Assessing Long-Term Volcanic Hazards to the Geologic Disposal of Nuclear Waste
Approximately 20,000 metric tons of radioactive and highly toxic waste, the by-product of this country's nuclear power and weapons industries, are stockpiled in temporary storage areas awaiting safe isolation from the environment. Environmentally responsible disposal of this waste is a continuing major concern. For 20 years many nations, including the United States, have committed themselves to the development of underground repositories to contain the material. These repositories, none of which have been built, will function under the premise that high-level radioactive waste can be isolated from the environment for 10,000 years or longer. Because hazards have never been evaluated for time scales of such magnitude, assessing the safety of a geological repository poses significant engineering and scientific challenges.
At Southwest Research Institute's Center for Nuclear Waste Regulatory Analyses (CNWRA), scientists are conducting a number of investigations related to the safety and likely performance of the proposed Yucca Mountain repository in southern Nevada. (See more on CNWRA.) Study results are submitted to the U.S. Nuclear Regulatory Commission (NRC) for consideration as it prepares to deliberate licensing of the repository at Yucca Mountain, currently the only site in the United States being considered for the disposal of high-level radioactive waste.
Situated about 150 km northwest of Las Vegas, Yucca Mountain is located in an arid region, the Western Great Basin. The groundwater table is hundreds of meters below the ground surface, and the proposed repository horizon is in the unsaturated zone well above the water table. Groundwater recharge and flow through the unsaturated zone are believed to be quite low. These conditions are favorable to the long-term isolation of radioactive waste. However, the region is also one of the most tectonically active areas in the country, which introduces the possibility of long-term geologic hazards.
Plans call for the repository to store 70,000 metric tons of waste within a 56 km2 area that will be excavated at a depth of 300 m beneath the crest of Yucca Mountain and 50100 m beneath the surrounding alluvial valleys. The mountain is a tilted block of densely welded ignimbrite, volcanic rock deposited by voluminous eruptions 1114 million years ago. The position of the repository within this tilted block is limited by faults that bound and penetrate Yucca Mountain.
Because Yucca Mountain is in a tectonically active area, CNWRA scientists are evaluating volcanic hazards in the area with two primary goals: to develop spatial probabilistic models suitable for comparison with geological models of the region, and to gain a better understanding of the consequences of volcanic activity for repository performance through studies of analogous, modern cinder cone eruptions. This work has resulted not only in a better understanding of volcanic hazards in the Yucca Mountain region and in an estimate of the likelihood of volcanic eruptions at the repository site, but in a better understanding of several types of volcanic processes and their hazards in general.
Volcanic Hazard Analysis
The analysis of volcanic hazards at Yucca Mountain is a unique problem because of the required duration of waste isolation. Volcanic hazard analyses are normally made for relatively short periods of time (from immediate to years or decades). For example, during the 1991 eruption at Colima Volcano, Mexico, volcanologists were most concerned about the intrusion of magma into the volcano and the short-term risks posed by the intrusion to several thousand people living on the flanks of the volcano. In contrast, volcanic hazards at Yucca Mountain are being assessed for time periods of 10,000 years or more. This is such a long time that many geological processes must be considered, including magma generation and supply, and the rate of change in these processes.
Volcanic hazards at the site involve the repository's proximity to small-volume basaltic volcanoes. In general, these types of volcanoes erupt in a single episode of activity that lasts for days to years. Renewed activity in the area will probably involve the formation of new volcanoes, such as cinder cones and lava vents. In contrast, nearly all previous volcanic hazard studies have concentrated on the likelihood of renewed eruptive activity at individual, large-volume volcanoes such as Colima Volcano. The dispersed nature of volcanism in the Yucca Mountain region has necessitated the development of new probabilistic approaches in hazard analysis. Probabilistic models treat the possibility of new eruptions and the formation of new volcanoes as random events, the distributions of which are governed by some underlying, but poorly understood, probability density function. Methods used to estimate probability density functions for volcanism and the application of the results in hazard analysis are often debated.
Styles of basaltic volcanism observed in modern eruptions range from relatively low energy eruptions that produce gently effusive lava flows, to high energy eruptions that sustain ash columns to heights of 510 km and that often ballistically propel large blocks of rock several kilometers from the vent. This range of activity, coupled with secondary thermal and geochemical effects, creates uncertainty about the consequences of volcanism for repository performance.
