Last year, Congress and the President approved the lone site characterized by the Department of Energy as suitable for the disposal of high-level nuclear waste at Yucca Mountain, Nevada. Now, large quantities of highly toxic waste produced at the nation’s nuclear power plants and the nuclear weapons complex may have a place to go-that is, if the Nuclear Regulatory Commission approves the location for licensing.DOE intends to submit a license application to the NRC by the end of 2004, though that date appears to be, in DOE lingo, “slipping.” (Actually, the law required the DOE to submit a license application within 90 days of the formal approval of Congress-that would have been September 2002-but who’s counting?) In light of the fact that many consider Congressional approval of the site to be the final word, let’s examine whether the site will actually be able to perform properly and keep the public safe from radioactive contamination for 10,000 years. A group of scientists at MIT’s Security Studies Program has found a number of unresolved technical issues that may affect the safety of a geologic repository at Yucca Mountain.
It is important to understand the enormous scope of the task at hand: namely, determining whether any particular site is suitable to host a nuclear waste repository. For one thing, we will never know whether the site has been successful because of the time scales involved. Moreover, the job of predicting repository performance over time is not an engineering endeavor-it is a science project based on the geological sciences. Geology explains the past but does not predict the future with any precision.
DOE maintains that it chose Yucca Mountain because the area was dry-the climate is arid and the water table deep-and therefore waste packages would be preserved. There are two problems with that. First, Yucca isn’t really that dry. Secondly, it turns out that wet conditions are actually better for extending the life of spent fuel; in fact, the United States is the only country pursuing a dry setting for a long-term nuclear storage repository. As a result of its policy, DOE has neglected to explore a number of significant issues associated with the corrosion of spent fuel.
There are other unanswered questions that could affect the performance of Yucca Mountain for holding nuclear waste. For one, it’s not clear that DOE has correctly predicted the increase in precipitation over the next ten centuries, leading to uncertainty about the probability that water could infiltrate the repository. In particular, DOE has not adequately accounted for the likely increases in temperature and rainfall that will occur due to anthropogenic climate change.
Moreover, DOE’s understanding of water transport through the rock above the water table remains in its infancy. In current models, water moves rapidly along fractures in the rock and much more slowly via grain boundaries. The question is: How is water transported along fractures-and which fractures would transport that water?
DOE relies heavily on the performance of Alloy-22, a chromium-nickel-molybdenum alloy, to endow the waste packages with a predicted 10,000 years of resistance to corrosion. This reliance stems from two years’ worth of corrosion research in the laboratory. But there is good reason to be skeptical about these results. No natural analogs were studied, because there are none. No long-term studies were conducted. And the complex conditions that will evolve over time around the waste package are still poorly understood.
DOE assumed that radionuclides in the nuclear waste, such as plutonium, would not move far from the repository because they are not soluble in water. This may be wishful thinking, however. We know, from evidence collected at the nearby Nevada Test Site, that plutonium can adhere to tiny particulate material called colloids and thus be transported long distances. But very little work has been done to investigate this phenomenon at Yucca Mountain, so a number of important technical issues are unresolved. In particular, we know very little about how radionuclides would be transported in the so-called saturated zone (that is, below the water table)-simply because we know very little about the saturated zone itself. This ignorance stems from the paucity of test wells in the area.
Nor is water the only major source of uncertainty. The potential for volcanism at Yucca Mountain remains a contentious issue as well. The Nuclear Regulatory Commission puts the probability for volcanism at the repository an order of magnitude higher than does DOE.
On the basis of these few issues, it is clear that DOE has much more work to do to show that Yucca Mountain is a suitable location for the long-term storage of nuclear waste. There is actually no great hurry in moving spent fuel away from reactors-it can be stored safely for many decades in dry casks. The United States should take its time to ensure that it has selected a reasonable location for nuclear waste disposal: the issue is too important to rush.