Site. Both Rainier Mesa and Yucca Mountain consist of alternating layers of welded and nonwelded volcanic rocks. These contain similar minerals that would slow the travel of radioactive particles. Rainier Mesa receives somewhat more rain per year and, therefore, provides a good analogue for studying seepage under wetter climate conditions, as well as for studying the ability of minerals to slow escaping radioactive particles.

Work remains to be done at several of the sites identified as natural analogues. At Peña Blanca, researchers plan to drill a borehole and remove rock core that will provide further information about the movement of radioactive particles. Data from the Yellowstone geothermal field and from Paiute Ridge, Nevada, are being used to understand how heat changes rock and mineral properties and affects the circulation of water. Ultimately, scientists hope that the data from these and other analogue sites will provide an enhanced confidence and a degree of independent validation for selected aspects of expected performance in a potential repository at Yucca Mountain.

1 The unsaturated zone of soil or rock is the region below the ground surface and above the water table in which the pore spaces are not completely filled with water. Instead, the pores contain water, air, and other gases. Although the water saturation of the pores is below 100 percent in this zone, there may be some sections of “perched water” (limited portions of the rock having 100 percent saturation) in the overall unsaturated zone as well.