Although future volcanic eruptions are unlikely to occur near the proposed geologic repository at Yucca Mountain, their occurrence would be highly deleterious. An understanding of the volcanic processes that have operated in the Yucca Mountain region in the past, and that may operate there in the future, must be fully elucidated by modern volcanological investigations. As far as possible, uncertainty about volcanic hazards must be resolved during the prelicensing stages of investigation, which will extend through the next several years.
Volcanism in the Yucca Mountain Region
The Yucca Mountain region has experienced tremendous volcanic activity during the last 14 million years. From approximately 14 to 11 million years ago, a series of cataclysmic eruptions deposited nearly 4,000 km3 of volcanic rock over the area, rock which now makes up the strata within which the proposed repository would be constructed. That episode of volcanism led to the formation of several large calderas, 20 km or more in diameter. Collectively, the calderas are called the Southwest Nevada Volcanic Field. Since the calderas were formed, basaltic eruptions have occurred across the region. These eruptions were quite different from the earlier, caldera-forming silicic eruptions, in that they were small in volume, occurred over a broad region, and continued intermittently until the most recent activity, about 100,000 years ago. Volcanoes formed by the basaltic eruptions include cinder cones, small isolated lava vents, spatter mounds, and the eroded remnants of such features. This most recent activity represents the greatest volcanic risk to the safe isolation of radioactive waste at Yucca Mountain.
More than 30 Miocene-Quaternary basaltic volcanoes, formed between approximately 9.4 and 0.1 million years ago and distributed over approximately 2,500 km2, are contained in the Yucca Mountain region. An initial problem in determining volcanic hazards is identifying and mapping these volcanoes, and then determining their ages. Recognizing and mapping volcanic vents is particularly difficult for many Pliocene (5.3- to 1.6-million year old) and older volcanic centers. Some of the late Miocene volcanic centers have been greatly eroded since their formation between nine and five million years ago, removing most of the evidence for vent locations. Alluvial sedimentation also has buried several Pliocene basaltic volcanoes in the Amargosa Valley, about 25 km south of Yucca Mountain; these volcanoes have been identified primarily by aeromagnetic surveys of the region. Although some of the aeromagnetic anomalies have been drilled, revealing Pliocene age basalt, others have not. Thus, there is uncertainty about the timing and extent of Pliocene volcanism, and this uncertainty impacts estimates of volcanic hazards.
Numerous research organizations have published more than 200 isotopic age determinations for the region's young basaltic rocks. Some controversy exists about the geochronology of basalt eruptions, but a general picture of volcanism has emerged most of the basaltic volcanoes formed in the last five million years are concentrated in three clusters.
The most active of the clusters is Crater Flat, the center of which is located 11 km from the crest of Yucca Mountain. The Crater Flat Cluster includes an 8-km long alignment of six vents, formed approximately 3.7 million years ago, and a 10-km long alignment of five volcanoes, formed approximately one million years ago. Such alignments are common features of basaltic volcanism. They indicate that eruptions occurred along pre-existing fractures, such as faults, or that the volcanoes are fed by a single set of dikes, sheets of magma that propagate through the crust. The repository site is located about 8 km from the closest of these one-million year old volcanoes.
A second cluster is located 40 km north-northwest of the site, in an area known as Sleeping Butte, on the Nellis Air Force Base Range. As with the Crater Flat Cluster, the Sleeping Butte Cluster consists of cinder cones that vary in age the youngest are 0.3 million years old.
A third cluster is centered approximately 30 km south of Yucca Mountain and includes the buried Pliocene volcanoes of the Amargosa Valley. Considerable controversy surrounds the history of the youngest volcano in the Amargosa Valley Cluster, Lathrop Wells.
A number of argon radiometric (40Ar/39Ar) age determinations made by the U.S. Geological Survey indicate that Lathrop Wells formed about 120,000 ± 20,000 years ago. Researchers from Los Alamos National Laboratory and the U.S. Department of Energy have used other dating methods, employing cosmogenic isotopes and thermoluminescence techniques, to suggest the volcano may have reactivated and erupted as recently as 40,000 years ago, or possibly even 8,000 years ago. The dating of a volcano as young as Lathrop Wells pushes each of these relatively new techniques to the limits of precision and accuracy. The possible reactivation of cinder cones long after their formation is a new idea in volcanology and will remain controversial until sufficient evidence can be found in the geologic record to evaluate the hypothesis. In the case of Yucca Mountain, the reactivation hypothesis is important because additional young eruptions at Lathrop Wells would indicate an increased state of volcanic activity in the region.
On geological time scales, the Yucca Mountain region is an active volcanic field. Volcanism has been concentrated there in three clusters during the last five million years. Each cluster contains cinder cones and lava vents that formed during the Pliocene, and each cluster has been the site of recurring volcanism within the last one million years. Such patterns in basaltic volcanism lend credence to probabilistic methods future volcanism is more likely to occur within or near existing clusters, rather than elsewhere in the Yucca Mountain region.
Geologic studies indicate that basaltic volcanism in the Yucca Mountain region behaves similarly to basaltic volcanism elsewhere in the Western Great Basin. Models describing the recurrence rate, or probability, of basaltic volcanism in the Yucca Mountain region should reflect the clustered nature of basaltic volcanism and shifts in the locus of that volcanism through time. Models should also be amenable to comparison with basic geological data, such as fault patterns and neotectonic stress. These factors may affect volcano distributions and the development of volcano alignments on a more detailed scale.
Using the probability models developed by SwRI researchers to address volcanic hazard analysis (see sidebar), the likelihood of a volcanic eruption occurring at the repository site is calculated to be between 15 × 104 in 10,000 years. These calculations are made for an 8-km2 area, including the area of the proposed repository and a small surrounding buffer zone.
The probability of future volcanism occurring at or near the proposed repository can be illustrated by analogy. Suppose an individual is asked to pick a number between one and 10,000 and is given one to five tries (it is uncertain how many) to guess the number. Of course, people play against even larger odds, and hope to win, all the time. It is more difficult to decide if odds are favorable when the probability of a catastrophic event such as a volcanic eruption is considered.
The probability range of 15 × 104 in 10,000 years is the same as, or slightly greater than, ranges indicated by many other research teams. However, this basic agreement must be tempered by an additional result of the probability models developed at the CNWRA, which indicates that the proposed repository is positioned on a probability gradient because of its proximity to Crater Flat. (See sidebar.) Immediately west of the proposed site, the probability of volcanism increases by about one order of magnitude, to more than 1 × 103 in 10,000 years, because of the presence of Quaternary volcanoes in Crater Flat Valley. The probability of a new volcano forming within an 8-km2 area 20 km east of the site is on the order of 1 × 105 in 10,000 years or less. Thus, within 20 km of the proposed site, and within a given 8-km2 area, the probability of volcanism during the next 10,000 years varies by more than two orders of magnitude. This rapid change in probability, resulting from clustering in volcano distribution, has important implications for the uncertainty associated with the use of probability models.
Returning to the analogy, the uncertainty of volcanic activity in the Yucca Mountain area would now be equivalent to asking a person to guess a number between one and 100,000, but not revealing whether there is one chance or 100 to guess the number. This high degree of uncertainty has led CNWRA volcanologists to explore additional, more deterministic aspects of volcanic hazards.
Beyond Probabilistic Hazard Analysis
No traditional volcanic hazard analysis relies solely on probabilistic methods. The methods developed at the CNWRA provide bounds on probability by treating volcanism as a spatio-temporal point process. However, additional relevant geologic information is sacrificed in this type of analysis. One such piece of information is the distribution of faults that may, under appropriate conditions, act as conduits for ascending magmas. Clearly, fault distribution and related information must be incorporated in a complete volcanic hazard analysis.
Dr. David Ferrill, a structural geologist at the CNWRA, and Dr. Alan Morris, an associate professor at the University of Texas at San Antonio, have developed a technique for analyzing the tendency of mapped faults to dilate in the lithospheric stress field. The technique involves using estimates of the crust's three-dimensional stress field and the orientation of faults within the field to map fault dilation tendency, which is calculated from the three-dimensional stress tensor and the normal stress vector across the dipping fault plane. If it is assumed that magmas will more likely follow faults with a high dilation tendency than not, a fault dilation tendency map can be calculated and combined with a volcano probability map, resulting in one map that shows the intersection of high dilatancy faults and zones of high eruption probability.
Application of this technique to hazard analysis at Yucca Mountain reveals that there is a large concentration of high dilation-tendency faults within the high probability zone defined by the Crater Flat cluster, that these faults extend into Yucca Mountain, and that existing volcanoes lie along, or form alignments parallel to, high dilation tendency faults near Yucca Mountain. One interpretation of the combined volcano probability and fault dilatancy maps is that structural conditions at Yucca Mountain differ little from those at Crater Flat or Lathrop Wells. If high dilation tendency faults act as conduits, then conduits are available for the transport of magma to the surface within or near Yucca Mountain. This implies that the probability of eruption is equal to or greater than the mean value of 15 × 104 estimated using probabilistic methods. The interpretation can be tested by further analyzing fault geometries and by investigating fault-dike interactions in geologically analogous areas.
Consequences of Basaltic Volcanism
Volcanism is traditionally viewed in terms of the frequency and occurrence of short- lived but violent episodes of eruption. In contrast, the risk of volcanism at Yucca Mountain is one element in a complex cost-benefit equation that affects much of our society and that will continue to affect our descendants. From a volcanic hazards perspective, there are two central issues in assessing the consequences of basaltic volcanism that are important in hazards assessment evaluation of the direct effects of volcanic eruptions on the repository and evaluation of the indirect effects of eruptions. Direct effects include the transport of radionuclides to the surface by magma and the dispersal of radionuclides by volcanic eruptions. Indirect effects include changes in the thermal, hydrologic, and geochemical settings of the repository as a result of nearby volcanism, which could cause accelerated movement of radionuclides out of the repository. Although these issues tend to be discussed separately, they are linked in any process model.
Cinder Cone Eruptions
The magnitude and consequences of direct volcanic eruptions at the repository site are unknown. Some investigators contend that the consequences of volcanism are slight and that lava might gently encapsulate waste as it rises to the surface. In such a benign scenario, a volcanic eruption might not disperse radionuclides at the surface. However, this view represents one end of a spectrum of eruption styles the kinetics of the eruptive process usually lead to more energetic activity. Rising magma often contains sufficient dissolved volatiles and ascends at a rapid enough rate for bubble nucleation and growth to greatly increase bulk volume and flow velocity. Eventually, bubble density reaches up to approximately 70 percent by volume of the magma-gas mixture, at which point the magma fragments. Fragmented magma easily erodes wall rock and results in explosive volcanic eruptions. Under these conditions, common at small-volume basaltic cinder cones, radionuclides could readily be transported to the surface and dispersed in the atmosphere. Interaction with groundwater could produce even more explosive eruptions.
Several explosive eruptions at cinder cones have occurred recently. In 1992, Cerro Negro, a small basaltic cinder cone in Nicaragua, erupted for several days, and the ash produced by the eruption forced the evacuation of almost 20,000 people from León, the country's second largest city. Approximately 3 × 107 m3 of magma was expelled, nearly all of it in an initial 18-hour phase. The convective eruption column was sustained at a height of approximately 8 km for the same period of time. Mass flow of magma from the crater was 300500 m3/s during those 18 hours, and steady thermal energy release was between 5 × 1011 and 2 × 1012 watts. While the eruption was occurring, the cone actually lost volume and elevation as the crater's diameter widened from 100 to 250 m. Ash was dispersed more than 60 km downwind by the eruption and accumulated to more than 1-cm thick, 30 km from the cone.
Observations of eruptions such as the one at Cerro Negro have led CNWRA volcanologists to develop an analog approach to a study of the consequences of volcanism on repository performance. The goals are to gather volcanological, geophysical, and geochemical data at several modern cinder cones, and to compare the data with observations made at Yucca Mountain cinder cones to further evaluate the range of eruptive styles that might impact the repository. Analogs selected include Cerro Negro, Parícutin Volcano in Mexico, and the Tolbachik cinder cones in Kamchatka, Russia. The Tolbachik cones, in particular, exhibited an incredible spectrum of eruptive activity during their formation in 1975, ranging from a comparatively gentle effusion of lavas, feeding lava lakes and creating fire fountains of lava less than 200 m in height, to explosive eruptions that injected ash into the stratosphere, sustained an eruption column at 12 km above sea level, and resulted in ash dispersal up to 1,000 km downwind.
An intuitive way to use data gathered at these volcanoes is to accept that the ranges of eruptive activity at modern analogs represent the ranges of activity that may have occurred in the Yucca Mountain region in the past. Dr. Brittain Hill, a CNWRA volcanologist, is leading the effort to quantify this relationship by comparing the rheologic properties of magmas at modern and ancient volcanoes. Indications of magma rheology are preserved long after eruptions in the composition, microcrystallinity, and vesicle size distribution in lavas and ash, and by related rock properties. Initial findings point to similarities between magma rheologies in many different geological settings and suggest that some of the eruptions near Yucca Mountain were quite explosive, rather than low energy and effusive. Continued investigations of the magma rheology of modern and ancient cinder cones will be an important aspect of estimating the impact of volcanic eruptions on radionuclide dispersal in the environment.
Cinder Cone Cooling and Degassing
Analog studies at the CNWRA also focus on the collection of volcanic gases and temperature measurements at cooling cinder cones, to assess the long-term impact of cooling and degassing on the hydrological and geochemical setting. For example, the increased thermal load from volcanism on the repository and the surrounding rocks could cause changes in groundwater flow. Similarly, volcanic gases could change the geochemical setting of the repository. These changes could result in accelerated dispersion of radionuclides.
The indirect effects of volcanism are as or more likely to impact repository performance than direct eruption of radionuclides, because indirect effects are still of consequence if igneous dikes are injected close to, but not within, the repository. The most direct way to study the longevity and scale of transitory heat and gas transfer processes is to make field measurements at cooling cinder cones. The CNWRA has undertaken such an effort at the Tolbachik cones, on the Kamchatka Peninsula of Russia, in close cooperation with Russian volcanologists from the Institute of Volcanic Geology and Geochemistry in Petropavlovsk, Kamchatka.
Even 20 years after the cessation of eruptive activity, the Tolbachik cones are surprisingly hot and continue to degas at a slow rate. Most degassing takes place at arcuate fractures formed on the crater rim of Cone I, the first cone to form during 1975. A nearly dry gas escapes from the fractures by forced convective flow at temperatures of up to 630° C. Measurements of major element chemistry and of oxygen and hydrogen isotopes indicate that the gas emanates from an extremely degassed basaltic magma, with little opportunity for interaction with meteoric groundwater.
The team constructed a series of temperature maps, monitored soil degassing over a broad region on and about the volcano, and made electromagnetic soundings of the cone to construct a view of the heat and mass transfer process within and beneath the volcano. These field measurements revealed in detail the structure and dynamics of cinder cone cooling and degassing. Electromagnetic soundings revealed a volume of rock within the cone that is still heated above the Curie point for basaltic scoria, approximately 550° C. This hot zone is as shallow as 100 m beneath the arcuate fractures on the cone rim and extends well below the base of the cone into the older, underlying rock.
Near the base of Cone I, the team monitored the cooling of an igneous dike that reached close enough to the surface to cause significant ground deformation and alteration. Temperatures in the basalt scoria above the dike reach 537° C at depths of less than 2 m. Such high temperatures are maintained because of the low thermal conductivity, high porosity, and related thermophysical properties of the scoria the dike intrudes.
The data and observations from Tolbachik will enable researchers to construct more accurate models of cooling and degassing processes than are now available. Dr. Peter Lichtner and Dr. Randall Manteufel of the CNWRA are building finite-element conduction and convection heat transport models describing phenomena observed and documented in Kamchatka. Of particular interest is the behavior of groundwater under such conditions phase changes that occur near the cooling dike, changes in groundwater flow, and the relationship between phase changes and the rate of wall rock alteration. Preliminary interpretations are strikingly different from previous models of cinder cone and dike cooling and degassing, emphasizing the complexity of volcanic systems and the need to use modern volcanoes in an investigation of potential hazards at the Yucca Mountain site
A concerted effort is under way at the CNWRA to identify and quantify volcanic hazards associated with the proposed long-term disposal of nuclear waste in a geologic repository at Yucca Mountain, Nevada. This type of hazard analysis is the first of its kind in volcanology it encompasses considerations of the dispersed nature of basaltic volcanism, the exceptionally long-term performance objective of the site, and the deleterious consequences of repository failure. As our society expands and becomes more complex, and as more facilities are designed with similar performance objectives, this type of hazard analysis will become increasingly necessary.
Analyses at the CNWRA indicate that the probability of volcanic eruptions within and near the repository site is on the order of 15 × 104 for a 10,000-year confinement period, but may be as high as 1 × 103, given the uncertainties of these estimates. The goal of continuing CNWRA volcanological research is to further address uncertainty concerning the probability and consequences of volcanic activity through the innovative application of modern volcanological techniques. Ultimately, it will be the responsibility of the NRC to weigh the risks posed by volcanism, quantified by investigations at the CNWRA and elsewhere, against the benefits of isolating radioactive waste at Yucca Mountain. (See acknowledgments.)
Published in the Summer 1995 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.